McGraw Hill Encyclopedia of Science & Technology

L

Libra — Lytic reaction

nal equinox, with daytime and nighttime of equal length, used to occur in Libra, though the precession of the equinoxes has moved that point to Virgo. See EQUINOX; PRECESSION OF EQUINOXES; VIRGO; ZODIAC. Libra’s brightest stars have the Arabic names Zubenelschamali and Zubenelgenubi—meaning northern and southern claws—that show that they once belonged to Scorpius. See SCORPIUS.

Libra The Scales, a southern zodiacal constellation (see illustration). It has no bright stars. Traditional pictures show a set of scales (a balance) with pans held by chains. Perhaps its name comes from the nearby constellation Virgo, who was the goddess of justice and who would have used scales to balance good and evil deeds. Also, the autum-

right ascension 16h

15h Magnitudes: 1.5–2.0

0°

2.0–2.5

SERPENS CAPUT

2.5–3.0

VIRGO OPHIUCHUS

3.5–4.0

ε

declination

–10°

4.0–4.5

δ

Zubenelschamali

β

4.5–5.0

ϑ λ

ν

ζ

Variable

α

tic

eclip

Zubenelgenubi

LIBRA

ι

κ

5.5–6.0

µ

γ η

5.0–5.5

ξ2 ξ1

48

–20°

3.0–3.5

σ

SCORPIUS

υ

HYDRA

τ –30°

LUPUS

CENTAURUS ◦

Modern boundaries of the constellation Libra, the scales. The celestial equator is 0 of declination, which corresponds to ◦ celestial latitude. Right ascension corresponds to celestial longitude, with each hour of right ascension representing 15 of arc. Apparent brightness of stars is shown with dot sizes to illustrate the magnitude scale, where the brightest stars in the sky are 0th magnitude or brighter and the faintest stars that can be seen with the unaided eye at a dark site are 6th magnitude. (Wil Tirion)

2

Lichens The modern boundaries of the 88 constellations, including this one, were defined by the International Astronomical Union in 1928. See CONSTELLATION. Jay M. Pasachoff

Lichens Symbiotic associations of fungi (mycobionts) and photosynthetic partners (photobionts). These associations always result in a distinct morphological body termed a thallus that may adhere tightly to the substrate or be leafy, stalked, or hanging. A thallus consists of layers, that is, a cortex and medulla made up of the fungus, and a photosynthetic layer of algal or cyanobacterial cells that are closely associated with fungal hyphae. Rhizoids anchor thalli to their substrates. Some lichens (Stereocaulon) with green photobionts have gall-like structures called cephalodia that contain cyanobacteria. See CYANOBACTERIA. Lichens are formed from specialized groups of parasitic fungi; this association is one of a controlled parasitism rather than mutualism. Thus, the photobionts that lichen fungi slowly parasitize should be considered victims and not partners. Lichen-forming fungi share two characteristics with fungi that parasitize plants: concentric bodies and specialized branches of hyphae (haustoria) that penetrate host cells and absorb nutrients from them. Distribution. Lichens have a worldwide distribution and grow on almost any inanimate object, such as bark, soil, roof tiles, or stone. Some also grow on living leaves of tropical plants and ferns, while a few grow inside sandstone rocks in the dry valleys of Antarctica. Lichens dominate about 8% of the Earth’s surface, especially in the northern boreal forests of Canada and Siberia, where reindeer lichens form vast uninterrupted carpets of growth. They are among the hardiest of organisms and thrive in some of the Earth’s harshest environments, such as polar regions, deserts, and high mountains. Classification. The name given to a lichen applies only to the mycobiont, while the photobiont has a separate name. Most of the 15,000 lichen-forming fungi are in the fungal class Ascomycotina (ascolichens); common genera of lichens include Cladonia, Parmelia, Lecanora, Lecidea, and Usnea. Approximately a dozen species of basidiomycetes form lichens. Dictyonema pavonium is a common, tropical basidiolichen with lobed thalli and the bluegreen Scytonema as photobiont. Omphalina forms gilled basidiocarps and two types of associations with the green alga Coccomyxa in the form of globular clusters (Botrydina-type thallus) or small lobes (Coriscium-type thallus). Lichens that do not have sexual reproduction (Lepraria) are placed in the Lichenes Imperfecti. See ASCOMYCOTA; BASIDIOMYCOTA. Photobionts of lichens are either green algae (Trebouxia, Trentepohlia) or cyanobacteria (Nostoc, Scytonema). The most common photobiont is Trebouxia. This unicellular green algae has never been

found in the free-living state. It is believed that Trebouxia is a lichenized and highly modified form of the filamentous alga Pleurastrum terrestre. Growth and reproduction. Lichens grow very slowly. Indeed, the fastest-growing lichen from a temperate zone is Ramalina menziesii, which has a maximum annual radial growth of 1 in. (28 mm). Most temperate-zone lichens grow only 0.1–0.2 in. (2–6 mm) per year or less. Lichen growth can be measured in terms of an increase in linear dimension or biomass of thallus. Growth rates of thalli vary greatly according to season, habitat conditions, latitude, and age of the thallus, and even between the lobes of a single thallus. Individual lichens may live for hundreds and even thousands of years. Fragmentation of thalli is a common way that lichens reproduce. Other asexual reproduction is by means of propagules that are simply outgrowths of the thallus and contain a few photobiont cells closely enclosed by fungal hyphae. Sexual reproduction is presumed to be common among lichen fungi because of their genetic variability and the presence of fungal sexual structures such as spermatia and ascogonial filaments. Ascospores, which are products of sexual recombination, are produced and discharged commonly by many lichens. What happens to the mycobionts after the spores germinate is not clear. Developing hyphae from spores may even steal photobiont cells from other lichens and use them to form their own thalli. See REPRODUCTION (PLANT). Physiology. The basic metabolic processes of lichens are photosynthesis, respiration, and nitrogen fixation. Lichens have adapted these processes to different conditions of light, temperature, day length, and water. The mycobiont causes the photobiont to excrete most of the carbon that it fixes during photosynthesis. Only a single type of compound is excreted. The mycobiont absorbs these compounds and converts them to mannitol, its own storage compound. When dry lichens absorb water, they respire at levels much higher than normal for several hours. The mycobiont respires stored polyols, in the case of chlorolichens, instead of depleting valuable proteins and other structural compounds. See PLANT RESPIRATION. Nitrogen-fixing lichens are common and contribute nitrogen to different ecosystems when they decay. In cyanolichens the mycobiont inhibits the nitrogen-assimilating enzymes of the cyanobiont, causing it to release most of the ammonia it produces. The ammonia is absorbed by the mycobiont and used to make proteins and nucleic acids. See NITROGEN FIXATION. Chemistry. Lichens produce several hundred secondary compounds that accumulate as crystals in the thalli, often at high concentrations. These compounds may protect the slow-growing thalli from harmful bacteria, fungi, and insects and may play a regulatory role in the interactions between bionts. Lichen compounds have been produced by freeliving fungi, especially species of Aspergillus, and by mycobionts growing alone in culture. Lichen

Lidar secondary compounds represent a new class of antibiotics in an age where standard antibiotics such as penicillin are becoming ineffective against antibioticresistant microbes. Secondary compounds are used extensively by taxonomists to characterize new taxa of lichens (chemotaxonomy). Pollution indicators. Lichens have become useful bioindicators of air pollution because of their sensitivity to pollutants such as sulfur dioxide, hydrogen fluoride, ozone, and peroxyacetyl nitrate (PAN) and particulate pollutants such as lead, copper, and zinc. The most widespread and damaging pollutant for lichens is sulfur dioxide, which is most toxic when the pH of the substrate is acidic and the thallus is wet. Nitrogen fixation is the most sensitive to sulfur dioxide, followed by photosynthesis and respiration. Acid rain adversely affects lichen communities but also opens up new niches for acid-loving species. Pollution zones around many major cities have been mapped by using lichens. Lichens have also been useful in measuring radioactive fallout from explosions of nuclear bombs and nuclear reactors. See AIR POLLUTION. Vernon Ahmadjian Bibliography. V. Ahmadjian, The Lichen Symbiosis, 1993; V. Ahmadjian and M. E. Hale (eds.), The Lichens, 1973; C. F. Culberson, Chemical and Botanical Guide to Lichen Products, 1969; M. Galun (ed.), CRC Handbook of Lichenology, 3 vols., 1988; M. E. Hale, The Biology of Lichens, 1983; T. H. Nash and V. Wirth (eds.), Lichens, Bryophytes, and Air Quality, 1988.

transmitted beam

reflected beam

secondary mirror

telescope beam splitter pulsed laser beam-steering mirror

collimating lens optical filter

optical detector

laser control line

focusing lens

optical detector start pulse range computer and control system

stop pulse

time-interval unit

Fig. 1. Block diagram of a typical monostatic system used for ranging or altimetry applications.

systems or laser radars. See LASER; OPTICAL DETEC-

A product obtained from the licorice plant (Glycyrrhiza glabra) of the legume family (Leguminosae). It is a perennial herb which grows wild and is cultivated in southern Europe and in western and central Asia. The roots are dried for several months and then packaged for shipment. Spain leads in the production of cultivated licorice roots. Licorice is used in medicine to mask objectionable taste and as a laxative; as a flavoring material in the brewing, tobacco, and candy industries; and in the manufacture of shoe polish. See ROSALES; SPICE AND FLAVORING. Perry D. Strausbaugh; Earl L. Core

TORS; RADAR.

The optical analog of radar. The term lidar is an acronym for light detection and ranging. Lidar systems employ intense pulses of light, typically generated by lasers, and large telescopes and sensitive optical detectors to receive the reflected pulses. They are most commonly used to measure the composition and structure of the atmosphere. The very narrow beamwidth, narrow linewidth, and ultrashort pulses of the laser make it possible to optically probe the atmosphere with exceptional sensitivity and resolution. When used to measure the range and velocity of hard targets, lidars are usually called laser ranging

primary mirror

field stop

Licorice

Lidar

3

Ranging and altimeter systems. The most common lidar configuration is the monostatic system (Fig. 1). The laser beam is either projected through the receiving telescope or propagates parallel to the optical axis of the telescope. If the system is designed for ranging or altimetry, the receiver measures the roundtrip propagation time of the laser pulse between the lidar and the target. The distance d to the target is given by the range equation below, where d=

cτ 2

c is the speed of light and τ is the roundtrip propagation time of the laser pulse. Precision timing is accomplished electronically by a device called the time interval unit. A beam splitter directs a small fraction ( 0.6)

n -type GaAs absorbing substrate

n contact Fig. 1. Cross-sectional view of a typical light-emitting diode structure. The 250–350-µm-thick GaAs substrate serves as a template for epitaxial growth of the active layers and provides mechanical strength to the completed device. The p-type and n-type AlxGa1−xAs upper and lower confining layers form the pn junction that provides the means to inject excess electrons and holes into the active region. These layers are usually 1–2 µm thick. The Al0.35Ga0.65As active region is where the excess electrons and holes recombine to generate photons. This layer is usually 0.1–0.5 µm thick. The p and n contacts are metal alloys or multilayer composites that are chosen specifically to make low-resistance electrical contacts to the adjacent semiconductor materials. (Copyright 1993 Hewlett-Packard Company; reproduced with permission)

column VI elements selenium or tellurium. The p-type dopant is usually a column II element, for example, zinc or magnesium; however, the column IV element carbon is now a viable alternative. For II–VI LEDs, the n-type dopant is the column VII (or group 17) element chlorine, and the p-type dopant is the column V element nitrogen. To facilitate electrical access, ohmic contacts are made by evaporating thin metallic films onto both n- and p-type layers. See BAND THEORY OF SOLIDS; CRYSTAL GROWTH; CRYSTAL STRUCTURE; MOLECULAR BEAMS; SEMICONDUCTOR HETEROSTRUCTURES. Device design. One of the primary objectives of LED design is to maximize external quantum efficiency, defined as the radiant flux (in watts) emitted from the device divided by the product of the photon energy (in electronvolts) of the peak emission and the current (in amperes) flowing through the device. For discussion purposes, it is useful to separate this quantity into two distinct parameters: the internal quantum efficiency and the light extraction coefficient. The internal quantum efficiency (the number of useful photons created per electron-hole pair injected) is limited by the transfer of excess carriers to indirect energy band minima, their leakage

29

30

Light-emitting diode outside the active region, and the trapping of carriers at heterointerfaces—all of which eventually lead to either nonradiative recombination or radiative recombination at undesirable wavelengths. The influence of these excess carrier loss mechanisms can be mitigated by the proper choice of active region materials and layer thicknesses. In the ideal case of a well-developed semiconductor material, combined with an optimized structure, the internal quantum efficiency may approach unity. The light extraction coefficient (or the fraction of photons that escape from the device) is constrained by total internal reflection at the semiconductor/air interface. Light generated by spontaneous emission at any point within the active region is uniformly distributed in all directions. Given the large difference in the refractive indices between semiconductor and air, only a small fraction of light impinges upon the interface at angles less than the angle for total internal reflection. This component, which escapes by simple refraction on its first pass through the device, is of the order of 4% for each of the six escape cones inscribed within the semiconductor parallelepiped (Fig. 2). Hence, even without any additional obstructions, the light extraction coefficient has a maximum value of only 24%. With normal epitaxial layer thicknesses and device cross-sectional area, however, almost all of the light emitted toward the four side walls is reflected off the top surface or absorbed in the substrate. Most of the light emitted directly upward toward the top surface is reflected back into the substrate or absorbed within the opaque metal contact pad. All of the light emitted downward toward the substrate is lost to absorption before reaching the back surface. Thus, in practice, a typical red-light-emitting diode with near-unity internal quantum efficiency has an external quantum efficiency of less than 5%. See LUMINESCENCE; REFLECTION OF ELECTROMAGNETIC RADIATION; REFRACTION OF WAVES. Applications. Conventional low-power, visible LEDs are used as solid-state indicator lights in instrument panels, telephone dials, cameras, appliances, dashboards, and computer terminals, and as light sources for numeric and alphanumeric displays. Modern high-brightness, visible LED lamps are used in outdoor applications such as traffic signals, changeable message signs, large-area video displays, and automotive exterior lighting. General-purpose white lighting and multielement array printers are applications in which high-power visible LEDs may soon displace present-day technology. Infrared LEDs, when combined in a hybrid package with solid-state photodetectors, provide a unique electrically isolated optical interface in electronic circuits. Infrared LEDs are also used in optical-fiber communication systems as a low-cost, high-reliability alternative to semiconductor lasers. Indicators and displays. The advantages of using LEDs to produce visible electromagnetic radiation are their high efficiency, small size, ruggedness, long life, and compatibility with silicon integrated circuits. The most appropriate measure of electricalto-optical power conversion for display applica-

top cone + 4 × (1/2) side cones

top cone (a)

top cone + bottom cone (c)

(b)

top cone + bottom cone + 4 side cones (d)

Fig. 2. Schematic diagram showing the means by which photons created at a point source inside a semiconductor parallelepiped escape to the outside environment. Photons incident on the semiconductor/air interface at angles larger than the critical angle are trapped by total internal reflection. The cones represent the solid angles over which photons can escape from the semiconductor after suffering only small Fresnel reflection losses. (a) Light escapes only from the top surface because the substrate is highly absorbing at the photon wavelength and the active region is too thin to allow escape from the sample edges. (b) Light escapes from both the top and bottom cones. A distributed Bragg mirror was inserted between the active region and the absorbing substrate so that photons emitted into the bottom cone would be reflected back toward the top surface. (c) Light escapes from the top surface and the upper halves of the four side cones because the active region was made thick enough that photons could reach the side walls of the parallelepiped before being trapped by total internal reflection off the top surface. (d) Light escapes from all six cones. The absorbing substrate was replaced by a transparent material, and thus photons emitted into the bottom cone and into the bottom halves of the four side cones can escape.

tions is luminous efficiency, defined as the luminous flux (in lumens) divided by the product of applied voltage and drive current (in watts). The luminous flux emitted from an LED is determined by measuring its corresponding radiometric quantity, that is, the radiant flux, and correcting it to account for the wavelength-dependent response of the eye. LEDs with power conversion efficiencies greater than 10 lumens per watt are now commercially available at discrete wavelengths covering the entire visible spectrum. Individual red-, green-, and blue-light-emitting diodes can be combined to produce an enormous variety of intermediate hues, and specially designed ultraviolet diodes can be used to excite an inorganic fluorescent coating for the purpose of generating white light (Fig. 3). Within the spectral window from 600 to 640 nm, the luminous efficiency of LEDs exceeds that of tungsten and halogen lamps and, in some laboratory demonstrations, approaches that of fluorescent lighting. This high level of performance is the consequence of many improvements in material quality and device design, particularly in the area of light extraction. Special current spreading layers, distributed

Light-emitting diode

wire bond to top contact

converter material in resin

blue lightemitting diode package

Fig. 3. Cross-sectional view of a light-emitting diode package designed to emit white light by combining blue, green, and red photons. The source of blue photons is the direct light emission from a AlGaN-GaInN double heterostructure LED chip. The sources of green and red photons are two distinct inorganic fluorescent materials embedded in the epoxy resin that encapsulates the LED chip. The green and red photons are generated by an energy down-conversion process; that is, blue photons emitted by the semiconductor LED are absorbed by the fluorescent materials, the excited electronic carriers lose energy through various internal mechanisms and drop down to an intermediate energy state, and then their electronic energy is converted back into photons by the process of flourescence. The intensity of the blue photon source is controlled by the choice of drive current passing through the semiconductor LED chip. The intensity of the green and red photon sources is set by adjusting the composition of the fluorescent materials in the epoxy resin.

Bragg reflectors, and wafer-bonded transparent substrates have been developed to move the light extraction coefficient closer to its theoretical limit (Fig. 4). Another important operating parameter is maximum light output power. The simplest way to increase the luminous flux from an LED is to raise its drive current. However, this is accompanied by an increase in Joule heating in the passive regions of the device, leading to a higher pn junction operating temperature. Thus, the output power of an LED lamp is limited by heat dissipation within the semiconductor chip, since an increase in junction temperature causes a reduction in internal quantum efficiency and shifts the emitted radiation to longer wavelengths. A luminous output as high as 20 lumens at 619 nm is sustainable by using a special high-power lamp package to improve thermal management. See ILLUMINATION; LUMINOUS EFFICIENCY; LUMINOUS FLUX; PHOTOMETRY. Optoisolators. In the simplest optical interface, the optoisolator, an LED, and a photodetector are optically coupled, but electrically isolated, in a small package. This device can be used, for example, at the interface between two different circuits, such as the switching equipment in a telephone central office and the connecting loop circuit which carries the signals to the telephone sets. The electric signal from the central office is converted to radiation by the LED, which in turn is converted back into an electric signal by the photodetector before it enters the loop circuit. This type of interface is traditionally provided by electromechanical relays or isolation transformers. The electrical isolation resulting from the optical path protects the central office from electromagnetic interferences such as lightning which hits

telephone wires or surge currents from electromechanical relays. LEDs are ideal for this application because they are very reliable and can be modulated to carry high-frequency signals. In a typical optoisolator structure, the LED and the phototransistor are mounted on separate metal lead frames, and the two components are coupled optically through a transparent plastic encapsulant (Fig. 5). This plastic is also the source of electrical isolation, typically on the order of 2500 V. Final encapsulation is completed with a black, opaque overmold which also provides mechanical stability. Optoisolators are compatible with silicon integrated circuits in size, reliability, and

n blocking layer

p - AlGaAs window ( λ and G∗ increases, and the rate constant decreases, with increasing driving force. This is the nonintuitive inverted regime. Marcus cross-relation. Marcus showed that the rate constant k12 for an electron-transfer reaction accompanied by a net chemical change is related to the rate constants k11 and k22 for the component self exchanges by Eq. (6). Here K12 is the equilibrium k12 = (k11 k22 K12 f12 )1/2 W12

(6)

[ln K12 + (w12 − w21 )/RT ]2   ln f12 = 
k11 k22 w11 + w22 4 ln + Z2 RT W12 = exp[−(w12 + w21 − w11 − w22 )/2RT ] constant for the cross-reaction, f12 is a factor that becomes significant at large driving forces, W12 is a factor that corrects for differences between the electrostatic work terms for the various reactions, w12 (w21) is the electrostatic work required to bring together the reactants (products) of the cross-reaction, w11 (w22) is the corresponding work to bring together the reactants in the self-exchange reaction, and Z is the collision frequency of two uncharged molecules in solution (1011 M−1 s−1). Equation (6) is based upon the assumption that the reorganization parameter for a cross-reaction is equal to the average of the reorganization parameters for the component self-exchange reactions, Eq. (7). λ12 =

λ11 + λ22 2

(7)

For example, if the oxidation of Ru(NH3)62+ by Fe(phen)33+ [reaction (4)] is the cross-reaction (k12, λ12), then the Ru(NH3)62+–Ru(NH3)63+ reaction (8) and 3+ Ru(NH3 )2+ 6 + Ru(NH3 )6 = 2+ Ru(NH3 )3+ 6 + Ru(NH3 )6

(8)

Fe(phen)32+–Fe(phen)33+ reaction (1) are the component self-exchange reactions (k11, λ11 and k22, λ22). When the driving force is not too large and the work terms cancel, then ln f12 ≈ 0 and W12 = 1. Under these conditions, Eq. (6) reduces to Eq. (9). k12 = (k11 k22 K12 )

1/2

(9)

Equation (9) is often referred to as the Marcus equation. It has been extensively applied to electrontransfer reactions involving metal complexes. The

stable oxidation states of transition-metal complexes generally differ by one electron, and the metal centers and surrounding ligands can be independently varied to encompass a wide range of electrontransfer rates and driving forces. For moderately exergonic electron-transfer reactions between similarly charged complexes, the agreement of the observed rates with those calculated from Eq. (9) is often remarkably good. At high exergonicities or when the net electron transfer involves oppositely charged complexes, it is necessary to use Eq. (6). The rates of highly exergonic reactions calculated with the extended equation are usually somewhat higher than the observed rates in the normal free-energy regime and are slower than the observed rates in the inverted regime. Reasons advanced for the failure of the cross-relation include changes in reaction mechanism, anharmonicity contributions, nuclear tunneling contributions, and noncancellation of nonelectrostatic contributions to the work terms. Examples of the application of the Marcus equation to organic or organometallic systems include the oxidation of tetraalkyltin, tetraalkyllead, and dialkylmercury by Fe(phen)33+; the oxidation of various methylarenes, ArCH3, by substituted Fe(phen)33+ complexes, reaction (10); and the oxidation of + 2+ ArCH3 + Fe(phen)3+ (10) 3 = ArCH3 · + Fe(phen)3

N-alkylpyridinyl radicals by [Ir(1,5-cyclooctadiene) (µ-pyrazolyl)]2+, reaction (11), where very good + C6 H5 N–R· + Ir+ 2 · = C6 H5 N–R + Ir2

(11)

agreement with the predictions for highly exergonic electron transfer is obtained. The Marcus equation has also been succesfully applied to reactions of alkylhydrazines, ferrocene derivatives, and p-phenylenediamine derivatives. Because the reorganization barrier is an intrinsic property of a redox couple and independent of its reaction partner, a simple relationship should exist between the homogeneous self-exchange rate constant of a redox couple and its exchange rate constant at an electrode. The relationship between homogeneous and heterogeneous electron-transfer rate constants predicted by the Marcus formalism also holds up well in practice. Proton, hydride, atom, and group transfers. Although originally derived for electron-transfer reactions, the Marcus formalism can also be applied to reactions in which bond making and bond breaking occur. If the net reaction involves transfer of a covalently attached X from a donor to an acceptor, then the crossreaction and self-exchanges need to be redefined in terms of X-transfer reactions as shown in Eqs. (12). AX + B = A + XB AX + A = A + XA

(12)

BX + B = B + XB For example, the oxidation of a methylarene, reaction (10), is followed by proton transfer from the methylarene cation radical to a substituted pyridine,

407

408

Margarine reaction (13). In this case, X = H+, A = ArCH2˙, and + ArCH+ 3 · + Y–py = ArCH2 · + Y–pyH

(13)

B = Y–py. The Marcus formalism yields a consistent value for the intrinsic barrier for the proton transfer. Other applications include hydride transfers, reaction (14), where AiN+R is a nitrogen heterocycle; gasphase atom transfers, reaction (15); and nucleophilic displacements (methyl transfers), reaction (16). A1 (H)NR + A2 N+ R = A1 N+ R + A2 (H)NR

(14)

RH + CH3 · = R· + CH4

(15)

ClCH3 + F− = Cl− + CH3 F

(16)

Since the transfer of the heavier particles generally involves strong electronic interactions, the X transfers are not well described by intersecting harmonic free-energy curves. Use of a bond-energy bond-order (BEBO) model for the X transfers results in a freeenergy expression similar to Eq. (5) when |G◦|/λ is small, but differs from it in not predicting an inverted regime at large |G◦|/λ. The λ parameter is found to have the same property as Eq. (5), and a cross-relation analogous to Eq. (6) is obtained. The expression for the free-energy of activation, Eq. (5), is applicable to reactions that can be described in terms of two weakly interacting harmonic free-energy curves. It does not apply when there is an appreciable contribution from bond making or rupture in the rate-determining step. The barrier averaging relation, Eq. (7), is, however, more general: intrinsic barrier additivity obtains in both electron and heavier-particle transfers. The factors determining reaction rates, particularly the additivity of intrinsic barriers, have proved invaluable in understanding and predicting the rates of a variety of electron-, atom-, and group-transfer reactions. Norman Sutin Bibliography. D. A. Astruc, Electron Transfer and Radical Processes in Transition-Metal Chemistry, 1995; L. Eberson, Electron Transfer Reactions in Organic Chemistry, 1987; R. A. Marcus, Annu. Rev. Phys. Chem., 15:155–196, 1964; R. A. Marcus, Pure Appl. Chem., 69:13–29, 1997; R. A. Marcus and N. Sutin, Biochim. Biophys. Acta, 811:265–322, 1985; S. F. Nelsen et al., J. Amer. Chem. Soc., 119:5900–5907, 1997; N. Sutin, Prog. Inorg. Chem., 30:441–498, 1983.

colored, flavored, fortified with vitamins, and otherwise formulated to have the same or similar taste, appearance, and nutritional value as butter. See BUTTER. Margarine is regulated by law in practically every country in which it is manufactured or sold. In 1977 the U.S. Food and Drug Administration revised and broadened its margarine Standards of Identity to conform to the agreements reached by the Codex Alimentarius Commission, an international body of food and legal authorities charged with the responsibility of unifying foods sold in international trade. Margarine produced in the United States must contain not less than 80% fat. The fats and oils must be edible but may be from any vegetable or animal carcass source, natural or hydrogenated. The required aqueous phase may be water, milk products, or solutions of dairy or vegetable protein, and must be pasteurized. Milk may be cultured after pasteurization. Vitamin A must be added to yield a finished margarine with not less than 15,000 international units per pound (0.45 kg). Optional ingredients include salt or potassium chloride for low-sodium diets, nutritive carbohydrate sweeteners, fatty emulsifiers, antioxidants, preservatives, edible colors, flavors, vitamin D, acids, and alkalies. See PASTEURIZATION. Margarines are formulated by blending two or more fats and oils in proportions designed to obtain the desired texture, consistency, and melting characteristics in the finished product. The various types of components include unhydrogenated or liquid, lightly, moderately, and highly hydrogenated oils. Meat fats are used in various bakery margarine formulations. See FAT AND OIL (FOOD). Low-calorie spreads designed to resemble margarine contain only 40–60% fat. The higher level of aqueous phase in the product requires the addition of emulsion stabilizers. These may be high levels of milk solids, casein, vegetable proteins, vegetable gums, or various emulsifiers. Low-fat spreads are not covered by U.S. Standards of Identity and cannot be labeled as margarine. See FOOD ENGINEERING. Theodore J. Weiss Bibliography. Code of Federal Regulations, 21, 166.110, 1983; H. Lawson, Food Oils and Fats: Technology, Utilization, and Nutrition, 1994; T. J. Weiss, Food Oils and Their Uses, 2d ed., 1983.

Marine biological sampling Margarine An emulsified fatty food product used as a spread and a baking and cooking fat, consisting of an aqueous phase dispersed in the fat as a continuous phase. Hyppolyte M`ege-Mouri´es invented margarine in 1869 and won the prize offered by Louis Napoleon for the development of an inexpensive butter substitute. Margarine is now considered a food in its own right and is manufactured in forms unknown to butter, such as plastic, soft, or fluid. However, the interrelationship is still acknowledged since margarine is

The collection and observation of living organisms in the sea, including the quantitative determination of their abundance in time and space. The biological survey of the ocean largely depends on specially equipped vessels. Sampling in intertidal regions at low tide is one of the few instances where it is possible to observe and collect marine organisms without special apparatus. A primary aim of marine biology is to discover how ocean phenomena control the distribution of organisms. Sampling is the means by which this aim is accomplished. Suppositions about the dynamics of

Marine biological sampling warp

net ring gauze

weight

(b)

cloth cuff

loading bar

net bucket "cod end"

closing wire

closing wire release stop release bar

release bar bucket

(a)

(c)

frame base

trigger plate

Fig. 1. Marine sampling devices. (a) Water sampling bottle with attached thermometer rack for recording water temperature at sampling depth. The brass messenger on the wire has just tripped the spring-loaded end closures (after S. Niskin, A reversing-thermometer mechanism for attachment to oceanographic devices, Limnol. Oceanogr., 9:591–594, 1964). (b) Conventional plankton net (after C. Schlieper, ed., Research Methods in Marine Biology, University of Washington Press, pp. 46–63, 1972). (c) Spring-loaded grab sampler (after W. Smith and A. D. MacIntyre, J. Mar. Biol. Ass. U.K., 33:257–264, 1954).

ocean life rest entirely on sampling methodology. Traditional techniques use samplers attached to wires lowered over the side of a ship by hydraulic winches (Fig. 1). These samplers include bottles designed for collecting seawater samples from particular depths, fine-meshed nets that are towed behind the ship to sieve out plankton and fish, and grabs or dredges that collect animals inhabiting the ocean bottom. These types of gear are relied upon in many circumstances; however, they illustrate some of the problems common to all methods by which the ocean is sampled. First, sampling is never synoptic, which means that it is not possible to sample an area of ocean so that conditions can be considered equivalent at each point. Usually, it is assumed that this is so. Second, there are marine organisms for which there exists no sampling methodology. For example, knowledge of the larger species of squid is confined to the few animals that have been washed ashore, or to the rare video recording. Third, the representativeness of the samples collected is problematic. The open ocean has no easily definable boundaries, and organisms are not uniformly distributed. The actual sampling is regularly done out of view of the observer; thus, sampling effectiveness is often difficult to determine. Furthermore, navigational systems are not error-free, so the position of the sample is never precisely known. All

developments in methods for sampling the ocean try to resolve one or more of these difficulties by improving synopticity, devising more efficient sampling gear, or devising methods for observation such that more meaningful samples can be obtained. Plankton. Single samples for phytoplankton and zooplankton are obtained by using sampling bottles or by towing a plankton net through the water horizontally, vertically, or obliquely (Fig. 1). These samples are preserved and then counted and identified using a microscope. Two developments in sampling planktonic organisms are continuous-recording systems, and multiple-net systems that collect zooplankton from various depths. See PHYTOPLANKTON; ZOOPLANKTON. Continuously recording systems. These methods for phytoplankton take advantage of the fact that chlorophyll a, the primary light-sensitive pigment used in photosynthesis, is a fluorescent molecule. This means that when chlorophyll-containing plant cells are illuminated (excited) by a particular wavelength of light, they reemit (fluoresce) light at another wavelength. Chlorophyll is excited by blue light and fluoresces red. Seawater is pumped aboard ship and passed through an instrument called a fluorometer that illuminates the water and records the level of fluorescence due to chlorophyll a in the

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Marine biological sampling towing bridle temperature-salinitydepth unit

attitude-control vanes

fluorometer

Fig. 2. Towed undulating vehicle system with some of its sensors. (Chelsea Instruments, Ltd., United Kingdom)

phytoplankton. This technique thus provides a continuous estimate of the total amount of plant biomass as the water is pumped through the fluorometer. Some fluorometers can be lowered over the side of the ship to measure directly the fluorescence in the water. A strobe light is used to illuminate a small volume near the device, the fluorescence is sensed, and a signal is sent up a conducting cable to shipboard recording devices. See CHLOROPHYLL; FLUORESCENCE; LUMINESCENCE ANALYSIS. Acoustic methods and video cameras can be used for estimating zooplankton abundances continuously. The sound sources used are derived from fish-finder sonars developed for commercial fishing. Sound energy is transmitted at multiple ultrahigh frequencies, usually between 10 kHz and 1 MHz, insonifying a volume of water within 1–2 m (3.3– 6.6 ft) of the transducer; and the sound that is reflected, or backscattered, from the organisms is then

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Marine biological sampling recorded electronically. In this range, organisms 0.1– 10 mm (0.004–0.4 in.) in size can be detected. Usually, the transducers for each frequency are triggered sequentially, and the acoustic backscatter measured from each. Backscattered sound depends on size. Smaller organisms can be detected only with high frequencies, while larger organisms can be detected at all frequencies. In the video method, a video camera with a synchronized strobe light is used to sample a volume of water at 60 frames per second. The recorded images are digitized and scanned for zooplankton. The fluorescence method for phytoplankton and the acoustic and video methods for zooplankton can be used while the ship is under way. For phytoplankton, water is normally pumped continuously to shipboard labs, whereas acoustic transducers are mounted to the hull or are towed just behind the ship near the surface. See SCATTERING LAYER; SONAR. The towed-undulating vehicle instrument can incorporate all three of the continuous-recording techniques as an undulating instrument package towed behind the ship (Fig. 2). Variations in plankton abundance (as acoustic backscatter or optical images for zooplankton, and as chlorophyll a for phytoplankton) along with environmental parameters such as temperature and salinity are recorded simultaneously in two dimensions (Fig. 3). These techniques are indirect and, with the exception of video methods, provide little information on what species are present. They do provide a detailed, nearly synoptic estimate of the smallscale variation in abundances of plankton over wide areas and with depth. Species analysis of phytoplankton requires bottle sampling (or in special cases, nets). Study of particular zooplankton organisms or groups can be accomplished by use of multiple-net systems. Multiple-net systems. These types of sampling gear are used to characterize the small-scale vertical distribution of zooplankton species. There are several designs in use. One of the most successful is the multiple opening/closing net environmental sensing system (MOCNESS). MOCNESS carries nine nets that are sequentially opened and closed at desired depths by sending electrical command signals down a conducting cable. The system is deployed with one net opened and is lowered to the desired depth. As MOCNESS is hauled in, nets are opened and closed sequentially at desired depths. The opening of one net automatically closes the previous net. For example, MOCNESS can be sent down to 1000 m (3000 ft) with one net open, and the remaining eight nets could be opened/closed at increments of 125 m (400 ft) as the net is hauled in. An electronic instrument package is mounted on the main frame and contains salinity, temperature, pressure (depth), light, and dissolved oxygen sensors for the recording of environmental data, as well as sensors that detect the flow of water through the net and the attitude of the net as it is towed. During a tow, all this information is recorded on a computer on the ship for later analysis. Every tow provides a series of eight zooplankton

samples, each from a different stratum, and one sample representing organisms collected over the entire depth that the net fished. Nekton. For catching the more active fishes and cephalopods and the larger crustaceans, large-sized nets are essential. The mouth opening of the MOCNESS has been enlarged for this purpose, to 215 ft2 (20 m2) for example, and represents an advance in nekton sampling because multiple depths can be fished. A similar design is the rectangular midwater trawl, combining multiple nets of 11 ft2 (1 m2) and others of 88 ft2 (8 m2), the two-sized nets being used to sample plankton and nekton, respectively. Other enhancements to these nets include specially designed cod ends that maintain the pressure and temperature of the sample as it is collected. By using this last modification, deep-living fish and crustaceans can be brought to the surface alive. A net that will catch larger squids and fishes inhabiting the deep has yet to be developed. For these organisms, video recording and direct viewing from deep-sea vehicles are the best options. See DEEP-SEA FAUNA. Benthos. Organisms that live on the bottom are sampled by grabs, dredges, trawls, and box corers. Grabs are basically a pair of jaws, triggered in various ways to close when they touch bottom, capturing a small (about 0.1 m2 or 3.3 ft2) sample of sediment. Several designs are available, depending on bottom type and depth of water (Fig. 1). The sample, after being returned to the deck, is washed and sieved for the organisms it contains. Dredges have a bag of coarse-meshed netting attached to a rectangular wrought-iron frame (0.6– 0.9 m or 2-3 ft wide) that forms the mouth of the net. The dredge is towed along the bottom by means of a movable bridle arrangement, while the lower side of the frame digs into the sediment to scoop up organisms living just below the surface. Bottom trawls are baglike nets tapering to a cod end. The mouth is kept open by otter boards, to which the two towing warps are attached. The otter boards also serve to disturb organisms off the bottom so that they become trapped in the net. Sometimes, counting wheels are fitted to trawls or dredges to give an indication of distance traveled. The box corer (Fig. 4) is a sediment-coring device. However, the same features that make it useful for the study of ocean sediments make it a valuable piece of equipment for benthic biological research as well. The advantages of the box corer over grabs and dredges are the precision with which it can obtain a bottom sample and the fact that it can bring a largely undisturbed sediment sample to the surface. Therefore, a better characterization of the environment of the organisms can be obtained. In addition, there is little bow wave as the corer approaches the bottom, and it can work in all types of sediment. The disadvantage is that it is large and heavy and thus requires handling by the most powerful winches. The box corer is allowed to free-fall to the ocean bottom. As the weight of the corer drives it into the bottom, the wire slackens. This action releases the spade, which swings to close off the bottom

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(c) Fig. 4. United States Naval Electronics Laboratory box corer. (a) Side view with spade in open position. (b) Side view with spade in closed position. (c) Disengaged core box for processing sample. (After R. R. Hessler and P. A. Jumars, Abyssal community analysis from replicate box cores in the central North Pacific, Deep-Sea Res., 21:185–209, 1974)

of the corer. The box corer is then brought to the surface. See MARINE SEDIMENTS. Bacteria and viruses. The use of fluorescent dyes, or fluorochromes, simplifies sampling and enumeration of marine bacteria. For this technique, a known volume of uncontaminated seawater or sediment (collected by water sampling bottles, grabs, or corers) is incubated for a short time with the fluorochrome, while bacteria cells absorb some of the dye. The cells are then collected on a filter and viewed under a high-powered microscope equipped with a light source that causes the dye to fluoresce. The bacteria appear as bright objects against a dark background and are counted directly. Acridine orange, a fluorochrome that binds to nucleic acids (RNA and DNA), is commonly used for enumerating marine bacteria. Not all bacterial cells seen under the fluorescent microscope are alive or capable of growth in laboratory culture, although the abundances measured correlate with other plankton variables. Viruses must be enumerated with an electron microscope, usually on samples that have been concentrated and filtered for larger forms. See MARINE ECOLOGY; MARINE MICROBIOLOGY; STAIN (MICROBIOLOGY). Other methods and observations. Direct and remote observation methods provide valuable information on the undersea environment and thus on

the representativeness of various sampling techniques. Personnel-operated deep-submergence research vessels (DSRVs) are increasingly used to observe ocean life at depth and on the bottom, and to determine appropriate sampling schemes. The deepsubmergence research vessels are used with cameras and television recording equipment and are fitted with coring devices, seawater samplers, and sensors of various types. Other cameras are operated unattended at the bottom for months at a time, recording changes occurring there. Scuba diving is playing a larger role, especially in open-ocean areas, and is used to observe marine organisms in their natural habitat as well as to collect the more fragile marine planktonic forms such as foraminifera, radiolaria, and jellyfish. Autonomous underwater vehicles (AUVs) continue to assume greater importance in sampling programs since they can go to greater depths than can divers, and they overcome a limitation in diving in that AUVs can be operated at night and can sample extensively under ice. Optical sensors carried aboard Earth-orbiting satellites can provide images of ocean color over wide areas. Ocean color is related to the turbidity as well as the amount of plant material in the seawater. This establishes a means by which sampling programs from ships can be optimized. See DIVING; SEAWATER FERTILITY; UNDERWATER PHOTOGRAPHY; UNDERWATER TELEVISION; UNDERWATER VEHICLE. John Marra Bibliography. P. F. Brodie, D. D. Sameoto, and R. W. Sheldon, Population densities of euphausiids off Nova Scotia as indicated by net samples, whale stomach contents, and sonar, Limnol. Oceanog., 23:1264–1267, 1978; C. S. Davis et al., Rapid visualization of plankton abundance and taxonomic composition using the Video Plankton Recorder, Deep Sea Res. II, 43:1947–1970, 1996; K. L. Denman and D. L. Mackas, Collection and analysis of underway data and related physical measurements, pp. 85–109 in J. H. Steele (ed.), Spaital Pattern in Plankton Communities, Plenum, New York, 1978; L. R. Haury, J. A. McGowan, and P. H. Wiebe, Patterns and processes in the time-space scales of plankton distributions, pp. 277–327 in J. H. Steele (ed.), Spaital Pattern in Plankton Communities, Plenum, New York, 1978; J. C. Kelly, Sampling the sea, pp. 361–387 in D. H. Cushing and J. J. Walsh (eds.), The Ecology of the Seas, W. B. Saunders, Philadelphia, 1976; J. Sherman et al., The autonomous underwater glider “Spray,” IEEE J. Oceanic Eng., 26:437–446, 2001.

Marine boiler A steam boiler designed to suit the marine environment and generally arranged to supply steam to the main propulsion machinery, ship’s service electric generators, feed-pump drivers, and other auxiliary services. See MARINE MACHINERY. Types of units. Marine boilers are usually of the two-drum water-tube type with water-cooled furnaces, superheaters, desuperheaters, and heat

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Fig. 1. Typical nonreheat, two-drum, down-fired welded-wall, double-superheater marine boiler. (Foster Wheeler Boiler Corp.)

recovery equipment of the economizer or air-heater type. The older fire-tube boilers, commonly referred to as Scotch boilers, and water-tube header types are no longer manufactured for marine service, although some of these boilers are still in use on older ships. See FIRE-TUBE BOILER; WATER-TUBE BOILER. The majority of ships are fitted with two boilers, although some large passenger ships may have three or more. Some cargo ships are fitted with only one boiler, and in some of these cases a smaller auxiliary boiler may be fitted for emergency or in-port steaming use. Design considerations. The pressure and temperature of the steam produced by marine boilers are dependent upon the steam cycle selected, which in turn relates to economic considerations such as the cost of fuel and the installed cost for the equipment for the propulsion power plant. Since the cost of fuel (energy) is high and rising as it relates to the installed cost of the equipment, there is a need for efficient steam plants, which in turn require high steam pressures and temperatures and efficient boilers. To attain high boiler efficiency it is necessary to minimize the heat loss up the boiler stack. This can be accomplished by utilizing heat recovery equipment such as

economizers or air heaters. See AIR HEATER; BOILER ECONOMIZER. Worldwide, most modern ships generate steam at a pressure of 850 lb/in.2 gage (5.86 megapascals) and a temperature of 955◦F (513◦C) at the superheater outlet and employ rotary regenerative air preheaters for the heat recovery equipment. However, in the United States most power plants employ the less efficient economizer as the heat recovery unit. Figure 1 shows a cutaway view of a typical twodrum marine boiler. It has a welded wall construction in way of the furnace and superheater; that is, the tubes are welded together and joined by ligaments so as to make the furnace and area in way of the superheaters gas- and airtight. See SUPERHEATER. In the upper boiler drum a submerged desuperheater is shown. The purpose of the desuperheater is to lower the steam temperature for a portion of the superheated steam that may be used for auxiliary purposes. Although they are not popular in the United States, a number of marine power plants employ a steam cycle using boiler steam reheaters. With this arrangement, steam that has passed through a portion of the turbine is directed back into the boiler for

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Fig. 2. Typical reheat, down-fired, welded-wall marine boiler containing primary superheater and reheater with economizer and gas damper control. The boiler gas air heater is not shown. (Foster Wheeler Boiler Corp.)

reheating in a device similar to the initial boiler superheater, except that the pressure is lower. The advantage of using this arrangement is that higher initial steam pressure may be employed, generally around 1450 lb/in.2 gage (10.0 MPa), and this advantage, coupled with the advantage of reheating the steam, can result in propulsion power plants where the fuel consumption may be 5–8% less than the conventional arrangement discussed above. See REHEATING. Figure 2 shows an economizer in the gas path; this would be installed in series with a gas air heater of the type shown in Fig. 3. The air heater shown is a rotary regenerative type and consists of a drum filled with a plate-type heat exchange surface that is rotated first through the boiler exhaust gas path, which heats the surface while cooling the gas, and then through the air that is discharged from the boiler-forced draft fan, which heats the air while cooling the surface. The heated air then passes to the boiler, where it supplies the air necessary for combustion. The use of an air heater of this type can permit boiler efficiencies to

be in the range of 91%, based on the lower heating value of the fuel. Although the heat exchange surfaces are coated with acid-resistant enamel, they deteriorate in time and must be replaced, particularly in the colder part of the heater. Thus arrangements must be made for easy replacement of the surfaces. Where economizers alone are fitted to serve as the boiler heat-recovery equipment, it is common to use steam air heaters. With this arrangement the air for combustion is forced through a heat exchanger containing extended surface tubes where heat is extracted from the steam to warm the combustion air. Special design requirements. In the design of marine boilers, special requirements must be considered. The units must be compact and economical with regard to space and weight. Also, they must be built to withstand the effect of the pitching and rolling motions of the ship, as well as vibration which is caused by the ship’s propeller. Reliability must be inherent in the design because of the requirements of safety at sea.

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Fuels. Marine boilers are generally arranged for oil firing. Oil is used extensively because of its simplicity of handling and storing and the fact that it can be stored in spaces that often cannot be used for carrying cargo. For economic reasons, coal was used in the United States for boats operating on the Great Lakes, but most of these installations have been converted to use oil as a fuel. Oil-fired boilers are arranged for forced draft; that is, the air for combustion is blown or forced into the boiler furnace by use of centrifugal, motor-driven fans. Coal firing generally uses a combination of forced and induced draft fans, and these boilers are operated such that the gas pressure in the boiler furnace is slightly negative with respect to the surrounding atmosphere. The oil normally burned is a residual fuel called No. 6 fuel oil or sometimes referred to as Bunker C. This fuel has the lowest cost of liquid fuel since it is what is left over after the refining process, which has extracted better and cleaner products such as gasoline, diesel oil, and home heating oil. The fuel from the ship’s bunker tanks is generally transferred to a settling tank, which contains about a

day’s supply. While in the tank, the fuel is heated, and water and heavy solids such as sand can be removed by settling before the fuel is pumped at high pressure through fuel oil heaters and to the burners. The purpose of the fuel oil heaters is to reduce the viscosity of the oil so that it may be atomized for burning. To assist this mechanical atomizing process, most fuel oil burners utilize steam under pressure to further break down the oil particles so as to provide better combustion characteristics. The oil being atomized at the burner is then mixed with the heated combustion air so that the combustion process can take place in the furnace. Coal is normally burned by using mechanical stokers and moving chain grates, although some consideration is being given to burning powdered coal, either in the solid form or as a powder mixed with oil in the form of a slurry. Automatic controls. Automatic controls regulate the feedwater flow to the boiler, the superheater outlet steam pressure and temperature, the fuel oil flow, and the quantity of combustion air. Generally the controls are of the solid-state electric type that sends signals to the electric or air power devices for positioning of valves and dampers. See BOILER

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Maintenance and repairs. Reliable and efficient performance of the boiler is required over the economic life of the ship, which is considered to be about 25 years. Normal maintenance and repairs are generally made while the ship is in port, while major repairs are made in a shipyard during the annual inspection periods. See DRYDOCKING. Boiler firesides must be cleaned of soot and ash deposits to obtain high boiler efficiency and proper superheater steam temperature. For this purpose soot blowers using steam as cleaning media are fitted in the superheater and the steam-generating and heat recovery sections of the boiler. These surfaces also must be water-washed periodically. Scale deposits and corrosion of the water sides of the boiler must be avoided. To accomplish this, chemically treated distilled water must be used for boiler makeup, and leaks of seawater to the boiler feedwater must be avoided. Periodic chemical testing of the boiler water and the feedwater must be conducted by manual or automatic means. See BOILER; BOILER FEEDWATER; STEAM-GENERATING UNIT. Robert P. Giblon

Marine conservation The management of marine species and ecosystems to prevent their decline and extinction. As in terrestrial conservation, the goal of marine conservation is to preserve and protect biodiversity and ecosystem function through the preservation of species, populations, and habitats. The importance of conserving marine species and ecosystems is growing as a consequence of human activities. Negative impacts on marine biological systems are caused by such actions as overfishing; overutilization, degradation, and loss of coastal and marine habitats; introduction of nonnative species; and intensification of global climate change, which alters oceanic circulation and disrupts existing trophic relationships. Such factors contribute to species decline and extinction in the sea. The growing human population puts direct pressure on marine biological resources through extraction of food and natural resources from the ocean; conversion of coastal habitats for housing, marinas, and other purposes; use of waterways for shipping and recreational purposes; intentional and unintentional translocation of exotic marine species; and pollution of coastal waters and their underlying sediments. In addition, humans put indirect pressure on coastal and marine systems through runoff, erosion, pollution, and eutrophication from terrestrial sources. Additional indirect pressures are imposed on marine systems, and on the entire biosphere, by the production of atmospheric emissions that contribute to global warming and increased penetration of ultraviolet radiation. Marine conservation biologists seek to reduce the negative effects of all these factors by conducting directed research and helping to develop management strategies for particular species, communities, habitats, or ecosystems.

The discipline of marine conservation has lagged behind terrestrial conservation in its development and application. This is due to many factors, including the logistic difficulties of working in the marine environment; practical difficulties of observing, counting, and tracking marine organisms and their offspring; and a relative lack of knowledge concerning many marine communities and habitats, especially those of the open ocean and deep sea. A variety of approaches and tools are used in marine conservation. These include population assessment; mitigation, recovery, and restoration efforts; establishment of marine protected areas; and monitoring programs. Many of these approaches overlap with those in terrestrial conservation. However, fundamental differences between terrestrial and marine environments in spatial dimension, habitat type, and organismal life history require that basic conservation techniques be modified for application to the marine environment. See BIODIVERSITY; ECOSYSTEM; MARINE ECOLOGY; OCEANOGRAPHY. Population assessment. Effective management requires knowledge of the size and status of populations. Trends in abundance can be detected through stock assessment methods first developed for marine fisheries and subsequently modified for application to other marine organisms. These methods use estimates of population size, reproduction, survivorship, and immigration to determine whether populations are increasing, decreasing, or stable. Small populations and populations with steeply declining numbers are typically thought to be at greater risk of extinction than larger, more stable populations. Population viability analysis is a specialized statistical assessment in which demographic and environmental information is used to determine the probability that a population will persist in a particular environment for a specified period of time. Population viability analysis can be used to guide management decisions, and has been used in efforts to manage marine mammals, turtles, seabirds, and other species. See ECOLOGICAL COMMUNITIES; ECOLOGICAL METHODS; POPULATION ECOLOGY. Mitigation, recovery, and restoration. Depleted, threatened, or endangered populations are often subject to mitigation or recovery efforts. The purpose of these efforts is to reduce the immediate threat of extinction or extirpation. This is typically achieved by direct human intervention to increase the size of a population or to prevent further decline in population size. Recovery can be promoted by reducing juvenile and adult mortality and by restoring critical habitat (that is, areas essential for feeding or reproduction). Methods used to achieve recovery of fish and marine invertebrate populations include reducing fishing quotas, restricting the use of certain types of fishing gear, restricting the seasonal or annual distribution of a fishery, or closing fisheries altogether. For species such as some marine mammals, recovery has been effected by legislation that greatly limits or prohibits their capture or lethal take. The eastern North Pacific stock of gray whale and the Bering-Chucki-Beaufort stock of bowhead whale, both of which suffered severe depletion due to

Marine conservation overexploitation in the nineteenth and twentieth centuries, are examples of successful recovery efforts. In contrast, for reasons that are not fully understood, attempts to increase the number of Hawaiian monk seals have been largely unsuccessful, and the species remains in danger of extinction. See ENDANGERED SPECIES; FISHERIES ECOLOGY. Recovery efforts can be most successful if they are based on multispecies or ecosystem-level management strategies. These strategies take into account positive and negative interactions between species, such as facilitation, competition, and predation. They further take into account interactions between species and their environment. An example of a multispecies interaction comes from the red sea urchin and the abalone, both of which have been the target of commercial and recreational fisheries in California. Juvenile abalone recruit most successfully beneath the spine canopies of adult urchins; thus, efforts to aid the recovery of abalone are most effective when the abundance of urchins is sufficiently high. An example of ecosystem-level interactions comes from the Steller sea lion, which has shown continued population decline in the Gulf of Alaska. Despite ongoing research and recovery efforts, the causes of this decline remain unknown, and attempts to increase the number of sea lions have been mostly unsuccessful. However, some believe that the decline has been caused in part by the combined effects of commercial fisheries that target the forage fish required by the sea lions, and by climate change, which has altered oceanic circulation patterns and thereby altered the distribution and reduced the availability of suitable prey. Key to the success of assessment and recovery programs is identification of the appropriate biological unit for conservation (for example, population, subspecies, stock, or evolutionarily significant unit). Maintaining genetic diversity is an important goal of conservation biology, because genetic diversity confers evolutionary potential. Thus, conservation efforts often are aimed at populations that are genetically distinct from other populations of the same species. Species become and remain genetically distinct through reproductive isolation (that is, by lack of interbreeding). A good example of reproductive isolation in marine species is found among salmon stocks in the Pacific Northwest of the United States. In this region, salmon of diverse stocks and species intermingle while they are feeding at sea. However, members of each stock return to a particular stream or river system to reproduce, thereby creating a high degree of genetic isolation between groups. Effective conservation strategies must take into account this genetic distinction by managing distinct stocks separately. Restoration efforts are aimed at returning habitats to an ecologically functional condition, usually consistent with some previous, more pristine condition. In the marine realm, restoration efforts have been limited to certain coastal and estuarine habitats. Seagrasses provide an important example of restoration efforts in coastal areas. Seagrasses are marine flowering plants that grow in large stands, or “meadows,”

in shallow bays and inlets. Seagrass communities are highly productive, and they perform essential ecosystem functions through provision of nursery areas for juvenile fish and other animals, and as feeding areas for both fish and birds. However, seagrass communities are being lost rapidly through coastal development. To offset these losses, restoration activities have been initiated in California, Texas, Florida, and other places. These efforts include transplanting seagrasses from healthy populations to sites where they can replace plants that were lost; regulating activities that contribute to habitat loss, such as the dredging or filling of tidelands; and modifying the design of docks and marinas to have a less deleterious effect on existing seagrass populations. See MARINE FISHERIES. Marine protected areas. Marine protected areas are set aside for the protection or recovery of species, habitats, or ecosystems. They include marine parks, marine reserves, marine sanctuaries, harvest refugia, and voluntary or legislated no-take areas. Some marine protected areas allow for consumptive use (such as fishing) or extraction of resources (for example, oil drilling), while others are closed to most human activities. The number of marine protected areas is growing. Two of the largest are Great Barrier Reef Marine Park in Australia, established to protect coral reef ecosystems, and the Gal´apagos Marine Park in Ecuador, which protects a unique temperate ecosystem. In the Antarctic, protection is provided to living and nonrenewable marine resources by principles codified in several international treaties. Other, smaller marine protected areas have been established to protect or promote the recovery of individual species or groups of species such as seahorses, dugongs, sea turtles, Mediterranean monk seals, Hawaiian monk seals, southern sea otters, rockfish, abalone, and seagrasses. However, many of these marine protected areas are so new that their effectiveness has not yet been demonstrated. Monitoring programs. Monitoring programs are necessary to determine the outcome of specific conservation actions and to guide future conservation decisions. Monitoring programs vary according to the objectives of specific conservation projects but typically include such activities as long-term surveys of population size and status, and the development of mathematical models to help predict specific outcomes. A good example of monitoring efforts are those concerned with the California gray whale. Repeated surveys of migrating whales allowed observers to estimate the number of animals in the population as well as trends in abundance over a 30-year period. The data showed the California gray whale population to be growing steadily and sufficiently large to allow its removal from the U.S. List of Endangered and Threatened Wildlife in June 1994. Terrie Klinger Bibliography. T. S. Agardy, Marine Protected Areas and Ocean Conservation, Academic Press, San Diego, 1977; E. A. Norse (ed.), Global Marine Biological Diversity, Island Press, Washington, DC, 1993.

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Marine containers

Marine containers Standardized rectangular boxes for the transport of marine cargo. Ocean transportation of cargo containers began in 1956, when the converted tanker IdealX of Pan-Atlantic Steamship Company left Newark, NJ, for Houston, TX, for the first time. On the vessel’s deck were fifty-nine 35-ft (11-m) containers holding cargo that until then had been brought to piers by truck or train, placed on pallets or in cargo slings, and lifted into the ships’ holds. This method of handling cargo, called break-bulk, was slow and laborintensive. Containers, in contrast, can be loaded at a remote site, sealed, moved to the pier, and lifted into the ship without the need for intermediate handling. The principal benefits other than reduced pierside handling cost are less cargo damage and reduced pilferage. See MERCHANT SHIP. Containerization has been the most significant change ever made to cargo transportation by sea. The now ubiquitous container transportation system has made break-bulk shipping almost obsolete on the major trade routes of the world. Even on routes serving minor outports, where cargo volumes have been insufficient to justify the landside infrastructure required for the container system, the phenomenon of containerization is increasingly important. Development. One aspect of the success of container transportation has centered on the standardization of equipment, which has allowed virtually any container to be carried anywhere in the world— by road, rail, or sea or any combination of these modes—without concern for the ability of the transport facilities to deliver the cargo, safely and untouched, to the ultimate destination, however remote it might be. The predominant materials of construction are steel and aluminum, although a small number of containers have included reinforced plastic side panels. All are built around a steel frame to provide the requisite strength for lifting and stacking, in vessel holds and on deck, as well as during intermodal portions of the journey. Through the mid-1960s, container transportation was marked by the use of boxes of a variety of lengths according to the needs of individual vessel operators and local statutory limits on over-the-road transportation. Containers with a length of 17, 20, 24, 30, and 35 ft (5.2, 6, 7.3, 9, and 11 m) were used during this period. These soon gave way to containers of International Standards Organization (ISO) dimensions, wherein lengths of 20 and 40 ft (6 and 12 m) were specified with heights of 8, 8.5, and 9.5 ft (2.4, 2.6, and 2.9 m). Width was standardized at 8 ft (2.4 m). At a later date, containers 45 ft (14 m) long were introduced, and later still, the length was increased to 48 ft (15 m) and finally 53 ft (16 m). The width of the 48- and 53-ft containers is 8.5 ft (2.6 m). Although these non-ISO boxes originated in the United States, where highway transportation practices and most state laws permit their use, they are now common in many other parts of the world. In addition,

attempts are being made to adapt the ISO standard containers to accommodate European standard pallets more efficiently; however, these have not been widely used in international shipping. An increasing number of containers are specially fitted with racks and other devices for dedicated services, of which the transport of motor vehicles and vehicle components is the most common. When completed vehicles are carried in ISO standard containers, they are usually stowed on movable racks to accommodate the maximum load per box. A lowheight 53-ft (16-m) container has been developed to permit motor cars to be driven in and secured without the need for special interior fittings. Where wider-than-ISO containers [mainly 48-ft (15-m) boxes] are to be carried below deck, it has been necessary to construct dedicated cellular guides to accommodate these boxes. On deck, the problem is less onerous, for container support points on deck and on hatch covers can readily be installed. In intermodal traffic, a number of special rail cars will accept containers of either width by the use of adjustable corner fittings. Although a majority of containers are used for general cargo, an increasing number of specialized boxes carry refrigerated or frozen meats, fish, fruit, and other foods. Refrigerated boxes are also used to carry critical medical supplies such as blood plasma and insulin. These containers are equipped with self-contained refrigeration units powered from the ship’s electrical system. Instrumentation systems are used to monitor and control cargo temperature. Tank containers are used to transport a vast array of oils, chemicals, and other liquids. Notable among these liquids are certain potable commodities and cryogenic gases, the latter typically carried in insulated, double-wall tanks. High-cube boxes are widely used for low-density commodities such as apparel. Apparel is carried on hangers that may be moved direct to retail outlets without the need for packaging. Half-height containers are used for special ladings, such as dense commodities. Open-top and open-flat containers are used for outsized loads, while a few double-height boxes have been used for the movement of outsized aircraft components. Stowage and handling. Containers are carried below deck on ships having cellular holds fitted with steel guides at the four corners, which provide longitudinal and transverse restraint as the ship rolls and pitches at sea. No other restraint is required. Stacking containers up to nine high is common in the holds of the largest ships. Deck loading, a unique characteristic of container ships, had been unavailable for most cargoes on break-bulk vessels. In the pioneering days of container transportation, deck stacks generally were limited to two tiers. As lashing methods and equipment were perfected, stack heights eventually rose to six high, consistent with adequate ship stability and unimpeded visibility from the wheelhouse. Many operators use the sixth tier to transport empty containers.

Marine containers Common to all containers are standardized corner fittings, which not only provide structural strength to permit stacking but also provide anchoring points for lashing. Lockable fittings are employed at each corner to provide rigidity throughout the entire stack. Lashings are conventionally chain or steel rods, with appropriate fittings at each end to mate with deck lashing points or container corners. The lashing assemblies include toggle or turnbucklelike devices to assist in securing the stack as an integral unit. Several ship operators have substituted structural frames for a portion of the conventional lashing devices in order to minimize time and labor in making deck-loaded containers ready for sea. These lashing frames are anchored to fixed ship structure and typically are placed between the lowermost and second tier of a deck stack, thus eliminating the need for chain or rod lashings on these tiers. A number of “hatch-coverless” container ships have been built for the purpose of reducing port time and the expense of handling hatch covers while loading and discharging. On such vessels, cell guides are extended upward above the hatch coamings to secure containers stowed above deck. The carriage of hazardous cargo in containers has received much attention. Regulatory requirements and common sense dictate how and where containers loaded with such commodities should be stowed. An important consideration in transporting hazardous cargo in this manner is to ensure that cargo manifests are accurate and the guidelines for stowage are strictly followed. Shoreside handling. In loading or discharging a container ship, individual boxes are typically lifted by shoreside or ship-mounted cranes using spreader frames having locking devices at the corners that engage the container corner fittings. The remoteoperated locking devices are under the control of the crane operator. Under certain circumstances, two containers are handled simultaneously; two 20-ft (6-m) boxes in tandem may be lifted using a single 40-ft (12-m) spreader frame, or two empty boxes, one above the other and secured to one another by locking fittings, may be lifted. Shoreside cranes are of the gantry type, capable of reaching to the extreme far side of the ship being worked. Over the first 30 years of the container era, shoreside cranes evolved to the centerpiece of an efficient handling for vessels of up to Panamax (106 ft or 32.2 m) beam, then the industry-wide standard. From the mid-1980s on, ships with a beam of up to 138 ft (42 m) have been built, making them unable to traverse the Panama Canal. As a result, the maximum outreach of newer shoreside cranes has been increased to handle these larger vessels. Cranes with an outreach of up to 180 ft (55 m) are being installed at major terminals, anticipating dimensions of future vessels. See HOISTING MACHINES. Geared container ships, that is, vessels having shipboard cranes, are generally limited to feeder vessels and smaller line-haul ships that call at ports not having extensive container-handling terminals. Although some multipurpose vessels, which may also

be used to carry break-bulk or bulk cargoes, have center-line mounted cranes, the cranes of geared container ships are typically installed on one side of the ship so that the vessel may be moored with cranes away from the pier in order to eliminate interference while containers are loaded or discharged by shoreside cranes. Handling and movement of containers at shore terminals has received close attention in an effort to reduce terminal dwell time. Two separate systems have evolved: placing each container on its own tractor-drawn chassis for eventual dispatch by road, and handling the containers within the terminal with specially designed straddle carriers, some of which can transport two containers one above the other. At some terminals, on-pier marshaling yards permit the loading of rail cars direct from the vessel for intermodal shipment. In-terminal transfer of containers from ship to ship or barge is also used extensively. The use of containers as an integral part of military logistics systems has received much attention, having been proven in both simulated and actual operations. Prepositioned military support vessels utilize containers for a major part of their cargo stowage. See HARBORS AND PORTS. Intermodalism. The concept of intermodalism, by which containers may be transported successively by sea, rail, and road, was the premise upon which containerization was conceived and achieved its greatest success. Intermodalism was most emphatically demonstrated in the United States, where the growth of the industry coincided with the establishment of the interstate federal highway network. The railroads, previously handicapped because of the atmosphere engendered by an archaic regulatory climate, eventually assembled an efficient containeron-flat-car transport system by which transocean containers could be carried between ocean terminals and inland points. Special rail cars were devised to permit up to two 40-ft (12-m) containers to be carried on each piece of rolling stock, and dedicated inland terminals were able to handle transfers between rail and road vehicles. Although the rail system was reasonably efficient, it was not until the development of the double-stack rail car fleet that per-mile costs were decreased sufficiently to make rail transport a truly competitive alternative. Articulated, drop-center cars permitted containers (including high-cube boxes) to be carried one atop the other, thus doubling the load carried per unit of train length. The western railroads of the United States, enjoying more liberal loading-gauge restrictions, were the earliest beneficiaries of this concept. Many eastern railroads, with their more congested facilities, found it necessary to alter bridges, tunnels, and other lineside impediments to take advantage of the double-stack equipment. The concept is in use with success over much of the United States, bringing the heartland within easy reach of deepwater ports. See RAILROAD ENGINEERING. Prospects. It is predicted that there will be significant increases in the level of both international and domestic container trade. Among the developments

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Marine ecology in container transportation that must accompany increased traffic will be further increases in the size of the container ship fleet, individual ship capacity, more efficient terminals and intermodal connections, and changes in container technology. Maximum ship capacity, now around 7000 TEU (twenty-foot equivalent units), may soon reach 8000 to 9000 TEU, and it is possible that true megacarriers, which carry over 10,000 TEU, may become the standard for major line-haul routes. Maximum ship capacity is closely tied to the ability of shore terminals and intermodal facilities to handle container throughput efficiently. Hence, improvements in the way that containers are handled throughout the landside transportation system, including ship loading and discharging, will be made. Critical to the ability of developed countries to handle increased traffic is the provision of adaquate and efficient port facilities. Assuming that container dwell time is constant, throughput at a terminal is a function of terminal area. The question must be raised: is sufficient additional area available at existing container ports to accommodate future growth? If not, is the construction of new ports, with appropriate intermodal connections, a viable alternative? Heretofore, the physical size of container ships has permitted the operation of port-to-port services, in which vessels call at multiple ports at each end of an ocean voyage. Ship size has increased significantly from the mid-1980s, leading to the emergence of hub- and-spoke operations, in which large line-haul ships call at a limited number of hub ports. From these, smaller feeder ships (which may be as large as yesterday’s line-haul vessels) distribute containers to nearby smaller outports. The expansion of the huband-spoke concept will continue as line-haul ship capacities increase to megacarrier (about 10,000 TEU) dimensions. The success of the hub port concept will depend upon the ability of the ship-port interface to decrease terminal dwell time to a commercially acceptable amount. There is also likely to be a substantial increase in development of intermodal systems in parts of the world where rail and highway facilities are being expanded. Similarly, the intermodal system will need to be made more efficient in developed countries as cargo volume increases and existing road and rail facilities become overloaded. Other changes are likely to be seen in the containers themselves. It is foreseen that collapsible containers will be developed for certain commodity loading to minimize the volume occupied in repositioning empty boxes, and specialized containers for dedicated cargoes will be utilized to a greater degree than that at present. Rod Vulovic

Marine ecology An integrative science that studies the basic structural and functional relationships within and among living populations and their physical–chemical en-

vironments in marine ecosystems. Although an outgrowth of natural history and the life sciences, marine ecology draws on all the major fields within the biological sciences as well as oceanography, physics, geology, and chemistry. See ENVIRONMENT. Historically, and to some extent today, emphasis within the field has been directed at the gathering and analysis of data and descriptive information about taxonomy, species distributions and abundances, natural history and population biology, and the physical–chemical characteristics of marine environments. However, emphasis has evolved toward understanding the rates and controls on ecological processes that govern both short- and longterm events, including population growth and survival, primary and secondary productivity, and community dynamics and stability. Marine ecology focuses on specific organisms as well as on particular environments or physical settings. Mangrove forests, salt marshes, seagrasses and seaweeds, and coral reefs form unique ecosystems defined by the dominant biological community. Lagoons, estuaries, bays, and the principal ocean environments of continental shelf, slope, abyssal plain, and mid-oceanic ridges form ecosystems based on physical–geological features. Together, these ecosystems occur in water depths ranging from the intertidal zone to the deepest ocean depths, about 33,000 ft (10,000 m), and encompass temperature extremes of 27◦F (−3◦C) in the deepest ocean waters to 104◦F (40◦C) in some tropical lagoons. See ATOLL; MID-OCEANIC RIDGE. Marine environments. Classification of marine environments for ecological purposes is based very generally on two criteria, the dominant community or ecosystem type and the physical–geological setting. Those ecosystems identified by their dominant community type include mangrove forests, coastal salt marshes, submersed seagrasses and seaweeds, and tropical coral reefs. Plankton are a principal component in all marine ecosystems and include organisms ranging in size from viruses to larval fishes that permanently, or at some stage in their life cycle, reside in the water column. Other communities include benthic (bottom–dwelling) communities in shallow and deep water, occupants of upwelling areas, and the specialized community that inhabits deep-sea hydrothermal vents. Marine environments identified by their physical–geological setting include estuaries, coastal marine and nearshore zones, and open-ocean–deep-sea regions (Fig. 1). See DEEP-SEA FAUNA; ECOLOGICAL COMMUNITIES; HYDROTHERMAL VENT; PHYTOPLANKTON; ZOOPLANKTON. Estuarine ecosystems. An estuary is a semienclosed area or basin with an open outlet to the sea where fresh water from the land mixes with seawater. The ecological consequences of fresh-water input and mixing create strong gradients in physical– chemical characteristics, biological activity and diversity, and the potential for major adverse impacts associated with human activities. Because of the physical forces of tides, wind, waves, and freshwater

Marine ecology estuary

barrier island

coastal plain

nearshore

20

100 50 0 −50 −100 −150

0 −20 −40

depth, ft

depth, m

40

(a) nearshore

coastal plain

open ocean

1

shelf

1

2 3

2

slope

4 plain

3

(b) Fig. 1. Cross-sectional profile of (a) estuarine and nearshore and (b) coastal and open-ocean marine environments. The nearshore region, although not precisely defined, is generally within kilometers of the coastline. 1 m = 3.3 ft.

fresh water seawater (a)

0

5

0

5

10

15

20

30

(b)

10

15

20

30

(c) Fig. 2. Salinity structure of the three general types of estuaries found throughout the world. (a) Strongly stratified system due to high surface fresh-water inflow. (b) Moderated stratified system showing lines of equal salinity in parts per thousand. (c) Well-mixed estuary showing vertically homogeneous salinity structure in parts per thousand.

depth, mi

0

0 depth, km

input, estuaries are perhaps the most ecologically complex marine environment. They are also the most productive of all marine ecosystems on an area basis and contain within their physical boundaries many of the principal marine ecosystems defined by community type. See ESTUARINE OCEANOGRAPHY. Salinity, temperature, submarine irradiance, that is, underwater light, and dissolved inorganic nutrients determine to large extent the biological composition of estuarine communities. In estuaries, salinity ranges from nearly fresh-water to oceanic concentrations. The actual salinity structure of estuaries varies both among and within estuaries and is determined by fresh-water inflow, tidal energy, and general estuarine circulation patterns. Estuaries are often classified into three groups, based on salinity structure (Fig. 2). The ecological significance of this is that most estuarine populations of plants and animals are physiologically limited by their salinity tolerance range, which restricts their distribution to particular zones. Temperature and submarine light play similarly important ecological roles by limiting or controlling important ecological processes. Temperature extremes generally limit the global distribution of species and seasonally determine the rates of metabolic processes within a given ecosystem. Submarine light plays an equally important role in governing both the rates of photosynthesis and the distribution of photosynthetic organisms. Dissolved inorganic nutrients, particularly nitrogen and phosphorus, not only limit the level of primary production but can influence species composition of estuarine communities. See SEAWATER. Estuaries are further characterized by their principal ecosystem components, which often form the basic ecological unit of study. Salt marshes, mangroves, seagrasses, plankton, and benthic ecosystems are principal components of estuaries and vary in relative importance or even occurrence within specific estuaries. Salt marshes and mangroves are ecosystems that are dominated by vascular plants, occur worldwide, and occupy the intertidal zone. Salt marshes have their greatest distribution and abundance in the temperate latitudes, whereas mangroves are restricted to the tropics. Both play important roles in aquatic productivity, trophic structure, and nutrient cycling and serve as natural erosioncontrol structures. Seagrasses, which are submersed aquatic vascular plants, occur subtidally throughout the world and grow at depths ranging from just below the water surface to more than 30 ft (10 m) in some tropical areas. These plants, evolved from terrestrial ancestors, play many of the same ecological roles as the intertidal salt marsh and mangrove ecosystems, and are limited principally by the availability of submarine light. Planktonic and benthic ecosystems occur in all aquatic systems and for most estuarine environments have the greatest areal extent. Both are important for aquatic productivity and nutrient cycling and support, at least in part, economically valuable fisheries resources. See MANGROVE; SALT MARSH.

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Marine ecology Coastal and nearshore ecosystems. Coastal and nearshore marine ecosystems are generally considered to be marine environments bounded by the coastal land margin (seashore) and the continental shelf 300– 600 ft (100–200 m) below sea level. The continental shelf, which occupies the greater area of the two and varies in width from a few to several hundred kilometers, is strongly influenced by physical oceanographic processes that govern general patterns of circulation and the energy associated with waves and currents. See CONTINENTAL MARGIN. Ecologically, the coastal and nearshore zones grade from shallow water depths, influenced by the adjacent landmass and input from coastal rivers and estuaries, to the continental shelf break, where oceanic processes predominate. Biological productivity and species diversity and abundance tend to decrease in an offshore direction as the food web becomes supported only by planktonic production. Estuarine–nearshore couplings include the advective transport of nutrients and organic matter into nearshore waters, and seasonal migrations of ecologically and economically important species between coastal waters and estuaries. Advective transport contributes organic matter and nutrients, particularly nitrogen, which are thought to augment both primary and secondary production. Net primary production in nearshore waters has often been shown inadequate to support estimated secondary production, and so riverine and estuarine sources provide organic matter and nutrients. In seasonal migrations, many species of shellfish and finfish that spawn offshore migrate or are transported by water into estuaries during the larval or juvenile stage. There, they grow and mature and then migrate offshore as adults to complete the cycle. Among the unique marine ecosystems associated with coastal and nearshore water bodies are seaweed-dominated communities (for example, kelp “forests”), coral reefs, and upwellings. Seaweed communities are distributed worldwide and occupy coastal areas from the intertidal zone to depths exceeding 30 ft (10 m). Green (Chlorophyceae), brown (Phaeophyceae), and red (Rhodophyceae) macroscopic algae dominate seaweed communities and show strong depth-dependent zonation patterns in response to various physical stresses, light requirements, and grazing pressure. Coral reefs are calcium carbonate structures made by reef-building (hermatypic) corals and crustose coralline algae and are best known for their high species diversity and structural complexity. They are limited to the tropical regions and require high light and temperature environments (greater than 68◦F or 20◦C). Upwelling regions are coastal areas where nutrient-rich bottom waters are moved toward the surface by persistent winds blowing parallel to the coast. As a result of the Coriolis effect and the Ekman spiral, surface water is transported offshore and replaced by bottom or middepth water, leading to a relatively continuous supply of nutrients to enhance primary production. Upwelling regions have historically been areas

of economically important fisheries. See REEF; UPWELLING. Open-ocean – deep-sea environments. Approximately 70% of the Earth’s surface is covered by oceans, and more than 80% of the ocean’s surface overlies water depths greater than 600 ft (200 m), making open-ocean–deep-sea environments the largest—yet the least ecologically studied and understood— of all marine environments. The major oceans of the world differ in their extent of landmass influence, circulation patterns, and other physical–chemical properties. Other major water bodies included in open-ocean–deep-sea environments are the areas of the oceans that are referred to as seas. A sea is a water body that is smaller than an ocean and has unique physical oceanographic features defined by basin morphology. Because of their circulation patterns and geomorphology, seas are more strongly influenced by the continental landmass and island chain structures than are oceanic environments. Within the major oceans, as well as seas, various oceanographic environments can be defined. A simple classification would include water column depths receiving sufficient light to support photosynthesis (photic zone); water depths at which light penetration cannot support photosynthesis and which for all ecological purposes are without light (aphotic zone); and the benthos or bottom-dwelling organisms. Occupying the depths between the surface and the average depth of the world’s oceans (12,000 ft or 4000 m) are a diverse group of organisms that include microbes, invertebrates, and vertebrates. Classical oceanography defines four depth zones; epipelagic, 0–450 ft (0–150 m), which is variable; mesopelagic, 450–3000 ft (150–1000 m); bathypelagic, 3000–12,000 ft (1000–4000 m); and abyssopelagic, greater than 12,000 ft (4000 m). These depth strata correspond approximately to the depth of sufficient light penetration to support photosynthesis; the zone in which all light is attenuated; the truly aphotic zone; and the deepest oceanic environments. The benthic fauna are classified in a similar manner as continental shelf and slope fauna, ocean basin or abyssal fauna, and fauna unique to the deep ocean basins and trenches. The open-ocean–deep-sea environments present themselves as both a challenge scientifically and as a world resource that demands cooperative international research and attention. Estuaries and coastal ecosystems, as defined previously, are more localized and lend themselves to regional scientific exploration and understanding. Oceans and seas, on the other hand, are a global resource, crossing all political and national boundaries and requiring international cooperation and participation in ecological studies. Marine ecological processes. Fundamental to marine ecology is the discovery and understanding of the principles that underlie the organization of marine communities and govern their behavior, such as controls on population growth and stability, quantifying interactions among populations that lead to persistent communities, and coupling of communities

to form viable ecosystems. The basis of this organization is the flow of energy and cycling of materials, beginning with the capture of radiant solar energy through the processes of photosynthesis and ending with the remineralization of organic matter and nutrients. Photosynthesis and autotrophic production. Photosynthesis in seawater is carried out by various marine organisms that range in size from the microscopic, singlecelled marine algae to multicellular vascular plants. Conversion of carbon dioxide into simple organic compounds and the subsequent synthesis of algal or vascular plant biomass forms the ultimate base of most food webs in the marine environment (omitted from this discussion are food webs based on chemosynthetic organisms). Because of that relationship, photosynthesis and primary production remain the focus of much marine ecological research. See FOOD WEB. The rate of photosynthesis, and thus the growth and primary production of marine plants, is dependent on a number of factors, the more important of which are availability and uptake of nutrients, temperature, and intensity and quality of light. Of these three, the last probably is the single most important in governing primary production and the distribution and abundance of marine plants. Consequently, the optical physics of seawater has been an important component of much research on photosynthesis and primary production in marine environments. The intensity and spectral quality of light in seawater is a function of incoming solar radiation, surface roughness, dissolved substances, suspended particulates, and water depth. Historically, measures of submarine light were based on photometric standards and units that had been developed for the human eye, such as footcandle, lumen, and lux, which have limited and questionable meaning in a biological context. Both physiologically and ecologically, light in the spectral region termed photosynthetically active radiation (PAR), at about 400–700 nanometers, is important and is measured as irradiance in units of energy flux (joules · m−2 · s−1) or photon flux (microeinsteins · m−2 · s−1), where one einstein (1 E) equals one mole of quanta or photons (6.02 × 1023). As solar radiation passes through a water column, it is reduced in energy content and changed in spectral quality because of absorption and scattering. The total effect on intensity, termed attenuation, is probably the most common optical property of water reported and is derived according to the exponential decay function expressed in the equation k1,2 =

ln (I2 ) − ln (I1 ) z2 − z1

where k is the attenuation coefficient between depths 1 and 2, I is the irradiance intensity at depths 1 and 2, and z is the depth. Generally, all depths are measured in meters below the water surface, and the attenuation coefficient is expressed in units of per-meter (m−1). PAR attenuation varies considerably depending on water type, with attenuation

photosynthetically active radiation, µE . m −2 . s −1

Marine ecology open ocean (k = 0.05)

1000

100

estuaries (k = 1.5) 1% light

10

nearshore regions (k = 0.75) continental shelf (k = 0.35)

1 0.1

1

10

100

depth, m Fig. 3. Photosynthetically active radiation intensity at depth for various values of the attenuation coefficient (k). The intersection of the 1% light level (the limiting light intensity for photosynthesis) with the decay curves marks the depth limit for the photic zone. In these examples, the depth limit ranges from approximately 300 ft (100 m) in the open ocean to 10 ft (3 m) in estuaries. 1 m = 3.3 ft.

coefficients of 0.03–0.04 for the clearest open-ocean waters and 3–4 for the most turbid coastal and estuarine waters. PAR attenuation determines water depths at which light intensity can support photosynthesis (photic zone) and ranges from a few meters in estuaries and turbid coastal waters to approximately 450– 600 ft (150–200 m) in the clearest open-ocean waters (Fig. 3). The relationship between photosynthesis and light intensity varies among the different primary producers in marine environments but can be defined generally by a hyperbolic relationship. Characteristic of this relationship is (1) an initial positive, linear increase in photosynthesis with increasing light; (2) a nonlinear response above a threshold light intensity where photosynthesis does not increase linearly with increasing light; and (3) an upper limit to the rate of photosynthesis (Pmax) at which increasing light has no effect and, if light intensity becomes high enough, may actually reduce or inhibit photosynthesis in some plants. Figure 4 illustrates the photosynthesis–light relationships for two hypothetical marine plants that differ in these fundamental characteristics. Autotrophic or primary production is the direct result of the biochemical and physiological processes of photosynthesis, but it is not equal to it, and results from the net positive growth of marine plants. Considering the high attenuation of light in water and the relationships between light intensity and photosynthesis, net autotrophic production is confined to relatively shallow water depths. The major primary producers in marine environments are intertidal salt marshes and mangroves, submersed seagrasses and seaweeds, phytoplankton, benthic and attached microalgae, and—for coral reefs—symbiotic algae (zooxanthellae). These principal autotrophic components of marine ecosystems differ in geographic distribution and abundance, rates of organic matter

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Marine ecology 3 photosynthesis, mg C . g . plant −1 . h−1

424

high-light species 2 low-light species 1

0

−1

−2

0

100 200 300 400 500 600 photosynthetically active radiation, µE . m −2 . s −1

Fig. 4. Photosynthesis–photosynthetically active radiation curves for two marine plants with fundamentally different light relations. The high-light plant, which has a higher Pmax but requires much more light, is typical of high-light environments such as those of the tropics and shallow water. The low-light plant has a lower Pmax but requires much less light and photosynthetically saturates more quickly. These plants are typical of more turbid environments or environments where light is limiting for significant portions of the photoperiod.

production, and the food webs they support relative to secondary or heterotrophic production, including fishery yields. Though imprecisely known and highly variable for specific environments, net primary production for the principal marine environments of the world’s oceans is summarized in the table. On an areal basis, estuaries and nearshore marine ecosystems have the highest annual rates of primary production. From a global perspective, the open oceans are the greatest contributors to total marine primary production because of their overwhelming size. The two other principal factors that influence photosynthesis and primary production are temperature and nutrient supply. Temperature affects the rate of metabolic reactions, and marine plants show specific optima and tolerance ranges relative to photosynthesis. Nutrients, particularly nitrogen, phosphorus, and silica, are essential for marine plants and influence both the rate of photosynthesis and plant growth. For many phytoplankton-based marine ecosystems, dissolved inorganic nitrogen is considered the prin-

cipal limiting nutrient for autotrophic production, both in its limiting behavior and in its role in the eutrophication of estuarine and coastal waters. See PHOTOSYNTHESIS. Food webs and heterotrophic production. The concept of a marine food web and heterotrophic, or secondary, production is based on the view that energy, as well as organic matter and nutrients, is transferred between organisms that occupy different levels in a heirarchical trophic or feeding structure. The complexity and specific interactions among populations within this conceptual framework form the basis of much marine ecological research and are a source of controversy among marine ecologists. What has evolved is that marine food webs and the processes leading to secondary production of marine populations can be divided into plankton-based and detritus-based food webs. They approximate phytoplankton-based systems and macrophyte-based systems. The classical view of the open-ocean food web involved planktonic algae (diatoms) being grazed by planktonic crustaceans (copepods), which were in turn preyed upon by other invertebrates and fish. Based on size, the final links in the food chain were relatively large organisms, and the classical view dictated that both energy flow and productivity in the oceans were due to these larger populations. To a large extent, this view resulted from the sampling gear and techniques available at the time. This simple, linear food-chain model has been replaced by recognition of the role played by smaller organisms, both autotrophic and heterotrophic, in primary and secondary production, and the cycling of carbon and nutrients. For planktonic food webs, current evidence suggests that primary production is partitioned among groups of variously sized organisms, with small organisms, such as cyanobacteria, playing an equal if not dominant role at times in aquatic productivity. The smaller autotrophs, both through excretion of dissolved organic compounds to provide a substrate for bacterial growth and by direct grazing by protozoa (microflagellates and ciliates), create a microbially based food web in aquatic ecosystems that had once been largely ignored. The consequences of the revised view are that the major portion of autotrophic production and secondary utilization in marine food webs may be controlled,

Areal and total estimates for net primary production in the principal marine environments of the world

Region

Area 106 km2∗

Net primary production, g · m−2 · yr−1

Open ocean Continental shelf Estuaries Seaweeds and coral Upwelling regions Total

332 27 1.4 0.6 0.4 361.4

125 300 1500 2500 500

∗1

km2 = 0.4 mi2.

Total net primary production 109 t · yr−1 41.5 8.1 2.1 1.5 0.2 53.4

Percent of area 91.9 7.5 0.4 0.2 0.1

Percent of total net primary production 77.7 15.2 3.9 2.6 0.4

Marine engine not by the larger organisms typically described as supporting marine food webs (including those leading to the major coastal and oceanic fisheries), but by microscopic populations, thus giving rise to the paradigm of the so-called microbial loop. Macrophyte-based food webs, such as those associated with salt marsh, mangrove, and seagrass ecosystems, are not supported by direct grazing of the dominant vascular plant but by the production of detrital matter through plant mortality. The classic example is the detritus-based food webs of coastal salt marsh ecosystems. These ecosystems, which have very high rates of primary production (see table), enter the marine food web as decomposed and fragmented particulate organics. The particulate organics of vascular plant origin support a diverse microbial community that includes bacteria, flagellates, ciliates, and other protozoa. These organisms in turn support higher-level consumers, which include gastropods, mollusks, polychaetes, crustaceans, and fish. The high productivity of estuaries, particularly those in the Mid-Atlantic temperate regions of North America and Europe and the northern coasts of the Gulf of Mexico, results not only from phytoplankton production in the estuaries themselves but also from the contribution of surrounding salt marsh ecosystems. The highest levels of both primary and secondary production are associated with estuarine and nearshore coastal marine ecosystems. Open-ocean systems are less productive on a per-area basis, and both pelagic (water column) and benthic food webs in deep ocean environments depend on primary production in the overlying water column. For benthic communities, organic matter must reach the bottom by sinking through a deep water column, a process that further reduces its energy content. Thus, in the open ocean, high rates of secondary production, such as fish yields, are associated with areas in which physical–chemical conditions permit and sustain high rates of primary production over long periods of time, as is found in upwelling regions. Regardless of specific marine environment, microbial processes provide fundamental links in marine food webs that directly or indirectly govern flows of organic matter and nutrients that in turn control ecosystem productivity and stability. See BIOLOGICAL PRODUCTIVITY; ECOLOGY; ECOSYSTEM; SEAWATER FERTILITY. Richard Wetzel Bibliography. J. W. Day, Jr., et al., Estuarine Ecology, 1989; M. L. Dring, The Biology of Marine Plants, 1982; J. W. Nybakken, Marine Biology: An Ecological Approach, 5th ed., 2000; I. Valiela, Marine Ecological Processes, 2d ed., 1995.

Marine engine An engine that propels a waterborne vessel. In all except the smallest boats, the engine is but part of an integrated power plant, which includes auxiliary machinery for propulsion engine support, ship services, and cargo, trade, or mission services. Marine engines

in common use are diesel engines, steam turbines, and gas turbines. Gasoline engines are widely used in pleasure craft. Only the propulsion engine will be discussed in this article. See BOAT PROPULSION; INTERNAL COMBUSTION ENGINE; MARINE MACHINERY. Factors in engine selection. The engine type selected for an application should depend on its characteristics, usually with reliability of prime importance. Other factors may include weight, volume, quality and type of fuel required, efficiency, maintenance requirements, durability, simplicity in installation and operation, noise and vibration, initial cost, and ability to sustain the required power levels. In some applications, selection has been made on the basis of expedience or convenience, usually because of the ready availability of an engine type; this explains the continued use of gasoline engines in pleasure craft. Weight and volume. These are usually the dominant criteria in warship applications and are also very important in fast merchant ships. Efficiency, or fuel economy, is tied to weight and volume of the machinery, since it is the range of operation that determines the amount of fuel that must be carried, by which the ship’s payload is correspondingly diminished. In large, relatively slow merchant ships, machinery weight and volume are less important. Fuels. Mostly, liquid petroleum fuels are used, with distillate fuels (including diesel fuel and gas oil) being the highest quality but most expensive, and residual fuel the cheapest but most difficult to use. Blended fuels of intermediate characteristics are often used, generally tending more toward the residual in cost and difficulty of use. While fuel-oil costs have varied widely over time, heavy blends and residuals have tended to cost about half as much as distillates, although experience has demonstrated an inverse correlation between maintenance cost and fuel quality. Coal and natural gas have application in certain trades, and the use of both is considered likely to increase in the future. Nuclear fuel is no longer considered viable for merchant ships, for economic and social reasons, but remains the dominant choice for the largest surface warships and for long-range submarines. See DIESEL FUEL; FUEL OIL; SHIP NUCLEAR PROPULSION. Power level. Power-level requirements depend on the application. One extreme is represented by most merchant trades, where an engine is required to sustain high power output between ports, without relief until arrival, and yet be capable of fairly rapid response over the whole power range while the ship is maneuvered. Another extreme exists with warships, where sustained performance is usually required at a low power level yielding a modest cruise or endurance speed, as well as at a much higher power for high speeds. The difference in power requirements is so great that two engines of different type may be selected. See SHIP POWERING, MANEUVERING, AND SEAKEEPING. Propellers. Most often, marine engines drive propellers, which must be operated at rotative speeds lower than those of turbines and of all but the

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Marine engine largest diesel engines. A speed-reducing transmission is therefore inherent with most marine engines, with reduction gearing the most common type, and electric drive or, in some small installations, belt or hydraulic drive also used. See PROPELLER (MARINE CRAFT). Diesel engines. Diesel engines of all types and power outputs are in use for propulsion of most merchant ships from the smallest to the largest, most service and utility craft, most naval auxiliary vessels, and most smaller surface warships and shorter-range submarines. The diesel engines most commonly used fall into either a low-speed category or the medium- and highspeed category. Low-speed engines (Fig. 1) are generally intended for the direct drive of propellers without any speed reduction, and therefore are restricted to a range of rotative speeds for which efficient propellers can be designed, generally below 300 revolutions per minute (rpm), and as low as 50 rpm

Fig. 1. Cross section of a low-speed diesel engine.

at rated power. Low-speed engines are two-stroke, crosshead engines, with 4–12 cylinders, which are always in-line, turbocharged, and aftercooled. The stroke-to-bore ratio of low-speed engines in current production is about 3:1. The largest engines are rated for power output of over 5000 kW (almost 7500 horsepower) per cylinder at about 100 rpm. These engines are heavy and very large, but they are well suited to operation on low-quality fuels and generally require only modest levels of maintenance. See TURBOCHARGER. Because of their higher rotative speeds, mediumand high-speed engines (Fig. 2) drive propellers through speed-reduction gears, but they are directly connected for driving generators in diesel-electric installations. With few exceptions, these are fourstroke, trunk-piston engines, which have up to 10 cylinders in-line or up to 24 in a V configuration and are mostly turbocharged and aftercooled. Stroke-to-bore ratios range from 1:1 to 1.5:1. Large medium-speed engines are capable of over 1500 kW (2000 hp) per cylinder at about 400 rpm. The upper limit of the medium-speed category, and the start of the high-speed category, is generally placed in the range of 900–1200 rpm, but there are no clear physical features that enable the distinction to be made. Many of these engines have a proven heavy-fuel capability, but most evidence indicates that maintenance costs are higher than those of low-speed engines that are run on fuels of similar poor quality. Some engines, especially those in the higher-speed category, are restricted to distillate fuels. Outside of the typical categories cited above are four-stroke, trunk-piston engines that are designed to be directly connected to propellers, and are indigenous, in both manufacture and application, to the Far East. These engines are built with six or eight cylinders in-line; they are rated for 70–700 kW (95–950 hp) per cylinder at speeds of 200–500 rpm. Another exception is a series of two-stroke, trunkpiston, medium-speed engines that dominate their field of application in United States waters, despite their requirement for distillate fuels. The highest rated of these engines, with 20 cylinders, has an output of more than 3500 kW (over 4500 hp) at 900 rpm. All of the low-speed diesel engines and many of the medium- and high-speed engines are direct-reversing engines, that is, capable of being adjusted, after being stopped, so that they can be started and run in the opposite direction. Nonreversing engines use reversing gearing, if reversing is not provided by electric drive or a controllable-pitch propeller. Most diesel engines have complex vibration characteristics and a tendency toward noisy operation, but much can be done to prevent these characteristics from becoming problematic. All diesel engines consume lubricating oil, generally at a rate of less than 1% of the fuel consumption. The thermal efficiency, which in the largest engines can exceed 45% even without the heat-recovery equipment generally used, is the highest of all current alternatives. The largest, low-speed engines offer this efficiency from

Marine engine low-quality fuel oils, with modest maintenance requirements and relative simplicity but with penalties of high weight, high volume, and high initial cost. The largest medium-speed engines offer comparable efficiencies but with lower weight, volume, and cost, and with a trade-off between fuel quality and maintenance costs. High-speed engines are lighter and more compact and have good efficiency, but generally require good fuels if intensive maintenance is to be avoided. See DIESEL ENGINE. Steam turbines. While steam-turbine plants cannot achieve the thermal efficiency of diesel engines, steam turbines of moderately high power levels (above about 7500 kW or 10,000 hp) offer efficient energy conversion from steam, which can in turn be produced by combustion of low-quality fuel oil, coal, or natural gas in boilers, or from a nuclear reactor. For high efficiency, high turbine speeds are required, typically 3000–10,000 rpm, with reduction gearing or electric drive used to achieve low propeller rotative speeds. The combination of turbine and reduction gear or electric drive has usually proven robust and durable, so that most oil-fueled steamships currently in service are held over from an earlier era. Others, more recently built, are capitalizing on the availability, in their trade, of a fuel unsuitable for diesel engines. In modern merchant ship plants, typical steam conditions at the turbine are a pressure of about 6 megapascals (870 lb/in.2), superheated to about 510◦C (950◦F), with exhaust to a vacuum condenser at about 5 kilopascals (1.5 in. Hg). Some highpowered plants have used reheat turbines, but generally cycle improvements are confined to two to four stages of regenerative feed heating from turbine extractions, and to economizers or rotary regenerative air heaters. See MARINE BOILER. In some applications, especially with electric drive, single-cased turbines are used, but more common is the division of the turbine into a small, high-speed, high-pressure unit and a larger, lowerspeed, low-pressure unit, each with 8–10 stages, independently connected to a single propulsion gear set (Fig. 3). This arrangement, known as a crosscompound turbine, provides more flexibility in design, allowing the high- and low-pressure stages to be better matched to the properties of the steam, which expands to about 400 times its initial volume from the throttle conditions cited above, to exhaust. For related reasons, impulse blading is favored for the high-pressure stages, with lower-pressure stages having increasing reaction. The first high-pressure stage is the control stage, and has partial admission, with the nozzles in groups, each controlled by a valve, enabling power output to be regulated by the number of nozzles in use. Throttle valve control is reserved for maneuvering. For reversing, separate astern stages are usually incorporated in the casing of the low-pressure turbine (Fig. 3), if reversing is not provided by an electric drive. See IMPULSE TURBINE; REACTION TURBINE; STEAM TURBINE. Gas turbines. Aircraft-derivative gas turbines have become the dominant type of propulsion engine for

427

Fig. 2. Cross section of a four-stroke-cycle V-engine.

medium-sized surface warships, including frigates, destroyers, cruisers, and small aircraft carriers. In all cases the turbines are multishaft, simple-cycle engines, with the power turbine geared to the propeller. In some installations, two to four turbines are the sole means of propulsion; in other cases, one or two turbines provide high-speed propulsion, while low-pressure turbine

astern stages high-pressure turbine

Fig. 3. Typical cross-compound steam turbine, with a double reduction gear, shown with upper halves of the turbine and gear casings removed. (Westinghouse Electric Corp.)

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Marine engine

Fig. 4. Gearset for a twin-engine, single-screw ship. (Philadelphia Gear Corp.)

diesel engines or smaller gas turbines are used for cruising speeds. Factors favoring the aircraft-derivative gas turbine in this application are low weight, compact dimensions, high power, rapid start and response, standardization of components, and maintenance by replace-

ment. The last two features are especially important in warships, since an engine can be exchanged in 2 days or less and overhauled by civilian experts ashore, permitting a smaller on-board crew. A major disadvantage of aircraft-derivative gas turbines, their requirement for high-quality distillate fuels, is of diminished importance in the warship application, since most warships, including steamships, use these fuels anyway. Other disadvantages include poorer performance in warmer climates, decreased efficiency at power levels below maximum, large air intake and exhaust ducts, and high exhaust temperatures. Reversing thrust must be provided in the gearbox, by electric drive, or by a controllable-pitch propeller. A further evolution of the aircraft-derivative gas turbine is an intercooled, regenerative-cycle unit, which offers higher thermal efficiency across the entire upper power output range, while retaining the other advantages cited above. Past applications of aircraft-derivative gas turbines and of simple-cycle and regenerative-cycle heavy-duty gas turbines to ships other than warships have had only limited success, and many of these ships were later converted to diesel propulsion. More recently, gas turbines have been used for very fast merchant ships. See GAS TURBINE; TURBINE PROPULSION. Mechanical reduction gears. Marine propulsion gearing is usually of the parallel-shaft, helical-tooth type. Single-reduction gearing (Fig. 4) is usually

bull gear

jacking gear motor

first reduction gear first reduction pinion

first reduction gear

forward second reduction pinions

aft

line shaft flange oil-spray nozzles

Fig. 5. Locked-train, double-reduction gearing for use with a cross-compound steam turbine or twin gas turbines. (Naval Machinery, U.S. Naval Academy, 1946)

Marine fisheries adequate for medium- and high-speed diesels, where the rotative speed ratio ranges from about 2:1 to 10:1. In a typical arrangement (Fig. 4), two engines drive a single propeller, with each engine connected to its pinion via one or more flexible couplings to reduce torsional vibration, and a clutch, which is usually pneumatic or hydraulic. Both double-helical teeth (Fig. 4) and single-helical teeth are common. The high speed of steam and gas turbines requires rotative speed ratios which can reach 100:1, so that double-reduction gearing is necessary (Fig. 3). For higher powers, with both steam and gas turbines, locked-train, double-reduction gearing is used (Fig. 5), which divides the input torque into parallel paths, permitting smaller, if more numerous, gears than would otherwise be required. Couplings of limited flexibility are provided between the first and second stages. Epicyclic gearing has been successfully used in propulsion applications, most often for high-speed stages in double- or triple-reduction steam turbine gearing. Two-speed gears, permitting a selection of rotative speed ratio, have been fitted in low-power applications, with diesel engines, where there is a large range of required ship speeds. Propulsion gearing is constructed to extremely close tolerances, with teeth of modified involute form, formed by hobbing or grinding processes, and either through-hardened or case-hardened, depending on the severity of the stresses. Forced lubrication of bearings, meshing teeth, and mechanical couplings is provided. Transmission efficiencies are high, with losses rarely exceeding 1–2% per stage. See GEAR; GEAR TRAIN. Electric drive. In this arrangement, the engine is directly coupled to a generator, and the electricity produced drives an electric motor, which is most often of sufficiently low rotative speed to be directly connected to the propeller shaft. Any number of engine-generator sets may be connected to drive one or more propulsion motors. Electric drive has been used with engines of all types, including low-speed diesels. Advantages of electric drive include flexibility of machinery arrangement, elimination of gear noise, high propeller torque at low speed, and inherent reversing capability. In ships with high electric requirements for cargo, mission, or trade services—for example, passenger ships, tankers with electric-motor-driven cargo pumps, or warships with laser weapons—there is an advantage in integrating propulsion and ship service support through a common electric distribution system. However, electric drive is usually heavier, higher in initial cost, and less efficient than direct or geared drive. Control arrangements. Automation has been widely applied to marine vehicles of all types; propulsion is normally under direct control from the wheelhouse. Alan L. Rowen Bibliography. R. L. Harrington (ed.), Marine Engineering, 1992; S. H. Henshall, Medium and High Speed Diesel Engines for Marine Use, 1972; E. C. Hunt et al., (eds.), Modern Marine Engineer’s Man-

ual, vol. 1, 3d ed., 1999, vol. 2, 2d ed., 1991; C. M. Plumb, Warship Propulsion System Selection, 1987; D. A. Taylor, Introduction to Marine Engineering, 2d rev. ed., 1996.

Marine engineering The engineering discipline concerned with the machinery and systems of ships and other marine vehicles and structures. Marine engineers are responsible for the design and selection of equipment and systems, for installation and commissioning, for operation, and for maintenance and repair. They must interface with naval architects, especially during design and construction. Marine operating engineers form part of the crew of large ships, where they are responsible for the operation and maintenance of the machinery. Aboard merchant ships, operating engineers are ship’s officers licensed by government agencies (in the United States, by the Coast Guard). Marine engineers are likely to have to deal with a wide range of systems, including diesel engines, gas turbines, steam turbines, boilers, heat exchangers, and pumps and compressors; electrical machinery; hydraulic machinery; refrigeration machinery; steam, water, fuel oil, lubricating oil, compressed gas, and electrical systems; equipment for automation and control; equipment for fire fighting and other forms of damage control; and systems for cargo handling. Many marine engineers become involved with structural issues, including inspection and surveying, corrosion protection, and repair. Marine engineers are generally mechanical engineers or systems engineers who have acquired their marine orientation through professional experience, but programs leading to degrees in marine engineering are offered by colleges and universities in many countries. Most marine operating engineers are graduates of these programs, although others have obtained their training through experience aboard ship. See BOAT PROPULSION; MARINE BOILER; MARINE ENGINE; MARINE MACHINERY; MARINE REFRIGERATION; NAVAL ARCHITECTURE; PROPELLER (MARINE CRAFT); SHIP DESIGN; SHIP POWERING, MANEUVERING, AND SEAKEEPING. Alan L. Rowen Bibliography. C. Drummond, The Remarkable Life of Victoria Drummond, Marine Engineer, 1995; R. L. Harrington (ed.), Marine Engineering, 1992; E. C. Hunt (ed.), Modern Marine Engineer’s Manual, 1991; D. A. Taylor, Introduction to Marine Engineering, 1990.

Marine fisheries The harvest of animals and plants from the ocean to provide food and recreation for people, food for animals, and a variety of organic materials for industry. Important products in marine capture fisheries include fish; mollusks such as oysters, clams, and squid; and crustaceans such as crabs and shrimps.

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Marine fisheries Marine mammals (whales) and reptiles (turtles) have been important historically in marine landings (the weight of the catch landed at the whart); and some plants, mostly seaweeds, are harvested in significant amounts. World landings began to increase substantially during the industrial revolution and rose at a rapid rate after World War II before leveling off in recent years. Marine Resources The oceans’ fish resources were once thought to be inexhaustible, essentially immune to the effects of fishing. As technological advances were made, bigger, more powerful vessels ranged ever farther from their home ports to exploit valuable fishery resources. The advent of steam and diesel engines, freezing capacity, and fish-finding capabilities added to the fishing power of specialized fleets, which by the 1960s fished for extended periods throughout the world’s oceans. By then, it was clear that fishing was a major factor controlling abundances of fish in the sea. Fishing is the last major food-hunting industry in the world in which wild animals are exploited in large quantities by nations with market economies. Marine fisheries provide >10% annually of animal protein consumed by humans, and additional large amounts of fish are processed to be fed to domestic livestock. As practiced in recent decades, unregulated or poorly managed fisheries can lead to collapse of major fish stocks. See FISHERIES ECOLOGY; FOOD MANUFACTURING. The annual marine fish catch, not including aquaculture and exclusive of plants and marine mammals, increased from 18.5 million metric tons (MMT) in 1950 to more than 93 MMT in 1997. Increases were particularly rapid in the 1960s as the rich ocean up-

welling regions off Peru came under exploitation, primarily for anchoveta (Engraulis ringens). Global fleets capable of industrial-scale fishing anywhere in the world dominated during this expansion phase (Fig. 1). Marine fisheries landings increased 7.2% per year during the 1960s, spurring false optimism that marine fish catches would ensure animal protein for a fast-growing human population. In the period 1969–1973, temporary drops in global landings were recorded, a reflection of combined heavy fishing and unfavorable El Ni˜ no climate conditions that devastated the anchoveta population off Peru, which had supported the world’s largest single-species fishery, peaking at more than 12 MMT in 1970. Subsequently, marine fish catches rose again, increasing at 1.3% annually in the period 1985–1997 (see table). This rate is less than the rate of human population growth in ˜ ; SEAWATER FERTILITY; UPthat period. See EL NINO WELLING. Most scientists believe that the global potential for marine fish catches has essentially been reached unless there is a major shift in fishing strategy. For traditional fisheries, approximately 100 MMT appears to be the limit of sustainability, and even that level depends upon effective habitat management and protection. A shift from fishing mostly large, carnivorous species to primarily species lower on the food chain that feed on plants or small animal plankton (which is already occurring to an extent) potentially could increase global landings. Targeting resources now hardly exploited, such as lanternfishes (family Myctophidae) in the deep sea or the shrimplike krill (euphausids) in the Antarctic, could also increase global landings. Whether developing such technologies will allow such shifts in strategy, whether the economics and marketing will be favorable, and whether it is ethically acceptable to develop these

100 Key: marine catch

90

excluding aquaculture 80 70 landings, 106 tons

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60 50 40 30 20 10 0 1950

54

58

62

66

70

74 year

78

82

86

90

94

1998

Fig. 1. Global landings from marine capture fisheries and marine aquaculture production, 1950–1997. Marine aquaculture contributed only small amounts until the mid-1980s. Marine plants (8.4 million tons harvested, mostly from aquaculture, in 1997) are not included in the graph. (Data from United Nations, Food and Agriculture Organization, Rome)

Marine fisheries World fisheries, 1985–1997, in millions of tons∗

Inland Capture Aquaculture Marine Capture Aquaculture Total world Human population (in billions) ∗Fisheries

1985

1990

1995

1997

Rate of increase, % per year

5.7 5.0

6.5 8.2

7.2 13.8

7.7 17.1

2.5 10.5

73.2 2.7 86.6

79.0 4.2 98.0

84.7 7.2 112.9

86.0 11.2 122.0

1.3 11.9 2.9

4.9

5.3

5.7

6.0

1.7

data from FAO (http://www.fao.org/docrep/w9900e/9900e00.htm). Data for 1997 are preliminary and subject to change.

fisheries in sensitive marine ecosystems, are questions that must be answered. To most scientists, it seems unlikely that the sustainable level of marine capture fisheries will be much higher than 100 MMT. See ANTARCTIC OCEAN; FOOD WEB. Biological productivity of ocean regions varies greatly, controlling the potential to produce fisheries products. Coastal regions and estuaries are more productive than the open ocean, which has been described in a comparative sense as a desert. Coastal areas of the sea where nutrient-rich water upwells, especially on the west coasts of continents, support many of the world’s greatest fisheries for sardinelike and anchovy species, even though these areas constitute less than 2% of the sea. Despite the infertility and low productivity of the open ocean, some of the world’s most valuable fisheries, for example tuna, are found there, often in weak but geographically extensive upwelling regions of the subtropical gyres in the world oceans. Most of the sea is being fished to some extent today. The deep sea has been relatively untouched because of technological challenges and dilute fish resources. But new fisheries at greater depths are developing that may change this situation. The vast expanses of the southern ocean support major krill (Euphausia superba) resources, which have been exploited only lightly to date because they are regulated by international treaties. If these krill were exploited heavily, the annual yield could be millions of tons. However, the role of krill as prey for fish, birds, and mammals in the Antarctic marine ecosystem is critical, and it seems unlikely that krill will support large fisheries in the near future. Marine aquaculture has become a major factor in marine fisheries production since the 1980s. The marine aquaculture sector has had the greatest relative increase in fisheries production since 1985 (Fig. 1; table) and will continue to increase during the twenty-first century. Ironically, the rapid rise in marine aquaculture production depends in part on landings of marine-capture fish, which are processed into fish meal, a major constituent of aquaculture feeds. Aquaculture is the sector of fisheries where large increases in production remain possible over the next 25 years. See AQUACULTURE. The character of fishing has changed significantly

since 1950. Traditional artisanal fisheries still exist in many parts of the world, although developing market economies have transformed these fisheries from subsistence fisheries to market industries. Since 1985, catches by developing nations have exceeded those of developed nations. Industrial fishing, dependent on fleets of large vessels and supported by sophisticated technologies, expanded primarily in developed nations, mostly in the Western Hemisphere, Japan, and the former Soviet Union. It was this sector of marine fisheries, especially its distant-water fleets, that was responsible for the rapid global expansion of fisheries. Marine recreational fishing has increased in importance. Statistics for recreational fishing are seldom accurate, but these fisheries account for a major fraction of landings for some species. While most recreational fishing is practiced in developed nations, its importance to tourist economies in developing nations has contributed to its increasing popularity. Recreational fishing is a major industry in the United States, where annual landings of recreationally caught marine fish have averaged more than 90,000 tons in recent years, and marine recreational anglers spent more than $7 billion dollars in 1997. Recreational fishers outnumber commercial fishers and have a powerful influence on fisheries management in the United States. Major components in the catch. The global catch (capture plus aquaculture) consists of hundreds of species partitioned into categories by the Food and Agriculture Organization (FAO). However, on a global basis 40% of the world landings and production of fish products (combined fresh-water, marine, and aquaculture) come from only 20 species. Thus, very few species dominate the marine capture fisheries. In 1997 five species contributed 22.1% to the marine landings: Peru anchoveta 7.7 MMT, Alaska pollock (Theragra chalcogramma) 4.4 MMT, Chilean jack mackerel (Trachurus symmetricus) 3.6 MMT, Atlantic herring (Clupea harengus) 2.5 MMT, and chub mackerel (Scomber japonicus) 2.4 MMT. All of these species are small, shoaling fishes that feed on plankton. Herrings, sardines, and anchovies make up the largest FAO category in global landing and production statistics, with more than 20 MMT landed

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Marine fisheries annually in recent years. Other major categories include cods, hakes, and pollocks (10 MMT), flatfishes and flounders (1 MMT), salmons (>2 MMT), and tunas, billfishes, and swordfish (nearly 5 MMT). Among the invertebrates, shrimps (>3 MMT), oysters (>3 MMT), squids (>3 MMT), and clams (nearly 3 MMT) are major contributors to these relatively high-value fisheries. The largest fraction of landings (70–75%) is consumed directly by humans. However, an important component, ranging 25–30% in recent years, is reduced to fishmeal and oil, which is used primarily as feed for domestic livestock. Small, oily fishes with low market value, such as sardines, anchovies, and capelin (Mallotus villosus), contribute most to these industrial meal and oil fisheries. A significant fraction of these landings and products are processed as feed for fish and shrimp in the aquaculture industry. Thus, inexpensive fish are converted into products that are used to produce cultured shrimp and salmon which have high market value. Some scientists question the wisdom of this approach, which requires heavy exploitation of small fishes to support a relatively small production of valued aquaculture products. Global status. Many marine fisheries are overexploited and less productive than they could be if managed better. On a global basis, FAO indicates that approximately 25% of the world’s fisheries were overexploited, depleted, or recovering during the mid-1990s. Another 44% were heavily or fully exploited, while approximately 31% were believed to be capable of sustaining heavier fishing. In the United States, the situation was similar in 1999, when 27% of its marine fisheries were known to be overexploited, 37% were heavily or fully exploited, and 15% were capable of sustaining heavier exploitation. The status of 21% of the United States stocks was unknown in 1999. Global demand. It is estimated that annual demand for fish to be consumed by humans will reach 95 MMT by the year 2010 if per capita consumption (15.7 kg in 1996) is maintained. Approximately 58 MMT of marine capture fish and 7 MMT of marine aquaculture fish are consumed annually in addition to approximately 25 MMT of fresh-water fish (aquaculture plus capture). Increased demand will require an additional 5 MMT to maintain the per capita consumption level. There is concern that wild fish stocks cannot meet that demand, given present fishing strategies and management problems. Marine aquaculture production will continue to increase, although its ultimate potential is uncertain and dependent to an extent upon landings of wild fish. Many authorities believe that an additional 5–10 MMT of marine capture fish could be landed sustainably if fishery management improved and overfished stocks were restored. Major Fishing Nations Major marine fishing nations fall into two categories: recent entrants and historical fishing nations. Each of the top five marine fishing nations landed more than 5 MMT in marine capture fisheries in 1997. In that

year, the top five nations were China (13.8 MMT), Peru (7.8 MMT), Japan (5.8 MMT), Chile (5.8 MMT), and the United States (5.0 MMT). Beginning in the 1980s, China has emerged as the number one fishing nation. China also has a major fresh-water and marine aquaculture industry. China’s total catch and production of fishery products was 35 MMT in 1997 (28.6% of world total). Although China has a long history of fishing and fresh-water aquaculture, its marine catch increased rapidly in recent years and continues to increase. In contrast, Peru was not a major fishing nation until the 1960s, when its anchoveta stocks came under exploitation. Peruvian fisheries concentrate on small pelagic fishes, including anchoveta, sardines, and jack mackerel, much of which is reduced to meal and oil. Landings and fish production fluctuate significantly under the positive influence of upwelled nutrients and are negatively affected by periodic El Ni˜ no oceanographic conditions. As a consequence, annual landings fluctuate markedly in Peru. Chile also is a recent entrant into the major fishing nation category, with landings resembling those of Peru, but with added diversity. Japan has a long history of fishing and was the number one fishing nation until the 1980s. Traditional fisheries in Japan are characterized by their diversity. Declaration of 200-nautical-mile Exclusive Economic Zones by most maritime nations in the 1970s had a negative impact on Japan’s distant-water fisheries. The United States, with extensive coastlines on both the Atlantic and Pacific coasts, also has a long history as a major fishing nation. Although levels of marine landings changed little in the 1990s (4–5 MMT), the United States has maintained its position among the top five or six fishing nations, which it has held for many decades. Many other nations have economies that depend significantly upon marine fisheries and marine aquaculture or have high per capita consumption of marine fishery products. These nations and their 1997 landings include the Russian Federation (4.4 MMT), Iceland (2.2 MMT), Norway (3.2 MMT), Thailand (3.5 MMT), Indonesia (3.3 MMT) and the Republic of Korea (2.6 MMT). In the past 25 years, there has been a significant shift in the dominance nations producing the most fish products. Historically, nations with developed economies landed the highest percentage of fish, but a shift occurred in the mid-1980s when nations with developing economies became the largest producers annually catching or producing approximately 70% of the global fish. Managing the Fisheries Managing marine fisheries is complex and often uncertain. Managers and the agencies or institutions that support them have jurisdiction over living resources that fluctuate naturally in abundance, often are highly mobile, and are only rarely privately owned. Many species (for example, tunas and billfishes) have ranges that extend over thousands of kilometers during their annual migratory cycle. The common property nature of marine fisheries

Marine fisheries spread the TAC over a season or to ensure that the catch is spread among numerous fishers, rather than being taken by the most powerful fishing vessels. In some managed stocks, only males are allowed to be landed—females carrying eggs must be released to protect spawning potential of the stock. Such measures are particularly popular in some crustacean (crab and lobster) fisheries. Maximizing sustainable yields. An objective of fisheries management is to achieve yields from a fishery that are both high and sustainable. In marine commercial fisheries, that objective historically was translated into a fishing strategy that allowed a maximum sustainable yield (MSY) from a stock that is in balance with the stock’s reproductive and growth capacities under a given set of environmental conditions. If the population growth rate of a species is known and is presumed to be primarily a function of the population’s size (expressed as its aggregate weight, or biomass), then the MSY can be predicted in relation to fishing effort (or fishing mortality rate) [Fig. 2]. See POPULATION ECOLOGY. For many years, MSY was set as a management target, which in retrospect often allowed catches that were too high, precipitating declines in the stock, leading to an unsustainable, overfished condition.

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complicates conventional management approaches that allow open access and require agreement among jurisdictions, and consensus on the best methods to regulate fishing and the allocation of the catch. Total Allowable Catch versus fishing effort control. The most common management approach is to assign an annual Total Allowable Catch (TAC). This requires setting a quota that, if exceeded, will terminate the fishery for that year. The TAC is set at a level to prevent a catch so large that the stock will be overfished. In an open-access fishery, there may be competition among fishers to secure a large fraction of the TAC, which can lead to “derby” fishing that is difficult to control. In international fisheries, the TAC is commonly apportioned among participating nations based on negotiated allocations and historical landings. Many variations of quota assignments, including dividing the TAC into seasonal allocations or apportioning it among areas, have been used. For example, in Pacific salmon management, individual TACs may be assigned to stocks that are fished near the mouths of spawning rivers. In this way, sufficient adults escape to insure the continued reproductive success of the individual stocks. A second common management approach is to control the amount of fishing effort. Controlling effort nominally limits the mortality rate attributable to fishing, which is presumed to be directly proportional to fishing effort. Effort regulation, when effective, fixes the fraction of a stock that is caught. Consequently, the number of fish landed may vary among years, with high catches during years of high abundance and lower catches when the stock is less abundant. Many fishery scientists and managers argue that effort management is preferred over TACs because TACs may allow landings that are too high in years when the stock is at low levels. Fishing effort can be controlled by limiting numbers of participants, by limiting amounts of fishing time or amounts of gear, or by legislating inefficiencies into the fisheries. Prohibiting fishing gears or methods that may, because of their efficiency, catch fish too rapidly or effectively may control fishers and allow an even distribution of landings throughout the fishing year. Frequently, TAC (output) and fishing effort (input) controls are applied together, especially in heavily fished stocks, for example, northwestern Atlantic stocks, such as cod (Gadus morhua) and haddock (Melanogrammus aeglefinus). Other, less direct methods may be applied as well. For example, size limits may be imposed to protect young fish, allowing them to mature and spawn. In some species (for example, some sturgeons), very large individuals may be protected from fishing to maintain the fecundity and spawning potential of a stock. Other controls which are becoming common include time and area restrictions, such as closure of nursery areas, protection of spawning habitats, and exclusion of fishers from polluted areas. In some cases, managers attempt to maximize the quality of the catch by specifying a fishing season when the resource is at its best condition (for example, oysters). Daily landing quotas, called bag limits, may be assigned to

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(b) Fig. 2. Concept of maximum sustainable yield (MSY). (a) Catch and revenue and cost versus fishing effort. (b) Stock biomass versus fishing effort. In a developing fishery, catch increases in relation to fishing effort. At an intermediate level of effort and stock biomass, the MSY can, in theory, be caught. At higher levels of effort, the sustainable level of catch declines and the stock is overfished. Fishing at effort levels near MSY is risky. Safe fishing is practiced at considerably lower levels of effort. The maximum economic yield (MEY) is taken at a level of effort where net profit is maximum. The MEY is taken at effort levels lower than the effort level associated with MSY. In unregulated, open-access fisheries, the level of effort may increase to the point where the “cost of fishing” line intersects the catch (=revenue) line.

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Marine fisheries Environmental variability and failed reproduction in years of unfavorable conditions make MSY an undesirable and risky strategy. However, estimating MSY remains an important objective, and recent management advice usually sets MSY as a threshold, a level of catch that, if approached, should signal caution rather than being considered a target for the fishery. Maximizing yield per recruit. A second common management goal has been to maximize or optimize the catch with respect to a given level of recruitment (production of young fish). In this approach, the size (or age) at which fish first enter the fishery is adjusted by regulating mesh sizes of fishing gears, and the fishing mortality rate is controlled by adjusting the amount of fishing effort. Given a fundamental knowledge of fish growth and an estimate of natural mortality rate, maximizing the yield per recruit can be easily achieved in theory, although it may be achieved at the risk of catching too many adults, thus reducing the biomass of the spawning stock to a level so low that future recruitment success is jeopardized (Fig. 3). Thus, fishing at a level that takes maximum yield per recruit is risky, and is now considered to be a threshold associated with too high a level of fishing mortality. The yield-per-recruit model also can be used to estimate spawning biomass per recruit relative to its level if there were no fishing mortality and under specified levels of fishing mortality (Fig. 3). Spawning biomass of a stock falls rapidly as fishing mortality rate increases, indicating that fishing mortalities must be set at relatively low levels to conserve high spawning potential and ensure future recruitment success. Biological reference points. Managing marine fisheries generally requires the designation of reference points, which serve as targets and thresholds. Biological reference points most often are specified fishing Risky fishing

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fishing mortality Fig. 3. Concept of maximizing yield per recruit (YPRmax). Yield per recruit increases as fishing mortality (or effort) increases until a maximum is reached, beyond which increases in effort result in declining YPR as the stock is overfished. Biological reference points (F%) are fishing mortality rates associated with specified yields per recruit. A stock is overfished at F levels higher than Fmax. Spawning biomass per recruit (SBR) declines as F increases. Safe fishing may require a relatively low F that ensures higher SBR (≥40% in the example). The F0.1 reference point, historically considered a safe target level, sets F at a level where the slope of the YPR curve is 1/10 of that at the origin.

mortality rates, spawning stock biomasses associated with fishing mortality rates, or catch levels relative to specified fishing mortality rates or stock biomass levels. Precautionary management rests on selecting target reference points that allow sustainable fishing at profitable levels, yet do not endanger the stock’s capacity to reproduce. In many fisheries, experience has demonstrated that fishing mortality rates must be limited to levels that maintain spawning biomass at levels at least 20–40% of that in an unfished stock. Thus, safe fishing may require limiting yields to levels that are less than the maximum sustainable yield and less than the maximum yield per recruit (Figs. 2 and 3). Economics. To be profitable, revenues from catches obviously must exceed costs, although governments often have subsidized fishing, an industry that provides human food. In a simple model, the maximum economic yield, or level of catch that provides the largest net profit, is achieved at an effort level where revenues most exceed costs of fishing. This effort level is considerably lower than the effort required to obtain maximum sustainable yield (Fig. 2). In this sense, it is a safer level at which to fish, in addition to maximizing profits. In openaccess fisheries, without strong management control, economists argue that effort will increase in a fishery until it equilibrates at a high level where net profits are zero (Fig. 2)—a clearly undesirable circumstance that also risks the biological collapse of the stock. Recreational fisheries. Marine recreational fishing is a valuable industry in many developed countries. Allocations of catch to recreational fishers are controversial in fisheries with competing users and different values, althouth principles of conservation and management for recreational and commercial sectors often are similar. In some cases, reference points in fisheries dominated by the recreational sector may be set conservatively to maintain quality (many and big fish) in the fishery. As marine recreational fishing increases in importance globally and contributes significantly to the economies of developing nations, conflicting management philosophy and practice between the recreational and commercial sectors seem certain to increase. Allocating rights and privileges. Many scientists and managers believe that to secure and stabilize marine fisheries their historical common property nature must be transformed to selectively grant user rights or privileges to participants in the fishery. Many examples of rights-based management are in effect throughout the world, such as licensing and permitting systems, and limiting entry to already overcapitalized fisheries to reduce or place a cap on the numbers of participants. An increasingly common but controversial management approach to limit access is the Individual Fishing Quota, which allocates shares to selected fishers based on their historical presence in a fishery. These shares of the Total Allowable Catch are often treated as a commodity that can be traded or sold. Management by Individual Fishing Quota, works on the assumption that participants,

Marine fisheries because they hold access rights and in a sense share ownership of the resource, will not engage in behavior that promotes overfishing or damages the marine environment. Institutions and agencies. A complex of local, regional, national, and international institutions is charged with management of marine fisheries. Many fish stocks are migratory. Fish stocks do not recognize jurisdictional boundaries and cannot be assessed or managed effectively if some jurisdictions are not included in management plans. The classic case exemplifying the need for interjurisdictional management is that of tunas, swordfish, and billfishes, which have been called “fish without countries” because of their migratory behaviors. International commissions provide management advice and in some cases hold management authority for highly migratory species, such as tunas, or for less migratory fish stocks in international waters, such as cod, haddock, and herring in the North Sea, Barents Sea, or Baltic Sea ecosystems. Examples of international commissions are the International Commission for the Conservation of Atlantic Tunas (ICCAT) and the Northwest Atlantic Fisheries Organization (NAFO). International commissions vary with respect to their research responsibilities and with respect to their advisory or regulatory roles relative to authority exercised by the signatory nations. See ARCTIC OCEAN; BALTIC SEA; NORTH SEA. Marine fisheries of most nations are usually managed by a national authority. In the United States, that authority rests in the Department of Commerce and its National Marine Fisheries Service. Nations with large, geographically dispersed fisheries may have regional management institutions responsibe for particular fish stocks. In the United States, regional management is vested in eight Regional Fisheries Management Councils, which develop management plans to govern particular fisheries. In addition, other regional institutions may manage marine fisheries in particular zones. For example, in the United States, the Atlantic States Marine Fisheries Commission is responsible for managing migratory stocks, such as striped bass (Morone saxatilis) and American shad (Alosa sapidissima), that primarily reside within 3 nautical miles of shore, but which may be fished along the coastlines of many states. There are also agencies within individual regions or states (or in local waters) that have authority to regulate fishing. These jurisdictional units sometimes have conflicting goals and approaches to fish stock management. In many cases, conflicts are resolved or avoided by agreements that ensure joint management responsibility among agencies. For example, in the United States, Atlantic coastal stocks of bluefish (Pomatomus saltatrix) are jointly managed by the Atlantic States Marine Fisheries Commission, the states, and three Regional Fisheries Management Councils. Ecosystem Approaches to Management During the 1990s there was increasing public concern about the status of marine fisheries and exploita-

tion practices. As more fisheries were overexploited, more habitats came under stress, and water quality issues came to the forefront. Most nations and management agencies adopted, at least nominally, precautionary approaches to the exploitation and management of marine fisheries. There emerged a sense that the burden of proof with respect to effects of heavier fishing pressure or implementation of new marine fisheries ought to be borne by fishers and not by management agencies. This shift in philosophy gained a foothold in the 1990s and has slowly become a guiding framework for marine fisheries management. The approach recognizes that fishing is a significant presence in marine ecosystems and that its ecosystem-level effects must be considered. Multispecies management. New models are emerging to account for multispecies interactions in marine fisheries that previously were neglected in traditional, single-species management. Predatorprey relationships dominate interactions among marine species and are emphasized in these models. Most fish die from being consumed by larger fish. Fishing that significantly alters abundances of predator and prey species can affect productivities of the stocks and their potentials to provide sustainable catches. In cases where several species of fish interact through predator-prey and competitive relationships, the effects of fishing on those interactions and on potential yields of individual species are difficult to predict and not always intuitive. Consequently, managers have been reluctant to adopt complex multispecies models as the basis for management. However, it is probable that such models will become a tool that managers regularly use to forecast potential effects of different fishing strategies on fisheries productivity and marine ecosystem structure. Ecosystem principles and fisheries management. Beyond multispecies management, the broader consequences of ecosystem effects of fishing need to be addressed. Consideration of fishing practices and their effects must be included in fisheries management plans. Water quality and critical spawning and nursery areas, or areas with critical habitats for prey or predators of a target species, must be protected. Catches of nontargeted organisms, called bycatch, whether they are unwanted species or are small individuals of a targeted species, must be reduced. The bycatch issue extends to the capture of birds, turtles, and marine mammals, adding impetus to solve the problem. The magnitude of the problem is highlighted by records that indicate more than 25 MMT annually of marine bycatch has been caught and discarded in recent years. See WATER CONSERVATION. Accounting for effects of fishing on ecosystems will depend on development of ecosystem models that not only examine a multispecies complex exploited by a fishery but include other components and aspects of ecosystems. In future management scenarios, ecosystem-level effects of fishing practices and alternatives will be explored by running ecosystem models in which fishing is simulated and ecosystem responses are evaluated. Such

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Marine fisheries and the towing speed to correct the net depth. Such complex systems are mainly meant for midwater trawling. Other applications are being developed. See SONAR. Echo sounding. One of the major problems of fishing is to find the fish when they are not visible at the surface. This was largely solved when, starting in the late 1930s, echo sounding was developed not only to measure the water depth but also to show fish. The principle of echo sounding consists in measuring the time that a sound impulse takes to travel to a target and the reflected echo to come back. Half the total time multiplied by the speed of sound (in water, about 0.9 mi/s or 1500 m/s) gives the distance. The echo strength depends, among other things, on the reflection properties of the target; for example, rocks reflect better than sand or mud and large fish (particularly with swim bladder), or fish schools better than small or single fish. Fish-finding echo sounders need more transmission power and echo amplification than purely navigational echo sounders and also a suitable monitor display, Historically, echograms were presented on recording or cathode-ray-tube (CRT) displays for echo discrimination of bottom and various fish sizes and concentrations. The normal echo sounder, covering the water column below the vessel, is applicable to all fishing methods. Horizontal, or, rather, oblique, echo sounding at various angles between the surface and the bottom forward and around the vessel is also known as sonar (sound navigation and ranging). It mainly serves in midwater trawling and purse seining (see below), for which the advance detection of fish is quite essential. Depending on type and power, the range for fish detection of echo sounders is down to about a 3300-ft (1000-m) depth, and of sonar up to about 9900 ft (3000 m) away from the vessel. The net sounder developed for midwater trawling is an echo sounder with the transducers or sounding units at the net rather than in the ship’s bottom (Fig. 3). The connection from net to trawler may be by cable or by an acoustic link (wireless). The more complex multinet sounder equipment for commercial fisheries includes forward transducers in addition to the down and up transducers, a net filling meter, and a thermometer at the net to check whether the water temperature is according to the preference of the species sought. Scientific multi-net sounder equipment has additional transducers across the trawl opening and further aft in the bag, a speed-through-water meter, and a transponder to study gear performance and fish reactions in detail. Purse seining. In addition to giving all fishers an eye to see under water and paving the way for aimed trawling, the development of underwater acoustics also opened a new dimension to purse seining. This fishing method uses a net which may be 490 to 4900 ft (150 to 1500 m) long and 100 to 660 ft (30 to 200 m) deep which is set in a circle around a fish school and then closed at the bottom like a purse, catching all fish inside. Unless it is done in connection with fish attraction (such as with light), purse seining is completely dependent on finding suitable

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fish schools. Schools of fish may be sited from the vessel or from airplane spotters who locate the schools and direct the vessel to them. For fish which are not visible at the surface, sonar-guided purse seining is the only solution and quickly led to a very significant expansion of this fishery, which is the largest bulk fish producer in the world. As in aimed trawling, the success of sonar-guided purse seining is largely dependent on the skill of the individual skipper. Consequently, computerized systems have been developed to assist the skipper and also improve the complex tasks of detecting and approaching a fish school, assessing its size and depth, and coordinating the relative movements of vessel and fish school during actual fishing. One such system employs a television screen (Fig. 6) to show the situation in the horizontal in relative or true motion as desired. The computer can also link the sonar to a selected fish school and calculate the depth of the school. Longlining. Longlines range (from a few hundred meters to many kilometers in length and carry baited hooks set at intervals. This gear has been adapted since the 1950s to effectively catch migratory species such as tunas and swordfish in the open sea. Development of accurate navigation and the Global Positioning System, and acquisition of sensors to detect temperature and other oceanographic features, have increased the ability of longliners to locate and remain in the environment of the target species. In addition, the use of chemiluminescent sticks light as a fish attractant has increased the effectiveness of the longline technique. See REMOTE SENSING; SATELLITE NAVIGATION SYSTEMS. Vessel trends. In addition to increases in size and horsepower, vessels in modern industrial fishing fleets have become more diversified to allow participation in fisheries that use more than one type of gear and target different kinds of fish. For example, in the Gulf of Alaska and Bering Sea fisheries, vessels may be rigged to alternatively trawl for groundfish, set

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Press, Washington, DC, 1999; Our Living Oceans 1999, National Oceanographic and Atmospheric Administration, National Marine Fisheries Service, Silver Spring, MD, 1999; Yearbook of Fishery Statistics, Food and Agriculture Organization, Rome, annually.

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distance from direction to ship to school center of school ship's path during speed and direction the last 10 min of school Fig. 6. Automatic tracking and true-motion situation display by a computer-integrated purse-seining system. Course is indicated in degrees, scale in meters, and speed of vessel in knots. Depth scale at left is in meters. 1 m = 3.3 ft. (SIMRAD AS, Norway)

traps for crab, or longline for halibut, depending on the season and potential profitablility of a particular fishery. Such versatility often is required to remain profitable in modern, highly competitive fisheries. See BERING SEA; OCEANOGRAPHIC VESSELS. Synthetic fibers. Apart from these special developments in trawling and purse seining, the introduction of synthetic materials for yarns, netting, and ropes had an even more general impact on fishing gears and fishing than the introduction of echo sounding, which started at about the same time. The main advantages of synthetics such as polyamide (for example, nylon), polyethylene, and polypropylene are their strong resistance against rotting. Some also have higher breaking strength, elasticity, and abrasion resistance than natural fibers such as cotton, manila, and sisal used hitherto. Higher breaking strength enables the use of thinner yarns, and synthetic monofilaments are almost invisible in water. This can significantly improve the catching efficiency of, for instance, trawls and other bag nets, gill nets, or angling gear. These advantages compensate for the higher price of synthetics, which have now almost completely replaced the traditional natural-fiber materials. See MANUFACTURED FIBER. Joachim Scharfe ¨ Bibliography. Ecosystem-Based Fishery Management, National Oceanographic and Atmospheric Administration, National Marine Fisheries Service, Silver Spring, MD, 1999; Fisheries of the United States, National Marine Fisheries Service, Silver Spring, MD, annually; National Research Council, Improving Fish Stock Assessments, National Academy Press, Washington, DC, 1998; National Research Council, Sustaining Marine Fisheries, National Academy

The study of the portion of the Earth beneath the oceans. Approximately 70% of the Earth’s surface is covered with water. Marine geology involves the study of the sea floor; of the sediments, rocks, and structures beneath the sea floor; and of the processes that are responsible for their formation. For the marine geologist, the presence of the oceans masks the principal features of interest. The average depth of the ocean is about 3800 m (12,500 ft), and the greatest depths are in excess of 11,000 m (36,000 ft; the Marianas Trench). The study of the sea floor necessitates employing a complex suite of techniques to measure the characteristic properties of the Earth’s surface beneath the oceans. Contrary to popular views, only a minority of marine geological investigations involve the direct observation of the sea floor by scuba diving or in submersibles. Instead, most of the ocean floor has been investigated by surface ships using remote-sensing geophysical techniques, and more recently by satellite observations. In the mid-1960s to the mid-1970s, the theory of plate tectonics was developed and verified based on geological and geophysical observations of the ocean floor and crust. Key observations included the magnetic characteristics of rocks, the disposition of submarine landforms, the distribution and ages of marine sediments, and the locus and nature of zones of earthquake and volcanic activity. Earthquakes served to define the principal plate boundaries. The axes of mid-oceanic ridges mark sites of plate divergence; trenches mark sites of plate collisions (subduction zones). Diverging plates allow for the birth of new oceanic crust, whereas colliding plates accommodate the destruction of equivalent amounts of crust. Large strike-slip faults exist (such as the San Andreas Fault) where plates slide by one another (Fig. 1). See EARTHQUAKE; PLATE TECTONICS. The ocean crust is relatively young, having been formed entirely within the last 200 million years (m.y). The process of renewing or recycling the oceanic crust is the direct consequence of plate tectonics and sea-floor-spreading processes. It is therefore logical, and perhaps essential, that the geologic history of the sea floor be outlined within the framework of plate tectonic tenets. Where plates move apart, molten lava reaches the surface to fill the voids, creating new oceanic crust. Where the plates come together, oceanic crust is thrust back within the interior of the Earth, creating the deep oceanic trenches. These trenches are located primarily around the rim of the Pacific Ocean (Fig. 1). The down-going material can be traced by using the distribution of earthquakes to depths of about 700 km (420 mi). At that level, the character of the subducted lithosphere

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Fig. 1. Map showing the tectonic structure of the Earth’s lithosphere, which is composed of about 10 rigid plates, each moving as a distinct unit. (After J. F. Dewey, Plate tectonics, Sci. Amer., 226(5):56–68, 1972)

is lost, and this material is presumably remelted and assimilated with the surrounding upper-mantle material. See GEODYNAMICS; LITHOSPHERE. Major Morphologic and Sediment Provinces The major features of the sea floor are the midoceanic ridges, basins, continental margins, large igneous provinces, marginal seas, and anomalous features. Mid-oceanic ridges. Most of the ocean floor can be classified into three broad physiographic regions,

one grading into the other (Fig. 2). The approximate centers of the ocean basin are characterized by spectacular, globally encircling mountain ranges, the midoceanic ridge (MOR) system, which formed as the direct consequence of the splitting apart of oceanic lithosphere. The small-scale morphologic characteristics of these mountain ranges depend somewhat upon the rate of separation of the plates involved. As is shown in Fig. 3, abyssal hill relief, especially within 500 km (300 mi) of ridge crest, is noticeably rougher on the slow-spreading Mid-Atlantic Ridge than on the

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Fig. 3. Topography of the mid-oceanic ridge. (a) Mid-Atlantic Ridge (25 N), spreading rate 2.6 cm (1.0 in.) per year. ◦ (b) East Pacific Rise (55 S), spreading rate 8.8 cm (3.5 in.) per year; sea-level rises causing transgression (arrows). 1 km = 0.6 mi.

fast-spreading East Pacific Rise. The profile of the East Pacific Rise is also broader and shallower than for the Mid-Atlantic Ridge. If the entire mid-oceanic ridge system were spreading rapidly, the expanded volume of the ridge system would displace water from the ocean basins onto the continents, and this may explain a well-known incident of marine transgression during the Cretaceous Period. The broad cross-sectional shape of this mid-ocean mountain range can be related directly and simply to its age. The depth of the mid-oceanic ridge at any place is a consequence of the steady conduction of heat to the surface and the associated cooling of the oceanic crust and lithosphere. As it cools, contracts, and becomes denser, the oceanic crust plus the oceanic lithosphere sink isostatically (under its own weight) into the more fluid asthenosphere. The depth to the top of the oceanic crust is a predictable function of the age of that crust; departures from such depth predictions represent oceanic depth anomalies (Fig. 4). These depth anomalies are presumably formed because of processes other than lithospheric cooling, such as intraplate volcanism. The Hawaiian island chain and the Polynesian island groups are examples of this type of volcanism. See ASTHENOSPHERE; ISOSTASY. Very detailed surveys have been completed for modest sized pieces of the mid-ocean ridge system in the North Pacific, South Pacific, and South Atlantic in an effort to compare and contrast the morphologic features of the crestal zones at fast-, intermediate-, and slow-spreading ridges. The amplitude of the small-scale component of relief is somewhat dependent on the associated rate of sea-floor spreading. This small-scale relief is born at the axis of the ridge. As it moves laterally by spreading, it evolves gradually, becoming more subdued as it is slowly draped by a “snowfall” of pelagic sediments. The resulting sea floor abyssal hills constitute the largest morphologic province in the world. The mapping studies have revealed that the midocean ridges are characteristically segmented at a variety of along-axis scales ranging from about 10 km

(6 mi) up to about 1000 km (600 mi). The depth of the near crestal regions and the degree of segmentation are thought to reflect the relative amount of magmatic melt available for injection and its temporal variability. Mid-ocean ridges’ variable subsidence character captures the largest scale component of ridge segmentation. The smaller scales of segmentation are manifest as contrasts in the along-axis depths and in their near-crestal morphology (Fig. 5). The loci of sea-floor volcanic activity create, most notably at or near the axis of the MOR system, an extreme environment of chemistry and temperature, with hydrothermal plumes (vents) having temperatures often in excess of 400◦C (750◦F) and containing dissolved and particulate minerals such as sulfides, lead, copper, and others but devoid of free oxygen and sunlight. These conditions define an environment previously presumed unsuitable for supporting life. Numerous diving vessel expeditions to these environments have sampled the fluids and mineral particles emanating like “black smoke” from large chimneylike structures. They have also recovered strange life-forms such as enormous clams and giant tubeworms that symbiotically exist with a type of bacteria called chemoautotrophs. Such observations have required geobiologists to reevaluate their presumptions about what other extreme environmental conditions may sustain life-forms. See HYDROTHERMAL VENT; MID-OCEANIC RIDGE; OCEANIC ISLANDS; VOLCANOLOGY. Basins. The deep ocean basins, which lie adjacent to the flanks of the mid-oceanic ridge, represent the older portions of the sea floor that were once the shallower flanks of the ridge (Fig. 2). The bulk of sediments found on the ocean floor can be broadly classified as terrigenous or biogenic. Terrigenous sediments are derived from drainage of adjacent landmasses and are brought to the sea floor through river systems. This sediment load is sometimes transported across the continental shelves, often using as pathways the submarine canyons that dissect the shelves, the continental slope, and the continental rise. Biogenic sediments are found in all parts of the

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ocean, intermixed either with terrigenous sediments or in near “pure form” in those areas inaccessible to terrigenous sedimentation. Biogenic sediments are composed mostly of the undissolved tests of siliceous and calcareous microorganisms, which settle slowly to the sea floor. This steady so-called pelagic rain typically accumulates at rates of a few centimeters per thousand years. The composition and extent of the input to the biogenic sediment depend upon the composition and abundances of the organisms, which in turn are largely reflective of the water temperature and the available supply of nutrients. The Pacific equatorial zones and certain other regions of deep ocean upwelling are rich in nutrients and correspondingly rich in the microfauna and flora of the surface waters. Such regions are characterized by atypically high pelagic sedimention rates. See UPWELLING.

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Fig. 5. Digitized contoured map of a ∼200 km (120 mi) ◦ ◦ stretch of the East Pacific Rise (Ridge) near 11 N, 104 W. The data were collected using a multibeam swath mapping system. The depth of the ridge crest varies from south to north and illustrates an intermediate scale segmentation of the ridge. The area slightly north of the map center shows an overlapping spreading center, another example of ridge segmentation. (Taken from Ridge 2000 MBS data and displayed using GeoMapApp software)

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Fig. 4. Ocean depth anomalies. (a) Schema of the lithosphere, asthenosphere, and varying ocean depths away from the axis of mid-oceanic ridge spreading system. (b) Parameters used to compute adjusted crustal depths and to analyze depth-age relationships. (c) Hypothetical crustal depth (DC) versus (age)1/2 plot showing linear relationship predicted by theory. Differences between the actual observations and the statistically best-fitted line represent residual depth anomalies. DC (predicted) yield positive or negative depth anomalies. WD = water depth; ST = sediment thickness; D0 = crustal depth at zero age crust; δDC = depth anomalies (difference between the observed and predicted crustal depths). ( After D. E. Hayes, Mapping oceanic depth anomalies: In search for indicators of asthenospheric convection, Lamont-Doherty Geological Observatory Yearbook, 1982–1983)

As the ridge flanks subside to great depths, often in excess of 6 km (4 mi) over a period of 100 m.y., the small-scale topography is gradually modified by pelagic sediments. Once sediments reach the sea floor, their ultimate disposition depends on local environment. Fine sediments can be held in suspension for long periods or can be transported along the bottom, ultimately to be deposited in a more tranquil environment. Spectacular bedforms such as sediment waves sometimes result and provide important information regarding the nature and vigor of present and ancient bottom-current activity. In some cases, the bottom currents can be so strong as to prevent the deposition of fine-grained sediments altogether, essentially scouring the sea floor free of any significant sediment. Over most of the deep-sea floor, bottom currents are sluggish, and most pelagic sediments

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Marine geology hydrates. Hydrates are frozen gases, typically composed of methane, higher-order hydrocarbons, and carbon dioxide, which exist in a stable state only within a relatively narrow range of temperature and pressure. The occurrence of such hydrates is now believed to be far greater than past estimates. Hydrates are often associated with a bottom-simulating reflector, which is thought to represent a sharp drop in acoustic impedance created by accumulations of free methane gas just below the base of the gas hydrate zone. See HYDRATE. There is growing evidence that the hydrocarbon gases (mostly methane), entrapped in sub-sea floor gas hydrates, constitute a significant component of global methane and is likely to be the focus of intense ongoing investigations. The potential of gas hydrates to contribute to the global energy inventory of recoverable hydrocarbons remains the subject of debate. See OIL AND GAS, OFFSHORE. Many sedimentary aprons or submarine fans are found seaward of prominent submarine canyons that incise the continental margins. Studies of these sedimentary deposits have revealed a number of unusual surface features that include a complex system of submarine distributary channels, some with levees. The channel systems control and influence sediment distribution by depositional or erosional interchannel flows. Fans also result from major instantaneous sediment inputs caused by large submarine mass slumping and extrachannel turbidity flows. See SUBMARINE CANYON. The present coastline shows no particular geological significance as a boundary, and continental and oceanic crustal structures may or may not lie close to the present shoreline. It has been well established that the sea level has changed many times, fluctuating by as much as 300–400 m (980–1300 ft) during the last 200 m.y. Such changes in sea level are primarily due to the presence or absence of major continental ice sheets, which store vast quantities of water, thereby reducing the total water available to fill the ocean basins. During periods of glaciation, sea level falls. See GLACIAL EPOCH. Low stands of sea level coincide with times of rapid transport of terrestrial sediments across the continental shelves and into the deep ocean basins. During interglacial periods, when little water is stored as continental ice sheets, sea level rises. Accordingly, much of the terrestrial material transported to sediments

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are deposited in a relatively tranquil environment as a uniform blanket of sediments on the existing seafloor relief. If steep sea-floor slopes exist, some sediments may slump off to the sides, effectively smoothing the primary topography upon which they are deposited. Where the processes of sedimentation have been acting long enough and the deposition locale is accessible to the sediments derived from the continents, the original relief of the ocean crust can become completely buried. Many terrigenous deposits represent relatively impulsive events (for example, major submarine landslides on the continental margin or large turbidity flows), where large volumes of material are transported great distances over very short periods. The principal mechanism of deposition is a leveling process that fills first the topographic lows and, ultimately, can lead to burial of the entire initial sea-floor relief. This result is an extremely flat, topographically featureless abyssal plain. Such abyssal plains characteristically lie between the continental margins and the exposed flanks of the mid-oceanic ridge system. The abyssal plains areas have gradients that are less than 1:1000 and are typically underlain by a variety of discrete layers of transported sediments. See BASIN; MARINE SEDIMENTS; TURBIDITY CURRENT. Continental margins. The continental margins lie at the transition zone between the continents and the ocean basins and mark a major change from deep to shallow water and from thin to thick continental crust. The continental margins consist of two broad categories: rifted margins and convergent margins. Rifted margins. Good examples of rifted margins are found bounding the Atlantic Ocean (Fig. 2). These margins represent sections of the South American and North American continents that were once contiguous to west Africa and northwest Africa, respectively. These supercontinents were rifted apart 160–200 m.y. ago as the initial stages of sea-floor spreading and the birth of the present Atlantic Ocean sea floor (Fig. 2). See CONTINENTS, EVOLUTION OF; SUPERCONTINENT. Continental margins are proximal to large sources of terrestrial sediments that are the products of continental erosion. The margins are also the regions of very large vertical motions through time. This vertical motion is a consequence of heating and subsequent cooling of the rifted continental lithosphere and subsidence. During initial rifting of the continents, fault-bound rift basins are formed that serve as sites of deposition for large quantities of sediment (Fig. 6). These sedimentary basins constitute significant loads on the underlying crust, giving rise to an additional component of margin subsidence. The continental margins are of particular importance also because, as sites of thick sediment accumulations (including organic detritus), they hold considerable potential for the eventual formation and concentration of hydrocarbons. As relatively shallow areas, they are also accessible to offshore exploratory drilling and oil and gas production wells. Also found in selected continental margin environments are the unusual formations known as gas

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Fig. 6. Schematic cross section of a rifted continental margin. Note dramatic change in total crustal thickness across the margin, fault-bounded sedimentary basins, and large thicknesses of land-derived sediments making up the continental rise.

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the sea is trapped on the broad continental shelves, and results in the outbuilding or progradation that characterizes many present-day continental margins. The interplay between sea-level changes and sedimentation yield gives rise to the complicated interfingering of sediment deposits along the continental margins. Another method of significantly altering sea level is related to large, global differences in the rate at which sea floor is created at the mid-oceanic ridge systems. When sea floor is created extremely rapidly, that is, the plates are moving apart very rapidly, the profile of the mid-oceanic ridge is relatively broad. Because the depths of the mid-oceanic ridges are a direct function of crustal age, fast spreading results in a greater fraction of the ridge appearing at shallow elevations than for a mid-oceanic ridge created by very slow spreading (Fig. 3). Hence, a mid-oceanic ridge system with a broad profile will effectively displace more water from the ocean basin regions than one with a steep profile. The displaced water must move onto the adjacent continents in association with the eustatic sea-level rise. This is one explanation for the occurrence of a major global transgression of the ocean onto the continental areas during the middle Cretaceous period (85–115 m.y.a.). See RIFT VALLEY. Convergent margins. In contrast to the rifted margins, the continental margins that typically surround the Pacific Ocean represent areas where plates are colliding (Fig. 1). As a consequence of these collisions, the oceanic lithosphere is thrust back into the interior of the Earth; the loci of underthrusting are manifest as atypically deep ocean sites known as oceanic trenches (Fig. 7). The processes of subducting the oceanic lithosphere give rise to a suite of tectonic and morphologic features characteristically found in association with the oceanic trenches. An upward bulge of the crust is created seaward of the trench that represents the flexing of the rigid oceanic crust as it is bent downward at the trench. The broad zone landward of most trenches is known as the accretionary prism and represents the accumulation of large quantities of sediment that was carried on the oceanic crust to the trench. Because the sediments have relatively little strength, they are not underthrust with the more rigid oceanic crust, but they are scraped off. In effect, they are plastered along the inner wall of the trench system, giving rise to a zone of highly deformed sediments. These sediments derived from the ocean floor are intermixed with sediments transported downslope from the adjacent landmass, thus creating a classic sedimentary melange. See CONTINENTAL MARGIN; SEDIMENTOLOGY. Anomalous features. In addition to the major morphologic and sediment provinces, parts of the sea floor consist of anomalous features that obviously were not formed by fundamental processes of seafloor spreading, plate collisions, or sedimentation. Such features are nonetheless important and represent significant components of the ocean-floor relief. Examples are long, linear chains of seamounts and islands. Many of these chains are thought to reflect

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(b) Fig. 7. Oceanic trench and associated tectonic and morphologic features. (a) Cross section of typical island arc system showing tectonic units and terminology (after D. E. Karig and G. E. Sharma III, Subduction and accretion at trenches, Geol. Soc. Amer. Bull., 86:377–389, 1975). (b) Schematic cross section showing the relationship of relative plate motions to plate boundaries and associated sea-floor features. At divergent plate boundaries (mid-oceanic ridges), shallow earthquakes occur. At convergent plate boundaries (trenches), shallow, intermediate, and deep earthquakes occur. Shallow earthquakes occur along active transform zones; fracture zones represent the relict traces of crustal discontinuities originally formed within a transform zone (after B. Isacks, J. Oliver, and L. R. Sykes, Seismology and the new global tectonics, J. Geophys. Res., 73:5855–5900, 1968).

the motion of the oceanic plates over hot spots that are fixed within the mantle. Hot spots carry magma through the oceanic crust to the surface, resulting in volcanic trails, which serve to define the relative motion of the plate over the hot spot. See MAGMA; SEAMOUNT AND GUYOT. The presence of large, anomalously shallow regions known as oceanic plateaus may also represent long periods of anomalous regional magmatic activity that may have occurred either near divergent plate boundaries or within the plate. Alternatively, many oceanic plateaus are thought to be small fragments of continental blocks that have been dispersed through the processes of rifting and spreading, and have subsequently subsided below sea level to become part of the submarine terrain. Large igneous provinces (LIPs) are noteworthy examples of vast outpourings of mafic lava that are found both on continents as large flood basalts and on the ocean floor primarily as large oceanic plateaus. While the existence of these features has been known for some time, their genesis was largely assumed to be the result of perturbations to normal plate tectonic processes. It is now believed that the

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Marine geology emplacement of many LIPs was so rapid (100 m.y.) oceanic crust has heat flow that asymptotically approaches a value of about 50 mW/m2. In relatively young ocean crust, some heat may be transported by processes other than conduction. Global observations of heat flow suggest that for crust younger than about 40 m.y., observed heat flow often falls well below that predicted by simple conductive cooling models. Hydrothermal circulation in the uppermost parts of the crust may serve to remove significant amounts of heat, thereby resulting in artificially low values of conductive heat flow. As the crust ages and gradually becomes covered by sediments, the effective plumbing system for hydrothermal circulation, the fractured upper crust, eventually becomes sealed, thereby preempting any additional heat loss except by conduction. The amount of heat lost through hydrothermal processes is unknown and can be estimated only by comparing observations with theoretical predictions. Hence, the total integrated annual heat flow through the sea floor is still somewhat uncertain. See EARTH, HEAT FLOW IN. Seismic reflection. The most commonly used technique for investigating the subsurface structure of the sea floor is seismic profiling. In principle, this technique is like depth sounding, but it involves the transmission of low-frequency acoustic energy to the sea floor and subsurface horizons. Low-frequency acoustic energy (about 10–100 Hz) is less rapidly attenuated within the sediments than the highfrequency energy of depth-sounding instruments. Hence, a portion of the outgoing acoustic signal penetrates the sea floor, eventually to be reflected to the surface at major geologic (lithologic) boundaries. Geologic materials with contrasts in density and seismic-wave velocity give rise to the seismic reflections. Modern seismic techniques involve the recording of the reflected energy at a large number of receiver locations, fixed within a seismic streamer that is towed behind the ship (Fig. 13). A typical streamer may consist of 240 or more independent recording channels, equally spaced in the streamer and extending 6 km (3.6 mi) or more behind the vessel. Information from different shots fired along the track and recorded at the various receivers can be gathered

Marine geology together by considering signals traveling through different offset distances but reflected from a subsurface common midpoint. This process is used to calculate velocity as a function of depth and to convert the reflection times of key reflecting horizons to depth. Also, by gathering, correcting, and adding the individual receiver signals (stacking), very weak or deep reflecting horizons can be detected. The common midpoint technique is the most popular method used in offshore oil exploration. It is also used extensively for seismic imaging of the structures of the deep ocean crust and the continental margins. Subsurface structures to depths of several kilometers below the sea floor can be imaged. It is the crustal layer thicknesses, the disposition of the reflecting horizons, and the relative amplitudes of the seismic returns that allow the acoustic properties of the oceanic sediments and crust to be interpreted in terms of geological properties and of marine geological history. Changes in the chemistry of the seawater, in bottom circulation, and in surface productivity, and impulsive sedimentation events can give rise to abrupt and dramatic changes in the sedimentary environment. Sedimentary deposits bounding such changes are usually characterized by contrasting physical properties. These contrasts in physical properties mark important events in the sedimentary history that give rise to discrete horizons that can be detected with seismic reflection techniques. Seismic techniques not only provide information regarding the total distribution of sediments but also allow correlation of key sedimentological events throughout time. Seismic horizons sampled in a small number of localities, but correlated and traced over very large regions, can be used to extrapolate limited sample results and to interpret the depositional history throughout entire ocean basins. See SEISMOLOGY. Seismic exploration technology continues to produce new and better tools for imaging the sub-sea floor. The Global Positioning System (GPS) has facilitated a higher quality and better resolution of surveys. The transition from conventional (2D) seismic surveys [Fig. 13] to 3D [Fig. 14] has been rapidly incorporated for use in offshore oil exploration. This precise and rapid acquisition methodology has in turn led logically to the time-dependent characterization (that is, 4D seismics) of known oil fields by seismic surveys. The availability of GPS now makes it practical to resurvey areas periodically and to reliably attribute observed differences in the images obtained to the temporal evolution of an oil field or perhaps to structural changes of an area of the sea floor following a tectonic event. New implications of remote sensing. Ongoing studies of the sea floor and sub-sea floor involve largely the application of sophisticated remote-sensing techniques. Most notable is the use of a variety of high-frequency seismic instrumentation designed to provide higher resolution of acoustic images of the sub-sea floor. Surveys using side-scan tools now have

the capacity to map the fine details of the sea floor and its shallow subsurfaces over much greater areal extent than before. Some of these tools provide deeper looks into the structure beneath the sea floor. Similarly, the improvement of multibeam topographic swath-mapping and associated side-scan sonar has allowed detailed features of the sea floor to be mapped over extensive areas. These map results are helping scientists to discriminate between competing models of volcanism and faulting regarding the formation of the relief of the small-scale topographic features (such as abyssal hills) characteristic of mid-ocean ridges. Paleobathymetry. Scientists are giving more attention to the importance of sea-floor relief in controlling the pathways of both shallow- and deep-ocean circulation. This circulation largely governs the meridional exchange of heat, which affects the pattern of global climate. With the advent of digital data files for bathymetry, crustal age, and sediment thickness, it is now possible to analyze the third dimension

Key: seismic source receiver locus of shot / receiver midpoints Fig. 14. Seismic profiling using four source arrays and two streamers (courtesy of Western Geophysical); hachured areas represent zones of the sea floor “swept over” by lines of shot/receiver midpoints, thereby creating an effective three-dimensional seismic reflection survey. (E. J. W. Jones, Marine Geophysics, Wiley, 1999)

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of plate reconstructions—paleobathymetry. Such reconstructions reveal the evolution of ridges, basins, and gateways and suggest their influences on variable paleoclimates. For example, although the Antarctic continent was located in a high-latitude position at least for the last 200 m.y., it has only been covered with ice for only about the last 35 m.y. The changing bathymetry of the circum-Antarctic must have played a profound role in modulating the climate in Antarctica and beyond (Fig. 15). Sampling. The ability to sample reflecting horizons, especially deep horizons, is limited. Standard piston cores recover only the 10–20 m (30–60 ft) of the near-surface sediment. Several thousand piston cores have been taken from the world ocean floor. In order to sample horizons lying deep beneath the sea floor, drilling/coring techniques must be used. The initial drilling program began in 1968 as the Deep Sea Drilling Project (DSDP), which transitioned in 1984 into the IPOD (International Phase of Ocean Drilling Program). The most recent phase of cooperative marine geological drilling in the deep sea floor is the Integrated Ocean Drilling Program (IODP). It continues with the goals of solving key problems of Earth science that are best (perhaps only) addressed by di-

rect deep sampling of the sub-sea floor using drilling. The IODP uses a dynamically positioned drilling vessel (the JOIDES Resolution) to drill into and sample the sediments and crystalline rock lying as much as 2000 m (6000 ft) below the sea floor. Over 35 years of international collaborative research is attributable to the ocean drilling program. Since its inception, nearly 800 sites and more than 1700 holes have been drilled. They provide the only direct sampling of the deep sediments and basement rocks that compose the sea floor. These drilling sites have led to breakthrough findings, including evidence of the nature and pattern of paleoclimate change, confirmation of the Earth’s paleomagnetic reversal time scale to ∼200 mybp, the composition of the upper oceanic crust, the history of deepsea sedimentation, the nature of past ocean circulation, hydrothermal processes at mid-ocean ridges and within the accretionary sediment wedges associated with subduction, and many others. The principal difference in the new IPOD phase of the program will be the incorporation of more than one drilling platform. The most dramatic addition will be the Japanese vessel, CHIKYU [over 200 m (650 ft) long], that will be capable of riser drilling. Riser drilling includes the ability to “seal off” the drill hole if dangerous conditions are encountered. Riser drilling capability opens up a variety of new target areas previously considered unsafe to drill because of the possibility of unstable sub-sea-floor conditions (such as hydrocarbons or other fluids/gases under high pressure). The next decade of research will see the implementation of “semipermanent” undersea observatories designed to make in-situ, real-time measurements of chemical, biological, and geophysical parameters relevant to sea-floor processes and phenomena. One specific example, the “Neptune Project,” will focus on monitoring the activities on and around the Juan de Fuca Plate, offshore NE Pacific Ocean. The proposed network will include ∼3000 km (2000 mi) of data and power cables laid on the sea floor which will service some 50 experimental sites through nodes onto the main cable network. Unmanned underwater vehicles will traverse the areas between experiment sites, and can be “repowered” from the cable nodes. They can also be deployed automatically or on command to sites of volcanic or tectonic events, relaying information to shore in real-time via direct links to the internet and facilitating control activities from shore-based labs. See UNDERWATER VEHICLES. The long-term unmanned sea-floor observatories, judiciously deployed in geologically active seafloor regions, will mark one important new approach to marine geological research in the coming decades. Dennis E. Hayes Bibliography. C. Emiliani (ed.), The Sea, vol. 7: The Oceanic Lithosphere, 1981; B. C. Heezen and C. D. Hollister, The Face of the Deep, 1971; J. Kennett, Marine Geology, 1982; E. Seibold and W. H. Berger, The Sea Floor: An Introduction to Marine Geology, 3d ed., 1996.

Marine machinery

Marine machinery All machinery installed on waterborne craft, including engines, transmissions, shafting, propulsors, generators, motors, pumps, compressors, blowers, eductors, centrifuges, boilers and other heat exchangers, winches, cranes, steering gear, and associated piping, tanks, wiring, and controls, used for propulsion, for ship services, and for cargo, trade, or mission services. Practically all marine machinery elements have nonmarine counterparts; in some cases, the latter were developed from marine applications, while in other cases specific equipment was “marinized.” For marine service, machinery may have to meet higher standards of reliability and greater demands for weight and volume reduction and access for maintenance. Marine machinery must be capable of withstanding the marine environment, which tends toward extreme ambient conditions, high humidity, sea-water corrosion, vibration, sea motions, shock, variable demand, and fluctuating support services. Even higher standards may apply for warship machinery. To improve system reliability, essential equipment may be fitted in duplicate or provided with duplicated or alternative support or control systems, while nonessential equipment may be fitted with bypasses, to permit continued operation of a system following a component failure. Isolation valves or circuit breakers are common, enabling immediate repair. Machinery on modern ships is highly automated, with propulsion usually directly controlled from the wheelhouse, and auxiliary machinery centrally controlled from an air-conditioned, sound-proofed control room, usually in the engine room. In the typical modern merchant ship (but not in passenger ships), the machinery operates automatically, and the controls are unattended at sea, with engineers called out by alarm in the event of malfunctions. Propulsion Machinery Propulsion machinery comprises an engine, usually a diesel engine, steam turbine, or gas turbine, with required gearing or other transmission system, and, for steam plants, steam generators. See BOAT PROPULSION; MARINE BOILER; MARINE ENGINE; PROPELLER (MARINE CRAFT); SHIP NUCLEAR PROPULSION. Shafting. Shafting (Fig. 1) is of steel, usually solid for fixed-pitch propellers. In modern practice, the propeller is fitted to a matched taper on the propeller shaft by a hydraulic press fit, without a key, since a keyway would be a source of stress concentrations. Shafting sections are flanged and bolted together. For controllable-pitch propellers, the shaft is hollow to accommodate the hydraulic or mechanical linkage, and the propeller is bolted to an outboard flange on the propeller shaft. Because the propeller shaft must then be withdrawn outboard, the inboard connection to the remaining shafting is usually made by a removable sleeve coupling. In modern commercial practice, all bearings, including those in the stern tube, are oil-lubricated, with special seals at both

ends of the stern tube; but in many applications, including warships, the stern-tube bearings have nonmetallic linings and are lubricated by water allowed to leak in around the shaft. See ANTIFRICTION BEARING; MACHINE KEY. Shafting is subject to large cyclic variations in load induced by the propeller and, in diesel plants, by the engines. An important consideration in shafting design is to prevent resonant frequencies of the shafting from coinciding with these cyclic variations, within the operating range of shaft speeds. Thrust bearing. The thrust developed by the propeller is transmitted through the shafting to the hull via a thrust bearing, comprising a collar on the shaft (Fig. 1) and a stationary housing mounted on a substantial foundation integral with the hull structure. For low-speed diesel engines, which are directly coupled to the shaft, the thrust bearing is built into the engine at its after end. In geared installations the bearing may be separate from or integral with the gear case, and forward or aft of the gear. Except in the lowest-powered installations, the bearings are of the pivoted-shoe, oil-lubricated type. Auxiliary Machinery In all but the smallest boats, the propulsion machinery is but part of an integrated power plant, which includes auxiliary machinery for propulsion-engine support and other ship services. Electric generating plant. The electric power required for all continuous or recurring purposes on a ship is about 5% of the propulsion power, when restricted to the supply of motor-driven auxiliaries, lighting, other hotel services including high-voltage alternating current and provision refrigeration, communication, navigation, and control equipment of simple cargo ships. Much higher percentages are required when such services as cargo refrigeration or cargo hold ventilation are provided. On passenger ships the electric load can rival or exceed the propulsion power. Peak power demand may occur intermittently if motor-driven thrusters are fitted for maneuvering. Ocean-going ships are required to have a minimum of two generators, with either one capable of supporting the essential electrical load, but most ships have at least three in order to improve ship reliability. A ship with a high electrical load may have three or more generators, with two or more operated in parallel to support the load. In addition, most ships are required to have a separate emergency source, usually a diesel generator, located outside the engine room, for emergency lighting, communication, navigation and control services, fire fighting, and often, steering. Power from the generators is distributed to motors, lighting, and other consumers through switchboards and distribution boards. On steamships, at least one of the generators, acting as the normal electricity source, is driven by an independent turbine supplied with steam from the same source as the propulsion turbines, and at least one will be a diesel generator. Most diesel-driven

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propeller shaft forward sternflange coupling tube bearing line-shaft bearing

bulkhead bulkhead stuffing box

stern-tube forward seal propeller

line-shaft bearing

stern tube aft peak tank bulkhead

thrust shaft thrust collar

bull-gear bearings

line-shaft bearing

flange coupling bull gear

Fig. 1. Propeller shafting for a single-screw merchant ship. (After R. L. Harrington, ed., Marine Engineering, Society of Naval Architects and Marine Engineers, 1992)

ships have diesel generators, as do many gas turbine– driven ships, although others have gas turbogenerators. Many ships are fitted with a generator driven from the propulsion gearing or shafting as the normal electricity source. In many high-powered dieselengine ships, sufficient waste heat can be recovered by an exhaust gas boiler to support a steam turbine, adequate for the normal load. Ships with alternatingcurrent electric drive may take power for ship services from the propulsion generator. See DIESEL ENGINE; GAS TURBINE; GENERATOR; STEAM TURBINE. Fuel systems. Generally, fuel is bunkered into tanks dispersed through the hull of a ship, using centrally controlled piping, and brought daily to settling tanks in or near the engine room by motor-

vented steam containing oxygen removed from water vent condensing water-spray nozzle

water-supply chamber water inlet

water-spray nozzles primary heating and deaeration chamber steam inlet

deflecting baffle conical water collector steam baffle plate stationary baffle

overflow

spring-loaded atomizing valve access hole

deaerated water storage

pump suction

Fig. 2. Deaerating feed heater. (After R. L. Harrington, ed., Marine Engineering, Society of Naval Architects and Marine Engineers, 1992)

driven transfer pumps, usually located in the engine room and using the same piping. Fuel systems are most complex on merchant ships with diesel engines, burning heavy fuel oil. On these ships, the fuel is heated in the tanks to facilitate pumping, then kept hot in the settlers to encourage the settling, by gravity, of water and solids. After further heating, settled fuel is passed through centrifugal separators to a day tank, from which it is supplied by motordriven service pumps, through final heaters and fine filters, to the engine. The heating is done by steam in immersed coils in the tanks and in shell-and-tube or plate heat exchangers. In parallel with the heavy fuel system is an abbreviated diesel fuel system supplying the generator engines and package boiler if these are not run on heavy fuel, and supplying the propulsion engine when necessary. The diesel fuel is not heated but is pumped to a settling tank, a centrifugal separator, a day tank, and filters. For ships using diesel fuel only, fuel heating is usually unnecessary and the system is simplified. Smaller plants are often not fitted with centrifugal separators, eliminating the need for a separate day tank. On steamships burning residual fuel, the fuel must be heated in the tanks, but is not centrifuged, so that the service pumps draw fuel directly from the settling tanks for supply to the burners via heaters and a fine strainer. See CENTRIFUGATION; DISPLACEMENT PUMP; HEAT EXCHANGER; PUMPING MACHINERY. Steam and feedwater systems. On steamships, superheated steam passes to the propulsion turbine and to the turbogenerator, then exhausts to the condenser, a shell-and-tube heat exchanger, which is cooled by seawater inside the tubes. The condenser is maintained under a vacuum, usually 5 kilopascals (1.5 in. Hg), by air ejectors to extend the useful range of energy recovery from the steam. Condensate is drawn from the condenser by motor-driven condensate pumps and discharged through one or more shell-and-tube heaters, each heated regeneratively by steam extracted from the propulsion turbine, to a deaerating feed tank. See STEAM CONDENSER; VAPOR CONDENSER. The deaerating feed tank (Fig. 2) uses extraction steam to heat the condensate, now called feedwater, to saturation temperatures, at which air and other noncondensable gases approach insolubility and can be vented from the system. This tank is also a surge

Marine machinery tank to accommodate differences in total volume of water and steam in the system as the ship is maneuvered. The deaerating feed tank distinguishes the marine steam plant from shoreside steam plants, which are not routinely subject to large load changes, where deaeration and limited surge can be achieved in a large condenser drainwell, not feasible aboard ship because of limited depth below the condenser. The deaerating feed tank is usually located high in the ship to provide a static head at the feedwater pumps, thereby preventing the hot feedwater from flashing to steam. On warships where this location is considered too vulnerable, the tank is lower, and feed booster pumps substitute for the static head. See BOILER FEEDWATER. Feed pumps are usually multistage centrifugal pumps directly driven by compact steam turbines. For boilers fitted with economizers, the feedwater passes directly to the economizer, but when air heaters are fitted the feedwater passes through one or more shell-and-tube, high-pressure heaters, heated regeneratively by steam extracted from the propulsion turbine. See AIR HEATER; BOILER ECONOMIZER. Often the turbogenerator has its own condenser and condensate pump. Sometimes the turbogenerator exhausts to the deaerating feed tank via an auxiliary exhaust line. Steam for some auxiliary services, such as for fuel heating and the distilling plant, is supplied from turbine extractions as the most efficient source. Steam for the feed-pump turbine is taken from the superheated steam line, in some cases directly, but more frequently via a desuperheater. All exhaust steam and all drains are collected to appropriate points in the cycle to recover the heat and the water. Steam systems on diesel-powered ships usually comprise a waste-heat boiler in the main engine uptake, in parallel with an oil-fired package boiler. Unless heat recovery is to be maximized, as when a waste-heat turbogenerator is fitted, the steam generated is saturated and is sufficient for fuel heating and other needs. Condensate is collected in a tank for return to the boiler via a motor-driven feed pump. On tankers, steam may be used for cargo pumps and for heating of the cargo. On steamships the cargo-pump turbines are supplied from the propulsion boilers directly, but to avoid contamination the cargo-heating steam may be supplied indirectly from a steam-to-steam heat exchanger. On diesel-powered tankers, large auxiliary, oil-fired boilers may be used for these services. On ships with no heating requirements for fuel, a low-pressure package boiler may be fitted or an alternative heating plant may be provided. Water-cooling systems. For steamships, very large quantities of seawater are required for the condensers. The seawater can be pumped through the condenser by large motor-driven centrifugal pumps, or induced to flow by pressure differences across the hull, which may have to be augmented or even created by a protruding lip at the outlet. Other cool-

ing requirements, such as those for lubricating-oil coolers and refrigeration condensers, are also met with seawater, directly or via an intermediate heat exchanger that cools freshwater for circulation to the auxiliaries. See CENTRIFUGAL PUMP. Except for the smallest engines, diesel-engine jackets are fresh-water-cooled in closed circuit, with the fresh water cooled in turn by seawater in a heat exchanger. Engine lubricating-oil coolers and aftercoolers and other auxiliaries may be circulated with seawater directly or via an intermediate freshwater loop. Cooling for gas turbines is largely internal, and cooling-water requirements are limited to lubricating-oil coolers for the engine and its reduction gear, and to the auxiliary machinery. Seawater and fresh water are circulated by centrifugal pumps, usually motor-driven, but often driven from the engine they serve. Heat exchangers of both shell-and-tube type and plate type are used. The selection of materials for equipment that handles seawater is critical, tending toward bronze pumps with stainless-steel impellers, and nonferrous or plastic-lined piping and heat-exchanger waterboxes. Heat-exchanger tubes are usually copper-nickel, but titanium has been used in naval applications. Plate-type heat exchangers are usually fitted with stainless steel or titanium plates. Lubricating-oil systems. The most complex systems are those on merchant ships burning heavy fuel in diesel engines. In general, large engines are operated with a dry crankcase, the oil draining to a sump tank below the engine, usually built into the double bottom. The oil is drawn from the sump by circulating pumps, which discharge through a cooler and a fine filter to bearings, pistons (if oil-cooled), camshaft, governor, and valve gear, from which the oil drains to the sump. Each engine has its own lubricating-oil system. Oil is drawn continuously to a centrifugal separator via a heater and then returned to the sump. Low-speed engines are fitted with a separate cylinder lubrication system, storing and supplying a high-viscosity oil of injection directly into the cylinders, where it lubricates the piston rings and cylinder liners before it burns. In trunk-piston engines, some of the circulating oil is used for this purpose. Reduction gearing is usually fitted with an independent sump, circulating pumps, filter, and cooler. Steam-turbine bearings are supplied from the same system that serves the reduction gear, but gas turbines usually use a different grade of oil and require separate pumps, coolers, and filters. Turbines and connected gearing, which unlike diesels cannot be brought to an immediate stop following an oil failure, are fitted with a gravity tank or other means of maintaining oil flow in an emergency. Stern tubes, line-shaft bearings, air compressors, auxiliary steam turbines, and some other auxiliaries may use the same grade of oil as the reduction gear on ships with turbines or medium- or high-speed diesel engines, or the same oil as the bearings of low-speed engines. However, diesel generators and refrigeration compressors each require a different grade of

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Marine machinery oil. Storage tanks of adequate capacity for several voyages must be provided for each grade of oil, as well as settling tanks for used oils that can be renovated onboard by settling and centrifuging, and usedoil storage tanks and a transfer pump for oils that must be discharged ashore after use. Lubricating-oil pumps are most often motor-driven rotary types, but centrifugal pumps are occasionally used. Some medium- and high-speed diesel engines are arranged to drive their own oil pumps, as are some reduction gears. Oil coolers may be shell-andtube or plate type. Compressed-air systems. Each of these systems comprises one or more motor-driven air compressors and one or more receivers. Most diesel propulsion and auxiliary engines and many gas turbines are started by using compressed air which, because of the volume required, is stored at medium or high pressures, typically 3 megapascals (435 lb/in.2) or higher. Air for miscellaneous services, such as for operating pneumatic hoists and tools, for recharging accumulators, and for supplying pneumatic controls, is usually stored at about 700 kilopascals (100 lb/in.2). The air for pneumatic controls is usually separately compressed and stored, and passed to services via a dryer. Compressors are usually motor-driven reciprocating units, but other types have been used for low-pressure service. See COMPRESSOR. Distilling plants. Fresh water is required for making up losses in steam systems and freshwater cooling circuits, as well as for drinking, washing, and cooking. Most modern ships also use freshwater for toilet flushing. Other ships, such as chemical tankers, require fresh water for cleaning of cargo tanks. In short voyages and in journeys on lakes and rivers, some of these needs might be met by taking water from shore, but most ocean-going ships generate their own freshwater from seawater. Steamships generally use multistage, low-pressure evaporators, heated by steam extracted from the turbine. Diesel-powered ships generally have single-stage evaporators heated by engine cooling water. Gas turbine ships with diesel generators may use the diesel cooling water or steam produced in a waste-heat boiler or an oil-fired package boiler. Alternatives to low-pressure evaporators are vapor-compression plants and reverseosmosis units. High-pressure evaporators are generally obsolete because of seawater scaling. See DISTILLATION. Bilge and ballast systems. Bilge systems consist of pumps, sometimes assisted by eductors, to remove water and other accumulated liquids from the engine room and other compartments in the ship, other than oil tanks. Where bilge water may be contaminated by oil, it is first pumped to a holding tank and discharged overboard slowly through an oil-and-water separator. Ballast systems comprise the pumps and piping used to fill and empty ballast tanks. (Ballast tanks are filled to improve the trim or stability of a ship which is not carrying cargo.) To reduce pollution, past practices of filling empty fuel and cargo tanks with ballast are no longer allowed. Bilge and ballast pumps are usually motor-driven centrifugal pumps,

although steam-reciprocating pumps are sometimes fitted for bilge service, while tanker ballast pumps are often steam-turbine-driven centrifugal units. In smaller plants the bilge pump may be driven via a clutch from the propulsion engine, and may double as a fire pump. Fire-fighting and inert-gas systems. In addition to portable extinguishers, a ship will be fitted with at least two centrifugal seawater pumps and a fire main to supply hose connections and sprinklers throughout the ship. Fire pumps are usually motor-driven, but in smaller plants a fire pump may be driven via a clutch from the propulsion engine and may double as a bilge pump. Most ships are required to have an emergency fire pump in a separate compartment, driven from the emergency generator or by an independent diesel engine. A separate system is fitted for smothering fires in machinery spaces and dry cargo holds, most often using bottled carbon dioxide and piping for gas distribution. On tankers and other ships with flammable liquid cargoes, a foam system is installed, with tanks of foam concentrate to be injected into seawater from the fire pumps and distributed via nozzles and monitors. On oil tankers, the space above the cargo in the tanks, or the whole tank when empty, is kept full of inert gas. Most often, the inert gas is boiler exhaust gas, passed through a seawater spray to remove sulfur and then blown into the tanks, but on tankers without boilers of adequate size an oil-burning inertgas generator is fitted. Refrigeration, heating, and ventilation. Refrigeration for provisions and air conditioning is generally provided by vapor-compression units, each consisting of a motor-driven compressor and a water-cooled condenser. The halogenated hydrocarbon refrigerants are being replaced by partially chlorinated refrigerants, which have lower ozone-depletion potentials, or by ammonia, previously considered obsolete. In systems suitable for most merchant ships, the refrigerant is expanded directly in evaporator coils in the provision store rooms or in the air-conditioning ducts; but for larger, more distributed systems, as on passenger ships, refrigerated-cargo ships, and fishing boats, an intermediate refrigerant, usually brine, is circulated through piping to the coils. See MARINE REFRIGERATION. Much refrigerated cargo is containerized, with each container equipped with its own motor-driven vapor-compression unit. Ships intended to carry such cargo have increased electrical generating capacity, sockets at the container locations, and increased ventilation capacity for containers carried below deck. See MARINE CONTAINERS. Heating for air conditioning is usually by steam or electric coils in the ducts, supplemented by local convectors. Ventilation systems on all ships are extensive, using motor-driven fans to force airflow through machinery, accommodation, and work spaces, and through cargo areas when appropriate, such as the enclosed vehicle decks of ferries and rollon/roll-off ships. See AIR CONDITIONING; COMFORT HEATING; VENTILATION.

Marine machinery Sewage treatment plants. Overboard discharge of raw sewage is no longer permitted, and craft for which holding tanks are impractical must be fitted with treatment plants. Most installations are of the biological type. In order to reduce the volume of sewage to be treated, clean drains from wash basins and showers are passed directly overboard. Further reductions can be made by using vacuum-flush systems, which are common on ships with large complements or with passenger accommodation. See SEWAGE TREATMENT. Incinerators. Garbage can no longer be dumped overboard, and most ships are therefore fitted with incinerators, most of which burn the garbage in a diesel oil-fired furnace. Most incinerators are equipped to burn used oil and sludge collected at the centrifugal fuel and lubricating-oil separators. Deck Machinery This machinery is more appropriately described as hull machinery, as it could encompass all machinery outside the engine room, including much of that cited above as auxiliary machinery. Much of the machinery described below is hydraulic. Such machinery is driven by electrohydraulic power packs, centered about a motor-driven hydraulic pump which supplies fluid under sufficient pressure to operate the rams or rotating hydraulic motors. Direct electric drive is also common, but steam-driven deck machinery is rarely found on modern ships. Steering machinery. Except for those of the smallest craft, rudders are turned by hydraulic power, usually rams operating directly on the rudder stock. The rams are under command of a telemotor, which transmits the motion of the manually operated steering wheel or lever in the wheelhouse, or signals from the automatic pilot. In general, steering machinery incorporates redundant systems and emergency controls. See SHIP POWERING, MANEUVERING, AND SEAKEEPING. Windlasses, capstans, and mooring winches. A windlass is a power-driven sprocket, permanently meshed to the anchor chain, used to raise the anchor; it is equipped with a brake to allow the anchor to be dropped in an orderly fashion. Capstans are vertical, power-driven drums around which wire or rope lines may be wrapped to assist in pulling and hoisting tasks. Mooring winches are power-driven horizontal spools, equipped with brakes, on which the wire or rope line for securing the craft to a dock is stored, and which are used to pay out the line and draw it taut as required. Some mooring winches are automatic, able to maintain a constant tension in the line. Most mooring winches are fitted with an auxiliary drum, called a warping or gypsy head, which is used like a horizontal capstan to assist in handling loose lines. Derricks and cranes. These are fitted for cargo handling and utility services on many types of ships, and may range from a boom pivoted at the base of a mast, used in conjunction with winches, to swiveling jib cranes with self-contained hoisting, luffing, and slewing drives, operated from an enclosed cab (Fig. 3).

hoisting and luffing motors

turntable base

operator's cab

jib

slewing motor

Fig. 3. General-purpose deck crane. (After D. A. Taylor, Introduction to Marine Engineering, 2d ed., Butterworth, 1990)

Power may be direct-electric or electrohydraulic. See DERRICK; HOISTING MACHINES. Boats, including lifeboats, are usually handled by davits, which are pairs of dedicated cranes that enable a loaded boat to be lowered safely to the surface of the sea. Specialized lifeboats, called free-fall lifeboats, are designed to be launched, fully loaded, from inclined ramps built onto a ship’s superstructure. Access and overhauling equipment. Apart from cargo handling, most ships are designed with carefully considered provisions to facilitate moving stores, garbage, and equipment on and off the ship and about the ship to locations such as workshops. These provisions include stores cranes, hatches or bolted plates, vertical trunks, side ports, monorails with trolleys, ramps for hand trucks or battery-powered forklift trucks, elevators, and strategically located pad eyes. Naval vessels are usually equipped with specialized equipment for underway replenishment of fuel, provisions, ammunition, and spares. Cargo pumping systems. These systems are installed in tankers and other ships carrying liquid cargo. In a large oil tanker, centrifugal pumps are located at the bottom of a pump room, which is located forward of the engine room bulkhead and extends the full height of the hull. The pumps are usually driven by steam turbines, but in some tankers by electric motors located in the engine room, with the shafts penetrating the bulkhead. The pumps are used to discharge the cargo, via mains with branches from each cargo tank, to flanged connections amidships, at both sides of the main deck, where hoses from the shore can be attached. During discharge, some of the oil is recirculated to tank cleaning machines, which spray it in a pattern to wash residue to the bottom of the tanks. At all times the space above the oil is kept full of inert gas. Stripping eductors, driven by the oil discharged from the pumps, are used to remove the last of the oil from the tanks. The ship is loaded through the same piping, with the oil under pressure from pumps ashore. All valves in the system are remotely controlled from a console in a cargo control room.

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Marine microbiology Some tankers, generally carriers of refined products or chemicals, have no pump room, but use individual pumps submerged at the bottom of each tank. These pumps may be driven by motors on deck via long shafts, or by hydraulic motors submerged in the tanks with the pumps. Stabilizers. Many types of ships are fitted with these units, including warships and passenger ships, intended to reduce the rolling motions of the ship by generating a moment to oppose the roll. Fin stabilizers are short, swiveling, streamlined blades which protrude from both sides of the hull, pivoted by hydraulic actuators controlled by a gyroscope. They may retract or fold back against the hull, or they may protrude at all times. Tank stabilizers are pairs of tanks, one on each side of the ship, which are kept half full of water and connected by a large pipe through which water is transferred. They may be passive, relying on gravity to transfer the water as the ship rolls, or active, using a gyroscope-controlled pump or compressed air. Thrusters. All types of ships are fitted with these units, usually at the bow but often at the stern as well, which function as maneuvering aids by providing a transverse thrust on command. The most common arrangement uses a propeller mounted in a transverse tunnel through the hull, driven by bevel gearing from an electric motor or diesel engine. Other thruster types, which may be rotated, are used for special applications, including dynamically positioned ships. Alan L. Rowen Bibliography. R. L. Harrington (ed.), Marine Engineering, 1992; E. C. Hunt (ed.), Modern Marine Engineer’s Manual, vol. 2, 2d ed., 1990; A. B. Neild, Jr. (ed.), Modern Marine Engineer’s Manual, vol. 1, 2d ed., 1965; D. W. Smith, Marine Auxiliary Machinery, 1983; D. A. Taylor, Introduction to Marine Engineering, 2d ed., 1990.

Marine microbiology An independent discipline applying the principles and methods of general microbiology to research in marine biology and biogeochemistry. Dealing, by definition, with organisms of microscopic dimensions, marine microbiology focuses primarily on prokaryotic organisms, mainly bacteria. Every cubic centimeter (0.06) of seawater contains about 1000–100,000 bacteria, and this to 1 million or more in nutrient-rich inshore waters. Because of their small size and easy dispersability, bacteria are virtually ubiquitous in the marine environment. Furthermore, natural populations of marine bacteria comprise a large variety of physiological types, can survive long periods of starvation, and are able to start their metabolic activity as soon as a substrate (food or growth-limiting nutrient) becomes available. As a result, the marine environment, similar to soil, possesses the potential of a large variety of microbial processes that degrade (heterotrophy) but also produce (autotrophy) organic matter. Considering the fact that the marine environment

represents about 99% of the biosphere, marine microbial transformations are of tremendous global importance. If the deep sea is defined as waters below 1000 m (0.62 mi), this area still comprises 75% of the global biosphere, with temperatures of 2–4◦C (34– 39◦F), total darkness, and high hydrostatic pressures. See BIOSPHERE. Heterotrophic transformations. Quantitatively, the most important role of microorganisms in the marine environment is heterotrophic decomposition and remineralization of organic matter. It is estimated that about 95% of the photosynthetically produced organic matter is recycled in the upper 300–400 m (1000–1300 ft) of water, while the remaining 5%, largely particulate matter, is further decomposed during sedimentation. Only about 1% of the total organic matter produced in surface waters arrives at the deep-sea floor in particulate form. In other words, the major source of energy and carbon for all marine heterotrophic organisms is distributed over the huge volume of pelagic water mass with an average depth of about 3800 m (2.5 mi). In this highly dilute medium, particulate organic matter is partly replenished from dissolved organic carbon by microbial growth, the so-called microbial loop. The rate of heterotrophic microbial decomposition of organic matter is primarily affected by three factors: nutrient concentration, temperature, and pressure. As a low-nutrient medium, seawater offers a growth advantage to bacterial populations that are characterized by high enzymatic affinities for their particular growth substrates. Morphological structures, such as increased surface area or stalks and organelles for the attachment of cells to solid surfaces, are other means of coping with the low-nutrient environment. Below depths of 300–400 m (1000– 1300 ft) the temperature is uniformly in the range of 2–4◦C (34–39◦F), furthering growth, and hence creating a predominance of low-temperature-adapted or psychrophilic microorganisms. While most surfaceborn mesophilic bacteria grow optimally within a range of 20–35◦C (68–95◦F), marine psychrophilic bacteria have growth optima of 8–16◦C (46–61◦F), and they are commonly killed at temperatures above 20◦C (68◦F). See SEAWATER. Hydrostatic pressure increases approximately 1 atm (100 kilopascals) for every 10 m (33 ft) of water depth. Most bacteria, marine and nonmarine, are barotolerant to various degrees and still grow at pressures of 300–600 atm (30–61 megapascals), corresponding to water depths of approximately 3000– 6000 m (2–4 mi). Pressure-adapted (barophilic) microorganisms grow beyond these pressures and are characterized by having growth optima at pressures higher than 1 atm (100 kPa). Obligately barophilic bacteria are species that are unable to grow at 1 atm (100 kPa). The various types and degrees of pressure adaptation (Fig. 1) do not eliminate a general gradual decrease of microbial metabolic activity with increasing pressure or depth in the ocean. In consideration of microbial adaptations for a deep-sea existence, temperature- and pressure-retaining samplers have been developed (Fig. 2).

Marine microbiology

different degrees of barotolerance

rate of growth

barophily

no growth at low or normal pressure

obligate barophily overall decrease of growth

hydrostatic pressure Fig. 1. Pressure-adapted growth in marine bacteria. The broken line represents a general decrease of microbial metabolic activity with increasing hydrostatic pressure (or water depth). (After H. W. Jannasch, R. E. Marquis, and A. M. Zimmerman, eds., Current Perspectives in High Pressure Biology. Academic Press, 1987)

Most microorganisms in a given sample of seawater lack their particular growth substrate or are not adapted to the prevailing environmental conditions and are therefore metabolically inactive. As a consequence, the mere number of microorganisms (as determined microscopically or by colony counts on agar plates) provides less information than direct testing of metabolic activity. The latter is done by measuring the rate of incorporation or turnover of a suitable radiolabeled substrate by the natural microbial population under natural conditions. The use of non-growth-stimulating labeled indicators, such as

Fig. 2. Pressure- and temperature-retaining bacteriological water sampler during recovery from a depth of 6000 m (about 4 mi). Transfer into prepressurized reaction vessels permits the measurement of microbial activity under field conditions. (Courtesy of S. Molyneaux)

adenine or thymidine as precursors of nucleic acid production, is one of the attempts to assess the unaltered natural activity of whole microbial populations. Of the large variety of organic material decomposed by marine heterotrophic bacteria, oil and related hydrocarbons are of special interest. Other environmentally detrimental pollutants (such as inorganic acids, organic solvents, pesticides, and herbicides) that are directly dumped or reach the ocean as the ultimate sink by land runoff are microbiologically degraded at varying rates. Techniques of molecular genetics are aimed at encoding genes of desirable enzymes into organisms for use as degraders of particular pollutants. A specifically marine microbiological phenomenon is bacterial bioluminescence, which may function as a respiratory bypass of the electron transport chain. Free-living luminescent bacteria are distinguished from those that live in symbiotic fashion in light organelles of fishes or invertebrates. See BIOLUMINESCENCE. Photoautotrophs and chemoautotrophs. The type of photosynthesis carried out by purple sulfur bacteria is an anaerobic process. It uses hydrogen sulfide (in contrast to water, which is used in green plant photosynthesis) as a source of electrons and thus produces sulfur, not oxygen. Photoautotrophic bacteria are therefore limited to environments where light and hydrogen sulfide occur simultaneously, mostly in lagoons and estuaries. In the presence of sufficient amounts of organic substrates, heterotrophic sulfatereducing bacteria provide the necessary hydrogen sulfide where oxygen is depleted by decomposition processes. Anoxygenic photosynthesis is also carried out by some blue-green algae, which are now classified as cyanobacteria. A genus of oxygenically photosynthesizing cyanobacteria (Synechococcus) was discovered to be common in surface seawater. See CYANOBACTERIA; PHOTOSYNTHESIS. Chemoautotrophic bacteria are able to reduce inorganic carbon to organic carbon (chemosynthesis) by using the chemical energy liberated during the oxidation of inorganic compounds (hydrogen, hydrogen sulfide, elemental sulfur, thiosulfate, ammonia, nitrite, and reduced iron and manganese). Their occurrence, therefore, is not light-limited but depends on the availability of oxygen and the suitable inorganic electron source. Their role as producers of organic carbon is insignificant in comparison with that of photosynthetic producers (exempting the processes found at deep-sea hydrothermal vents). The oxidation of ammonia and nitrite to nitrate (nitrification) furnishes the chemically stable and biologically most available form of inorganic nitrogen for photosynthesis. Genera of nitrifying bacteria include Nitrosomonas, Nitrosococcus, Nitrosospira, Nitrobacter, other Nitrosovibrio, Nitrosolobus, and Nitrospina. See NITROGEN CYCLE. In contrast to soil and fresh water, the effect of acidification by the microbial formation of sulfate and nitrate is minimized in the marine environment by the high buffering capacity of seawater.

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Marine microbiology By the same token, highly acidophilic sulfuroxidizing bacteria lose out in competition with facultatively chemoautotrophic bacteria, which produce nonacidifying polythionates as their end product of sulfur oxidation. The generation of methane and acetic acid from hydrogen and carbon dioxide stems from anaerobic bacterial chemosynthesis, and is common in anoxic marine sediments. See METHANOGENESIS (BACTERIA). Marine microbial sulfur cycle. Sulfate is quantitatively the most prominent anion in seawater. Since it can be used by a number of heterotrophic bacteria as an electron acceptor in respiration following the depletion of dissolved oxygen, the resulting sulfate reduction and the further recycling of the reduced sulfur compounds make the marine environment microbiologically distinctly different from freshwater and most soils. Sulfate is far more important in seawater for microbial anaerobic respiration, than nitrate (denitrification, resulting in nitrogen gas production) or carbonate (methanogenesis, resulting in methane production). The assimilatory reduction of sulfate to organic sulfur compounds is quantitatively of minor importance in the marine environment compared to the bacterial respiration of sulfate. The resulting hydrogen sulfide is reoxidized to sulfate in the presence of oxygen with the concomitant production of organic carbon. This chemosynthetic production is considered secondary, since it depends on the primary photosynthetic production of organic carbon and oxygen. The marine anaerobic, heterotrophic sulfatereducing bacteria are classified in three genera; Desulfovibrio, Desulfotomaculum, and Clostridium. Microbial sulfate reduction is particularly important in the Black Sea, the world’s largest anoxic basin with a maximum depth of 2000 m (6000 ft), where the oxygen-sulfide interface lies at a depth of about 100 m (300 ft). This microbially produced phenomenon has turned the Black Sea into one of the most studied marine environments. See BLACK SEA. The marine aerobic sulfur-oxidizing bacteria fall into two groups: the thiobacilli (Thiobacillus and Thiomicrospira) and the morphologically conspicuous filamentous or unicellular organisms (Beggiatoa, Thiothrix, Thiovulum, Thiospira, Thioploca, and Achromatium). While the former comprise a wide range from obligately to facultatively chemoautotrophic species (requiring none or some organic compounds), few of the latter have been isolated in pure culture, and chemoautotrophy has been demonstrated in only a few. Thioploca and Beggiatoa have been observed to form massive microbial mats on the sea floor, the former off the coast of Chile in depths of 60–200 m (80–600 ft), the latter in the vicinity of sediment-covered hydrothermal vents at a depth of 2000 m (6000 ft). Deep-sea hydrothermal vent bacteria. Two types of hydrothermal vents have been investigated: warm vents (8–25◦C or 46–77◦F) with flow rates of 1–2 cm (0.4–0.8 in.)/s, and hot vents (260–360◦C or 500– 600◦F) with flow rates of 2 m (6.5 ft)/s. In their

immediate vicinity, dense communities of benthic invertebrates are found with a biomass that is orders of magnitude higher than that normally found at these depths and dependent on photosynthetic food sources. This phenomenon has been explained by the bacterial primary production of organic carbon through the chemosynthetic oxidation of reduced inorganic compounds. The chemical energy required for this process is analogous to the light energy used in photosynthesis and is provided by the geothermal reduction of inorganic chemical species. The specific compounds contained in the emitted vent waters and suitable for bacterial chemosynthesis are mainly hydrogen sulfide, hydrogen, methane, and reduced iron and manganese. There appear to be three major sites where microbial growth takes place: within the subsurface vent system, in microbial mats covering surfaces exposed to the vent plume, and in various symbiotic associations. In the first case, bacterial cells and visible cell aggregations are emitted with the vent waters and become available to filter-feeding sessile and planktonic animals; Thiomicrospira is the most commonly isolated genus. In the second case, grazing animals can be observed on various types of surfaces feeding on Beggiatoa and Thiothrix-like filamentous bacteria. Isolated from mat material are manganeseoxidizing, extremely thermophilic methanogenic as well as methylotrophic bacteria (Fig. 3). The third case leads to the production of the major portion of animal biomass: prokaryotic chemoautotrophic symbiosis is found in the gill tissue of the vent clam (Calyptogena magnifica) and mussel (Bathymodiolus), as well as in a special tissue, the trophosome, of

1 µm

Fig. 3. Transmission electron micrograph of methylotrophic bacteria within iron-manganese deposits of microbial mats from deep-sea hydrothermal vents. (From P. A. Rona et al., eds., Hydrothermal Processes at Seafloor Spreading Centers, Plenum Press, 1984)

Marine mining the mouthless and gutless pogonophoran tubeworm (Riftia pachyptila). Similar symbiotic systems have subsequently been found in estuarine environments, where hydrogen sulfide occurs as a product of bacterial sulfate reduction. The extremely thermophilic microorganisms isolated from hydrothermal vents belong, with the exception of the genus Thermotoga, to the Archaebacteria. Of eight archaeal genera, growing within a temperature range of about 75–110◦C (165–230◦F), three are able to grow beyond the boiling point of water, if the necessary pressure is applied to prevent boiling. These organisms are strictly anaerobic. However, unlike mesophilic bacteria, hyperthermophilic marine isolates tolerate oxygen when cooled below their minimum growth temperature. See ARCHAEA; HYDROTHERMAL VENT. Virology. Observations have shown that seawater may contain 105–108 virus or phage particles per cubic centimeter (0.62 in.3). While one hypothesis purports that up to 60% of bacterial mortality may by due to viral infection, it is generally assumed that bacterial communities are dominated by cells that are resistant to their co-occurring phages, the latter maintaining themselves by infecting relatively rare sensitive cells. See BACTERIOPHAGE. Biotechnology. Marine microbial biotechnology is an applied science. As an example, thermostable deoxyribonucleic acid–cleaving polymerases from the above-mentioned hyperthermophilic vent bacteria are produced commercially and used extensively in molecular biology, particularly in the polymerase chain reaction. Another example is the use of thermostable enzymes in high-temperature processes, such as the use of amylases from hyperthermophilic archaea in the conversion of cornstarch to glucose and fructose in the beverage industry. See BACTERIA; BIOTECHNOLOGY; MICROBIOLOGY. Holger W. Jannasch Bibliography. R. M. Atlas and R. Bartha, Microbial Ecology, 4th ed., 1997; A. T. D. Brock (ed.), Thermophiles: General, Molecular and Applied Microbiology, 1986; R. A. Herbert and G. A. Codd, Microbes in Extreme Environments, 1986; G. Rheinheimer, Aquatic Microbiology, 4th ed., 1993; H. G. Schlegel and B. Bowien, Autotrophic Bacteria, 1989.

Marine mining The process of recovering mineral wealth from seawater and from deposits on and under the seabed. While the world’s demand for mineral commodities is increasing at a rapid rate, most of the developed countries have been thoroughly explored for surface outcroppings of mineral deposits. The mining industry has needed to improve its capabilities for exploring and exploiting low-grade and unconventional sources of ore. Corresponding advances in oceanography have highlighted the importance of the ocean as a source of minerals and indicated that the technology required for their exploitation is in some cases already available.

Manganese nodules. Undersea mining became an important, but unsuccessful, diversification for many major oil and aerospace companies in the late 1960s. Over 80 separate exploration activities were reported in shallow coastal areas worldwide, but few resulted in new operating mines. Attention turned to the deep seabeds, and much of the debate and controversy in the deliberations of the United Nations Conference on the Law of the Sea (UNCLOS) revolved around the exploitation of manganese nodules. Conflicts over the rights of management of these minerals in international waters led to attempted moratoria on deep-seabed mining activities. Anticipating agreement, several countries, including the United States and Germany, prepared interim legislation to permit nationals of the respective countries and reciprocating states to mine the deep seabed for nodules containing manganese, copper, nickel, and cobalt. Several major multinational consortia were active in testing deep-seabed mining systems, including companies from the United States, Canada, United Kingdom, Australia, Germany, France, Belgium, Netherlands, and Japan. See MANGANESE NODULES. Metalliferous sulfides. In 1979, an extraordinary discovery of massive deposits of metalliferous sulfides containing zinc, copper, silver, and gold was made at 21◦N on the East Pacific Rise at a depth of 3000 m (10,000 ft). This confirmed projections, based on previous discoveries of metalliferous muds in the Red Sea, that the boundaries between spreading oceanic plates would be host to similar hydrothermal mineral deposits. As oceanic plates move apart, molten basaltic magma is thrust up between them, resulting in extensive fracturing of the adjacent flanks as much as several kilometers from the axis. Cold seawater percolates through the fractures, deep into the oceanic crust, and in its then superheated state dissolves many of the metals contained in the basalt. Normal convection causes the now hot, metal-rich solution to rise back to the seabed along the axis. The temperature and pressure decrease as the solution rises, and the contained minerals may be differentially precipitated as they pass through the cooler rocks. The fluids rise from the seabed at 300–350◦C (570–660◦F) into cold seawater at 4◦C (37◦F) to form black or gray smokers (hydrothermal vents), from which the remaining minerals are precipitated to form vent chimneys or to rain down as fine particles of mineral-rich sediments. Intensive examination of parts of the Juan de Fuca/Gorda Ridge spreading centers off the northwestern United States resulted in further discoveries, including goldrich sulfide deposits off Papua, New Guinea. See HYDROTHERMAL VENT; ORE AND MINERAL DEPOSITS; PLATE TECTONICS. With commercial interest in the manganese nodules waning, because of depressed metal markets and continuing uncertainties over the terms of the UNCLOS, and claims of 370-km (200-nautical-mile) exclusive economic zones (EEZ) proliferating, emphasis changed during the 1980s to assessing the potential of minerals within the EEZs. Prime targets

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Marine mining Hydrates. More recently, there has been widespread interest in the research, identification, assessment, exploration, and development of methane hydrate resources. In the United States, methane hydrates had been regarded as an academic curiosity, or as an expensive nuisance causing the clogging of oil and gas pipelines. The frozen, crystalline gas hydrates, also known as clathrates, have been demonstrated to be more widely distributed in the marine environment than was originally anticipated, and that this ubiquitous material represents a potential source of energy, on a global scale, that is significantly greater than all other forms of fossil fuel energy combined. The formation and stability of the crystals are dependent on the combination of high pressure, low temperature, and a source of gas. Fossil hydrates formed during the ice age are found onshore and throughout the continental shelf in high

were the high-cobalt manganese crusts, containing up to 2% cobalt, which were found to be widely distributed throughout the Pacific basin on the flanks of islands and seamounts at depths between 800 and 2400 m (2600 and 7900 ft). Others were the metalliferous sulfide deposits formed at spreading centers and in back-arc basins within the economic zones, as well as the more traditional nearshore deposits of unconsolidated heavy minerals containing gold, tin, and titanium, and extensive bedded deposits of phosphorites projected in the U.S. Atlantic margin (Fig. 1). Meantime in certain countries, government interests in the manganese nodules was maintained, mainly in connection with environmental issues. In 1988, France, India, Japan, and the Soviet Union registered as pioneer investors under the terms of the United Nations convention. Since then, China and Korea have been added. 140° E 170° W 140°

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Occurrence of minerals Mineral Likely Known Sand and gravel Placers Phosphorites Manganese _ nodules Cobalt _ crust Massive sulfides

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minimum continental shelf 5-year planning areas delineated by U.S. Minerals Management Service planning areas seaward of minimum continental shelf minimum continental shelf regions seaward of planning areas

Fig. 1. Offshore mineral areas of the United States.

1000 2000 km 500 1000 mi

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Marine mining latitudes. The deep seabeds are host to hydrate deposits that are forming now. Methods of extraction of these deposits, which may be up to hundreds of meters thick over areas of tens of square kilometers, are being studied in India and Japan, where they are considered serious contenders as national sources of energy. One proposed method of recovery would be to drill underneath the deposit and remove the gas below the hydrates. The subsequent reduction in gas pressure would lead to sublimation of the crystalline hydrate cap, resulting in a continuous release of additional gas. The hydrate deposits may also constitute a hazard to oil and gas operations in terms of sea-floor instability, and it is suggested that they may be a significant factor in the presence of atmospheric greenhouse gases as the result of temperature changes in the oceanic environment, causing major gaseous releases from the seabed and influencing global climate. See HYDRATE. Environment and minerals. While mineral resources to the value of trillions of dollars do exist in and under the oceans, their exploitation will not be simple. Many environmental problems must be overcome and many technical advances must be made before the majority of these deposits can be mined in competition with existing land resources. The International Seabed Authority (ISA) has prepared environmental guidelines for the development of manganese nodules and the International Marine Minerals Society (IMMS) has compiled similar guidelines for the development of metalliferous sulfides. The marine environment may be logically divided into four significant areas: waters, deep seabeds, continental shelf and slope, and seacoast. Of these, the waters are the most significant, both for their mineral content and for their unique properties as a mineral overburden. Not only do they cover the ocean floor with a fluid medium quite different from the solid earth or atmosphere and require entirely different concepts of ground survey and exploration, the constant and often violent movement of the surface waters combined with unusual water depths presents formidable deterrents to the use of conventional seagoing techniques in marine mining operations. The mineral resources of the marine environment are of three types: dissolved minerals of the ocean waters; unconsolidated mineral deposits of marine beaches, continental shelf, and deep seabed; and consolidated deposits contained within the bedrock underlying the seas. These are described in the classification outline below, which shows also subclasses of seabed and subseabed deposits, characteristics that have a very great influence on the economics of exploration and mining. I. FLUID Seabed Freshwater Seawater Magnesium Sodium Uranium

Bromine Salts of 26 other elements Subseabed Hydrothermal fluids II. UNCONSOLIDATED Seabed Industrial materials Sand and gravel Shells Aragonite Heavy-mineral placers Magnetite Ilmenite, rutile Chromite, monazite Nodules Manganese Phosphorite Muds and oozes Metalliferous Carbonaceous Siliceous Calcareous Barite Subseabed Heavy-mineral placers Gold Platinum Cassiterite Gemstones Bedded deposits Phosphorites III. CONSOLIDATED Seabed Crusts Phosphorite Cobalt Manganese Mounds and stacks Metallic sulfides Subseabed Disseminated, stratified, vein, or massive deposits Coal Sulfur Phosphates Carbonates Potash Ironstone Limestone Metallic sulfides Metallic salts Gas hydrates See MARINE GEOLOGY; SEAWATER. As with land deposits, the initial stages preceding the production of a marketable commodity include discovery, characterization of the deposit to assess its value and exploitability, and mining, including beneficiation of the material to a salable product. Exploration. Initial requirements of an exploration program on the continental shelves are a thorough study of the known geology of the shelves and adjacent coastal areas and the extrapolation of known

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Marine mining metallogenic provinces into the offshore areas. The projection of these provinces, characterized by relatively abundant mineralization, generally of one predominant type, has been practiced with some success in the localization of certain mineral commodities, overlain by thick sediments. As a first step, the application of this technique to the continental shelf, overlain by water, is of considerable guidance in localizing more intensive operations. Areas thus delineated are considered to be potentially mineral bearing and are subject to prospecting by geophysical and other methods. A study of the oceanographic environment may indicate areas favorable to the deposition of authigenic deposits in deep and shallow water. Some deposits may be discovered by chance in the process of other marine activities. Field exploration prior to or following discovery will involve three major categories of work: ship operation, survey, and sampling. Ship operation. Conventional seagoing vessels are used for exploration activities with equipment mounted on board to suit the particular type of operation. The use of remotely operated vehicles (ROVs) will eventually augment or replace existing techniques, but they are not yet advanced sufficiently for commercial exploration. See UNDERWATER VEHICLE. One of the most important factors in the location of undersea minerals is accurate navigation. Ore bodies must be relocated after being found and must be accurately delineated and defined. The accuracy of survey required depends upon the phase of operation. Initially, errors of 300 m (1000 ft) or more may be tolerated. However, once an ore body is believed to exist in a given area, maximum errors of less than 30 m (100 ft) are desirable. These maximum tolerated errors may be further reduced to centimeters in detailed ore-body delineation and extraction. A variety of types of electronic navigation systems is available, with accuracies from 900 m (3000 ft) down to approximately 1 m (3 ft). Loran, Lorac, and Decca are permanently installed in various locations throughout the world but are becoming obsolete as the differential Global Positioning System (GPS) has completely revolutionized the capabilities for positioning with high accuracy in any part of the world’s oceans. See SATELLITE NAVIGATION SYSTEMS. During sampling and mining operations, the vessel must be held steady over a selected spot on the ocean floor. Two procedures that have been fairly well developed for this purpose are multiple anchoring and dynamic positioning. A three-point anchoring system is of value for a coring vessel working close to the shore. A series of cores may be obtained along the line of operations by winching in the forward anchors and releasing the stern anchor. Good positive control over the vessel can be obtained with this system, and if conditions warrant, a four-point anchoring system may be used. Increased holding power can be obtained by multiple anchoring at each point. Dynamic positioning is useful in deeper water,

where anchoring may not be practical. The ship is kept in position by use of auxiliary outboard propeller drive units or transverse thrusters. These can be placed both fore and aft to provide excellent maneuverability. Sonar transponders are placed on the bottom or held submerged at a depth of minimum disturbance, or the system may be tied to shore stations. The auxiliary power units are then controlled manually or by computer to keep the ranges at a constant value. See MARINE NAVIGATION; SONAR; UNDERWATER NAVIGATION. Survey. The primary aids to exploration for mineral deposits at sea are depth recorders, side scan sonars, subbottom profilers, magnetometers, and bottom and subbottom sampling systems. Their use is dependent upon the characteristics of the ore being sought. For the initial topographic survey of the seabed, and as an aid to navigation in inshore waters, the depth recorder is indispensable. It is usually carried as standard ship equipment, but precision recorders having a high accuracy are most useful in survey work. Integrated equipment packages to record bathymetric features using side-scan sonar similar to radar imagery, and precision contouring using multiple, narrow-beam depth recorders have been among the most significant advances in seabed mapping. By using sophisticated image enhancement, a direct fallout from the space program, these systems are able to map in real time at rates up to 25,000 km2 (9700 mi2) a day, depending on the precision required. Combined with the GPS, these developments have brought seabed mapping much closer to the capabilities attained in terrestrial mapping from the air, at least where the water is deep enough to allow relatively wide swaths to be recorded. In the search for marine placer deposits of heavy minerals, the subbottom profiler is probably the most useful of all the exploration aids. It is one of several systems utilizing the reflective characteristics of acoustic or shock waves, and is a development of standard geophysical seismic systems for reflection surveys, used in the oil industry. Profilers for shallow penetration use a variety of energy sources including electric sparks, compressed air, gas explosions, acoustic transducers, and electromechanical (boomer) transducers. The return signals as recorded show a recognizable section of the subbottom. Shallow layers of sediment, configurations in the bedrock, faults, and other features are clearly displayed and require little interpretation. The maximum theoretical penetration is dependent on the time interval between pulses and the wave velocity in the subbottom. A pulse interval of 0.5 s and an average velocity of 2400 m/s (8000 ft/s) will allow a penetration of 600 m (2000 ft), the reflected wave being recorded before the next transmitted pulse. See SEISMIC STRATIGRAPHY. Penetration and resolution are widely variable features on most models of wave velocity profiling systems. In general, high frequencies give high resolution with low penetration, while low frequencies

Marine mining give low resolution with high penetration. The general range of frequencies for mineral exploration varies from 1 to 10 kHz (typically 3.5 kHz), and the general range of pulse energy is 100–25,000 joules for nonexplosive energy sources. The choice of system will depend very much on the requirements of the survey, but for the location of shallow placer deposits on the continental shelf the smaller lowpowered models have been used with considerable success. With the advent of the flux gate, proton precession, and the rubidium vapor magnetometer, all measuring the Earth’s total magnetic field to a high degree of accuracy, this technique has become much more useful in the field of mineral exploration. Anomalies indicative of mineralization such as magnetic bodies, concentrations of magnetic sands, and certain structural features can be detected. Although all three types are adaptable to undersea survey work, the precession magnetometer is more sensitive and more easily handled than the flux gate, and the rubidium vapor type has an extreme degree of sensitivity which enhances its usefulness when used as a gradiometer on the sea surface or submerged. See GEOPHYSICAL EXPLORATION. Once an ore body is indicated by geological, geophysical, or other means, the next step is to sample it in area and in depth. Mineral reserves cannot be confirmed without collecting samples. Sampling. Mineral deposit sampling involves two stages. First, exploratory or qualitative sampling to locate mineral values and allow preliminary judgment to be made. For marine deposits, this will involve such simple devices as snappers, drop corers, drag dredges, and divers. Accuracy of positioning is not critical at this stage, but of course is dependent on the type of deposit being sampled. Second, the deposit must be characterized in sufficient detail to determine the production technology requirements and to estimate the profitability of its exploitation. This quantitative sampling requires much more sophisticated equipment than does the qualitative type, and for marine work few systems in existence can be considered reliable and accurate. However, in particular cases, systems can be put together using available hardware that give the accuracy required. Specifically, qualitative sampling of any mineral deposit offshore can be carried out with existing equipment. Quantitative sampling of most alluvial deposits of heavy minerals (specific gravity less than 8) can be carried out at shallow depths (less than 107 m or 350 ft overall) using existing equipment, but cannot be carried out with reliability for higher-specific-gravity minerals such as gold (specific gravity 19). Advances have been made in sampling for diamonds. Any system that will give quantitative samples can be used for qualitative sampling, but in many cases heavy expenses could be avoided by using the simpler equipment. To obviate the effects of the sea surface environment, the trend is toward the development of fully submerged systems, but the deficiencies in sampling heavy placer minerals are not due to the marine en-

vironment. Even on land, the accuracy of placer deposit evaluation is not high and the controlling factors not well understood. There is still a need for more intensive research in this area. Quantitative sampling of consolidated deposits offshore has been confined to water depths of no more than a few hundred feet mainly because of the expense. Whereas the Ocean Drilling Program has the capability to drill in thousands of meters of water, only one system presently exists that can be placed on the bottom from a conventional vessel, to drill small holes in the rock to depths of about 50 m (165 ft), which is a prerequisite for even the most simple deposit characterization of metalliferous sulfides. Similarly, the characterization of metalliferous crust deposits depends on multiple, accurately positioned samples of the crust material and the underlying bedrock. Until such systems are better developed, the true nature of the many discoveries of deep-seabed consolidated deposits will remain speculative. Evaluation of surficial nodule deposits on the deep seabed at depths of 5000–6000 m (16,500–19,800 ft) may be carried out by using combinations of optical or acoustic imagery and sampling. Except for box corers that are used for geotechnical sampling, devices lowered from a stationary vessel have given way to the use of free-fall (boomerang) samplers equipped with corers, grabs, or cameras, and discharged at preselected points in groups of 8–12 for later retrieval. Also widely used are towed-television or multiple-shot photographic cameras that give a fully or nearly continuous coverage along selected tracks. Nodules are commonly assayed aboard ship and the data from all systems analyzed with onboard computers. A variety of seabed maps showing bathymetry, seabed topography, nodule distribution, and grade may be produced while the vessel is still on site. Ultimately, methods for continuous in-place assaying while under way should be perfected, which will allow the requisite data maps to be produced in real time. Exploitation. Despite the intense interest in undersea mining, new activities have been limited mostly to conceptual studies and exploration. The volume of production has shown little change, and publicity has tended to overemphasize some of the smaller, if more newsworthy, operations. All production to date has come from nearshore sources, namely, seawater, beach and nearshore gold and diamond placers, and nearshore-consolidated deposits of coal, sulfur, and potash. Minerals dissolved in seawater. Commercial separation techniques for the recovery of minerals dissolved in seawater are limited to chemical precipitation and filtration for magnesium and bromine salts and solar evaporation for common salts and freshwater production on a limited scale. Other processes developed in the laboratory on pilot plant scale include electrolysis, electrodialysis, adsorption, ion exchange, chelation, oxidation, chlorination, and solvent extraction. The intensive interest in the extraction of fresh water from the sea has permitted

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Marine mining TABLE 1. Mining regions for dissolved mineral deposits offshore Mineral

Location

Sodium, NaCl Magnesium, metal Mg, MgO, Mg(OH)2 Bromine, Br Freshwater

Worldwide United States, United Kingdom, Germany, Russia Worldwide Middle East, Atlantic region, United States Canada

Heavy water

additional research on the recovery of minerals, but successful commercial operations will require continued development of the combination of processes involved for each specific mineral. Three minerals or mineral suites are extracted commercially from seawater: sodium, magnesium, and bromine (Table 1). Of these, salt evaporites are the most important. Japan’s total production of salt products comes from the sea. Magnesium extracted from seawater accounts for 75% of domestic production in the United States. Freshwater compares with bromine in total production value, and heavy water, or deuterium oxide, has been produced from seawater in Canada for many years. Extensive work has been carried out in Japan on the extraction of uranium. Unconsolidated deposits. Unconsolidated deposits include all the placer minerals, on the seabed and in the subseabed, as well as the authigenic deposits of manganese nodules found at moderate to great depths. See AUTHIGENIC MINERALS. The mining of unconsolidated deposits became widely publicized with the awareness of the potential of manganese nodules as a source of manganese, copper, nickel, and cobalt, and because of developments in the exploitation of offshore diamonds in South-West Africa in the late 1960s. Unconsolidated deposits for many years presented a major source of tin offshore in South East Asia. So far, the methods of recovery (which have been used or proposed) have been conventional; bucket or clamshell

dragline

bucket ladder

that is, dredging using draglines, clamshells, bucket dredges, hydraulic dredges, or airlifts. All these methods (Fig. 2) have been used in mining to maximum depths of 60 m (200 ft), and hydraulic dredges for digging to 90 m (300 ft) have been built. Extension to depths much greater than this does not appear to present any insurmountable technical difficulties, and in Japan, reserves of sand and gravel are being mapped for exploitation to depths of 300 m (1000 ft). In the deep ocean three industrial consortia successfully tested dredge systems for manganese nodules in water depths over 4500 m (15000 ft). Dredging operations have exploited such diverse products as diamonds, gold, heavy mineral sands, iron sands, tin sands, lime sand, and sand and gravel. The most important of all of these commodities is the least exotic; 60% of world production from marine unconsolidated deposits is involved in dredging and mining operations for sand and gravel. Other major contributors to world production are the operations for heavy mineral sands (ilmenite, rutile, and zircon), mostly in Australia, and the tin operations in Thailand and Indonesia, which account for more than 10% of the world’s tin. Economics of these operations are dictated by many conditions. The spectacular range of costs offshore results from the effects of different environmental conditions. In general, offshore operations are more costly than similar operations onshore, but the factoring of secondary environmental costs may change this. Offshore diamonds represent a $1.5 billion per year industry along the coasts of Namibia and South Africa. Most of the marine diamonds are high-quality gemstones, having traveled thousands of kilometers from their source inland and then been subjected to intensive wave action over several cycles of sea-level change. Much of the world’s heavy mineral sand production is from beach sand operations in Australia, Sri Lanka, and India. Only two oceangoing dredges are used. Most of the others are pontoon-mounted hydraulic dredges, or draglines, with separate washing plants.

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Marine mining Lime shells are mined as a raw material for portland cement. Two U.S. operations for oyster shells in San Francisco Bay and Louisiana employed bargemounted hydraulic cutter dredges of 41- and 46-cm (16- and 18-in.) diameter in 9–15 m (30–50 ft) of water. The Iceland Government Cement Works in Akranes uses a 46-m (150-ft) ship to dredge seashells from 40 m (130 ft) of water, with a 61-cm (24-in.) hydraulic drag dredge. In the United Kingdom, hopper dredges are used for mining undersea reserves of sand and gravel. Drag suction dredges up to 97 cm (38 in.) in diameter are most commonly used with the seagoing hopper hulls. Similar deposits have been mined in the United States, and the same type of dredge is employed for the removal of sand for harbor construction or for beach replenishment. Some sand operations use beach-mounted draglines for removal of material from the surf zone or beyond. For deep-seabed mining at depths below 5000 m (3 mi), systems have been tested using airlift or suction lift and a mechanical system using a continuous loop of synthetic line from which buckets are suspended every 100 m (330 ft) or so. The gathering devices tested on the bottom with hydraulic systems include towed and self-propelled miners (Fig. 3), the latter incorporating a means to crush and slurry the nodules before pumping. Tests to identify the environmental effects of mining have not been conclusive, but tend to indicate that adverse effects are minor and can be mitigated. Consolidated deposits. The third and last area of offshore mineral resources has an equally long history. The production from in-place mineral deposits under the sea is quite substantial, particularly in coal deposits. Undersea coal accounts for almost 30% of the total coal production in Japan and just less than 10% in Great Britain. For most of the bedded deposits that extend from shore workings a shaft is sunk on land with access under the sea by tunnel (Fig. 4a). Massive and vein deposits are also worked in this manner. Normal mining methods are used, but precautions must be taken with regard to overhead cover. Near land and in shallow water a shaft is sunk at sea on an artificial island (Fig. 4b). The islands are constructed by dredging from the seabed or by transporting fill over causeways. Sinking through the island is accompanied by normal precautions for loose, waterlogged ground, and development and mining are thereafter conventional. The same method is also used in oil drilling. Offshore drilling and in-place mining (Fig. 4c) are used only in the mining of sulfur, but this method has considerable possibilities for mining other minerals (such as hydrates, metalliferous sulfides, or deep placers) for which leaching or slurrying are applicable. Petroleum drilling techniques are used throughout, employing stationary platforms constructed on piles driven into the sea floor or floating drill rigs. See OIL AND GAS, OFFSHORE; UNDERGROUND MINING. Minerals produced by offshore mining include dissolved minerals, unconsolidated minerals, and con-

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Fig. 3. One-tenth-scale remotely piloted vehicle test miner; note Archimedes’ screw drive cyclinders. (Ocean Mineral Company)

solidated minerals (Table 2). Though this is only a fraction of world mineral production, the percentages will increase as problems of technology, environment, and infrastructure are resolved.

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(c) Fig. 4. Methods of mining for exploitation of consolidated mineral deposits offshore. (a) Shaft sunk on land, access by tunnel. (b) Shaft sunk at sea on artificial island. (c) Offshore drilling and in-place mining.

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Marine mining TABLE 2. Summary of production from mineral deposits offshore

Type

Minerals

Dissolved Sodium, magnesium, minerals calcium, bromine Unconsolidated Diamonds, gold, heavy minerals mineral sands, iron, sands, tin sands, lime shells, sand and gravel Consolidated Iron ore, coal, sulfur, minerals hydrates

Percent of world production value 25 1

1

The future. Despite the technical problems to be overcome, the future of the undersea mining industry is potent. With the advent of national exclusive economic zones claimed by most sovereign coastal states, there is a strong impetus to understand the nature of the resources that are potentially available from these vast new jurisdictional areas. The distinction between shallow and deep-water seabed minerals is no longer political, as in continental-shelf and international seabeds, because many of the exclusive economic zones extend to abyssal depths. The most significant discoveries of marine minerals since the early 1970s have included metalliferous muds in the Red Sea, manganese nodules in the Pacific and Indian oceans, massive metalliferous sulfide deposits in the Atlantic and Pacific oceans, widespread seabed encrustations of high-cobalt metalliferous oxides in the Pacific, and gas hydrates in continental margins and deep ocean basins. Seawater represents the greatest accessible resource of many elements known and dissolved mineral extraction from seawater by conventional means results in a significant portion of the world’s mineral production. As appropriate technology is developed, it is likely that the extraction of minerals directly from the sea will increase. The presence of hydrothermal fluids rich in minerals, associated with sea-floor spreading centers and other volcanic phenomena, presents an additional potential for the production of minerals and geothermal energy. The mining of unconsolidated deposits nearshore will continue to be significant for the supply of construction materials and heavy minerals and will benefit from improved technology, particularly as deposits are sought in deeper water, further from shore, and in high latitudes. In the deep seabeds, extensive testing of systems for recovering manganese nodules, during the late 1970s, indicated almost conclusively that the nodules are a recoverable resource that await only a suitable economic climate and political infrastructure. Similar conclusions may be drawn from the successful testing of systems to mine the metalliferous muds in the Red Sea. Consolidated deposits may call for a variety of new mining methods that will depend on the type, grade, and chemistry of the deposit, its distance from land, and the depth of water (Fig. 5). The possibility of direct sea-floor access at remote sites through shafts drilled in the sea floor was given consideration under

the U.S. Navy’s Rocksite program during the 1960s and will be directly applicable to some undersea mining operations. In relatively shallow water, shafts could be sunk by rotary drilling with caissons. In deeper water, the drilling equipment could be placed on the sea floor and the shaft collared on completion. The laying of large-diameter undersea pipelines has been accomplished over distances of 40 km (25 mi) and has been planned for greater distances. Subestuarine road tunnels have been built using prefabricated sections. The sinking of shafts in the sea floor from the extremities of such tunnels should be technically feasible under certain conditions. The most likely methods for the development of hard-rock submarine ore bodies will be some variation of solution mining technology, involving fracturing of the rock mass in place and removing the valuable materials in solution through a pattern of boreholes. Major problems to be overcome include the control of the fracture pattern and the selective extraction of the desired elements from complex ores. See SOLUTION MINING. In the late 1990s, Australian scientists discovered rich deposits of gold, silver, copper, and zinc sulfide in water at a depth of 1200–1700 m (4000–5700 ft) in the Manus Basin off the coast of Papua New Guinea.

land 16 km (10 mi) distant shaft chamber prefabricated tunnel sediments

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area shattered by nuclear blast ore body (c) Fig. 5. Possible future methods of mining consolidated mineral deposits under the sea. (a) Shaft sinking by rotary drilling from tunnels laid on the seabed. (b) Breaking by nuclear blasting and dredging. (c) Shattering by nuclear blasting and solution mining.

Marine navigation Metal content of samples from the areas being studied have ranged 10–15% copper, 3–26% zinc, 15–21 g of gold per ton, and 130–200 g of silver per ton. The deposits are surficial masses and chimneys formed around high-temperature hydrothermal vents of mineralized fluids or smokers resulting from tectonic activity associated with the movement of oceanic plates. With drilling for oil and gas now becoming state of the art in similar water depths, the transition to active mining development using off-the-shelf equipment appears likely in the near future. Exploration for similar deposits is being carried out in other areas. Major technological advances achieved will enhance the possibility of mining in the future, including underwater surveying, advanced underwater vehicle systems, and hydrometallurgical processing. The most important of these may be in the combination of seabed imagery and global positioning, for without adequate maps the characterization and development of the resources of the seabed could not be reasonably pursued. Outlook. There have been some significant advances in understanding minerals of the seabeds. The ability to manage these minerals as sustainable resources has been enhanced by the passage of legislation. The most important understanding is that, despite the difference in the geological environment of the seabeds compared to land, the potential for mineral occurrence, hectare for hectare, is equivalent in both. On this basis, the global mineral resource base has recently been increased by a factor of 4, and three-quarters of it is underwater. The most important legislation has been the Law of the Sea, in what is probably the most far-reaching redistribution of natural resources in human history. This law has resulted in a peaceful subdivision of jurisdictions, which should significantly affect the world’s mineral markets and the balance of economic powers, at least in the Pacific, for as long as humans are dependent on the use of mineral raw materials. Michael J. Cruickshank Bibliography. H. R. Cooper, Practical Dredging, 1981; D. S. Cronan, Underwater Minerals, 1980; M. J. Cruickshank, Law of the Sea and minerals development, Ocean Yearbook, vol. 13, pp. 80–106, Dalhousie Law School, 1998; M. J. Cruickshank, Marine mining, in SME Mining Engineering Handbook, pp. 1985–2027, 1992; M. J. Cruickshank, Mining, and mineral recovery, U.S.T. Handbook Directory, pp. A/15–A/28, 1973; M. J. Cruickshank and C. L. Morgan, Synthesis and Analysis of Existing Information Regarding Environmental Effects of Marine Mining, Consulting Report to Continental Shelf Associates for U.S. Department of the Interior, Minerals Management Service, OCS Study MMS 93-0006, 1992; Deep Seabed Polymetallic Nodule Exploration: Development of Environmental Guidelines, Proceedings of the International Seabed Authority’s Workshop, Sanya, Hainan Island, People’s Republic of China, June 1–5, 1998; F. C. F. Earney, Marine Mineral Resources, Routledge, 1990; P. Hoagland III and J. Broadus, Seabed Material

Commodity and Resource Summaries, Woods Hole Oceanog. Inst. Tech. Rep. WHOI-87-43, October 1987; R. H. Joynt, R. Greenshields, and R. Hodgen, Advances in sea and beach diamond mining techniques, S.A. Min. Eng. J., p. 25 ff., 1977; A. Krull, Ocean Mining and Engineering: New Directions and Opportunities, 1983; Society of Mining Engineers, Mineral Resources of the Pacific Rim, 1982; United Nations (ed.), Seabed Minerals Series, 3 vols., 1983; U.S. Congress, Office of Technology Assessment, Marine Minerals: Exploring Our Ocean Frontier, 1987.

Marine navigation The process of directing a watercraft to a destination in a safe and expeditious manner. From a known present position, a course is determined that avoids dangers, and on this course estimates are made of time schedules. The task is to make periodic adjustments using en route measurements of position or heading and speed. The method used will depend on the type of vessel and on its role or mission. The devices available range from a simple compass to a host of sophisticated electronic systems. In all cases, the navigator must plan and prepare by setting instruments in order and by checking for predictable current and tidal effects and hazards to navigation en route. This preparation includes having the latest, correct charts and reviewing pertinent sections of sailing directions, tabulations of navigational lights, and tide and current tables. The prudent navigator of a ship planning to cross an ocean will consider the particular requirements of each segment of the voyage. A voyage may be broken into six phases: preparatory, departure, confluence area after departure, high seas, confluence near destination, and terminal. The methods used to fulfill the requirements of these phases fall under one of the following broad categories of navigation: dead reckoning, piloting, celestial navigation, and electronic navigation. The first three categories have become somewhat standardized; the fourth category has been under constant and innovative development. Electronic navigation is rapidly evolving into integrated systems providing all navigation functions. The publishing of full portfolios of electronic navigational charts and related publications, and the establishment of international standards for systems that display and integrate information, have allowed navigators and maritime administrators to consider reliance on full electronic solutions. The following discussion covers the methods and technologies, and how they may be applied onboard ships. Dead reckoning. A navigator maintains a reckoning of present and future positions of a ship that is deduced from measurements of direction and speed along the track. In the absence of systems that provide position-fixing capability, this reckoning may be the only way of estimating position on a voyage.

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Marine navigation Position fixing is always a past event. Hence, a projection of position requires the use of dead reckoning. It is customary to do this by hand by using pencil, paper, and plotting instruments. However, there are computerized electronic devices that have brought dead reckoning to a high state of sophistication. Among these are marine course computers that will automatically provide such positions besides indicating the command heading, speed made good, distance to destination, estimated time of arrival, and steering signal to gyropilot. See DEAD RECKONING. Inertial navigation is a position-fixing technique that estimates current position by accumulating position changes since the last known position. This selfcontained method of navigation eliminates the need for any external references and is passive, that is, nonradiating. The basic principle of inertial navigation is to measure the components of a craft’s acceleration along precisely defined axes. If the initial conditions are known, successive integration will produce the velocities and distances along those axes. The key inertial component is the gyroscope with its characteristic “rigidity in space” and its phenomenon of precession. The basic instrument for measuring acceleration is the accelerometer. A digital computer is used for converting accelerations into position. See ACCELEROMETER; GYROSCOPE; INERTIAL GUIDANCE SYSTEM. Another self-contained positioning system is the Doppler sonar navigator. In this system, ultrasonic beams are transmitted to the ocean floor in the forward, aft, starboard, and port directions, and reflected by the ocean floor. Relative motion between the craft and the ocean floor produces Doppler shifts in the frequencies of the reflected signals, the amount of the shift of each beam being directly proportional to the speed of the craft in the direction of that beam. A digital computer converts the Doppler shift of each frequency to speed, and if a reliable reference direction indicator is available, the resultant velocity—speed and direction—of the craft is determined. Distance traveled is determined by integration of the speed with respect to time. Velocity thus determined is with respect to the ocean floor. If the depth of water is greater than about 400 m (220 fathoms or 1320 ft), signals are returned by water reverberation rather than by reflection from the ocean floor, and the velocity is therefore relative to the water. The system can be adapted as an aid in the docking maneuvers of ships. In this use, additional transducers are employed to provide bow, stern, and athwartships velocities. See DOPPLER EFFECT; SONAR. Piloting. This method of navigation is used when the ship is close to land and involves the use of landmarks and seamarks for frequent determination of position. Such marks may be observed by visual, radio, or sound techniques. Visual piloting consists of sighting identifiable landmarks at known positions. The radio system most widely used in this piloting sequence is radar that, besides giving a chartlike pictorial display, can provide bearings and distances for position fixing. The acoustical approach utilizes ul-

trasonic depth finders. These instruments provide depth measurements that may be compared with those on the chart. Such comparisons can be used to confirm positions or can be used as forewarnings of possible stranding. See PILOTING; RADAR. Celestial navigation. The high-seas phase of a voyage involves the use of either celestial navigation or satellite navigation systems. Sometimes a combination of the two is used. Celestial navigation was formerly considered the primary method of fixing the position of a craft when it was out of sight of land. When using celestial navigation, the navigator follows a routine schedule of a morning twilight observation for a fix, a morning Sun observation of both altitude and azimuth for a morning Sun line of position from which an estimated position can be determined using the dead-reckoning plot, as well as a check on the errors of magnetic compasses and gyrocompasses, an observation of the Sun at meridian transit for a latitude line, and afternoon and evening twilight observations similar to those in the morning. Altitude observations are made with a marine sextant and are timed to the nearest second. For each body observed, the altitude, time, assumed position, and pertinent data from a nautical almanac are converted to a line of position by using prepared tables or a computer. Although two lines of position suitably distributed in azimuth can provide a fix, mariners typically require at least four lines of position unless that is not possible under the prevailing conditions. The celestial bodies observed are the Sun, stars, planets, and the Moon when it is in favorable position relative to other bodies. The principal limitations of celestial navigation are the need for a well-defined natural horizon and for visible celestial bodies. Celestial navigation serves as a backup technique on most vessels and is usually practiced by aficionados or students. The practice of celestial navigation is of value in educating mariners to better understand satellite navigation, since the effects of geometry and measurement quality are similar whether one is navigating by a constellation of satellites or a collection of celestial objects. See CELESTIAL NAVIGATION. Electronic navigation. This division of navigation previously encompassed only radio navigation systems. It now includes navigation involving any electronic device or instrument. A system that utilizes radio signals to provide lines of position or to fix the position of a craft involves the use of transmitters at accurately defined positions on the surface of the Earth or in satellites. Most common today is the use of satellite navigation using the Global Positioning System (GPS). In the near future new signals will be added to GPS, and the European Union is building a compatible system called Galileo. See ELECTRONIC NAVIGATION SYSTEMS. Loran C. Loran C is a hyperbolic system operating at 100 kHz. It combines the pulse-matching techniques of the former Loran A with phase comparison for greater accuracy. Each chain of Loran C stations consists of a master station operating with two or more secondary stations that transmit synchronized

Marine navigation signals. The useful range extends outward to the limit of ground-wave reception, about 1500–2200 km (810–1190 nmi) from the transmitters. Increased accuracy can be obtained by using Loran C in the differential mode. In this mode the reading of a monitor receiver at a known position is compared with the predicted reading, and any difference is then transmitted to vessels in the vicinity and applied as a correction to their readings. Loran C can also be used in a range-range mode, in which position is determined from the ranges to two transmitters, if the user has a stable, accurate time indicator synchronized with the time sources at the transmitters. Loran C has been designated the official system for the United States confluence and coastal regions. In the mid-1990s the advent of satellite navigation led to decisions by the federal government to decommission Loran C by the year 2000. However, the lifetime of the system has now been extended indefinitely. The need for Loran C as a backup system for navigation and timing is being examined by the U.S. Department of Transportation. The Northwest European Loran-C System (NELS) organization is undertaking a similar examination regarding future operations, with further complications arising from the circumstance that several countries host stations. See LORAN. Global Positioning System. The United States’ Global Positioning System (GPS) is an advanced navigation satellite system whose space segment consists of 24 satellites in 12-h orbits, 20,200 km (10,907 nmi) above the surface of the Earth. The control segment consists of several monitor stations to continuously track the satellites above the horizon, a master control station (with one or more additional for redundancy) to receive the tracking data and update the orbital data and satellite clock drifts, and upload stations to transmit the data to the satellites daily or more often. The user segment receives the signals transmitted by the satellites and computes pseudorange and pseudorange rate, which it converts to three-dimensional position of the craft. The average velocity of the craft during a measurement interval is the change in carrier phase of the GPS signal averaged over that interval. Time is available to 0.1 microsecond. With 24 functioning satellites in orbit, five or more are above the horizon virtually all the time. Signals from four satellites are sufficient for computation of three-dimensional positions. Users authorized to have access to the Precise Positioning Service (PPS) can expect absolute accuracy of positions of 5–10 m (16–32 ft). Other users have access to Standard Positioning Service (SPS), providing 20 m (65 ft) accuracy. In the differential mode an accuracy of 3– 5 m (10–16 ft) can be achieved. The GPS can be integrated with other systems for redundancy and integrity measurement. See SATELLITE NAVIGATION SYSTEMS. Galileo. The European Union is building a satellite navigation system called Galileo. Galileo is generally similar to GPS. Significant differences in the programs are the level of civilian control of policy in Galileo, a greater number of available services, and

the upfront development of applications that take advantage of Galileo. Automatic Identification System. The Automatic Identification System (AIS) was developed under the guidance of the International Maritime Organization (IMO), primarily to improve maritime safety by assisting the navigation of ships, the protection of the environment, and the operation of vessel traffic services (VTS). AIS serves in a vessel-to-vessel mode for collision avoidance, as a means for coastal states to obtain information about a ship and its cargo, and as a ship-to-shore method for vessel traffic management. The AIS device includes a Global Positioning System (GPS) receiver, a microprocessor, and a very high frequency–frequency-modulation (VHF-FM) transceiver. The microprocessor takes data from a ship’s sensors and packages it with the vessel’s identification into a digital signal. The signal is automatically broadcast. When other broadcasts are received, the processor prepares the received data for display. Each device determines its own transmission schedule, and, in the presence of other stations, uses receipt of previous messages to ensure that stations do not transmit simultaneously. This self-organizing logic allows many AIS units to operate without interfering with one another. AIS information includes static, dynamic, and voyage-related elements. Static information is manually entered on installation and seldom changed. Dynamic information is automatically entered from ship sensors, while voyage-related information is manually entered and updated as appropriate. The primary emphasis of AIS is collision avoidance. AIS is expected to improve vessel-operator situational awareness. In comparison to radar and automatic radar plotting aids (“own ship” information), AIS information is provided directly from the “other” ship’s sensors with each vessel’s name and call sign. AIS will also provide an efficient method to monitor vessel movements for purposes of environmental safety, coastal security, or traffic management. Though there is as yet no agreement on the definition of an AIS base station, coastal and harbor monitoring will probably be accomplished through a local network of AIS stations forming an “AIS Reception Network.” On a regional or national level, the multiple, local networks might be linked through a wide-area network. Vessel traffic services. Vessel traffic services (VTS) have been established in a number of heavily trafficked ports throughout the world in an attempt to reduce the number of collisions and strandings and safeguard the environment. Generally the service provides marine traffic management of an advisory nature, but in an especially hazardous situation it may be necessary for the VTS to exercise emergency control of vessel movements. The usual system components of such a service include AIS base stations, high-resolution radars, general-purpose computers, and synthetic video displays, all combined to provide real-time forecasts of vessel movements, lane stray alerts, collision and grounding warnings, and a

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Marine navigation description of potential congestion areas. A ship-toshore voice communications link permits information to flow between VTS operations personnel and identified ships within the traffic surveillance area. These voice communications are often recorded on audio tape units. The system tracks stationary and moving traffic to establish position coordinates, and monitors the position of floating navigational aids in the surveillance area to permit detection of misplaced or drifting buoys. See VESSEL TRAFFIC SERVICE. Traffic-separation schemes. Traffic-separation schemes have been established in a number of high-traffic density areas throughout the world, primarily to decrease the risk of collision at sea. International navigation rules govern the operation of vessels within these traffic separation schemes. A typical scheme consists of the establishment of parallel traffic lanes separated by an intervening buffer zone, analogous to a divided freeway on land. In some areas, surveillance centers provide information on the positions and movements of vessels in the area, and other hazards. Although compliance with such schemes is generally advisory only, a dramatic reduction in collisions in the areas where they have been established has contributed to increased acceptance by mariners, and the present trend is toward mandatory compliance, analogous to air-traffic control, but generally leaving the mariner greater freedom of selection of route, speed, and time of encounter of other craft. Shipboard systems. The human–machine interface for the practice of marine navigation is evolving. Once the navigator was an expert on a complex set of publications, making precise optical measurements, and plotting the results with the accuracy of a draftsman. Now the navigator retains these skills and must also learn to work with complex computer-based systems that integrate several sources of real-time information and compare the results to large quantities of stored hydrography and cartography. The trend is toward more integration with an accompanying assessment of the integrity or quality of separate information sources. This integration is becoming possible through redundancy of the essential information regarding one’s own ship and other nearby ships. For example, a crossing situation with another ship can be evaluated with both radar and AIS information. Basic sensors in these systems include GPS receivers, radar, AIS transceivers, a gyrocompass or transmitting magnetic compass, and normally some type of autopilot for steering and perhaps speed control. In ascending order of sophistication, a brief description of the major systems follows: Electronic chart system. An electronic chart displays on a video screen the same type of hydrographic information that mariners seek in a traditional nautical chart. Both electronic and paper charts offer graphic representations of water depths, shorelines, topographical features, aids to navigation, and hazards. An electronic chart database and display offers improvement over the paper chart when it is combined with other information. At a minimum, the ship’s po-

sition (from electronic navigation) and planned track are needed. With such a display, the navigating officer can determine at a glance the ship’s position relative to its intended track, the shoreline, the waterway, and any hazards or threats. Electronic charts integrated with a range of information, and with hardware and software that can process a hydrographic database to support decision making, are classified as electronic chart display and information systems (ECDIS). Thus, electronic charts are a component of an ECDIS. In addition to displaying a real-time picture of the vessel’s position in the waterway, an ECDIS manages navigational and piloting information (typically, vesselroute-monitoring, track-keeping, and track-planning information) to support navigational decision making. The term ECDIS is specifically used to indicate an approved device for carriage onboard internationally regulated commercial vessels. The term “electronic chart system” (ECS) is somewhat more generic and is used to indicate a range of systems on various vessels. Collision avoidance systems. Collision avoidance systems (CAS), also known as automatic radar plotting aids (ARPA), of varying degrees of sophistication have been developed to reduce the work load of the navigator and eliminate human error. Typically, such a system consists of a digital computer that receives inputs from the ship’s radar, compass, and log, and determines and displays collision threats, and in some installations provides a recommended avoiding action. Integrated navigation systems. Integrated navigation systems integrate the functions of the electronic chart system and collision avoidance systems, and normally some level of automated control of vessel course and speed. The advent of the AIS gives a big technical push towards integration in order to correlate the AIS-derived collision avoidance information with the traditional radar tracks. Automation of navigation tasks through an ECDIS combined with the accuracy of plotting intercepts in an integrated collision avoidance system gives the navigator better information and more time to consider and act versus manually plotting and correlating disparate sources. Some systems may provide a visual display of traffic lanes and possible navigation hazards. They may also perform prevoyage route planning and then continuously maintain a dead-reckoning position. Other features available in some are the computation of course to steer to the next waypoint and the use of adaptive automatic steering wherein the vessel’s reaction to the outside environment is translated into corrective steering changes designed to optimize the track and decrease the possibility of damage to the ship and its cargo. Intelligent systems. There are a variety of intelligent systems deployed aboard automated ship’s bridges: piloting expert systems, engineering and vibration expert systems, neutral network systems for adaptive and intelligent steering control, and automated intelligent docking systems. Piloting expert systems provide advice for navigation and collision avoidance by

Marine refrigeration combining the knowledge and reasoning processes of expert ship’s captains, ship’s officers, and pilots so as to assist navigators during trips. Such systems make use of real-time positioning information, nautical rules of the road, local knowledge, and organizational procedures and regulations to recommend best courses of action for the navigator. See EXPERT SYSTEMS; NEURAL NETWORK. Shipboard piloting expert systems are integrated into conventional and advanced bridge designs so as to provide intelligent ship control systems. Such systems use input from radar, automated radar plotting aid (ARPA), ECDIS, and bridge instrumentation, and employ heuristic or qualitative reasoning, rather than strict application of deterministic rules. Integrated bridge systems. These systems are designed to allow the wheelhouse to function as the operational center for navigational and supervisory tasks aboard the ship. These bridges in many cases become ship’s operations centers, incorporating controls and monitors for all essential vessel functions, including navigation, engine control, and communications. Other ship functions such as cargo loading, monitoring, and damage control may be included. Many routine navigational tasks, such as chart updating, position plotting, and steering, may be automated. The integrated ship’s bridge is thus a unified federation of systems supporting vessel navigation, communications, steering, administration, collision avoidance, safety, and monitoring and control of ship’s systems. See NAVIGATION. Jay Spalding; Alton B Moody; Martha R. Grabowski; Richard Greenspan Bibliography. S. F. Appleyard, Marine Electronic Navigation, 2d ed., 1988; N. Bowditch, American Practical Navigator: An Epitome of Navigation, National Imagery and Mapping Agency, H. O. Publ. 9, 2002; T. J. Cutler, Dutton’s Nautical Navigation, 15th ed., 2004; R. R. Hobbs, Marine Navigation, 4th ed., 1998; F. J. Larkin, Basic Coastal Navigation, 2d ed., 1998; A. B Moody, Navigation Afloat: A Manual For the Seaman, 1980; National Research Council, Minding the Helm: Marine Navigation and Piloting, 1994; H. H. Shufeldt, G. D. Dunlap, and B. A. Bauer, Piloting and Dead Reckoning, 4th ed., 1999.

Marine refrigeration Marine refrigerating equipment is used for shipboard refrigeration of products as well as for air conditioning the quarters of passengers and crew. Shipboard refrigeration is necessary for the preservation of perishables in transit and foodstuffs to be used by passengers and crew. Marine refrigeration is also used for maintaining certain cargo products in liquid form that would otherwise evaporate when stored at ambient conditions. A typical ship stores or cargo refrigeration plant consists of a compressor, motor, condensor, receiver, evaporators, and controls. A typical air-conditioning unit consists of a compressor, motor, condenser, receiver, water chiller, and

controls. See AIR CONDITIONING; REFRIGERATION. Refrigerants. The common refrigerants that have been in use for years, known as CFCs (chlorofluorocarbons) and HCFCs (hydrochlorofluorocarbons), are still being phased out because they contain chlorine, which depletes Earth’s stratospheric ozone layer. The stratospheric ozone layer protects all life from ultraviolet radiation. In recent years, holes have been detected in the ozone layer in both hemispheres in late winter and early spring. CFC and HCFC refrigerants have been determined to be major contributors to the holes. Common CFC refrigerants include R-11 and R-12. R-22 is a common HCFC refrigerant. Replacements for these refrigerants known as HFCs, which are non-ozone-depleting, have been developed. These refrigerants contain fluorine in place of chlorine, and do not destroy the ozone layer. The replacement HFC refrigerants normally provide similar to slightly better efficiencies than the CFC refrigerants they replace. The exception is that HFC-134a, which replaces CFC-12 (R-12) in low-temperature applications, decreases plant capacity approximately 35%. This problem is solved by increasing the compressor speed or changing the compressors. For some systems, the cargo itself is used as the refrigerant, such as when carrying ammonia or petroleum products such as propane or butane. Preservation of perishables. Cargoes such as perishable foodstuffs and chemicals requiring refrigeration are normally carried in insulated holds. The holds are usually cooled by direct-expansion evaporators which may be gravity-type cooling coils or forcedconvection cooling units located in the holds. As an alternative to this system, cargoes may be carried in insulated containers stowed on deck. Liquid cargoes. Liquid cargoes, such as ammonia, and low-pressure petroleum gases, such as propane and butane, are carried in insulated tanks kept cool by allowing the cargo to evaporate. This process involves drawing off the vapors from the tanks, compressing and condensing the fluids, and returning them to the tanks. Hydrocarbon cargoes of very low temperature, such as liquefied natural gas, are stowed in well-insulated tanks and also are kept cold by allowing the cargo to evaporate. The gases are drawn off and may be burned in the ship’s power plant as a partial source of energy for propulsion, or used to power an engine-driven refrigerating unit to assist in keeping the product cold while minimizing the amount of product that must be evaporated for cooling. Refrigerating systems. For refrigerating systems using insulated holds, liquid refrigerant is delivered to cooling coils, where the refrigerant is expanded, and enters the evaporator as a low-pressure liquid. There it evaporates by absorbing heat from the space being cooled. The refrigerant vapor is then drawn from the evaporator by a compressor, which delivers the refrigerant to a condenser as a high-pressure, high-temperature vapor. The condenser, normally a shell-and-tube heat exchanger cooled by seawater flowing through the tubes, removes the heat

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Marine sediments

Marine sediments The accumulation of minerals and organic remains on the sea floor. Marine sediments vary widely in composition and physical characteristics as a function of water depth, distance from land, variations in sediment source, and the physical, chemical, and biological characteristics of their environments. The study of marine sediments is an important phase of oceanographic research and, together with the study of sediments and sedimentation processes on land, constitutes the subdivision of geology known as sedimentology. See MARINE GEOLOGY; OCEANOGRAPHY; SEDIMENTOLOGY. Environments of Deposition

Modern rotary vane HFC-143a ship stores refrigeration unit. (York International Corp.)

absorbed in the evaporator and the heat of compression from the refrigerant as it condenses to a highpressure liquid. The refrigerating cycle is then repeated. For the deck-stowed container system, each container has its own refrigeration system consisting of a compressor, air-cooled condenser, cooling coil, and circulating fan. For liquid cargoes in insulated tanks, the latent heat of vaporization is released as the evaporating liquid changes state to a gas. This is the refrigeration system used in liquid cargo systems. Ship stores. A modern ship stores refrigeration unit is shown in the illustration. Refrigeration systems for ship stores are relatively small compared to cargo refrigeration systems or air-conditioning systems. The refrigerant is circulated through evaporators to maintain the desired temperatures in the various storerooms needed for chilled and frozen products. Two complete refrigeration units are usually employed, with one unit operating and the second on standby. Air conditioning. For air-conditioning applications, the cooling coils used for refrigeration are replaced with a water chiller. The water chiller is the evaporator where water that is circulated throughout the ship is cooled. The chilled water is circulated to cooling coils situated in various fan rooms for passenger comfort as well as the cooling of equipment. Multiple air-conditioning units are normally employed, and sometimes the units are located in different areas or zones of the ship. For large ships with many passengers, reciprocating compressors are too small; high-tonnage centrifugal-compressor air-conditioning plants are used instead. Reciprocating compressors are normally less than 100 tons, whereas centrifugal compressors can have several hundred tons capacity. Screw compressors can be used for moderate- as well as high-capacity airconditioning plants. John H. Merold

Traditionally, marine sediments are subdivided on the basis of their depth of deposition into littoral (0– 66 ft or 0–20 m), neritic (66–660 ft or 20–200 m), and bathyal (660–6600 ft or 200–2000 m) deposits. This division overemphasizes depth. More meaningful, although less rigorous, is a distinction between sediments mainly composed of materials derived from land, and sediments composed of biological and mineral material originating in the sea. Moreover, there are significant and general differences between deposits formed along the margins of the continents and large islands, which are influenced strongly by the nearness of land and occur mostly in fairly shallow water, and the pelagic sediments of the deep ocean far from land. Sediments of continental margins. These include the deposits of the coastal zone, the sediments of the continental shelf, conventionally limited by a maximum depth of 330–660 ft (100–200 m), and those of the continental slope. Because of large differences in sedimentation processes, a useful distinction can be made between the coastal deposits on the one hand (littoral), and the open shelf and slope sediments on the other (neritic and bathyal). Furthermore, significant differences in sediment characteristics and sedimentation patterns exist between areas receiving substantial detrital material from land, and areas where most of the sediment is organic or chemical in origin. See DEPOSITIONAL SYSTEMS AND ENVIRONMENTS. Coastal sediments. These include the deposits of deltas, lagoons, and bays, barrier islands and beaches, and the surf zone. The zone of coastal sediments is limited on the seaward side by the depth to which normal wave action can stir and transport sand, which depends on the exposure of the coast to waves and does not usually exceed 66–99 ft (20–30 m); the width of this zone is normally a few miles. The sediments in the coastal zone are usually land-derived. The material supplied by streams is sorted in the surf zone; the sand fraction is transported along the shore in the surf zone, often over long distances, while the silt and clay fractions are carried offshore into deeper water by currents. Consequently, the beaches and barrier islands are constructed by wave action mainly from material from fairly far away, although local erosion may make a contribution, while the lagoons and

Marine sediments bays behind them receive their sediment from local rivers. The types and patterns of distribution of the sediments are controlled by three factors and their interaction: (1) the rate of continental runoff and sediment supply; (2) the intensity and direction of marine transporting agents, such as waves, tidal currents, and wind; and (3) the rate and direction of sea level changes. The balance between these three determines the types of sediment to be found. See DELTA. On the Texas Gulf Coast, rainfall and continental runoff decrease gradually in a southwesterly direction. The wind regime favors considerable wave action and a southwesterly drift of the nearshore sand from abundant sources in the east. Since sea level has been stable for several thousand years, the conditions have been favorable for the construction of a thick and nearly closed sand barrier that separates a large number of bays from the open Gulf. This barrier is constructed by marine forces from sediments from distant sources and varies little in characteristics along its length. The bays, on the other hand, receive local water and sediment. In the east, the supply of both is fairly abundant, and since the streams are small, the sediment is dominantly fine; the bays have muddy bottoms and brackish waters. Conditions are fairly stable, and a rich, but quantitatively not large fauna is present, including oyster banks. At the southwestern end, continental runoff and sediment supply are negligible. The only sediment received by the bays comes from washovers from the barriers and is therefore mainly sandy, and the virtually enclosed bays with no runoff are marine to hypersaline. Locally, this yields chemical precipitates such as gypsum and calcium carbonate, and is also conducive to the development of a restricted but very abundant fauna, which produces significant deposits of calcareous material. The sediments of bays and lagoons are often more stratified than those of the open sea as a result of fluctuating conditions. The textural and compositional characteristics depend on local conditions of topography, shore development, and wave and current patterns. They range from coarse

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10 fathoms (18.3 m )

nmi 0 ske leta gr l c a l c a r ap esto ne oolites

gravel and cobbles on rocky beaches fronting the open sea to very fine clayey silt in the interior of quiet lagoons. See ESTUARINE OCEANOGRAPHY. The effects of sea-level changes are imperfectly known, but it is easily comprehended that the development of coastal sediments is to a large extent a function of the duration of this environment in a particular place. Thus if sea level rises or falls rapidly, there is no time for extensive development of beach, barrier, and lagoon deposits, and discontinuous blankets of nearshore sands, with muds behind them, are formed. As the rate of change decreases, open barriers, consisting of widely spaced low sand islands, tend to form, which imperfectly isolate open shallow lagoons in which essentially marine conditions prevail. A prolonged stability is required to produce thick, closed barriers and completely isolate the lagoon environment. Entirely different nearshore deposits are found on shoals where supply of sediment and freshwater from the land is absent, either because land areas are small (Bahamas), the drainage is directed elsewhere (southern Cuba), or there is no rainfall (Arabian Peninsula). Calcareous muds and coarse calcareous sands then make up the lagoon and beach deposits. If, in addition, the shoal borders directly on the deep ocean without transitional shelf (Fig. 1), cool water is driven onto the shoal, where it warms up and precipitates calcium carbonate. In the turbulent water of the shoal, this precipitation either takes place in the form of oolites or it cements organic debris together in small aggregates (grapestone). At the edge of the shoal, the presence of cool, nutrientrich ocean water is favorable for the growth of coral and algal reefs that are bordered by a zone of skeletal sand derived from broken calcareous organisms. The inner sheltered portions receive the finest calcareous sediments. The types of sediment and their distribution patterns are controlled mainly by the shape of the shoal, in particular the position of its edge, by the prevailing wind, and by the location of sheltered or somewhat deeper quiet areas. Shelf and slope sediments. The continental shelf is a gently seaward sloping plain of greatly varying width,

Isle of Pines

s k e l et coral reefs Caribbean Sea

al

r calca 0

20 km

Fig. 1. Shoal water sediments as examples of sedimentation without supply of land-derived sediment. (a) The Bahamas. (b) The Gulf of Batabano, which is in southwestern Cuba. Both of these shoals border directly on the deep ocean.

40

473

474

Marine sediments ranging from less than a mile (1.6 km) along steep rocky coasts to several hundred miles, for example, in the western Gulf of Mexico. A distinct break in slope at 330–1600-ft (100–500-m) depth marks the transition to the continental slope which descends somewhat more steeply (5–10◦) to the deep-sea floor. In areas of active tectonism, for example, off the coast of southern California, the shelf is narrow and separated from the continental slope by a wide zone of deep basins alternating with shallow banks and islands. Submarine canyons cut the edge of the continental shelf in many places and sometimes reach back into the nearshore zone. See SUBMARINE CANYON. During the Pleistocene, the continental shelf was subjected to repeated transgressions and regressions. During each interglacial, sea level was high and the shoreline was located near its present position; during each glacial period, much water was withdrawn from the ocean and the shoreline occurred near the edge of the shelf. The last low sea-level stand occurred approximately 19,000 years ago, and the present shoreline was established as recently as 3000–5000 years ago. On most shelves, equilibrium has not yet been fully established and the sediments reflect to a large extent the recent rise of sea level. Only on narrow shelves with active sedimentation are present environmental conditions alone responsible for the sediment distribution. Sediments of the continental shelf and slope belong to one or more of the following types: (1) biogenic (derived from organisms and consisting mostly of calcareous material); (2) authigenic (precipitated from seawater or formed by chemical replacement of other particles, for example, glauconite, salt, and phosphorite); (3) residual (locally weathered from underlying rocks); (4) relict (remnants of earlier environments of deposition, for example, deposits formed during the transgression leading to the present high sea-level stand); (5) detrital (products of

Key: shelf, depositional slope shelf, nondepositional shelf margin reefs

Fig. 2. Sediment distribution of the northwestern Gulf of Mexico as an example of the sediments of a shelf with abundant land-derived sediment. Shelf depositional sediments and slope deposits are silty clays; nondepositional area is covered with relict sediments. Arrows show generalized circulation.

weathering and erosion of land, supplied by streams and coastal erosion, such as gravels, sand, silt, and clay). See AUTHIGENIC MINERALS. On shelves with abundant land-derived sediment, the coastal zone is composed of deltas, lagoons, bays, and beaches and barriers. Outside the beaches and barriers, a narrow strip of wave-transported sand, usually less than 2 or 3 mi (3 or 5 km) wide, fringes the coast. On the open shelf, the sediment deposited under present conditions is a silty clay, which, near deltas, grades imperceptibly into its bottomset beds. Usually, the silty clay, which results from winnowing near the coast, is carried no more than 20–30 mi (32–48 km) offshore by marine currents, so that the zone of active deposition is restricted. If the shelf is narrow, all of it will fall into this zone, but if it is wide, the outer part will be covered by relict sediments resulting from the recent transgression. These relict sediments were deposited near the migrating shoreline and consist of beach sands and thin lagoonal deposits. They have been extensively churned by burrowing animals and wave action, resulting in a mottled structure, and authigenic glauconite has formed in them. See TRACE FOSSILS. On many shelves, small calcareous reefs (shelfedge reefs) occur at the outer edge. These reefs apparently depend on the presence of deep water for their growth, although it is not certain that they are growing vigorously at the present time. In the Gulf of Mexico, where they are particularly abundant, they mark the tops of salt domes in the subsurface. Beyond the reefs begins the zone of slope deposition, where in deeper and quiet water silty clays with abundant calcareous remains of open water organisms are being slowly deposited. Thus, there are in principle four parallel zones on each shelf: an inner sandy zone; an intermediate zone of clay deposition; an outer shelf zone of no deposition, where relict sediments occur, terminating in edge reefs; and a slope zone of calcareous clays. This parallel zonation is often strongly modified by special current patterns, which carry fine sediments farther out across the shelf, as in the western Gulf of Mexico; by rapidly advancing deltas that provide a sediment source far out on the shelf, as in the Mississippi delta; or by exposure to unusually vigorous wave action that prevents fine sediments from being deposited, even though a supply is present. The fine-grained deposits that are being formed tend to be deposited more rapidly near the source than farther away, and as a result contain more biogenous material with increasing distance from the source, so that they become more calcareous (Fig. 2). On shelves with little or no land-derived material, the only available sources of sediment are biogenous and authigenic. These sources provide far less material than rivers do, and as a result sedimentation rates are much lower. Even on shelves with abundant supply of land-derived material, the areas of nondeposition are extensive, often 40–50% of the total area. On the calcareous shelves, relict sediment may cover up to 75% of the entire area. Near the shelf

Marine sediments algae-coral reefs

0 25 50 75 nmi

ms ho

10 fat f

al

ico ex tite M f lcilu o a it e c f l en ral Gu c ar l inife a m c let fora - pel oolite e renit alca c l ta s ele thom sk 10 fa m) 3 . 8 (1

0 50 100 150 km

fer

ll calcarenite she es renite lcilutit alca nite and ca lc ca re ga cal al

ini am or ico of Mex

100 fathoms (183 m) (a)

Florida

and tz s quar it e al tion ren t r a n s i c al c a q u a r tz ose

Gulf

edge, these relict sediments are shallow-water deposits formed during the last low stand of the sea, and consist of small algal reefs and oolites as described for the Bahamas. See OOLITE. Landward of this zone, the deposits, formed when sea level rose rapidly, are thin blankets of calcareous debris, consisting of shell material, bryozoa, coral debris, and so forth (Fig. 3). The only active sedimentation zones occur very near the shore, where calcareous and sometimes land-derived sand, silt, and clay are being deposited at present, and at the outermost shelf margin and continental slope, where a blanket of fine-grained calcareous mud very rich in planktonic Foraminiferida is found (foraminiferid calcilutite and calcarenite). Calcareous shelves occur in the Persian Gulf and off the coasts of Australia. The rise of sea level has, in many regions, severely restricted the supply of sediment to the continental shelf. Along the eastern coast of the United States, the valleys of many rivers have been flooded, and during the present stable sea level, barriers have been built across them which restrict the escape of sediment from the estuaries. As a result, sedimentation on this shelf is slow, and relict transgressive sands occur nearly everywhere at the surface. Much of the fine-grained sediment transported into the sea by rivers is not permanently deposited on the shelf but kept in suspension by waves. This material is slowly carried across the shelf by currents and by gravity flow down its gentle slope, and is finally deposited either on the continental slope or in the deep sea. If submarine canyons occur in the area, they may intercept these clouds, or suspended material, channel them, and transport them far into the deep ocean as turbidity currents. If the canyons intersect the nearshore zone where sand is transported, they can carry this material also out into deep water over great distances. See SUBMARINE CANYON; TURBIDITY CURRENT. Complex sediment patterns form in areas of considerable relief, for example, the borderland off southern California, where very coarse relict and residual sediments on shallow banks alternate rapidly with silty clays and calcareous deposits in the deep troughs. Such cases, however, are rare along the continental margins. Unimportant, but striking and geologically interesting, are the calcareous sediments associated with coral reefs and atolls. Usually, they occur on islands in mid-ocean, where clear water with abundant nutrients is available, and landderived sediments are absent; but fringing and barrier reefs with associated calcareous sediments also occur along coasts with low sediment supply. See CONTINENTAL MARGIN; REEF. Tjeerd H. Van Andel Deep-sea sediments. Sediments covering the floor of the deep sea were first systematically described and classified during the late nineteenth century by J. Murray and A. F. Renard (1884, 1891) after their observations during the Challenger Expedition (1872– 1876). Their classification included two principal sediment types, terrigenous (sediments deposited near to and derived from continental areas) and

475

Yucatan Peninsula 100 fathoms (183 m) (b)

Fig. 3. Sediments on shelves with little land-derived sediment supply. (a) Off western coast of Florida. (b) Off northwestern Yucatan Peninsula.

pelagic (sediments, principally fine grained, accumulated slowly by settling of suspended material in those parts of the ocean farthest from land). Both categories include biogenic and nonbiogenic material, as well as sediment derived from continents, that is, terrigenous, making the classification, at best, difficult to apply. Some of the terms, such as pelagic red clay, however, have remained in general usage but with meanings somewhat modified from the original definitions. A problem with most classifications has been in distinguishing descriptive categories, for example, red clay, from genetic categories, that is, those that include an interpretation of sediment origins, for example, volcanic mud. In addition, classifications are difficult to apply because so many deep-sea sediments are widely ranging mixtures of two or more end-member sediment types. The following sections will briefly describe the most important end members, their manner of origin, and some of the factors that control their distribution. Biogenic sediments. Biogenic sediments, those that were formed from the skeletal remains of various kinds of marine organisms, may be distinguished according to the composition of the skeletal material, principally either calcium carbonate or opaline silica. The most abundant contributors of calcium carbonate to the deep-sea sediments are the planktonic foraminiferids, coccolithoforids, and pteropods. Organisms that extract silica from the seawater and whose hard parts eventually are added to the sediment are radiolaria, diatoms, and to a lesser degree, silicoflagellates and sponges. The degree to which deep-sea sediments in any area are composed of one or more of these biogenic types depends on the organic productivity of the various organisms in the surface water, the degree to which the skeletal remains are redissolved by seawater while settling to the bottom, and the rate of sedimentation of other types of sediment material. Where sediments are composed largely of a single type of biogenic

476

Marine sediments

60°

20° 60°

120°

180°

120°

60°

0°

20°

60° Key: nonbiogenic sediment

siliceous ooze

calcareous ooze

(a)

60°

20° 60°

120°

180°

120°

60°

0°

20°

60° Key: 30%

(b)

60°

20° 60°

120°

180°

120°

60°

0°

20°

60° Key: 70%

(c) Fig. 4. Distribution of deep-sea sediments. (a) Major types. (b) Kaolinite concentration. (c) Montmorillonite concentrations. The concentrations are in the 1000 (1830) Decomposes 1088 (1990) for CeH1.73

0.77 1.396 1.902 3.78 5.45

-----------

790 (1454) 425 (797) 960 (1760) 630 (1166) 750 (1382) for CeH2.2

10.95

---

440 (824)

Dark gray powder

∼ 1050 (1922) at 580 atm (58.8 MPa) Decomposes

Lanthanum nickel hydride

LaNi5 H6

---

---

PdH0.66

Metallic

Decomposes

∼ 10.8

---

Diborane Silane Stannane Arsine Stibine Tellurium hydride

B2 H6 SiH4 SnH4 AsH3 SbH3 TeH2

Colorless gas Colorless gas Colorless gas Colorless gas Colorless gas Colorless gas

⫺165.5 (⫺265.9) ⫺185 (⫺301) ⫺150 (⫺238) ⫺113.5 (⫺172.3) ⫺88.5 (⫺127.3) ⫺51 (⫺59.8)

⫺92.5 (⫺135) ⫺111.8 (⫺169.2) ⫺52 (⫺62) ⫺55 (⫺67) ⫺17 (1.4) ⫺4 (25)

Aluminum hydride Copper hydride

AlH3 CuH

White solid Dark brown solid

Decomposes Decomposes slowly even at 25 °C (77 °F)

0.438 at bp 0.68 at mp ----2.2 at bp 2.7 at ⫺18 °C (⫺0.4 °F) --6.39

0 (32) [2.5 atm at 25 °C or 0.25 MPa at 77 °F ] 25 (77) for PdH0.56 , ⫺78 (⫺108) for PdH0.83 -------------

Palladium hydride

-----

-----

sodium hydride has the same type of structure as sodium chloride. See CRYSTAL STRUCTURE. The thermal stability of the alkali metal hydrides decreases, and their chemical reactivity increases, in the order LiH, NaH, KH, RbH, and CsH. The hydride ion in these compounds has a radius of approximately 0.14 nanometer (about the same size as the fluoride ion, F−). The anion from heavy hydrogen, that is, the deuteride ion D−, tends to be slightly smaller than H−, when in an equivalent environment. When molten lithium hydride is electrolyzed, hydrogen is evolved at the anode, since H− is the migrating species. A common way to represent a reversible metalhydrogen system is to plot the composition against hydrogen pressure (called the dissociation pressure) at a fixed temperature. Figure 2 shows the 500 and 575◦C (932 and 1067◦F) isotherms for the sodium-hydrogen system. Pure liquid sodium at 575◦C (1067◦F) exists at point A, and as hydrogen is admitted, it dissolves in the metal up to the limit at point B. This is a solution of NaH (10%) in molten Na (90%). When more hydrogen is pumped in, a solid appears, whose composition is indicated by point C (NaH0.90, that is, sodium hydride deficient in hydrogen). During this stage (points B to C), the hydrogen pressure remains constant at 36.6 atm (3.7 MPa). The dissociation pressures at the various plateaus may be computed for any temperature by the equation log P = (−5960/T) + 8.59, where P is the pressure in atmospheres and T the absolute temperature. Further introduction of hydrogen finally yields pure NaH at point D. In the plateau region (B to C), each mole of hydrogen which reacts liberates 27 kcal (113 kilojoules) of heat. More heat is liberated in

the cases of lithium hydride (43 kcal or 180 kJ/mole of H2) and CaH2 (42 kcal or 176 kJ). See PHASE EQUILIBRIUM. The ionic hydrides are all exceedingly reactive reducing agents. Their reaction with water yields the metal hydroxide and hydrogen, which sometimes ignites at the high temperature resulting from the reaction. Sodium hydride dissolved in molten sodium hydroxide is employed in descaling steel and titanium. Calcium hydride is a source of hydrogen in remote areas, for example, to fill meteorological balloons. CaH2 is also used as a desiccant in transformer oil and in the reduction of certain oxides to metals, such as tantalum. Transition metal hydrides. This group of compounds is less well understood than the ionic and

80 70 hydrogen pressure, atm

Lithium hydride Sodium hydride Calcium hydride Titanium hydride Cerium hydride

Boiling point, ( °F) °C

Density, g/cm3

Temperature at which dissociation pressure is 1 atm or 102 kPa, °C ( °F)

60 D

50 40

575°C

B

C

30 20 A

10 0

0

500°C 20

40

80

60

composition, % NaH ◦

◦

Fig. 2. Sodium-hydrogen system. F = ( C × 1.8) + 32; 1 atm = 105 Pa.

100

754

Metal hydrides covalent hydrides. When lanthanum is heated in hydrogen, the gas is readily taken up, forming first a material whose composition is approximtely LaH1.86 (the “dihydride” phase) and finally LaH3. The electrical conductivity does not change radically between La and LaH1.86, but it falls dramatically from LaH1.86 to LaH3. The hydrogen is generally thought to form hydride ions, with radius about 0.13 nm, but some of the valence electrons remain free or mobile in the dihydride. This accounts for the conductivity of this substance. These mobile electrons are said to be in the conduction band. They are progressively consumed as the dihydride is converted to the trihydride. A single phase exists between LaH1.86 and LaH3, and it is possible to arrest the addition of hydrogen at any intermediate composition, such as LaH2.63. Such a substance is sometimes referred to as a solid solution and sometimes as a compound which represents extreme nonstoichiometry. These features (electrical conductivity and nonstoichiometry) characterize the transition metal hydrides. See NONSTOICHIOMETRIC COMPOUNDS. The ionic hydrides, such as NaH, exhibit much more limited nonstoichiometry. When sodium is converted to pure NaH, all electrons in the conduction band are consumed and the product is white and insulating. The hydrides of the rare-earth metals are the easiest to describe in terms of an ionic model. The hydrides of the actinide metals are known through curium, the best known being uranium hydride. All are dark-gray powders which ignite spontaneously in the air. Large single crystals of cerium hydride are fairly stable in air. Copper hydride, CuH, is unique in that it is the only hydride which can be precipitated from aqueous solution. It forms a dark-brown sediment which decomposes when dried. Copper hydride has a hexagonal crystal structure and is essentially a covalent compound. Titanium, zirconium, vanadium, and niobium all react with hydrogen, forming dihydrides at the limiting compositions. The materials are conducting at all compositions. Except for palladium, the metals to the right of titanium in the periodic table show decreased reactivity toward hydrogen. Metal of the vanadium family and also chromium absorb hydrogen almost to the monohydride stage. The transition metals in the manganese, iron, cobalt, and nickel groups dissolve only small amounts of hydrogen, except for palladium, which readily absorbs hydrogen to the limit PdH0.66. The palladium-hydrogen system has been much studied, because of its importance in catalytic hydrogenation. Thorium hydride, Th4H15, becomes superconducting at approximately 8 K (−445◦F). See PALLADIUM; SUPERCONDUCTIVITY. When hydrogen (protium, 1H) in a metal hydride is replaced with its isotope deuterium, the product (deuteride) is usually less stable. The dissociation pressures of the deuterides are about twice those of the corresponding hydrides at the same temperature in the great majority of cases. The inverse effect is known, however. For example, at 40◦C (104◦F) vanadium hydride (VH1.5) has a dissociation pressure of

4.8 atm (490 kPa), while VD1.5 has a dissociation pressure of 1.8 atm (182 kPa). The uranium compounds of the three isotopes of hydrogen (UH3, UD3, and UT3) are frequently used in the laboratory as sources and reservoirs for these gases and for their purification. Titanium hydride also serves in this capacity. Some of the hydrides serve as a starting point for the synthesis of halides, sulfides, and so forth, of the metal. For example, this is represented by reaction (1). Deuterides and tritides of actinide metals UH3 + 3HCl → UCl3 + 3H2

(1)

probably play an important role in thermonuclear (hydrogen) weapons. Common metals such as steel and copper dissolve small quantities of hydrogen at elevated temperatures. On cooling, the gas comes out of solution and results in severe degradation of the mechanical properties of the metals. This is called hydrogen embrittlement and can be prevented by degassing the metal while it is still molten. Problems from hydrogen embrittlement also arise in high-pressure catalytic units employing hydrogen gas and at electrode surfaces where hydrogen ion is reduced to hydrogen, as in some electroplating processes. The cause of this embrittlement is the introduction of dislocations and stacking faults by the hydrogen atoms. The result is to suppress dislocation glide near cracks, preventing stress relaxation. See CRYSTAL DEFECTS. A number of alloys or intermetallic compounds are known which react with hydrogen and form ternary hydrides. Examples are LaNi5H6, Mg2NiH4, AlTh2H2, and CaAg2H. LaNi5H6 permits an attractive means of portable hydrogen storage. This might be important if hydrogen is generated on a massive scale as a pollution-free energy source for fuel cells and internal combustion engines. Storage of hydrogen as compressed gas or as liquid is not convenient. The energy density of LaNi5H6 is twice that of liquid hydrogen and 12 times that of the compressed gas. The ternary hydride LaNi5H6 is readily formed reversibly from lanthanum-nickel alloy and hydrogen. Pure rareearth metals are not necessary in making the nickel alloy; the commercial material (misch metal, mostly Ce, La, and Nd) is satisfactory. After charging with hydrogen, these materials provide practical gas pressure (a few atmospheres) from −30 to +30◦C (−22 to 86◦F), and are thus attractive for powering hydrogenburning engines of automobiles. Owing to the highly reversible character of these metal-hydrogen systems, they have recently been studied as heat pumps and energy storage devices. To operate an air-conditioning unit, hydrogen is pumped from one metal hydride unit (such as LaNi5H6), which provides cooling, to an alloy unit (LaNi5), where heat is evolved on formation of hydride. Alternatively, solar or waste thermal energy could be converted into mechanical work. Another important ternary hydride is the Zr-U-H system, which is employed in pulsed reactors of the Triga type. The role of the hydrogen atom is to moderate fast neutrons. Another type of ternary hydride

Metal matrix composite is illustrated by the thorium carbohydrides, Th2Ch2 and Th3Ch4. See RARE-EARTH ELEMENTS. Covalent hydrides. Most evidence indicates that in ionic and metallic hydrides an electronic pair is associated primarily with the hydrogen as H-, while in covalent hydrides the electron pair is shared between the hydrogen atom and an atom of another element. In these compounds hydrogen is considerably smaller (radius 0.03 nm) than in the ionic hydrides. Covalent hydrides usually consist of small molecules, in which case they are gases (SiH4 and SbH3), but some form high polymers, in which case they are nonvolatile solids (AlH3 and ZnH2). All tend to decompose irreversibly rather easily on heating. Covalent hydrides are generally synthesized indirectly, not from direct combination of the elements. Gaseous hydrides such as SiH4, PH3, and AsH3 can be generated by heating a solid mixture of the corresponding oxide and LiAlH4. See CHEMICAL BONDING. The hydrides of boron deserve special mention, as their bonding posed a difficult problem in chemical theory which has been resolved only recently. In diborane, B2H6, four of the hydrogen atoms are bonded to the two boron atoms by ordinary single, covalent bonds. The other two hydrogen atoms are linked to both boron atoms, the four bonds being formed by only two electron pairs. The structure of diborane is illustrated in Fig. 3. The hydrogen atoms which bridge between the two boron atoms do not form an ordinary hydrogen bond but another type usually described as an electron-deficient bridging bond. There are nearly two dozen higher boron hydrides, of which B4H10, B5H9, B6H10, and B10H14 are representative. A group of compounds, the carboranes, has been discovered in which some of the boron atoms of the higher hydrides are replaced with carbon atoms. An example is B10C2H10. See BORANE; CARBORANE. Numerous derivatives of the boron hydrides have been prepared, of which sodium tetrahydroborate (or tetrahydridoborate or borohydride), NaBH4, is the most important. It is used as a selective reducing agent in organic chemistry. This hydrido complex is moderately stable in water, especially in alkaline solution. This stability permits its use in recovery of heavy metals from industrial waste streams. See HYDROBORATION. Aluminum hydride, AlH3, normally exists as an insoluble, nonvolatile polymer. Its derivative, lithium tetrahydroaluminate (or lithium aluminum hydride), LiAlH4, is an important reducing agent in both organic and inorganic chemistry. For example, silane can be prepared H H

H B

B

H

H H Fig. 3. Structure of diborane, B2H6.

as in reaction (2). Lithium tetrahydroaluminate ether

SiCl4 + LiAlH4 −−−→ SiH4 + LiCl + AlCl3

(2)

reacts vigorously with water. An ether is generally used as the solvent. Sodium cyanotrihydroborate, NaBH3CN, can be used in acidic medium. A family of aluminum-based reducing agents is now available commercially including sodium diethyl-dihydroaluminate, NaAlH2(C2H5)2; sodium tri-tert-butoxohydroaluminate, NaAlH(O-t-C4H9)3; sodium bis(2-methoxyethoxo)dihydroaluminate, NaAlH2(OCO2CH2OCH3)2; diethylaluminum hydride, (C2H5)2AlH; and di-iso-butylaluminum hydride, (i-C4H9)2AlH. All are soluble in aromatic hydrocarbons. They are rather expensive, but their specific reducing powers make them attractive for synthesizing high-value products, such as pharmaceuticals, flavorings, fragrances, dyes, and insecticides. There are thousands of hydrocarbons, which represent hydrogen compounds of carbon, the first element in group 14. The simplest of these compounds is methane, CH4. Other 14 hydrogen compounds are silane, SiH4; germane, GeH4; stannane, SnH4; and plumbane, PbH4. This series is progressively less stable, with stannane decomposing at room temperature. Higher hydrides of silicon are known, Si2H6 and so on up to Si10H22. These compounds, SinH2n+2, are analogous to the corresponding alkanes, CnH2n+2. Various silane derivates, such as SiH3Hl and (SiH3)3N, have been synthesized. A parallel situation exists in the case of higher germanium hydrides. Unstable distannane, Sn2H6, has also been made. Mixed compounds such as SiH3GeH3 can be prepared. The group 15 hydrides show the same stability trends as the preceding group, with stability decreasing in the order NH3, PH3, AsH3, SbH3, and BiH3. Group 16 hydrides, including H2Se, H2Te, and H2Po, also grow less stable down the series, the same trends continuing in the case of hydrogen compounds of the halogens. These compounds in the last two series, such as H2Se and HI, are ordinarily considered as acids. See HYDRIDE. James C. Warf Bibliography. A. Dedieu, Transition Metal Hydrides, 1992; E. L. Muetterties (ed.), Boron Hydride Chemistry, 1975.

Metal matrix composite A material in which a continuous metallic phase (the matrix) is combined with another phase (the reinforcement) that constitutes a few percent to around 50% of the material’s total volume. In the strictest sense, metal matrix composite materials are not produced by conventional alloying. This feature differentiates most metal matrix composites from many other multiphase metallic materials, such as pearlitic steels or hypereutectic aluminum-silicon alloys. See ALLOY. The particular benefits exhibited by metal matrix composites, such as lower density, increased specific

755

756

Metal matrix composite strength and stiffness, increased high-temperature performance limits, and improved wear-abrasion resistance, are dependent on the properties of the matrix alloy and of the reinforcing phase. The selection of the matrix is empirically based, using readily available alloys; and the major consideration is the nature of the reinforcing phase. Matrices and reinforcements. A large variety of metal matrix composite materials exist. The reinforcing phase can be fibrous, platelike, or equiaxed (having equal dimensions in all directions); and its size can also vary widely, from about 0.1 to more than 100 micrometers. Matrices based on most engineering metals have been explored, including aluminum, magnesium, zinc, copper, titanium, nickel, cobalt, iron, and various aluminides. This wide variety of systems has led to an equally wide spectrum of properties for these materials and of processing methods used for their fabrication. Reinforcements used in metal matrix composites fall in five categories: continuous fibers, short fibers, whiskers, equiaxed particles, and interconnected networks. Continuous fibers. Several continuous fibers or filaments are used in metal matrix composites. Their elastic moduli vary significantly, depending on the nature of the fiber and its fabrication process. For example, silica-alumina spinels and microcrystalline or amorphous polycarbosilane-derived fibers possess significantly lower elastic moduli than do pure alumina or crystalline β-silicon carbide produced by chemical vapor deposition. Carbon fiber strength and modulus also vary significantly with processing, depending on the level of graphitization of the microstructure. See ALUMINUM; ELASTICITY; GRAPHITE; SPINEL. Short fibers. Short fibers are less expensive, especially when they are mass-produced for other applications such as high-temperature thermal insulation. Their physical properties can be similar to those of continuous fibers; however their reinforcing efficiency in the matrix is also far lower. Short fibers used in engineering practice include chopped carbon fibers and alumina-silica fibers. Whiskers. Whiskers are single-crystal short fibers, produced to feature highly desirable mechanical properties due to lack of microstructural defects. Whiskers have typically been made of silicon carbide, and they are often priced far higher than short fibers. The high price and toxicity of most whiskers have prevented their application in engineering practice. Single-crystal whiskers, because of the absence of grain boundary defects, offer much higher tensile strength than other types of discontinuous reinforcements, and thus they are preferred for certain applications of discontinuously reinforced metal matrix composities. The whiskers can be aligned to a preferred orientation by conventional metallurgical processes; higher directional strengths can be achieved in finished components where fabrication is by extrusion, rolling, forging, or superplastic forming. Whiskers tend to produce anisotropic

properties due to their alignment during processing, while particulate materials usually produce essentially isotropic properties. See CRYSTAL WHISKERS; GRAIN BOUNDARIES; SILICON. See also EXTRUSION; FORGING; METAL ROLLING; SUPERPLASTICITY. Equiaxed particles. Equiaxed particles of several ceramics, including those containing silicon carbide, aluminum oxide, boron carbide, and tungsten carbide, do not provide the possibility for preferential strengthening of the matrix along selected directions; however, their price is low and their combination with the metal is relatively easier. These reinforcements are therefore used in many metalmatrix composite systems, including mass-produced aluminum matrix composites. Interconnected cellular networks. These can be produced by several methods, such as by chemical vapor deposition of ceramic onto a pyrolizable polymer foam or by conversion of a preceramic polymer foam prior to infiltration with the molten matrix. Alternatively, some processing techniques for in-place metal matrix composites, including directional oxidation of aluminum melts, produce interconnected reinforcing networks. In-place metal matrix composites comprise those in which the reinforcement is produced from the matrix; although these materials are actually in some cases produced by alloying, their designation as metal matrix composites has resulted from the special morphology of the second phase (for example, some directionally solidified eutectics that have fibrous second phase) or from the ceramic nature of the second phase (for example, titanium-carbidereinforced steel produced by solidification of irontitanium-carbon alloys). Microstructure. The microstructure of a metal matrix composite comprises the structure of matrix and reinforcement, that is, the interface and the distribution of the reinforcement within the matrix. The distribution of the reinforcement within the composite can be controlled to some extent during processing. Large modifications can be handled individually and distributed with relatively great precision within the composite. Continuous fibers of smaller diameter, in the range of 10 µm or less, are generally combined with the matrix by infiltration. The fibers then tend to concentrate into regions of high-volume fraction, near 50 vol %, within which individual fibers are relatively randomly distributed. Greater control of the reinforcement volume fraction can be exerted with short fibers and whiskers because of bending of fibers between their contact points. In particle-reinforced metal composites, the reinforcement distribution is affected by solidification of the matrix in liquid-phase processes, because the growing solid phase may push the particles as it grows. In powder metallurgical processes, reinforcement particle clustering may result from unequal sizes of matrix and reinforcement powder particles. At the interface between matrix and reinforcement, intimate atomic-level bonding is typically achieved between matrix and reinforcement.

Metal matrix composite Microstructural evolution of the matrix is governed by the same basic laws that govern the microstructural evolution of all metals; however, the reinforcement can exert a strong influence on the course taken by the evolving matrix microstructure. In solidification, for example, the reinforcement has been shown to strongly reduce microsegregation in the matrix, to influence the stability of plane solidification during cooling, and to alter the coarsening of matrix microstructure. In solid-state deformation processes, the reinforcement has been shown to alter the evolution of texture in the matrix, the kinetics of recrystallization, and the grain size after recrystallization. See METAL. Composite properties. Composite properties depend first and foremost on the nature of the composite; however, certain detailed microstructural features of the composite can exert a significant influence on its behavior. Physical properties of the metal, which can be significantly altered by addition of a reinforcement, are chiefly dependent on the reinforcement distribution. A good example is aluminum-silicon carbide composites, for which the presence of the ceramic increases substantially the elastic modulus of the metal with-

apply coating continuous fiber

continuous strand of short aligned fibers

out greatly affecting its density. Elastic moduli for 6061 aluminum matrix composites reinforced with discrete silicon carbide particles or whiskers have been calculated by using the rule of mixtures for the same matrix reinforced with two types of commercial continuous silicon carbide fibers. As a result, several general facts become apparent. First, modulus improvements are significant, even with equiaxed silicon carbide particles, which are far less expensive than fibers or whiskers. However, the level of improvement depends on the shape and alignment of the silicon carbide. Also, it depends on the processing of the reinforcement: for the same reinforcement shape (continuous fibers), microcrystalline polycarbosilane-derived silicon carbide fibers yield much lower improvements than do crystalline β-silicon carbide fibers. These features, of influence of reinforcement shape, orientation, and processing on modulus, are quite general, being also observed for example in metal matrix composites reinforced with aluminum oxide or carbon. Other properties, such as the strength of metal matrix composites, depend in a much more complex manner on composite microstructure. The strength of a fiber-reinforced composite, for example, is

filament wind coated fibers or co-deposit matrix

hot press

transverse microstructure

draw through melt, slurry plasma spray electroplate vapor deposit alternate layers of fibers and matrix (sheet)

diffusion bond

alternate layers of fibers and matrix (powder)

hot press

infiltrate matrix in liquid stage

eutectic liquid

solidify

solidify unidirectionally

solid liquid Fig. 1. Examples of methods used to make metal matrix composites. (Space Sciences Laboratory, General Electric Valley Forge Space Technology Center, King Prussia, Pennsylvania, 1967)

757

758

Metal matrix composite determined by fracture processes, themselves governed by a combination of microstructural phenomena and features. These include plastic deformation of the matrix, the presence of brittle phases in the matrix, the strength of the interface, the distribution of flaws in the reinforcement, and the distribution of the reinforcement within the composite. Consequently, predicting the strength of the composite from that of its constituent phases is generally difficult. See BRITTLENESS; PLASTIC DEFORMATION OF METAL. Design considerations. In general, metal matrix composites exhibit good creep resistance (deformation under constant stress or load), fatigue properties (response to cyclic loading), and toughness (ability to absorb energy on impact without failure). These properties, in combination with strength and stiffness, are used by the designer in selection of a material for structural applications. Also important to the designer is the fact that metal matrix composites are anisotropic; that is, the mechanical properties depend critically on the orientation of the fibers with respect to the applied force and on the nature of the stress (tensile or compressive). Anisotropy can be an advantage or disadvantage, depending on the state of stress on the part in service. See CREEP (MATERIALS); METAL, MECHANICAL PROPERTIES OF; STRESS AND STRAIN. Production. A variety of techniques are available for the production of continuous or discontinuous metal matrix composites. These may be broadly classified as diffusion processes, deposition processes, and liquid processes (Fig. 1). Diffusion processes. These include the pressing and sintering of a powder metal matrix and bare or coated fibers, and the hot or cold pressing of the reinforcement between thin foils of the matrix metal or alloy. In diffusion bonding, a stack of matrix foils are pressed around rows of relatively large-diameter fibers. Another solid-state process blends powdered matrix and reinforcement, and the matrix is densified around the reinforcement by conventional sintering or hot-pressing processes. See POWDER METALLURGY; SINTERING. Deposition processes. Primary techniques are electrode-positioning of the matrix around fibers, plasma

spraying of the matrix around fibers, or vacuum deposition of the matrix around fibers. In each case, it is usual to complete the process by cold or hot pressing. See ELECTROPLATING OF METALS; VACUUM METALLURGY. Liquid processes. The important techniques in this category are infiltration of the liquid matrix around fibers, the pressing of powdered matrix and fibers at a temperature above the melting point of the matrix, and unidirectional solidification of eutectic alloys. See EUTECTICS. Other processes. Spray casting processes (another class of liquid-state processes) involve spraying the matrix with or onto the reinforcement; solidification is achieved by heat extraction to the supporting substrate. There are additional processes that involve a partly liquid matrix. These include compocasting (a dispersion process using the matrix in semisolid form) and liquid-phase sintering processes. In-place processes create the reinforcement from within the matrix. This can be accomplished by simple alloying and solidification, as is the case with directionally solidified eutectic alloys. Alternatively, the reinforcement can be formed by reaction of elements contained within the matrix in another phase such as a gas. Fabrication. Composite processing methods combine the reinforcement with the matrix. This is accomplished while the matrix is either solid or liquid. Typical liquid-state processes include the dispersion processes, which are casting techniques. A second set of processes involves liquid-metal impregnation; these include squeeze casting, where a preform or a bed of dispersoids is impregnated by molten alloy under hydraulic pressure. A third set comprises spray processes. In one of these, a molten metal stream is fragmented by means of a high-speed cold inert-gas jet passing through a spray gun, and dispersoid powders are simultaneously injected. A stream of molten droplets and dispersoid powders is directed toward a collector substrate where droplets recombine and solidify to form a high-density deposit. In rapid solidification processing of composites, a jet of liquid alloy–particle slurry impinges under pressure on a water-cooled copper wheel (Fig. 2),

pressure

consolidation billet

can composite melt collection

extrusion extrusion

water-cooled copper wheel

can

Fig. 2. Steps involved in rapid solidification processing of metal matrix composites.

conventional forging

Metal rolling Some composite components with proven potential Composite Aluminum–silicon carbide (particle)

Aluminum–silicon carbide (whiskers) Magnesum–silicon carbide (particle) Aluminum–aluminum oxide (short fibers)

Aluminum–aluminum oxide (long fibers) Copper-graphite

Aluminum-graphite Aluminum–titanium carbide (particle) Aluminum–fiber flax Aluminum–aluminum oxide fibers–carbon fibers

Components

Advantages

Piston Brake rotor, caliper, liner Propeller shaft Connecting rod

Sprockets, pulleys, and covers Piston ring Piston crown (combustion bowl) Connecting rod Electrical contact strips, electronics packaging, bearings Cylinder, liner piston, bearings Piston, connecting rod Piston Engine block

and the resulting flake powders are collected. The powder is put into cans and consolidated into billet or extruded to form a dense composite with higher yield strength, ultimate tensile strength, and ultimate ductility. See METALLIC GLASSES. Depending on the process, the desired microstructure, and the desired part, metal matrix composites can be produced to net or near-net shape; or alternatively they can be produced as billet or ingot material for secondary shaping and processing. Applications. The combined attributes of metal matrix composites, together with the costs of fabrication, vary widely with the nature of the material, the processing methods, and the quality of the product. In engineering, the type of composite used and its application vary significantly, as do the attributes that drive the choice of metal matrix composites in design (see table). For example, high specific modulus, low cost, and high weldability of extruded aluminum oxide particle–reinforced aluminum are the properties desirable for bicycle frames. High wear resistance, low weight, low cost, improved hightemperature properties, and the possibility for incorporation in a larger part of unreinforced aluminum are the considerations for design of diesel engine pistons. See COMPOSITE MATERIAL; HIGH-TEMPERATURE MATERIALS. Mel M. Schwartz Bibliography. American Society for Metals, ASM Metals Handbook, vol. 2, 9th ed., 1991; S. M. Lee, Reference Book for Composites Technology, 1989; S. Ray, Review synthesis of cast metal matrix particulate composites, J. Mater. Sci., 28:5397–5413, 1993; T. Rienhart, Engineered Materials Handbook: Composites, vol. 1, 1988; M. M. Schwartz, Composite Materials Handbook, 2d ed., 1992; S. Suresh, A. Mortensen, and A. Needleman (eds.), Fundamentals of Metal Matrix Composites, 1993.

Reduced weight, high strength and wear resistance High wear resistance, reduced weight Reduced weight, high specific stiffness Reduced reciprocating mass, high specific strength and stiffness, low coefficient of thermal expansion Reduced weight, high strength and stiffness Wear resistance, high running temperature Reduced reciprocating mass, high creep and fatigue resistance Reduced reciprocating mass, improved strength and stiffness Low friction and wear, low coefficient of thermal expansion Call resistance, reduced friction, wear, and weight Reduced weight and wear Reduced weight and wear Reduced weight, improved strength and wear resistance

Metal rolling Reducing or changing the cross-sectional area of a workpiece by the compressive forces exerted by rotating rolls. The original material fed into the rolls is usually an ingot from a foundry. The largest product in hot rolling is called a bloom; by successive hotand cold-rolling operations the bloom is reduced to a billet, slab, plate, sheet, strip, and foil, in decreasing order of thickness and size. The initial breakdown of the ingot by rolling changes the coarse-grained, brittle, and porous structure into a wrought structure with greater ductility and finer grain size. Process. A schematic presentation of the rolling process, in which the thickness of the metal is reduced as it passes through the rolls, is shown in Fig. 1a . The speed at which the metal moves during rolling changes, as shown in Fig. 1b, to keep the volume rate of flow constant throughout the roll gap. Hence, as the thickness decreases, the velocity increases; however, the surface speed of a point on the roll is constant, and there is therefore relative sliding between the roll and the strip. The direction of this relative velocity changes at a point along the contact area, this point being known as the neutral or no-slip point. At the neutral point, roll and strip have the same velocity; to the left of this point (entry side) the strip moves more slowly than the roll; and to the right of this point (exit) it moves faster. Hence, the direction of frictional forces acting on the strip are opposite in these two regions, as shown in Fig. 1a. The net frictional force acting on the strip must be in the direction of exit to enable the rolling operation to take place. Although friction is a disadvantage in many metal-working processes, it is a necessity in rolling; without friction the rolls cannot pull the strip through the roll gap. It has been observed that in hot rolling the coefficient of friction may be as

759

760

Metal rolling roll

Vf

Vi

(a)

Vf Vroll Vi (b)

(c) Fig. 1. Rolling process. (a) Direction of friction forces in the roll gap. (b) Velocity distribution. (c) Normal pressure acting on the strip in the roll gap.

much as 0.7, while in cold rolling it generally ranges 0.02–0.3. The normal pressure distribution on the roll and hence on the strip is of the form shown in Fig. 1c. Because of its particular shape this pressure distribution is known as the friction hill. The overall slope of this hill depends on the coefficient of friction and the ratio of roll-strip contact length to the thickness of the strip. The size of the friction hill (area under the curve) represents the magnitude of the roll separating force per unit width that tends to push the rolls apart. This force can be reduced by increasing the workpiece temperature, reducing friction, taking smaller bites, and using smaller roll radii. Another method of reducing this force is by applying tension to the strip; this lowers the apparent compressive flow stress of the material. The tension can be back tension (entry) or front tension (exit) or both. Depending on the magnitude of these tensions, the neutral point shifts from its original position. The roll-separating force can become so great that it is not possible to reduce further the thickness of the strip during the particular pass. With such a force the rolls deform elastically (roll flattening). In such cases rolling may be accomplished by improving lubrication, rolling a stack of sheets at the same time, or annealing the strip. The maximum possible draft (the difference between the initial and final thickness of the strip) is a function of the friction coefficient an roll radius; it can be shown analytically that maximum draft is equal to the coefficient of friction squared times the roll radius. The workpiece may become wider during rolling; this is called spreading. It is found that spreading increases with the thickness-width ratio of the workpiece and with decreasing coefficient of friction and decreasing radius-thickness ratio of the roll.

Practice. A great variety of roll arrangements and equipment are used in rolling. Some basic types are shown in Fig. 2. In another method (Steckel rolling) the strip is pulled through idler rolls by front tension only. The proper reduction per pass in rolling depends on the type of material and other factors; for soft, nonferrous metals, reductions are usually high, while for high-strength alloys they are small. Temperatures in the hot rolling of various metals and alloys are similar to those in forging, namely, for aluminum alloys, 750–850◦F (400–450◦C); copper alloys, 1500◦F (820◦C); alloy steels, 1700–2300◦F (930–1260◦C); titanium alloys, 1400–1800◦F (760– 980◦C); and refractory metal alloys, 1800–3000◦F (980–1650◦C). Rolling speeds range up to about 5000 ft/min (1500 m/min). Although hot rolling has the advantage of lowering forces and increasing the ductility of the metal, cold rolling gives smoother finish, closer tolerances, and increased strength. The thickness of a rolled sheet is identified by gage number: The smaller the number the thicker the sheet. Actual thickness depends on the particular standard used; there are different standards, depending on the type of metal. The terminology for

+ + +

+

+

+ +

+

+

(c)

(b)

(a)

(d)

+

+

+

+

+

+

(e)

support roll

planetary rolls

cage (f) Fig. 2. Basic types of roll arrangement. (a) Two-high. (b) Three-high. (c) Four-high. (d) Cluster. (e) Tandem rolling with three stands. (f) Planetary mill.

Metallic glasses hardness for rolled sheets is shown in the table. A sheet that is reduced in thickness by one gage number from the annealed condition is called quarter hard, by two gage numbers half hard, and so on, as listed in the table. The dead-soft condition is fully annealed. Successful rolling practice requires a careful balance of factors such as reduction per pass, control of temperature, lubrication, roll size and finish, and intermediate annealing. Requirements for roll materials are mainly strength and resistance to wear. Common roll materials are cast iron, cast steel, and forged steel. In cold rolling, cemented carbide is also used for small rolls. To avoid variation in strip thickness due to bending of the rolls, rolls are ground to a particular geometry, called camber, whereby the center of the roll has a diameter of a few thousandths of an inch greater than the diameter of its ends. Hot rolling is usually carried out without a lubricant, although graphite or grease may be used. Cold rolling is carried out with low-viscosity lubricants, paraffin being a suitable lubricant for nonferrous metals. The type of residual stresses in the strip depends on the reduction and also on roll radius. Small rolls or small reductions produce compressive residual stresses on the surface of the strip and tensile stresses in the central portions, whereas large rolls produce the opposite residual stress pattern. A number of defects (wavy edges, zipper breaks, edge cracks, alligatoring) can result during the rolling process. These are usually eliminated or reduced by changes in operating variables or by special techniques. Miscellaneous processes. Most seamless tubing is produced by a technique based on the principle that the inside of a round rod is subjected to secondary tensile stresses when compressed radially (Fig. 3a). A simple demonstration of this can be made by rolling an eraser, removed from the end of a pencil, between a flat surface and a ruler that is moved back and forth (Fig. 3b). In a short time a hole is produced in the center of the eraser. The roll-piercing of round bars to make seamless tubing (Mannesmann process) is based on this principle (Fig. 3c). The compressive radial forces are supplied by two rolls with their axes in parallel planes, but at an angle to each other so as to move the workpiece through the rolls. The rolling action causes the center of the billet to rupture. The purpose of the mandrel is to expand the tube and

Hardness terminology for rolled sheets

Terminology

Increase in gage number

Annealed (dead-soft) 1 /8 hard 1 /4 hard 1 /2 hard 3 /4 hard Hard Extra hard Spring hard Extra spring hard

0 — 1 2 3 4 6 8 10

Reduction in thickness, % 0 6 11 21 29 37 50 60 69

(a) (b)

(c)

Fig. 3. Development of the Mannesmann process for producing seamless tubing (a–c explained in text).

improve the surface finish of the inside. For this process to be successful, the billet material should be of high quality and free of defects. In the ring-rolling process, a thin, large-diameter ring is produced from a thicker and smaller-diameter ring. This is accomplished by placing the ring between two rotating rolls; the reduction in thickness of the ring is compensated by an increase in diameter; there is little or no change in the width of the ring. Sections such as railroad tracks and I beams are also rolled by passing the stock through a number of specially designed rolls with their axes placed in different directions. Roll-pass design for such sections requires considerable experience to avoid defective products and to obtain desirable properties. Metal powders are also rolled into sheets or strip by special techniques. See POWDER METALLURGY. Serope Kalpakjian Bibliography. R. Pradhan and G. Lyudkovsky (eds.), Hot- and Cold-Rolled Sheet Steels, 1988; W. L. Roberts, Cold Rolling of Steel, vol. 2, 1978.

Metallic glasses Metals and metallic alloys having an amorphous structure on the atomic scale. Although the word “glass” is commonly used to refer to the familiar transparent oxide glasses (such as silicate glasses used to make windowpanes), in a more general sense a glass is any solid obtained from a liquid that does not crystallize upon cooling. As a result, a glass retains an atomic-scale structure in which the atoms are more or less randomly arranged, similar to that of the liquid state. Because most metals crystallize quickly, special care must be taken with processing and alloy design to produce a metallic glass. Metallic glasses produced directly by quenching the liquid are quite similar in terms of structure and properties to amorphous alloys (of the same chemical composition) produced by other means, such as physical vapor deposition, irradiation, mechanical alloying, or solid-state reaction. See AMORPHOUS SOLID; CRYSTAL; GLASS; GLASS TRANSITION. Preparation. When cooled below its equilibrium melting temperature, a liquid becomes unstable and there is a tendency for the stable crystalline phase (or phases) to form. Because of the significant structural change that occurs, crystallization does not happen

761

Metallic glasses

liquid

temperature

melting temperature

1

crystalline

supercooled liquid

glass transition temperature

3 glass 0

2 time

Fig. 1. Time required for crystallization of amorphous alloys below the equilibrium melting temperature.

instantaneously, and it is therefore possible to cool a liquid below its melting temperature without crystallizing it, for a limited time (Fig. 1). Such a liquid is said to be supercooled. Crystallization occurs slowly at temperatures just below the melting temperature, where it is difficult to nucleate new crystals, and at low temperature because atomic diffusion is slow. There is an intermediate temperature at which crystallization occurs most rapidly. If a liquid is cooled slowly but continuously (curve 1 in Fig. 1), crystallization will begin at a temperature below the melting temperature and will be completed at a later time (and lower temperature). A liquid may be cooled so quickly that there is no time for crystallization to occur at all (curve 2). Instead, at a lower temperature called the glass transition temperature, the supercooled liquid transforms to an amorphous solid, or glass. The slowest rate at which the liquid can be cooled and form a glass, while avoiding crystallization, is called the critical cooling rate (curve 3). The critical cooling rate is a measure of how easily a material forms a glass, with low critical cooling rates indicating easy glass formation. In materials with complicated crystal structures or in which the required atomic rearrangements are difficult, crystallization occurs slowly. Such materials, including many oxides and polymers, form glasses easily and have critical cooling rates on the order of 10−2 to 10−4 K/s (10−2 to 10−4 ◦F/s). Most metals have relatively simple crystal structures and crystallize readily. The critical cooling rate for pure metallic elements is on the order of 1013 to 1014 K/s (1013 to 1014 ◦F/s), rates which can only be achieved in very small volumes of material under specialized laboratory conditions. The critical cooling rate of an alloy can be much lower than that of a pure element, making production of metallic glasses feasible. The first true metallic glass, produced in 1960, was an alloy of gold and silicon and was cooled at a rate of approximately 2 × 106 K/s (4 × 106 ◦F/s). While still quite high, cooling rates on this order can be easily achieved by a variety of rapid solidification techniques, such as single-roller

melt spinning. These techniques enabled the production of commercial quantities of metallic glasses, beginning in the 1970s. A significant limitation of the high cooling rates required was that the early metallic glasses could be produced only in forms in which one dimension of the material was small (typically 50–100 µm), such as ribbons, foils, wires, and powders. Recently, more complex alloys have been developed in which the critical cooling rate is as low at 1 K/s (2◦F/s) or even lower. These cooling rates can be achieved via common metallurgical processing techniques, such as die casting into a metallic mold, enabling the production of metallic glass components with minimum thicknesses in excess of 1 cm (0.4 in.). These easily processed alloys are commonly called bulk metallic glasses. Furthermore, a bulk metallic glass can be reheated to a temperature above its glass transition temperature, without crystallization, causing it to soften dramatically and allowing it to be easily formed into complex shapes in much the same way as is commonly done with amorphous thermoplastic polymers. However, because the crystalline form is still the stable state, all metallic glasses will crystallize if subjected to elevated temperatures for long times. See METAL CASTING. Structure. The differences between the highly regular atomic positions characteristic of a crystal and the disordered structure of a liquid or glass can be explored using techniques based on the scattering of radiation such as x-rays, electrons, or neutrons. In crystalline materials, the regular atomic positions cause constructive or destructive interference among the scattered waves, leading to the formation of sharp, intense diffraction peaks at particular scattering angles that are characteristic of the structure of the material and of the wavelength of the radiation (Fig. 2a). The peaks from amorphous materials by comparison are quite broad and weak (Fig. 2b). A truly random structure (such as that of a monatomic gas) would show no peaks at all, so the presence of the peaks is proof that there is short-range order (over a few atomic distances) in the structure of glasses and liquids. Fourier transformation of the scattering data produces a radial distribution function (Fig. 3), which describes the atomic environment around an

intensity, a.u.

762

crystalline

amorphous 0

10 20 30 scattering angle, degrees

Fig. 2. X-ray diffraction patterns for crystalline and amorphous Pd40Ni40P20 alloys.

40

radial distribution function, atoms/Å

Metallic glasses ume of material), given by Eq. (2), is called the re-

30 25

Uel =

20 15 10 5 0 0

1 2 3 4 5 distance from average atom, Å

6

Fig. 3. Radial distribution function for amorphous Pd40Ni40P20. The shaded region represents the first coordination shell of atoms around an average atom in the glass.

average atom in the material. From the radial distribution function, one can determine the number of atoms in the nearest-neighbor shell (the coordination number) as well as the distance between the average atom and its nearest neighbors. If the constituent elements are similar in size, then the average coordination number is approximately 12, just as it is in a close-packed crystalline structure. That metallic glasses are nearly as close packed is also reflected in their densities, which are typically just 0.5–2% lower than crystalline phases of similar chemical composition. See FOURIER SERIES AND TRANSFORMS; X-RAY DIFFRACTION. Metallic glasses are also notable for their lack of structure over length scales longer than the atomic. Conventional crystalline alloys have microstructural features such as grains and precipitates with typical sizes of 0.1–100 µm; these features make fabrication of very small components difficult and lead to anisotropic mechanical properties that can be problematic. Because metallic glasses have no structural features on these length scales, they should be well suited for highly miniaturized devices, such as for micro-electro-mechanical systems (MEMS) [Fig. 4]. The lack of microstructure also means that metallic glasses have low ability to suppress vibrations (damping capacity). See MICRO-ELECTRO-MECHANICAL SYSTEMS (MEMS). Mechanical properties. At low stresses and at temperatures well below the glass transition, metallic glasses display isotropic linear elastic behavior in which the strain ε, Eq. (1), is directly proportional to ε=

σ E

(1)

the stress σ through Hooke’s law, where the proportionality constant E (Young’s modulus) describes the resistance to elastic (reversible) deformation, or stiffness. The disordered atomic structure of a metallic glass enables slight rearrangements of the atomic positions in response to an applied load, which would not be possible if the material were crystalline. This has the effect of reducing the stiffness slightly, so that E for metallic glasses is typically 20–30% lower than that for a crystalline alloy of the same composition. Distortion of the atomic bonds requires energy, and the elastic strain energy (per unit vol-

σ2 1 σε = 2 2E

(2)

silience of the material. The high yield stress of metallic glasses, together with low modulus, means that metallic glasses can sustain large elastic strains of 2–3% (compared with 0.1–0.2% for a typical crystalline metal) and have exceptionally large values of resilience. See HOOKE’S LAW; STRESS AND STRAIN; YOUNG’S MODULUS. At stresses beyond the elastic limit (or yield stress), metallic glasses (like crystalline alloys) experience permanent, or plastic, deformation. The degree of plastic deformation that a material can sustain before fracture is called ductility. Conventional crystalline alloys that are useful in structural applications combine high strength with good ductility. Both the strength and ductility of crystalline metals are determined largely by the properties of defects in the crystalline structure called dislocations. Although the dislocations reduce the yield strength of a crystalline metal below that predicted solely on the basis of the strength of the atomic bonds, interactions among the dislocations cause the material to strengthen (by strain hardening) as it deforms. This property gives many crystalline metals good ductility and toughness. In contrast, amorphous alloys have no dislocations (due to their disordered structure) and so they can be quite strong, often three to four times as strong as a crystalline alloy of similar composition (see table). However, the lack of dislocations means that metallic glasses do not strain-harden, and therefore have almost no ductility in tension. Instead of deforming uniformly, as crystalline metals do, plastic deformation in metallic glasses (at temperatures well below the glass transition temperature) is concentrated into very narrow shear bands. This problem can be partially overcome by reinforcing the metallic glass with particles of a ductile crystalline metal that impede shear band motion, although some strength is sacrificed. See PLASTIC DEFORMATION OF METAL. Besides strength, fracture toughness—the ability of a material that may have flaws (such as small cracks) to support a load without fracturing—is of critical importance for many structural applications. Although plastic deformation (which requires energy and therefore enhances toughness) is concentrated into shear bands, the plastic deformation in each shear band can be extensive, and there can be considerable branching of the shear bands ahead of a crack tip. As a result, the fracture toughness (Kc) of some metallic glasses (20–90 MPa · m1/2) is comparable to that of crystalline alloys, although for other amorphous alloys the fracture toughness can be quite low (∼1 MPa · m1/2), comparable to engineering ceramics. Furthermore, annealing some metallic glasses tends to make them more brittle, for reasons that are not well understood. At elevated temperatures, approaching the glass transition temperature, the strength of a metallic glass drops and the deformation becomes homogeneous instead of being concentrated into shear

763

764

Metallic glasses

Mechanical properties of some amorphous alloys, together with properties of common crystalline engineering alloys

Density, g/cm3 (lb/in.3)

Young’s modulus, GPa (106 psi)

Zr41Ti14Cu12.5Ni10Be22.5 Mg65Cu25Tb10 Pt57.5Cu14.7Ni5.3P22.5

6.12 (0.221) 3.98 (0.144) 15.0 (0.541)

101 (14.6) 48 (7.0) 95 (14)

Steel (4340, quenched and tempered) Titanium (Ti-6Al-4V, solutionized and aged) Aluminum (7075-T6)

7.85 (0.283)

∗Elastic

Poisson’s ratio

Yield strength, MPa (103 psi)

Tensile strength, MPa (103 psi)

Percent elongation

Fracture toughness, MPa · m1/2 (ksi · in.1/2)

Amorphous alloys 0.34 1900 (275) 0.31 700 (100) 0.42 1400 (200)

1900 (275) 700 (100) 1470 (213)

2∗ 1.5∗ 1.5∗

25 (23) 2 (1.8) 90 (83)

207 (30)

Crystalline alloys 0.30 1620 (235)

1760 (255)

12

50 (46)

4.43 (0.160)

114 (16.5)

0.34

1100 (160)

1170 (170)

10

55 (50)

2.80 (0.101)

71 (10)

0.33

505 (73)

572 (83)

11

24 (22)

deformation; under tensile loading, most metallic glasses fail without measurable plastic deformation.

bands. If heated above the glass transition, a metallic alloy becomes a fluid, the viscosity of which can be controlled over a wide range of temperature. Because the stresses required to cause deformation are low, either of these states can be useful for processing metallic glasses into complex shapes. See METAL; MECHANICAL PROPERTIES OF. Electrical, optical, and magnetic properties. Like crystalline metals, metallic glasses are electrically and thermally conductive, due to the presence of conduction electrons. However, the disordered atomic structure and the high alloy content mean that metallic glasses are not especially good conductors, with an electrical resistivity on the order of 10−6 · m, as compared to about 10−8 · m for copper at room temperature. However, the resistivity of metallic glasses does not vary strongly with temperature (except near absolute zero, where some amorphous alloys become superconducting). The presence of conduction electrons also allows metallic

(a)

(b)

(c) Fig. 4. Micrographs of (a) Replica of silicon micro-electromechanical device produced by warm-pressing into a metallic glass. (b, c) Submicrometer features are cleanly reproduced. [Courtesy of J. Lewandowski and A. Shamimi Nouri, Case Western Reserve University (J. J. Lewandowski, M. Shazly, and A. Shamimi Nouri, Scripta Materialia, 54:337–341, 2006)]

glasses to scatter and absorb incident light, giving metallic glasses the shiny luster typical of metals. Thus, unlike common oxide glasses, metallic glasses are not transparent to visible light. See ELECTRICAL CONDUCTIVITY OF METALS. Some metallic glasses, particularly alloys based on iron, can be ferromagnetic. Although the presence of alloying elements decreases the saturation magnetization relative to that of a pure ferromagnetic element, for many applications the lack of crystal structure is a distinct advantage. In particular, amorphous alloys based on the transition metals (iron, cobalt, and nickel) have low coercivity because there are no crystalline grain boundaries (which can pin magnetic domain walls) and because there is no magnetocrystalline anisotropy. Also, because the electrical resistivity is high, eddy-current losses associated with high-frequency magnetization and demagnetization are minimized. Many amorphous alloys show large magnetostriction (shape change upon magnetization). See MAGNETIC MATERIALS. Applications. Because the early metallic glasses could only be produced in limited shapes (such as thin ribbons and foils), applications were mostly limited to those making use of the advantageous magnetic properties of iron-based ferromagnetic alloys. For instance, these alloys can be used to make high-efficiency cores for electrical transformers, and the large magnetostriction is useful for making antishoplifting tags. The more recent development of bulk glass-forming alloys has opened up a much wider range of applications that make use of the unique mechanical properties of metallic glasses. In particular, the ability to soften the glass by heating it above the glass transition temperature makes it possible to form complex shapes and castings with thin sections. This, together with the high strength and stiffness (compared to polymers), makes metallic glasses attractive for applications such as cases for cellular telephones and other electronic devices. High strength is related to high hardness. This, together with the ability to form fine features, makes metallic glasses well suited for precision knife-edges, such as scalpels. Also, the high resilience and low

Metallocenes damping capacity mean that metallic glasses make good springs. One example is the use of zirconiumbased metallic glasses as golf club heads. Although current bulk metallic glasses tend to be rather expensive in terms of the raw cost of materials (including both the base elements such as zirconium, and the alloying elements such as beryllium), the processing costs can be low because the glasses can be easily processed at temperatures above the glass transition. Future applications are likely to be driven by these economic considerations as much as the unique mechanical properties of metallic glasses. Todd C. Hufnagel Bibliography. T. H. Courntey, Mechanical Behavior of Materials, McGraw-Hill, 2000; S. R. Elliott, Physics of Amorphous Materials, Longman, London, 1984; B. Lemley, Glassy metal, Discover, pp. 46–51, April 2004; M. Telford, The case for bulk metallic glass, Mater. Today, pp. 36–43, March 2004.

Metallocenes Bis-cyclopentadienyl derivatives of transition metals whose bonding involves overlap of ns, (n − 1)d, and np orbitals of the metal with molecular orbitals of appropriate symmetry of each cyclopentadienyl ring. The resulting complexes often possess two parallel rings (sandwich structure), but in some cases, for example those involving the titanium subgroup of metals, the rings are canted (Fig. 1). Numerous monocyclopentadienyl complexes (halfsandwich complexes) also exist, as do systems containing three and even four η5-bonded cyclopentadienyl rings; these latter systems are not metallocene complexes. Moreover, compounds constituted of parallel ring systems that are not both η5bonding cyclopentadienyls (mixed-sandwich complexes) are not metallocene complexes. Metals in the periodic table commonly known to form metallocene complexes are titanium, zirconium, hafnium, vanadium, chromium, molybdenum, tungsten, manganese, iron, ruthenium, osmium, cobalt, rhodium, and nickel. See COORDINATION COMPLEXES. Bonding. Each cyclopentadienyl ligand has three filled bonding molecular orbitals and two low-lying empty antibonding molecular orbitals. The valence levels of iron(0) consists of 4s, 3d, and 4p orbitals containing eight electrons. In the molecular-orbital bonding scheme for ferrocene, three filled molecu-

Fe

Ti

Cl Cl

lar orbitals from each of the cyclopentadienyl rings overlap with the 4s, the three 4p, and two of the 3d orbitals of correct symmetry to form six new bonding orbitals. Further stabilization results from backbonding from the filled d orbitals of the iron to the empty antibonding orbitals of the rings. This gives a total of 18 electrons in nine orbitals (18-electron rule), 10 of which come from the two cyclopentadienyl rings (in their conceptual neutral state) and 8 of which come from iron(0). Only six of the orbitals contribute significant bonding. See CHEMICAL BONDING. Structure and properties. The first transition-metal series of metallocenes from chromocene through nickelocene forms a continuous series of solid solutions; each metallocene possesses a melting point of 173◦C (343◦F). In the crystalline state the cyclopentadienyl rings of ferrocene when viewed down the axis joining their centers exhibit the staggered conformation, but in the gas phase the rings are eclipsed. For ruthenocene and osmocene, the conformations in the crystalline state are eclipsed, possibly reflecting the greater ring-metal bond distances. For ferrocene, the gas-phase barrier to rotation has been found to be only 4 ± 1 kJ · mol−1. Ferrocene structurally possesses a C C bond distance of 0.140 nanometer and a C Fe bond distance of 0.204 nanometer. Ferrocene can be sublimed under reduced pressure at 100◦C (212◦F) and is stable to temperatures >400◦C (750◦F). Ferrocene is somewhat stable to H2SO4; with HNO3 it forms a cation known as the ferricinium ion. Cobaltocene can be easily oxidized to a very stable cation, (C5H5)2Co+, which can be recovered unchanged from aqua regia. Ferrocene is oxidatively stable, cobaltocene and nickelocene much less so, but they can be handled for brief periods in air. Chromocene, on the other hand, is dangerously pyrophoric when exposed to air. See SOLID SOLUTION. Preparation. The best-known preparation of metallocenes, here exemplified by the formation of ferrocene, is by combination of the appropriate transition-metal salt with the cyclopentadienyl Grignard reagent [reaction (1)]. This Grignard reagent 2C5 H5 MgBr + FeCl2 −→ (C5 H5 )2 Fe + 2MgBrCl

(1)

can be formed by an exchange reaction between cyclopentadiene and C6H5MgBr. Useful modifications of this procedure involve treatment of cyclopentadiene with sodium or sodium hydride in tetrahydrofuran to form the sodium salt, followed by addition of a transition-metal halide, carbonyl, or related derivative. The thallium salt, C5H5Tl, can also be used. A milder set of conditions is possible; for example, a strong organic base can be used as an acceptor for HCl [reaction (2)]. 2C5 H6 + CoCl2 + 2(C2 H5 )2 NH −→

(a)

(b)

Fig. 1. Metallocene structures. (a) Staggered sandwich structure of ferrocene. (b) Canted cyclopentadienyl ring structure of titanocene dichloride. The distribution of the ligands about the Ti atom is tetrahedral.

(C5 H5 )2 Co + 2(C2 H5 )2 NH2 + Cl−

(2)

See GRIGNARD REACTION. Reactions. The reactions of metallocenes can be divided into two classes: the first is typified by the

765

766

Metallocenes COCH3

Li

tion are shown in reactions (3) and (4). In each case, (C5H5)2Co + CCl4

Fe

(CH3CO)2O mo H+ no ac yla tio n

CH3(CH2)3 Li

CCl3 H

Fe

ion

+ (C5H5)2Co +Cl −

Co

at thi

i ol

n

mo

O Fe COCH3

Li

lation

bis li

cy

Fe

bis a

thiat

ion

COCH3

Hg(O2CCH3) CH2O (CH3)2NH

Fe

Co

(4)

O

Hg(O2CCH3)2

SO3H

O

(C5H5)2C0 +

Fe Li

H2SO4

O

(3)

Fe

cobalt has gone formally from a Co(II) species to a Co(I) species, and in each of the products cobalt has the 18-electron configuration. Nickelocene reacts in similar fashion to reduce its coordination sphere by a factor of 2 to attain the 18-electron rare-gas configuration [reactions (5) and (6)]. 2C2H5OH

(C5H5)2Ni + Na/Hg

(C5H5)2Ni + H3CO2C

C

C

(5)

Ni

CO2CH3

CH2N(CH3)2 H (6)

H3CO2C

Fe

H3CO2C

Ni

Fig. 2. Aromatic reactions of ferrocene.

iron triad, and comprises essentially the reactions of aromatic molecules; the second consists of the reactions of the other metallocenes where the 18electron rare-gas configuration is not found. Reactions in these latter systems often lead to a product where the 18-electron rule is obeyed. Ferrocene is a very electron-rich system, and undergoes electrophilic substitution with great rapidity. For example, acylation proceeds about 106 times faster than that of benzene under similar conditions. Ferrocene also undergoes several other typical aromatic substitution reactions besides acylation, including sulfonation, dimethylaminomethylation (Mannich reaction), metalation, and the like (Fig. 2). Bis substitution tends to factor a product where each ring is monosubstituted although several cases are known where two substituents are introduced into one ring. Ferrocene is oxidized and deactivated under conditions for nitration and halogenation. The other metallocenes often react as if to relieve the instability of their electronic configurations; those that are electron-rich lose coordination sites on the ligand (or the entire ligand) in order to reduce their outer-shell electron count to 18, the krypton configuration. Reactions in accord with this formula-

A somewhat similar rationale can be used for the few reactions of manganocene that have been studied. Manganocene, in contrast to the other metallocenes, is only a 17-electron system and so reacts with electron-rich systems such as biphenyl radical anion, resulting in displacement of the cyclopentadienyl ring [reaction (7)]. − (C5H5)2Mn +

(7) Mn

Vanadocene is so electrophilic that it inserts into π bonds in carbenelike fashion [reaction (8)]. (C5H5)2V + CS2

S (C5H5)2V

(8)

C S

Structures. Metallocenes can possess both axial and chiral atom asymmetry. For example,

Metallochaperones α-ketotetramethyleneferrocene (1) was the first fer-

O Fe

(1) rocene resolved; a number of chiral transition-metal metallocenes in the titanium subgroup such as (2) R

O

in solid rocket fuels. Lastly, a cyclopentadienyl complex, CH3C5H4Mn(CO)3, briefly replaced tetraethyllead as an octane booster and antiknock agent in liquid fuels. See ORGANOMETALLIC COMPOUND. Donald W. Slocum Bibliography. J. Collman et al., Principles and Applications of Organotransition Metal Chemistry, 2d ed., 1987; F. A. Cotton et al., Advanced Inorganic Chemistry, 6th ed., 1999; R. H. Crabtree, The Organometallic Chemistry of the Transition Metals, 3d ed., 2001; F. G. A. Stone and R. West (eds.), Advances in Organometallic Chemistry, vol. 10, 1972; M. Tsutsui et al., Introduction to Metal π-Complex Chemistry, 1970.

Ti CI

Metallochaperones

(2) also are known. Other novel structural features are illustrated by the bis-complexation of nickel by pentalene to form (3) and by triple-decker metallocenes of type (4).

Ni

Ni

(3) + Ni

BF4−

Ni

(4) Decasubstituted ferrocenes [C5(CH3)5]2Fe and [C5(C6H5)2]Fe and pentasubstituted metallocenes such as C5(C6H5)5FeC5H5 have been prepared; the pentasubstituted cyclopentadienyl ligands constitute an interesting extension of the possibilities of metallocene chemistry, offering greater air and thermal stability and other property modification. Uses. Decamethylzirconocene reacts with nitrogen to form complex (5). Acid hydrolysis of this

N

N

Zr

N

N

Zr

N

N

(5) complex provides ammonia and hydrazine. Although not commercial, this species has the potential for acting as a nitrogen-fixing complex that could be used directly in the soil. Chromocene can be reacted with alumina to make a polymerization catalyst for ethylene. Ferrocene and some alkylsubstituted ferrocenes have been used as moderators in high-temperature combustions such as occur

A family of proteins that shuttle metal ions to specific sites within a cell. The target sites for metal delivery include a number of metalloenzymes, or proteins that bind metal ions, such as copper, zinc, or iron, and use these ions as cofactors to carry out essential biochemical reactions. It was long thought that metalloenzymes could acquire their metal ion without the assistance of an accessory metallochaperone. The metal ion was believed to freely diffuse within the cell until it contacted its metalloenzyme target, at which point the metal ion inserted itself into the protein. However, several discoveries in the late 1990s led to a radical change in the thinking of how metalloenzymes acquire their metal ion. The insertion of the metal is now known to require an auxiliary protein, the metallochaperone. Metallochaperones escort the ion to a specific intracellular location and facilitate incorporation of the metal into designated metalloenzymes. See BIOINORGANIC CHEMISTRY; CELL (BIOLOGY). Although the bulk of current knowledge on metallochaperones is restricted to copper, it is reasonable to assume that a distinct class of proteins is responsible for the incorporation of other metal ion cofactors into metalloenzymes. The heavy metals iron, manganese, and zinc are also required as cofactors for metalloenzymes, and like copper, these metals are presumably present at trace quantities inside living cells. It is therefore highly likely that new families of metallochaperones will emerge in the near future. Copper chaperones. Among the metallochaperones that have been studied in detail are a family of three copper chaperones. These molecules operate in eukaryotic (nucleated) cells to direct copper to distinct intracellular locations: the mitochondria, the secretory pathway, and the cytosol. All three copper chaperones were originally identified through genetic studies of bakers’ yeast, Saccharomyces cerevisiae. Their discoveries led to rapid identification of the human homolog proteins that are closely related in function and structure. One of the first copper chaperones identified, COX17, is a small protein that specifically directs copper to the mitochondria. Mitochondria, often called the power plants of the cell, are the sites where energy is produced from

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Metallochaperones respiration. The copper delivered by COX17 is inserted into the metalloenzyme cytochrome oxidase, needed for respiration. A second copper chaperone identified was ATX1, which carries copper to the secretory pathway, a cellular compartment that functions to shuttle proteins toward the cell surface. The metal delivered by ATX1 is incorporated into copper enzymes destined for the cell surface or the extracellular milieu. The most recently identified copper chaperone is CCS, which specifically delivers copper to a single metalloenzyme, superoxide dismutase. This copper-requiring enzyme is located in the soluble cytosolic compartment of the cell and acts to detoxify harmful reactive oxygen species. See MITOCHONDRIA. The COX17 copper chaperone was discovered by D. M. Glerum and A. Tzagoloff in 1996 in their search for yeast genes involved in the assembly of cytochrome oxidase. Each molecule of COX17 can carry two ions of copper by coordination of the metal to cysteine residues (sulfur-containing amino acids) present in the COX17 polypeptide. COX17 has been localized to the cytosolic compartment of the cell as well as to the intermembrane space of the mitochondria, in part explaining its ability to shuttle copper ions across the cytosol to the mitochondria. ATX1 was originally identified by S. Lin and V. C. Culotta in 1995 as a gene which when expressed to very high levels in yeast would prevent oxidative damage. However, this apparent antioxidant activity was later found not to be the primary function of ATX1. Rather, this protein acts to specifically shuttle copper ions across the cytosol to an intracellular copper transporter located in the secretory pathway. In humans, this intracellular copper transporter or copper pump is known as the Wilson or Menkes protein. A number of copper-requiring metalloenzymes that are exported from the cell rely on the Menkes/Wilson proteins and human ATX1 to acquire their copper. Included in this list are copper enzymes involved in connective tissue formation, brain function, and iron use. The ATX1 copper chaperone is

Cu-ATX1 Cu

C T R

Cu-CCS

WD

Cu-Cp Cu-SOD1

Cu-COX17 Cu-Cox

A family of copper metallochaperones direct trafficking of copper ions inside the cell. The schematic shows three distinct pathways of copper movement within a eukaryotic cell. Copper is first transported into the cell by the CTR and is then bound as cargo to one of three copper metallochaperones: (1) ATX1 delivers the metal to an intracellular copper transporter [such as the human Wilson disease (WD) gene product], which in turn pumps the copper into the secretory pathway where the metal is inserted into copper enzymes destined for export from the cell [such as human ceruloplasm (Cp)]. (2) CCS specifically delivers copper to the cytosolic superoxide dismutase enzyme (SOD1). (3) COX17 carries copper to the mitochondria where the metal is incorporated into cytochrome oxidase (Cu-Cox).

known to bind a single copper ion via two cysteine residues present in the ATX1 amino acid sequence MXCXXC (M = methionine, X = any amino acid, C = cysteine). This copper-binding site is not unique to ATX1, but is also found in the Wilson/Menkes copper transporters that serve as targets for ATX1. Studies by T. V. O’Halloran and colleagues revealed that ATX1 physically interacts with the copper transporter and deposits copper by transferring the metal from the MXCXXC site in the copper chaperone to the same site in the copper pump. See AMINO ACIDS. The CCS copper chaperone discovered by Culotta, J. D. Gitlin, and coworkers in 1997 specifically inserts copper into a single enzyme, the soluble copper- and zinc-requiring superoxide dismutase (SOD1). Interestingly, CCS contains the same MXCXXC copperbinding site found in ATX1, but in addition the CCS protein contains a region that is quite similar to its target of copper delivery, SOD1. CCS is believed to specifically recognize SOD1 by virtue of the SOD1like region in the CCS protein. This region of CCS has been shown to physically interact with SOD1 and thereby serve as the “molecular glue” that holds SOD1 in place while the MXCXXC copper-binding region of CCS mediates copper transfer. Although the three copper chaperones vastly differ in their respective targets of copper delivery, they are similar in several important aspects. These molecules are small proteins that bind copper by cysteine residues. They predominantly localize to the cytosol and function to deliver copper from the cell surface to a specific target deep within the cell. Genetic studies in yeast have revealed a family of cell-surface copper transporters (CTRs) that are responsible for the uptake of copper into the cell and delivery of the metal to the copper chaperones. It is presently unclear whether the copper chaperones go directly to the “front door” to pick up copper from the CTRs or whether a “middle molecule” exists that directly supplies the copper chaperones with their metal ion “cargo.” An additional property common to all copper chaperones is that these proteins are necessary only when intracellular levels of copper are low. When cells are exposed to high concentrations of copper, the metalloenzymes can acquire their copper in a chaperone-independent manner. Yet intracellular copper is normally present at exquisitely low levels, and activation of copper enzymes is wholly dependent upon copper chaperones. Copper not only is an essential nutrient but also is quite toxic to living cells, and elaborate detoxification mechanisms prevent the free metal ion from accumulating to any substantial degree. The copper-requiring metalloenzymes cannot compete for these vanishingly low levels of available metal, explaining the requirement for the copper metallochaperones (see illus.). Implications for human disease. The copper metallochaperones may have important relevance to specific diseases in humans. For example, a number of neurodegenerative diseases have been associated with deficiencies in mitochondrial cytochrome oxidase, and it is conceivable that mutations in the human COX17 gene may contribute to one or more

Metallography of these disorders. With regard to ATX1, the intracellular copper transporters that serve as targets of ATX1 action have already been linked to two important disorders of copper metabolism in humans. Wilson disease results from mutations in the Wilson copper transporter and primarily presents as chronic copper toxicity in the liver. Menkes disease, resulting from mutations in the Menkes copper transporter, is a fatal disorder associated with severe copper deficiency. It is therefore possible that additional inherited disorders of copper metabolism result from abnormalities in human ATX1 itself. Moreover, the CCS copper chaperone may have important implications regarding the fatal motor neuron disease amyotrophic lateral sclerosis (ALS), commonly known as Lou Gehrig’s disease. A subset of familial cases of amyotrophic lateral sclerosis have been linked to mutations in the human gene encoding SOD1. This disease does not result from a loss of SOD1 activity but from a new toxic property unique to the mutant enzyme. Although the cause of SOD1-linked amyotrophic lateral sclerosis is not clear, evidence to date suggests that disease results from the toxic copper ion bound to the mutant SOD1 metalloenzyme. Thus, patients suffering from SOD1-linked amyotrophic lateral sclerosis could conceivably benefit from treatments that abrogate the action of the CCS copper chaperone. Valeria Cizewski Culotta Bibliography. J. R. Conner (ed.), Metals and Oxidative Damage in Neurological Disorders, Plenum Publishing, New York, 1997; V. C. Culotta and J. Gitlin, Disorders of copper transport, in C. R. Sriver et al. (eds.), Molecular and Metabolic Basic of Inherited Disease, McGraw-Hill, 1999; M. C. Linder, Biochemistry of Copper, Biochemistry of the Elements series, vol. 10, Plenum Publishing, New York, 1991.

Metallography The study of the structure of metals and alloys by various methods, especially light and electron microscopy. Light microscopy of metals is conducted with reflected light on surfaces suitably prepared to reveal structural features. The method is often called optical microscopy or light optical microscopy. A resolution of about 200 nanometers and a linear magnification of at most 2000× can be obtained. Electron microscopy is generally carried out by the scanning electron microscope (SEM) on specimen surfaces or by the transmission electron microscope (TEM) on electron-transparent thin foils prepared from bulk materials. Magnifications can range from 10× to greater than 1,000,000×, sufficient to resolve individual atoms or planes of atoms. Metallography serves both research and industrial practice. Light microscopy has long been a standard method for observing the morphology of phases resulting from industrial processes that involve phase transformations, such as solidification and heat treatment, and plastic deformation and annealing. Microscopy, both light and electron, is also indispens-

able for the analysis of the causes of service failures of components and products. Light microscopy. Microstructural features observed in photomicrographs include the size and shape of the grains (crystals) in single-phase materials (Fig. 1a and Fig. 2a), the structure of alloys containing more than one phase such as steel (Fig. 2b, c), the effects of deformation (Fig. 1b), microcracking (Fig. 1c), and the effects of heat treatment (Fig. 2b, c). Other structural features investigated by light microscopy include the morphology and size of precipitates, compositional inhomogeneities (microsegregation), microporosity, corrosion, thickness and structure of surface coatings, and microstructure and defects in welds. The selection of representative specimens for metallographic examination is important. For example, if a part such as a forged shaft has directional properties, transverse and longitudinal sections should be examined. Sheet, wire, electronic components, and other small specimens are mounted in fixtures or plastic mounts. A specimen is initially ground on a series of abrasive papers of decreasing grit size, or on laps. It is then polished on one or more cloth-covered wheels with an abrasive such as aluminum oxide or diamond dust. Automated polishing equipment is also used. The specimen surface becomes progressively more scratch-free and mirrorlike; however, polishing, even when properly carried out, leaves a thin layer of distorted metal. See ABRASIVE. Electrolytic polishing, which consists of controlled anodic dissolution, is an alternative to mechanical polishing. Once the operating conditions have been established, electrolytic polishing is a simple operation; and the polished surface may be superior to a mechanically polished surface, particularly for soft metals. Polished specimens reveal only a few structural features, such as inclusions, microcracks, and microporosity. Etching with an appropriate reagent is generally necessary to reveal microstructural details, and it also removes the distorted layer from mechanically polished specimens. Etching reagents are mainly acids and bases (and occasionally other compounds) dissolved in water or alcohol. Most reagents act by dissolution, a few by selective deposition of reaction products (staining). In a single-phase material, etching is effective because it attacks different parts of the structure selectively. Grain-boundary regions may dissolve preferentially to the body of the grains (Fig. 2a); or adjoining grains may develop facets of different orientation, which reflect differing amounts of light (Fig. 1). In multiphase alloys, the phases are attacked selectively (Fig. 2b, c). Etching may also be carried out electrolytically by making the specimen the anode. Etching by heating in a vacuum or oxidizing atmosphere has proved useful. Cathodic etching, in which the specimen is bombarded by ions of an inert gas, is effective with some materials that resist etching by other methods. In the standard metallographic microscope, a beam of light normal to the surface illuminates the

769

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(a)

Metallography

500 µm

500 µm

(b)

500 µm

(c)

Fig. 1. Photomicrographs of typical microstructures of brass (70% Cu–30% Zn). (a) Annealed; grains with twin bands. (b) Reduced 40% by cold rolling; distorted grains. (c) Stress-corroded, with horizontal crack. (Courtesy of W. R. Johnson)

specimen. The objective lens of the microscope serves as a condensing system for the incident light and as the image-forming system for the light reflected by the specimen; this is known as brightfield illumination. In dark-field illumination, a hollow beam of light converges on the specimen; a mirrorlike surface appears dark, whereas a rough surface appears bright. Polarized light reveals differences in constituents that are optically anisotropic; useful (and often beautiful) color variations can be observed with white light. Differential interference contrast is increasingly used; optical interference permits quantitative investigation of surface contours, and color is produced by inserting a sensitive tint plate in the illumination system. Hot-stage microscopy allows many phase transformations to be followed as they occur. Most specimens must be protected from oxidation by a protective atmosphere or vacuum, and the objective lens must be kept cool. Reflecting objectives are particularly suitable because of their long working distance. Thermal etching reveals the structural features. See REFLECTING MICROSCOPE. Images are normally photographed in black and white or color. Video microscopy is common, and

(c)

(b)

(a) 500 µm

digital image acquisition and image processing methods are also used. Quantitative microscopy methods can be used for the quantitative characterization of microstructures. The basic problem to be solved is the conversion of measurements on a two-dimensional section into quantities representing the three-dimensional structure of the material. The quantities most frequently measured are grain size, relative amounts of phases in a multiphase material, total area of grain boundaries, particle size of a dispersed phase, and spacing of lamellae and dispersed particles. The use of desktop computers, special software packages, and digital image processing have greatly advanced the speed and accuracy of quantitative microscopy methods. Nevertheless, valid quantitative description of microstructures requires close attention to the selection of representative microsections from the original material or product. See GRAIN BOUNDARIES; OPTICAL MICROSCOPE. Electron microscopy. The electron microscope offers improved depth of field and higher resolution than the light microscope, as well as the possibility of in-place spectroscopy techniques. The scanning electron microscope images the surface of a

100 µm

100 µm

Fig. 2. Photomicrographs of typical structures of iron and steel. (a) High-purity iron. (b) Steel containing 0.85% C, slowly cooled; pearlite structure, consisting of alternating layers of light ferrite and dark cementite. (c) Steel containing 0.80% C, quenched and tempered; dark plates of tempered martensite. (Courtesy of W. R. Johnson)

Metallography material, while the transmission electron microscope reveals internal microstructure. Images produced by the scanning electron microscope are generally easier to interpret; in addition, the instrument operates at lower voltages, offers lower magnification, and requires less specimen preparation than is necessary for the transmission electron microscope. Consequently it is important to view a specimen with light microscopy and often with the scanning electron microscope before embarking on transmission electron microscopy. However there are some disadvantages. Electron microscope specimens are viewed under vacuum, the instruments cost significantly more than light microscopes, electron beam damage is always a danger, and representative sampling becomes more difficult as the magnification increases. See ELECTRON MICROSCOPE. Scanning electron microscope. The scanning electron microscope scans a fine beam of electrons across the surface of a specimen; and images are formed by collecting either the electrons that are backscattered from the surface or, more usually, secondary electrons knocked out of the surface atoms. The electron signal is then used to modulate a television screen scanning synchronously with the electron beam. The images contain topographic information, so a major use of the scanning electron microscope is to study rough specimens, such as fracture surfaces and semiconductor devices, which cannot be observed in the light microscope with the whole field of view in focus. Fortunately, secondary electron emission mimics the behavior of reflected light, and the human eye can generally interpret the images produced by a scanning electron microscope in a straightforward manner. The best scanning electron microscopes offer maximum useful magnifications of about 100,000×. Such a high-resolution instrument, which uses a field emission electron gun, is able to resolve surface detail on a dimensional scale of 1–2 nm. However, most scanning electron microscopes are used in the range of 1000× to 20,000×, resolving detail at the 5–10-nm level with excellent depth of field and improved resolution compared to the light microscope. Specimen preparation for scanning electron microscopes may consist of metallographic polishing if the topography is unimportant, or if direct comparison has to be made with the light microscope. However, the specimen can usually be observed in its as-received condition with minimal cleaning, if necessary. If electron-beam charging occurs because the specimen is insulating or contaminated, the image resolution degrades catastrophically, so a thin conductive coating is evaporated on the specimen surface (Fig. 3). However, field emission gun scanning electron microscopes can be operated at very low voltages (1 MeV) are rare; but they offer the possibility of very high resolution, the study of thicker specimens, and the option of carrying out in-place experiments such as straining, heating, or cooling the specimen, in order to watch changes occur dynamically. Spectrometry. Electron microscopes offer much more information than images and diffraction patterns. The ionizing nature of electron irradiation means that x-ray spectrometry and electron spectrometry, both powerful tools in their own right, can be performed in both scanning electron microscopy and transmission electron microscopy. The various signals detected spectroscopically can also be used to form images of the specimen, which reveal elemental distribution among other information. In particular, the characteristic x-ray signal can be detected and processed to map the elemental distri-

bution quantitatively on a micrometer scale in the scanning electron microscope and a nanometer scale in the transmission electron microscope. Electron spectroscopic signals permit not only elemental images to be formed (Fig. 4) but also images that reveal local changes in bonding, dielectric constant, thickness, band gap, and valence state. See ELECTRON SPECTROSCOPY. Limitations. Ultimately the information gathered in both the scanning electron microscope and the transmission electron microscope is limited by electron damage, which is also a consequence of the ionizing nature of the beam. Thus the characteristic of electrons that creates the many useful signals for imaging also places limits on the applicability of the instruments. See METALLURGY. David A. Thomas; David B. Williams Bibliography. American Society for Metals, Metals Handbook, vol. 9; Metallography and Microstructures, 9th ed., 1985; J. I. Goldstein et al., Scanning Electron Microscopy and X-ray Microanalysis, 2d ed., 1992; L. Reimer, Transmission Electron Microscopy, 4th ed., 1997; G. F. Vander Voort, Metallography, Principles and Practice, 1999; D. B. Williams et al., Images of Materials, 1991.

Metalloid An element which exhibits the external characteristics of a metal but behaves chemically both as a metal and as a nonmetal. Arsenic and antimony, for example, are hard crystalline solids that are definitely metallic in appearance. They may, however, undergo reactions that are characteristic of both metals and nonmetals. Certain of their oxides dissolve in either acids or bases, and are said to be amphoteric in character because they behave either as a base or an acid. Many elements form compounds that are amphoteric. However, only when this dualistic chemical behavior is very marked and the external appearance metallic is the element commonly called a metalloid. See METAL; NONMETAL. Francis J. Johnston

Metallurgy The technology and science of metallic materials. Metallurgy as a branch of engineering is concerned with the production of metals and alloys, their adaptation to use, and their performance in service. As a science, metallurgy is concerned with the chemical reactions involved in the processes by which metals are produced and the chemical, physical, and mechanical behavior of metallic materials. Metallurgy has played an important role in the history of civilization. Metals were first produced more than 6000 years ago. Because only a few metals, principally gold, silver, copper, and meteoric iron, occur in the uncombined state in nature, and then only in small quantities, primitive metallurgists had to discover ways of extracting metals from their ores. Fairly large-scale production of some metals was carried

Metallurgy out with technical competence in early Near Eastern and Mediterranean civilizations and in the Middle Ages in central and northern Europe. Basic metallurgical skills were also developed in other parts of the world. The winning of metals would have been of little value without the ability to work them. Great craftsmanship in metalworking developed in early times: the objects produced included jewelry, large ornamental and ceremonial objects, tools, and weapons. It may be noted that almost all early materials and techniques that later had important useful applications were discovered and first used in the decorative arts. In the Middle Ages metal working was in the hands of individual or groups of craft workers. The scale and capabilities of metal working developed with growth of industrial organizations. Today’s metallurgical plants supply metals and alloys to the manufacturing and construction industries in a variety of forms, such as beams, plates, sheets, bars, wire, and castings. Technologies such as communications, nuclear power, and space exploration continue to demand new techniques of metal production and processing. The field of metallurgy may be divided into process metallurgy (production metallurgy, extractive metallurgy) and physical metallurgy. According to another system of classification, metallurgy comprises chemical metallurgy, mechanical metallurgy (metal processing and mechanical behavior in service), and physical metallurgy. The more common division into process metallurgy and physical metallurgy, which is adopted in this article, classifies metal processing as a part of process metallurgy and the mechanical behavior of metals as a part of physical metallurgy. Process metallurgy. Process metallurgy, the science and technology used in the production of metals, employs some of the same unit operations and unit processes as chemical engineering. These operations and processes are carried out with ores, concentrates, scrap metals, fuels, fluxes, slags, solvents, and electrolytes. Different metals require different combinations of operations and processes, but typically the production of a metal involves two major steps. The first is the production of an impure metal from ore minerals, commonly oxides or sulfides, and the second is the refining of the reduced impure metal, for example, by selective oxidation of impurities or by electrolysis. Process metallurgy is continually challenged by the demand for metals which have not been produced previously or are difficult to produce; by the depletion of the deposits of the richer and more easily processed ores of the traditional metals; and by the need for metals of greater purity and higher quality. The mining of leaner ores has greatly enhanced the importance of ore dressing methods for enriching raw materials for metal production. Several nonferrous metals are commonly produced from concentrates. Iron ores are also treated by ore dressing. See ELECTROMETALLURGY; HYDROMETALLURGY; ORE DRESSING; PYROMETALLURGY; PYROMETALLURGY, NONFERROUS.

Process metallurgy today mainly involves largescale operations. A single blast furnace can produce crude iron at the rate of 3000–11,000 tons (2700– 9900 metric tons) per day. A basic oxygen furnace for steelmaking can consume 800 tons (720 metric tons) of pure oxygen together with required amounts of crude iron and scrap to produce 12,000 tons (11,000 metric tons) of steel per day. Advanced methods of process analysis and control have been applied to such processing systems. The application of vacuum to extraction and refining processes, the leaching of low-grade ores for the extraction of metals, the use of electrochemical reduction cells, and the refining of reactive metals by processing through the vapor state are other important developments. See IRON METALLURGY; STEEL MANUFACTURE. Because the production of metals employs many different chemical reactions, process metallurgy has been closely associated with inorganic chemistry. Techniques for analyzing ores and metallurgical products originated several centuries ago and represented an early stage of analytical chemistry. Application of physical chemistry to equilibria and kinetics of metallurgical reactions has led to great progress in metallurgical chemistry. Physical metallurgy. Physical metallurgy investigates the effects of composition and treatment on the structure of metals and the relations of the structure to the properties of metals. Physical metallurgy is also concerned with the engineering applications of scientific principles to the fabrication, mechanical treatment, heat treatment, and service behavior of metals. See ALLOY; HEAT TREATMENT (METALLURGY). The structure of metals consists of their crystal structure, which is investigated by x-ray, electron, and neutron diffraction, their microstructure, which is the subject of metallography, and their macrostructure. Crystal imperfections provide mechanisms for processes occurring in solid metals; for example, the movement of dislocations results in plastic deformation. Crystal imperfections are investigated by x-ray diffraction and metallographic methods, especially electron microscopy. The microstructure is determined by the constituent phases and the geometrical arrangement of the microcrystals (grains) formed by those phases. Macrostructure is important in industrial metals. It involves chemical and physical inhomogeneities on a scale larger than microscopic. Examples are flow lines in steel forgings and blowholes in castings. See CRYSTAL STRUCTURE; METALLOGRAPHY; X-RAY DIFFRACTION. Phase transformations occurring in the solid state underlie many heat-treatment operations. The thermodynamics and kinetics of these transformations are a major concern of physical metallurgy. Physical metallurgy also investigates changes in the structure and properties resulting from mechanical working of metals. See PLASTIC DEFORMATION OF METAL. The composition of metallic objects is often characterized by the presence of impurities, nonuniform distribution (segregation) of solute elements

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Metameres and nonmetallic inclusions, especially in steel. These phenomena, which originate in the production process, can have important effects on the properties of metals and alloys. They illustrate the close relation between process and physical metallurgy. The applications for which a metal is intended determine the properties that are of practical interest. For use in machinery and construction, mechanical properties including deformation and fracture behavior are of greatest importance. In transportation equipment the strength-to-weight ratio deserves special consideration. In other applications, electrical or thermal conductivity or magnetic properties may be decisive. Resistance to corrosion is a common requirement and accounts for another close link between metallurgy and chemistry, especially electrochemistry and surface chemistry. See CORROSION; METAL, MECHANICAL PROPERTIES OF. General principles. An old distinction between ferrous and nonferrous metallurgy, although still of some practical significance, is no longer considered fundamental. Certain general principles have become established and apply to all metals. In some respects, however, there are great differences between metals which has led to specialization in research and industrial practice. On the other hand, an underlying structural science is developing to bring all materials—metallic and other inorganic as well as organic materials—within a unified framework. Metallurgy occupies a position at the juncture of physics, chemistry, mechanical engineering, and chemical engineering. It also borders electrical, civil, aeronautical, and nuclear engineering. Metallurgical knowledge can be relevant to fields that are as removed from engineering as archeology, crime investigation, and orthopedic surgery. In the field of materials science and engineering, metallurgy takes its place as one of the oldest and most highly developed disciplines. The area of concern of metallurgy widened as problems of materials availability were recognized and focused attention to the need for new production processes and the recycling of secondary metals. Environmental considerations require new technology for pollution abatement. Energy conservation favors more efficient processes in metals production. Conservation of materials also calls for more effective utilization of metals and the substitution of more plentiful for scarce metals. For more information on metallurgy and some associated techniques see articles on individual metals and their metallurgy. Michael B. Bever Bibliography. C. S. Barrett and T. B. Massalski, Structure of Metals, 3d rev. ed., 1980; G. F. Carter, Principles of Physical and Chemical Metallurgy, 1979; G. E. Dieter, Mechanical Metallurgy, 3d ed., 1986; P. Haasen, Physical Metallurgy, 2d ed., 1988; M. A. Meyers and K. K. Chawla, Mechanical Metallurgy: Principles and Applications, 1984; J. J. Moore, Chemical Metallurgy, 2d ed., 1983, reprint 1993; R. E. Reed-Hill and R. Abbaschian, Physical Metallurgy Principles, 3d ed., 1992.

Metameres The successive subdivisions along the length of the body axis in bilaterally symmetrical animals; also called somites or segments. Common examples are the muscles and spinal nerves in the human body and in the body and tail of many mammals, snakes and lizards, salamanders, and fishes. It also occurs in other chordates, and in arthropods and annelid worms. It never involves reproductive organs, and thus differs from strobilization in tapeworms and certain jellyfish. This serial repetition of parts (metamerism or segmentation) arises either from a bilateral series of coelomic pouches which form the segmental muscles, kidneys, and body cavities of lower forms, or from mesoblastic somites which form the skeletal and muscular segments of vertebrates. Repetitive features of the nervous system are acquired secondarily through the influence of mesodermal metameres upon adjoining ectodermal tissues. Several primitive embryonic somites become fused in the heads of adult arthropods and vertebrates. See ANIMAL SYMMETRY; COELOM; MUSCULAR SYSTEM; NEURULATION. Howard L. Hamilton

Metamict state The state of a special class of amorphous materials that were initially crystalline. W. C. Broegger first used the term metamikte in 1893 to describe minerals that were optically isotropic with a “glasslike” fracture but still retained well-formed crystal faces. In 1914 A. Hamburg correctly attributed the transition from the periodic, crystalline state to the aperiodic, metamict state as induced by alpha-decay damage. In minerals, this damage is the result of the decay of naturally occurring radionuclides and their daughter products in the uranium and thorium (238U, 235U, and 232Th) decay series. A wide variety of complex oxides (for example, pyrochlore structure types), silicates (such as zircon, thorite, and yttrialite), and phosphates (for example, xenotime) are reported as occurring in the metamict state. All of these structures can accommodate uranium and thorium. A renewed interest in the metamict state has been stimulated by concern for the long-term stability of crystalline materials (nuclear waste forms) that will serve as hosts for actinides (for example, plutonium, americium, curium, and neptunium). Various crystalline materials (phases) may appear in a single waste form; each phase may or may not suffer radiation damage. For some nuclear waste-form phases, the radiation-induced transformation to the metamict state has been stimulated by doping phases with highly radioactive plutonium-238 or curium-244. See ACTINIDE ELEMENTS; ALPHA PARTICLES; RADIOACTIVITY. Mineral properties. Metamict minerals are generally optically isotropic but may show varying degrees of anisotropy. Metamict phases lack cleavage, and conchoidal fracture is characteristic.

Metamict state

l / l0 ρ

a

metamict

There is a decrease in the hardness (approximately 30%) and in the elastic modulus with increasing alpha-decay dose. However, the introduction of defects causes a substantial increase in the fracture toughness. Figure 1 shows a micrograph of (001) zone in a petrographic thin section of zircon that exhibits differences in birefringence. The birefringence decreases with increasing alpha-decay dose until the material becomes optically isotropic. Different zones in this sample have experienced different alphadecay doses because of variations in the uranium and thorium concentrations. The system of microfractures are perpendicular to the zones, they are restricted to low-dose zones and terminate in zones that have alpha-decay doses ≥ 0.5 × 1016 alpha events/mg (approximately equal to 1025 alpha events/m3). Fractures terminate in the high-dose zones in which fracture toughness has increased. The crystalline structure may be reconstituted by heating. Fully metamict material usually recrystallizes to a polyphase aggregate, but if the material is only partially damaged, it is possible to recrystallize to the original, single crystal. During recrystallization, several new phases may form; the particular phase assemblage is dependent on the conditions of recrystallization (for example, temperature and atmosphere). In many cases, the original premetamict phase may not recrystallize because of compositional changes caused by subsequent alteration. Annealing is exothermic, releasing the stored energy associated with the alpha-decay damage. In rare cases, recrystallization is pyronomic, releasing so much energy that the sample glows incandescently on heating. Metamict minerals contain uranium and thorium, although contents may be quite low. The presence of uranium and thorium distinguishes metamict minerals from other naturally occurring amorphous materials that have not experienced this radiation-induced transformation. Lanthanide elements are also common (in some cases over 50 wt %) and water of hydration may be high (up to 70 mol %).

crystalline

Fig. 1. Optical micrograph of (001) zone in a petrographic thin section of zircon. The variations in the shading are due to differences in the birefringence. (From W. Lutze and R. C. Ewing, eds., Radioactive Waste Forms for the Future, North-Holland, 1988)

a, p, l / l 0

0.1 mm

Partially crystalline metamict minerals display distinct broadening of diffraction maxima. Shifts in the position of diffraction maxima correspond to the increase in unit cell volume and the decrease in density. Volume expansion can be up to 8% and may cause microfracturing (Fig. 1). Fully damaged material (alphadecay doses in excess of one displacement per atom) is x-ray- and electron-diffraction amorphous; that is, the fully damaged material is completely aperiodic and thus unable to diffract x-rays or electrons. See ELECTRON DIFFRACTION; X-RAY DIFFRACTION. Alpha-decay damage. The radiation damage caused by the alpha-decay event is the result of two separate but simultaneous processes: (1) An alpha particle with an energy of approximately 4.5 MeV and a range of 10,000 nanometers dissipates most of its energy by ionization; however, at low velocities near the end of its track, it displaces several hundred atoms, creating Frenkel defect pairs. (2) The alpha-recoil atom with an energy of approximately 0.09 MeV and a range of 10 to 20 nm produces several thousand atomic displacements, creating tracks of disordered material. These two damaged areas are separated by thousands of unit cell distances and have different effects on the crystalline structure. Local point defects cause an increase in the distortion; therefore, there is an increase in the strain in the structure. Alpha-recoil tracks create regions of aperiodic material that at high enough alpha-decay doses (usually 1024 to 1025 alpha-decay events/m3) overlap and finally lead to the metamict state. The former causes broadening of x-ray diffraction maxima and an increase in unit cell volume (and a decrease in the density); the latter causes a decrease in diffraction peak intensities. Typical changes in density (ρ), unit cell parameter (cell edge; a), and diffraction intensity (I/I0) are shown in Fig. 2. The radiation-induced transition

transition zone

dose, dpa Fig. 2. Schematic representation of the change in the diffraction intensity (I/I0), cell edge (a), and density (ρ) as a function of increasing alpha-decay dose. (After G. R. Lumpkin and R. C. Ewing, Alpha-decay damage in minerals of the pyrochlore group, Phys. Chem. Miner., 16:2–20, 1988)

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2 nm (a)

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5 nm (c)

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Fig. 3. High-resolution transmission microscopy images showing progressive structural damage to microlite as a function of increasing alpha-event dose (alpha-events/mg, uncorrected for annealing). (a) 0 is the width of the state. This means that the time-dependent function not only rotates in the complex plane at a rate determined by the value of Er but also decreases exponentially in modulus at a rate determined by the value of . In a metastable state, therefore, the probability of finding the system with some specified values of the coordinates decreases exponentially with increasing time without changes in the state’s coordinate probability distribution. In other words, the metastable state decays; the decay lifetime of the metastable state, that is, the time interval required for the probability to decrease to 1/e of its original value (where e = 2.71828 is the natural base of logarithms), is inversely proportional to . See COMPLEX NUMBERS AND COMPLEX VARIABLES; E (MATHEMATICS). Although the foregoing is quite generally correct, in practice, especially in atomic and nuclear physics applications, the designation metastable state usually is reserved for states whose lifetimes are unusually long. For example, the excited states of atoms usually decay with the emission of a single photon, in a time of the order of 10−8 s. However, the necessity for angular momentum and parity conservation forces the second excited state (2S1/2) of atomic hydrogen to decay by simultaneous emission of two photons; consequently, the lifetime is increased to an estimated value of 0.15 s. Thus, the 2S1/2 state of atomic hydrogen is usually termed metastable, but most other hydrogenic states are not. Similarly, emission of a gamma-ray photon by an excited nucleus usually occurs in 10−13 s or less; however, the lifetime of one excited state of the 113In nucleus, the state that customarily is termed metastable, is about 100 min. Since radiative transition probabilities for emission of photons generally decrease rapidly with decreasing frequency, a low-lying atomic or nuclear excited state may have a lifetime longer than most excited states of atoms and nuclei and yet not be metastable in the practical sense just described, because photon emission from the state may not be

Metazoa hindered by any general requirement or selection rule, such as is invoked for the 2S1/2 state of hydrogen. See CONSERVATION LAWS (PHYSICS); SELECTION RULES (PHYSICS). Physical interpretation. Mathematical analysis supports the following physical interpretation of the existence of metastable states in quantum-mechanical systems: Decay of the metastable state characteristically involves escape of some part of the system to infinity. In radiative decay, for instance, photons are emitted to infinity; the spontaneous decay of uranium-238, with a lifetime of billions of years, is accompanied by emission of an alpha particle. In essence, the system is in a metastable state rather than a discrete truly stationary state (wherein  would be zero and the decay lifetime infinite) because there exists some mechanism coupling the putative discrete stationary state to the continuum. In radiative decay, this coupling mechanism is the interaction between the charged particles in the system and the radiation field; in alpha-particle emission the coupling mechanism is the quantum-mechanical ability of the alpha particle to tunnel through a potential barrier at the nuclear surface, a barrier which the particle could not possibly traverse in classical mechanics. In this alpha-particle emission illustration, the quantum-mechanical metastability can be associated with the fact that the corresponding classical motion of the particle inside the nucleus is stable with respect to small energy perturbations (which keep the particle energy below the barrier peak) but unstable to large energy perturbations (which permit the particle to rise in energy above the barrier peak and thereby to roll down to infinity along the extranuclear tail of the potential). Other illustrative classical physics analogs of quantum-mechanical metastable states include the resonant modes of the electromagnetic field inside a cavity with imperfectly reflecting walls (so that some fraction of the radiation incident on the walls from within the cavity can escape the cavity) and the oscillating modes of a weightless, frictionless string, one of whose end points is attached to a very heavy chain rather than, as is more usual, tied to a fixed immovable point (which permits some of the energy on the string to escape to infinity along the chain). See CAVITY RESONATOR; EXCITED STATE; NONRELATIVISTIC QUANTUM THEORY; NUCLEAR ISOMERISM; RADIOACTIVITY; VIBRATION. Edward Gerjuoy Bibliography. R. B. Leighton, Principles of Modern Physics, 1959; A. Messiah, Quantum Mechanics, vol. 1, 1961; A. Temkin (ed.), Autoionization, 1966.

Metatheria An infraclass of therian mammals including a single order, the Marsupialia. The Metatheria are distinguished from the Eutheria (the placental mammals) by numerous characters. The full metatherian dentition is I 5/4 C 1/1 Pm 3/3 M 4/4, for a total of

50 teeth. The braincase is small, the angular process of the mandible is inflected, and a pair of marsupial bones articulates with the pelvis. Almost all living marsupials have a pouch on the belly of the female in which the young are carried after birth. The Metatheria arose from unknown therians in the Cretaceous or earlier at about the same time as eutherian mammals. For a time the two groups evolved side by side, but the marsupials were unable to compete with the more progressive later placental forms and died out except in South America and Australia, where they were isolated by water barriers. See DENTITION; EUTHERIA; MAMMALIA; THERIA. D. Dwight Davis; Frederick S. Szalay Bibliography. R. M. Nowak, Walker’s Mammals of the World, 6th ed., 1999.

Metazoa The monophyletic group of eukaryotic organisms comprising all multicellular animals, but not those in which their cells form colonies, such as choanoflagellates. In colonies, all the cells need to feed because they are not in contact with each other, such that nutrients cannot be transported between them. In multicellular organisms, cells are engaged in a more specialized division of labor because nutrients can be transported between them. Therefore metazoans have specific characters related to cell recognition, adhesion, communication, and skeletal elements responsible for maintenance of the shape of the whole body and its organs. The need for cell-to-cell communication via neurotransmitters evolved into one of the most intriguing systems in all organisms, the nervous system. Other specific characteristics of metazoans are connected to sexual reproduction, such as the origin and structure of haploid gametes, fertilization, and development from the zygote to the adult organism. For most recent authors, Metazoa is a synonym of Animalia. General morphology. Metazoans are divided into about 35 major lineages that represent unique body plans. Each one of these lineages (named phyla; phylum in singular) can be defined by certain unique characteristics not found in any other phyla. The simplest metazoans are the sponges (phylum Porifera) without a well-defined axis of symmetry, few cell types, with collar cells (choanocytes; flagellate cells lining the cavities of a sponge) similar to those of choanoflagellates, and without true tissues. Sponges are sessile and feed by filtering small organisms from the water by using the collar cells. With a few exceptions, the remaining metazoans have well-defined axes of symmetry; some examples are the radial symmetry of the phylum Cnidaria (sea anemones and jellyfish), the pentaradial symmetry of the phylum Echinodermata (sea stars and sea urchins), or the bilateral symmetry of most other Metazoans. See PORIFERA. Complex metazoans have tissues and organs, including muscle cells that allowed the animals to

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Metazoa become independent from the substrate. This motility probably triggered an arms race of predators and prey escape mechanisms. The same race is probably responsible for the evolution of increased body complexity, size, and development of jaws and skeletons. Prominent examples of animals with exoskeletons are arthropods (including crustaceans, arachnids and insects) and mollusks (snails, clams and their relatives). Endoskeletons are less common among the vertebrates, but are a feature of some mollusks and echinoderms. Different types of jawlike structures evolved in many lineages, and probably played a key role in the evolution of vertebrates. See ANIMAL KINGDOM; SKELETAL SYSTEM. History. Metazoans evolved during the Precambrian (Proterozoic) in the primitive oceans. Most preserved Precambrian animals had simple body plans, probably similar to those of extant sponges and cnidarians. It is believed that most of the extant animal body plans also originated later during the Precambrian times, but that these were inconspicuous animals that did not leave well-preserved fossil evi-

dence. Large metazoans flourished during the Cambrian, and many exceptionally preserved fossil remains are well known from famous deposits such as the Burgess Shale in British Columbia, Canada, or the exquisite Chengjiang localities in Yunnan Province, China. Colonization of terrestrial ecosystems by metazoans is evidenced by fossils in the Silurian Period (ca. 420 million years ago), with likely animal traces in soils even older. Even nowadays most phyla are strictly marine, and only a few have colonized terrestrial environments although still having aquatic relatives or even aquatic life stages. Only one animal phylum, the Onychophora (velvet worms), has no aquatic extant representatives. See BURGESS SHALE. Gonzalo Giribet Bibliography. R. C. Brusca and G. J. Brusca, Invertebrates, 2d ed., Sinauer Associates, Sunderland, 2003; K. M. Halanych, The new view of animal phylogeny, Annu. Rev. Ecol. Evol. System., 35:229–256, 2004; C. Nielsen, Animal Evolution, Interrelationships of the Living Phyla, 2d ed., Oxford University Press, 2001.