Extinction events in earth history: proceedings of the Project 216, Global Biological Events in Earth History

Lecture Notes in Earth Sciences Edited by Somdev Bhattacharji, Gerald M. Friedman, Horst J. Neugebauer and Adolf Seilacher

30 Erie G. Kauffman Otto H. Walliser (Eds.)

I[I

Proceedings of the Project216: Global Biological Events in Earth History

Extinction Events in Earth History

Springer-Verlag Berlin Heidelberg NewYork London ParisTokyo Hong Kong

Editors Prof Dr Dr. h.c. Erie G. Kauffrnan University of Colorado Department of Geological Sciences Campus Box 250 Boulder, C O 80309, USA Prof Dr. Otto H. Walliser University of Gottlngen Institute and Museum for Geology and Palaeontology Goldschmidt-Str. 3, D-3400 Gothngen, FRG Layout' Gabrlela Meyer, G~)ttlngen

ISBN 3-540-52605-6 Sprmger-Verlag Berhn Heidelberg New York ISBN 0-38?-52605-6 Sprlnger-Verlag NewYork Berlin Heidelberg

This work is subject to copyright All rights are reserved, whether the whole or part of the material ts concerned, specifically the rights of translation, reprmhng, re-use o{ ~llustrat~ons,rec~tahon, broadcasting, reproduction on microfilms or in other ways, and storage m data banks Duplication of this pubhcatlon or parts thereof is only permitted under the provisions of the German Copynght Law of September 9, 1965, ~nits version of June 24, 1985, and a copyright fee must always be paid Violahons fall under the prosecution act of the German Copyright Law © Springer-Verlag Berhn Heidelberg 1990 Printed in Germany Printing and binding. Druckhaus Beltz, Hemsbach/Bergstr 2132/3140-543210 - Printed on acid-free paper

PREFACE

This volume presents results from members of the Project 216 "Global Biological Events in Earth History" of the International Geological Correlation Programme (IGCP). The project, initiated by the elder editor (O.H.W.) within the framework of the International Palaeontological Association (IPA) in the late 70s, was officially established in 1984. Subsequently, it led to the first three conferences on Global Bio-Events, and their respective symposia volumes: 1) In G6ttingen, West Germany in 1986 (WaUiser, O. H., Ed., 1986, Global Bio-Events, Springer-Verlag); in Bilbao, Spain in 1987 (Lamolda, M. A., Kauffrnan, E. G., and Walliser, O. H., Eds., 1988, Paleontology and Evolution: Extinction Events; Rev. Espafiola de Paleont., n. extraord.); and in Boulder, Colorado, U.S.A. in 1988 (this volume). The next meeting, on Innovations and Revolutions in the Biosphere, is planned in Oxford, England in 1990, to be hosted by Martin Brasier. During the history of this project, the focus of our research has shifted significantly. Initial focus was on specific global mass extinctions (e.g. the Precambrian/Cambrian, Frasnian/Fammenian, Cretaceous/Tertiary, and Eocene/Oligocene events) to a broader treatment of Phanerozoic mass extinctions, their differences or unifying factors, and their causal mechanisms. Subsequent meetings have attempted to focus attention on a fuller spectrum of global bio-events in Earth history. The Boulder Conference, and this volume, although still strongly influenced by the excitement of mass extinction research, expresses these new trends in bioevent studies. The Boulder conference, held on May 16-23, 1988, focused on a broad spectrum of Abrupt Changes in the Global Biota. Over 100 participants from 13 nations attended this meeting, representing diverse disciplines of palaeobiology, palaeoclimatology, palaeoceanography, sedimentology, geochemistry, and a broad spectrum of the stratigraphic and geological sciences. Four days of talks were supplemented by field trips to the continental Cretaceous/Tertiary boundary in the Raton Basin, New Mexico, and to the Cenomanian/Turonian mass extinction interval exposed near Pueblo, Colorado. The Conference itself was characterized by a great diversity of approaches to bio-event research, and the phenomenon of mass extinction. In particular, interactive causes involving both extraterrestrial and earthbound (tectonic, oceanographic, climatic) forces were discussed, and each major Phanerozoic mass extinction was treated by specialists in the field. In addition, many presentations focused on the causal mechanism and patterns of bio-event development that were not restricted to mass extinction intervals, but which could cause regional to global biotic response at any time in Earth history. Thus, both the conference, and this volume, focus attention on climatic and oceanic perturbations from anoxia, advection, rapid thermal change, toxic chemical enrichment, and energy shock from impacts and giant tsunamis as forcing mechanism for regional to global bio-events. The delicate balance of perched ocean/ctimate~fe systems under typical warm equable non-glacial Phanerozoic conditions, and their susceptibility to shock from even small perturbations, was a philosophical theme that ran throughout the meeting. The case for extraterrestrial forcing of tectonic, volcanic, and biological events was greatly strengthened by new data presented at this conference, with special concern for the effects of small comet/meteorite impacts in the oceans, and their chemical/physical/biological signature which might be used, in the absence of shocked minerals, microspheres or trace metals, to identify extraterrestrial events associated with global and regional bio-events. The conference benefitted from the introduction of much new data at high levels of resolution, especially from poorly studied mass extinction intervals. Interactive discussions, and many new ideas characterized the meeting. The new scientific results of this meeting are exciting; they are reviewed in the Conference Report published in Episodes (1988, v. 11, n. 4, p. 289-292). Most of the key papers presented at the Boulder meeting appear in this volume.

IV What lies ahead in bio-event research? Clearly, a great deal of excitement and an age of discovery. We have only touched the surface of this new and dynamic field. We are starting to comprehend the dynamics of global mass extinctions, integrating detailed geochemical, physical and biological data into scenarios of cause and effect. But in the years ahead lies the job of understanding the whole spectrum of regional bioevents preserved in the ancient record, and especially the application of this research to solutions of the critical problems inherent in global change and the modern biotic crisis. Future directions for research at this conference include the investigation and modeling of abrupt chemical and thermal shifts in the ocean, the effects of impacts at deep ocean sites, the documentation of successful survival strategies and repopulation patterns following biotic crises, the deep ocean record of bio-events, and focus on alternative forces other than impacting to account for mass extinction events. This volume introduces some of these new pathways in bio-event research.

ACKNOWLEDGEMENT

We are extremely grateful for the encouragement and support of our colleagues, the International Palaeontological Association, the International Union of Geosciences (IUGS) and the UNESCO. We further thank the University of Colorado and its Department of Geological Sciences for hosting the 3rd International Conference on Global Bio-events in Boulder. We received valuable help from Mrs. Dipl.Geol. L. Alberti and from Mrs. C. Kaubisch. Special acknowledgement is due to Gabriela Meyer who made any effort to type and to design this volume.

Erie G. Kauffman Host of the 3rd International Conference of IGCP Project 216

Otto. H. WaUiser Leader of IGCP Project 216

CONTENTS

PREFACE

...........................................................................................................................................................

GENERAL

III

ASPECTS

WALLISER,O. H.: HOW to define "global bio-events". ................................................................................. 1 BOUCOT, A. J.: Phanerozoic extinctions: How similar are they to each other? ....................................... 5

KITCHELL,J. A.: Biological selectivity of extinction .................................................................................. 31 LEARY, P. N. & RAMPINO, M. R.: A multi-causal model of mass extinctions: increase in trace metals in the oceans .................................................................................................................................

45

LEARY, P. N. & HART, M. B.: Important considerations in the investigation of global bioevents ...... 57 RICE, A.: Shock pressures in igneous processes: Implications for K/T events ....................................... 59 WILDEr P., QUINBY-HUNT, M. S. & BERRY, W.B.N.: Vertical advection from oxic or anoxic water from the main pycnocline as a cause of rapid extinction or rapid radiations ........................ 85 YANG, Z.: An astronomical explanation of anomalous concentrations of iridium element during catastrophic extinctions ............................................................................................................... PALAEOZOIC

99

EVENTS

BIERNAT, G. & BEDNARCZYK,W.: Evolutionary crisis within the Ordovician acrotretid inarticulate brachiopods of Poland .................................................................................... 105 BERRY, W. B. N., WILDE, P. & OUINBY-HUNT, M. S. : Late Ordovician graptolite mass mortality and subsequent Early Silurian re-radiation ........................................................................ 115 BOUCOT, A. J.: Silurian and pre-Upper Devonian bio-events ................................................................ 125 TRUYOLS-MASSONI,M., MONTESINOS,R., GARCIA-ALCALDE, J. L. & LEYVA, F.: The Kacakotomari

event and its characterization in the Palentine domain (Cantabrian Zone, NW Spain) 133

LOTrMANN, J.: The Middle G i v e t i a n p u r n i l i o - E v e n t s

- A tool for high time resolution

and event-stratigraphical correlation ...................................................................................................

145

SCHINDLER,E.: The late Frasnian (Upper Devonian) Kellwasser Crisis ............................................. 151 MC GHEE, G. R. Jr.: The Frasnian-Famennian mass extinction record in the eastern ....................... 161 United States RACKI, G.: Frasnian/Famennian event in the Holy Cross Mts, Central Poland: Stratigraphic and ecologic aspects .......................................................................................................

169

KALVODA, J.: Late Devonian - Early Carboniferous paleobiogeography of benthic Foraminifera and climatic oscillations .................................................................................................

183

SIMAKOV, K. V.: Major evolutionary events among the spiriferids at the DevonianCarboniferous boundary ........................................................................................................................

189

PALAEOZOIC/MESOZOIC

EVENTS

TEICHERT,C.: The Permian-Triassic boundary revisited ........................................................................ 199 BENTON, M. J.: Mass extinctions in the fossil record of late Palaeozoic and Mesozoic tetrapods ...239

VI

MESOZOIC

EVENTS

WHATLEY,R.: The relationship between extrinsic and intrinsic events in the evolution of Mesozoic non-marine Ostracoda .........................................................................................................

253

COLLOM,C. J.: The taxonomic analysis of mass extinction intervals: an approach to problems of resolution as shown by Cretaceous ammonite genera (global) and species (Western Interior of the United States) ................................................................................ 265 HAR~ES, P. J. & KAUFFMAN, E. G.: Patterns of survival and recovery following the Cenomanian-Turonian (Late Cretaceous) mass extinction in the Western Interior Basin, United States ............................................................................................................................... 277 RoY, J. M., MCMENAMIN, M. A. S. & ALDERMAN, S. E.: Trophic differences, originations and extinctions during the Cenomanian and Maastrichtian stages of the Cretaceous .................. 299 JOHNSON, C. C, & KAUFFMAN, E, G.: Originations, radiations and extinctions of Cretaceous rudistid bivalve species in the Caribbean Province ............................................................................ 305 MESOZOIC/CENOZOIC

EVENTS

HART,M. B. & LEARY,P. N.: Periodic bioevents in the evolution of the planktonic foraminifera..325 CALDEIRA, K., RAMPINO, M. R., VOLK, T. & ZACHOS, J. C.: Biogeochemical modeling at mass extinction boundaries: atmospheric carbon dioxide and ocean alkalinity at the K/T boundary ...............................................................................................................................

333

FLEMING, R. F. & NICHOLS, D. J.: The fern-spore abundance anomaly at the CretaceousTertiary boundary: a regional bioevent in western North America ................................................. 347 NICHOLS, D. J., FLEMING, R. F. & FREDERIKSEN,N. O.: Palynological evidence of effects of the terminal Cretaceous event on terrestrial floras in western North America ............ 351 KAUFFMAN, E. O., UPCHURCH, G. R. & NICHOLS, D. J.: The Cretaceous - Tertiary boundary interval at south table mountain, near Golden, Colorado .................................. :............ 365 LANOLDA, M. A.: The Cretaceous-Tertiary boundary crisis at Zumaya (Northern Spain). Micropaleontological data .....................................................................................................................

393

NANSEN,T. A. & UPSHAW, B.: Aftermath of the Cretaceous-Tertiary extinction: Rate and nature of the early Paleocene molluscan rebound ............................................................................. 401 SWAIN, F. M.: Species duration and extinction patterns in Cenozoic non-marine Ostracoda, Western United States .......................................................................................................

411

AGUSTI,J. & MOYA-SOLA,S.: Mammal extinctions in the Vallesian (Upper Miocene) ................... 425

HOW

TO DEFINE

"GLOBAL

BIO-EVENTS"

NE~U O~ A

contribution to Proiect

GLOBAL IJ

W.~LLISER, Otto H. *)

BIO EVENTS

Abstract : Regarding the manifold pattern of global bio-events, the corresponding terms should not be defined too restrictive.

I N T R O D U CT I O N :

Sciencen e e d s

a common

vocabulary and definitions of t h e

used terms. But a

definition must not be necessarily extremely restrictive. Sometimes a relatively wide definition is more adequate or even necessary, e.g. if the term has to describe a large variety of parameters or patterns or, as another example, if the knowledge of the investigated subject is still on a low level. On account of these reasons, the need of a wider definition also applies for those happenings which we call biological or geological events. Therefore I plead for a non-restrictive usage of the term 'global bio-event', as explained in the following.

G 1 o b a I : Extinctions are serf-evidently global, even if the last representative of the extinct taxon lived in a res~icted area for a longer time. Therefore, the problem is to prove whether the disappearance of a certain taxon, recognized in a certain area, is due to local or regional mortality, to migration or to real extinction. The disappearance of certain taxa in the Devonian often was thought to be only of local nature, but later turned out to be a real extinction, i.e. global. Another, but even more important question in connection with global bio-events is whether a globally distributed taxon disappears contemporaneously elsewhere. Up to now, normally we only can confine the corresponding time interval of extinctions in the order of a biozone, i.e. in general about 1 - 0,25 million years. In some other cases (e.g. the Kellwasser Event = KW Event; see Schindler, 1990, this volume) the time interval of stepwise extinctions can be globally restricted to the order of 100 ky and less. Even in the first mentioned case, we may call it contemporaneous with regard to the geological time scale.

E v e n t : Of course, each change, disappearance, new occurrence, interruption, origination etc., i.e. each happening, is an event. In connection with global biological or geological events we should use this term for exceptional happenings which took place in a relatively short time span; 'relatively' in comparison with the tong time intervals between the events. But what does exceptional mean? A few examples may help to elucidate this: significant higher ex~nction rate or origination rate compared with the average rate of background extinctions or background origination respectively; sudden changes of facies; intercalation of an anoxic layer in a sequence with well developed communities. Now I want to take up again the question of the relative shortness of an event. A tsunami-caused layer (compare Lottmarm, 1990, this volume), or most probably also the main Iridium layer at the K/T boundary, are products of an extremely short happening of days or even hours. In relation to that, the KW Event with a duration of several 100 Ky is extremely long. However, comparing the time-span of the KW Event with the

*) Institut und Museum fiir Geologic und Pal/iuntologie, Unlversit~it G~ttingen, Goldschmidt-Str. 3, D - 3400 GiSttingen, F.R.G.

preceding Frasnian time, in which over several million years the changes have been much less, the time interval of the KW Event is relatively very short and justifies the term 'event'. The above mentioned bio-events are connected with high extinction rates or even with mass extinctions. As pointed out earlier (e.g. Walliser, 1986), an extinction event often is followed - after a certain time interval - by radiations. If this phase is short in comparison with the following time of nomismogenesis (during which a "normal" background rate of origination can be observed), we may call it an event, too, in this case a r a d i a t i o n e v e n t . The sudden occurrence and spreading-out of a new biological construction ("Banplan", e.g. the ftrst ceiling of bactritids, leading to goniatites) is an event, too, in this case abiological i n n o v a t i o n event. Another difficulty with the term event may be demonstrated by the otomari Event (compare TruyolsMassoni et al., 1990, this volume), which also has been called Kacak Event by House (1985). The cephalopod limestone sequences of the Rheiulsche Schiefergebirge (Germany) show intercalated dark to black sediments (shales and sometimes limestones), called Odershausen Formation. At the very base of that formation occur Nowakia otornari (sensu lato; it is an early form) and Cabrieroceras sp., formerly called C. rouvillei. The base of the Odershausen Fro. has been taken as the base of the upper Middle Devonian ("Givetian") in the pelagic realm since the beginning of this century. On account of the observation, that the occurrence of "anoxic" sediments can be recognized world-wide at this time level, I called it otornari or rouvillei Event. In the meantime I recognized the real C. rouvillei (v. Koenen, 1886) (from the Montagne Noire, France) is a younger synonym to C. crispiforme (Kayser, 1879), but not identical with the Cabrieroceras from the Odershansen Fro. C. crispiforrne occurs already within the late australis Zone and disappears at the end of the kockelianus Zone. T k. kockelianus and a early form of the P. ensensis (or P. xylus group, respectively) overlap in one limestone layer in the sections Bou Tchrafme (SE-Morocco) and in the Eifel Mountains (Germany; Weddlge, 1988). On account of holostratigraphic considerations this layer represents a time-span in the order of 10 to 20 Ky. In both mentioned regions, also the end of the "anoxic" intercalation is time-equivalent: P. hemiansatus occurs very short after the first limestone layer above, the black shales. What is the otornari Event? Is it at position a of Fig. 1 (corrsponding to Truyols-Massoni et al., 1990, this volume), or at b or c, or is it the time-span between, e.g., b and c? The original intention was to designate the beginning of the black shales (b in Fig. 1) as 'otomari Event', most probably being connected with a transgressive pulse. Of course, also the end of the black shale (c in Fig. I) is not only a lithological, but also a biological event, which could be named separately. Current investigations on the changes between australis and late ensensis Zones are promising in respect to a better knowledge of the taxonomy and ranges of the involved taxa. Therefore, closer definitions will be made after completion of these investigations. Until then the terra 'otomari Event' should be used in the mentioned original, not restricted sense.

C r i s i s : If the extinction rate is higher than the rate of originations over a longer period of time, we shouldn't call this process an event but rather a crisis. Again we shouldn't make too restrictive definitions, because there are all possibilities and transitions between a relatively short crisis and a relatively long event. A good example again is the KW Event (Schindler, 1990, this volume): It is preceded by a crisis, lasting several million years; the KW Event itself has a stepwise pattern, i.e. it can be subdivided in several special events, of which the last, culminating one, may have been extremely short. The sequence 'crisis - stepwise event - final culmination' seems to occur often in earth history such as at the end of the Cretaceous.

Category/Order: The fact that global events of quite different impact on biosphere or sedimentary regime appeared in earth history, allows us to distinguish different categories or orders of events. Boucot (1990, this volume) lists such different orders, fortunately without giving strict def'mitions. Fortunately, because as soon as we would base the def'mitious on exact percentages of extinctions or originations per time-unit, we would no longer be able to rank the events, unless having reached an utopian high

h~

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WHEN SHOCK HITS ROCK/AIR INTERACE A TENSION WAVE OF MAXIMUM STRENGTH 0 " N r IS REFLECTED BACK TOWARD SOURCE

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BOTH OUTGOING COMPRESSIVE STRESS AND INGOING TENSION STRESS YIELD RESULTANT

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SEVERAL SLABS MAY BE THROWN OFF. IF THE SLABS ARE JOINTED OR INCOMPETANT THEY MAY BREAK UP

WHEN THE COMBINED STRESSES OF THE OUTGOING AND REFLECTED SHOCK EXCEED THE TENSILE STRENGTH GTS OF THE ROCK, THE ROCK SLABS AND MOVES OFF WITH THROW VELOCITY UTH

Figure 1 : Depiction of an idealized shock wave proceeding from detonation point to surface of, say, a quarry wall. To conserve momentum a tension wave is reflected from the rock/air interface back toward the detonation point. When the combined compressive and tensile stresses of both outgoing and reflected waves exceed the tensile strength of the rock, the rock fails at that point, This process keeps repeating itself back into the rock, i.e., the quarry wall fails retrogressively; from the sur-face inward. Mt. St. Helens failed in a similar fashion. Relationships describing shock failure indicate Mt. St. Helens should have fallen in sections about 500 m thick which the volcano did, Mt. St, Helens fell due to shock disruption indicating explosive volcanism to be a possible source of shocked minerals.

(e.g., Larson, 1977). Such a particle velocity implies a shock velocity less than sonic (e.g., Teller et al., 1968) by the time the shock reached the surface of the mountain. In solids shock may stow to speeds less than that of sound and is often accompanied by an elastic precursor (see Rice, 1985, 1987 for details a~d references). Immediately before the failure of the north slope, the mountain "rippled and churned" for a number of seconds (Lipman & MuUineaux, 1981). This would be indicative of an elastic precursor which further indicates that shock induced the failure of the north slope. Sound speed in alluvium is ca. 0.5 km/s and picking this value for the shock speed yields an estimate of the shock pressure p at the mountain's surface from p =

90Uv

ca. 150 bar

where v is the particle velocity, U the shock velocity and the mountain material density is taken as 9 0 ca. 1.5 x 103 kg/m3 (e.g., Hino, 1959). This pressure is somewhat larger than the expected tensile strength of the material of which the north slope is composed and provides a boundary condition to obtain an estimate of the detonation pressure Pd from P = Pd (a/r)2 sin2 ~ where we take the cone angle Ct of the crater to be ca. 20°, the reduced radius a ca. 200 m and the depth of the detonation point r = 4.5 kin. This yields a detonation pressure ca. 700 kbar. This pressure estimate is

(52 conservative because 1) some workers are placing the depth of the explosion as far down as 7 km (e.g., Rutherford et al., 1985) and 2) the estimation of north slope throw velocity herein assumed no friction in the slide kinetics. Carter et al. (1988) report multiple shock planes are formed in quartz at shock pressures of 28 kbars and 20* C but only single lamellae at temperatures of 400° C. Such pressures are then more than sufficient to form lameUae structure in shocked minerals. It should be noted however that shock features are not expected to form in the magma or in wall rock that is hot enough to anneal out these features. The source of shocked minerals must be from the surrounding country as indicated in Fig. 2. The above results were obtained by accessing Hugoniots that have been established from considerable experimental work on materials of a geologic nature (e.g., see Teller et al., 1968). Some modes of analysis have become practice in the science of explosives and these also provide thermodynamic relations to yield pressure. The pressure history of a detonation is separated in several stages for treatment. The most extreme pressure in a detonation is termed the spike pressure Ps which proceeds into the undecomposed explosive to ignite it. The spike pressure def'mes the front of the advancing detonation wave and the front of the reaction zone within which decomposition takes place. The specific volume of the charge is smallest at the spike pressure but recovers, i.e., expands through the reaction zone. The end of the reaction zone defines the "Chapman-Jouguet" or C-J plane at which decomposition is complete. The pressure at the C-J plane is defined as the detonation pressure Pd = Ps/2. Pressure continues to drop, however and the specific volume continues to expand to the "explosion" pressure Pe = Pd/2. The explosion pressure is defined by the return of the volume of the charge and the volume of the inerts to their pre-detonation values. The volume of the inerts here constitute the covolume (e.g., Cook, 1968). The explosion pressure is then given by Pe (v- Ct ') = nRT where v is the pre-detonation specific volume, Ct' the pre-detonation covolume, n the number of moles, R the gas constant and T temperature. A CO 2 content of 10 w/o yields an explosion pressure of about 60 kbar. Regardless the equation of state, the detonation pressure is about twice the explosion pressure (e.g., Hino, 1959), i.e., about 220 kbar in this case. The spike pressure should be about 240 kbar. A concommitant water content of 3 w/o would add another 75 kbar or so to the detonation pressure, raising the spike pressure to about 400 kbar. The independent results of the slide kinetics given above and the seismic inferences given below suggest the volatile content of the explosive section of the Mt. St. Helens magma chamber to be higher than employed here. A CO 2 content of 30 w/o would be more in line with these other results. The Fig. 2 : Shown above is the industrial experience of quench supersaturation which occurs in solidifying ingots. A quench margin is formed upon pouring the ingot which impedes rapid heat transfer from the remaining melt. Note that the top of the ingot does not founder even though the solidified portion is heavier than the underlying melt; this because rapid cooling and formation of many small crystals at the top so increase the viscosity near the top that the top becomes competent before it has had a chance to pull away and fall through the melt. As the ingot continues to solidify inward from the sides, volatiles are forced into the remaining melt as their solubility is many orders of magnitude less in the solid. Eventually the last of the melt becomes supersaturated in volatiles and supercooled. The initiation of a nucleation site causes volatiles to come out of solution creating a local pressure spike that provides activation energy to form more nucleation sites. This autocatalytic reaction sweeps through the melt as a shock front, leaving gas and solids behind. Pressures to 10 kbar are seen in industry in ingots that are dimunitive in comparison to magma chambers. The carry over of this phenomena to the magma chamber environment is shown on the left and is brought about in a different fashion as the chamber is zoned with more silicic material at the top and more marie material at the bottom. The compositional variation through the chamber as well as the temperature variation, cooler at the top, warmer at the bottom, dictates double diffusive layering develop in the chamber. As explained in more detail in Figs. 5 and 6, this layering provides a stepwise distribution in temperature

63

through the chamber, this because each convecting layer is well mixed. Temperature is indicated to increase to the right. The variation of melting point with depth is also depicted. The more mafic material at the bottom of the magma chamber has the highest melting point and is generally always close to its liquidus in nature. As the chamber cools the effectiveness of the convection as a heat transfer agency sees to an undercooling of the marie material before the melting point of the more silicic material at the top of the chamber is approached. The lowermost layers are the layers with the propensity to explode. See Figs. 5 and 6 for further detail.

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not to scale

ROCK

64 above exercise independently provides pressures similar to those previously determined. Another completely independent assessment is given below confirming these pressures. Little of the energy of an underground explosion is expended in seismic energy. The coupling coefficient between explosive energy and generated seismic energy runs about 10-3. Most of the energy of an underground explosion goes into inelastic deformation, local brecciation, phase changes. Kanamori et al. (1984) report the magnitude of the Mr. St. Helens event to be M s = 5.2. There are numerous relations in the literature equating magnitude of earthquake with energy (e.g., see Kasahara, 1981). That employed by the U.S. Geological Survey seems to provide the most conservative values of energy, i.e., logEs = 1.5 Ms +4.4 For the 18 May 1980 Mt. St. Helens seismicity, this yields E s ca. 1.6 x 1012 J. An estimate of the give of the magma chamber can be had from E = FdR. Kanamori et al. (1984) indicate the source of the 18 May 1980 quake to be a downward vertical force of 2 x 1012 N from which dR ca. Im. From these approximations p = E/dV = E/4r~ R 2 d R = 103Es/4iI: R2 dR ca. 6 kbar Bath's relationship seems to yield the largest energy for a given earthquake magnitude, i.e., Log E s = 1.44 M + 12.24 This provides an E s of 5.4 x 1012 J for Mt. St. Helens. Assuming a cylindrical magma chamber which pistons the downward directed 2 x 1012 N force of Kanamori et al. (1984), then with dV = I~ r 2 dx, a 0.3 m displacement of a magma chamber floor about 0.5 km wide yields pressures approaching a megabar. The discovery of shocked minerals in volcanic discharge (e.g., Carter et al., 1986), in particular the Mt. St. Helens' ash that fell near Pasco, WA, would favor the relationship of Bath as do the results from the slide kinetics. As a frame of reference, it is to be noted that 1 kiloton of TNT in wet tuff yields pressures of about 1 mbar and in granite, 2 mbar. The U.S. Geological Survey reports the energy release of the 18 May 1980 Mt. St. Helens explosion to be equivalent to 24 megatons ofTNT (Lipman & Mullineaux, 1981). The great majority of this energy, however, can be attributed to the fall of the north slope. In general, it isn't until the collapse of the broken material such as a quarry wall that the gas generated by the explosion begins to escape. As events proceed rapidly to the eye, the initiation of gas release may be incorrectly identified with the detonation itself when in fact the detonation may have taken place many seconds beforehand and the surrounding material already comminuted. The north slope of Mr. St. Helens had pretty well collapsed before there was appreciable venting of gases (Rice, 1985). The characteristics of the failure of the north slope were in complete keeping with breakout of burden by a buried explosion and are totally within the confines of the mining engineering experience.

A MECHANISM

FOR ENGENDERING

HIGH

PRESSURE

VOLCANIC

EXPLOSIONS

There is little doubt from the observational properties of Mt. St. Helens that pressures high enough to form shocked minerals attend explosive volcanism. This holds regardless any lack of knowledge of the mechanism that engendered these pressures. It would be satisfying to have in hand, however, a mechanism that might yield such pressures. Chemical processes seem forbidden because of the low oxygen fugacity available in the chamber (e.g., A. Huffman, 1988, personal communication). The conditions under which melt coolant interactions can occur are so stringent as to forbid them from the volcanic environment. These

65 conditions are discussed in the appendix which also includes an example of their misapplication to explain peculiar seismic phenomena thought associated with a melt - coolant explosion. These difficulties suggest examining another phenomena which has the advantage that it commonly occurs in the industrial environment and has support from experience in solidifying igneous melts. It is common industrial experience that solidifying melts often develop very high volatile pressures within them and to the point of bursting through several centimeters of solid steel. This phenomenon is depicted in Fig. 2 which shows also the postulated carry over to the magmatic environment. When an ingot of steel is poured and set to cool, a quenched layer is immediately frozen to the sides, bottom and top of the container. The quench margin impedes heat flow and slows further cooling and freezing of the interior. Note the solid cap at the top of the ingot container which does not founder on forming even though it is heavier than the underlying melt. Quenching the surface melt of the ingot greatly increases the temperature dependent viscosity of the melt at the top and also generates innumerable small crystals at the top which so thickens and immobilizes the surface melt that it solidifies and becomes competent before it can sink. Industrial melts invariably contain volatiles. However, volatile solubility is many orders of magnitude smaller in the solid phase then in the liquid phase. As the ingot freezes inward, the volatile content is pushed into the melt remaining in the interior, supercharging the remaining melt with gas. The final melt becomes supercooled before freezing (as do all melts). Hence the last melt ends up supersaturated in gas and in liquid phase. Further cooling causes loss of this metastable state with the formation of a crystal nucleation site that initiates the freezing. Volatiles are ejected from this site into a high temperature environment too quickly to be diffused into a new phase (the situation is even worse in silicic melts whose diffusion times are on the order of many, many years). This generates a local pressure spike which provides activation energy to generate more nucleation sites which in turn generate more gas and an autocatalytic chain reaction sets in driven by the pressure associated with the dumping of the gases from the melt. This pressure sweeps through the melt as a shock which spawns solidification and gas release in its wake. The volatiles in the melt are dumped within the time it takes for the shock to traverse the remaining melt. This is in the same time frame as observed in industrial melts (Rice, 1985, for references to the industrial literature). It is the common observation that fluids possessing both a temperature gradient and a concentration gradient break up into layers of different density, the density of each layer increasing with depth. The conveeting system in the fluid is then one of many layers, each layer well mixed such that they are of uniform composition and temperature throughout. A fluid of uniform composition would comprise a single layer only bounded by the top and bottom of the fluid. In the case of varying composition there are gradients only at the boundary layers separating each layer of convection. This gives the vertical variation in temperature and composition a staircase like distribution, both temperature and mafic content increasing with depth. A representative temperature profile is shown in Fig. 2 as well as a representative melting point distribution in the magma chamber. The more silicic material at the top of the chamber has a lower melting point than the more mafic material near the bottom, the variation in composition shown by the grading in the idealized representation of the magma chamber. As the magma chamber cools, the staircase temperature distribution shifts to the left, i.e., to lower temperatures. Note that the lower portions of the chamber become cooled below their melting point, setting up a situation similar to that in the ingot depicted on the right. In the magma chamber case, however, it is a layer in the lower portion of the magma chamber that explodes. Offgassing will flash cool other layers in the vicinity which can lead to subsequent explosions by these layers also. There were apparently two explosions at Mt. St. Helens, about two minutes apart. In industry, mechanical disturbance such as vigorous stirring of supercooled melts does not lead to precipitation of the

66 solid phase whereas crystal seeding will. For instance, a supercooled metallic melt can be vigorously stirred with a ceramic rod without initiating freezing. However, pitching a nail into the melt will instantaneously lead to massive precipitation of solid phase and explosive exsolution of volatiles (see Rice, 1985). Rayleigh number considerations suggests the convecting layers in the magma chamber to be on the order of 10 m thick which is the same estimate provided by the blasting literature for the thickness of the exploding layer. The above mechanism which is suggested to apply in magma chambers is an outgrowth of earlier considerations (e.g., Rice & Eichelberger, 1976; Rice, 1981). Once open to atmospheric conditions, the magma chamber may continue to offgas in a "boiling" mode rather than an explosive one and continue to boil for some time after the explosion which exposed magma to atmospheric conditions. It is unlikely that shock features would be retained in phenocrysts or other material in the magma chamber due to the elevated temperatures of the magma which would anneal out such features. The same applies to the magma chamber walls. However, further out into the country rock where it is cooler, such features could be retained. Several tens of meters away from the magma chamber wall the temperature will have fallen off to several hundred degrees (vivid evidence of such poor thermal conductivity of rocks is demonstrated by those scientists who sample magma by poking through the thin solid crust across which they have walked in ordinary shoes). Rice (1987) estimated the shock features to be derived from surrounding country rock to distances of the order of 500 meters out from the magma chamber. Equilibrium estimates of the stishovite transition indicate its formation could run to several hundred meters from the magma chamber, well into a region cool enough for its retention. However, nothing is known of the effect of impurities or load rate on the formation of stishovite. It may well occur even further out. Jointing cracks or angularities in country rock can serve as stress concentrators which would extend this region considerably further. If the country rock is not of magmatic composition, shocked minerals from it would not reflect magmatic origins. The mass transfer associated with the offgassing of solidifying melts can be quantified from relations developed in industry. For example, the transport coefficient Kw is given by Kw0C j1/2 VbD1/2 where Vb is the bubble volume, D is the diffusion coefficient and the rate of bubble formation is given by J = z [exp(- A H/kT)

"~f6y / (3-b) r~ m ] [exp {-16r~ 5/ 3/3kT(Pe_ pa)2}]

where b = (Pc - Pa)/Pc (e.g., Richardson, 1974), z is the number of exsolving species per cm3, A H is the heat of formation of one molecule of vapor from the melt, m is the mass of the vapor molecule, k is Boltzmann's constant, Pc is the pressure in a bubble of critical size such that the partial pressure of the gas in equilibrium with the fluid exceeds the sum of the ambient pressure Pa and the pressure due to the bubble surface tension, 2 5/ /r. A 10" K drop in temperature can lead to an increase in the rate of bubble formation of 1032 times (e.g., Katz & Sliepovich, 1971). Gases are known to disassociate in industrial melts, e.g., CO2 ----> CO + O. For magmatic melts and assuming a heat of formation of C + CO ---->CO 2 from the melt of about 70 kcal/mole, the overpressure above ambient is about 500 kbar. The proposed mechanism then yields pressures as inferred from the dynamics of the Mt. St. Helens eruption. Further, shock in industrial melts can also disassociate CO into C and O (e.g., Nellis et al., 1981) which may provide explanation for the association of soot with the K/T boundary. It is known that strong impact (as opposed to stirring) can induce sudden freezing in supercooled fluids (e.g., Lovett et al., 1982). The reaction rate equations that describe freezing phenomena are general and

67 also describe explosive kinetics which, too, can be initiated by impact. For detonating explosives, the reaction rate K is coupled both to the energy equation and to the shock velocity. Assuming a planar reaction front normal to the x direction, the shock coupling has form (e.g., Courant & Friedrichs, 1967; Fickett & Davis, 1979) Ude/dx = K where e is the percent of reacted material in the reaction zone. K has the usual form for reaction rates,

K = A(kT/h)exp [- a G/kT] where A is a transmission factor, k is Boltzmann's constant, T is temperature and A G is the activation energy which is the sum of the surface energy of the nucleation site and the difference in the free energy in the volume of the nucleation site between the liquid and solid state (e.g., Knight, 1967). The free energy has the usual form dH - SdT where it is useful to note the form of the enthalpy dH = dQ + vdP which contains change in pressure. The factor A is related to the inverse of the viscosity and a geometric factor (e.g., Willnecker et al., 1986). Taking the effect of viscosity and G = 161% 3( 3 T2 m/3(L3 m A T 2) (e.g., Davies, 1973), an overpressure on the order of kbars as computed above will be sufficient to drive explosive crystallization of mafic magmatic melt.

TEMPERATURE*C I0 A~DRY

r

f b

!7!/ :1;1

:1:!

i I: I

r-,,,o,,,

I---

Iiquidus

I0

0L , t , t tlI, ~00 600 800 I000

= I 7 51-

go

Ill ~ 1200

400

3O

1 /,

i',;

irl

',. !

i ',

',1

, ,

600

£

,,

longed build up of supersaturation does occur in the industrial foundry if the sides of the retort are

I- if? I

oL,

0 ~

800

Here T m is the melting point, L m is the latent heat and A T is the degree of undercooling. It is to be noted that the reaction rate equation yields an exponentially increasing pressure curve as temperature drops below the freezing point. Experimentally derived pressure versus undercooling relations show a similar response for solidifying magmatic melts, pressures exceeding the confinement strength of the experimental apparatus, i.e., 35 kbars (Yoder, 1976). The above equation more properly describes homogeneous nucleation which may occur if the magma walls are glassy. Pro-

II

-~10 I000 1200

'

10

Figure 3 : Phase relations for wet and dry magmas are shown above. The arrowed line running along the bottom of the lower diagram depicts supercooling at possible magma chamber depths. Many melts can be supercooled many tens of degrees below their solidus. If this occurs for either rhyolite or basalt as shown above, an apparent unbounded oressure is available.

glassy and do not provide nucleation sites to bleed of volatiles. Such a situation is extremely dangerous and care is taken to prevent the sides of the retort from becoming smooth. Although undercoolings of over 300° have been obtained in metallic melts (e.g., Thompson & Spaepen, 1983) it is much easier to attain such deep uudercoolings in silicic material which will go to glass unless cooling times are on the order of 103 to 106 years depending on their viscosity (volatile content can alter this). However, such cooling times are not out of reason in the magma chamber context. The

68 depths of undercooling available in magmatic melts indicate that if phase equilibrium fields did apply, they would also yield pressures similar to those indicated above (see Figure 3, courtesy of P. Wiley). If a mafic melt with water content of 3 w/o at depth of 5 km were supercooled to 600° K, then the arrowed line indicates the direction in which the pressure line would be intersected. As these lines run nearly parallel, it is apparent that extremely high volatile pressures would eventually be obtained. Quench supersaturation explosions, i.e., the mechanism described above, is akin to "second boiling" as described by Morey in 1922 wherein high pressures may be generated by exsolving gases within a solidifying melt. Loper and McCartney (1988) refer to other work wherein pressures to 35 kbar were measured while cooling confined igneous melts, the limitations of the experimental equipment preventing the observation of higher pressures. These situations do not represent metastable, supercooled conditions and such pressures could not be obtained without the confinement. As will be discussed, Loper and McCartney's attempt to apply these results to the volcanic environment is incorrect.

ARGUMENTS

AGAINST

VOLCANISM

AS A SOURCE

OF SHOCK

PRESSURES

It has been suggested that the growth of the bulge on the north slope of Mt. St. Helerts was eventually responsible for the failure of the north slope and shock processes were not involved (Kanamori, personal communication, 1987). Rice (1985) has shown, however, that the north slope landslide of Mt. St. Helens did not have characteristics of a gravitational failure. The run and coverage were far too great and precursors were absent such as accelerating downward creep. For that matter, the significant pre-avalanche deformation that did occur, i.e., that of the bulge, was upward and outward and was of a de-accelerating nature (Voight et al., 1981). A slip plane slope stability analyses of Mt. St. Heleus just prior to the May eruption which includes the effect of the bulge, pore pressure estimations, etc. (Rice et al., 1988) strengthens the previous conclusion that the north slope could not have fallen by itself but required disruption from within (Rice, 1985). Kanamori (personal communication, 1987) has adhered to the original interpretation offered by the U.S. Geological Survey concerning the mechanism of the Mt. St. Helens eruption, i.e., the release of overburden due to the failure of the north slope ostensibly led to the flashing of the volatiles in the magma chamber (e.g., U.S.G.S. Prof. Paper 1250). The lateral exhaust through the side of the volcano was taken by the U.S. Geological Survey to be a directed blast in the horizontal. In analysing the near field seismic data Kanamori and Given (1982) indicated the arrivals of only Love and Rayleigh waves to be consistent with this interpretation. It should be recognized that landslides by themselves are effective Love wave generators (see Rice, 1985, for references) and that inversions and source characteristics determinations are not unique. Further, explosive sources are accompanied by a local shadow zone with respect to body waves and the lack of significant P arrivals at near field stations is consistent with an initiating explosion as was the situation at Mt. St. Helens. There is concurrence that the initiating earthquake preceeded the failure of the north slope by approximately I1 s which is consistent with the use here of a detonation point 4.5 km deep and shock propagation velocities of 0.5 km/s. Kanamori's own analyses show the earthquake to preceed the north slope failure (see, for instance text and fig. 16 in Kanamori et al., 1984). This point is dwelt upon here as Kanamori (personal communication, 1988) believes the landslide occurred before the quakes but this author is unable to establish how to arrive at this conclusion. In contrast, the U.S. Geological Survey considers the earthquake to have been responsible for the loss of the north slope (e.g., Voight et al., 1981). Two minutes after the initial quake - beneath all the rubble and overburden of what had beeffthe north slope - there was another magnitude 5 explosive - source earthquake and subsequent explosive venting. This contradicts the

~521 release of overburden as an explanation for volcanic explosions. The necessity of uncapping magma chambers to engender explosions leaves unexplained events in which no slide was involved such as Le~mington in New Guinea. The top of Leamington was uniformly blown off such that the resulting crater is completely symmetrical (P. Lippman, personal communication, 1985). Neither are slides involved in the explosions at Sakurajima (Ishihara, 1985) wherein explosion source quakes preceed by several seconds explosive eruption of the crater floor. These observations greatly strengthen the conjectures herein that an explosion in a magma chamber was the initiating event on 18 May 1980 at Mt. St. Helens. Further, the explosion on 18 May 1980 at Mt. St. Helens has been placed as deep as 7 km (Rutherford et at., 1985) which makes it even more difficult to appeal to the removal of north slope overburden as the initiator. Kerr (1987) reported in Science that the appearance of particles of shocked minerals as large as 100 m at the K/T boundary constituted an "insoluble problem for advocates of volcanic catastrophe" in explnlnlng the events at the end of the Cretaceous. Kerr states that the work of Lionel Wilson (e.g., Wilson & Huang, 1979) indicates grains of such size would be lofted no more than 500 km from the point of explosive volcanism. It is Kerr's conclusion that only a meteor impact could provide the worldwide dispersal that seems to attend the shocked minerals found at the K/T boundary and has been taken by others as proof of impact (e.g., Bohor et al., 1987). Kerr's conclusion and those of Wilson are incorrect. Bailey et al. (1984) report large concentrations of giant sized mineral particles in the present Arctic atmosphere. These concentrations appear to be uniformly distributed to 5 km in height. Some of these particles are reported to have the appearance of voleanic ash. Concentration does vary with size, i.e., the inverse cube of the particle diameter. From their data, these workers estimate the fall speed of a particle 100 m in diameter should be 0.05 m/s and much lower for plate llke aggregates. These fall speeds are considerably less than those of 0.25 m/s for equally large diameters as predicted by Wilson and Huang (1979). For that matter, quartz crystals of Asian origin of size around 300 m are known to be transported by wind from Asia to Hawaiian (Duce, personal communication, 1987), mineral particles in the 50 m range can be transported by wind distances in excess of 10.000 kin, and particles as large as 100 m have been transported from the Sahara to Miami, Florida (e.g., Betzer et al., 1986; Duce, 1986; Uematsu et at., 1985; Dauphin, 1983; Carder et al., 1986). It is to be noted that this material is of similar density, composition and even greater size than the shocked minerals from the K/T. It has recently been reported that explosions never develop pressures in excess of that provided by the tensile strength of the confining material or the hydrostatic head provided by overburden therefore volcanoes can never be a source of shocked minerals as confining pressures run to only several kilobars (de Silva & Sharpton, 1988). If explosions never developed pressures in excess of the confining strength, then open air blasts would never greatly exceed atmospheric pressure. Only a cursory examination of the blasting literature indicates that open air detonations of TNT and other high explosives can yield pressures at the detonation point on the order of a megabar (e.g., Cook, 1968). Not even confusion between detonation and deflagration attends the allegations of de Silva and Sharpton. Detonation driven reactions are carried forward at shock velocities in the reactant such that the reaction is over before there is any dynamic response of the reaction products. That is, pressure many orders of magnitude greater than that provided by confinement are released long before the confinement has a chance to respond. Gunpowder deflagrates, i.e., burns. Untamped gunpowder will simply flare. However, confined gunpowder allows a build up of pressure that greatly aecelerates the burning rate, carrying it to completion so rapidly that overburden and tensile strengths are considerably exceeded before the confinement fails. The reactions considered here to be responsible for fast rise time, high pressures in the volcanic environment are detonation driven; that is, driven by shock.

70 The observation that explosive discharge may be laden with phenocrysts has provided an objection to quench supersaturation explosions. Homogeneous nucleation of an undercooled melt driven by detonation kinetics is not expected to be charged with crystals. The speed of the shock driving the exsolution exceeds by many orders of magnitude crystal growth rates. Further, the existence of crystals indicates solidification which proceeded more as a heterogenous nucleation mechanism. In the latter case deep undercooling with attendent metastable conditions that are potentiaUy explosive would be difficult to obtain. This because of the available nucleation sites. The magma would off gas continuously in a much more subdued fashion as it cooled. The fact that magma chambers are zoned, however, provides an environment for phenocryst laden discharges from the upper reaches of the chamber as the result of explosive exsolution of supercooled melt nearer the bottom. This environment secures most of the crystal population in the upper portions of the chamber. There exists recent theoretical literature which would oppose such a population in spite of geologic evidence that it occurs. These views are discussed later. There are instances, however, wherein the phenocryst population is concentrated mostly at the top of the chamber. There is an approximately 20 meter thick mat of phenocrysts extending downward from the solidified surface of the Kil~uea Iki lava lake, with decreasing populations into a melt region approximately 20-30 m thick. Apparent deviations from this distribution such as the Shamsan caldera (e.g., Cox et al., 1979) wherein the crystal concentration appears to come from the bottom of the chamber will be dealt with elsewhere ~ i c e , in prep.) wherein it is shown that the increasing crystal content of the chamber exudate reflects its cooling history and not settling. The melt region of the lava lake is underlain by solid material thought to be the result of crystal settling. Although the lake is believed to initially have a depth of 100 m, apparently not until nearly two decades after emplacement were attempts made to sound the lake by several geophysical means. No consistent results were obtained (e.g., see Helz, 1987). It is not clear that the bottom of the present melt is not a surface that

APPROXIMATEVERTICAL DISTRIBUTION OF PHENOCRYST CONTENT FOR $HAMSAN CALDERA ROCKS {FROM COX ET AL, 1979), NOTE THE BOTTOM PORTION OF THE MAGMA CHAMBER WAS DEVOIDOF CRYSTALS

DEPTH BELOW SURFACEOF K1LAUEAIKI LAVA LAKE 1

......

1

ooy ij

> "o

..........

0D

i11

Figure 4 : The pbenocryst distribution in the above unit are concentrated near the top, suggesting the evolution of a magma chamber as shown in Fig. 6. Figure 5 : Phenocryst distribution is confined to the region above the melt at Kilauea Iki Lava lake (from Helz, 1987). This distribution is similar to that given in the previous Figure, Fig. 4, and sinailarly suggests the evolution of a magma chamber as shown in Fig. 6.

1

o

1.8 0 OL

CPX

/

E 5 0 0 /

I

0

31o

112

3.0

TOTALFELDSPAR

ORE

71 had been flooded. It is clear that the phenocryst distribution from the bottom of the melt to the surface of the lava take is in accord with models of magma chamber solidification from the top down which also yield banded structures (e.g., Rice, 1981, 1985). A qualitative depiction of the phenocryst distribution above the 40 m depth is given in Fig. 5. Note further that vesicle population correlates strongly with crystallization. Attention is now directed to Fig. 6 in which is depicted an idealized magma chamber whose chemical variation with depth again secures within it a multi-diffusive layering. Shown in Fig. 6 is the change in temperature (T), composition (C) and viscosity (V) with depth. The distribution of each of these entities (T, C and V) is as before (Fig. 2) a staircase function that increases with depth for T and C but decreases for it is only at the boundaries separating each layer that significant gradients exist. Shear entrainment at the separating boundaries does secure transport between layers as indicated by arrows crossing the boundaries: for instance, material from layer 2, i.e. C2, is engrained in layer 1 and vice versa. It is anticipated that the upper portion of the chamber is more acidic than the lower portion, hence the upper portions have lower melting points Tim than the lower portions. Material Cn has a higher melting point than C 1.

Figure 6 : A model convecting magma chamber depicTi ted above as suggested by MAXIMUM .':1. "° ., • :.," ". . . . . . % .;. ". e2entr;ined Into'GI , ~ Figs. 4 and 5 as well as VISCOSITY ¢~, • "manyl~anoer~sts C" "" ":" *'" 1"2 • ' . ~ m : ' . ." IN UPPER theoretical considerations. MOST LAYER The chamber is zoned. • , I . . ,.." [Tm2 ! C1 entrainedlntoC2 le 2 '. • : :. ! ! Lighter material is on the top and has composition C1 (e.g., is most silieic) which also fixes its melting point, T1. Take special note however of the dots representative of phenocrysts of material that has come up from depth which is of different ! C~'nn-1 entrained composition and h i g h e r melting point than In- 1 trained into Cn- 1 the material in which it LOWEST resides. As the chamber is VISCOSITY no phenocryst8 I CI~ cooling from the top, this IN LOWER ,Vn MOST layer also possesses the LAYER increasing lowest temperature T 1 and . is the most viscous material "Yl in the chamber, possessing viscosity ~t"Proceeding downward through the chamber, the magma becomes heavier and at the bottom has composltion C n (e.g.,most mafic) which implies material with the highest melting point in the chamber. This is also the warmest part of the chamber with temperature T n. It also possesses the lowest viscosity,V n. Being zoned, the chamber possesses both compositional and temperature variationwith depth which secures multi-dkffitsiveconvection, that is,the chamber supports a multi-layered system rather than a single layer system of convection. Physical properties are then distributed in a "staircase"fashion from top to bottom of the chamber as indicated above. As shown by the broad arrows, material from one layer can be entrained into another, and in fact be cascaded up or down through the layers of the chamber. Material moving upward into cooler,more viscous material willcool, crystallizeand if possessed of sufficientvolatilecontent, vesiculate.This due to its high melting point. Material moving downward from the top, however, is of lower melting point and ifithas crystallized,itwill have a proclivity to be resorbed. Moving up and down the staircase can lead to zoning and/or to reversed zoning. As the chamber cools, the staircasetemperature distributionmoves to the left.The distributionof melting point temperatures should not change significantly. Increasingci r~

Tmi

~

~ )lnCreraSlnl

72 Some C 1 is engrained into layer 2 (second from top) but as it enters a higher temperature environment it gets further from its liquidus. Crystals of C 1 engrained in layer 2 therefore have an increased likelyhood of being resorbed. For that matter some crystals of C 1 could cascade all the way down to layer n near the bottom where they would be completely resorbed as the highest temperatures in the magma chamber occur here and are above the melting point Tin1 of C 1. In a similar fashion, crystals of C2 may be engrained into layer 3, later 4, etc. Some C2 will also be engrained into layer 1 (at the top) where it will have a propensity to crystallize out. This because C2 has a higher melting point than C 1 and it enters a colder environment. Similarly for compositions of other layers, e.g. C4 engrained into layer 3, etc. It is possible for Cn material to cascade by entrainment all the way to the top of the chamber, crystallizing on the way up as it encounters colder and colder temperatures. There is evidence that there is a significant population of mafic crystals engrained in the more silicious material at the top of the chamber (e.g. Eichelberger, personal communication, 1981). Moving up and down the convection ladder will generate reaction rims, embayments, zoning, resorbtion in crystals. Most of the crystals reside in the upper, cooler reaches of the magma chambers. The lower regions of the chamber are too hot to support a crystal population. They would melt. The quench supersaturation explosion mechanism can take place in the crystal free region at the bottom of the magma chamber when these regions of the chamber have cooled sufficiently to reach the nucleation temperature. Note that the higher melting point of the material in the lower region of the chamber allows this material to become undercooled even though it is still the hottest part of the magma chamber. Mafic magma does not seem to occur at temperatures far removed from its liquidus which facilitates this undercooling of melt in the bottom of the magma chamber. A large temperature drop across the entire chamber itself is not anticipated because of the heat transfer efficacy of the convections. The upper portions of the chamber then, also being much further removed from their liquidus, will not be subject to undercooling until long after the material at the bottom of the chamber. Such evolution can be confirmed theoretically. The thickness of the exploding layer of magma is estimated to be on the order of some tens of meters (Rice, 1985). Although there is experimental variabilitywith regard to heterogeneous nucleation preventing the development of a supercooled melt (there is even some question as to whether homogeneous nucleation ever really exists), assuming the exploding layer to be phenocryst free in a magma chamber 1 km deep attributes to this layer only 10% of the discharge of the volcano. This assumes of course that material from this layer even reaches the surface. Fig. 7 indicates the situation from which it can be ascertained that phenocryst laden discharge is to be anticipated from a quench supersaturation explosion. Further, the propensity for ash to be lofted further than heavier phenocrysts offers the hazard of a skewed distribution of phenocryst content of the magma if the massive volumes of ash carried off by atmospheric processes are not taken into account. In any case, except close in to the lip of the volcano an explosive eruption through the compositional variation of the chamber will disrupt it to yield a mix as shown in Fig. 7. Obviously the outfaU may not accurately represent the distribution of material as it existed in the magma chamber. Such a distribution of crystals as indicated in Figs. 5, 6 and 7 runs cotmter to some recent work which has been proferred as supportive of segregation by crystal settling. It is surprising such effort is undertaken in view of the fact that one commonly sees advertised hydraulic or pneumatic transport systems carrying particles of coal, rock, minerals, limestone, etc. in sizes up to 8 cm as uniform suspensions in the working fluid up vertical hoists of height greater than a k m or over distances of 25 km. This in media orders of magnitude less viscous than magmas hence far more likely to facilitate settling out (e.g., Siemag, Inc.). Efforts that would support crystal settling still leave unexplained marginal border groups and heavier

73

material overlying light (e.g., as in the Jimberlana) unless one requires that somehow the magma chamber be rotated on its sides and roof occasionally to secure it. Further such efforts ignore a vast array of industrial literature regarding solidifying melts in which analogues of border groups and cryptic variation are

5

• C1

' .

•

..

•

-~C .

e2

n-l,. ...,

"

AS.

• C34 "

") j x~J

CARRIED OFF

"-3

"'~"

commonplace as well as the occurrence of heavier material overlying light. The industrial phenomena however is due to convective fractionation with no crystal settling involved at all (see Rice, 1981, 1985 for refer-

ences). The effort of Weinstein et al. (1988) is representative of work attempting to support crystal settling as an important phenomena in magma chambers. Their theoretical study however, is restricted to laminar flow whereas magma chambers possess Rayleigh numbers readily of the order of 1014, about ten orders of magnitude higher than the transition to turbulent flow. The study then is restricted to melts about a meter thick hence have no application to magma chambers at all. This applies even to deep seated magma chambers. If the chamber is about a kilometer thick, a miniscule temperature of only one degree across the chamber will yield a Rayleigh number of 1012, well into the turbu-

DEGREE OF UNDERCOOLING

Figure 7 : After some time convective heat transfer drops the temperature of the magma chamber such that the lower layers become undercooled, (i.e., take on temperatures below their melting point), although the lower layers remain the warmest portion of the chamber. This sets up a potentially explosive situation in the bottom of the chamber. If the super cooled layer blows, it will yield a mix of material from all levels.

lent regime. It should be kept in mind that the environs in which the chamber is immersed will be 15° C cooler at the top of the chamber than at the bottom simply due to the geothermal gradient. Particularly remarkable however, is the manner in which settling is theoretically secured in this work. The computer tracks the particles, tests to see if they are close to the bottom and if they are, the computer program, without appeal to physics, simply removes the particle from circulation and calls it settled. Harry Hess, as long ago as 1960, noted in his discussion of the Pzlisade sill that experimental evidence indicated strong vertical transport off the floor of a convecting body. In an attempt to experimentally verify such results, these authors fred support only when they cease heating the fluid. That is, settling of glass beads in syrup does occur but only when the convection comes to a stop. Otherwise the spheres remain entrained in the

74 fluid even though the relative density difference exceeds by an order of magnitude that to be found in magma chambers and even though the glass beads scale to "crystal" sizes of tens of centimeters. "Sticky bottom" is the term employed by these workers for the artifice of "logic gating" to settle particles. We have applied the same paradigm to "prove" that crystals accumulate on the "sticlq?' top of the chamber similar to crystal growth in industry which does occur in the more viscous chilled boundary layer at the top. These authors however indicate "the effects of boundary conditions are not important", hence dismiss the most important manifestation of convection itself, the boundary layer. The use by Weinstein et al. of the terms "homogeneous nucleation" to describe uniform crystal growth throughout the chamber and "heterogeneous nucleation" to describe growth confined at the top of the chamber indicates ignorance of solidification processes wherein these terms have quite different but such specific and singularly important meanings in the solidification literature (see for instance, Carter, 1979) that such confused use would adamantly be avoided. Further, the dismissal of the viscosity change of the upper boundary layer eliminates a powerful retention mechanism. Crystal retention correlates to increased viscosity of magmas as noted by other workers (Marsh & Maxey, 1985). The mechanism which glues crystals to the roof in the industrial setting is the crystal content of the melt adjacent to the roof. When this crystal content exceeds 65%, there is a sudden snap - through increase of viscosity by orders of magnitude (this has been noted also in the geological literature. See Shaw, 1965). Crystal content will taper off away from the wall but a "glue front" will advance down into the magma chamber with crystal content of 65%. It is interesting to note that it seems all orthocumulate structure has abut 65% crystal content imbedded in the matrix. Few lava flows contain 65% crystals. There is other work indicating results similar to Weinstein et al. that is also flawed. The treatment of this work is relegated to another paper (Rice, in prep.) to keep this one of reasonable size. It has been alleged by Shoemaker (personal communication, 1980) that explosive volcanism is a decompression process. If this were the case, first arrivals of P waves from such an event would be down. First arrivals from the Mt. St. Helens explosion were up, indicating to the contrary a compressive, i.e., an explosive event and not a venting event. Work on decompressive processes such as Wilson (1980) has been held up as an example of results calling into question high pressures inferred for magmatic processes from ballistic calculations (e.g., de Silva & Sharpton, 1988). Wilson cautions however that his inferences have no application to fuel-coolant interactions whose intensities are similar to the detonation processes discussed here. However, the inference of supersonic discharge from volcanic vents (Wilson et al., 1980) is at odds with flashing blow down experience in industry wherein choking limits the exit velocities to the speed of sound of the material of the flow (e.g., Please et al., 1986). Hydrofracking from a magma chamber to the surface of the earth may set up a blowdown situation such as occurs when a line of pressurized superheated water fails. Decompression occurs with a refraction wave passing back through the system wherein the attendant pressure drop causes massive flashing of the water with two phase flow exiting the break (e.g. Weisman, J., 1987). Although a diverging portion of the path may become supersonic, eventually the flow must enter the atmosphere and become choked. Exit flow with velocities greater than Mach number M > 1 occur only when PIAc/P2A2 > 2 using the ratio of specific heats of water. A c is the minimum area of the nozzle, P1 the pressure driving the flow. A 2 is the exit area, P2 the exit pressure. If the flow eventually dumps to the atmospheric then A2 is large enough to secure M < 1. Inclusion of slip between the components of the multiphase flow indicates gas velocities should be several times larger than magma velocities, an aspect neglected by Wilson et al. (1980). Further, there exist correlations that indicate the need for frictional corrections due to effects beyond viscous head loss: e.g. material colliding with walls, etc. These have forms Pt/Pv = 1 + R/K where R is the mass ratio of particles to gas, K an empirical constant, Pt is the total pressure drop, and Pv is the pressure drop of the vapor phase only as deduced from ordinary

75 viscous situations. These relations imply corrected pressures some tens of times larger but do not take in another important phenomena also neglected in the work of Wilson et al. (1980). Beyond the work of Shaw (1965) relating to magmas, it is well known elsewhere that when solid content gets beyond 50% of the volume of the flow, the effective viscosity of the flow begins to rise in a fashion even stronger than exponential. For instance, in slurries the effective viscosity can rise by a factor of 200 with a doubling of particulate content (e.g. Wohlbier, 1980). Inclusions of this effect in all assessments of magma chamber pressures from discharge rates, ballistic and otherwise, raises these assessments as high as a megabar, a pressure commonly associated with many commercial explosions. However, there is a fundamental difficulty in the mechanism of Wilson and coworkers that will not allow even the above problems to manifest themselves. Rapid degassing as the magma rises cannot take place as the diffusion coefficients of volatiles in melts are miniscule. For the same reason an oversaturated diver may not experience the bends until some time after returning to the surface, degassing due to loss of overburden head as magma rises is impeded by the time it takes to diffuse the gas from the melt. A volatile diffusion coefficient of 10_5 cm2/s in a melt 10 m across should take over a year to degas. In fact, the diffusion times of volatiles from magma is so great that it restricts the magma to boiling off over many, many days instead of acquiring within seconds the bouyancy of 70% volume gas content. The rise time of the magma will be further restricted as the viscosity increases greatly due to flash cooling as the magma boils (see Rice, 1985, for estimates). The mechanisms of Wilson and coworkers cannot occur. These latter difficulties also attend a model put forth by Loper and McCartney (1988) to generate volatile release and attendant pressure rise through magma mixing. Solidification by this process does not entail shock propagation speeds and must proceed by diffusional mechanisms alone which even when abetted by convection require at the minimum months to significantly degas. This excludes the similar dissipative cascade time scale necessary to break up large turbulent eddies into small. Driving gas out of a volume of solidified melt 1 micron across under near equilibrium conditions entails diffusion coefficients of 10_12 cm2/s which in turn implies a gas bleed-off time of many hours. For larger size melts, the diffusion time becomes monumental. Some workers (e.g. Wilson et al., 1980) contend degassing will not begin until depths of only 5 km have been reached and this initiates only slowly. In addition, the suggestion by Loper and McCartney that shock features may be formed by slow rise in pressure is incorrect and does not correspond to fact. Although shock features do not arise in minerals of relevance to K/T events until pressures exceed 20 kbars (Carter et al., 1989), shock features are never obtained unless there is a load rate sufficiently high to generate a shock front. Although a shock wave may have considerable length, shock fronts themselves (but not elastic precursors) rise within interatomic distances (e.g., Greene & Toennies, 1964). That is, they possess rise times of considerably less than a millisecond and not days as proposed by Loper and McCartney. There are other criticisms. While not applicable to detonation type reactions proposed here (in which the full pressure develops before the confinement can dynamically respond), De Silva and Sharpton (1988) have noted that the explosive mechanisms of Loper and McCartney are deficient. Although Loper and McCartney cite literature indicating laboratory confinement of pressures to 35 kbar developing in a solidifying magmatic melt, it is the confinement of the experimental apparatus that allows such pressures to develop in near equilibrium processes. The experimental apparatus did not have the strength to confine higher pressures. It is noted by De Silva and Sharpton that these relatively slow developing pressures exceed the overburden and tensile strength of the material confining magma chambers and cause it to fail before total ex.sotution of the volatiles. This failure opens the chamber to atmospheric conditions. Pressure then bleeds off with the confined volatiles never exceeding the local ambient pressure. Only slow bleed off of volatiles can take place

76 by the mechanism of Loper and McCartney leading to the gradual build-up of pressure until hydrofrak takes place to release the pressure. As pressure release exsolution is not expected to commence until the magma is some 5 km below the surface of the earth (e.g. Wilson et al., 1980) the mechanism of Loper and McCartney cannot be expected to be responsible for deep-seated crypto-explosions. Pressures attainable by the mechanism of Loper and McCartney are limited to that of the overburden: perhaps 2 kbar. The length of time it takes magma to diffuse off its volatile content has important ramifications, dictating that some sort of metastable conditions be set up conducive to instantaneous exsolution of volatiles. Huffman (personal communication) indicates that there isn't sufficient oxygen to appeal to chemical reaction. These situations dictate there can be only one mechanism by which explosive volcanism occurs: metastable undercooling, i.e. quench supersaturation.

CONCLUSIONS: SOURCE

OF

PERIODICITY K-T

IN

MANTLE

CONVECTION

THE

POSSIBLE

VOLCANISM

Several independent methods of demonstrating that volcanic processes are capable of producing shocked minerals have been reviewed here. It has also been shown here that a number of efforts to reach opposite conclusions, also independent of one another, are flawed and upon correction strengthen assessments that indicate shock may accompany exploding magma chambers. Some of these assessments made clear the importance of CO 2 in generating volcanically derived pressures sufficient to form shocked minerals. These results compliment the discovery of CO 2 inclusions along shock lameUae in minerals of the Vredefort and Sudbury (Medenbach et at., 1987; Fricke & Schreyer, 1987). Such inclusions do not seem to be reported in shocked minerals from underground nuclear testing which further supports suggestions that CO 2 is crucial to cryptoexplosions (e.g., Nicolaysen, 1985). Augmenting this view is the proposal that the Deccan Trap represented mantle CO 2 outgassing that led to a greenhouse effect which was deletorious to large body life forms (e.g., McLean, 1985). The Deccan Trap is now known to lie athwart the K/T boundary strengthening the suggestion that these massive flows may be a source of the K/T iridium (e.g., Vandamme et al., 1986). The extensive kimberlite pipe fields emplaced at the end of the Cretaceous could also provide an atmospheric CO 2 dump sufficient to overload oceans that at the time had little buffering capacity (Kurtz & Rice, 1988). It has been argued that although the marie flows of the Deccan Trap may be a source for the iridium, they could not be a source for the shocked minerals. There is, however, a common intimacy between marie flows and silicic flows, the latter which could be the source of explosive volcanism. This intimacy did exist with the emplacement of the Deccan Trap (e.g., Lightfoot et al., 1987; Setlma & Battiwala, 1977). For that matter, Sukheswala and Sethna (1977) note the appearance of patchy extinctions in phenocrysts in silicic outpourings associated with the Deccan Trap. Patchy extinctions are characteristic of mosalcism which is a diagnostic of extreme shock (see any reference to Carter herein). Although marie outpourings themselves are not to be an expected source of explosive volcanism, the converse is not necessarily true. Iridium can be expected from explosive volcanism. Note in Fig. 2 that the zoned magma chamber is mafic at the bottom and hence could also be a source of iridium. In this context, it is important to note that iridium has been found in volcanic ash layers preserved in Antarctic blue ice and at levels comparable to some K-T sites, e.g., 10 ppb (C. Koeberl, University of Vienna, personal communication, 1988). Although the Deccan Traps have attracted considerable attention with regard to the K-T debate and have been suggested as the manifestation of a single mantle plume that breaks away from the core - mantle boundary with a regularity of 30 my ~I.oper & McCartney, 1988), it has been noted (e.g., C. Officer,

77 personal communication, 1988) that not only were the Deccan Traps involved in the K/T events but there was also extensive explosive volcanism in the high latitudes of the Southern Hemisphere, e.g. Walvis Ridge, Kerguelan Plateau, Mand Rise, and Haiti. Although not well dated, both the diatreme fields in North America and Australia are of Late Cretaceous/Early Tertiary Age. There are kimberlite swarms of similar age in Africa as well as manifestations of volcanic ash throughout Europe, and Western North America. The source of the latter may have been obliterated with the San Juan volcanics. It is therefore incorrect to look just to the Deccan Traps as the volcanic cause of the K/T events as have Loper and McCartney (1988). The experiments which led Loper and McCartney (1988) to infer plume behaviour in the mantle leading to Deccan Trap type volcanism actually establish the opposite: i.e. the unlikelihood that plumes stream from the core-mantle interface through the mantle. Their experimental configuration of water below syrup overturns in a single blob without entrainment unless something akin to a silk membrane is placed between the two fluids. It cannot be expected then that the mantle is a prototype of this model unless a silk membrane exists between the core - mantle interface. The driving mechanism for this experiment are the interstices in the silk membrane which supplement the bouyancy head by an amount approximately 2 "y/r where y is the surface tension and r is the distance across the interstices. The plume will have greatest propensity to rise at the smallest grid. The Bond number for the mantle will be close to infinity and such mechanism is not expected there. These experiments fall in with a rash of others appearing in the earth science literature wherein no attempt has been made to assess if there is any figment of dynamical similitude with earth processes. No scaling is provided with this work which leaves one with no immediate sense if there is any applicability whatsoever and therefore ungraciously imposes a waste of time upon the reader. In addition, the experiment is far removed from the mantle at least in one manner. The experiment is not convecting. It has been shown elsewhere that attempted plume formation simply contributes to the mainstream flow and as in the ease of Loper and McCartney follow pre-existing routes (long established upwellings) and make no new ones for themselves (e.g. Bradley, 1986). The two dimensional theoretical analyses by Loper and Stacey (1983) suggesting plume formation is artificially constrained by boundary conditions convenient for analysis and could just as well be representative of an upwelling sheet. There are other difficulties. Theoretical attempts at modelling the core-mantle boundary layer indicate it most pronounced in the upwellings, in opposition to seismic inferences thast the opposite occurs (e.g. Lay, 1989). Further, plume mechanisms require cooling of the core, which is deemed to generate little energy but that of latent heat if ft is solidifying at a significant rate. It is accepted however, that most of the heat generated within the earth derives from the internal heating of the mantle and it is the heat source of the mantle that is responsible for mantle convection. The mantle then controls the thermal state of the core and until the mantle exhausts its internal energy sources the mantle dictates the thermal regime of the core and the heat flux from it if any (Tozer, 1989). This further implies that but for phase changes imposed by pressure (which must have been emplaced early in the history of the earth), no other phase changes can occur associated with heat flux wihtin the core until the mantle exhausts its own energy sources. This in turn means that there will be no transport of latent heat or otherwise from the core to the mantle to provide for plume formation a difficulty well recognized (e.g., Lay, 1989). Fully developed natural convection flow characterized by convection ceUs has also been known to display periodicity in transport for large enough Rayleigh numbers (e.g. Krishnamurti, 1970) and appeal need not be made to plumes of the nature of Loper and McCartney. Scaling to mantle conditions yields a range of periodicities depending on viscosity, depth and temperature differences (e.g., Rice, 1975, 1985). These periodicities are of geological time scales which include the magic number 30 million years. It is not surprising that boundary layer analyses yield time scales of similar order of magnitudes boundary layers are

78 the characterizing features of convection. Although perhaps not providing much geometric detail the treatment of boundary layers alone has yielded a great deal about expected transport rates and characteristic times of the systems. Although convection cells do possess periodicity, a combination of cells with different phases are more likely to yield a record that would appear episodic, generating a geologic signal perhaps closer to reality. There are two modes of convection in a layer of fluid: one in which the fluid rises in the center of a cell and sinks at the sides, the other in which fluid rises at the sides of the cell and sinks in the center. The latter can occur in internally heated fluids (e.g. Tritton, 1977). Appeal tO such type of convection cell in the mantle allows a surge in the upwelling along the perimeters of the cell which will yield manifestation of apparent plume activity all along the boundaries of the cell. This mode of convection will have a much larger geographic distribution than can be had by upwellings restricted to a central region. Appeal to this commonplace process in convection rather than the artifice of placing a silk membrane between the core and the mantle resolves the problem of the widespread distribution of K-T volcanics. Although Loper and McCartney note multiple plume formation in their experiments, these multiple sources converge on their way up to reach the surface as a single entity with no large surface distribution. Widespread jumps in sea floor spreading initiating at sea floor spreading ridges has been correlated with major rifting events, increased volcanism (both subareal and subacqueous), and sea level changes (e.g., see Campsie et al., 1984, and Hallam, 1984, for a literature review). Periodicities to these variations have been suggested (Vogt & Perry, 1981). The accompanying volcanism is thought to have brought about climatic changes by CO 2 injection into the atmosphere. It has been proposed that significant warming of ocean waters from episodic spreading ridge volcanism occurs even today with important consequences (e.g., El Nino) on ocean circulation and weather (Shaw & Moore, 1988). The points discussed here and the recent proposal that an Eocene CO 2 greenhouse event can be tied to enhanced or pulsed sea floor hydrothermal activity (Owen & Rea, 1985) gives indication that Shiva's abode may at last be discovered: the mantle. Footnote: It has been proposed that acid rains contributed to the K-T mass extinctions. However, volcanic CO 2 injection may be far more important than injection of acid into the atmosphere. Acid rains of pH 1.7 are common in the Los Angeles area (Roth et al., 1985) and non-anthropogenic rains of pH 3 occur in Alaska (Klinger, personal communication, 1988). Although it is estimated that the 1783 Lakagigar eruption dumped approximately a million tons of sulphuric acid into the atmosphere, the death of one quarter of the Icelandic population and three quarters of their livestock is attributed to dry flourine and dry sulfer products (Thorarinsson, 1969). Further, highly explosive volcanism apparently contains little acid (Stothers & Rampino, 1983).

APPENDIX

The difficulties in generating melt - coolant explosions in the magmatic context are briefly reviewed here (see Rice, 1985, for detail and references). The preponderance of industrial literature indicates explosive melt - coolant interactions to take place in less than a millisecond. If the explosion is due to heat transfer from the melt to the coolant, then intimate mixing must occur within this time frame that requires amongst things that the Weber number be large enough to break the melt up into droplets much less than a millimeter in diameter. This to secure a high enough surface to volume ratio to assure fast enough heat transfer to meet obvserved detonation time constraints. These observations negate speculation that lava

79 flows into the ocean will yield steam explosions of hydrogen bomb proportions as was feared might take place at Heimaey (Kanamori, 1986, personal communication). For that matter vast amounts of lava enter the sea from Hawaii without generating an explosive response, SCUBA divers filming red hot magma advancing across the sea floor under water. The constraints imposed by the industrial experience are not to be met by naturally occuring volumes of magma. For instance, pillow basatts are not of millimeter size and their occurrence argues against explosive melt-coolant interaction between magma and sea water. Otherwise they would have been blown to bits. There are further difficulties. Water above 80° C will not explode no matter how high the temperature of the melt. If the temperature of the melt is extremely high, there will be no explosion regardless the water temperature (e.g., Dutleforce et al., 1976). In addition, dry melts, i.e., melts sparged of volatiles never explode. The hosing down of white hot slag to granulate it is a case in point. This has led to the suggestion that the cause of these explosions is quench supersaturation (e.g., Rice, 1985), i.e., explosive offgassing during solidification. The quench supersaturation mechanism provides explanation for the observation that explosions will not occur for coolants too hot or melts too hot. In both cases it is likely that film boiling occurs which greatly inhibits the heat transfer from melt to coolant (by factors of 1000 or greater). This allows the melt to cool slowly in the coolant rather than be quenched which in turn allows volatiles to bleed off quietly. The above constraints will be even more rigid in the ground water context, the effective "viscosity" of earthen materials being considerably greater than that of water and impeding by as equally large factors the required rapid intimate mixing of magma and pore water in earthen material. This has relevance to a proposed explosion source for a peculiar quake off Tiro Shima which was thought to have been brought about by the flashing of water in sea floor sediments on contact with magma (e.g., Kanamori et al., 1986). The magma will pond between basement and sea floor sediments as its density may be anticipated to be somewhat larger than that of the overlying sediments. A necessary but not sufficient condition for the magma to penetrate the overlying sediments is the rough engineering rule of thumb given by (see Rice, 1981, for references)

v > \/2gha~/~ where V is the upward velocity of the magma and h the depth of the overlying material. As this estimate excludes yield strengths, virtual mass, viscosity,, etc., it is extremely conservative and for a sedimentary overburden of 3 km thickness, indicates a required penetration velocity considerably greater than 2.5 x 102 m/s. In any case, it is unlikely sediment or melt ~ break up into millimeter size particles within a millisecond as indicated to be required (see Rice, 1985) for an explosive interaction. For that matter, argillaceous sediments this thick as found in the Gulf Coast would have interlayer and pore water squeezed out of them at the basement boundary and in addition may have gone over to claystones. It would take perhaps an hour at 1100" K to cook out structural water at atmospheric pressure, this temperature needing to rise with increasing depth, i.e., pressure (R. Reynolds, Dept. of Geology, Dartmouth College, 1987, personal communication). It can be anticipated, however, that the influx of magma at the sedimentbasement boundary Hill hydrofracture the interface, allowing the magma to flood along the basementsediment boundary. This because the resisting horizontal stress 0 h = Ko c~ v where 0 v is the hydrostatic head will likely have K o very close to zero for sedimentary materials and even if K o = 1, the hydrofracturing in the vertical would be small in comparison to that in the horizontal (N. Y. Chang, Dept. of Civil Engineering, University of Colorado Denver, 1987, personal communication). Therefore the predominate source motion would be the lifting of overburden which is consistent with the predominance of the SV signal in comparison to P and SH motion reported for this earthquake. These analyses apparently

80 came from only LP data suggesting there was little SP component in the signal which in turn would be strong evidence that an explosion was not involved. If little SP component attended the signal, than a displacement with slow rise time is suggested. It is unusual for an earthquake of this magnitude (5.5) to generate tsunamis of the size reported for Tiro Shima unless it is a source with a rise time of some length, say 100s (e.g., Kanamori, 1972). It is likely that an overpressure above ambient of only several bars will see to separation of sediment from basement. If a dike is feeding the hydrofrac between basement and overlying sediment, an estimate of the time necessary to inject magma to a certain thickness can be had from A p = 13 (V212) (L/D) f where V is the average flow velocity, p is the melt density, L the crack length and D the hydraulic diameter is taken here to be half the thickness of the injected magma. The flow should stop when the injected sill is thick enough to overcome the overpressure above local hydrostatic conditions, i.e., 5-10 m thick. A large crack implies a small roughness ratio and iteration indicates a Reynolds number in the turbulent regime suggesting a friction factor f = 0.01. This will imply that a horizontal crack extending a kilometer either side of the dike could be filled to about 10 m thick in somewhat over a minute. This type of filling may also provide explanation for the peculiar directivity of the radiation pattern of this earthquake. Using E = Apdz to estimate the energy release, the above estimate of dz, i.e., 5-10 m and an overpressure above ambient of several bars yields an energy expenditure of ca. 1020 dyne cm if the effected area is some 6 - 7 km 2. It was assumed above that the sill extended in the horizontal 1 km either side of the crack, i.e., was 2 km across. Taking the flooded area to be about 3 km long provides the effected area used above. Using the relation log E s = 1.44 M + 12.24 for this magnitude 5.5 quake yields an energy release of 1.5 x 1020 ergs, similar to the first estimation above. Kanamori et al. (1986) indicate the source of the Tiro Shima quake to be a compensated linear doublet W I T H O U T a couple, hence the moment M o of this quake should be close to the above estimates of the energy release. Kanamori et el. (1986) report, however, a moment four orders of magnitude greater. The details of their estimation of moment are not spelled out but Kanamori and Anderson (1975) provide a graphical relation of M s vs M o. Kanamori et al.'s (1985) moment versus magnitude places one at the extreme left hand edge of the graphical relation provided by Kanamori and Anderson (1975), hence the curve cannot be followed out to lower moments. Further, the value of shear modulus p. assumed for this graphical relationship is 3 x 1011 dyne/era 2 whereas that of sea floor clays is about eight orders of magnitude less at a consolidation stress of ca. 370 kPa and at 5% shear strength. The deepest sediments will lie closer to the value used in the graph but if the following relationship (e.g., Pilant, 1979) is employed l o g M o = 1.5 M s + 11.8-1og(~] ~ / ~ ) we obtain a moment without couple closer to that of the above energy estimates, i.e. ca. 1020 dyne cm. This assumes a seismic efficiency "~ ca. 1 (since lifting, not sliding, is the form of displacement, little friction is assumed to be encountered in opening the crack). It also assumes ~ ca. 1 bar and p. ca. 105 dyne/era2. Hanks and Thatcher (1972) indicate a moment of this value to have a source dimension of a k m or so which supports the effected area assumed above as do the empirical formulations of Utsu and Seki (1955) and others. Little information is given regardhlg the characteristics of the tsunami associated with the Tiro Shima quake. Tide gauges don't often reflect the full height of a seismic sea wave. Apparently, the run up on the beach did not yield a bore of great height. A conservative estimate would be to pick the maximum stable height of the wave to be 1.5 m suggesting a wave height of 1 m just before landfall. Assuming constant depth

81

d, a rough guess as to the initial wave height might be had from H/K s Kf = H o where H = i m, K s is the shoaling factor, and Kf = fHo'Ks A x/d2 where f is the friction factor, H o' = the equivalent deep water wavelength, A x is normally the fetch but taken here to be the projected source dimension on the sea surface. Using values from Breitschneider (1969) suggests an initial wave height of ca. 2 m. If the directivity of the sediment displacement of 5-10 m is preserved, such an initial wave height is plausible. The point of this discussion is to emphasize that explosive volcanism is unique in its characteristics and that other mechanisms can be raised for seismic events that do not fall within these characteristics.

REFERENCES

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82 HaUam, A. (1984): Pre-Quaternary sea-level changes. - Ann. Rev. Earth Planet. Sci., 12, 205-243. Hanks, T. C. & Thatcher, W. (I972): A graphical representation of seismic source parameters. - J. Geophys. Res., 77, 4393-4405. Helz, R. T. (1987): Differentiation behavior of Kilauea I16 lava lake, Kilauea Volcano, Hawaii: An overview of past and current work. - In: Mysen, O. B. (ed.): Magmatic Processes: Physicochemical Principles. Geochemical Society Special Publication, No. 1. Hess, H. H. (1960): Stillwater Igneous Complex. - Geological Society of America Memoir, 80, 230 pp. Hino, K. (1959): The Theory and Practice of Blasting. Nippon Kayaku. Yamaguchi-Ken. Ishlhara, K. (1985): Dynamical analysis of volcanic explosion. - Journal of Geodynamics, 3, 327-349. Kanamori, H. (1972): Mechanism of tsunami earthquakes. - Phys. Earth Planet. Interiors, 6, 346-359. Kanamori, H. & Anderson, D. L. (1975): Theoretical basis of some empirical relations in seismology. - Bull. Seis. Soc. Amer., 65, 1073-1095. Kanamori, H. & Given, J. W. (1982): Analysis of long-period seismic waves excited by the May 18, 1980, eruption of Mount St. Helens - a terrestrial monopole? - J. Geophys. Res., 87, 5422-5432. Kanamori, H., Given, J. W. & Lay, T. (1984): Analysis of seismic body waves excited by the Mount St. Helens eruption of May 18, 1980. - J. Geophys. Res., 89, 1856-1866. Kanamori, H., Ekstrom, G., Dziewonski & Barker, J. S. (1986): An anomalous seismic event near Tori Shima Japan - a possible magma injection event. - Trans. AGU, 67, 1117. Kasahara, K. (1981): Earthquake Mechanics. - Cambridge University Press, London, 248 pp. Katz, D. L. & Sliepovich, C. M. (1971): LNG/water explosions: cause and effect. - Hydrocarbon Processing, 240-244. Kerr, R. A. (1987): Asteroid impact gets more support. - Science, 236, 666-668. Kurtz, J. & Rice, A. (1988) (in prep.): The terminal Cretaceous kimberlite pipe swarms: implications for atmospheric CO 2 content. Larson, D. B. (1977): The relationship of rock properties to explosive energy coupling. - Lawrence Livermore Laboratory Report UCRL 52204, Livermore, CA. Lay, T. (1989): Structure of the Core-mantle Transition Zone. - Eos, 24 Jan., pg. 49-55. Lipman, P. W. & Mullineaux, D. R. (eds.) (1981): The 1980 eruptions of Mount St. Helens, Washington. U.S. Geol. Survey Prof. Paper 1250, xxvii + 844 pp. Loper, D. E. & McCartney, K. (1988): Shocked quartz found at the K/T boundary: a possible endogenous mechanism. - Eos, vol. 69, pgs. 961. Lovett, G. M., Reiners, W. A. & Olson, R. K. (1982): Cloud droplet deposition in subalpine balsam fir forests: hydrological and chemical input. - Science, 218, 1303-1304. Marsh, B. D. & Maxey, M. R. (1985): On the distribution and separation of crystals in convecting magma. Journal of Votcanology and Geothermal Research, vol. 24, 95-150. McLean, D. M. (1985): Deccan Traps mantle degassing in the terminal Cretaceous marine extinctions. Cretaceous Research, 6, 235-259. Medenbach, O., Fricke, A. & Schreyer, W. (1987): Fluid inclusions along shock-induced planar elements in minerals from the basement rocks of the Vredefort structure: fingerprints of an endogenic origin? International Workshop on Cryptoexplosions and Catastrophes in the Geological Record, 6-10 July, Parys, So. Africa. Moore, J. G. & Rice, C. J. (1984): Chronology and character of the May 18, 1980 explosive eruptions of Mount St. Helens. - In: Explosive Volcanism: Inception, Evolution and Hazards. - National Academy of Sciences Press, Washington, DC. Morey, G. W. (1922): The development of pressure in magmas as a result of crystallization. - Washington Acad. Sci. Journal, 12, 219-230. Morris, S. (1987): Fluid dynamic constraints on time - dependent convection in the earth's mantle. - Terra Cognita, Mathematical Geophysics Symposium, Osterbeek, June, 1986. NeUis et al. (1981): Chem. Phys., 75, 3055. Nicolaysen, L. (1985): Renewed ferment in the earth sciences - especially about power supplies for the core, for the mantle and for crises in the faunal record. - South African Journal of Science, 81, 120-132. Owen, R. M. & Rea, D. K. (1985): Sea Floor hydrothermal activity links climate to tectonics: the Eocene carbone dioxide greenhouse. - Science, 227, 166-169. Pilant, W. L. (1979): Elastic Waves in the Earth. - Elsevier Scientific Publishing Co., New York, 493 pp. Please, C. P., Wheeler, A. A. & Wilmott (1986): A mathematical model of cliff blasting, SIAM.

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VERTICAL WATER OF

ADVECTION FROM

RAPID

THE

MAIN

EXTINCTION

FROM

OXIC

PYCNOCLINE OR

RAPID

OR AS

ANOXIC A CAUSE

RADIATIONS

•• lJ

A contribution Proiect

to

GLOBAL BIO EVENTS

WILDE, Pat, QUINBY-HUNT, Mary S. & BERRY, Willim~ B. N. *)

Abstract: The vast majority of oceanic biomass fives in the surface wind-mixed layer (0-100 m) of the ocean,

trophicalty dependent on light or primary production based on photosynthesis. Waters from the main pyenocline (100-1000 m) or deeper naturally contain decay products from sinking organic matter as a function of the oxidation state of the waters. Such products, in proper concentrations, can inhibit photosynthetic growth or are toxic or debilitating to respirors. Usually, physical oceanographic processes of vertical circulation are slow or volumetrically small enough to permit "conditioning" or mixing of toxicants of the deep waters with surface waters so that the deleterious effects of deep water are neutralized or localized. However, rapid global to regional scale vertical advection of deep waters into the surface mixed layer could create an ecologic crisis for various marine groups through a combination of: (1) direct toxicity; (2) reduction or modification of nutrient and food supplies through inhibition of photosynthesis; 3) chronic debilitation caused by contact with such toxic waters; or (4) increased predation by more adaptive or less effected taxa. Such events are not necessarily universally deleterious as they could offer new opportunities for taxa ecologically restricted under prior conditions. During cool climates with oxic deep waters, a crisis may be caused by upwelling of metals concentrated with depth and resulting in reduced primary productivity, as well as metal toxic and/or chronic reactions in higher groups. During warm climates with anoxic to dysaerobic waters in the pycnocline, a crisis may result from contact with anoxic waters with a maximum effect on respirors and a minimal to enhanced effect on phytoplankton. Upwelling may come from three redox zones: I- oxic; II- nitric; and III- sulfatic. Each zone would be the source of waters of differing chemistries that could be advected into the photie zone. The effect on spedtfie taxa will be selective as a function of the depth and volume of source water, as different organisms have different tolerance limits or preadaptive capabilities. In the geologic record, significant upwelling events would be recorded initially as a general reduction in diversity, followed by mass extinctions in some groups and the possibility of rapid radiation in opportunistic groups. The ecologic requirements of both the extinct taxa and the newly enhanced taxa might be used to identify the type of any given major upwelling event.

I NTRODUCTION

Wilde and Berry (1984, 1986) discussed the oceanographic conditions which could contribute to rapid changes in marine communities and to apparent mass mortalities. They proposed physical oceanographic processes which bring chemically different waters from depth into the life range of fossil-producing organisms. Wilde and Berry (1984) concentrated on the physical mechanisms required to produce rapid overturn of deep water. Expanding on this theme, Wilde and Berry (1986) showed that chemical variability in the water colnmn with depth, produced by both oxic and anoxic decomposition of surface-produced organic matter, when introduced into surface waters, could trigger rapid changes in the marine biota. This paper discusses how the scale of vertical advection, both in depth and areal extent, influences the riving conditions of marine biotas ranging from essentially marked enhancement to rapid extinction.

*) Marine Sciences Group, University of California, Berkeley, CA, 94720, U.S.A.

86 GENERAL CHEMICAL CONDITIONS The ocean is generally stratified with respect to density (temperature and salinity) and light. Most organisms live in the photic zone. Organisms are exposed to and adapt to different chemical environments, depending on time, geographical location and where they live in the water column. In the modern ocean, phytoptankton and other organisms in the surface layers consume and sequester various metals and compounds keeping the surface waters low in nutrients (Figs 1 a-d) and dissolved metal ions (Vinogradov, 1953; Goldberg, 1957; Knauer & Martin, 1983). At depth, as organisms decay, nutrient species are released causing their concentrations to increase.

200

20

Retire,

40

400 =

=

Pacific O c e a n near H a w a i i a n Islands

600

60

Q 80O

80

1000

100

A

1200 0

120

-

10

20

Nitrate

30

40

50

0

(omollkg)

Concentration

1.0 Phosphate

2,0

3,0

Concentration

4.0

(pmot/kg)

200

200

400

400 £ o

Q

600

600

800

800

1000

1000 0

20

40 Silicate

00 Concentration

80 (IJmol/kg)

"~00

120

0 [ 0

50 I 1

100 I 2

150 pmol/kg I 3 mLIL

200 I 4

250 I 5

I 6

D.O, Concentration ( p m o l / k g )

Figure 1 : Distribution of the nutrients in the water column in the tropical-subtropical Atlantic and Pacific Oceans. A. Nitrate. B. Phosphate. C. Silicate. D. Dissolved 0 2. From Ouinby-Hunt and Wilde (1987) with permission of Marcel Dekker, copyright 1987. Data from the GEOSECS expedition.

87 The oxygen concentration in the oceanic water column varies significantly with depth (Fig. 1 d). At the surface the oxygen concentration is a function of the atmospheric concentration, temperature and salinity. Through the photic zone, oxygen is consumed or released by differewt organisms resulting in a subsurface mmfimum. Below the photic zone, as a result of the consumption of oxygen by oxidation of decaying organic material, the concentration declines dramatically. Below the pycnocline, the oxygen concentration in the modern ocean increases due to an injection of high latitude deep and bottom waters. Oxic waters are defined as having chemically active dissolved oxygen as the primary oxidant. If sufficient organic matter is present, all dissolved oxygen will be consumed and other agents will be used to oxidize organic matter. In the oceans, the thermodynamic sequence of available oxidants after free oxygen are the oxygenated nitrogen species followed by oxidized sulfur species, principally sulfate. Fig. 2 shows the areas of the modem ocean with dissolved oxygen below 0.4 mL/L. In the modem ocean the result of oxidation by oxygenated nitrogen species may be seen in the eastern tropical Pacific (Fig. 3).

.

III11~ e a wi~ maximum den~tT" of E. eximia ~ " ~ /Yea with maximum density of E. distinguenda

Figure 2 : Hachured regions show the distribution of modern marine waters with dissolved 0 2 < 0.4 mL/L (After Denser, 1975). Shaded areas show characteristic species endemic to oxygen-poor waters in the eastern tropical Pacific (After Brinton, 1980). Projection is Goode's Homosline equal-area, copyright, University of Chicago. (From Berry et al., 1987, with permission of Goological Society of Denmark,

copyright, 1987). In the nitric zone, nitrate and nitrite act as oxidizing agents producing nitrite, nitrogen and zmmonia. The concentration of nitrite in the nitric zone initially increases with depth, then decreases as nitrate is consumed, and subsequently, nitrite assumes the role as oxidant (Fig. 3). In the suffatic zone, sulfate is the oxidizing agent resulting in production of sulfides and ammonia. The sulfatic zone exists today in the Cariaco Trench, the Black Sea and in some tjords, such as Lake Nitinat. In the ancient oceans, particularly in warm, calm conditions, the extent of the nitric and sulfatic zones would have been greatly expanded. In the photie zone, various metals and compounds are taken out of sea water during primary production and released back into sea water below the photic zone by oxidation of sinking organic matter (Redfield, Ketehum & Richards, 1963). The distribution of such metals in the water column generally follow the concentrations of dissolved nutrients with depth, but as a function of their solubility (Fig. 3).

88 For oxic conditions seen in the modem ocean, Quinby-Hunt and Wilde (1987) reported that the concentrations of Cr, Ni, Zn, Ge, Se, Sr, Cd, I, and Ba correlate quantitatively with the concentrations of the nutrients N, P, or Si. In addition, Be, Mn, Fe, Co, Cu, Ga, As, Pd, rare earth elements, Hg, Rn, and Ra also are partially nutrient-related, but no relatively simple expression has been developed relating these elements with nutrients. As the concentration of the nutrient-related elements generally increases with depth in oxic waters, their effect on organisms living in the photic zone as a function of concentration can be related to the depth of vertical advection or upwelling. The concentrations of some of these elements, notably Fe and Mn, also controlled by scavenging and redox considerations, tend to decrease below the mixed layer, although the concentration may increase to some degree in the oxygen-minimum zone. Under anoxic conditions, the distribution of nutrient-related elements may differ from that under oxic conditions. If the waters are from the nitric zone (Wilde, 1987), such as the denitrification zone of the Eastern Tropical Pacific and the Northern Arabian Sea of the Indian Ocean, then the concentrations of many dements may be expected to increase. Notably, the concentrations of Fe and Mn might be expected to increase because neither the O =, or S =, which tend to precipitate these elements are present. Other elements whose concentration in the oxic and sulfatic zones is solubility controlled will also increase. If anoxia has reached the stage of sulfate reduction, those elements in sea water which form sulfides will form mineral complexes and be removed from the water column. Thus, Cu, Cd, and Zn are depleted below the redoxocline in the Black Sea (Brewer & Spencer, 1974) and the Cariaco Trench (Jacobs et al., 1987) (Figs. 4 b, 4 c). In anoxic waters, metals that form sulfides are depleted. In the sulfatic zone, Fe is more depleted than in the nitric zone, but appears in significantly higher concentrations than in the oxic zone (Fig. 4 a) due to greater solubility of the iron sulfide vs ferric hydroxide. Manganese occurs in greater concentrations in the nitric and sulfide zones than in the oxic zone (Fig. 4 d). Oxygen Minimum Zone Off Peru Micromoles/Liter 02

0

250

NO~ 0 5 0 '~a.,~&.;---~,

500 10 ,

,

,

Au~ 2 0 0

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600

i o2 i ;

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, 20

, NO 3

~ 40

.o3 ~. , ~ T 60

600

oo,,L .-.M.~ ~ 320

a

800 280

240

200

160

120

80

40

0

Distance From Shore (kilometers)

Figure 3 : Vertical distribution of nitrate, nitrite and dissolved 0 2 .in the eastern tropical Pacific off Peru (After Anderson et al., 1982). From Berry et al., 1987, with pernnsslon of Geological Society of Denmark, copyright, 1987.

Figure 4 (opposite side) : Comparison of the vertical distribution of trace metals in the oceanic water column in the modem oxic ocean and the Cariaco Trench. A. Iron (N. Pacific: Gordon et al., 1982). B. Copper (N. Pacific: Bruland, 1980). C. Cadmium (N. Pacific: Bruland, 1980). D. Manganese (N. Pacific: Martin & Knauer, 1984). All data for the Cariaco Trench are from Jacobs et al.. 1987.

89

Cu conc. (nM/kgJ

Fe conc. {nM/kgJ

i

t

l

l

t

r

(

l

l

l

l

o o o o o o l t l

l

/ f Cariaco Trench

Cariaco Trench

N.Pacific

o o

N.Pacific

A Mn conc. (nM~

Cd conc,(pM/kg) (Thousands)

o

~

~

~

~

g

~

~

~

~

-

=

I

Cariaco Trench

N.Pacific

N.Pacific

o

o

o Cariaco T r e n c h

C Figure 4 (see opposite side for explanation)

D

90 PHYSICAL

PROCESSES

OF V E R T I C A L

ADVECTION

Wilde and Berry (1986, p. 81) discussed three major geographical scales of upwelling produced by several physical processes: I. Planetary (Oceanic); II. Regional; and Iit. Local. Upwelling on a oceanic scale may be the result of divergence of water by Ekman transport produced by change in direction of zonal surface winds at boundaries of major climatic belts. The effective maximum depth for upweUing in this case is that of mixed layer (ca. i00 meters); the vertical rise would be 10 to 80 meters/month. (Apel, 1987, p. 270, 10-6 m/see = 30 m/yr, equatorial). Divergence due to Ekman transport occurs at the equator and the temperate/polar boundary (60°N and 60"S). Vertical advection on an oceanic scale also may occur as a result of displacement by continual renewal of water masses at source. In this case, the vertical rise is of the order of milllmeters/day. Thus, one mixing cycle occurs about every 1000 years (3 m/yr: Apel, ].987). Overturn of deep water to the surface could also occur on an oceanic scale. The vertical rise probably would be rapid, greater even than for Ekman transport. Horizontal movement is from high to lower latitudes. Regional (].00 to 1000 km) upweUing may be due to seasonal Ekman transport: for example, due to atmospheric high pressure off western coasts. Regional upweUing also may be due to off-shore advection, that is, entrainment by major oceanic currents moving off-shore or by vertical Coriolis deflection and surfacing of Equatorial undercurrents as eastward flowing undercurrents rise. Wilde and Berry (1986, p. 8].-82) listed some of the physical processes that can produce local ( < 100 kin) upweUing. Obstruction of horizontal current flow by banks or seamounts produce vertical advection in Taylor columns along the leading edge of the obstruction. Closed cyclonic eddy circulation, that is, oceanic "weather" spin-off, and migration of cold-core eddies from Rossby waves in major currents may cause vertical advection of deep waters causing local upwelling. Bernoulli uplift, by flow through constricted straits; breaking internal waves or internal surf also cause local upwelling.

BIOLOGICAL

IMPLICATIONS

DEPTH AND REDOX

OF T H E V A R I A T I O N

OF CONCENTRATION

WITH

CONDITIONS

The effects of rapid global to regional vertical advection of deep waters into the mixed layer could cause ecologic crisis due to: (1) direct toxicity or enrichment; (2) reduction/modification of nutrient or food supply;, (3) chronic debilitation due to contact with deep waters; and (4) increased predation by taxa adapted to the new water mass chemistry. The effect of a dissolved chemical on a biotic community is a function of the concentration of that chemical, the temperature and pH of the water, and other constituents present in the water column. A particular compound may be inert (no effect), limiting (required at a certain minimum concentration), inhibiting (also referred to as sublethal or chronic, that is debilitating for short exposures or with lifeshortening consequences), or toxic (lethal at certain concentrations). Elemental concentrations do not always indicate whether the above effects will occur. Chelating agents, antagonistic or synergistic elements (antagonistic elements reduce the effect of an element; synergistic elements enhance the effect), redox state, pH or temperature modify the actual chemical configuration or activity experienced by the organism (Bewers & Yeats, 1977; Anderson & Morel, 1978, 1982; Thomas et al., 1980). Thus, the activity of the chemical species rather than the absolute concentration is the important consideration (Jackson & Morgan, 1978). Fe, Mn, and Zn are limiting for a number of organisms. Diatoms, eoccolitt~, dinoflagellates, and

91

cyanobacteria are Fe-limited in neritie waters for activities less than 10-7 M and in pelagic ocean waters at activities less than 10-9 M (Brand et al., 1983). The concentration of dissolved Fe in the open North Pacific Ocean is much less than 10-9 (Gordon et al., 1982). The same groups of organisms are Mn-limited at activities less than 10-10 M in neritie waters. In the oceanic pelagic realm, all but coccolitlas are limited at less than 10-10 M. In the open ocean, dissolved Mn is about 0.2-4 x 10-9 in the mixed layer (Landing & Bruland, 1980; Martin & Knauer, 1984). Concentrations of both Mn and Fe are sufficiently low in the open ocean that they may be controlling for some protic species (Martin & Gordon, 1988; Foster & Morel, 1982). In neritic waters, Brand et al. (1983) reported that certain neritic diatoms are Zn-limited at activities less than 10-11"5 M; but in the pelagic oceans, other phytoplankton (including diatoms, coccoliths, dinoflagellates and cyanobacteria) are not limited even at activities as low as 10-13 M (Brand et al., 1983). Although metals such as Zn and Cu are limiting at low activities, they may be toxic at high activities (Sunda & Guillard, 1976). For example, the deleterious effects of copper on the diatom, Thalassiosirapseudonana, are chronic above activities of 3 x 10"11 M, and toxic above 5 x 10"9 M. The concentrations of Cu in the upper open ocean are of the order of 10-9; therefore organisms with sensitivities similar to T. pseudonana exist in conditions where Cu concentrations, even if strongly chelated, are inhibiting to growth. Not all of the nutrient-related elements have either a plastic or catalytic role (senso Dietrich, 1963, p. 246) in the biology of marine organisms, particularly those in higher trophic levels. Some elements or compounds are sequestered passively in the lipids of living organisms (for example, the highly toxic methyl mercury, Boney, 1975, p. 103). For lower trophic levels, such concentrations may be harmless. But by concentration in successive trophic levels, certain dements or compounds may reach chronic or toxic levels detrimental to organisms ha higher trophic levels. Provasoli (1963) found such a situation for the toxicity of dinoflagellates to higher trophic level during Red Tide upwellings off the southwest coast of Africa. Morel (1986) has noted that for algae, which in the ocean are the major primary producers, Cd, Pb, and Hg (all non-essential elements) are toxic at low concentrations. Accordingly, a crisis could be produced by upwelling of enriched metals or other compounds from deeper water into the surface layers. UpweUed toxic waters that inhibit photosynthesis could cause a crisis in groups with short food chains by reduction of primary production. During warm climates and potentially anoxic waters in the pycnoctine below the mixed layer, the crisis could be due to low levels of oxygen and its related chemistry so that more active organisms would be affected more than sessile or low-oxygen tolerant organisms. Conversely, upwelling of nutrient species including those trace metals whose concentrations are limiting, could result in enhanced living conditions for some species, resulting in a bloom. In either case, the species composition may change (Sunda et al., 1981). Sunda et al. (1981) have shown that in water from off the North Carolina coast in which the diatom Nitzschia and the green flagellate Chaetoceros where common elements of pelagic communities, Cu additions caused a shift in species dominance from the diatoms to the flagellate.

POTENTIAL

I M P A C T S OF U P W E L L I N G E V E N T S

The effects of upwelling on biotas depend on the depth of upwelling which will in turn control the physieo-chemical conditions of the upwelled water. For modern open ocean conditions with well-ventilated deep waters, generally oxic conditions occur regardless of the depth of upweUing. In restricted locations where a major denitrilication zone occurs, such as the Eastern Tropical Pacific (see discussion in Berry et al., 1987), waters representative of the nitric zone might be vertically advected during enhanced upwelling.

92 In basins such as the Cariaco Trench, the Black Sea, or tjords such as Lake Nitinat, upwelling of sulfatic waters could occur. Brongersma-Sanders (1957) and Richards (1965) have given various incidences of mass mortalities due to the introduction of sulfides into the water col~lrnn: For the Phanerozoic, Wilde (1987) proposed that three redox zones: oxic; (2) nitric, i.e. anoxie, with no sulfide production; and sulfatic, i.e. anoxic, with sulfate-reduction, existed in the open ocean on a regional to planetary scale. This zonation assumes a wind-mixed oxygenated surface layer of 50 to 100 meters below the surface. Variations in redox occurred in the underlying pycnoeline as a function of the efficacy of deep ventilation (Wilde & Berry, 1982), which, presumably, was related to climate. Upwelling of increasing depth would result in penetration into the superimposed oceanic redox zones. Thus, for an ocean with a discrete anoxic layer below the surface mixed layer, such as proposed by Wilde (1987; Wilde & Berry, 1982), there is a potential for u p w e 11 i n g f r o m a 11 t h r e e z o n e s at the same location, depending on the depth of upwelling. Upwelling, in order to influence extinction or killing events, must be sufficiently rapid that chemical eqttilibrium is not attained during the rise.

Upwelling from the Oxic Zone

With duration of upwelling and/or increasing depth of the source of upwelled water, more decay products would be brought into the photie zone as oxidized metals. Therefore, with increasing depth, increased quantities of the dissolved micronutrients, N (as nitrate), P, and Si wilt be brought to the surface layer. Increased levels of Cd, Cu, Zn, Co, Ni, Se, Cr, Ba, Ge, As, Pal, Te, I, REE and organic decay products are also expected. Lower concentrations of dissolved oxygen would be brought to the surface from the oxygen minimum zone. Fe and Mn concentrations generally decline with depth, although there can be increased concentrations in the oxygen-minimmn zone. Thus, Cu and Cd algal toxicity would increase. However, the availability of nitrate, phosphate and silicate would increase. The upwelling of Mn-poor waters, such as those of the Sargasso Sea, may result in acute Mn deficiency (Sunda & Huntsman, 1983). The conjunction of waters low in Mn and higher in Cu could result in enhanced toxicity as Cu is antagonistic to Mn-usage in some diatoms (Sunda & Huntsman, 1983). A similar relationship exists between Cd + 2 and Fe + 3: Cd toxicity can be reversed if there is sufficient Fe + 3 present (Foster & Morel, 1982). Upwelling of deep oxic waters depleted in Fe + 3 or Mn, but with high levels of Cd + 2 or Cu could have greater to,deity for some species than would nitric zone waters that would have dramatically increased levels of Fe and Mn. Due to the influx into the photic zone of unchelated metal ions, such as Cu or Cd (Barber et al., 1971; Terry & Caperon, 1982) and low levels of 02, photosynthesis would initially be suppressed even though additional nutrients also would be entrained. If the depth of upwelling is great enough, suppression might continue to drastically reduce primary productivity. This would be particularly critical in middle to high latitude locations if the upwelling occurs during the normal time of plankton growth when seasonal light levels also are critical. With the suppression of primary productivity, effects would be felt in the higher trophic levels in the water column and eventually among the benthic seston feeders. Under most situations involving upwelling of oxic waters, a phytoplankton bloom (due to increased levels of nutrients), and an increase in zooplankton grazers would be expected to follow the initial suppression. These upwelling conditions could be advantageous to benthos above the oxygen minimum zone. If the oxygen demand of the bloom is excessive, eutrophication may occur causing death of respirors and benthos if anoxia reaches the bottom. Blooms of certain dinoflagellates may be inherently toxic to predators.

93 Upwelling from the Nitric Zone

In the nitric zone (where nitrate is the oxidant), concentrations of micronutrients (P as phosphate, Si as silicate and N as nitrite and ammonia) increase with increasing depth. Certain trace met',d concentrations, Fe, M.n, Cd, Cu, increase with depth because of the lack of precipitating anions such as sulfide. The nitric zone occurs at relatively shallow depths at the top of the pycnocline, about 100 m. The increased concentrations of micronutrients, as well as reduced nitrogen compounds 0~ppley et al., 1969) and the limiting trace metals, Fe and Mn, would increase productivity and may suppress Cu or Cd toxicity. In the nitric zone, there would be reduced nektonic activity, and little or no benthic megafauna. The oxygen-poor waters would inhibit respiring organisms, especially nekton. Such conditions would give advantage to specialized low-oxygen tolerant types, such as the copepods Euphausia distinguenda and E. eximia (Brinton, 1980), which occur in the eastern tropical Pacific off Peru (Fig. 2). The sequence of biological events as a consequence of upwelling from the nitric zone initially would be enhanced primary productivity, but rapid reduction in active grazers and nekton. Increased productivity would secondarily add to the oxygen demand in the water column and cause a reduction in benthic populations, even with additional seston food supply.

Upwelling from the Sulfatic Zone

In the sulfatic zone, micronutrient concentrations, N (as ammonia), phosphate, and silicate, increase with depth due to continuing decay of organic matter. Concentrations of Fe would be greater with depth than in oxic ocean waters, though probably less than in nitric zone waters (Fig. 4 a). Mn concentrations are greater in the sulfatic zone than in the oxic zone, and similar to those in the nitric zone, based on observations from the Cariaco Trench (Fig. 4 d). The concentrations of Cu, Cd, Zn, and any cations that form sulfides will decline due to sulfide precipitation (Figs. 4 b, 4 c). Ni is unchanged with depth. Sulfatic zone waters are oxygen-depleted, and with depth have increasingly higher concentrations of toxic hydrogen sulfide. During upwelling of sulfatic zone water, primary productivity will increase due to increased levels of ammonia, particularly for those phytoplankton that are Fe- or Mn-limited. On the other hand, primary productivity may decrease among other phytoplankton due to low levels of trace nutrients, such as Zn and Cu. For animals, acute hydrogen sulfide toxicity will cause losses of nekton and benthic megafauna with an excellent chance of preservation of large organisms in anoxie bottoms. Thus, upwelling from suIfatic waters would be characterized by enhanced primary productivity and elimination of nekton and respiring benthos.

P O T E N T I A L E X A M P L E S IN G E O L O G I C RECORD

Evidence of extinctions or killing-events in the geological record would appear as reductions in diversity, mass extinction of some taxa, followed by rapid radiations among surviving and newly adapted organisms if conditions remained stable. Survivors could return if preexisting conditions were reestablished. Evidence for mass mortality/radiation events due to vertical advection from the oxie, nitric or sulfatic zones may be found in the rock record and are discussed below. Possible examples of vertical advection from the oxic zone are found in sequences from the end of the Ordovidan and from the end of Triassic. A mass mortality/radiation event in the late Weuloek could be due to upwelling from the lower nitric zone. The extinctions evidenced in the early Jurassic Toareian shales may be the result of upweUing from the sulfatic zone.

94 Advection from the Oxie Zone

At the end of the Ordovician, graptolites suffered mass mortality at various tropical localities, including Dob's Lima, Scotland, Anhul, China, and Mirny Creek, USSR (Berry et al., this volume). The latest Ordovieian graptolites were restricted to limited areas in the tropics of the time. Each incidence of near extinction was a cousequence of a local event. At each site, the environment changed from one inhabited by graptolites to one in which graptolltes were virtually excluded. That change involved introduction of oxic waters into the oxygen-poor waters inhabited by graptolites resulting in graptolite mass mortality. Such oxygenation could be the result of vertical advection of pyenoclinal waters vent~ated during the glaciation that occurred at the end of the Ordovidan. Advection of colder, more oxygenated, and more metalliferous water may have affected the graptolites themselves or their food supply. In the late Triassic, House (1985) has shown a massive, world-wide decline of ammonites in the interval of the latest Norian through Rhaetian, the latest part of the Triassic. By the close of the Triassic, ammonites had become virtually extinct. Triassic ammonites lived primarily on the margins of the tropical shelves of the time. They seemingly lived primarily in low oxic (dysaerobic) waters under similar environmental conditions as those inhabited by graptolites at an earlier time. During the late Triassic, major marine transgressions began across many of the low-lying continental areas (Sellwood, 1978). Anderton et al. (1979) described a major transgression in Britain that began in the south and spread initially over lagoons. Volcanism occurred in southern Europe, and rocks from the Aquitaine and New England suggest that an initial phase of North Atlantic rifting began in the late Triassic (Anderton et al., 1979). Accordingly, the new ocean became connected to the world ocean with better circulation. As the Atlantic opened, the rise in sea level presumably was accompanied by advection of more oxic waters into ammonite habitats. Introduction of these waters resulted in ammonite extinctions that took place over a several million year interval. The length of time of the extinctions suggests that the demise of the ammonites may have resulted from a long-term chronic condition. Throughout the Jurassic, ammonite diversity increased, notably in species better adapted to oxygenated waters. Apparently, as a result of the opening of the Atlantic, new patterns of vertical advection supported the radiation of these species.

Advection from the Nitric Zone

In the late Wenlock, mass mortalities of tropical graptolites (Rickards et al., 1977) occurred concurrently with development of massive carbonate reefs after a long interval of limited reef growth (Berry & Boucot, 1970; Ziegler et al., 1974), as did the development of land plants with vascular tissue (Cooksonia) (Edwards & Feehan, 1980). Both marine events were of relatively short duration. The marine events took place in water oceanward from the shallow subtidal zone. The oldest vascular tissue was found in early Ludlow strata in Wales, which was tropical in the Silurian. These events can be explained by expansion of the nitric zone toward the surface. UpweUed waters from the deep nitric zone would contain no oxygen and would be elevated in ammonia (see above). Vertical advection of anoxic waters could have caused mass mortalities of graptolites via a number of mechanisms. Mortalities might be caused by lack of sufficient oxygen to support either graptotites or their food. If graptolites were forced into upper levels of the mixed layer, they may have been exposed to greater predation. By the time ammonia was advected into shallow waters, it would have been diluted sufficiently to be a nutrient rather than a toxicant. This diluted ammonia would have increased productivity, thereby providing increased food supply for the reef-building corals.

95 Increased levels of ammonia could also increase the alkalinity sufficiently to favor carbonate precipitation necessary for coral reef building. Nutrient levels of ammonia in tidal waters may have provided the necessary fertilization for the development of vascular land plants in the nearshore environment. As upwelling from this zone ceased, the graptolites reradiated, and coral reefs receded.

Advection from the Sulfatie Zone

Hallam (1967, 1975, 1977, 1981), Jenkyns (1985), Wilde et al. (1986), and Riegel et al. (1986) discussed the Lower Jurassic Toarcian extinctions in terms of physical conditions relating oceanic redox conditions with transgression and regression. The extinctions can be explained in terms of an upweUing of sulfidic waters. Toarcian rocks in Great Britain and Europe are organic-rich shales with a non-bituminous argillaceous facies above and below. The fauna is characterized by nektonic ammonites, belemnites and fish scales; and mierofossils: ostracods, foraminifera, and radiolarians (Kauffman, 1978, 1981). The benthic fauna are of limited diversity with large populations. Massive mortalities, consistent with upwelling from the sulfidic zone, are observed in Yorkshire at the base of the Jet Rock (Hallam, 1967; Jenkyns, 1985) and in Germany, in the Posidonia Shale (Riegel et al., 1986). In the Posidonia shale, pyritic concretions, indicative of the presence of sulfidic waters, are common in the lower bifrons zone (Riegel et al., 1986).

CONCLUSIONS Vertical advection or upwelling is a mechanism that could cause selective extinctions or killing events. By this mechanism, step-wise or gradual extinctions are readily explained as are survivals. The impact of upwelling is a function of depth, temperature and the chemical properties of the source water. The redox properties, that is whether the water is ode (oxygen as oxidant), nitric (oxygenated-nitrogen species as oxidant), or sulfatic (sulfate as oxidant) control the effect of such waters on the biota. The chelating tendencies of the water, organismal tolerances, and the presence or absence of antagonistic or synergistic elements may determine whether an organism thrives, exists or dies. Impacts equally depend on the taxa present, their tolerance limits and preadaptive capabilities. The results of massive upwelling events would be characterized in the geologic record by decreased diversity of species, and, possibly, certain areas depanperate of species followed by radiations.

ACKNOWLEDGEMENTS

The authors wish to thank Profs. A. Boucot, W. Holser, E. Kauffman and O. Walliser for useful discussions and encouragements at Boulder. M. Krup drafted the figures and coordinated the preparation of the manuscript with her usual efficiency. This is contribution number MSG-88-003 of the Marine Sciences Group, University of California, Berkeley.

REFERENCES

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AN A S T R O N O M I C A L

EXPLANATION

LOUS CONCENTRATIONS DURING CATASTROPHIC

OF IRIDIUM EXTINCTIONS

OF

ANOMAELEMENT

•? U

A to

contrlbutlon Project

GLOBAL BIO EVENTS

YANG, Zhengzong *)

Abstract: In this paper a new possible explanation of iridium anomalies in the extinction boundaries is proposed. I found that the impact of an object is not necessary to be in the theory when we try to explain the iridium anomaly.

INTRODUCTION

Kepler's law states the solar system revolves around its galactic center and therefore must be affected by many astronomical factors. These effects can bring about many anomalous events in the solar system, which can, in turn, affect the Earth. There are many astronomical factors that can ~ffect the Earth directly in the solar system. These combined effects may cause many occurrences of rare events on Earth. Rare events are also produced by the Earth itself. Many catastrophic events of varying magnitude have occurred since the Earth was formed. When these events occur, they may reflect certain galactic laws. In this paper, the anomalous concentration of Ir in the K-T boundary clay is discussed. There are many causes of catastrophic events. Many astronomical phenomena, for example the impact of an asteroid or a comet showers, cosmic dust storms, etc. can result in a series of rare events including anomalous concentrations of Ir, mass extinction, and glacial ages. We have no definite explanation for the anomalous concentration of Ir at the K-T boundary. Several models have been proposed to explain this phenomenon, such as the impact of an asteroid or a comet, the explosion of a supernova, etc. Here we propose a model for the occurrence of a large meteoric storm at the end of the Cretaceous.

A

POSSIBLE ASTRONOMICAL EXPLANATION OF I R I D I U M ANOMALIES

What is the cause of iridium anomalies at the extinction boundaries? What was its influence throughout geological history? Many workers have tried to interpret the iridium anomaly found at the K-T boundary. Some think it was generated by a reaction inside the Earth (e.g. volcanism). But this hypothesis has difficulty in explaining some of the facts (e.g. trace element composition, microsphere, multiple shock lamellae). Some workers have ascribed it to an extraterrestrial cause. There are four major extraterrestrial hypotheses that have been proposed: the explosion of a supernova (Napier & Clube, 1979), the impact of an asteroid (Alvarez et al., 1980), the impact of a comet (Urey, 1973; Hsu, 1980), and periodic comet showers triggered by close passage of solar companion star or field star. Other hypotheses exist (Gartner & Keany, 1978; McLean, 1978; Hut et al., 1987; Rampino & Volk, 1988). Alvarez et al. (1980) proposed that an asteroid impacted on the Earth, causing the iridium anomaly. However this hypothesis has a problem: An asteroid is a solid mass. When it impacts on the Earth, the kinetic energy transferred creates a huge *)

Beijing Astronomical Observatory, Academia Sinlca, Beijing, China

100

meteorite crater and impact debris. This hypothesis fails to explain why the density of iridium in some areas, or craters is less than the normal predicted density of Ir. In 1973, Urey proposed the cometary hypothesis. Hsu (1980) elaborated on it further. This hypothesis has some advantages. But it has similar difficulties as the Alvarez et al. (1980) hypothesis, the main characteristic of the two hypothesis is the impact. In this paper a new hypothesis is proposed: Anomalies of iridium at the K-T and other extinction boundaries were the result of large meteorite storms. The process is described as follows. A meteoric shower (not a comet shower) is composed of many meteors falling into the atmosphere in a short period of time. A large meteorite storm is judged by the scale of the meteoric stream. There are several sources which may produce a large meteoric storm: a) A comet may collapse in on itself after revolving round the sun for about 106 years. The collapsed comet turns to many meteoroids. Newly formed meteoroids distribute nonuniformly into Earth's orbit. The large meteoroid group is more nonuniform in distribution. The meteoroids scatter because of radiations. A large meteoric shower would occur if the Earth ran into a large meteoroid group, b) Owing to the perturbations of stars in the Galaxy and planets in the solar system, a comet which is normally far from the sun could enter the Earth's gravitational field. The Earth's tidal action and radiative heating of the sun could force the comet to break apart if it is made of brittle matter. The collapsed comet becomes many meteoroids. If the Earth attracts these meteoroids into the atmosphere in a short period of time, a large meteorite storm will occur. The influence of a large meteoric storm on the Earth can be estimated by a simple calculation. The mass of a large meteorite storm is nearly the mass of a comet, about 1013 tons. In normal situations the total mass of the fallen meteors is about 2 x 105 ton each year. When a large meteoric storm occurs the total mass is 108 times higher than normal. The total volume of the meteors in a large meteorite storm is: M

V -- - ~ = 1019em 3

(1)

where M is the total mass, which is about 1019g, and D is the density of the meteoric stream which hasn't scattered; this is the same as the density of the cometary head, about I g/cm3. Thus, after the occurrence of a large meteoric storm, the thickness of the meteoric dust deposited on the Earth's surface is: h= ~

V

= 1.96era

(2)

where R is the radius of the Earth. This estimate of the thickness is nearly at the upper limit (h varies with M). The fallen dust of meteoric debris would be very thick, and sufficient to be one of the causes of iridium anomalies. After a large meteorite storm appears, the time of suspended meteor dust in the atmosphere can be calculated. According to the Newton's law, we have dv m - ~ = m g - 67r~rv

(3)

where ~ is the viscous coefficient of the atmosphere (we take it as 180 x t0-6p), r is the radius of the dust (the dust is spheric in the model), and m is the mass of the dust. The equation can be rewritten as dv ~ + c v =a

(4)

101

where

c = 6~r~?r/m.

We assume that v = v0 at h0, the equation is solved v=

~ ( e ° ~ + v ° - - £c - 1)e - ° t g

(5)

ho = tt + ~(~0 + ~)(I- e -°')

(6)

and

C

Once we knov¢ c, we can fred the settling

180 x lO.Sp (atmosphere)

1.5x 10"2p (sea)

0,01

2.7

225

0.001

270

225 x 102

0.0001

27000

225 x 106

time t of the dust. Putting the corresponding data into the above equations, we can obtain the constant c and settling time t.

200 km (atmosphere)

100 km (atmosphere)

1000 m (sea)

0.01

0.6

0.3

0.3

0.001

64

32

26,5

Table 2 : particles.

17.5 years

culates c. In Table 2, the settling times are calculated. During the settling time the

Table 1 : The value of constant c.

0.000t

Table 1 gives reasonable 3] and r and cal-

8.75 years

7,3 years

dust is floating in the atmosphere, and may block radiation from the sun. This effect can be calculated as follows. The decrease of the radiation flux in a layer dl is proportional to the number of dust particles and size of the particle. So we have:

The settling time of different radius of dust

dI I

N=r2dl

(7)

where I is radiation flux and l is penetration length, which is h 0 in our case. Solving this equation we derive: I = Ioe - m ~

(8)

= e-x = T

where T is the fraction of the solar radiation penetrating the dust. N I is the number of dust particles per unit area on the Earth's surface. If we take the mass of the comet is 1015g and r is 0.001 cm, we have T = 1. If we take the mass of comet is 1018g, we have T ~ 0. Here we can see that if a meteoric storm caused by a collapsed comet is small (M = 1015g) and has only slight influence; but in a large storm caused by rather big meteoric (M = 1018g), the situation will be very serious and mass extinction may occur.

THE

EFFECT

OF A LARGE

METEORIC

STORM

ON THE

EARTH

When a large meteoric storm appears in the atmosphere, one possible effect on the Earth would be to change the temperature. The temperature would increase in a short time and then decrease. The low temperature phase would remain for a long time. The explanation is as follows. When a meteor falls into the atmosphere, it is burning due to friction. Finally, it may totally become dust before impact. The dust remains suspended in the atmosphere. In this process the potential energy of the meteor turns into kinetic energy and then turns into heat. The heat increases the temperature of the

102

I

Cornet (in the spacesphere)

I

Meteoroids (in the spacesphere)

l1

t

''' A laq~ n ~ t ~ storm (inthe spacesphete) i

XCN + 0 2 . . . . . • + CO2 C02 in the meteoroids (in the atorosphere)

I RichIr in the dust (in the lithoshphe~e) The anomalyof Ir coverslargearea aroundthe world

Reductionof radiativeenergy I I from the sun (in the atomsphere)

T

1

"'

....,,

t

I

Lessdeathsof marine t t I Lessdeaths I Jbiocommunilies J I°!f'~n"=~l

I

balanceof CO2 (in the atomspehre andwatersphere)

Damageof (ntPh°~¢:Y~o~hpe~ei']Sre)I~eari~r~2:here,,I

!

t

Increaseof the temperature J (in the atomspher)

I

Damageof CCD (in the hy~oc~phere) I

t

I~°fterrestlia'

I

t

T

I Deathsof calcurcous marinefauna 1 Figure 1 : The Extinctional Process by a Large Meteoric Storm.

atmosphere because of conduction and circulation of the air. With the diffusion of the dust into the atmosphere, the atmosphere becomes densely saturated. According to my calculations, the intensity of light would be nearly zero on the ground. If the surface of the Earth cannot receive radiative energy from the sun, very low temperatures will result. The affect of this scenario on the biosphere is obvious. The dust prevents the plants from receiving adequate sunlight, the plant's photosynthesizing process weakens, and plants broadly die out. The rapid decimation of the plants destroys the base of the food chain, and as a result many kinds of animals die out. Depending on the nature in the atmosphere the meteoric dust may be distributed uniformly, causing a global effect for darkness, fallout debris and iridium anomalies. After a large meteoric storm occurs, changes in hydrosphere are the same as predicted for a comet falling into the sea in that meteoric dust will eventually fall into the hydrosphere, contaming the water down to the bottom of the sea. When a comet collapses, its chemical composition does not essentially change. After many meteors fall into the atmosphere, its chemical properties should also not change. The process of extinction by a large meteorite is described in Fig. 1.

CONCLUSION The difference in the present hypothesis and previous ones is in astronomy. According to our hypothesis the iridium anomaly at extinction boundaries is explained by a large meteoric storm. This hypothesis is supported by astronomical calculations, which may explain why we have failed to definitely fred the impact crater causing the K-T anomaly iridium, although the Manson structure is a strong candidate.

103

We agree that impact events do occur in the Earth, which can be confirmed by comparing the Earth's surface with other planets, and actual craters on Earth. However, these impact events may not be the sources of the anomalous iridium. In my theory an impact is not necessary to explain anomalous iridium levels. The Ultimate effects of a meteorite storm on Earth geology and paleontology in my theory are similar to those of other theories. The current theory is a reasonable alternative to other hypotheses.

REFERENCES

Alvarez, L. W. et al. (1980): Extraterrestrial Cause for the Cretaceous-Tertiary Extinction. - Science, 208, 1095-1108. Davis, M. et al. (1983): Berkeley Laboratory, preprint LBL-17298. Gartner, S. & Keany, J. (1978): The terminal Cretaceous event. A geologic problem with an oceanographic solution. - Geology, 6, 708-712. Hsii, K. J. (1980): Terrestrial catastrophes caused by cometary impact at the end of Cretaceous. - Nature, 285, 201-203. Hut, P. et al. (1987): Comet showers as a cause of mass extinction. - Nature, 329, 118-126. McLean, D. M. (1978): Land Floras: The Major Late Phanerozoie Atmospheric Carbon Dioxide/Oxygen Control. - Science, 200, 1060-1062. Napier, W. N. & Clube, S. V. M. (1979): A theory of terrestrial catastrophism. - Nature, 282, 455-459. Rampino, M. R. & Volk, T. (1988): Mass extinctions, atmospheric sulphur and climatic warming at the Kfr boundary. - Nature, 332, 63-65. Urey, H. C. (1973): Cometary Collisions and Geological Periods. - Nature, 242, 32-33. Whitmire, D. P. & Jackson, A. A. (1984): Are periodic mass extinctions driven by a distant solar companion? - Nature, 308, 713-715.

EVOLUTIONARY CRISIS WITHIN ClAN ACROTRETID INARTICULATE OF POLAND

A contribution

THE ORDOVIBRACHIOPODS

to

U

Project

GLOBAL BIO EVENTS

BIERNAT, Gertrude *) & BEDNARCZYK,Wieslaw **)

Abstract: Acrotretidae, a group of short time range, dynamically evolved during the Cambro-Ordovician

times and became extinct by the end of the Ordovician except for three genera of the Scaphelasma line going up to the Silurian and of the Torynelasma line going on to the Lower Devonian. In Poland, acrotretids are very diversified in Ordovician, especially in the Lower Ordovieian/Upper Tremadocian-Llanvirnian), similarly as e.g. in Balto-Scandia (Sweden, Estonia, Leningrad environs). The main factors responsible for their mass extinction are discussed: they comprise, among others, changes of sea level, changes of climate in connection with glaci-enstatic events and fluctuations in the rate of the sea-floor spreading.

INTRODUCTION

Acrotretids within the Ordovician shelly faunas of Poland constitute a distinctive and largely diversified fossil group. Available records reveal that these minute inarticulate brachiopods have developed a wide range of variation in morphology, sometimes of bizarre appearance. Only some genera appear to be of rather wide geographic distribution but all are, as a rule, of limited stratigraphic range. These lithofacies dependent animals appear to have been susceptible to the environmental changes caused by various factors, including, among others, the Late Ordovician cooling. The presented brief review of the aerotretid taxa throughout the Ordovician of Poland illustrates sufficiently well their very discontinuous ranges and points out their stepwise and almost complete extinction by the end of the Ordovician.

GENERAL

R E M A R K S ON T H E O R D O V I C I A N

IN P O L A N D

In Poland, three basic Ordovician sedimentary regions are distinguished (Fig. 1). These are as follows: 1. Pre-Vendian East-European Craton comprising northern and eastern Poland (Bednarczyk, 1979, 1986; Podhalanska, 1980); 2. Kielce region, southern part of the Holy Cross Mountains (Bednarczyk, 1964; Bednarczyk & Biernat,

19'78); 3. Koszalin-Chojnice to the Lysog6ry region skirting the south-western margins of the Craton and lying on the protracted Oslo-Seanian belt (Erdtmann, 1965; Bednarczyk, 1974; Jaanusson, 1979). All the sedimentary processes in the above mentioned regions and those influencing the development of the fauna associations appear to be comparable to those of Scandinavia. Main events connected with the transgressive-regressive phenomena which are recorded from north-western and central Poland appear to coincide with those occurring in global scale (i.e. Erdtmaun, 1986; LindstrSm & Vortisch, 1983; Walliser,

1984). The Acrotretidae which are considered here derive from the two first of the three sedimentary regions *) **)

Zaklad Paleobiologii PAN, A1. Zwirki i Wigury 93, PL 02-089 Warszaw, Poland Institute of Geological Sciences PAN, A1. Zwirki i Wigury 93, PL 02-089 Warszaw, Poland

106

mentioned above, starting there with the Tremadocian deposits. Only very few informations come from the third region where the Ordovician deposits begin with Ltanvirnian-Llandeilian silts with graptolites (Bednarczyk, 1974).

~'~cs e ~ ~

.~

w~

~

~ e r l b a l t i c Depression

~

T ~

~Q

--

~. "~ e.~ \ •.-...~ "o. \

-~..~ .\

~

.. t

O

%. ~ xi~.~"~,~=z:~ "~,, \ - T"TT/.J" ">