Advances in Geophysics ~ Estuarine Physics and Chemistry-Studies in Long Island Sound, Vol. 22

ADVANCES IN

GEOPHYSICS

VOLUME 22

Contributors to This Volume ROBERTC. ALLER L. K. BENNINGER HENRYJ. BOKUNIEWICZ J. K. COCHRAN ROBERTB. GORDON RICHARDJ. MCCAFFREY JOHN THOMSON K. K. TUREKIAN

Advances in

GEOPHYSICS VOLUME 22

Estuarine Physics and Chemistry: Studies in Long Island Sound Edited by

BARRY SALTZMAN Department of Geology and Geophysics Yale University New Haven, Connecticut

1980

Academic Press A Subsidiary of Harcourt Brace Jovanovich, Publishers

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80 81 82 83

98 7 6 5 4 3 2 1

CONTENTS LIST OF CONTRIBUTORS ................................................................. FOREWORD.................................................................................. PREFACE.....................................................................................

ix xi xiii

The Sedimentary System of Long Island Sound

ROBERTB. GORDON 1. Introduction ............................................................................. 2 . Geological History .................................................................... 3 . Sea Level ................................................................................ 4 . Physical Oceanography .............................................................. 5. Sedimentation .......................................................................... 6. Further Research ...................................................................... References ...............................................................................

1 2 12 20

25 33

35

Storm and Tidal Energy in Long Island Sound

HENRYJ . BOKUNIEWICZ AND ROBERTB. GORDON 1. Introduction ............................................................................. 2. Tidal Energy ............................................................................ 3 . Storm Energy ........................................................................... 4 . Water Level Deviations ............................................................. 5. Conclusions ............................................................................. Appendix I . Formulation of the Energy Balance in an Embayment .... Appendix I1. Estimate of Tidal Dissipation of All of Long Island Sound .................................................................. References ...............................................................................

41 43 48 55

60 61

65 67

Sediment Transport and Deposition in Long Island Sound

.

HENRYJ BOKUNIEWICZ AND ROBERT B. GORDON 1. Introduction

............................................................................. 2 . Power Sources ......................................................................... 3. Sediment Sources ..................................................................... 4 . Sediment Transport and Bottom Stability ...................................... 5 . Sediment Deposition and Distribution ........................................... 6. Comparison with Other Estuaries ................................................ References ............................................................................... V

69 70 84 87 95 99 104

vi

CONTENTS

Sand Transport at the Floor of Long Island Sound

HENRYJ . BOKUNIEWICZ

1. Introduction

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

2 . Background ............................................................................. 3 . Long Island Sound .................................................................... 4. Sediment Transport ................................................................... 5 . Formation of the Transition Zone ................................................ 6 . Discussion ............................................................................... 7 . Summary and Conclusions ..........................................................

References

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

107 107 110 113 116 122 124 126

The Sources and Sinks of Nuclides in Long Island Sound

K . K . TUREKIAN. J . K . COCHRAN. L . K . BENNINGER. AND ROBERTC . ALLER 1. Introduction ............................................................................. 129 2. Sources of Trace Metals Delivered to Long Island Sound ................. 131 3 . The Distribution of Trace Metals in Long Island Sound Sediments .... 137 4. Trace-Metal Distributions in Mussels and Oysters: An Index of the Composition of Suspended Particles ................................................... 142 5. Processes Affecting the Deposition and Accumulation of Trace Metals in Long Island Sound Sediments .................................................. 147 6. Processes Affecting the Vertical Distribution of Nuclides in the Sediment Pile ........................................................................... 153 161 7 . Summary ................................................................................. References ............................................................................... 163

A Record of the Accumulation of Sediment and Trace Metals in a Connecticut Salt Marsh

RICHARDJ . MCCAFFREY AND JOHNTHOMSON 1. Introduction ............................................................................. 2 Experimental Methods and Results .............................................. 3 . Discussion ............................................................................... 4. Summary and Conclusions ..........................................................

.

References

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

165 169 189 227 229

CONTENTS

vii

Diagenetic Processes Near the Sediment-Water Interface of Long Island Sound .I . Decomposition and Nutrient Element Geochemistry (S. N. P)

ROBERTC . ALLER 1. Introduction ............................................................................. 2 . Location of Study and Station Description .................................... 3 . Sampling ................................................................................. 4 . Methods .................................................................................. 5 . Results .................................................................................... 6. Discussion ............................................................................... 7 . Summary ................................................................................. Appendix A . Macrofauna (>1 mm) Sieved from Flux-Core Boxes ..... Appendix B . Box-Core and Gravity-Core Data from Long Island Sound .................................................................. Appendix C . Flux-Core Data ...................................................... List of Symbols ........................................................................ References ...............................................................................

238 238 250 252 257 272 317 320 322 340 343 343

Diagenetic Processes Near the Sediment-Water Interface of Long Island Sound.11. Fe and Mn

ROBERTC . ALLER 1. 2. 3. 4.

Introduction ............................................................................. Location and Description of Study Area ....................................... Sampling ................................................................................. Methods .................................................................................. 5 . Results .................................................................................... 6. Discussion ............................................................................... 7 . Summary ................................................................................. List of Symbols ........................................................................ References ...............................................................................

351 352 353 353 355 367 406 409 410

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

417

Index

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LIST OF CONTRIBUTORS Numbers in parentheses indicate the pages on which the authors' contributions begin.

ROBERT C. ALLER,* Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520 (129, 237, 351) L. K. BENNINGER,~ Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520 (129) HENRYJ. BOKUNIEWICZ,Marine Sciences Research Center, State University of New York, Stony Brook, New York 11794 (41, 69, 107) J. K. COCHRAN, Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520 (129)

ROBERT B. GORDON,Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520 (1, 41, 69)

RICHARDJ. MCCAFFREY,'Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520 (165) JOHNTHOMSON,'Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520 (165)

K. K. TUREKIAN, Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520 (129)

* Present address: Department of Geophysical Sciences, The University of Chicago, Chicago, Illinois 60637. Present address: Department of Geology, University of North Carolina, Chapel Hill, North Carolina 27514. * Present address: Graduate School of Oceanography, University of Rhode Island, Kingston, Rhode Island 02881. Present address: Institute of Oceanographic Sciences, Wormley, Godalming, Surrey GU8 SU8, England. ix

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FOREWORD Occasionally, a complete volume of Advances in Geophysics will be devoted to a single topic of special interest. This volume, dealing with some of the important physical, chemical, and biological processes occurring in estuaries, does so. The works included here have an additional common feature: all are based on studies made in Long Island Sound, which poses problems prototypal of those encountered in many estuarine settings. We are still a long way from establishing the kind of general quantitative theory of the estuarine variables that is necessary for the understanding and effective management of these coastal environments. The articles in this volume expose the rich variety of phenomena and interactions that will have to be included in such a theory:Although particular emphasis is placed here on the transport, physicochemical structure, and evolution of the bottom sediments, the relationship of these factors to their broader geological and hydrodynamical contexts is also considered and clarified. BARRYSALTZMAN

xi

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PREFACE Estuaries are usually classified in terms of the characteristics of their circulation, broadly speaking, as “salt wedge,” and “partially mixed.” Estuaries differ in many other ways-for example, in the nature and amount of sediment they transport and the composition and diversity of the biological communities they support. No two estuaries, no matter how close in affinity they may be according to hydrologic classification, are ever identical. Long Island Sound is partially mixed, but its estuarine circulation is due to a “freshwater” source that is actually the brackish water of New York Harbor-water that receives large quantities of wastes from New York City. The two major rivers entering the Sound are the Connecticut and the Housatonic. Their hydrologic importance is local, but they are the principal sources of the “natural” sediment entering the Sound. The eastern passes of the Sound are the major source of its tidal flow, although the western pass is also of local importance. The energy of the tides and estuarine circulation has a major influence on sediments and animal communities and is harnessed most dramatically at times of strong wind stress. The atmosphere also is a courier of pollutants to the Sound, mainly from New York City and the industrial area southwest of it. The chemicals transferred through the atmosphere leave their imprints on the materials deposited in the Sound, together with those transported hydrographically. We see Long Island Sound as an “urban” estuary occupying a basin-like setting, protected from the highly energetic encounters with the open ocean, and therefore capable of retaining a memory of estuarine processes in its deposits over its entire existence. This collection of articles represents a part of the work that has been going on in the Department of Geology and Geophysics at Yale over the past 15 years. It builds on the classic work done there in earlier years by members of the Bingham Laboratory, notably the group associated with Gordon Riley. Our work has been aimed at understanding the fluxes of materials to and within the Sound as influenced by natural forces and the activities of people. The approach has been both chemical and physical. The articles herein refer to a considerably larger body of research at Yale and at other institutions. In particular the work of our Yale colleagues Robert A. Berner and Donald C. Rhoads and their research groups must be cited. Our four research groups have pursued investigations of Long Island Sound with perspectives and methodologies characteristic of each group, but the net effect on our understanding of the system has been far

xiii

xiv

PREFACE

greater than a simple summing of the individual efforts. The references cited in the contributions of this cluster clearly attest to this. We feel that we now have a much better understanding of how one estuary functions as a whole. This understanding yields a sense of satisfaction, but does not leave us in a state of complacency; much remains to be learned about the complexities of Long Island Sound, let alone about the other estuaries of the world. Nonetheless, we believe that many of the methods we have used to examine the Sound will prove useful in studies elsewhere. Funding for these studies has come from several sources. These include government agencies, particularly the Department of Energy (and its antecedents), the U.S. Army Corps of Engineers, and the National Science Foundation. In addition, the United Illuminating Company provided financial support as part of its interest in the environmental consequences of the construction of a new power plant adjacent to New Haven Harbor. These and other benefactors are cited in each contribution as appropriate. KARLK. TUREKIAN ROBERTB. GORDON

ADVANCES IN

GEOPHYSICS

VOLUME 22

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THE SEDIMENTARY SYSTEM OF LONG ISLAND SOUND ROBERTB. GORDON Deparfment of Geology and Geophysics Yale University New Haven, Connecricuf

I. 2. 3. 4. 5. 6.

Introduction . . . . . Geological History . . Sea L e v e l . . . . . . Physical Oceanography Sedimentation . . . . Further Research . . . References . . . . . .

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1

2 12 20 25 33 35

1 . INTRODUCTION

Water-transported products of erosion from the land surface (both solutes and particulates) reach the sea by passing through estuaries. The flow of water from a source area through a river system to an estuary is governed by the balance of the downhill component of the gravitational force and the friction between the fluid and the bed of the stream. Sediment moves intermittently with the fluid flow; most is in motion during periods of high discharge. A wider range of physical, chemical, and biological phenomena are active in an estuary than in a river, and the processes of sediment transport and deposition become both more diverse and complicated. These include periodic water motions due to the tide and to waves, both those generated in the estuary and the swell from the sea. Salt is encountered and may have both chemical and physical effects on the form of the erosion products. Estuarine circulation results from the density difference between fresh and salt water. The length scales of the flow are increased in the estuary and geostrophic effects begin to be important. There may be large lateral flows and sharp boundaries between different water masses in the estuary. Sediments entering an estuary encounter marine animals which ingest them and transform their physical form; in shallow water, interaction with marine plants may facilitate sediment deposition. As a result of all these processes the sediment transported by a river may be processed into a new form within the estuary, the distribution between elements in solution and those adsorbed on solid particles may be changed, some materials may be stored tem1 ADVANCES IN GEOPHYSICS. VOLUME

22

Copyright 8 1980 by Academic Press. Inc. All rights of reproduction in any form resewed, ISBN 0-12-018822-8

2

ROBERT B. GORDON

porarily or permanently in the estuary, and what is exported may be of different size, shape, or composition than that supplied by the river. The number of physical processes that control the behavior of erosion products in an estuary is large and most of these processes are sensitive to the boundary conditions that obtain in each individual estuary. Description of them is likely to be of only parochial interest and the difficulties in the way of generalization are great. There are satisfactory schemes for the hydrographic classification of estuaries (Dyer, 1973, Chapter 2), but none as yet for the classification of estuarine sedimentary systems. In fact, the full scope of the interaction among physical, chemical, and biological processes in an estuary has hardly been examined. Such an examination is one objective of the articles in this volume. The selection of Long Island Sound for this purpose is due partly to geographical convenience; nevertheless, the Sound does display many estuarine characteristics in relatively simple form and is in many ways typical of the estuaries on glaciated terrain that surround the North Atlantic. Often these are the estuaries that have suffered most from the impacts of industrial society. We believe that many of the specific results obtained for Long Island Sound, and most of the methods developed to study it, will find application in the study of other estuaries. The quantitative description of the processes that control the passage of the products of erosion through the estuary will provide a basis for the efficient intercomparison of other estuaries, i.e., the development of an estuary classification scheme. This article is a review of previous research on the geology and related physical oceanography of Long Island Sound. 2. GEOLOGICAL HISTORY

The basin occupied by Long Island Sound is a product of the period of prolonged erosion of eastern North America that occupied the late Mesozoic and the Cenozoic eras. Because the onshore geological record consists of erosion surfaces, and the products of this erosion are now submerged on the continental shelf, reconstruction of a detailed geological history is not possible. However, the main events are well established. When the Atlantic Ocean began to open, about 180 MY BP, the Southern New England land surface had high relief, as shown by block faulting of sediments in the Connecticut Valley, which persisted into the Jurassic. Between the mid-Jurassic and early Cretaceous this relief was reduced to no more than about 100 m (McMaster and Ashraf, 1973, and other references given therein). The eroded surface produced then is now the Fall Zone surface (Flint, 1963) and its northern bound is the Fall Line.

--

THE SEDIMENTARY SYSTEM OF LONG ISLAND SOUND

3

The Fall Zone surface defines the top of the bedrock beneath Long Island Sound. A simple sequence of events that will account for the formation of the basin now occupied by the Sound is illustrated in the series of sketches in Fig. I , After opening of the Atlantic Ocean (Fig. la), cooling of the continental margin resulted in its subsidence (Fig. lb) at a rate that decreased as the continent moved away from the mid-ocean ridge. Additional subsidence was caused by the weight of the sediment accumu-

',

....... ....:...*..: ..:,;:;. .......::..

. I : : .

%.

(f)

2 MY bp

I.

I

1

(0)

today

.*$.... ......

Now England '/ I T - -$ .r:X+ ,,. Upland Surfacr Fall Zonr fau

F a . I . Schematic diagram of the evolution of the basin of Long Island Sound showing: (a) Start of opening of Atlantic Ocean: (b) subsidence of the continental margin due to cooling as the continent moves away from the mid-ocean ridge; (c) accumulation of sediment on the continental margin causing further subsidence; (d) reduction in sea level due to worldwide reduction in the rate of sea-floor spreading: (e) erosion of coastal plain sediments due to lowered base level of rivers: (0 glaciation: and (9) present configuration. (See text for sources.)

4

ROBERT B. GORDON

lating on the continental shelf (Watts and Ryan, 1976). This contribution to the subsidence can be described as resulting from downward, elastic flexure of the continental crust about a hinge line, now identified as the Fall Line. The mean downward tilt of the Fall Zone surface today is 9.4 x lo-'. If this developed over the full 170 Myr available, the average tilt rate would be only 5 x lo-" yr-I, which can be easily accounted for by cooling of the continental margin and deflection under the accumulating sediment load. [Tilting rates along the east coast of North America are observed to be as great as 2 x yr-' at present (Brown, 1978); this suggests that fluctuating rather than slow, steady tilts have contributed to the long-term average tilt rate.] Since the downwarping of the continental margin did not require uplift inland of the Fall Line, erosion rates on much of the land surface supplying sediment to the continental margin must have remained relatively low. Menard (1961) estimated that 7.8 x lo6 km3 of rock must have been removed from the Appalachians over 125 Myr to account for the sediment now on the continental terrace and rise and on the abyssal plains off the east coast of North America. The mean sediment yield required to produce this material is -0.2 kg/(m2 yr). (For comparison, this is about the same as the sediment yield of the Missouri River drainage basin today.) Matthews (1975) has used the more extensive data on sediment thicknesses off the Atlantic Coast now available to estimate that the sediment yield of eastern North America over the past 60 Myr was 0.012 kg/(m2 yr) for the northern half of the coast and 0.067 for the southern half. These sediment yields can be attained with a land surface relief of a few hundred meters under temperate climate conditions and so are consistent with the hypothesis that both the elevation and relief of most of the land surface have remained moderate since the opening of the Atlantic Ocean. Continued erosion of the New England upland and deposition on the continental margin resulted in the configuration bedrock, sediment, and sea surface shown in Fig. lc. Sediment cover extended at least to the Fall Line (Sharp, 1929, p. 38; Johnson, 1931, p. 39), while the shore line was not far removed. According to Pitman (1978) the shoreline began to move rapidly seaward starting at about 65 MY BP because of an acceleration in the rate of fall of the sea-surface elevation caused by diminished world average sea-floor spreading rates. The base level for the rivers on the New England upland surface was lowered and the coastal plain sediments covering the upper part of the old Fall Zone surface werre removed by erosion, as shown in Figs. Id and e, to form a cuesta at the south side of the new valley. As the coastal plain sediments were removed, the rivers from New England again flowed over the Fall Zone surface. According to McMaster and Ashraf (1973) they in part returned to their old, pre-

THE SEDIMENTARY SYSTEM OF LONG ISLAND SOUND

5

Cretaceous valleys. These authors find that the major valleys on the landward end of the Fall Zone surface join the valleys on the adjacent Fall Zone surface still covered by coastal plain sediments. River valleys on the Connecticut highlands usually follow the rock formations of lowest erosion resistance (Flint, 1963) and it may be supposed that the same is true offshore on the Fall Zone surface. The removal of the coastal plain sediments was probably accomplished by drainage into an easterly flowing river (Dana, 1890), which turned south near the present eastern end of Long Island (see Fig. 2 ) . Rivers crossing the Fall zone surfacejoined this stream from the north. Sediment from the south was delivered by streams flowing down the steep face of the coastal plain cuesta. Excavation of sediment to form the valley to the cuesta as mapped in Fig. 2 required denudation at a mean rate of about 8 x lo7 kg/yr. For comparison, about this much sediment is carried now by streams entering Long Island Sound west of the Connecticut River. One difficulty with this hypothesis for the denudation of the bedrock surface under the Sound is that there are deep basins north of the mapped edge of the cuesta that could not have been excavated by the proposed river system. These are shown by the shaded areas in Fig. 2 . They are thought to be formed on the Fall Zone surface but they may be closed off by outliers of Coastal Plain sediments. Some river valleys on the Fall Zone surface have been overdeepened by subsequent glacial erosion (the Quinnipiac River valley both south and north of New Haven contains basins up to 250 m deep, for example), but it is unlikely that the deep areas on the Fall Zone surface could have been formed this way since the shape of the basins is not elongated in the direction of ice flow. More detailed mapping of the topography of the Fall Zone surface under Long Island may help resolve this problem. The geological history of the region for the past several million years has been dominated by successive continental glaciations and their accompanying changes in sea level. Deposits formed during both pre-Wisconsinan and Wisconsinan glaciations have been identified on Martha’s Vineyard (Kaye, 1964), but there is only limited evidence of the location of the southern edges of any of these ice sheets other than the Wisconsinan. At many localities in Connecticut two distinct till layers are found. The older rests directly on bedrock, is compact and weathered at its upper surface, and is thought to be of early Wisconsinan-Illinoian age (J. P. Schafer, personal communication, 1978; Pessl and Schafer, 1968). Older till generally forms the cores of drumlins in southern New England (Schafer and Hartshorn, 1965). Several islands in Long Island Sound (including Falkner Island, 8 km offshore) are composed of older till (R. F. Flint, personal communication, 1975). On Long Island, the Mannetto and

THE SEDIMENTARY SYSTEM OF LONG ISLAND SOUND

7

Jameco formations were probably deposited by a pre-Wisconsin glaciation. Thus, it is likely that at least one ice sheet older than Late Wisconsinan covered the basin now occupied by Long Island Sound. Some of the Pleistocene deposits in the basin of the Sound may be pre-Wisconsin in age, but nothing definite can be said because no deep borings have been made. The late Wisconsinan ice sheet extended to the Ronkonkoma end moraine on Long Island (see Fig. 3a). The end moraines on Long Island, and their westward extensions, are not independently dated, but were formed before about 16,000 yr BP (Schafer, 1979). If the profile of the Antarctic ice sheet is fitted to an end point on the Ronkonkoma moraine, the ice thickness over southern Connecticut is found to be in excess of 1 km. Evidence of glacial erosion and deposition is found on the tops of the highest mountains throughout New England, proving a substantial ice thickness. No satisfactory estimate of the total amount of glacial erosion of southern New England has been made yet because the total volume of glacial drift has not been measured. Maps of the bedrock surface of Connecticut show that the erosion was concentrated along the valleys of existing rivers. In places these have been overdeepened by more than 100 m. Schafer and Hartshorn (1965) erstimate that perhaps as much as 20 m of rock was removed from the southern New England surface by glacial erosion. For rock of density 2.4 Mg/m3, this is erosion of 5.2 x lo4 kg/ m2. If it was removed in four episodes of glaciation each lasting 50,000 yr, the erosion rate was 0.26 kg/(mzyr), not very different from Menard's estimate of the long-term, mean denudation rate of the Appalachians. The deglaciation of southern New England is believed to have taken place by stagnation-zone retreat (Koteff, 1974) rather than by either melting down in place or by an active ice margin building recess moraines. According to this model a band of stagnant ice is supplied with rock debris by active ice advancing against its up-ice end. This debris is converted by the action of meltwater to ice-contact drift on and adjacent to the stagnant ice and to outwash deposits further away from the margin. A characteristic unit of stratified drift-a valley train-results. Small end moraines may form at the boundary between the live and stagnant ice (Schafer and Hartshorn, 1965). When the stagnation zone retreats to a new location, a new stratified drift unit is initiated. Former ice margins are thus marked by small end moraines or the heads of stratified drift units. The locations of the dated organic remains most useful in establishing the chronology of the deglaciation of the Long Island Sound region are shown in Fig. 3b. The most important of these is the date of 14,240 yr BP, the oldest from a series of dates on deposits in Rogers Lake shown

m

FIG.3(a). Contours show the depth in meters below sea level of the top surface of the glacial drift (principally ice contact drift and outwash) in the basin of Long Island Sound. End moraines are shown in solid black; the Elmhurst Moraine (EM) mapped by Newman (1977) is shown by the heavy dashed line. Dotted areas are shoreside deposits of outwash and ice contact drift. The abbreviations are: C1, Captain Islands; NI, Norwalk Islands: MM, Madison moraine; LM, Ledyard moraine; OSM, Old Saybrook moraine; CM, Charlestown moraine; HHM, Harbor Hill moraine; RM, Ronkonkoma moraine. Locations of end moraines in eastern Connecticut based on Flint and Gebert (1976); field work now in progress may show some revisions necessary (Schafer, 1979, also personal communication).

t4’030‘ (b)

5900 vr bs

-

FIG.3(b). Location of sources of dated organic material used to establish the chronology of deglaciation (crosses) and sea-level chang (squares). Depths are measured from mean high water (i.e.,marsh surface). Triangles mark the saddle points on the sills bounding the ba! of the central Sound. Abbreviations: TB, Totoket bog; RL, Rogers Lake; BI, Block Island; MS, Mattituck sill. The Bloom and Stuiver s level curve was established for location B-S. Dates based on Davis (1%5) with additional data from Schaffel (1971) and Newman (197 Other data sources are given in the text.

10

ROBERT B . GORDON

to be younger than all nearby till (Stuiver et al., 1963; Davis, 1969). Two lines of minor moraines, and their offshore extensions, have been identified in southern Connecticut by Flint and Gebert (1976). These are the Madison and Old Saybrook moraines shown in Fig. 3a. (The Ledyard moraine does not reach Long Island Sound, but may correlate with the Madison moraine.) They are interpreted as the boundary between live and dead ice. The Old Saybrook moraine is south of Rogers Lake and must have been formed, therefore, before 14,240 yr BP. The position and orientation of the moraine line suggests that all the central and eastern Sound must have been clear of ice by this date. Extensive sub-bottom acoustic reflection profiling through the eastern Sound has failed to reveal any trace of submerged or buried end moraine segments between the Harbor Hill moraine and the Old Saybrook moraine (J. A. Gebert, personal communication, 1976). Thus, the ice retreat across this part of the Long Island Sound basin must have been more nearly continuous than that across southern Connecticut. The Captain and Norwalk Islands (see Fig. 3a) were examined by Flint and Gebert and identified as end-moraine segments. They may correlate with the Lordship outwash and the Madison moraine (J. P. Schafer, personal communication, 1979). Additional moraine segments have been mapped on the western end of Long Island by Newman (1977). During retreat of the ice sheet across Long Island Sound large quantities of ice-contact drift and outwash sand must have been released by the melting ice. Such deposits can be studied in detail in southern Connecticut. Their extent along the coast is shown in Fig. 3a. Beneath Long Island Sound they form the principal horizon on which subsequent lacustrine and marine sediments were deposited. The ice-contact drift and outwash sand are good acoustic reflectors and have been mapped in some detail by means of acoustic reflection profiling (Grim et af., 1970; Bokuniewicz el al., 1976). The slopes of the outwash surface along the shore of central Long Island Sound and of the nearby valley trains are listed in Table I. Also listed in the table for comparison are the slopes of the Fall Zone surface and of the New England upland surface. All these slopes will have been increased subsequent to deglaciation by tilting of the land surface due to viscous rebound after removal of the ice load. An estimate of this increase in slope can be made from the tilt of the shorelines of glacial Lake Hitchcock, which occupied much of the valley of the Connecticut River and was drained about 10,700 yr BP (Flint, 1956). We will assume that the regional tilt due to rebound since that time is the same as the 0.8 x 10-’-rad tilt (Emerson, 1898; Jahns and Willard, 1942; Koteff, 1968) of the lake shorelines. Hence, the present slopes would have to be reduced by 0.8 x

11

THE SEDIMENTARY SYSTEM OF LONG ISLAND SOUND

TABLEI. MEANSLOPES OF OUTWASH SURFACE ALONG CENTRAL LONGISLANDSOUND SHORE AND NEARBY VALLEY TRAINS Surface Fall Zone surface (Flint, 1963) New England upland (based on Barrel], 1920) Lake Hitchcock shoreline (Emerson, 1898) Outwash surface (based on topographic map of the Guilford, Conn. quadrangle) Valley trains (lower ends where concave up): Mill River (based on Lougee, 1938, Plate XII) West River (based on Flint, 1971, Fig. 4) Farm River (based on Flint, 1964, Fig. 5) Quinnipiac River (based on Flint, 1965, Fig. 7) Submerged outwash sands: South of New Haven (based on Fig. 3 of Bokuniewicz ef nl., 1976) South of shore East Haven to Madison extending to 6.8 km , offshore (based on Fig. 6 of Bokuniewicz et ~ l . 1976)

Slope 9.4 x 4.4 0.82 3.8

(rad)

2.0 2.8 2.2 1.1 3.2 2.9

rad to find the slopes at the time of deglaciation. The present slope of the outwash surface on land is slightly greater than the slopes of the valley trains and the offshore extensions of the outwash. Although the slopes of the surfaces of the outwash sands on land and offshore are nearly the same where they have been compared in the eastcentral Sound region, the two surfaces are not continuous. Comparison of the elevations of these surfaces is complicated by the need to refer the marine surveys to the same datum as that used in mapping the land surface. Absolute elevation control was not maintained in the available bathymetric surveys and it is estimated that there may be an error of 1-2 m in matching the data of on-land and marine surveys. Figure 4 shows the configuration of the surface of outwash deposits near the shoreline at one location in Madison, Connecticut (C. Sullivan, personal communication, 1977). The projection of the onshore land surface seaward is 7.6 m above the surface of the now-submerged sands offshore. This difference is large compared to errors that may result from establishment of the survey datum. This elevation difference is interpreted as the amount of the outwash that has been removed by wave erosion during the rise in sea level that followed deglaciation. The eroded sand was probably incorporated in the marine mud that was being deposited simultaneously in the deeper water further offshore. On the basis of the evidencejust presented, the sand deposits extending from 6 to 10 km out from the Connecticut shore are interpreted as continuations of the onshore deposits of outwash sand and it is hypothesized that they were formed at the time when the ice margin was north of the

12

ROBERT B. GORDON

FIG.4. Cross section through glacial outwash sands (dotted) along the Connecticut shore (at longitude 72"34'W). Marine mud deposited on the outwash is shown shaded. (Based on data obtained by C. Sullivan, personal communication, 1977.)

Madison and Old Saybrook moraines. The volume of the outwash sand deposit can be estimated from the altitude and inclination of the Fall Zone and outwash surfaces. It is 2.1 x lo5 m3 per meter of shoreline (including that subsequently removed by marine erosion). The time for the ice to move across the Sound was not more than 4000 yr (and probably was much less). The outwash extends about one-quarter of the way across the Sound. If it were formed in 1000 yr and the source area for the rock debris were 100 km long, the requisite erosion rate is 3.5 kg/(m2 yr), which is very large compared to the rates discussed earlier. It could be reduced by perhaps a factor of 4 by allowing a longer formation time or a larger source area. For comparison, note that Boulton (1974) measured an abrasion rate on basalt of 2.7 kg/(m2 yr) under ice 40 m thick flowing at a speed of 9.6 m/yr. This suggests that the glacial erosion rates are either much higher during deglaciation than during glacial advance, or that the long-term, average glacial erosion rates for southern New England have been estimated too low. The former hypothesis is favored on the basis of Boulton's (1974) demonstration that glacial erosion rates are greatest for intermediate ice thickness. 3. SEALEVEL At the maximum of the late Wisconsinan glaciation, sea level was much reduced and the sea margin was well out on the continental shelf. The subsequent rise of the sea relative to the land determined the marine history of Long Island Sound. Local field data reveal only the more recent parts of the sea level history of the region, so reliance must be placed on

THE SEDIMENTARY SYSTEM OF LONG ISLAND SOUND

13

geophysical arguments, or historical data from other localities, to define the sea level in the Sound directly after deglaciation began. Changes in sea level are the result of several physical effects. Melting of the ice sheets increases the volume of water in the ocean basins. The volume of the basins changes as isostatic compensation adjusts the elevation of the sea floor to the increased water load. Gravitational attraction between the ice and the sea tilts the water surface. The elevation of the land surface changes in response to the removal of the ice load. Each of these effects must be known if a sea level curve is to be computed for a given locality. A self-consistent theory of sea level changes that takes into account these factors (except the local gravitational attraction) and allows for the spherical shape of the earth was developed by Cathles (1975). The problem has been studied further by Farrell and Clark (1976) and by Peltier and Andrews (19761, who reach similar conclusions to those of Cathles about the distribution of viscosity within the earth that will match the theory of glacial rebound to the available field data. In order to compute a sea level curve for Long Island Sound we use the meltwater curve obtained by Cathles from geological evidence for the extent of the ict sheets as a function of time during deglaciation and his Model I ocean basin volume adjustment. These combine to give eustatic sea level curve C in Fig. 5 . Morner’s (1969) eustatic curve M, based on the interpretation of extensive field data in Scandanavia, is shown for comparison. To construct a curve of land elevation, we use local sea level data and a physical model to interpolate between the available data points. Where the sediment supply is adequate (as it is in Long Island Sound), salt marsh surfaces grow up to the level of mean high water (McCaffrey and Thomson, this volume). Radiometrically dated salt marsh peat taken from borings therefore gives sea level at former times. The most extensive data of this type in the area are from the Hammock River Marsh (B-S in Fig. 3b); Bloom and Stuiver (1963) established a sea level curve extending back 7000 yr for this locality. Additional data points are available for the Quinnipiac River (Upson et al., 1964),the south pier of the Throgs Neck Bridge and a bore in Flushing Bay (Newman, 1977), and one deep boring made in Long Island Sound off Manhasset Neck (Schaffel, 1971). (Locations are shown in Fig. 3b.) There are no data for ages greater than 12,300 yr. To get a starting point for a land elevation curve, we use the total uplift expected for the latitude of Long Island Sound from Cathles’ model 2 curve for the total ultimate uplift expected. Curve R in Fig. 5 starts at this elevation. The solid part of the curve is constructed from the data of Bloom and Stuiver; the two branches show the likely error in the radiometric dating. It is likely that the initial portion of the land

14

ROBERT B. GORDON

c

1

I6

I2

4

8

rear

bp

FIG.5. The eustatic sea level curves proposed by Cathles (1979, C, and Morner (1969), M. compared with land elevation curves R derived as explained in the text. The curve R-25 is drawn 25 m below R .

uplift curve is influenced by the elastic deflection of the crust near the ice margin (Walcott, 1970). The forebulge caused by this deflection is expected to follow the retreating ice with little delay and to cause an initially rapid rise in the land surface. The dashed portion of curve R is estimated on the basis of the available data points referenced earlier, an elastic deflection at the ice margin of 10 m, and the slowest physically reasonable viscous rebound from an initial crustal depression of 85 m. The dash-dot curve is drawn with no allowance for elastic deflection. The present configuration of the surface of the glacial drift in Long Island Sound is a deep central basin bounded by sills on the east and west. The submergence history of the Sound depends on the elevations of the lowest points on these sills relative to the sea level curve. On the Mattituck sill (to the east) this elevation is now -25 m. Sand is now being transported from east to west across the Mattituck sill and it is possible that the sill is now at a higher elevation than it was immediately after retreat of the ice. (More detailed acoustic reflection profile studies of the internal structure of the sill may answer this question.) The lowest point on the sill to the west (which has not yet been surveyed in as much detail) is higher than - 20 m. (The locations of the saddle points on the eastern

THE SEDIMENTARY SYSTEM OF LONG ISLAND SOUND

15

and western sills are shown by the triangles in the map, Fig. 3b.) It is likely, therefore, that the sea entered the central part of the Sound from the east when the sill depth dropped below sea level. The curve R-25 in Fig. 5 shows the elevation of the lowest point on the Mattituck sill relative to sea level. Until 8000 yr BP it was probably above the sea surface and the central part of the Long Island Sound basin would have been occupied by a lake. Because of the steep rise of the sea surface elevation curve for this time, error in the estimate of the sill depth will have a relatively small effect on the estimated submergence data. However, the possibility of an earlier submergence, around 12,000 yr BP, cannot be ruled out with the data now available. It would certainly have occurred if there were no elastic forebulge. Such an earlier submergence may have been temporary. The western end of the Sound was submerged by 10,000-12,000 yr BP, as shown by Newman’s and Schaffel’s data, but the high ground separating the western and central parts of the Sound (western triangle in Fig. 3b) probably remained about sea level until after 8000 yr BP. Several water level recorders have been operated nearly continuously in Long Island Sound over the past 40 years. It should be possible to use their records to determine the submergence rate of the coast. Water level depends on the tide, the season of the year, on the passage of storm centers, local wind stress, and changes in the elevation of the recorder such as might be due to subsidence of the ground. It is generally considered that comparison of yearly averages eliminates all but the last of these effects from the record. [Kaye and Stuckey (1973) show how the tidal range is affected by the 18.6-yr tidal period and suggest that there is also an effect on mean sea level. If there is, it is very small.] The yearly mean sea level for New London, computed from the National Ocean Survey tide gauge records, is shown in Fig. 6. [It is generally similar to that published by Hicks and Crosby (1974).] For comparison, the mean sea level for the five winter months is also shown. The rise is about 3 mm/yr. (A much longer record is available for New York City and this shows a generally steady rise of sea level there since 1930.) If inferences about the rebound of the land surface or eustatic changes in sea level are to be drawn from these data, it is necessary to be sure that all influence of meteorological events on the elevation of the water surface has been removed by the averaging. Several aspects of the data in Fig. 6 indicate that this is not the case. The variability of the winter-months curve is greater than that of the yearly average curve. The rate of sea level rise for the winter months (3.2 mm/yr) is different than that for the yearly mean (3.0 mm/yr). Several of the high and low water levels are for years in which there as an unusually large amount of storm activity. The data on water level changes due to storms presented in the next article in this volume,

16

ROBERT B. GORDON

c -1

1-61

40-41

50-51

60-61

70-71

YEAR

FIG.6. Annual mean sea level (solid circles) and mean sea level for the winter months (November-March, open circles) computed from tide gauge records taken at New London.

p. 41, show that a small change in the proportion of offshore and onshore winds in a year can be expected to have an effect on the mean water level comparable to the amplitude of the variability of the mean sea level curve in Fig. 6. It is likely that meteorological events are not adequately eliminated from tide gauge records by yearly averaging and that changes in the annual mean water level may be due to changes in storm-track distributions as well as to changes in eustatic sea level and the elevation of the land surface. Brown (1978) has shown that there is a large discrepancy between the tilting of the east coast of the U. S. revealed by water level and geodetic surveys. Tide gauge data may not be as reliable an indicator of changes in the elevation of the land surface as are precise leveling measurements. The surfaces of salt marshes are generally considered to grow up to the elevation of mean high water (Chapman, 1960). Different species of salt marsh plants are tolerant to different amounts of immersion and so grow at different elevations on the marsh. Adams (1963) has shown that narrow elevation zones of marsh plant species occur along the North Carolina coast. A series oflevelingmeasurementswas done on a salt marsh surface in Stony Creek, Connecticut, to see if a similar relationship could be established for the marshes of Long Island Sound. The marsh studied is located near the tide gauge at the Yale Field Station (station 2773a in Vol. I1 of the Admiralty Tide Tables, where the tidal characteristics are sum-

THE SEDIMENTARY SYSTEM OF LONG ISLAND S O U N D

17

marked). Figure 7 shows the range of elevations in which each of the principal plant species on this marsh is found. There is some overlap of the ranges, but it is clear that the high marsh species mark well the elevation of mean high-water springs, which is also the average elevation of the largest extent of nearly flat marsh surface. This is well below the highest astronomical tide. The marsh surface is also flooded by storm tides. The amount of tidal immersion for each elevation range can be read off the diagram in Fig. 8, which is computed for the tidal constants for this station. If the marsh surface is keeping up with changes in sea level, there should be measurable changes in its elevation over a span of a few years. Harrison and Bloom (1977) describe how these elevation changes are detected by measuring the depth of burial of marker beds placed on the

I

--MHWS

--MHWN

-0

M SL

FIG.7. Ranges of elevation in which different plant species occur on a salt marsh near Guilford, Connecticut. Datum is mean sea level (MSL). The tidal ranges shown are the highest astronomical tide (HAT), mean high-water springs (MHWS). and mean high-water neaps (MHWN).

18

ROBERT B. GORDON

-2

0

6

12 IMMERSION TIME (hr)

18

FIG.8. Average immersion time as a function of elevation for the salt marsh of Fig. 7.

marsh surface each year. They find 10-yr average burial rates ranging from 2 mm/yr near the mouth of the Connecticut River to 6 mm/yr on the Connecticut shore of central Long Island Sound. These rates are larger than those revealed by other types of evidence. The marsh surface cannot grow above the level at which submersion by sea water prevents invasion by upland plant species. McCaffrey and Thomson (this volume) show that there is no self-compaction of the salt marsh peat. Thus, a burial rate in excess of the rate of rise of sea level cannot be sustained. Since the rate of peat formation along Long Island Sound is determined by the supply of nutrients (Steever et al., 1976) and, perhaps, by the amount of storm energy dissipation, the burial rate of the marker beds used by Harrison and Bloom may be variable on a time scale long compared to the 10-yr sampling interval, and that the observed high burial rates are a follow-on to a previous interval of low rates. The burial measurements should be continued over a longer time span to resolve this question. Sea level changes are the dominant factor influencing the evolution of salt marshes on the coast along Long Island Sound. The salt marsh surface grows upward with the rise of sea level, but the marsh can survive only if it receives protection from erosion of its seaward edge by waves. Thus, most marshes are found behind protective barriers and, if this protection is lost for any reason, destruction of the marsh by wave action follows. Figure 9 illustrates a marsh that is evolving in this way. Extensive salt marsh extends up the valleys of the rivers shown in the map, but the face

24

THE SEDIMENTARY SYSTEM OF LONG ISLAND SOUND

19

of the marsh exposed to the sea is being cut back by erosion due to waves. The marsh formed on the surface of an extensive plain of outwash sands bounded laterally by bedrock and end-moraine segments. It is inferred that the marsh once extended out to location B (bottom sampling shows submerged salt marsh peat between A and B) and that there was a protective barrier of sand extending from the end-moraine segment nearly to the bedrock outcrop, where the river mouth was located. Some additional protection may have been derived from another end-moraine segment now reduced by wave erosion to the boulder field shown on the map. Rise of sea level evidently caused these barriers to be overtopped and their remnants are the sand deposit seaward of B and the boulder field shown in the cross section and map of Fig. 9. It is estimated from

PEAT SAND END MORAINE BEDROCK BOULDER

4. 4. 4.

FIG.9. Map and section showing evolution of the salt marsh at Guiiford Harbor, Connecticut. When sea level was several meters lower, a protective barrier of sand extended westward past B and the marsh formed behind this barrier. When sea level rose above the barrier, the face of the marsh began to retreat under the attack of waves leaving the erosion surface between B and A. Retreat of the exposed marsh face is continuing while the marsh surface simultaneously grows upward with rising sea level.

20

R O B E R T B. GORDON

the sea level curve that this may have happened about 1000 years ago. In that time span the marsh face has retreated from B to A, where erosion is still active. Destruction of some unprotected marsh faces around the shore of the Sound at a comparable rate is expected as long as sea level continues to rise. 4. PHYSICAL OCEANOGRAPHY

Study of the tides of Long Island Sound began early in the nineteenth century and by 1932, when Le Lacheur and Sammons published their summary volume, a substantial body of data had been collected. It was recognized that the period of longitudinal oscillation of the Sound is close to 12.4 hr and that the tidal characteristics of the Sound are those of a resonant basin. Thus, the amplitude of the tidal height increases, and the speed of the tidal stream decreases, to the west; the greatest speed of the stream occurs when the water surface is nearly level and the time of high and low water is nearly synchronous throughout the Sound. Harmonic constants for the tide height and the speed of the tidal stream have been determined for a number of additional stations in the Sound since 1932 and we summarize these in Table 11. The dominant constituent is the lunar semidiurnal tide, M2. The amplitude of M 2 for the tide height increases by about a factor of 3 between the eastern and western ends of the Sound, while the amplitude of the stream decreases by almost as much. At the far-western end, shallow water components become relatively large (Bowman, 1976b). A calculation of the tidal exchange between successive sections across the axis of the Sound is given by Koppelman et al. (1976). A refined model of the tidal oscillation in Long Island Sound must allow for geostrophic and frictional forces. Rotary tides are observed in the center of the Sound, as illustrated by the current-meter record shown in Fig. 10. Each arrow in this diagram is the average velocity vector for a 20-min time interval. The anticlockwise rotation is in the direction expected for the effect of the earth’s rotation on a resonant basin (Doodson and Warburg, 1941). A theory of the damped resonant oscillation of the Sound with a linearized representation of the friction was published by Redfield (1950) and was further developed by Ippen and Harleman (1966). The average tidal dissipation calculated from Redfield’s model at mean tidal range is 460 MW. It is shown on p. 66 of the next article in this volume that an estimate of the dissipation based on direct measurements of water height and speed is 455 MW, that the Q-’ of the tidal oscillation is 0.32, and that the average specific dissipation is 0.060 W/mz. Direct measurements of the specific dissipation have been made for only a few

TABLE11. PRINCIPAL HARMONIC CONSTANTS OF THE TIDEAND TIDALSTREAM FOR LONGISLAND SOUND'

M2 Name

Lat . 41'21' 41"IO' 4176' 41"15' 41"17' 40"57' 41'10' 40°48'

0 1

g

H

(deg)

(m)

0.07 0.08

133 144

0.10

144 150

0.05 0.05 0.06 0.06 0.06

Long. Source' ~

New Londonb Plum Island Saybrook Jetty Hoadley Pt. New Havenb Port Jeffersonb Bridgeport Willetts Point

KI

s 2

72'06' 72"12' 72'21' 72'44' 72%' 73'05' 73"Il' 73"47'

(I) (I) (I) (1,2) (1,2) (I) (1)

(I)

~

273 287 307 319 325 328 32 1 334

~~

0.36 0.37 0.51 0.79 0.95

0.94 O.%

1.12

278 295 318 330 345 348 345 353

0.07 0.07 0.09 0.14 0.18 0.16 0. I6 0.20

104 111 104 I10 1I3 123 117 120

0.08 0.10 0.09 0.09 0.10

142 147 147

0.07 0.06

0.06

150

Stream

M2 g

Station name (73- 15) (72-2) (72-5) (EN-A)

Lat. 41%' 41W7' 41W' 41"OO'

Long. Source 72"32' 72"53' 72"58' 73"26'

(2) (2) (2) (3)

(deg) 47 55

KI

s 2

uo

(mdsec)

50

400 228 154

56

150

g

(deg) 66 66 61 74

uo

(mdsec) 71 40 27 34

g

(deg) 190

227 174 342

01

uo

(mrn/sec) 28 19 35 6

g

(deg) 220 267 214 93

uo

(mrnkec) 17 11

21 0.4

'References: ( I ) Admiralty Tide Tables, Vol. I1 (1978); (2) Gordon and Pilbeam (1975); (3) Bokuniewicz et a / . (1977). Tide gauge within harbor; may not be representative of conditions on the open coast. Notation: g is the phase angle and H and UOthe amplitudes of the tidal constituents; see the Admiralty Tide Tables for descriptions of these constituents.

22

ROBERT B. GORDON no

FIG.10. Velocity vectors recorded for successive 20-min time intervals by a current meter 2 m above the bottom near the geometrical center of Long Island Sound. The rotary character of the tide and the net drift of bottom water due to the estuarine circulation are shown.

other localities and are listed in Table 111; comparison shows that, while Long Island Sound is a tidally dominated estuary, it is one of intermediate specific dissipation. The tide advances up the Connecticut and the Housatonic Rivers as a progressive wave (Le Lacheur and Sammons, 1932). Tides and circulation in the Thames River are described by Tolderlund (1975). The East River is a tidal straight with most of its tidal prism derived from New York Harbor (Bowman, 1976a). Long Island Sound is connected to the sea by three passes at its eastern end and by the East River (through New York Harbor) at its western end. The principal source of fresh water entering the Sound is the Connecticut River, which enters near the eastern end, as shown in Fig. 11. Thus the Sound does not have the conventional configuration of an estuary with TABLEIll. SPECIFIC TIDALDISSIPATION Chandeleur Sound (Hart and Murray, 1978) Narragansett Bay (Levine and Kenyon, 1975) Long Island Sound Irish Sea (Taylor, 1919) Bay of Fundy (McLellan, 1958)

0.006 (Wlm’) 0.03 0.06 1.3 1.9

THE SEDIMENTARY SYSTEM OF LONG ISLAND SOUND

T

TI

23

1.

FIG.1 I . Entry of fresh water (excluding that from the Hudson River) and sewage effluent into Long Island Sound, by sections. (After Bowman, 1975.)

a river at its head end. Nevertheless, there is a well-developed estuarine circulation with less-saline water flowing eastward at the surface and more-saline water flowing westward at the bottom. This was first shown by Riley (1952, 1956, 1967) in his comprehensive study of the physical oceanography of the Sound. The water flow due to the estuarine circulation deduced from the salt balance is 25,000 m3/sec in the eastern Sound and 3500 m3/sec in the western end. Although it is a tidal strait rather than a river, the East River plays an important role in the maintenance of the estuarine circulation in the Sound because there is a net export of salt at the average rate of 1.2 x lo4 kg/sec through it (Bowman, 1975). This

24

ROBERT B. GORDON

maintains the longitudinal salinity gradient throughout the Sound. A small, additional residual flow is due to nonlinear tidal effects (Ianiello, 1977). Wilson (1976) has used a one-dimensional model to compute the gravitational circulation resulting from the density gradient determined by the salinity and temperature distributions and the friction due to tidal stream turbulence. He finds good agreement with the circulation deduced by Riley. The presence of this circulation is confirmed by drifter returns (Gross and Bumpus, 1972; Paskausky and Murphy, 1976) and currentmeter observations (Gordon and Pilbeam, 1975). However, Paskausky and Murphy’s inference that bottom-water flow does not penetrate into the central part of the Sound in the summer is not confirmed by other sources of data. Layers of surface and bottom water separated by a well-defined interface are present in the central part of Long Island Sound during much of the year. This interface is shown by the temperature and salinity profiles reproduced in Fig. 12. The Sound is large enough that lateral effects in the circulation are quite important. For example, the boundary between surface and bottom water is tilted upward to the north in response to the Coriolis force, while in the shallow water along both shores tidal mixing is strong enough to eliminate the interface between surface and bottom water. Bottom water flows laterally into these shoreside mixing zones, causing upwelling along the north and south coasts of the Sound (Gordon and Pilbeam, 1975). Sharp frontal boundaries between water masses are also present in the waters of Long Island Sound. One of the most persistent forms between river and sound water off the mouth of the Connecticut River during periods of high discharge or ebb flow (Garvine, 1974, 1975, 1977; Garvine and Monk, 1974). [At low river discharge salt penetrates several kilometers up the Connecticut River, which is itself a small estuary (Meade, 1966).] Bowman and Esaias (1977) have found a front separating the waters of Smithtown Bay, Long Island, from the waters of the central Sound. They suggest that instabilities at the front are responsible for the periodic injection of patches of high concentrations of phytoplankton into the Sound. In addition to receiving fresh water, large quantities of wastes and sediment are inserted into the Sound. The injection of sewage wastes and, for comparison, the inflow of fresh water, along the axis of the Sound is shown in Fig. 11 (after Bowman, 1975). New York City is the dominant source of wastes and, because they are inserted where the natural circulation is weakest, they represent the main cause of environmental degradation of the Sound at the present time. Bowman (1976a) has shown how this problem could be resolved by the construction of tidal locks across the East River. The principal source of sediment entering the

THE SEDIMENTARY SYSTEM OF LONG ISLAND SOUND

25

FIG.12. Temperature and salinity gradients measured near the geometrical center of Long Island Sound in the summer. The elevation above the bottom is Z . The bottom water layer is colder and more saline.

Sound is the Connecticut River, which drains most of central New England and enters the Sound near its eastern end. This is an area of strong mixing (because of a high level of tide-stream turbulence) and sediment brought down the river is mixed into the bottom water that subsequently flows into the central Sound. The extensive deposits of silt-clay sediment in the central basin of the Sound are formed, therefore, as a consequence of the estuarine circulation. 5. SEDIMENTATION

The mechanics of sediment transport and deposition in Long Island Sound are discussed in some detail in subsequent articles in this volume. Only an overview of the principal results is presented here.

26

ROBERT B. GORDON

The sediments of the Sound consist principally of glacial sands and marine muds. A margin of sand is found along both the Connecticut and the Long Island shore except for some localities where the shore consists of bedrock (see Fig. 3). The marine mud, deposited since the sea reentered the Sound, occupies most of the central basin. If lake sediments exist in the Sound, they are now buried under deposits of the marine mud. Long cores that would permit their detection have never been collected in the Sound. Throughout most of the Sound the lateral boundary between the mud and the sand is quite sharp, as shown on the map on p. 97 of this volume, but toward the eastern end the transition from muddy to sandy bottom is gradual. This is because sand carried into the central Sound by the combined action of the tidal stream and the estuarine circulation is being incorporated into the accumulating marine mud. A quantitative model of this transition zone is presented later in this volume. The volume of the marine sediment in Long Island Sound has been measured by acoustic-reflection profiling (Bokuniewicz et al., 1976). The method works well in the Sound because the upper surface of the glacial drift (the “sub-bottom”) is a strong acoustic reflector and the marine mud is sufficiently gas free to be acoustically transparent. Illustrations of acoustic-reflection profiles showing deposits of marine mud over outwash sand and over a submerged end-moraine segment are shown in Fig. 13. All available reflection profile data were used to construct the contour map of the surface of the glacial sands shown in Fig. 3a. The volume of silt-clay sediment contained in the marine mud is estimated by subtracting the volume of the contained sand (based on the analysis of the sand content of cores taken throughout the Sound) from the measured total volume of sediment. The volume of silt-clay sediment in the Sound is 5.3 x lo9 m3 and, since the density is about 0.8 Mg/m3, the mass is about 4.2 x 1OI2 kg. A small amount of sediment in the Sound is of biogenic origin and some originates from erosion of the north shore of Long Island, but most is supplied by the rivers that drain into the Sound. The Connecticut River is the principal source of fresh water (71% of the drainage basin area) and since all the rivers entering the Sound flow over generally similar terrain, it is likely that the sediment contribution of each is approximately proportional to its discharge. The present rate of supply of sediment to the Sound per unit area of existing mud bottom calculated from available data on the sediment yield of the rivers entering the Sound is 0.26 kg/(m2yr). The sediment yield of the Connecticut River is low compared to that of most rivers because much of its drainage basin is on glacial till, which is very erosion resistant. The principal source of sediment entering the river is bank collapse where the river flows over the beds of old glacial

FIG. 13. Structure of the bottom of Long Island Sound revealed by acoustic reflection profiles made with 7-kHzacoustic pulses. (Upper echo is produced by a 200-kHz echo sounder.) (a) Section of end moraine capped by boulders and almost buried by marine mud. (b) Thick deposit of marine mud in central Long Island Sound on top of outwash sand with reflector above thought to be surface of lacustrine deposits. (c) Sand-to-mud transition zone in central Long lsland Sound. In all records each division on the vertical scale is 600 mm.

FIG. 13b. See p. 27 for legend.

M

FIG.13c. See p. 27 for legend.

30

ROBERT B. GORDON

lakes (Gordon, 1979). This source is insensitive to land use and it is expected that the sediment yield has not changed much during the marine regime of the Sound. We will assume that the sediment supply rate over the past 8000 yr is about the same as the present rate, 0.26 kg/(m2 yr). The average, long-term mean rate of sedimentation in the Sound computed from the amount of accumulated silt-clay sediment is 0.29 kgl(m2 yr). It follows that the trapping efficiency of the Sound for sediment delivered by rivers must be nearly loo%, since sediment is accumulating at a rate slightly greater than the rate at which it is being delivered to the Sound by the rivers. Further evidence of high trapping efficiency is obtained from the recently completed radiometric determination of the sedimentation rate in the Sound by Benoit et al. (1979). The sediment accumulation rate calculated from their data is -0.4 kg (mZ yr), which exceeds the rate at which sediment is supplied by the rivers entering the Sound. The principal sources of error in the determination of the trapping efficiency of the Sound are the possibility that shore erosion may contribute more sediment to the Sound than our estimate and the possibility that the sea entered the Sound at an earlier date than that inferred from the data presented in Fig. 5 . The sediment-trapping efficiency of Long Island Sound is high because of the combined effects of several factors. First, the volumetric capacity of the Sound to store sediment is large compared to the sediment yield of the rivers entering it. The large capacity is a consequence of the deepening of the Sound basin by both preglacial erosion and by glacial overdeepening. The low sediment yield of the rivers is a consequence of the erosion-resistant character of the glaciated terrain of central New England. For comparison, estuaries along the southeast coast of the U.S. are much less likely to have such a large sediment storage capacity because of the high sediment yield of their drainage basins. They were filled nearly to capacity as they were flooded by the postglacial rise of sea level. An important factor in keeping the sediment storage capacity of Long Island Sound large is that sea level is rising faster than sediment is accumulating in the estuary. Where the rise of sea level is small, or the coast is emergent, the storage capacity of an estuary is much more likely to be exceeded as, for example, in the estuaries of the Lune and the Mersey on the west coast of the U. K. Finally, the specific dissipation in the Sound (0.06 W/ m2) is low enough to permit accumulation of sediment on the existing mud bottom. In estuaries having a much higher specific dissipation, such as the Bristol Channel of the U. K. (-6 W/m2),delivered sediment is not retained on the bottom. Sediment trapped in Long Island Sound is not necessarily isolated from contact with the ambient water or the waters of the continental shelf. The

THE SEDIMENTARY SYSTEM OF LONG ISLAND SOUND

31

specific dissipation in the Sound is large enough to regularly resuspend some of the trapped sediment, and, as is shown on p. 93, the amount of resuspension may be substantially increased during storms. The material resuspended in the water column consists of pelletized silt-clay sediment produced by the benthic animals of the Sound. This pelletized material forms a mantle about 10-mm thick on top of cohesive mud deposits. The amount of resuspension depends both on the level of the specific dissipation and on the properties of the pelletized material. The critical erosion velocity of the pellets shows seasonal variation in response to the seasonal changes in the production of organic binding by bacteria in the sediments (Rhoads et al., 1979). The data available on both the properties and the excitation of the sediment are not as yet adequate to permit a quantitative description of the amount of resuspension. Observations show that the mantle of pelletized material is fully excited into the water column only infrequently. New silt-clay sediment entering the Sound is processed to pellets by the benthic animals almost at once. The residence time of the silt-clay sediment in the pelletized mantle is about 10 years (see p. 98), after which it is converted into cohesive mud (or muddy sand) that is not subject to resuspension into the water column and is effectively isolated from the ambient water. The characteristics of the sedimentary regime of the Sound have several important consequences for the management of its marine resources. The first is that the sedimentary system is not likely to be much altered by any of the ordinary range of engineering works-dredging, construction of dams on the tributary rivers, or a bridge over the Sound-likely to be undertaken in the area. However, construction of tidal locks across the East River, as suggested by Bowman, would be extraordinary because the resultant alteration of the salinity of the Sound would change the benthic animal population responsible for the maintenance of the pelletized layer of sediment on the bottom. Materials entering the Sound attached to silt-clay sediment particles (such as heavy metals) are likely to be retained in the Sound and not exported to the sea. The Sound has a very large capacity to store these materials, but during the first 10 years or so of storage there will be frequent direct contact with the ambient water because of resuspension. Resuspended sediment may intermix, through tidal exchange, with suspended sediment in the waters of the continental shelf. Qualitative data are lacking. Akpati (1974) has examined the composition of sediment in Fishers Island Sound (which is actually the eastern extremity of Long Island Sound) and finds that material from both the adjacent land and from offshore sources is present, although in what proportions remains unknown. The pelletized mantle subject to resuspension

32

ROBERT B. GORDON

in the deep water of the Sound is also a reservoir of sediment available for accumulation in dredged channels. Sawhney and Frink (1979) have shown that the clay mineralogy of the sediments in New Haven Harbor is like that of the sediment reservoir in the central Sound and unlike that of the sediment carried by the rivers entering the harbor. Resuspended sediment is exchanged with that in harbors by the lateral circulation of the Sound and the local estuarine circulation of the harbor. Thus, continued sediment accumulation in the harbors is expected as long as the pelletized mantle of sediment in the central Sound exists. Once the silt-clay sediment is incorporated into the permanent mud bottom of the Sound, subsequent contact with the water above will be limited to molecular diffusion through the interstitial water. Contaminated sediment released from a point source in the Sound will be rapidly dispersed throughout the mantle of pelletized material by tidal mixing, so that large, local concentration gradients are not expected to persist around sediment disposal sites (Bokuniewicz and Gordon, 1979). However, regional gradients in the heavy metal content of the sediment are present. The capacity of the Sound to store and retain contaminants attached to sediment particles is a consequence of its high trapping efficiency. Determinations of the trapping efficiency of other estuaries are not generally available, although for some, such as the Bristol Channel, it is near zero, so that comparison of this aspect of the Sound with other environments is not yet possible. In the context of the longer span of geological time, the present sedimentary processes in the Sound are anomalous. Before glaciation, sediment was removed from the land surface at average rates comparable to the present-day sediment yield of drainage basins of moderate relief and elevation. The debris produced was transported to the continental shelf and margins. During the late Cenozoic glaciations there were periods of rapid denudation in the land surface, with rates which may have been as high as 3.5 kg/(m2yr). The sedimentation rate in glacial Lake Hitchcock, for example, was at least three times greater than the present rate in Long Island Sound (Gordon, 1979). In the interglacial periods the denudation rate became very small because the debris produced during the periods of rapid erosion was not cleared from the land surface. The present is one of those periods. Most of the sediment transported by the river system draining into Long Island Sound is debris produced by the late Wisconsin ice and left behind as the ice retreated. Most of this material is quite erosion-resistantand the regional denudation rate is small in consequence. The sediment yield of the streams draining into Long Island Sound is remarkably small by any standard. Because of the high trapping efficiency of the Sound, this material is not reaching the sea and probably will not

THE SEDIMENTARY SYSTEM OF LONG ISLAND SOUND

33

until sea level begins to fall at some time in the future. Because of its depth and configuration, Long Island Sound will probably be one of the last estuaries on the east coast of North America to lose its trapping efficiency during a fall of sea level. 6. FURTHER RESEARCH

A number of problems have arisen in our discussion of the evolution and operation of the sedimentary system of Long Island Sound that remain unresolved. Further field work and analysis are needed on these. We summarize below some of the more interesting ones. The origin of many features of the drainage system of Southern New England remains obscure. A record of an earlier stage of the development of this system is preserved beneath the sediments in the Sound. This record could be revealed by acoustic reflection profiling and used to establish the relationship of the ancient to the present drainage system. It may be possible to use microtopography determined from the reflection profiles to identify the continuation of bedrock formations offshore beneath the sediments in the Sound; this would permit the determination of the relation of the ancient drainage pattern to lithology. It would be of particular interest to trace the path of the ancestral Housatonic River across the Sound basin and to find where it passes under Long Island. These data should help resolve the question of how the overdeepened basins on the bedrock surface beneath the Sound were formed. It is likely that the older till found in Connecticut extends underneath the Sound; it may be continuous with some of the tills found on Long Island. Because it is highly compacted, the older till is expected to have a relatively high sound speed. It should show up as a distinct acoustic horizon and it should be possible to trace it under the Sound by acoustic methods. Data on the distribution of the older till would be most helpful in the interpretation of the early glacial history of the Long Island Sound basin. The thickness of marine sediments in the Sound has been measured by reflection profiling. A corresponding set of data on the thickness of the glacial drift could be used to determine glacial erosion rates in Southern New England. Since acoustic penetration to the rockhead can be attained throughout the Sound, there should be no technical difficulty in obtaining the requisite data. It should be possible to differentiate between the compact tills and outwash materials on the basis of their acoustic properties. Some horizons can be sampled by following them to outcrop, but some number of deep drill holds will undoubtedly be required to properly iden-

34

ROBERT B. GORDON

tify other horizons. A related problem that can be approached by the same methods is the internal structure of the Mattituck sill. It is important to determine whether or not the sill depth has been built up by the accretion of tidally transported sand or has remained nearly unchanged since deglaciation, since the sill depth determines the date of the return of the sea to the Sound. Reflection-profiler surveys should be made to better locate the sill at the western end of the Long Island Sound basin and to determine whether or not its altitude has been altered by marine processes. The history of sea level change in the Sound is reasonably well established for the past 5000 years, but there are substantial uncertainties about it before that time. The recovery of organic remains suitable for dating from deep bores within the Sound are needed to resolve this question. Because of its particular geographical situation, the Sound is an ideal place to test theories about rebound of the crust near an ice margin. It is likely that a description of regional rebound could be assembled by utilizing, in addition to the data from the Long Island Sound area, published information on the tilting of glacial lakes (such as Lake Hitchcock). Additional information could be obtained from careful leveling to obtain the inclination of the lake bottoms; excellent exposures are available in Southern New England. These data may also help to resolve the apparent discrepancy between the relative uplift rates east and west of the Connecticut Valley revealed by sea level and geodetic data. It is particularly important to determine whether or not there was appreciable elastic rebound due to a forebulge at the margin of the retreating ice sheet since, if this rapid rebound was small, the marine history deduced for the Sound will require substantial modification. The evolution of salt marshes along the Connecticut and Long Island shores continues to be of practical as well as academic concern; much useful data could be obtained at relatively low cost by continuing marker bed experiments (such as those initiated by Bloom), by assembling historical data on the geography of the marshes, and by making repeated surveys of the marsh margins. It would also be of interest to determine the absolute elevation of marsh surfaces with respect to mean sea level and the tidal range (such as presented in Fig. 7) for localities at the eastern and western ends of the Sound. The principal uncertainty in the determination of the amount of sediment entering the Sound arises from a lack of information about the composition of the materials being eroded from the north shore or Long Island. Field work and sampling along this shore are needed. Before it is incorporated into the permanent bottom of the Sound, sediment is available for resuspension for a number of years. The amount and frequency of resuspension depends on both physical and biological factors, which can

THE SEDIMENTARY SYSTEM OF LONG ISLAND SOUND

35

be studied individually in the laboratory. However, there are no long-term data of sufficiently high sampling density to permit description of the amount of resuspension of the muddy bottom that takes place in the Sound. The physical and biological factors that control the susceptibility of muddy sediment in the Sound to resuspension are sufficiently complex, and their interaction sufficiently uncertain, that one could not have confidence in an evaluation of the stability of the muddy bottom based on laboratory data alone; good quality field observations are necessary. Resuspended sediment is available for exchange with sediment in Block Island Sound or the continental shelf, but the amount of the intermixingwhich is important in determining how efficiently the Sound traps effluent materials attached to the sediment particles-is not known at all. Extensive observations of sediment concentrations and water movements in the eastern end of the Sound would be needed to answer this question. Available data show that Long Island Sound retains nearly all of the sediment delivered to it, its trapping efficiency is near 100%. The factors responsible for this are the size and depth of the basin, the magnitude of the specific energy dissipation, and the rapid biological processing of the sediment delivered by the rivers. If this identification of the factors responsible for the high trapping efficiency is correct, it should be possible to use an evaluation of these same factors to determine the trapping efficiency of other estuaries. Measurements of the sediment mass balance of other representative estuaries is needed to test these ideas. ACKNOWLEDGMENT 1 thank Henry Bokuniewicz, Walter Newman, and J. P. Schafer for valuable comments on topics discussed in this chapter.

REFER E N cE s Adams, D. A. (1963). Factors influencing vascular plant zonation in North Carolina salt marshes. Ecology 44,445-456. Akpati, B. N. (1974). Mineral composition and sediments in eastern Long Island Sound, New York. Marit. Sediments 10, 19-30. Barrel], J. (1920). The Piedmont terraces of the northern Appalachians. Am. J . Sci. [4] 49, 227-258. 327-362. 407-428. Benoit, G. J., Turekian, K. K.. and Benninger, L. K. (1979). Radiocarbon dating of a core from Long Island Sound. Estiiarine Coastal Mar. Sci. 9, 171-180. Bloom, A. J., and Stuiver, M. (1963). Submergence of the Connecticut coast. Science 139, 332-334. Bokuniewicz, H. J., and Gordon, R. B. (1979). Containment of particulate wastes at openwater disposal sites. In “Ocean Dumping and Marine Pollution” (H. D. Palmer and M. G. Gross, eds.), pp. 109-129. Dowden, Hutchinson L Ross, Inc., Stroudsburg, Pennsylvania.

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Bokuniewicz, H. J., Gebert, J., and Gordon, R. B. (1976). Sediment mass balance of a large estuary, Long Island Sound. Estuarine Coastal Mar. Sci. 4, 523-536. Bokuniewicz, H. J., Dowling, M., Gebert, J., Gordon, R., Kaminsky, P., Pilbeam, C., and Tuttle, C., (1977). “Aquatic Disposal Field Investigations Eatons Neck Disposal Site Long Island Sound Appendix A: Investigation of the Hydraulic Regime and the Physical Characteristics of Bottom Sedimentation,” Tech. Rep. D-77-6. U.S. Army Engineer Waterways Experiment Station, Vicksburg, Mississippi. Boulton, G. S. (1974). Processes and patterns of glacial erosion. I n “Glacial Geomorphology” (D. R. Coates, ed.), pp. 41-87, State University of New York at Binghamton. Bowman, M. J. (1975). Pollution prediction model for Long Island Sound. Proc. Ocean Eng., 3rd, 1975 pp. 1084-1103. Bowman, M. J. (1976a). Tidal locks across the East River: An engineering solution to the rehabilitation of Western Long Island Sound. I n “Estuarine Processes” (M. Wiley, ed.), Vol. 1, pp. 28-43. Academic Press, New York. Bowman, M. J. (1976b). The tides of the East River, New York. J. Geophys. Res. 81, 1609- 1616.

Bowman, M. J., and Esaias, W. E. (1977). Coastal jets, fronts and phytoplankton patchiness. 8th Liege Colloq. Ocean Hydrodyn., 1977 pp. 255-268. Brown, L. D. (1978). Recent vertical crustal movement along the coast of the United States. Tectonophysics 44, 205-23 1 . Cathles, L. M., 111 (1975). “The Viscosity of the Earth’s Mantle.” Princeton Univ. Press, Princeton, New Jersey. Chapman, V. J. (1960). “Salt Marshes and Salt Deserts of the World.” Wiley (Interscience), New York. Dana, J. D. (1890). Long Island Sound in the Quaternary era, with observations on the submarine Hudson River channel. Am. J . Sci. [3]40,425-437. Davis, M. B. (1%5). Phytogeography and palynology of the northeastern United States. In “The Quaternary of the United States” (H. E. Wright, Jr. and D. G. Frey, eds.), pp. 377-401. Princeton Univ. Press, Princeton, New Jersey. Davis, M. B. (1969). Climatic changes in southern Connecticut recorded by pollen deposition at Rogers Lake. Ecology 50,409-422. Doodson, A. T., and Warburg, H. D. (1941). “Admiralty Manual of Tides.” HM Stationery Ofice, London. Dyer, R. R. (3973). “Estuaries, A Physical Introduction.” Wiley. New York. Emerson, B. K. (1898). Geology of Old Hampshire County, Massachusetts. U.S . Geol. Surv., Monogr. 29, 1-790. Farrell, W. E., and Clark, J. A. (1976). On postglacial sea level. Geophys. J. 46, 647-667. Flint, R. F. (1956). New radiocarbon dates and late-Pleistocene stratigraphy. Am. J. Sci. [5] 254, 265-287.

Flint, R. F. (1%3). Altitude, lithology and the fall zone in Connecticut. J. Geol. 71,683-697. Flint, R. F. (1964). The surficial geology of the Branford Quadrangle. Conn., State Geol. Nat. Hist. Surv., Quadrangle Rep. 14. Flint, R. F. (1965). The surficial geology of the New Haven and Woodmont Quadrangles. Conn., State Geol. Nat. Hist. Surv., Quadrangle Rep. 18. Flint, R. F. (1971). The surficial geology of the Guilford and Clinton Quadrangles. Conn., State Geol. Nat. Hist. Surv., Quadrangle Rep. 28. Flint, R. F., and Gebert, J. A. (1976). Latest Laurentide ice sheet: New evidence from southern New England. Geol. SOC.Am. Buli. 87, 182-188. Garvine, R. W. (1974). Physical features of the Connecticut River outflow during high discharge. J . Geophys. Res. 79, 831-846.

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Garvine, R. W. (1975). The distribution of temperature and salinity in the Connecticut River estuary. J. Geophys. Res. 80, 1176-1 182. Garvine, R. W. (1977). Observations of the motion field of the Connecticut River plume. J. Geophys. Res. 82, 441-454. Garvine, R. W., and Monk, J. D. (1974). Frontal structure of a river plume. J. Geophys. Res. 79, 2251-2259. Gordon, R. B. (1979). Denudation rate of central New England determined from estuarine circulation. Am. J. Sci. 279, 632-642. Gordon, R. B., and Pilbeam, C. C. (1975). Circulation in central Long Island Sound. J . Geophys. Res. 80, 414-422. Grim, M. S., Drake, C. L., and Heirtzler, J. R. (1970). Sub-bottom study of Long Island Sound. Geol. SOC.Am. Bull. 81,649-666. Gross, M. G., and Bumpus, D. F. (1%9). Residual drift of near-bottom waters in Long Island Sound. Limnol. Oceanogr. 17,636-638. Hamson, E. Z., and Bloom, A. L. (1977). Sedimentation rate on tidal salt marshes in Connecticut. J. Sediment. Petrol. 47, 1484-1490. Hart, W. E., and Murray, S. P. (1978). Energy balance and wind effects in a shallow sound. J. Geophys. Res. 83, 4097-4106. Hicks, S. D., and Crosby, J. E. (1974). Trends and variability of yearly mean sea level. NOAA Tech. Mem. NOS 13, 1-14. laniello, J. (1977). Nonlinearly induced residual currents in tidally dominated estuaries. Ph.D. Thesis, University of Connecticut, Storrs. Ippen. A. T., and Harleman. D. R. F. (1966).Tidal dynamics of estuaries. In “Estuary and Coastline Hydrodynamics” (A. T. Ippen, ed.), pp. 493-545. McGraw-Hill, New York. Jahns, R. H., and Willard, M. E. (1942). Late Pleistocene and Recent deposits in the Connecticut Valley, Massachusetts. Am. J. Sci. [5] 240, 161-191, 265-287. Johnson, D. (1931). “Stream Sculpture on the Atlantic Slope.” Columbia Univ. Press, New York. Kaye, C. A. (1964). Outline of Pleistocene geology of Martha’s Vineyard, Massachusetts. U.S . , Geol. Surv., Prof. Pap. 401-C. Kaye, C. A., and Stuckey, G . W. (1973). Nodal tidal cycle of 18.6 Yr. Geology 1, 141-144. Koppelman, L. E., Weyl, P. K., Gross, M. G., and Davies, D. S. (1976). “The Urban Sea: Long Island Sound.” Praeger, New York. Koteff, C. (1968). Postglacial tilt in Southern New England. Geol. SOC. Am., Spec. Pap. 101 (abstr.). Koteff, C. (1974). The morphological sequence concept and deglaciation of southern New England. In “Glacial Geomorphology” (D. R. Coates, ed.), pp. 121-144. State University of New York at Binghamton. Le Lecheur. E. A., and Sammons. J. C. (1932). Tides and currents in Long lsland Sound and Block Island Sound. U.S . Coast Geodetic Surv., Spec. Publ. 174. Levine, E. R., and Kenyon, K. E. (1975). The tidal energetics of Narragansett Bay. J. Geophys. Res. 80, 1683-1688. Lougee, R. J. (1938). Physiography of the Quinnipiac-Farmington Lowland in Connecticut. Colby Coll. Monog. No. 7. pp. 1-64. McLellan, H. J. (1958). Energy consideration in the Bay of Fundy System. J. Fish. Res. Board Can. 15, 115-134. McMaster, R. L., and Ashraf, A. (1973). Subbottom basement drainage system of inner continental shelf off southern New England. Geol. SOC.Am. Bull. 84, 187-190. Mathews, W. H.(1975). Cenozoic erosion and erosion surfaces of eastern North America. Am. J . Sci. [5] 275, 818-824. Meade, R. H. (1966). Salinity variations in the Connecticut River. Water Res. 2, 567-579.

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Menard, H. W. (1961). Some rates of regional erosion. J. Geol. 69, 154-161. Morner, N. (1969). The late Quaternary history of the Kattegatt Sea and the Swedish west coast, deglaciation, shoreline displacement, chronology, isostasy and eustasy. Arsb., Sver. Geol. Unders., Ser. C63, No. 3, 404-453. Newman, W. S. (1977). Late Quaternary paleoenvironmental reconstruction: Some considerations from northwestern Long Island, New York. Ann. N . Y. Acad. Sci. 288, 545-570. Oldale, R. N., and Uchupi, E. (1970). The glaciated shelf of northeastern United States. U . S . , Geol. Surv., Prof. Pap. 700, B167-8173. Paskausky, D. F., and Murphy, D. L. (1976). Seasonal variation of residual drift in Long Island Sound. Estuarine Coastal Mar. Sci. 4, 413-522. Peltier, W. R., and Andrews, J . T. (1976). Glacial-isostatic adjustment. 1. The forward problem. Geophys. J . 46,605-646. Pessl, F., and Schafer, J. P. (1968). Two-till problem in Naugatuck-Torrington area, Western Connecticut. In “Guidebook for Fieldtrips in Connecticut” (P. M. Orville, ed.), Guidebook No. 2. State Geological and Natural History Survey of Connecticut, Hartford. Pitman, W. C., 111 (1978). Relationships between eustacy and stratigraphic sequences on passive margins. Geol. SOC.Am. Bull. 89, 1389-1403. Redfield, A. C. (1950). The analysis of tidal phenomena in narrow embayments. Pap. Phys. Oceanogr. Meteorol. 11, 1-35. Rhoads, D. C., Yingst, J. Y., and Ullman. W. J. (1979). Seafloor stability in central Long Island Sound. Part I. Temporal changes in the erodibility of fine-grained sediment. In “Estuarine Interactions,” (M. L. Wiley, ed.), pp. 221-244. Academic Press, New York. Schafer, J. P. (1979). The late Wisconsinan Laurentide ice sheet in New England. Geol. SOC. Am., Abstr. Programs 11, 52. Riley, G. A. (1956). Oceanography of Long Island Sound 1952-1954. 11. Physical oceanography. Bull. Bingham Oceanogr. Collect. 15, 15-46. Riley, G. A. (1967). Aspects of oceanography of Long Island Sound. 11. Transport and mixing processes in Long Island Sound. Bull. Bingharn Oceanogr. Collect. 19, 35-61. Sawhney, B. L., and Frink, C. R. (1979). Clay minerals as indicators of sediment source in tidal estuaries of Long Island Sound. C/ays, Clay Miner. (in press). Schafer, J. P. (1979). The late Wisconsinan Laurentide ice sheet in New England. Geol. SOC. Am., Absir. Programs 11,52. Schafer, J. P., and Hartshorn, J. H. (1965). The Quaternary of New England. I n “The Quaternary of the United States” (H. E. Wright, Jr. and D. G. Frey, eds.), pp. 113-128. Princeton Univ. Press, Princeton, New Jersey. Schaffel, S. (1971). Reconstruction of late glacial and post-glacial events in Long Island Sound. Ph.D. Thesis, New York University. Sharp, H. S. (1929). The physical history of the Connecticut shoreline. Conn., Stare Geol. Nut. Hisi. Surv. Bull. 46, 1-97. Steever, E. 2..Warren, R. S., and Niering, W. A. (1976). Tidal energy subsidy and standing crop production of Spartina alterniflora. Estuarine Coasial Mar. Sci. 4,473-478. Stuiver, M., Deevey, E. S., Jr., and Rouse J. (1963). Yale natural radiocarbon measurements. VIII. Radiocarbon 5, 312-341. Taylor, G. I. (1919). Tidal friction in the lrish Sea. Phiios. Trans. R. SOC.London, Ser. A 220, 1-33. Tolderlund, D. S. (1975). “Ecological Study of the Thames River Estuary (Conn.),” Rep. No. RDCGA575. U. S. Coast Guard Academy. Upson, J. E., Leopold, E. B. and Rubin, M. (1964). Postglacial change of sea level in New Haven Harbor, Connecticut. Am. J . Sci. [ 5 ] 262, 121-132.

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Walcott, R. J. (1970). Isostatic response to loading of the crust in Canada. Can. J . Earth Sci. 7 , 7 16-727. Watts, A. B . , and Ryan, W. B. F. (1976). Flexure of the lithosphere and continental margin basins. Tectonophysics 36, 25-44. Wilson, R. E. (1976). Gravitational circuiation in Long Island Sound. Estuarine Coastal Mar. Sci. 4, 443-453.

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STORM AND TIDAL ENERGY IN LONG ISLAND SOUND HENRY J. BOKUNIEWICZ Marine Sciences Research Center State University of New York Stony Brook, New York

AND

ROBERT B. GORDON Department of Geology and Geophysics Yale University New Haven, Connecticut

1.

2. 3. 4. 5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tidal Energy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Storm Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Water Level Deviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Appendix 1. Formulation of the Energy Balance in an Embayment . . . . . . . . . Appendix 11. Estimate of Tidal Dissipation of All of LIS . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. .

41 43 48 55 60 61 65 67

1. INTRODUCTION

Power is used in an estuary to mix fresh and salt water, to resuspend bottom sediment, to transport sand and mud, and to maintain turbulence in the tidal stream. The requisite mechanical energy may be derived from the oceanic tide, the gravitational potential of the moon, the inflow of fresh water, or the wind stress acting on the water surface of either the estuary or the adjacent ocean. If the dominant power source for an estuary is tidal, the temporal variability of mixing and of sediment resuspension and transport should be regular; it should be possible to analyze their variation in terms of tidal constituents in much the same way that water level changes are analyzed. This is not possible when other power sources are important. Some alternative means of describing the time variation of power supplied to the estuary is then required for the analysis of estuarine mixing and transport processes, or before a program of observation can be designed to measure these reliably. Thus, determination of the temporal variation of the power supplied to an estuary can be of 41 ADVANCES IN GEOPHYSICS, VOLUME

22

Copyright 0 1980 by Academic Press, Inc. All ri&s of reproduction in any form reserved.

ISBN 0-12-018822-8

4,.JOa

74.00'

7 b w

I

73-00

I

72-30'

I

7200'

FIG.1. Map of Long Island and Block Island Sounds showing the locationsof tide gauges,anemometers,and current meters. The tidegaugeswere at New London (NL), New Haven (NH), Bridgeport (Bpt), Port Jefferson (PJ), New Rochelle (NR), Montauk (M), and Sandy Hook (SH). Newport (Np) is 67 km east of NL and not shown on the map. Current meters were operated at locations J, D, S, X, and Y; a water level recorder was also operated at J. Stratford Point Is St. Anemometer locations are shown by open circles. Power calculations are done for the section between A and B.

STORM AND TIDAL ENERGY IN LONG ISLAND SOUND

43

practical as well as scientific interest. Here we address this problem for Long Island Sound (LIS). The Sound (Fig. 1) is a large estuary in which the dominant source of power is thought to be the tide. However, observations of sediment resuspension and transport in water much too deep for the bottom to be affected by waves generated by the wind, show temporal variability that cannot be accounted for in terms of tidal constituents (see following article, this volume). Significant mechanical power must be entering the Sound from other sources. Long Island Sound receives fresh water principally from the Connecticut River. The resultant estuarine circulation was first described by Riley (1952, 1956) and has been calculated in a one-dimensional model by Wilson (1976). The circulation and mixing in the central Sound has been measured by Gordon and Pilbeam (1975). Because the natural period of longitudinal oscillation of LIS is very near 12.4 hr, the Sound has a resonant co-oscillating tide with an amplitude in the western end about four times that at the eastern entrance. Tidal currents in the Race reach speeds up to 4 knots. The tidal currents and water heights vary by about a factor of 2 between neap and spring tide. The Sound is subject to frequent winter storms and occasional hurricanes.

2. TIDALENERGY The first quantitative analysis of the tide in LIS was made by Redfield (1950), who represented the energy dissipation by linearized friction. The tidal oscillation used in his model has a node at longitude 72" 45'W and an apparent reflecting surface at 73" 30'W, about two-thirds of the way along the longitudinal axis of the Sound. The average rate at which energy is dissipated by Redfield's resonant co-oscillating tide, calculated according to the method derived by Ippen and Harleman (1966), is 460 MW at the mean tidal range. The relative rate of energy dissipation in an oscillating system may be described by the quality factor Q, which is 2n times the maximum energy stored per tidal cycle divided by the energy dissipated per cycle. (Q is also 2n times the number N of cycles required to dissipate the energy stored in the system.) For LIS, maximum energy stored in a tidal oscillation of mean range is 12.3 x lo6 MJ and Q is 3.8. The tidal oscillation is heavily damped ( N = 0.6). The only data used in Redfield's analysis are tide height observations. If current measurements are also available, the power P passing over any section across the Sound perpendicular to the current direction can be calculated by the method derived by Taylor (1919). If D is the water depth

44

HENRY J. BOKUNIEWICZ AND ROBERT B. GORDON

at mean sea level, h the height of the water level above D, and v the velocity normal to the section taken positive for inward flow, then (2.1)

P

=

pgDs(hv)

where p is the density, g the acceleration due to gravity, and s the length of the section. It is assumed that h 6 D in the derivation of Eq. (2.1). When the water level changes and the current velocities are solely due to the tide, we set P = P,, which is the tidal power crossing the section. The symbol (hv) indicates that the product is averaged over an appropriate time interval. In Taylor’s original analysis the averaging of h v was done over a semidiurnal tidal cycle. Because of the diurnal inequality and the spring-neap variation in tidal amplitude, there is a net gain (or loss) of energy from cycle to cycle as the tidal amplitude changes. To allow for this, we use an averaging period of 15 days in calculating the tidal energy flux. It is recognized that cross terms in (hv)due to lower frequency tidal constituents may not be negligible, but longer spans of data were not at hand. Simultaneous measurements of v and h were made at location J (Fig. 1) on the north-south section A-B through the middle of LIS. The measurements covered a 16-day period during which the winds were calm. The water depth was measured with a Bass model WG-100 wave gauge each hour and the current, with a General Oceanics model 20-10 current meter. Both instruments were fitted with quartz-crystal clocks so that timing errors were small, The current meter was set 2 m above the bottom on a taut mooring. The north-south component of the current at this location is only 15% of the east-west component, and the estuarine circulation carries bottom water westward into the Sound at a speed of about 5 cm/ sec; there is a corresponding outward flow of less-saline surface water (Gordon and Pilbeam, 1975). The east-west component of the measured velocity is used for v. Since in the calculation of (hv) there must be no net flow of water across the north-south section over an integral number of tidal cycles, this mean flow component was removed from the current meter record. There is no significant flow of tidal energy across any other boundary of the Sound to the west of section A-B so Pt is the tidal power passing into the Sound west of the section. If h and v measured at location J are representative of the entire section A-B, the energy crossing the section and entering LIS west of 72” 53’W in time T is the sum pgDs Z hv At, where At is the interval between successive, simultaneous measurements of h and v , and the summation is over all At in T. This sum is shown in Fig. 2 as a function of T. The indicated tidal power (i-e., the slope of the curve in Fig. 2) is larger at springs and smaller at neaps, as expected. The average observed power is 53 MW. The decrease in energy

STORM AND TIDAL ENERGY IN LONG ISLAND SOUND

45

10

1

N

-0

w

ou-

h

P

%,'

w

40-

20 0 (

100

200 .

- ~.

300

J

400

T (hr)

FIG.2. The total apparent tidal energy-crossing section A-B as calculated from h and v measured at J during calm weather. Curve L is the energy used in work done on the moon by the water within section A-B.

after T = 270 hr in Fig. 2 is unexpected, since there is no obvious source from which power could flow seaward across section A-B for times longer than a semidiurnal tidal cycle. The negative slope of the energy curve could arise from errors in the measurement of the phase difference between h and v , from release of energy stored in low-frequency components of the tidal oscillation, or from spatial variation in the flux of tidal power across section A-B. The data needed to check these possibilities are not at hand. However, the second seems quite unlikely, the first and third, about equally likely. Improvement in the measurement of the phase difference between h and v, if possible, would require a very long run of data because of the relatively large fluctuating component of v (see preceding article, this volume). Lateral variation of power along the section is discussed below. To find the actual P, for section A-B from the results presented in Fig. 2, allowance must be made for: (1) the variation of the amplitude and phase of v through the water column at location J; (2) the lateral variation of the magnitude of h and v across the section; and (3) the change in the phase of h and w across the section.

The variation of the magnitude and phase of v with height is due to friction at the bottom and the estuarine circulation. Data from a vertical current

46

HENRY J. BOKUNIEWICZ AND ROBERT B. GORDON

meter array at location S were used to evaluate (hv)for the upper and lower half of the water column. Current meter records for locations S, X, and Y spaced along the section were used to calculate (hv) at these locations. The records are not simultaneous and no concurrent measurements of h are available. It was necessary to compute h from the tidal constituents at J because tide height data for the southern part of section A-B are not available and tidal intervals for the shore of Long Island published in tide tables do not yield a consistent pattern that can be used for interpolation. The phase of the tide measured at J and that predicted for location A, the New Haven Harbor entrance, is nearly the same and is, therefore, assumed to be constant across the section. Le Lacheur and Sammons (1932) report that the tidal range is only 5% less on the Long Island shore than on the Connecticut shore. On the basis of these computations it is estimated that the P, shown in Fig. 2 should be decreased by 20% to get the actual energy flow across the section. When this correction is applied, the average, net tidal power through section A-B becomes 43 MW. Variation in the lateral distribution of the tidal power flow across the section between spring and neap conditions is not ruled out and may be the cause of the negative slope in the energy curve in Fig. 2. Part of the energy flowing into western LIS across section A-B is used in mixing surface and bottom water, part is used in work done on the moon, and the rest is dissipated by friction. The power required for the mixing of surface and bottom water in the western half of LIS is estimated by the method of McLellan (1958) to be about 1 MW, or 2% of the mean tidal power. The work done on the moon is substantially larger, however. The method of calculating this work was derived by Taylor (1919). Taylor’s method has been widely used, but it has recently been shown to be incorrect (Garrett, 1975). The work done on a volume of water of surface area A inside a boundary of length S and mean water depth D by

FIG.3. Definition of the phase angle used in Eq. (2.3).

STORM AND TIDAL ENERGY IN LONG ISLAND SOUND

47

TABLEI. TIDALPOWERCHARACTERISTICS OF WESTERNLIS AVERAGED OVER THE SPRING-NEAP PERIOD Measured tidal power over section A-B as measured at location J Corrected tidal power for section A-B Power used in mixing surface and bottom water Power used in work done on the moon Power dissipated through friction

53 MW

43 1 17

25

the moon’s attraction during two complete lunar semidiurnal tides is (2.2)

Em

=

pDS(sl~)- PA

J

h dfl

where n is the gravitational potential of the moon, the integration is over all changes in in two semidiurnal tidal cycles, and the average is also taken over two cycles. We show how this expression can be used to calculate the mechanical energy balance of an embayment in Appendix I. If the tide height and tidal stream speeds can be represented by sinusoidal terms, Eq. (2.2) reduces to MR’ (2.3) E m = 3pG 7cos2d cos’ B($DSvoP sin 24$, - h A H sin 2 4 ~ ~ ) Dm where G is the gravitational constant, R the Earth’s radius, D , the radius of the moon’s orbit, M the mass of the moon, 0 the latitude, 2H the tidal range, P the tidal period, and d the declination of the moon. The angle cb0, defined in Fig. 3 , is measured from the time interval between the moon’s meridian passage and high water; 4; is measured from the meridian passage to slack water. High water occurs before the moon’s meridian passage inLIS so c $ > ~ 0 and, in the western Sound, the second term in Eq. (2.3) is larger than the first and work is done on the moon by the tide. High water is nearly simultaneous and the variation of tidal amplitude is small throughout LIS west of section A-B. Calculation of Emto the requisite accuracy may then be made with the aid of tide tables and the nautical almanac. This calculation was done numerically for the period in which the data in Fig. 2 were obtained and is shown as curve L; over the spring-neap cycle Em averages - 17 MW or 40% of the energy flow across section A-B. This leaves 25 MW as the average tidal power dissipated by friction in the western half of LIS. The results are summarized in Table I. Data that would permit computation of the influx of tidal power to all of LIS are not at hand, but an estimate can be made. It is given in Appendix 11.

48

HENRY J. BOKUNIEWICZ AND ROBERT B. GORDON

3. STORMENERGY

Storms passing over southernNewEngland cause water level deviations at the shore ranging up to -1 m (Miller, 1958). These deviations result principally from set-up, i.e., sloping of the sea surface due to wind stress. Easterly storm winds also generate strong alongshore currents in the waters of the continental shelf; the pressure gradient due to the set-up and the alongshore current are found to be nearly balanced (Beardsley and Butman, 1974). Miller found that the water level deviation at any given observing station can be separated into a regional component (due to sea surface tilt) and a local component, which is influenced by the configuration of the land near the site of observation. This implies that energy from a storm reaches the shore zone from both local winds and regional winds blowing over the waters of the continental shelf. In order to evaluate the changes in the energy flow into LIS due to storms we first examine records for one major winter storm. The “northeaster” of 15-16 December 1972 is chosen because simultaneously recorded water height, current meter, and wind velocity data are available for it. Deviations of observed water levels from predicted tidal heights (“residuals,” Ah) at New Haven (NH), New London (NL), and Newport (Np) were calculated for an eight-week period starting in mid-December 1972 by a regression method using 18 tidal constituents. Additional water level residuals were supplied to us by J. Ianiello (personal communication, 1975) for the days of the storm of 15-16 December. Two current meters set 2 m above the bottom and located 3 km apart were in operation at D and S in Fig. 2 nearly on section “A-B,” throughout the study period. These meters were in water sufficiently deep to be unaffected by waves at the water surface. The recorded velocities were resolved into E-W (u,) and N-S (u,) components. Each of these components was then divided into mean, tidal, and fluctuatingparts with the aid of the regression analysis. Wind data are available from the U. S. National Weather Service station on Stratford Point, from several Coast Guard Stations around the Sound, and, in more detail, from a weather tower at apower plant adjacent to New Haven Harbor. The locations of all observing stations are shown in Fig. 1 . The residual water levels at NH, NL, and Np, the residual currents (the sum of the mean and fluctuating components), and the square of the E-W and N-S components of the wind speed measured at Stratford (assumed proportional to the components of the wind stress on the water surface, a,, a,) are shown for the period of the storm in Fig. 4. The water levels at NH, NL, and Np rise while the wind has a strong easterly component; the subsequent fall in level begins when oxreverses. The times of the maximum and zero Ah for these three water level stations are nearly

49

STORM AND TIDAL ENERGY IN LONG ISLAND SOUND

--

WATER LEVEL RESIDUAL I

1000-(WIND SPEED^

--N-S

bE

N -

-----

'

New Haven New London Newport

500-ig N-

a W

w n

o=-,

v)

-

'F

5OO-ggIn 1000

14

I

I

I

15

I

I

16 DECEMBER 1972

I

17

I I

18

FIG.4. Departure of observed from predicted water level at NH, NL, and Np; the mean flow recorded 2 m above the bottom at location D, and the relative wind stress as measured at NH during a winter storm, December 1972.

coincident, but the magnitude of Ah increases to the westward. Cross correlations were computed for the water level residuals at NH, NL, and Np for a 15-day period beginning 15 December 1972. The results are summarized in the following tabulation and show that there is a very close correlation between the water level deviations at these three stations: Harbor

Correlation

NL vs. Np NH vs. Np

0.96 0.83

50

HENRY J. BOKUNIEWICZ AND ROBERT B. GORDON

The time required for a shallow-water wave to travel from Newport to New Haven is about 2.5 hr, from New London to New Haven, about 1.3 hr. Since the greatest Ah occurs earlier at New Haven than at New London, the rise in water level in LIS is not a surge advancing as a progressive wave from the sea. Evidence of a storm surge in LIS is found only for very intense, rapidly moving storm systems, such as the 1938 hurricane (Redfield and Miller, 1957). The water level residuals observed at successive times during the 15-16 December 1972 storm are shown in Fig. 5 for all observing stations. Station locations are projected onto a line running along the axis of LIS (direction 075 true from Throgs Neck) to construct the abscissa of the graphs. Wind speeds and directions are shown on an adjacent set of maps. The residual water levels at Bridgeport and Port Jefferson, nearly opposite each other across the Sound, are almost the same throughout the storm. Hence, there is little change in water level across the Sound and the levels shown in Fig. 5 define the longitudinal slope of the water surface throughout the study area. During the period of easterly wind, the water level throughout the region is raised and the water surface slopes upward to the west. The water level residuals at Montauk Point and Sandy Hook, on the open ocean, are also shown in Fig. 5. The dashed line connecting them represents the slope of the sea surface in the direction 075 outside of LIS and Block Island Sound (BIS); it is nearly the same as the slope inside the two Sounds. The water level and wind data suggest that the following sequence of events occurred during the December 1972 storm: easterly winds set water on the continental shelf in motion towards the west, increasing water levels along the coast and tilting the sea surface upward, as observed for similar storms by Beardsley and Butman (1974). Similar changes in water level and surface slope occur in BIS and LIS. (There is also an increase in water level along the coast due to the reduced barometric pressure, but this is small compared to the change due to wind stress; no correction for local barometric pressure has been made in calculating the residual water levels.) The movement of the storm center, and the resulting change in wind speed and direction, are sufficiently slow that approximate balance between wind stress and surface slope is maintained. The inflow of water to LIS is seen in the current meter records (Fig. 4) as a residual flow to the west (u, < 0) during the period of water level rise. The observed residual flow across section A-B accounts for the observed increase in water volume within LIS westward of the section to within the accuracy of measurement. The water level at the extreme western end of LIS is influenced by the net flow through the East River, but the volume involved is negligible compared to that entering through the eastern passages to

STORM AND TIDAL ENERGY IN LONG ISLAND SOUND

51

BIS and the Atlantic Ocean. When the alongshore component of the wind reverses direction, the excess water level begins to fall and, as the intensity of the west wind increases, the surface slope both inside and outside the two Sounds reverses. An outward net flow of water from LIS is then indicated by the current meters. The mean tidal prism of LIS is 5.47 x lo9 m3. The greatest excess volume of water in the Sound during the 15-16 December storm is 3.4 X lo9 m3 or 62% of the mean tidal prism. The potential energy of the excess water at maximum 6h is 18 x lo6 MJ; the potential energy at the top of the tidal oscillation of mean range is 12.3 x lo6 MJ. The power-crossing section A-B during the storm can be evaluated from the current meter records from locations D and S and tide gauge data, but the quality of the data is not as high as that shown in Fig. 2 for two reasons. First, mechanical clocks were used in the current meters and interpolated corrections for their rates are required. Second, the available water level data are from a tide gauge within New Haven Harbor (rather than one at a current meter site), and a correction for the phase and amplitude difference of the tide between this location and that of the current meters is required. The tide height was corrected by a factor of 1.3, determined by a direct comparison of simultaneous water level measurements at the New Haven harbor tide gage and location J. The phase of the water level observed at NH was then corrected so that the tidal power dissipation during a calm period of about 25 hr duration that occurred about four days after spring tides on both the December 1972 and the October 1975 records, was the same on both records. This required that the phase angle be corrected by a factor of 0.78, which corresponds to a shift of about 1". The lunar work rate increased by about 3% as a result of this phase correction. Figure 6 shows the power-crossing section A-B as measured at locations J and D after the power used in lunar work and in fresh water-salt water mixing has been removed from each record. Both curves represent energy that must be dissipated by friction, curve 1 for calm conditions and curve 2 for time that includes the storm. The positions of the curves on the time axis have been shifted so that the times of spring tides coincide. (Maximum spring tides occur 130 hr after the start of both curves.) The slopes of the two curves are nearly identical for the time after the storm, which shows that the corrections to h based on the 24-hr calm periods, as described earlier, are reasonable. The agreement of the slopes also suggests that the decrease in the total energy near the ends of both curves is not due to measurement errors, but results from inadequate correction for the lateral variation of the energy flux across section A-B. During the stormy period, the first 85 hr of the record, the power crossing

FIG. 5. Left-hand graphs show the water level residual along the central axis of LIS at successive times during the December 1972 storm; wind velocities are shown on the righthand side. The dashed line shows the water level outside of the Sound.

STORM AND TIDAL ENERGY IN LONG ISLAND SOUND

IWO

3

2

-0.45-pm fraction (associated with particles) adds to the 3 pg/l delivered from the “uncontaminated” reservoirs. (Because of the possibility of atmospheric transport of metal contaminants, it is not likely that any nearby reservoir is truly uncontaminated.) Figure 2 shows that the bottom sediments of the system, sampled in 1973, are strongly impacted by the trace-metal injections from industry. Although the silver concentration is very high in the sediments close to the source of industrial contamination, this effect is strongly attenuated downstream. Indeed, as we shall see, the effect is not discernible in New Haven Harbor where other sources predominate. This implies that most of the metals are retained behind the dams and a relatively small fraction escapes to impact the estuary. About 15 km to the west of New Haven Harbor the Housatonic River with its heavily polluted tributary, the Naugatuck River, empties into the Sound. (The confluence occurs below the last dam on the Housatonic.) This river supplies a significant amount of trace metals to the adjacent part of Long Island Sound, mainly in particulate form (Turekian, 1971). This contrasts sharply with the Quinniapiac River and demonstrates that the construction of dams is certainly one important factor in inhibiting transfer of metal-polluted sediments to the estuarine zone. The Connecticut River, although the most important river draining into Long Island Sound, seems to be least important in the transport of metals

133

FIG.1. Silver in the Quinnipiac River (Connecticut) system in 1965 (previously unpublished Yale University data). All concentrations determined on unfiltered samples except where indicated. The measurements were made by emission spectrography after silver-free sodium chloride was added and the solution freeze-dried.

from human activities to the Sound. This is no doubt due in part to the extensive damming along the course of the river and in part to the minimal amount of metal fabrication along its length.

2.2. Sewer Outfalls: New Haven Harbor Applequist et al. (1972) showed that the mercury concentration in the sediments of New Haven Harbor varied in relation to distance from the several sewage-treatment plants discharging into the harbor (Fig. 3). As is the case with most of the older New England cities, storm sewers are combined with sanitary sewers and the effluent is processed through the sewage-treatment plants. During periods of high discharge associated with large storms, the treatment plant is bypassed and the unprocessed effluent

134

K. K. TUREKIAN et a / .

d BROAD BROOK RES.

FIG.2. The concentrations of silver, lead, and copper in sediments of the Quinnipiac River (Connecticut) system (previously unpublished Yale University data). Analyses made by emission spectrography. Concentrations in parts per million.

is debouched directly into the harbor, resulting in organic carbon and metal enrichment of the sediment around the outfalls. In an unpublished report from Yale (Turekian et al., 1972), it is shown that a relation exists between high concentrations of Pb, Zn, Cu, and organic matter in sediments and proximity to a sewer outfall on the eastern shore of New Haven Harbor. A more cursory survey of the sediments on the west side of the harbor showed the same thing (unpublished results). Thus the pattern established by our mercury study, we believe, can be extended to the other elements associated with sewage sludge. Figure 4 is the representation of the relationship between Zn concentration and the weight lost on ignition (a rough expression of the combination of water loss from clay minerals and the degradation of organic matter) for a traverse up the New Haven Harbor shipping channel. This cuts across the two sewer outfall regions discussed earlier (see Fig. 3), and the strong correlation of Zn and the volatile solids concentration reflects their influence.

NUCLIDES IN LONG ISLAND SOUND

135

FIG.3. The distribution of mercury (in ppm) in the tops of sediment cores raised from New Haven harbor. The dominant control on mercury concentration is proximity to sewagetreatment plant outfalls, marked by arrows. (After Applequist el a/., 1972.)

"

0

1

2

3

4

5

6

7

8

9

0

% VOLATLE SOLIDS FIG.4. The relation of zinc concentration to volatile solids (mainly organic matter with some adsorbed water in clays) in sediments from the New Haven harbor channel. Data from the U.S. Corps of Engineers files (New Haven Harbor Project: Report on Environmental Sampling and Testing, 1972).

136

K. K. TUREKIAN et a/.

2.3. Atmospheric Supply: The Record in a Salt Marsh

Salt marshes are a common feature of the Long Island Sound coast. Where they remain protected from wave erosion their surface is an index of high tide. If coastal submergence occurs over time, as has been the case in New England for at least the last 100 years, the protected marsh grows upward to maintain its surface at high tide and provides a record of previous environmental conditions. The surface of a marsh is exposed to the atmosphere most of the time. The highest point of the tidal cycle immerses the surface only about 5% of the time. Unlike marshes in other parts of the east coast, Long Island Sound marshes are not dominated by detrital sediment, but are constructed of the fibrous framework of the marsh vegetation. Because of this, burrowing by organisms does not appear to perturb the sedimentary record as occurs in the muddy sediments at the bottom of Long Island Sound (see later). As the marsh grows upward in response to the rising sea level, each layer should preserve a record reflecting the depositional environment of the time. Changes in detritus supply from streams, for example, should be recorded by sediment trapped in the fibrous framework. Similarly, the atmospheric flux records of metals delivered to the marsh surface should be maintained in the layers. McCaffrey (1977) and McCaffrey and Thomson (this volume) have shown that the 210Pbchronology from a vertical 'IoPb profile in a Connecticut salt marsh agrees with tide-gauge data (Fig. 5). This shows that during the past 100 years the sea level has been rising relative to the Connecticut coast. The agreement between the '"Pb and tide-gauge data is especially striking because the rate of coastal submergence has not been constant over the past 100 years. In addition, these researchers showed that the calculated Z'oPbflux, as determined by the standing crop of unsupported "OPb in the salt marsh, equaled the atmospheric flux as determined for New Haven by Benninger (1978). This implies that: (1) the trace-metal distribution vertically in the salt marsh reflects the changing flux over time and that no vertical migration of the trace metals is expected by diffusion or biological activity; and (2) that there should be an atmospheric flux of trace metals recorded in the growing salt marsh. Indeed, the calculated fluxes of Cu, Pb, and Zn (Fig. 6) appear to be almost solely atmospheric as the predicted fluxes of these metals are in agreement with the estimated atmospheric fluxes (Table I). The implication from both *IOPband trace-metal data is that the marsh surface, exposed above the sea surface most of the time, behaves like an atmospheric collector and can be used to monitor the changing atmospheric flux of trace metals over time.

137

NUCLIDES IN LONG ISLAND SOUND

A G E (yr) 40

0 0 -

+.

I20

80

160

200 '1

c

**

-? c E

0

10

-

20

-

-.

Smoothed Tide-Gouge Record Salt-Marsh Pb2"

Record

-

....-.

-

1

W

> 0)

d (u

b

z

0 W

m

-

r

c

4

W

0

ji/_

, ;

e4% e49

*

,

, , ; ,

,

e8

,

, ; ,

\B8

,

, , ; ,

\B8

,

I

,

,

4b0

\

i

YEAR

FIG. 5. Depth versus time curves for the Farm River (Connecticut) salt marsh based on "'Pb dating with a comparison with the tide-gauge record for New York City. (From McCaffrey and Thomson, this volume.)

3. THEDISTRIBUTION O F TRACEMETALS IN LONGISLAND S O U N D SEDIMENTS

Greig et af. (1977)have recently made a detailed study of the distribution of a number of trace metals (Sb, Cd, Co, Cr, Cu, Pb, Mn, Ni, Ag, Sc, Zn) in the top 4 cm of Long Island Sound sediments collected using a Smith-McIntyre grab sampler. The 4-cm sampling fortuitously represents, to within a centimeter, the rapidly reworked portion of the sediments as determined using 234Th(Aller and Cochran, 1976). Figures 7-9 show concentration maps for Cu, Zn, and Pb constructed from the data of Greig et al. (1977). The primary control on the trace-metal concentrations is the grain size of the sediment. This can be seen by comparing the trace-metal maps with a grain-size distribution map (Fig. 10) for the Sound. The sand-rich sediments have the lowest trace-metal content. There is, however, an im-

138

K . K.TUREKIAN et

0

1972

4

8

12

4

al.

12

8

0

4

8

I

1912

a 1892

a

Zn

1852

1792 FIG. 6. The “excess” flux of copper, zinc, and lead as a function of time at the Farm River (Connecticut) salt marsh. (From McCaffrey and Thomson, this volume.)

portant second-order effect related to the coastal sources of trace metals. Sediments adjacent to Throgs Neck, the Housatonic River (to the west), and New Haven Harbor are higher in trace metals than other sediments of the same grain size. These three areas are heavily impacted either by sewer outfalls or direct injection of industrial sewage along a contiguous channel (as in the Naugatuck-Housatonic system). A number of cores collected from central Long Island Sound have been analyzed for trace metals as a function of sediment depth (Thomson et al., 1975; Turekian, 1979; Benninger et al., 1979). They show roughly the same patterns for Cu, Zn, and Pb (Fig. 11): a roughly exponential decrease in concentration with depth. At greater depths there are occasional peaks of high concentrations. TABLEI. CALCULATED EXCESS METALFLUXTO THE SURFACE OF THE FARMRIVERSALT MARSHCOMPARED TO MEASURED ATMOSPHERICDEPOSITION RATESAT SELECTED SITES

Site and date of collection Branford, Connecticut, salt marsh (1972) New York City (1969-1970) Nantucket, Massachusetts (1966- 1967)

Cu 8

f

9.8 5.6

Zn

2

12 f 3 32 7.6

Pb

Source

7 +- 4 McCaffrey (1977); and McCaffrey and Thomson (this volume) 35 Volchok and Bogen (1971) 8.5 Lazrus et a/. (1970)

NUCLIDES IN LONG ISLAND SOUND

139

COPPER pglgrn DRY SEDlMM

FIG. 7. Map of copper concentrations in surface sediments of Long Island Sound. (Constructed from the data of Greig et al., 1977.)

ZINC pglgm DRY SEDIMENT

FIG.8. Map of zinc concentrations in surface sediments of Long Island Sound. (Constructed from the data of Greig et al., 1977.) 1

LEAD pg/gm DRY SEDIMENT

m

'Do

wm

0-m

n

> o (Aller and Cochran, 1976; Benninger et al., 1979; Aller et al., 1980). DB can also be considered approximately constant in the top 5 cm (Aller and Cochran, 1976), allowing simplification of Eq. (6.8) in this case to

ac,/at

(6.9)

cs

=

DB(d’c^Jdx’)

+R

where is the metal concentratiodvolume total bulk sediment. Because of physical disturbance, the solid-phase Mn and Fe profiles at FOAM are irregular and cannot be effectively treated quantitatively. The distributions at NWC and DEEP are sufficiently regular to be modeled and those at NWC are generally repeatable enough that an assumption of steady state for Eq. (6.9) seems reasonable (Figs. 8 and 9). Steadystate solid-phase distributions are assumed for DEEP as well. It is not possible to determine an absolute reaction rate from any of these profiles because the percentage of the total metal that is diagenetically mobile at any given depth is unknown. A relative net reaction rate for the upper region compared to deeper sediment can be calculated by subtracting an assumed background concentration value from the concentration in the zone to be modeled. This means that only a profile of “excess” Mn or Fe is utilized to estimate reaction. The net reaction rate will therefore represent an average dissolution rate (R negative) of excess Mn or Fe in the modeled zone. This will be a good estimate of the actual dissolution rate if most reprecipitation takes place at or outside the boundaries of the modeled regions. No estimate of Fe dissolution can be made at DEEP because total Fe decreases toward the interface. Relative Mn loss rates for the upper 4 cm at NWC are determined as follows. The Mn concentration (pg/gm) between 3 and 4 cm was sub-

386

ROBERT C. ALLER

tracted from each of the overlying sample intervals. These “excess” concentrations were converted into units of mass/total volume by assuming a sediment-particle density of 2.5 gm/cm3and utilizing the measured water contents (Appendix B, Part I). Excess concentrations were plotted and found to be adequately fit by simple exponential functions; A t . , = ACso exp( -ax) (Fig. 18; Table V). These functions were then substituted into Eq. (6.9) and an estimate of R was obtained using D B = 0.43 x cm2/ sec (Aller et al., 1980). This D e is the average, not instantaneous, value for the upper 5 cm at NWC during summer; winter-spring values at NWC or summer values at nearby stations do not differ significantly from this D B(Aller and Cochran, 1976; Aller et al., 1980). Resulting estimates of dissolution rate functions of the form R = R, exp( -ax) in the upper 4 cm at NWC are listed in Table V. The lowest rate is found for NWC-1, whereas the estimates from the three other cores are in fairly good agreement. It is possible that a small amount of erosion (e.g., 0.5 cm) could have occurred at NWC prior to collection of NWC-1 and thereby caused a lower estimate (see, for example, Benninger et al., 1979). The Mn profile at DEEP was treated in the same way, except that only the top 3 cm was considered. DB from 234Thexcess activity profiles (3 cm) is -0.15 x cm2/secat this station (Aller et al., 1980). Resulting exponential functions for excess Mn (At.,) and reaction rates from Eq. (6.9) are listed in Table V. It is possible to estimate a turnover time T for excess Mn in the top few centimeters L at each station from the excess Mn profiles Ats and the estimated reaction rates R. T is given by (6.10)

T

=

( L j k A t s dx)/(Jo“R dx)

The calculated values of T are listed in Table V. Except for the high estimate from NWC-1 these show that the excess Mn present in the top few centimeters of sediment is completely dissolved on the order of every 60-100 days. This is a more rapid estimated turnover than distributions

FIG. 18. Excess Mn and Fe profiles for cores NWC-2 and NWC-4. The curves plotted are those used to calculate solid-phase dissolution rates. Note that the vertical axis for excess Fe is offset to a depth of 1 cm.

TABLEv. Mn AND Fe EXCESSDISTRIBUTIONS AND REACTION RATES

Core NWC-I NWC-2 NWC-3 NWC-4

DEEP-I

Core NWC-2 NWC-3 NWC-4

Model interval (cm) 0-4 0-4 0-4 0-4 0-3

Excess Mn (~dcm’) 11I exp( - .077x) 477 exp( - 1 . 2 ~ ) 366 exp( - 1 . 0 8 ~ ) 501 exp( - I .35x) 162 exp( - 1 . 5 ~ )

Model interval (cm) 1-4 2-5 1-4

Mn dissolution rate (~glcm’lday) 2.4 exp( - 0 . 7 7 ~ ) 26 exp( - 1 . 2 ~ ) 16 exp( - I .08x) 34 exp( - I .35x) 4.7 exp(- 1 . 5 ~ )

Excess Fe (mg/crn3) 2.6 4.3 2.2

+ 0 . 4 7 ( ~- 1) - 0.61(~+ 0.40(~- 2) - 0 . 8 5 ( ~- 2)2 + 0.41(~- I) - 0.18(~- 1)’

Turnover time T (days) 185

13 92 59 103

Fe dissolution rate (pglcrn3/day1 45 63 13

First-order dissolution rate const. k (/day)

Production-supported flux estimate JMc. (mmoles/mz/day)

0.022 0.054 0.043 0.067 0.029

0.57 3.9 2.7 4.6 0.57

Turnover time (days) 104 I16 232

T

Production-supported flux estimate J F e (mmoles/m2/day) 24 34 7

388

ROBERT C. ALLER

from other environments predict (e.g., Robbins and Callender, 1975), due in part to the availability of biogenic mixing coefficients (D.) from 234Th activity profiles in this study. Previous calculations have assumed average long-term sedimentation rates as the major transport agent of particles within the sediment column. The present estimates are in agreement with laboratory experiments, which showed that Mn profiles similar to those found in the field could form in -45 days (Aller, 1978). In some studies it has been assumed a priori that Mn2+production rates or, equivalently, oxide dissolution rates, can be represented as a firstorder reaction where the rate of solid-phase dissolution is proportional through a constant k to the quantity of excess or sometimes total oxide present, that is, R = - k ( A e , ) (Holdren et al., 1975; Elderfield, 1976). Substitution of this function into Eq. (6.9) shows that this is the same as assuming an exponentially decreasing dissolution rate having an attenuation coefficient = (k/D,)”’. The attenuation coefficients a found in this study by direct substitution of solid-phase distribution data into Eq. (6.9) can be equated to (k/D.)”’to obtain values of k for comparison with other work. The values of k = a2DBare listed in Table V and range from 2.6-7.8 x lO-’/sec or 8.2-25/yr. These are about 103-104times greater than model values calculated for other near-shore sediments (Holdren et al., 1975). One internal check on whether the Mn2+production rates are reasonable can be made by calculating the maximum sediment-water flux of Mn2+ that could be supported by these rates. At steady state, this flux JMnis simply equal to the average production rate over the modeled interval L, which from Table V and Eq. (6.9) is (6.11)

JMn= (Ro/a)[l- exp(-aL)]

This requires that at steady state, what Mn2+leaves the sediment can be no greater than that produced. Because of precipitation of oxides at the interface, the flux out of the sediment into overlying water can, of course, be less than predicted by (6.11). The maximum steady-state sediment-water fluxes predicted by (6.11) and the estimated production rates are listed in Table V under JMn.Estimates range from 0.6 mmoles/m2/dayat DEEP to 5 mmoles/m’/day at NWC and are in excellent agreement with the magnitudes of actual measured fluxes (Table I). This suggests that: (1) the production rate estimates made from the solid phase are reasonable, and (2) that a large fraction of the Mn’’ produced in these sediments near the interface escapes into the overlying water and is reprecipitated there rather than in the sediment deposit. The agreement may be more apparent than real because only minimum production rates can be estimated from the excess metal profiles. Additional checks on the calculated rates come from the pore-water distributions as developed later.

DIAGENETIC PROCESSES. 11.

389

Estimates of Fe2+ production at NWC can also be made in the same way as done for Mn”. Only cores NWC-2, 3, and 4 have solid-phase Fe profiles that lend themselves to modeling of this type (Fig. 8). Slightly different depth intervals were used for calculation of excess Fe for each core because of small differences in the forms of the profiles. In all cases the top 0-1 cm was considered an obvious region of net precipitation and not used. The modeled depth intervals are 1-4 cm (NWC-2, NWC-4) and 2-5 cm (NWC-3). The Fe content of the bottom-most sample in each selected interval was subtracted from overlying samples to obtain measThe bottom sample selected as a base concenures of excess Fe, tration is either a minimum value in the respective Fe profile or representative of a relatively constant Fe concentration below that depth. Excess concentrations were plotted and found to be fit relatively well as a function of depth by a second-order polynomial (Fig. 18, Table V). Insertion of these functions into Eq. (6.9) results in an estimate of a constant dissolution rate for Fe in each modeled interval. These range over 0.013-0.063 mg/cm3/dayand are best considered order of magnitude rates because of the subtraction of the large background value from the profile in each case. Values of the turnover time 7 for excess Fe in each interval were calculated using Eq. (6.11) and vary from -100-200 days, somewhat longer but similar to those found for excess Mn. The maximum expected steady-state flux of Fe2+from NWC sediments into overlying water is approximately J F e = RL, where L is the thickness of the modeled interval and R is the constant Fe dissolution rate in that interval. Calculated fluxes range from 7-34 mmoles/m2/dayand are considerably higher (10-100 x ) than directly estimated fluxes from incubated box cores (Tables I and V). This is consistent with the rapid oxidation kinetics of Fe” in seawater (Aston and Chester, 1973; Kester et al., 1975) and the expected precipitation of most mobilized Fez+ from solution as the oxygenated interface is approached. One major conclusion from these calculations is that there is at least as much Fe mobilized as Mn and probably as much as 10 times more. This redistributed Fe is much harder to analytically detect above its lithologic background.

AeS.

6.4.2. Mnz+ and Fez+ Reaction Rates from Pore- Water Distributions. The dissolved Mn2+ and Fez+ profiles can also be modeled by transport-reaction equations equivalent to Eq. (6.9) or (6.10) (Anikouchine, 1967; Spencer and Brewer, 1971; Michard, 1971; Robbins and Callender, 1975; Holdren et al., 1975). This allows independent checks on the form and magnitude of MnZ+and Fe2+ production rates estimated from the solid-phase distributions. In addition, modeling allows estimation

390

ROBERT C. ALLER

of precipitation rates and evaluation of the effects of biogenic irritation on pore-water profiles. It was shown in Part I that it is possible to describe solute profiles in the bioturbated zone by defining an average sediment microenvironment and determining solute distributions within it. Because the sediment is composed of many such microenvironments, an average vertical solute profile found in the sediment must correspond to that in a single microenvironment. The average microenvironment is assumed to be a finite hollow cylinder or annulus of sediment oriented vertically in the deposit. This portion of sediment essentially corresponds to that associated with a single ideal infaunal animal. The inner radius of the cylinder r , is determined by the average size of macrofauna inhabiting the sediment and irrigating burrows, and the outer radius of the annulus r2 is determined by the effective abundance of individuals. In the simplest case where animals are immobile tube dwellers and are evenly distributed, r , is the average tube radius and r z is simply half the distance between individuals as measured from the tube axis (Aller, 1977, 1978, 1980, Part I). The vertical length of the cylinder microenvironment L corresponds to the effective thickness of the bioturbated zone. The transport-reaction equation describing pore-water distributions in the imaginary cylinder is (cylindrical coordinates) (6.12)

'a = D-a(r$) at

r ar

ac

+ D y + R

ax

where all symbols are as before and r is the radial dimension measured from the cylinder axis. Porosity is assumed to be approximately constant with respect to pore-water volume and is not explicitly written into the equation. Advection is also ignored because pore-water distributions in the top -20 cm over short time periods only are considered here (e.g., Lerman, 1975). Adsorption is not included because only the steady-state case will be considered (Berner, 1976). D is the molecular diffusion coefficient in the bulk sediment and is assumed both isotropic and constant [equivalent to D , of Berner (19801. If there are no or very few irrigated burrows present in the sediment, lateral diffusion is not significant and the r dependence of Eq. (6.12) can be ignored. In that case, the equation becomes the more traditional onedimensional transport-reaction equation used to model pore-water solute profiles where advection is relatively unimportant (Berner, 1971; 1980; Lerman, 1979). Both the cylindrical microenvironment model and the onedimensional Cartesian coordinate model will be used here to quantify the Mn2' distributions at NWC and DEEP.

DIAGENETIC PROCESSES. 11.

39 1

The reaction term R in Eq. (6.12) is determined as follows. Mn2+ is produced by the dissolution of solid-phase Mn oxide and is subject to reprecipitation as either an oxide or reduced phase. Because oxide reduction begins very close to the sediment-water interface, I assume that little reprecipitation as an oxide actually takes place within the deposit or that reprecipitation takes place so near to the interface that it cannot be differentiated from a boundary condition. Therefore, the Mn2+distribution can be considered as influenced dominantely by production and anoxic precipitation reactions over most of the sampled interval. The production term was shown in the previous section to be of the form R = R o exp( -ax) where R o and a are constants and x is the depth in the deposit. Precipitation reactions are commonly assumed to follow firstorder or pseudo-first-order kinetics such that R = k l ( C - Ceq)where k , is a first-order rate constant and C,, represents a depth-dependent equilibrium concentration (Holdren et al., 1975;Robbins and Callender, 1975). In LIS sediments the concentrations of many anions such as HCO; ,which might precipitate with Mn”, are roughly constant over the top -20 cm of sediment. This is true in particular at NWC and DEEP (Part I). It will therefore be assumed that C , , is constant over the depth interval of interest and that its value is the concentration to which a profile asymptotes at depth. Taken together these considerations suggest that an appropriate reaction term for Mn2+in the present case is R

=

Roexp(-aLu)

- kl(C -

Ceq)

Equation (6.12) will be solved only for the steady-state case (dClat = 0) because the dimensions of the effective cylinder microenvironment are such that steady-state solute distributions are quickly achieved (Part 1). The boundary conditions on the cylinder are taken as (6.13a)

x=O,

r=rl,

(6.13b)

r =

(6.13~)

x = L,

r2,

C=CT

aClar = 0 aclax

=

o

Condition (6.13a) specifies that the solute concentrations along the sediment-water interface and within the ideal burrow are equal and constant. The second condition, (6.13b), requires that concentrations go through a maximum or minimum halfway between individual microenvironments. Condition (6.13~)matches the bioturbated zone with the underlying unburrowed zone by requiring a continuity of flux between the two regions. In this case, the gradient is taken as -0 because of the requirement that

392

ROBERT C. ALLER

C asymptote to a constant C,, and because the gradients observed in cores are approximately zero at the base of the zone of interest. The solution to Eq. (6.12) with conditions (6.13a)-(6.13c) and R = R o exp( - a)- k , (C - C,,) is (by separation of variables)

with

The functions ZJz) and K,(z) are the modified Bessel functions of the first and second kind, respectively, of order v (see e.g., Abramowitz and Stegun, 1964, for values). The vertical concentration gradient measured in the sediment pore waters corresponds to the average concentration in the effective microenvironment over the finite depth interval x , - x 2 and is given by (6.15)

=

( Z . ~ i : . . . . ) / ( l . ~ ~ ~ r ~ r ~ ~ )

Values for r l , r2, and L in the present case are taken as those found by modeling NH; distributions in the same cores analyzed for Mn2+(Part I). It is assumed that NH; is not subject to precipitation reactions and can be used to obtain the effective geometry of diffusion. Once obtained, by using a relatively conservative element this geometry can be used to model Mn” distributions and estimates of reaction rates can be made. To illustrate the behavior of the cylinder model and also to demonstrate how irrigated burrows can be expected to influence Mn2+profiles, a representative vertical profile predicted by Eqs. (6.14) and (6.15) for Mn” has been plotted for the case k , = 0 (Fig. 19). The production rate for Mn”, r l , r 2 , and L are those for core NWC-4 based on the solid-phase dissolution rate of Section 6.4.1 (Table V; divided by average porosity ~0.750)and the cylinder-model values of Table V in Part I. The value of D is estimated from the molecular diffusion coefficient at infinite dilution T = 19°C (Li and Gregory, 1974), multiplied by a correction factor for sediment structure of 0.56. This factor was approximated by cp2 with an average porosity of cp = 0.75 (Manheim, 1970; Manheim and Waterman, 1974; Krom and Berner, 1980). For comparison, the one-dimensional

DIAGENETIC PROCESSES. 11.

393

0

2 4

c

,k'

10

t

Q

l2

n

14

a,

16

no p r e c i p i t o t l o n

18

FIG.19. Comparison of the one- and two-dimensional models for Mn2' distribution in the top 0-18 cm of sediment at NWC. The production rate in both cases is that found for core NWC-4. The anoxic precipitation rate is assumed to be zero. The effective cylinder geometry used in the two-dimensional model is that determined for NH; in Part I; r l = 0.14 cm, r 2 = 4.5 cm. The basal gradient is constrained to be zero. The diffusion geometry created by irrigated burrows results in a vertical pore-water solute profile exhibiting apparent precipitation.

case (rl + 0; r2 --* m) is plotted for the same upper and lower boundary conditions. The cylinder model demonstrates that the presence of irrigated burrows can produce a decrease of Mn2+concentration with depth that would commonly be interpreted as evidence for precipitation when, in fact, no such precipitation is occurring. In order to estimate precipitation rate constants k l for each core, the production rates determined in Section 6.4.1 from solid-phase distributions and the model cylinder geometries determined from NH: distributions were first used to fix values of R0,ct, r l , rz, and L. Estimates of D in each case were made by modifying the infinite dilution value at the appropriate core collection temperature (Li and Gregory, 1974) by the cpz correction factor 0.56 as outlined previously. The average porosity at both NWC and DEEP is -0.75 in the modeled zone, assuming a sediment particle density of 2.5 gm/cm3. CTis -0.2 pit4 based on flux box cores (Appendix C, Part I). C,, was estimated from the asymptotic concentration in each core and is -25-27 pA4 at NWC and -125 at DEEP. These values and the corresponding values of k , determined by profile fitting for each core are listed in Table VI. Plots of model profiles are given in Figs. 20 and 21. Summer and fall profiles at NWC are fit quite well by the model. The winter profile is not; this may reflect an increase in Mn" oxidation rate

TABLEVI. PORE-WATER Mn2+ MODELVALUES'

Cylinder Model

Sample

TCC)

Inner radius r , (cm)

NWC-2 NWC-3 (dash line) (dot line) NWC-4 DEEP- 1

13.2" 3"

0.14 0.14

19" 18.5"

0.14 0.2

Outer radius r 2 (cm) 5.9 14 4.5 4

D RO a L (cm2/day) (pmoles/cm3/day) (/cm) (cm) 0.244 0.169 0.287 0.283

0.631 0.389 0.825 0.114

1.2 1.08 1.35 1.5

18 16 18 16

Anoxic precipitation Estimated flux CT C,, const. k , acrossx = 0 (pM) (pM) (/day) (mmoles/mz/day) 0.2 0.2 0.2 0.2

One dimension

Sample N WC-2 NWC-3 NWC-4 DEEP-I a

See Section 6.4.2.

Anoxic precipitation Estimated flux across x = 0 const. k l (/day) (mmoles/m*/day) 0.95 0.85 0.67 1

2.1 1.3 3.O 1

25

0.57

1.7

27 50

0.6 0.7 0.45

0.99 0.82 2.4 1.2

25 125

0.5

395

DIAGENETIC PROCESSES. 11.

MnZ+ b M 1

MnZ' (uM) 100

18

200

100

300

200

300

h!

FIG.20. Pore-water Mn" profiles predicted by the two-dimensional cylinder model (dash or dot bars) and one-dimensional model (continuous profile line) compared with the actual measured profile in cores NWC-2 and NWC-3 (solid bars). Model values are given in Table VI.

near the interface or is due to an overestimate of Mn2+ production rate because 234Thmixing rates can integrate over a several-month period. The profile at DEEP is relatively insensitive to most model values and largely reflects the choice of Ceq.The subsurface maximum in Mn2+not predicted by the production-rate function in this case demonstrates that production of Mn2+ is underestimated at depth. All profiles predict values of k , O.5-0.7/day or 3.5-49 x 10-4/min. Except for the winter core at NWC (NWC-3), the sediment-water Mn" fluxes predicted by the model pro-

-

Mnz+(uM) 100

200

300

Mn"(uM1 400 I

6

12 14 16

FIG.21, Pore-water Mn2' profiles predicted by the two-dimensional cylinder model (dash or dot bars) and one-dimensional model (continuous profile line) compared with the measured profile in cores NWC-4 and DEEP-1 (solid bars). Model values are given in Table VI.

396

ROBERT C. ALLER

files are in the range, -1-2 mmoles/m2/day, observed by direct measurement (Table 11). The magnitudes of k , determined from these model fits are lo2-lo3 times the highest values yet reported (Robbins and Callender, 1975; Elderfield, 1976). This is due predominantly to the higher estimates of R o used here than in previous work. As stated previously, the general agreement of the direct flux measurements with the estimated range of production rates indicates that the present model values are at least internally consistent with all available data. In order to determine how estimates of k l would change if the effect of irrigated burrows on diffusion geometry were ignored, Eq. (6.12) was also solved at steady state for the one-dimensional case, that is

-=

(6.16)

at

o = Da2c T+ ROe--M- kl(c- ceq) ax

The solution to Eq. (6.16) with the boundary conditions (6.13a)-(6.13~) is (6.17) C(x) = A cosh[A(x - L ) ] + Ceq - [Rd(azD- kdle-" with A =

(kl

lD)1'2j A = [(C,- Ceq

+ Ro)/(aZD- k~)]/cosh(-AL)

The resulting model profiles are plotted as the continuous curves in Figs. 20 and 21. All appropriate model parameters (CT, Ceq,Ro, a,L, and D ) are identical to those used for the cylinder calculations. Values of k I were varied to obtain the plotted curves. Estimates of k , ranged from 0.7-11 day and are generally higher than, but within a factor of, 1.5-2 of those determined by the cylinder model. The estimate of DEEP is relatively insensitive and k , ' s in the range 0.5-l/day give reasonable fits. Sediment-water fluxes estimated by the one-dimensional model are also in the range observed (Table VI). The use of the one-dimensional model allows estimates of reaction rates to within a factor of 1.5 of those predicted by the more complex twodimensional model. This close agreement is due largely to the rapid attenuation of MnZ+production rates with depth and suggests that in such cases the distribution of a nonconservative pore-water constituent can be modeled reasonably accurately by use of the transport-reaction equation in one dimension. With this in mind, a one-dimensional transport-reaction model will be used to obtain corresponding reaction rate constants for Fez+.These are very approximate because of the large uncertainty in Fez+ production rates, oxidation rates, and variation in sulfide production over the zone

-

DIAGENETIC PROCESSES. 11.

397

of interest (Goldhaber et al., 1977; Aller and Yingst, 1980). It is imagined in this case that the sediment column can be divided into three vertical zones of fixed thickness. In the upper zone near the sediment-water interface, Fe2+distribution is controlled by diffusive loss and precipitation as an oxide. The rate of oxidation depends on the 0,concentration and pH, which change rapidly as the sediment-water interface is approached (Stumm and Lee, 1961; Kester et a/., 1975). It will be assumed that an average rate constant can be applied in this zone to characterize precipitation. In zone 2, Fe2+is produced at an approximately constant rate as calculated in Section 6.4.1 and is also subject to precipitation as a sulfide (Part I). Fe-sulfide precipitation kinetics are complex (Rickard, 1974), but it will be assumed here that at constant pH and approximately constant pS the kinetics can be described as first order in Fe2+with a decreasing rate as an equilibrium value C,, is approached. Zone 3 is taken as dominated by the same net precipitation of Fe2+ as in Zone 2, but without further production. The equations describing these distributions are at steady state: a2C zonel, O = D ? - k z C , 05xSL1 ax a2C (6.18b) zone 2, 0 = D 7 + R o - k3(C - Ceq), L I 5 x 5 L Z ax a2C L2 5 x 5 w (6.18~) zone 3, 0 = D 7 - ka(C - Ceq), ax (6.18a)

where k 2 and k 3 are the oxic and anoxic reaction constants, respectively, R o is a constant FeZ+production rate, C,, is an equilibrium Fe2+ concentration presumably determined by sulfide equilibria, and all other symbols are as before. The boundary conditions are (a)

x = 0,

(b)

x = LIP

(el

x = L2,

(0

x--,

MI

c

= CT

Czone 1 = Czone 2

(ac1ax)zone 2

=

(ac/dx)zone 3

C + Ceq

These require continuity of flux and concentration across zone boundaries as well as a constant boundary value at the surface and an asymptotic

398

ROBERT C. ALLER

concentration at depth. The solutions are (6.20)

zone 1, zone 2, zone 3,

C C C

+

= A 1 sinh(ulx) CTcosh(ulx) = A 2 sinh(u2x) B2 cosh(u2x) = A 3 expf - w ) C,,

+ +

+ Rdk, + C,,

where (TI

= (kz/D)'",

0 2

= (k3/D)1'2

F; = uIcosh(a2L cosh(ulLl) - u2sinh(o2L sinh(o1L A2 =

(Ce,

+ R/k3)u1cosh(all, I) - [R0/[k3exp(u2L2)1- UICT exp(cr2LI)[ulcosh(ulLl) + uz sinh(uILI)l

B 2 = -A2 - Ro/[k3exp(u2L2)l

A1 =

- A I exp(u2LI ) + [ 1 - cosh(u& I)lexp(u2L2)lRdk3 sinh(utLI) CT cosh(alli) + Ceq -sinh(ulL I)

A 3 = -A2

+ [exp(uzL2)- cosh(u~~~)lRdk3

The constants used in the model are as follows. The D terms were estimated in the same way as done for Mn by correcting the infinite dilution values of Li and Gregory (1974) to the core collection temperature and multiplying the corrected D in each case by the factor 0.56 corresponding to 'p2 (Lerman, 1978). Ro, L , , and L z were determined from solid-phase distributions as discussed in Section 6.4.1. A porosity of 0.75 was assumed in correcting R o to a rate relative to pore-water volume. k 2 does not significantly alter the shape of the model profiles because zone 1 is relatively thin. A value of Uday was used in all cases. This is 1/10 the magnitude of the water column precipitation constant kFe, estimated in the flux experiments (Section 5.4) and found to result in sediment-water fluxes in the range of those given in Table 1. CT was fixed at 0.02 p M [based on flux-core blank samples (Appendix C, Part I)] and C,, at 1.1 F M ,corresponding to the observed asymptotic concentration at depth in cores NWC-2, NWC-3, and NWCd. With all other variables fixed, k , was then varied to produce as close a fit as possible to the observed porewater profiles. Fez' profiles predicted by the model are plotted in Fig. 22. Corresponding model values are listed in Table VII. Estimates of the anoxic precipitation rate constant k , range from about 10-120/day and are the

399

DIAGENETIC PROCESSES. 11.

Fe2'bM) 10

20

Fez' (JIM)

Fe2+(NM) 10

30

20

2

10

30

20

30

40

2 -

4

-E

-

6

8

u 10 c c

12

0"

14

a

16

8 1 10

NWC-3 10

--

16

18

FIG.22. Pore-water Fez+ vertical'concentration profiles (continuous line) predicted from the three-zone, one-dimensional model compared with the measured profile (solid bars). Model values are listed in Table VII.

same magnitude as those reported for Fe" oxidation in well-aerated seawater (Kester et al., 1975; Murray and Gill, 1978). These k , values are 10-100 times the anoxic precipitation rate constants determined for Mn, a fact consistent with the higher production rates calculated for Fez+. The use of the square wave production rate distribution is an obvious approximation and a Gaussian distribution for the production rate, such as used for Mn" by Robbins and Callender (1975), would be more realistic. A smoothly increasing, then decreasing, Fe" production-rate distribution would result in better fits to the actual data at the boundaries of zone 2 and lower estimates of k,. k , would also be slightly smaller if the cylindrical coordinate model were used as previously illustrated for Mn2+.However, because of the very approximate nature of the production rates and the simplifications made concerning the kinetics of oxic and anoxic precipitation, more sophisticated modeling is not warranted. In summary, the pore-water Fez+ and Mn2+profiles can be used together with solid-phase distributions, measurements of sediment-water exchange, and knowledge of the biogenic diffusion geometry as obtained from modeling relatively conservative solutes to place constraints on reaction rates. If anoxic precipitation kinetics are assumed to be first order, then rate constants are calculated for both Mn2+and Fez+losses that are larger than any others reported in the literature of which I am aware. Both the shapes of the pore-water profiles, particularly at NWC, as well as the apparent rapid reaction rates indicate that most anoxic precipitation of Fez+and MnZ+takes place in the zone of solid-phase excess (e.g., Figs. 2-7). Precipitation rates and production rates could therefore easily be minima.

TABLEVII. PORE-WATER Fe" MODELVALUES"

Sample

T ("C)

D (cm2/day)

LI (cm)

L2 (cm)

NWC-2 NWC-3 NWC-4

13.2" 3" 19"

0.256 0.184

1 2 1

4 5 4

a

See Section 6.4.2.

0.297

R (mmoles/cm3/day) 1.1 1.5

0.31

CT

C., (pM)

0.02 0.02 0.02

1.1 1.1 1.1

Oxic precipitation k2 (/day) 1 1

1

Anoxic precipitation k , (/day)

Estimated flux acrossx = 0 (mmoles/m2/day)

1 20

0.01

80

0.001 0.06

9.5

40 1

DIAGENETIC PROCESSES. 11.

6.5. Flux of Mn2+and Fez+ into Overlying Water

The flux of Mn” into the overlying water varies seasonally in a generally regular fashion and is a strong function of temperature (Fig. 23, Table I). An exponential function of the Arrhenius type used in Part I to describe the temperature dependence of NH: fluxes can also be used in this case to describe the temperature dependence of Mn2+ fluxes. If J = J’ exp( -E/gT), where J is the flux, J’ the constant preexponential factor, T the absolute temperature, E the apparent activation energy, and g the gas constant, then the apparent activation energies and preexponential factors are FOAM:

E = 18.2 kcallmole;

J’

NWC:

E = 45.9 kcaYmole;

J’ = exp(80.6)

DEEP

E = 40.7 kcal/more;

J’

= exp(30.4)

= exp(69.7)

The average yearly Mn” fluxes calculated from these activation energies and assuming T = 285 - 10 cos(2vt) [t is time (yr)], are 0.23, 2.6, and 0.36 mmoles/m2/day at FOAM, NWC, and DEEP, respectively. Values of E calculated from NH$ fluxes were shown to agree well with those found for the temperature dependence of microbial metabolic activity of -19 kcal/mole (Part I). In contrast, the apparent activation energies of Mn2+ fluxes can be much higher even though Mn2+production should also be controlled directly or indirectly by metabolic rates. One of the likely reasons for this difference is that Mn2’ release from the sediment is influenced by abiogenic precipitation reactions in addition to production. The changing relative importance of oxic or anoxic consumption reactions and Mn” production rates from season to season can be expected to produce a temperature dependence of sediment-water

T (“C)

FIG.23. Directly measured Mn2+flux out of bottom sediment at each station as a function of temperature (season).

402

ROBERT C. ALLER

Mn2+ flux different from that controlled by production alone. For example, during winter the surface sediment is more highly oxygenated than at other times of year because of lowered microbial activity. As a result, Mn2+oxidation rates near the interface should be relatively high at that time compared to production rates. This change in the relative rates of consumption-production reactions would result in a greater decrease of the flux from the sediment than could be explained by the temperature dependence of production. Consumption of Mn" near the sediment-water interface can be demonstrated in part by discrepancies between flux predictions made using Fick's first law and actual measured fluxes. Minimum flux estimates expected on the basis of concentration gradients alone can be made by assigning the MnZ+concentration in the top 0-1 cm of each core to a depth of 0.5 cm and calculating a linear concentration gradient between that depth and overlying water (x = 0, Mn2+ 0.2 pM).This gradient is a minimum, assuming no precipitation reactions, because in some cores the maximum Mn2+concentration must occur closer to the interface than 0.5 cm (e.g., all summer cores), whereas in others, not accounting for the finite sampling interval minimizes the gradient. Diffusion coefficients were estimated as before by correcting the infinite dilution values of Li and Gregory (1974) to the flux-core temperatures giving 6.38, 5.32, and 3.65 x cm2/secfor 22", 15", and 4"C, respectively. These were then multiplied by the factor cp2 to approximately correct for sediment structure (Lerman, 1978; Krom and Berner, 1980); cp2 = 0.44 at FOAM and 0.56 at NWC and DEEP as approximated from average water contents (see Part I, Section 6.9). Ficks first law gives the diffusive flux in sediments as (Berner, 1980)

-

(6.21)

J,=o = -cpI)(dC/dx),=o

The minimum fluxes calculated from (6.21), assuming cp = 1 at x = 0, and values of I) and (dcldx) determined as described above are listed in Table I along with the directly measured fluxes. At FOAM the minimum flux predicted on the basis of concentration gradient is always larger than the measured flux. Similar overestimates occur for winter cores at both NWC and DEEP and the fall core at DEEP. These discrepancies are consistent with a partial loss of Mn2+from solution in the top centimeter or at the sediment-water interface in each case. It should be noted that although the calculated flux estimates are approximate and either overestimate or underestimate the flux for the reasonsjust discussed, they do predict the measured flux to within a factor of 2-6 in all cases.

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Because of physical disturbance the solid-phase Mn profile at FOAM is irregular and a production rate could not be estimated from it (Section 6.4.1). An order of magnitude production rate can be inferred from the flux data as follows. There is overall agreement in the magnitude of estimated production rate, measured fluxes, and estimated fluxes at NWC and DEEP. Mn” flux at FOAM is lower, but similar in range to NWC and DEEP. In addition, measured fluxes at FOAM are within a factor of 3 of those predicted from pore-water gradients indicating that precipitation losses are not unusually high compared to the other stations. Taken together these facts indicate that Mn2+ production rates at FOAM are in the same range as at NWC and DEEP. In addition to seasonal differences in flux due to temperature-controlled reactions and transport, the magnitude of the flux changes with depositional environment. Fluxes in this case are highest at NWC and lowest at FOAM. Subsequent measurement of Mn2+fluxes made during summer 1977 at a variety of stations in LIS has shown that the magnitude is directly correlated with the abundance of organic matter or fine-grained sediment in the top few centimeters (Aller, 1979). The Mn2+flux can also be increased by biogenic reworking as demonstrated in laboratory experiments (Aller, 1978). This increase presumably comes about because of sediment irrigation as well as the continual transport of oxide-rich particles into otherwise reduced regions of a deposit. The presence or absence of deposit-feeding organisms is also correlated with grain size and depositional environment (e.g., Sanders, 1956). Mobile deposit feeders such as protobranch bivalves are most abundant at NWC and may be partly responsible for relatively high Mn” release rates there compared with other stations. Because of the rapid attenuation of production rates away from the interface, the construction of burrows per se does not greatly influence the Mn2+flux in a steady-state system. At DEEP, the cylindrical model predicts that as much as -40% of the flux could come from burrows, whereas at NWC only 3-7% has a radial source. The use of average concentrations within sampling intervals to calculate gradients will lower the effective radial component still further. This relatively small effect, given measurement uncertainties, illustrates why the flux estimates made previously using the molecular diffusion coefficients are reasonably close to that actually observed. The contribution of radial diffusion to the flux could change if reaction rates are different around burrows than in surrounding sediment (Aller and Yingst, 1978) or if the rate is not rapidly attenuated with depth. The magnitude of the Mn2+fluxes from different areas of LIS are in general agreement, although usually somewhat higher than those reported

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from other near-shore regions. Graham et al. (1976) report Mn2+fluxes from Narragansett Bay, Rhode Island also taken in June-July, 1975. They obtained an average for this time of about 0.4 mmoles/m2/day, which is lower than that measured at any LIS station. No information was given on the depositional environment so direct comparison with stations in the present study is not possible; nevertheless, the values obtained are not dissimilar to those found at FOAM. Benthic Mn2' fluxes reported from the Chesapeake Bay range from 0.3-7 mmoles/m2/dayin summer (July, 1977) to 0.01-0.02 mmoles/m2/day in spring (April, 1978) (Eaton, 1979). The high value of 7 mmoles/m/day was taken in a region of anoxic bottom water and is not directly comparable to measures made in LIS. The lower values are similar in range to those found in this study, but again no description of environment was given. Elderfield (1976) estimated a range of fluxes from near-shore sediment of 0.003-0.3 mmoles/m2/daybased on pore-water profile data of Calvert and Price (1972). He implies that this flux is to surface sediment and that it does not represent Mn*+,which actually escapes into overlying water. If it is assumed that the Mn2+does escape, these estimates are generally lower than the yearly average fluxes calculated in this study. Because of the large uncertainty in Fe2+flux measurements, no spatial patterns in magnitude are discernable. Any regular seasonal pattern is also obscured, except that Fe2+fluxes are definitely lowest in winter. The measured fluxes listed in Table I have been corrected for precipitation loss in the core boxes and in some cases are 10-100 times higher than that predicted by the actual uncorrected measured concentration increase in the flux core water (Section 5.4). Oxidation is therefore so rapid that the release might best be conceptualized as a colloid flux. The expected flux of Fe" from the sediment if no precipitation at the interface takes place can be estimated from the pore-water concentration gradients in a similar way as done for Mn2+.A linear concentration gradient was estimated for each core by assigning the measured Fe2' concentrations at 0- 1 and 1-2 cm to the midsample depth and calculating the corresponding gradient. The overlying water Fe2+concentration was assumed to be zero. Values of DFez+were determined in the same way as described for Mn2+using the same sediment correction factors (cp'). The infinite dilution values are: 6.59, 5.56, and 3.94 x cm2/sec at 22", 15", and 4"C, respectively (Li and Gregory, 1974). The calculated fluxes are listed in Table I. Fluxes calculated from the linear gradient are within a factor of 2 of corrected measured fluxes at 22°C. Gradient calculations predict lower fluxes at 15°C and higher at 4°C than the corrected measured fluxes. Approximate agreement during warm periods and overestimates during cold

DIAGENETIC PROCESSES. 11.

405

periods are consistent with relatively low and high oxygenation of surface sediments at those times and correspondingly low and high precipitation rates near the interface. The discrepancies of 3-100 times between the two estimates during the fall ( W C ) period are not readily explained except by measurement errors and assumptions, or by requiring production of Fe2+along the sediment-water interface and burrow walls as a result of biogenic reworking and possibly oxidation of sulfides. Uncertainties in the measurement of Fe2+fluxes can be reduced in future studies by determining the flux of both particulate and dissolved Fe from the bottom sediments and by more stringent monitoring of precipitation rates within the flux-core boxes than done in this study. The release of Mn" and Fe2+into the water column, their subsequent oxidation, and the constant resuspension of particles coated with freshly precipitated Fe, Mn oxides as a result of diagenetic remobilization should play a major role in controlling the water chemistry of the estuary (Aston and Chester, 1973; Turekian, 1977). Continual scavenging of Fe- and Mnoxide-seeking elements, such as Zn, Cu, Co, and Ra, by coprecipitation or adsorption on colloids and larger sedimentary particles must take place. This sweeping-out process will maintain low standing concentrations of many metals even in the presence of significant natural or anthropogenic inputs into the basin (Broecker et al., 1973; Aller et al., 1980). Benthic fluxes of Mn2+and reprecipitation are known to influence the spatial distribution of solid-phase Mn within near-shore basins (Evans et al., 1977; Grill, 1978). These fluxes may also influence the distribution of Mn within the ocean basins as a whole. For example, Turekian (1977) calculated that if the annual Mn2+flux found in LIS (-1 mmole/m2/yr) were assumed to apply to -1% of the sea floor and if a few percent of this annual flux were lost to the deep sea, then the excess Mn accumulation rate of -1 pg/cm2/yr(Bender et al., 1970; Elderfield, 1976) in deepsea sediments could be accounted for. The salient conclusion is not necessarily that near-shore sediment-water Mn2+ fluxes cause excess Mn accumulation in the deep sea but that these fluxes can be high enough to affect large-scale distributions. Because of the lithologic abundance of Fe, it is not possible to readily demonstrate the effect of diagenetically released dissolved and colloidal Fe on sea-floor distributions. However, remobilization can be reflected in Fe contents of suspended sediment (Murray and Gill, 1978). Dissolved and colloidal Fe release from sediments may also be important in explaining seaward migrations or isotopic exchange of Fe from one basin to another as suggested by the distribution of "Fe inventories (Labeyrie et al., 1976). Fallout 55Fein turn may be a useful tool for quantifying such large-scale Fe mobilization.

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7. SUMMARY (1) Pore-water profiles of Fez+ and MnZ+from three stations in Long Island Sound have general depth-dependent concentration distributions similar to those reported from other sedimentary basins: concentrations rise above seawater values to a maximum below the interface and then decrease again or remain constant deeper in the deposit. Beyond these general features, specific features of the profiles reflect the internal transport-reaction regime effective at each station. (2) The production of Mn2+ in pore waters is directly related to the rate of reduction of Mn oxides during the decomposition of organic matter, both as a function of depth in the sediment as well as seasonally. Fez+, on the other hand, is produced both by the reduction of Fe oxides and by abiogenic or biogenic oxidation of Fe sulfides. The result of different sources for the two dissolved metals is a different seasonality of the interstitial water profiles near the sediment-water interface. The temporal changes in both Mn2+and Fez+profiles are repeatable from year to year. During the summer, pore-water MnZ+in the top few centimeters reaches the highest concentration of the year. In the fall, MnZ+concentrations are lowered in magnitude throughout the sediment column as a result of both decreased production and a relative increase in the effect of biogenic transport processes that exchange sediment solutes with overlying waters. During the winter, Mn2+ profiles reflect lowered rates of production. Concentrations near the sediment-water interface decrease because of the increased dominance of precipitation reactions and diffusive loss to overlying water compared with production rates. Overall MnZ+production mimics the seasonal and, to a lesser extent, the depth-dependent production patterns of metabolites such as NH: (Part I). Fez+ is also formed in abundance near the sediment-water interface during the early summer when the overlying water first begins to warm. Because of the associated increase in sulfide production, Fez+ does not always rise to its maximum yearly concentration at this time. In the fall, like many other ions whose source is in the sediment, Fez+ concentrations below the top few centimeters drop to their lowest value of the year. Unlike Mn", during the winter Fez+ may reach its maximum standing concentration of the year at some stations. This presumably results from a net oxidation and loss of FeS and FeSzfrom surface sediment and an associated release of Fez+ that does not immediately depend on strongly temperature-controlled microbial metabolism. (3) Both Mn2+ and Fe2+ react rapidly with anions formed during decomposition. Equilibrium calculations and direct measurement of solidphase sulfides indicate that away from oxidized portions of sediment, Fez+

407

DIAGENETIC PROCESSES. 11.

concentration is determined in part by the formation of FeS-FeS,, whereas Mn2+ concentrations appear to be related in some cases to the formation of MnC03. Other reduced phases may also influence Fe+ and Mn+ solubilities. (4) Although Mn” and Fez+ may maintain a set mass action relation with a given solid phase, the overall magnitudes of their concentrations reflect transport processes within the sediment. Biogenic transport is particularly important in this regard because the buildup at depth of anions that can precipitate Mn2+or Fez+ is determined to a large extent by burrow construction and irrigation (Part I). If, for example, it is assumed that Mn2+ concentrations are controlled by MnCO, solubility, then the increased Mn” below 10 cm at deep water relative to inshore stations could result from the decreased alkalinity buildup associated with increased biogenic transport offshore (Part I). ( 5 ) The solid-phase Mn and Fe profiles, together with estimates of biogenic reworking rates obtained from 234Thdistributions, allow calculation of solid-phase dissolution rates or equivalent Mn2+and Fez+production rates in the top 5 cm at two stations. The rates obtained indicate that the turnover time of excess Mn above the average sediment background is -60- 100 days. Excess Fe turns over on a similar time scale of 100-200 days. Excess Mn dissolution rates decrease exponentially beneath the sediment-water interface and can be described by functions of the form R = R o exp( - ax)where R o 5-30 pg/cm3/day,a 1-1 S/cm in magnitude, and x is depth in the sediment. Fe dissolution was estimated only as an average rate over selected depth intervals from 1-4 or 2-5 cm beneath the interface. A range of Fe dissolution of -10-60 pg/cm3/day was calculated. (6) Pore-water profiles were used together with solid-phase dissolution rates in diagenetic models to determine first-order anoxic precipitation rate constants for both Mn and Fe. A two-dimensional cylindrical coordinate model was employed to account for the effects of biogenic irrigation of burrows on pore-water MnZ+distributions. Two-dimensional diffusion can result in a decrease in Mn” with depth that would be interpreted as evidence for precipitation and cause overestimation of precipitation rates in a one-dimensional model. Anoxic precipitation rate constants of -0.5-0.7/day are estimated from the cylindrical coordinate model. A traditional one-dimensional model predicts rates 1.5-2 times higher. Agreement is relatively close between the two models in this case because Mnz+ production rates attenuate rapidly with depth, Fez’ precipitation rates of about lO-l20/day are estimated from a three-layer one-dimensional model. (7) The rapid oxic and anoxic precipitation of both Mn” and Fez+

-

-

-

-

408

ROBERT C. ALLER

apparently results in substantial reprecipitation within the zone of excess solid-phase Mn and Fe. This means that the estimated production rates are probably minima although agreement between production rates, the estimated sediment-water fluxes of MnZ+and Fez+that they could support, and the actual measured benthic fluxes are evidence that the production rates are approximately correct. (8) Both MnZ+and Fez+escape into overlying water. Mn" flux is highest in the summer and lowest in the winter. The seasonal range encompassing values at all stations is 0.01-4 mmoles/m2/day and the yearly average flux range is 0.23-2.6 mmoleslmz/dayin the central Sound. MnZ+ fluxes can be predicted to within a factor of 2-6 by use of Fick's first law of diffusion and assuming a linear concentration gradient from the top 0-1 cm of sediment to overlying water. Diffusion coefficients were estimated from the infinite dilution value multiplied by the factor Q' (Manheim, 1970; Lerman, 1978). This general agreement indicates that precipitation loss lowers the flux of Mn2+from sediment, but not radically. The high reactivity of Fez' under oxygenated conditions caused major problems in measurement of its flux out of the bottom. Because of this it is not possible to say with certainty what the magnitude or seasonality of the fluxes are, although the lowest fluxes definitely occur in winter. Measured fluxes corrected for precipitation loss range from about 0.001 to 0.5 mmole/m2/day. (9) The flux of Mn2+ from near-shore sediments is sufficiently high to influence both small- and large-scale distribution of Mn in the ocean basins, The lithologic Fe background prevents easy recognition of Fe deposition patterns resulting from diagenetic remobilization, but it is likely that an absolute quantity of Fe similar to or greater than that found for Mn is mobile. (10) In both Part I and I1 it has been shown that consistent explanation and quantitative modeling of diagenesis near the sediment-water interface can be made if a wide range of biological, chemical, and geological data are available for a given area. The importance of interrelationships between benthic communities, physical depositional environment, and chemical properties of the elements under consideration in controlling diagenesis and sediment-water exchange has been documented and emphasized. ACKNOWLEDGMENTS This article is based on a portion of a Ph.D. dissertation done in the Department of Geology and Geophysics, Yale University. Modifications and, I hope, improvements were made at the University of Chicago while I was supported by an Alfred P. Sloan Foundation Fellowship. M. B. Goldhaber and J . Y. Yingst deserve special thanks as valued co-workers during

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critical stages of this project. I also wish to thank L. K. Benninger, J. K. Cochran, G. R. Holdren, and J. K.Rosenfeld for much help and discussion. My major diving partners over the years were J. Y. Yingst, W. J. Ullman, and M.Pimer. R. Wells was an indispensable aid in the field and laboratory. M. Pimer and M. Reed captained the boats used in sampling. K. K. Turekian, D. C. Rhoads, R. A. Berner (Yale), and V. Barcilon (Chicago) provided critical comments, guidance, and advice at various stages. Thanks to G. R. Holdren and an unidentified reader for critical review of the manuscript. J. Pasdeloup deciphered and typed the final manuscript copy. Research support was predominantly by ERDA grant EY-764-02-3573 (K. K.Turekian, principal investigator) and by NSF grant GA-42-838 (D. C. Rhoads, principal investigator). Personal support was additionally provided by a NSF fellowship, a Yale Graduate Fellowship, ERDA grant EY-764-02-3573 and EPA grant R804-909-010 (D. C. Rhoads, principal investigator).

LISTOF SYMBOLS Constant Attenuation constant Constant Concentration Pore-water solute concentration at x, r, and t Concentration in solid-phase (masshnit volume total sediment) Excess Mn or Fe concentration in solid-phase above background (masslunit volume total sediment) Average pore-water solute concentration over defined interval Asymptotic concentration of pore-water solute at depth in the sediment Molecular diffusion coefficient modified for porosity and turtuosity (charge coupling and ion pairing assumed neglible) Biogenic particle-mixing coefficient Activation energy Constant Gas constant; 1.99 cal/deg/mole Solute flux at depth x Preexponential factor for temperature dependence of J, First-order solid-phase dissolution rate constant First-order anoxic precipitation rate constant for solute Mn2+ in sediment First-order oxic precipitation rate constant for solute Fez+ in sediment First-order anoxic precipitation rate constant for solute Fe2+ in sediment First-order oxic precipitation rate constant for solute Fez+in water overlying sediment First-order oxic precipitation rate constant for solute Mn2+ in water overlying sediment Thickness of modeled sediment interval, usually the intensively bioturbated zone Integer summation variable Porosity General reaction term Reaction constant Radial distance from central axis of hollow cylinder (burrow) Temperature Time

410 7

w X

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Excess Mn or Fe characteristic turnover times in sediment Sedimentation rate, thicknessltime Vertical depth in sediment measured positively from sediment-water interface.

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Goldhaber, M. B., Aller, R. C., Cochran, J. K., Rosenfeld, J. K., Martens, C. S., and Berner, R. A. (1977). Sulfate reduction, diffusion and bioturbation in Long Island Sound sediments: report of the FOAM group. Am. J. Sci. 211, 193-237. Goto, K., Komatsu, T., and Furukawa, T. (1962). Rapid colorimetric determination of manganese in waters containing iron. Anal. Chim. Acta 21, 331-334.

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INDEX

A

Acoustic-reflection profiling, 26-29, 33-34 Aerobic respiration, in decomposition reaction, 273 Alabandite, 375-376, 379, 381 Alkalinity, near sediment-water interface,

256-260, 262-264, 284, 317-318, 322-339 oxidation of sulfides, 371-375 Ammonia, near sediment-water interface, NH; sampling, 254-260, 262,

264-266, 268, 283-293, 2%-298, 300-302, 308-319, 322, 324-342 Amphipod, 242 Anaerobic decomposition of organic matter,

272, 274 Anemometer, 42,72 Anoxic precipitation of iron and manganese,

375-376, 397-399, 407 Antarctic ice sheet, 7 Appalachians, erosion, 4 Atmosphere, trace metal deposition from,

130, 136, 159, 161-162,210-220, 228

Microenvironment model of bioturbated zone Bivalvia, 242, 244-245, 281, 320-321 Block Island, 9 Block Island Sound, 42, 50,76 Bloom and Stuiver sea level curve, 9,13 Bottom currents, 115 Bottom disturbance, by wind and storm, 54,

74-75 Bottom stability, and sediment transport,

87-95 Bottom water, 24-25, 72-73, 114 nontidal displacement, 82-83 wind stress, 76-82, 84 Box-core sampling, 250-252, 259-267, 353,

357-359 tabulation of data from, 322-336 Bridgeport, Connecticut tide gauge, 42 trace metals in oysters, 143 water level deviations, 50 Bristol Channel, specific dissipation, 30 Burrows and burrowing animals, 281-282,

293-297, 300-303, 390, 392-393 burrowing crab, in salt marsh, 200-201 Bursting phenomena, 109

B Bay of Fundy, tidal dissipation, 22 Beach sand, in shore materials, 85 Bedrock, in shore materials, 85 Belle (hurricane), 76-78, 89,279 Benthic fauna, 240, 242-249, 320-321 HPO:- excretion rates, 314 NH: excretion rates, 311-312 and pore-water distributions, 286-303 sediment and fluid transport, 280-282 sediment processing, 31, 91, 93, 98,

101-102, 114 Benthic manganese fluxes, 404-405 Beryllium, 149-150, 153-154 Biogenic sediment and fluid transport,

279-282, 284,407 Bioturbation, 192, 285-303, 390; see also

C

Cable and Anchor Reef, 71, 74-75 Cadmium, in oysters and mussels, 143, 146 Calcium, near sediment-water interface,

254,256,259,264,268,285,326-327, 332-333, 335-339 Calcium carbonate, near sediment-water interface, 254, 256, 285, 322-323, 329,

334, 364 Captain Islands, 8, 10 Carbonates, solubility products, 375,377-381 Carbon-14, in sediments of Long Island Sound, 131, 155-158 Carbon-14-carbon-12 ratio, in sediment of Long Island Sound, 131

417

418

INDEX

Carbon-14 dating technique, salt-marsh sam238-241, 246-250, 252-253, 255, ples, 166-167 258-316, 352-353, 356-403 Carbon-nitrogen ratios, of decomposing orbox-core data, 334-336 ganic matter, 316-317 flux-core data, 342 Cathles’ sea level curve, 13-14 gravity-core data, 339 Cenozoic glaciations, 32 macrofauna, 321 Cesium, 131 Deglaciation, Long Island and southern New Chandeleur Sound, tidal dissipation, 22 England, 7, 9-12 Chao Phya estuary, 102 Deposit-feeding fauna, 240, 242, 244, 246, Charlestown moraine, 8 281-282 Chem box core, for bottom samples, 250, Diagnetic processes near sediment-water 252-254 interface Chesapeake Bay, 101, 109 decomposition and nutrient element geomanganese fluxes, 404 chemistry, 237-342 Chloride, near sediment-water interface, box-core and gravity-core data, 322-339 254, 256, 268, 325-327, 331-336, 339 decomposition reactions, 272-274 Clay band, of salt marsh sample, 173, 178, flux-core data, 340-341 207, 225-226, 228 location, methods, and results of study, Cobalt, in streams, 132 238-272 Connecticut macrofauna, 320-321 glacial erosion, 7 nutrient flux between sediment and moraines, 10 water, 308-315 outwash deposits, 11-12 organic material, supply and reactivity, sea level changes, 15-17 274-215 till layers, 5 , 33 pore-water composition, abiogenic reConnecticut highlands, 5 action controls, 303-308 Connecticut River, 10,22,24-26,43,86-87, pore-water distributions, models of, 130 285-303 metal transport, 132-133 pore-water profiles, variation in, 282-285 trace metals in mussels, 145 products of decomposition, 276-279 Continental shelf, as sediment source in stoichiometry of decomposition,315-3 17 Long Island Sound, 85-86, 99 transport processes, 279-282 Copper iron and manganese concentrations, in Long Island Sound, 137, 139, 141, 351-415 159- 162 flux into overlying water, 401-405 in New Haven Harbor, 134 location, methods, and results of study, in oysters and mussels, 143-145 352-367 in salt marsh, 136, 138, 168, 183-186, 188, precipitation reactions and saturation 191-192, 201-207, 212-219, 221, states, 375-382 227-228 production of, 368-375, 384-400 in streams, 134 seasonality, 382-384 Crustacea, 320-321 Dredge spoils, in sediment, 131 Current meter, 42, 44,46, 48, 51, 75-79 Dredging, effect on sediment storage capacCurrents, in Long Island Sound, 75-82, 115 ity, 103 unidirectional particle transport, 108 Current velocity, 115-1 16

E D Dammina of rivers. 132-133 DEEP station, central Long Island Sound,

Easterly winds, 50 East River, 22-23 tidal locks, effect on sedimentary regime, 31

419

INDEX

Eddy, in estuary, 116 Eddy-diffusion coefficient, in sand transport, 118-121, 124-125 Effective fetch, 72 Elmhurst moraine, 8 Embayment, energy balance formulation, 61-65 End moraine, 7-8, 10 acoustic reflection profile, 27 in shore materials, 84-85 Energy, in estuarine sedimentary processes, 99-101 Energy balance, in embayment, formulation Of, 61-65 Erosion glacial, of New England, 7, 12 rate, on land surface supplying sediment to east coast of North America, 4, 12 river banks, 87 shoreside, as sediment source, 84-85, 101 Estuarine circulation, I , 23-25, 43, 75 salinity gradient, 82 and sand flux, 115 Estuary, 130 sediment storage capacity, 30 sediment systems, 1-2, 99-103 Eustatic sea level curve, 13-14

F Falkner Island, 5 , 71-72 Fall Line, of Long Island Sound, 2, 4, 6 Fall Zone surface, of Long Island Sound, 2-5, 10-11 Farm River, 11 Farm River salt marsh, Connecticut dating of deposits, 193-198 history, 170-172 Lack of disturbance and chemical immobility, 199-210 peat properties, 172-181 sediment and trace metals, 161, 168-169, 181-189 atmospheric sources, 210-220 silt and clay sources, 225-227 Fauna, benthic, see Benthic fauna Fecal pellets, in sediments, 31-32, 91-93, 102, 114, 241 Fermentation, in organic decomposition, 273

Ferrous oxide ion activity products, 376-378 reduction, 368-369, 384, 406 Ferrous sulfide, at sediment-water interface, 257, 270-271, 276-278, 317, 322-323, 329-33 1, 334, 377 oxidation, 369-370, 374, 384, 406 Fishers Island Sound, 31 Floods, and sediment transport, 109 Flow velocity, see Current velocity Fluid mud, 102 Flux-box core, for bottom samples, 250, 253-255, 354, 364-367 data, 340-341 macrofauna, 318-319 FOAM station, central Long Island Sound, 238-244, 252-253, 255, 258-316, 352-353, 356-403 box-core data, 323-327 flux-core data, 340 gravity-core data, 337 macrofauna, 320

G Gastropoda, 242, 320-321 Glacial drift, 7-8, 14 Glacial sand, 26 Glaciations, eastern North America, 5,7,32 Grass, of salt marsh, 165-167, 173,221-224, 229 Gravity-core sampling, 252, 254, 257-259, 306-307, 353, 355-357 tabulation of data from, 322, 337-339 Great Marsh, Connecticut, 167 Greigite, 276, 375, 377

H Hammock River Marsh, 13 Harbor Hill moraine, 8, 10 Hauerite, 381 Heavy metals, in sediments, 32 Hermit crab, 242 High marsh, 165-166 Hitchcock, Lake, 10-11, 32 Housatonic River, 22 trace metals, 132, 143, 145 Hurricane, effect on bottom water and sediment, 76-78, 89, 279

420 Hydrogen sulfide in salt marsh, 168 near sediment-water interface, 322-324 Hydroxyapatite, 305 Hydrozoa, 320-321

INDEX

atmospheric fluxes, 210-220 dating of deposition, 189-198 in streams, 132, 134 Ledyard moraine, 8, 10 Long Island Sound acoustic reflection profiles, 26-29 bedrock geology, 6 I contours of water depth, 71 Ice-contact drift, 7, 10 currents, 75-82, 115 Industrial wastes, 101, 130, 132 dated organic matter, location of, 9 Interglacial period, denudation rate of land diagenetic processes near sediment-water surface, 32 interface, 237-415 Irish Sea, tidal dissipation, 22 decomposition and nutrient element Iron, see also Ferrous oxide; Ferrous sulfide geochemistry, 237-342 reduction, in organic decomposition, 273 iron and manganese concentrations, in salt marsh, 168-169, 178-181,183-186, 35 1-4 15 190-192,201-203,210,214-215,217, estuarine circulation, 43 227-228 geological history, 2-12, 122, 130 near sediment-water interface, 254-255, geometric and tidal characteristics, 104 257, 268, 274, 276, 323-415 glacial drift, depth of, 8 flux into overlying water, 401-405,408 islands, composition of, 5 location, methods, and results of study, nuclides, sources and sinks of, 129-164 352-367 physical oceanography, 20-25 precipitation reactions and saturation power characteristics, 84 states, 375-382 river flow, 82 production, 368-375,384-400,406-408 salinity, 24-25 seasonality, 382-384, 406 sand content variation, 120-121 in streams, 132 sand transport at floor, 107-128 sea level rise, 12-20 sedimentary system, 1-39 J sedimentation parameters, 98 sediment sources, 84-87 Jameco formation, 7 sediment transport and deposition, 69-106 K shore materials, 85 storm energy, 48-55 Knight Inlet, 100 temperature, 24-25 tidal dissipation estimate, 65-66 L tidal energy, 43-47, 60, 70, 84 water level deviations, 55-60 Lacustrine deposits, 27, 95 waves, 70, 72-75 Lag effect, in sediment transport, 108 Lordship outwash, 10 Land elevation curve, 13-14 Low marsh, 165-166 Lead in Long Island Sound, 131, 137, 139, 141, Lunar semidiurnal tide, 20-21 149- 150,153- 159,161-162,280-28 1, 284 in mussels, 146, 152 M in New Haven Harbor, 134 in salt marsh, 136-138, 168-169, 181-188, Mackinawite, 276, 375, 377, 380 191-392, 201-202, 227-228 Madison moraine, 8, 10

42 1

INDEX

Manganese reduction, in organic decomposition, 273 in salt marsh, 168, 183-186, 190-192,

Near-bottom flow velocity, 116 New England, geological history, 4-5, 7,

201-204,206,208-210,214-215,217, 227-228 near sediment-water interface, 254-255, 274, 323-415 flux into overlying water, 401-405, 408

New Haven, Connecticut clay mineralogy of sediments, 32 fluid mud, 102 population growth, 172 sand-mud ratio, 119, 121 sewer outfalls, 133-135 tide gauge, 42, 51 trace metals in oysters and mussels, 143,

location, methods, and results of study, 352-367

precipitation reactions and saturation states, 375-382 production, 368-375,384-399,406-408 seasonality, 382-384,406 Manganese oxide reduction, at sedimentwater interface, 368-369, 406 Mannetto formation, 5 Marsh, Long Island Sound shoreline, 85 Martha’s Vineyard, glaciations, 5 Mattituck sill, 9, 14-15, 34, 71, 110-111, 122-123, 125

glacial outwash, 95-96 sand flux, 90 sand waves, 114-1 15 Meltwater curve, 13 Mercury, in New Haven Harbor, 133-135 Mersey River, 103 Metals, see Trace metals Methane production, in organic decomposition, 272-273 Microenvironmentmodel of bioturbated zone, 294-303, 390-396 Mill River, 11

Mineral material in sediment, 91 Montauk, New York, 42, 50 Moraine, 7-8, 10, 84-85 acoustic reflection profile, 27 Morner’s sea level curve, 13-14 Mud, 26-27, 87, 89, 101-102, 110, 114; see also Sand-mud transition zone deposition, 95-98 transport, 91-95 Mussels, trace metals in, 142, 144-147, 152

N Narragansett Bay, 22, 404 Nauerite, 375 Naugatuck River, 132

10-12

145

water level deviations, 48-50 New London, Connecticut mean sea level, 15-16 tide gauge, 42, 55 trace metals in oysters, 143 water level deviations, 48-50, 55-60 Newport, Rhode Island, water level deviations, 48-49 New Rochelle, New York, 42 Nickel, 148 in mussels, I46 Nitrate reduction, in organic decomposition, 273 Nitrogen-carbon ratios, of decomposing organic matter, 316-317 Nitrogen concentration, near sedment-water interface. see Ammonia Noank, Connecticut, trace metals in oysters, 143

Norwalk, Connecticut, trace metals in oysters, 143 Norwalk Islands, 8, 10 Nuclides, see Trace metals Nucula annulata, 93, 242, 244, 320-321 NWC station, central Long Island Sound, 238-241,244-247,250,252-253,255, 258-316, 352-353, 356-403

box core data, 328-333 flux-core data, 341 gravity-core data, 338 macrofauna, 321

0

Ocean basin volume adjustment, 13 Old Saybrook moraine, 8, 10 One-dimensional model of pore-water dis-

422

INDEX

tributions, 285-293, 396-399 Organic matter dated remains, location of, 7, 9 at sediment-water interface, 269-270, 323, 328-331, 334 decomposition products, 276-277 decomposition reactions, 272-274 stoichiometry of decomposition, 315-317 supply and reactivity, 274-275 Outwash deposits, 7, 10-12, 26-27, 85, 95-96 Oysters, trace metals in, 142-144

P Peat, in salt marsh, 13, 166-167, 172-181, 199, 220-227, 229 trace-metal analysis, 185-189 Pelletized material, see Fecal pellets Phosphate mineral formation in sediments, 304-308 near sediment-water interface, HP0:sampling, 254-260, 262, 261-268, 283-284, 305, 308-31 I , 314-319, 323-342 Plankton, 24, 283-284, 314-316 Plutonium, 131, 154-156 Polluted streams, and trace metals in Long Island Sound, 132-135 Polonium, in mussels, 152 Polychaeta, 242,244-245,247,281,297,299, 320-321 Pore water, near sediment-water interface of Long Island Sound, 252-268, 282-308, 317-318 box-core data, 323-336 gravity-core data, 337-339 iron and manganese concentrations, 353-362, 394-395 Port Jefferson, New York, 42, 50 Potomac estuary, 76, 100-101 Pyrite in salt marsh, 168-169, 178-181,203,227 near sediment-water interface, 257, 271-272,216-277,322-323,329-331, 334 oxidation, 369-371, 314

Q Quinnipiac River, 5 , 11, 13 metal concentrations, 132-134

R Race, the, Connecticut, 71 Radionuclides, see Trace metals Radium, in salt marsh, 181-184, 187 Reddingite, 305, 307-308, 381 Resonant basin, 20 Resuspended sediment, 31-32, 34-35, 5 5 , 87-88, 93-95, 114, 125, 240, 279 Rhodochrosite, 375-376, 379, 381-382, 407 Rivers flow and discharge into Long Island Sound, 82, 86-81 metal concentrations, 131-1 34 power, in estuarine sedimentary processes, 100 sediment supply rate, 98 Rogers Lake, 7, 9 Ronkonkoma moraine, 7-8 Rotary tides, 20, 22 S

Sachem Head, Connecticut, sand-mud ratio, 119, 121 Salinity, Long Island Sound, 24-25, 82, 114 Salt hay, see Spartina patens Salt marsh accretion, theories of, 220-227 dating of samples, 189-198 elevation, 16-20, 34 sediment and trace metals, 136-138, 161, 165-236 Salt thatch, see Spartina alterniflora Sand, 26, 85, 87, 89, 241 distribution, in Long Island Sound, 110, 112-113 grain size, 113 trace-metal content, 137 transport, 89-91,96-97, 107-128 Sand-mud transition zone, 27, 113 formation, 116-122, 125 sand distribution, 123-124 Sand wave, 89-91, 96, 114-115

423

INDEX

Sandy Hook, New York, 42, 50 Savannah River, 103 Sea-level changes, 9, 11-20, 34 rise, and sediment storage capacity, 30 and salt-marsh accretion, 192-198,222-225 Sea level curve, 13-14 Sea-surface elevation, 4, 15 Seawater transport, biogenic, 282 Sediment accumulation rate, 30, 130 bulk density, 175, 177-178, 183-184 composition, in Long Island Sound, 110-113 iron and manganese concentrations, 353-355 Long Island Sound’s sedimentary system, 1-39 resuspended, 31-32, 34-35, 55, 87-88, 93-95, 114, 125, 240, 279 in salt marsh, 165-236 sources, 84-87, 101 storage capacity, 30-31, 102-103 thickness, in sand-mud transition zone, 117 trace metals, 137-142, 147-163 transport and deposition, 25-33, 69-106, 108-110, 113-116, 279-282 and bottom stability, 87-95 power sources, 70-84 in stormy period, 5 5 , 61 Sediment trapping efficiency, of Long Island Sound, 30, 32-33, 35, 98-99 Sediment-water interface, diagenetic processes near, see Diagenetic processes near sediment-water interface Sediment yield, 4 of rivers entering Long Island Sound, 26, 30, 32 Severn estuary, 102 Sewage wastes, 23-24, 101, 130, 133-135 Shell debris, in sediments, 241, 243-247, 249, 281-282 Shoals, and sediment transport, 109 Shoreside erosion, 84-85, 101 Siderite, 375-378, 381 Silt, deposition of, and salt-marsh accretion, 220, 223 Silt-clay sediment, 25-26,30-32,91,93,102, 24 1

deposition rate, 97 of salt marsh, 225 Silver, in streams, 132-134 Spartina alterniflora, 165,167,200,209,222, 224 Spartinapatens, 165-166,169,173,176,190, 193, 200, 209, 222 Specific dissipation, 30-31, 99-101, 103 Stagnation-zone retreat, in deglaciation of New England, 7 Stony Creek, Connecticut, salt marsh leveling measurements, 16-17 Storm energy, 48-55 Storms and flow velocity, 114 resuspension of sediment, 31, 279 and sand distribution, 123-124 and sediment transport, 109 Stratified drift, 7 Struvite, 304-305 Sulfate near sediment-water interface, SOf sampling, 254-261, 283-284, 317, 322-339 SO:- concentrations during anoxic incubation, 375-376 Sulfate-chloride ratio, in salt marsh, 188-189, 191, 203, 209, 227 Sulfate reduction in organic decomposition, 273-278, 316-317 in salt marsh, 168-169 Sulfide oxidation, at sediment-water interface, 369-375 in salt marshes, 168-169, 202-203, 227 solubility products, 375-381 Surficial sediment, in Long Island Sound, 110 Suspension-feeding fauna, 242, 244, 247, 281-282 Susquehanna River, 132

T Tay, Firth of, 100 Temperature, and flux of iron and manganese into overlying water, 368, 401-403

424

INDEX

Thames estuary, 102 Thames River, 22 Thiobacillus , 369 Thorium, 131, 149-151, 153-154, 161-162, 280-281, 284 Tidal characteristics of Long Island Sound, 20-22 Tidal currents, 1 I3 and sand flux, 115 Tidal dissipation estimate, for Long Island Sound, 65-66 Tidal energy, 41, 43-47, 60,70, 84 Tidal locks, effect on sedimentary regime, 31 Tidal stream, 75, 79-80 Tidal velocity, 79-80, 108-109, 116, 122 Tidal wave, and sediment transport, 109 Tide, 113 salt marsh submersion, 21 1 and sedimentary processes, 101, 107-109 Tide gauge data, 15-16, 42, 51, 55 Till, 5, 33 Tilt rate, east coast of North America, 4, 10 Totoket bog, 9 Trace metals in Long Island Sound, 129-164 distribution in sediments, 137-142 in mussels and oysters, 142-147 processes affecting deposition and accumulation in sediments, 147-161 sources of, 131-136 in salt marsh, 165-236 atmospheric supply, 136-138 Two-dimensional model of pore-water distributions, 294-303, 390-3% U

Uca pugnax, in salt marsh, 200-201 Unbound sediment, 98

V Valley train, 7, 10-1 1 Vivianite, 305-306, 308, 380

W Water column scavenging of trace metals, 149- I50 Water level, in Long Island Sound, 15-16, 48-53, 55-60 Water velocity at bottom, 72-73 in stormy period, 54 Wave gauge, 44 Wave recorder, 74 Waves, 70, 72-75, 87, 89, 100 West River, 11 Whitlockite, 305-306,308 Winds, 70, 72-75 and currents, 76-82, 84 and flow velocity, 114 and sand distribution, 123 and sedimentary processes, 100-101, 109 and tidal stream velocity, 54 and water level deviations, 48 in winter storm, 50-51 Wisconsinan glaciations, 5, 7, 32

X

X-ray box core, for bottom samples, 250, 252-253

Y Yoldia limatuia, 93, 242, 244, 246, 311-312, 314, 320-321, 383

2 Zinc in Long Island Sound, 137, 139, 141, 161-162 in New Haven Harbor, 134-135 in oysters and mussels, 142-143, 146 in salt marsh, 136, 138, 168, 183-186, 188, 191-192, 201-202, 213-219, 227-228