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Diatom paleoproductivity and sediment transport in West Antarctic basins and the Neogene history of the West Antarctic Ice Sheet (WAIS)
Scherer, Reed Paul, Ph.D.
The Ohio State University, 1992
UMI 300 N. Zeeb Rd. Ann Arbor, MI 48106
DIATOM PALEOPRODUCTIVITY AND SEDIMENT TRANSPORT
IN WEST ANTARCTIC BASINS AND THE NEOGENE
HISTORY OF THE WEST ANTARCTIC ICE SHEET (WAIS)
DISSERTATION
Presented in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in the Graduate School of The Ohio State University
********
By
Reed Paul Scherer, B.S., M.S.
The Ohio State University
1992
Dissertation Committee:
Dr. P.-N. Webb, Chairman Approved by
Dr. W.l. Ausich
Dr. L.A. Krissek
Dr. L.G. Thompson Adviser Department of Geological Sciences Dr. K. Jezek This dissertation is dedicated to my wife, Lene. We met in Denmark in 1978 and saw each other again the following summer in Southampton, New York. We then lost contact for nearly a decade. Before the long absence (and our marriage in 1989), Lene sent me a book of short verse, called Grooks, by Danish poet and physicist Peit Hein. One couplet from the Grook entitled T.T.T., came to mind frequently during the last few years of graduate school:
When you feel how depressingty slowly you climb, It’s well to remember that "Things Take Time" ACKNOWLEDGEMENTS
I wish to express my gratitude to Professor Peter-Noel Webb for the guidance and
support he provided, on many levels, throughout the course of this project. I look forward to
continued collaboration with Dr. Webb into the future. Dr. Webb, the Department of Geological
Sciences and the Byrd Polar Research Center provided the platform from which many scientific
opportunities related to this reserarch became possible. For many stimulating scientific
exchanges I thank my friends and colleagues at Ohio State and elsewhere, notably Dr. David
Harwood, Dr. Amy Leventer-Reed, Dr. Scott Ishman and Mr. Charles Hart, all of whom
reviewed various parts of this dissertation. Thanks to Drs. William Ausich, Lawrence Krissek,
Kenneth Jezek, Lonnie Thompson and D. Graham Jenkins for many helpful comments
regarding the dissertation, and Drs. Donald Blankenship, Ian Whillans and Richard Alley for
many animated and enlightening glaciological discussions. Special thanks to Mrs. Judith
Jenkins for editorial reviewing, Mr. John Nagy for drafting assistance and Michelle Jones for
laboratory assistance.
Sediments analyzed for this dissertation were made available by (1) Mr. Dennis
Cassidy of the Antarctic Sediment Core Facility of Florida State University (piston cores from
DF82 and DF81; gravity cores from RISP), (2) Dr. Robert Bindschadler of NASA Goddard
Space Flight Center (CIR) and (3) Dr. Hermann Engelhardt and Dr. Barclay Kamb of California
Institute of Technology (UpB). Financial support for this project came from many sources,
including but not limited to NSF grants (Division of Polar Programs) to Dr. Webb (DPP-
8716261, DPP-8919910) and myself (DPP-8919910), the Ocean Drilling Program (USSAC), the
Department of Geological Sciences (research-related support from the "Friends of Orton Hall" fund and stipend from Teaching Assistantship) and the Byrd Polar Research Center of The
Ohio State University. VITA
May 9, 1957 ...... Born - Brooklyn, New York
1975 ...... Graduate, Sheepshead Bay High School, Brooklyn, N.Y., Special Marine Science Program
1979...... B.S., Geology Southampton College, Long Island University Southampton, N.Y.
1979-1982...... Research Assistant in Micropaleontology Lamont-Doherty Geological Observatory, Columbia University, N.Y.
1983 ...... M.S., Geology University of South Carolina, Columbia, SC
198 4...... Research Associate in Basin Analysis Earth Sciences and Resources Institute, Univ. of South Carolina, Columbia SC
1985 ...... Research and Teaching Assistant, Department of Geology, Univ. of South Carolina, Columbia, SC
Summer, 1984, 1985 ...... Visiting Scientist, Centre National des Researches Scientifique, Gif-sur-Yvette, France
1986-1991 ...... Graduate Research and Teaching Associate Department of Geological Sciences and Byrd Polar Research Center, The Ohio State University Columbus, OH
iv PUBLICATIONS
SCHERER, R.P., Cohen, A.D., Andrejko, M.J., Raymond,R., Gooley.R., 1982. Freshwater diatom assemblages from surface peats of the Okefenokee swamp-marsh complex of southern Georgia. GSA Abst. Prog., 14(7):609 (abstract).
SCHERER, R.P., 1983. Freshwater diatom assemblages and paleoecology of the Okefenokee swamp/marsh complex, southern Georgia. MS Thesis, University of South Carolina, Columbia, 126pp.
SCHERER, R. P. and Cohen, A. D., Diatoms in Okefenokee Swamp peats., 1984. (In) The Okefenokee Swamp: Its Natural History, Geology, and Geochemistry, Cohen, A.D. et al. (eds.) Wetland’s Surveys, Los Alamos, NM, pp. 456-467.
SCHERER, R.P., 1984. Freshwater diatom assemblages and ecology/paleoecology of the Okefenokee swamp-marsh complex of southern Georgia, USA. Eighth International Symposium on Fossil and Recent Diatoms, Paris, France, Conference Abstract.
Lazarus, D., SCHERER, R., and Prothero, D., 1985. Evolution of the radiolarian species-complex Pterocanium. Jour. Paleontology, 59(1):183-220.
SCHERER, R.P., 1985. Pogo Strikes Back: Diatom responses to human influences in the Okefenokee Swamp. South Carolina Academy of Sciences Annual Meeting Clemson, SC. S. C. Acad. Sci. Bull. 46:63 (abstract).
SCHERER, R.P., 1987. Biogenic sedimentation and bottom transport in the Bransfield Strait, Antarctic Peninsula. GSA Abst. Prog. 19(7):832 (abstract).
SCHERER, R.P., 1987. Paleoenvironmental studies of non-marine diatoms in Quaternary antarctic sediments. Antarctic Jour. U.S. 1987 Review, 22:35-37.
Harwood, D.M. and SCHERER, R.P., 1988. Diatom biostratigraphy and paleoenvironmental significance of reworked Miocene diatomaceous clasts in sediments from RISP Site J-9. Antarctic Jour. U.S. 1988 Review, 23:31-34.
SCHERER, R.P., Harwood, D.M., Ishman, S.E. and Webb, P.-N., 1988. Micropaleontological analysis of sediments from the Crary Ice Rise, Ross Ice Shelf. Antarctic Jour. U.S. 1988 Review, 23:34-36.
SCHERER, Reed P., 1988. Freshwater diatom assemblages and ecology/paleoecqlogy of the Okefenokee swamp-marsh complex of southern Georgia, USA. Diatom Research. 3(1)115-143.
SCHERER, R.P., 1988. Estimating diatom paleoproductivity and sedimentation rates: Comparison of Holocene and lower Miocene Antarctic sediments. SEPM Abstracts, 5th
v annual midyear meeting, Columbus, Ohio. 5:48.
SCHERER, R.P., Harwood, D.M., Webb, P. and Greene, D., 1988. New microfossil data on marine sediments beneath the Ross Ice Shelf and Miocene paleoenvironment of West Antarctica. SEPM Abstracts, 5th annual midyear meeting, Columbus, Ohio. 5:48.
SCHERER, R.P., 1988. Paleoproductivity and particle flux in the Bransfield Strait, Antarctic Peninsula. 2nd Polar Diatom Colloquium, Bremerhaven, Germany. Conference Abstracts, Alfred Wegener Institute.
SCHERER, R.P., 1988. Sediments underneath the Ross Ice Shelf and their paleo environmental significance. 2nd Polar Diatom Colloquium, Bremerhaven, Germany. Conference Abstracts, Alfred Wegener Institute.
SCHERER, R.P., 1989. Microfossil assemblages in "deforming till" from Upstream B, West Antarctica: implications for ice stream flow models. Ant. Jour. U.S., 1989 Review, 24:54-55.
SCHERER, R.P., 1989. Paleoenvironments of the West Antarctic interior: microfossil study of sediments below Upstream B. Antarctic Jour. U.S., 1989 Review. 24:56-57.
SCHERER, R.P., 1989. Microfossil study of active till and relict sediments beneath ice stream B, West Antarctica: implications for antarctic paleoclimates and modern ice stream flow. GSA Abst. Prog. 21(7):262 (abstract).
SCHERER, R.P., 1989. Geologic analysis of sediments from beneath the West Antarctic Ice Sheet and the Provenance Envelope concept. AGU Fall Meeting, EOS 70(43)1081 (abstract).
Harwood, D.M., SCHERER, R.P. and Webb, P.-N., 1989. Multiple Miocene marine productivity events in West Antarctica as recorded in upper Miocene sediments beneath the Ross Ice Shelf (Site J-9). Marine Micropaleontology. 15:91-115.
Ocean Drilling Program Leg 124 Shipboard Scientists, 1989. Origins of marginal basins. Nature. 338:380-381.
Ocean Drilling Program Leg 124 Shipboard Scientists, 1989. Leg 124 researchers drill marginal basins. Geotimes. 34(5) :15-17.
Rangin, C., Silver, E SCHERER, R..., et al., 1989. Proc. Ocean Drilling Program, Initial Reports, 124, College Station, TX (Ocean Drilling Program), 916 pp.
Smith, R., Betzler, C., Brass, G. Huang, Z., Linsley, B, Merrill, D. Muller, C, Nederbragt, A, Nichols, G, Pubellier, M., Sajona, F., SCHERER, R. Shibuya, H., Shyu, J., Spadea, P., et. al., 1990. Depositional history of the Celebes Sea from ODP Sites 767 and 770. Geophysical Research Letters. 17(11):2061-2064.
Nichols, G., Betzler, C., Brass, G. Huang, Z., Linsley, B, Merrill, D. Muller, C, Nederbragt, A, Pubellier, M., Sajona, F., SCHERER, R. Shibuya, H., Shyu, J., Smith, R., Solidum, R., Spadea, P., et. al., 1990. Depositional history of the Sulu Sea from ODP Sites 768, 769 and 771. Geophysical Research Letters. 17(11):2065-2068.
SCHERER, R.P., 1990. Fossil diatoms beneath the West Antarctic Ice Sheet and possible implications. 11th internat. Symp. Living and Fossil Diatoms, San Francisco, (abstract).
SCHERER, R.P., 1991. Miocene radiolarians of the Sulu Sea, ODP Leg 124. Proc. Ocean Drilling Program, Scientific Results, 124: College Station, TX (Ocean Drilling Program), pp. 359-368.
SCHERER, R.P., 1991. Radiolarians of the Celebes Sea, ODP Leg 124, Sites 767 and 770. Proc. Ocean Drilling Program, Scientific Results, 124: College Station, TX (Ocean Drilling Program), pp. 345-358.
Hsu, V., Merrill, D., Muller, C. Nederbragt, A, SCHERER, R., Shibuya, H. and Shyu, J., 1991. Chronostratigraphic synthesis, ODP Leg 124. Proc. Ocean Drilling Program, Scientific Results, 124: College Station, TX (Ocean Drilling Program), pp. 11-35.
SCHERER, R.P., 1991. Quaternary and Tertiary microfossils from beneath Ice Stream B: Evidence for a dynamic West Antarctic Ice Sheet history. Antarctica in Global Change: an Ocean Drilling Perspective, Santa Barbara, CA. (abstract).
SCHERER, R.P., 1991. Quaternary and Tertiary microfossils from beneath Ice Stream B: Evidence for a dynamic West Antarctic Ice Sheet history. Palaeogeography, Palaeoclimatology, Palaeoecology (Global and Planetary Change 90:395-412. Section),
FIELDS OF STUDY
Major Field: Geological Sciences TABLE OF CONTENTS
Acknowledgements...... iii Vita ...... iv List of Tables ...... xiii List of Figures...... xv List of Plates ...... xviii Introduction...... 1
CHAPTER PAGE
I. THE WEST ANTARCTIC ICE SHEET AND ’GLOBAL CHANGE’...... 6 INTRODUCTION...... 6 WEST ANTARCTIC ICE SHEET EVOLUTION...... 10 ICE SHEET MODELLING...... 14 THE ROSS ICE STREAMS...... 15 MICROPALEONTOLOGIC ANALYSIS OF SUB-ICE SEDIMENTS... 21
II. METHODS: NEW APPROACHES TO ANTARCTIC DIATOM ANALYSES.. 23 DIATOMS IN GLACIAL DIAMICTONS: SCIENCE FROM THE TRASH HEAP...... 23 Drawing a Distinction Between Methods in Biostratigraphy and Paleoceanography ...... 26 METHODS FOR THE EXTRACTION OF DIATOMS FROM GLACIAL SEDIMENTS FOR BIOSTRATIGRAPHY...... 28 BIOSTRATIGRAPHIC GOALS...... 28 Washing ...... 28 Sieving...... 32 Heavy liquid flotation ...... 33 DIATOM IDENTIFICATION AND BIOSTRATIGRAPHY...... 34 A NEW METHOD FOR THE DETERMINATION OF ABSOLUTE DIATOM ABUNDANCE IN SEDIMENTS...... 38 Introduction...... 38 Previous methods...... 39 New method...... 41 Step 1: weighing...... 42 Step 2: chemical attack ...... 42 Step 3: slide preparation ...... 43 Step 4: counting and abundance calculation ...... 49 Results ...... 50 Reproducibility...... 50 Accuracy ...... 52
III. DIATOM ABUNDANCE IN HOLOCENE SEDIMENTS OF THE NORTHERN ANTARCTIC PENINSULA: A MODEL FOR THE INTERPRETATION OF NEOGENE ANTARCTIC PALEOPRODUCTIVITY...... 57 INTRODUCTION...... 57 Setting...... 58 The Diatom Assemblage ...... 65 METHODS AND MATERIALS...... 71 RESULTS...... 75 Surface Sediments...... 75 Chaetoceros Abundance as aTracer of Paleoproductivity.... 82 Late Holocene Paleoproductivity Record of the Northern Antarctic Peninsula ...... 84 DISCUSSION...... 94 The Holocene Record...... 94 Application to Neogene Antarctic Paleoenvironments 99
IV. BIOSTRATIGRAPHIC STUDY OF SEDIMENTS FROM ROSS ICE SHELF PROJECT (RISP) Site J-9...... 101 INTRODUCTION...... 101 METHODS AND MATERIALS...... 108
ix BIOSTRATIGRAPHIC RESULTS...... 110 DISCUSSION OF DIATOM BIOSTRATIGRAPHIC OBSERVATIONS...... 118 SUMMARY...... 121
V. BIOSTRATIGRAPHIC STUDY OF SEDIMENTS FROM THE CRARY ICE RISE (CIR), ROSS ICE SHELF...... 129 INTRODUCTION ...... 129 METHODS...... 130 RESULTS...... 131 DISCUSSION OF BIOSTRATIGRAPHIC RESULTS...... 133
VI. MICROPALEONTOLOGIC ANALYSES OF SEDIMENTS FROM BENEATH THE WEST ANTARCTIC ICE SHEET AT UPSTREAM B (UpB)...... 140 INTRODUCTION...... 140 METHODS...... 143 MICROFOSSILS IN UPSTREAM B SEDIMENT...... 144 Non-marine Diatoms ...... 146 Calcareous Microfossils ...... 149 Marine Diatoms ...... 150 DISCUSSION...... 154 CONCLUSIONS AND IMPLICATIONS...... 162 VII. PALEOENVIRONMENTS OF THE WEST ANTARCTIC INTERIOR 174 INTRODUCTION: THE CHRONOSTRATIGRAPHIC FRAMEWORK...... 174 MIOCENE PALEOENVIRONMENTS OF THE ROSS EMBAYMENT FROM RISP AND CIR CLASTS...... 180 Paleoproductivity: The Chaetoceros record...... 180 Inferences Regarding Sea Ice Extent...... 185 DESCRIPTIVE ANALYSES AND PALEOENVIRONMENTAL INFERENCES FROM UpB DIATOM ASSEMBLAGES...... 186 ADVANTAGES AND LIMITATIONS OF PALEOENVIRONMENTAL
x INTERPRETATIONS OF GLACIAL SEDIMENTS...... 189
VIII. GLACIAL SEDIMENTATION AND TRANSPORT MODELS: IMPLICATIONS FOR ICE STREAM BED PROCESSES AND WEST ANTARCTIC ICE SHEET RECONSTRUCTION...... 190 INTRODUCTION...... 190 ICE STREAM FLOW MODELS: TILL DEFORMATION VS. BASAL SLIDING...... 191 PROVENANCE ENVELOPES...... 197 Preliminary Provenance Envelope Definitions...... 200 Ross Ice Shelf Project Site J-9 (RISP)...... 200 Crary Ice Rise (CIR) ...... 204 Upstream B (UpB) ...... 205 Byrd Station ice core basal debris (BYRD) ...... 205 AN INTEGRATED GEOLOGICAL7GLACIOLOGICAL MODEL FOR THE ROSS SECTOR OF THE WAIS...... 208
SUMMARY AND CONCLUSION...... 219
REFERENCES...... 223
APPENDICES...... 247 A. COMPREHENSIVE TAXONOMIC LIST WITH SUMMARY OF SPECIES OCCURRENCES...... 248 B. BIOSTRATIGRAPHIC RANGES OF KEY DIATOMS...... 255 C. FIELD PLANNING FLOW CHARTS...... 257 D. TABLES OF ANTARCTIC PENINSULA PISTON CORE DATA.... 262
xi LIST OF TABLES
TABLE PAGE
1. The sediments evaluated for reproducibility of diatom absolute abundances ...... 51
2. Six replicate counts of diatom absolute abundance in five different sediments...... 53
3. Surface sediment diatom, organic carbon and lithic abundance data from the northern Antarctic Peninsula ...... 72
4. Percent organic carbon of Ross Ice Shelf Project (RISP) matrix and clasts. Data for matrix samples are averages from Sackett (1986) and Wrenn and Beckman (1981) ...... 111
5. Qualitative analyses of diatoms and other siliceous microfossils from clasts and matrix from RISP Core 78-16 ...... 112
6. Quantitative counts of matrix sediments and clasts from various RISP cores...... 113
7. Percent organic carbon in CIR matrix and clast samples ...... 132
8. Quantitative counts of diatoms in CIR matrix and clasts ...... 138
9. Diatom species list and cumulative abundances in Upstream B sediment...... 145
10. Chaetoceros spore abundance and organic carbon content in Piston Core DF82-34...... 263
11. Chaetoceros spore abundance in Piston Core ..DF82-47...... 264
12. Chaetoceros spore abundance in Piston Core ..DF82-50 ...... 265
13. Chaetoceros spore abundance in Piston Core ..DF82-60 ...... 266
14. Chaetoceros spore abundance in Piston Core DF82-71 ...... 267
15. Chaetoceros spore abundance in Piston Core DF82-98 ...... 268
xii 16. Chaetoceros spore abundance in Piston Core DF82-100 ...... 269
17. Chaetoceros spore abundance in Piston Core DF82-150 ...... 270
18. Chaetoceros spore abundance in Piston Core DF82-182 ...... 271
19. Chaetoceros spore abundance and organic carbon content in Piston Core DF81-18...... 272
xiii LIST OF FIGURES
FIGURES PAGE
1. East and West Antarctica ...... 4
2. West Antarctic crustal blocks ...... 8
3. Configuration of the West Antarctic Ice Sheet ...... 11
4. Ross ice streams ...... 16
5. Deforming till/till delta model of Alley et al. (1986; 1989a) ...... 19
6. Diatom Zones of Harwood and Maruyama, 1991 ...... 36
7. Glassware modified for quantitative diatom slide preparation 44
8. Apparatus for simultaneously preparing 12 samples ...... 47
9. Diatom abundances (valves per gram) calculated for six observations on five samples ...... 54
10. Map of the Antarctic Peninsula showing strong climatic gradients...... 60
11. General pattern of surface current flow along the northern Antarctic Peninsula ...... 62
12. Piston core distribution and generalized bathymetry of the northern Antarctic Peninsula ...... 73
13. Plot of percent organic carbon vs. depth for surface samples 76
14. Plot of absolute abundance of lithic grains greater than 20pm vs. absolute abundance of Chaetoceros spores in surface samples 78
15. Absolute abundance of Chaetoceros spores vs. depth in surface samples from the Gerlache and Bransfield Straits ...... 80
16. Chaetoceros spore abundance (cells per gram) downcore in outer Bransfield Basin Cores DF82-34 and DF82-50 ...... 86
17. Chaetoceros spore abundance in inner Bransfield Basin
xiv Cores DF82-47, DF82-60, and DF82-71...... 89
18. Chaetoceros spore abundance in piston cores from Gerlache Strait (DF82-100, DF82-151), Hughes Bay (DF82-98) and James Ross Island region (DF82-182)...... 92
19. Chaetoceros spore abundance in the Western Bransfield Basin near Low Island (DF81-18)...... 95
20. Plot of Chaetoceros abundance vs. percent organic carbon for DF81-18...... 97
21-. Location of RISP Site J-9 on the Ross Ice Shelf...... 102
22. Clasts ( >500 micrometers) recovered from approximately 100 cc of sediment from RISP 78-16, 72-77 cm...... 106
23. Biostratigraphic ranges of key diatom species in RISP sediments recovered from clast and matrix samples with identification of specific ages ...... 114
24. Biostratigraphic ranges of key diatom species in CIR sediments recovered from clast and matrix samples with identification of specific ages ...... 134
25. Location of Upstream B (UpB) relative to the Ross Ice Streams, RISP and CIR...... 141
26. Schematic range chart for selected diatoms in Upstream B sediment...... :...... 151
27. Benthic foraminiferal oxygen isotope signals that extend to Stage 11, 400,000 years ago, compiled from recent literature ...... 158
28. Coastal oniap curve with interpreted eustatic sea level for the Plio-Pleistocene...... 160
29. Summary of biostratigraphic ages identified in southern Ross Embayment sediments (RISP, CIR, UpB)...... 176
30. Eustatic sea level curve of Haq et al. (1988) with position of Neogene diatom assemblages identified in Ross Embayment sediments...... 178
31. Simplified cartoon outlining the differences between the "Basal Sliding" model and the "Till Deformation" model of ice stream flow ...... 192
xv 32. Provenance Envelope for RISP and CIR sediments...... 201
33. Provenance Envelope for UpB ...... 206
34. Detailed bathymetry of the Ross Embayment ...... 212
35. Stylized stratigraphic cross section of the Ross Embayment showing the West Antarctic Ice Sheet (WAIS) and submarine and sub-ice strata ...... 214
36. Flow chart of field and laboratory procedures for collection and analyses of sediment cores from beneath ice sheets ...... 258
37. Flow chart of some geological analyses that may be performed on Antarctic glacial sediments ...... 260
xvi LIST OF PLATES
PLATE PAGE
I. A comparison of standard and enhanced diatom preparation methods...... 29
II. Typical Chaetoceros spores from Antarctic Peninsula sediments 66
III. Marine diatoms from sediment clasts and matrix sediments recovered by RISP...... 123
IV. Marine diatoms from sediment clasts and matrix sediments recovered by RISP...... 125
V. Marine diatoms, silicoflagellates and ebridians from sediment clasts and matrix sediments recovered at RISP Site J-9 ...... 127
VI. Marine diatoms from Upstream B (UpB) ...... 164
VII. Marine diatoms from Upstream B ...... 166
VIII. Marine diatoms from Upstream B ...... 168
IX. Marine and non-marine diatoms and other microfossils from Upstream B ...... 170
X. Microclasts of diatomaceous non-marine sediment from Upstream B ...... 172
XI. Microfossils from Byrd Station icecore basal debris ...... 209 INTRODUCTION
Very little is known about the geology and geologic history of West Antarctica with regard to tectonic evolution, paleoceanography or ice sheet history. The interior sedimentary basins hold a fundamental archive of this history, particularly for the Cenozoic and, perhaps most significantly, for the Late Neogene. Unfortunately, most of the geologic record of West Antarctica is obscured by as much as 4000 meters of ice. An undisturbed stratigraphic record from within these sedimentary basins can only be recovered by drilling beneath the ice. Evidence that these deposits exist, in the form of recycled microfossils and other sedimentary particles, is found in glacial sediments that lie beneath the current ice sheet.
The modern West Antarctic Ice Sheet (WAIS) is grounded below sea level across most of its bed, but enough ice exists above flotation level to raise global sea level by six meters, should the ice sheet disappear. The debate about WAIS history and its stability has become particularly relevant in recent years, given concerns that anthropogenic greenhouse gasses might be influencing global temperatures and sea levels. Most glaciologists believe that the current configuration of the West Antarctic Ice Sheet would be profoundly affected by even minor variations in sea level and ocean temperature. A relatively small rise in sea level, coupled with bdsal melting of the ice shelf, would lead to a retreat of the grounding line, a decay of the ice shelf, and an eventual disintegration of the ice sheet.
The configuration of the West Antarctic Ice Sheet (WAIS) has been interpreted by some scientists as stable for the last 6 million years, whereas others believe that the
WAIS has a history of highly dynamic behavior, including many cycles of advance and retreat. In fact, many believe that the West Antarctic Ice Sheet collapsed during the penultimate interglacial (oxygen isotope Stage 5.5, 120,000 years ago). Most reconstructions of past WAIS configurations have been based on (1) proxy marine
(Neogene) and ice core (Late Pleistocene) records, (2) glacial geologic evidence (Plio-
Pleistocene), which is overwhelmingly dominated by records of maximum glacial extents and lacks evidence of glacial minima, and (3) ice sheet modelling, which is based on boundary conditions that are defined in part by the equivocal geologic data.
The best way to determine if and when the WAIS has disintegrated in the past is to find direct, datable evidence of open marine conditions in the West Antarctic interior.
Sediments from beneath the ice sheet and ice shelves contain evidence of marine conditions in the form of microfossils, particularly diatoms. Diatoms are single-celled brown algae that form ornate siliceous tests that are preserved in great abundance in
Antarctic sediments. Assemblages of fossil diatoms provide excellent biostratigraphic and paleoenvironmental markers. The occurrence of Pliocene and Pleistocene marine fossils in West Antarctic interior sediments, beneath the ice, would directly demonstrate that marine conditions were present across West Antarctica during certain intervals, thus disproving the "long-term stability" scenarios. However, the apparent absence of fossils of these ages in sediments from beneath the ice sheet would not disprove instability models, due to a myriad of possible taphonomic and stratigraphic complexities.
To date, sediments have been collected from beneath four regions of the WAIS.
The sediments collected are all glacial diamictons that contain a mix of particles that were eroded from distinct sedimentary, igneous and metamorphic source rocks. Fossil diatoms and other microfossils within these sediments provide an opportunity to evaluate the ages and environments of Cenozoic marine and non-marine sedimentary source rocks. This study reports the findings of micropaleontologic analyses of these sediments.
Biostratigraphic analyses of these sediments provide a discontinuous record of marine productivity events in the West Antarctic interior. The fossils were initially deposited during deglacial or preglacial conditions in West Antarctica. By contrast, marine fossils were not deposited in the West Antarctic interior during periods of extensive glaciation, due to ice cover. Paleoenvironmental conditions in the West Antarctic basins during certain Cenozoic deglacial intervals are interpreted here, based on quantitative and qualitative micropaleontologic observations and comparison with quantitative analysis of microfossils in modern Antarctic marine basins. Figure 1 is a map of Antarctica identifying regions discussed in the text. Figure 1. Map of East and West Antarctica with identification of the Ross Embayment and the AnWctic Peninsula.
4 5
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-i- i * ' S * o'e
Figure 1 ^ CHAPTER I
THE WEST ANTARCTIC ICE SHEET AND ’GLOBAL CHANGE’
INTRODUCTION
Glacial and glacial-marine sediments in and around West Antarctica contain a wealth of paleoenvironmental and paleoglaciologic information, particularly in the form of recycled microfossil assemblages (Webb, 1990). Recycled microfossils provide a window into the geologic history of vast regions that remain covered by ice today (Harwood,
Scherer and Webb, 1989; Harwood, 1991; Webb et al., 1984; Webb, 1990; Truswell and
Drewry, 1984). Much of the information archived in these sediments has relevance to some of the most pertinent questions in Antarctic and global earth science, particularly with regard to Tertiary, Quaternary and Holocene sea levels and the history and dynamics of ice sheets.
New results from high resolution paleoclimatic records indicate that major cold/warm transitions can occur on time scales as short as decades-to-centuries
(Fairbanks, 1990; Dansgaard et al., 1989; Thompson et al., 1989) and that rapid ice decay is a significant factor in these transitions (Miller and Kaufman, 1990; Teller, 1990;
Fairbanks, 1990; Lambeck, 1990; Nakada and Lambeck, 1988). West Antarctica hosts the last remaining large marine ice sheet. The configuration of this ice sheet makes it inherently unstable (Weertman, 1974; Hughes, 1977; Mercer, 1978; Bindschadler, 1990;
1991; Alley and Whillans, 1991). The bed of the ice sheet is well below sea level, typically 500m to 600m below sea level. However, the bed deepens toward the center of the ice sheet, reaching more than 2500m in parts of the Byrd subglacial basin and the
Bentley Trough. Ice thicknesses over these basins exceeds 4500m (Drewry, 1984).
Consequently, retreat of the grounding line along the continental shelf will tend to accelerate as depth to the grounding line increases. Once a threshold is reached, positive feedback from rising sea levels can lead to catastrophic collapse of the WAIS (Weertman,
1974; Hughes, 1977; Mercer, 1978; Thomas and Bentley, 1978; Thomas et al., 1979;
Engelhardt et al., 1990).
The hypothesized scenario of rapid collapse of this marine ice sheet, along with coincident eustatic sea level rise, has been widely discussed in governmental and media forums as well as among the scientific community. Several large-scale scientific initiatives have been proposed for the study of global climate change. Programs such as the
International Geosphere-Biosphere Project (IGBP) and the WAIS Initiative have outlined a framework for study of the stability and sensitivity of the global environment. The West
Antarctic Ice Sheet plays a central role in these initiatives (Scientific Committee on
Antarctic Research (SCAR) Steering Committee for IGBP, 1989; Bindschadler (West
Antarctic Ice Sheet Initiative), 1991; Ice Core Working Group, 1989; National Research
Council Committee on Glaciology, 1984, 1985).
With more than 98 percent of Antarctica covered by ice, much of the geology and geologic history of the southern continent remains a mystery. While East Antarctica is a vast continental craton, West Antarctica is a complex mosaic of tectonically active crustal blocks (Fig. 2; Storey, 1991), and is characterized by mountainous regions and deep, ice- filled basins and troughs (Dalziel and Grunow, 1982; Grunow et al., 1987). In addition to the four main West Antarctic crustal blocks [(1) Marie Byrd Land, (2) Antarctic Peninsula- eastern Ellsworth Land, (3) Ellsworth Mountains-Whitmore Mountains and Hagg Nunataks, Figure 2. West Antarctic crustal blocks: Marie Byrd Land (MBL), Antarctic Peninsula - eastern Ellsworth Land (AP), Ellsworth Mountains - Whitmore Mountains and Hagg
Nunataks (EWM, HN)), Thurston Island - Western Ellsworth Land (Tl) and major bathymetric features: Ross Sea Embayment (RSE), Weddell Sea Embayment (WSE), Byrd
Subglacial Basin (BSB) and Bentley Subglacial Trough (BST). The 500 meter bathymetric contour is shown (from Storey, 1991).
8 9
«/£ V-WEDDELL
ANTARCTICA
a
-S O O m ■ ■ ■ ■ ■ -1500 m -2000 m —I 2 f * o a *.
Figure 2 10
and (4) Western Ellsworth Land-Thurston Island], there are likely to be several additional
small ice-covered blocks that have lithologies and orientations distinct from those of the
larger blocks (Storey, 1991; Grunow et al., 1987). In the absence of an ice sheet, West
Antarctica would be covered by a broad open sea, punctuated by islands (Fig. 3). These
open seaways would provide fertile habitats for seasonal plankton productivity (Harwood,
1991; Harwood, Scherer and Webb, 1989; Webb, 1979, Taylor, 1914).
WEST ANTARCTIC ICE SHEET EVOLUTION
Many glaciologists have long considered the West Antarctic Ice Sheet (WAIS) to
be inherently unstable. They believe that because it is a marine ice sheet (grounded well
below sea level), a relatively small rise in sea level (eustatic or regional), or increased
basal melting, could lead to break-out of the buttressing ice shelves and rapid grounding-
line retreat. According to this scenario, once a threshold is reached, catastrophic ice
sheet "collapse" is possible (Bindschadler, 1991; Lingle et al., 1991; Mercer, 1978;
Hughes, 1977), which, given the current configuration, would raise global sea level by as
much as 6 meters (Alley and Whillans, 1991).
Looking to the geologic record of Pleistocene eustatic sea level changes, Mercer
(1978) suggested that the WAIS collapsed during the last peak interglacial (oxygen
isotope Stage 5e) (Fig. 3). There is abundant evidence of moderately higher eustatic sea
levels during the penultimate interglacial, ~120,000 years ago. Minimum estimates of sea
level elevation range from more than 2m to considerably more than 6m (Matthews, 1990).
The most often quoted estimate is 6m (Matthews, 1990), which is approximately Figure 3. Configuration of the West Antarctic Ice Sheet, (a) current configuration, (b) with
WAIS in retreat (redrawn from Mercer, 1978).
11 ^ ice Shelves % Grounded Ice km i------1 0 1,000 13 equivalent to the amount of water sequestered in the WAIS above flotation level (i.e., disintegration of the WAIS would raise sea levels by 6m).
In contrast, many paleoceanographers, basing their concepts largely on their interpretation of oxygen isotopic records from deep sea sediments with low accumulation rates, have traditionally considered the Antarctic ice sheets to have been relatively stable through the Late Neogene. For example, Kennett (1977) and Kennett and Barker (1990) argue that the West Antarctic Ice Sheet formed in latest Miocene time (~6 Ma) and has not retreated significantly beyond its current borders since that time. This view of ice sheet stability has not been universally accepted by the paleoceanographic community
(e.g., Abelmann et al., 1990). These contrasting interpretations of West Antarctic Ice
Sheet history are based on data from proxy sedimentary records, such as sea level records from non-polar regions and oxygen isotope stratgraphy of deep sea sediments.
Proxy data suffer from two major limitations. (1) Neither sea level nor oxygen isotope records can be calibrated to a universal, time-independent standard, due to a myriad of possible local effects. (2) While proxy records may contain a global signal, they cannot provide any indication of the source of the changes that controlled the signal. For example, a six meter change in eustatic sea level might be the result of ice volume loss unrelated to the WAIS, such as in Greenland (Koerner, 1989), or ocean basin variation due to tectonic or isostatic effects. 14
ICE SHEET MODELLING
In addition to suggesting that the WAIS may have collapsed during the last interglacial period, ~ 120,000 years ago (oxygen isotope Substage 5e), Mercer (1978) discussed the potential impact of increasing atmospheric carbon dioxide in the post industrial (i.e., current) interglacial on stability of this marine ice sheet. Mercer (1978) recognized the growing importance of computer models for predicting the response of the
WAIS to climate change. While there have been great advances in General Circulation
Models since Mercer’s classic paper was published, most current models for development of the Antarctic ice sheets (e.g., Lindstrom and MacAyeal, 1987; Oglesby, 1989;
Huybrechts, 1990) fail to reproduce large scale ice sheet fluctuations through time, despite geologic evidence suggesting that there have been major ice sheet advances and retreats.
This descrepancy suggests that the models still need considerable refinement (Elliot et al., in press). Computer simulations of ice sheet evolution tend to rapidly build large ice sheets that become stable. The failure of these models is largely due to insufficient consideration of ice sheet dynamics, particularly basal processes.
A wealth of data is available marking maximum extents of the West Antarctic ice sheets (e.g., Cooper et al., 1991; Melles, 1991; Denton and Hughes, 1981; Kellogg and
Truesdale, 1979). However, there is a distinct lack of direct evidence regarding previous interglacial ice minima, due to the presence of the Recent ice sheet. Because of this inequity in data quality, a bias may have developed among glacial geologists and ice sheet modellers emphasizing the build-up of ice. Another common interpretive bias comes from our inherent tendency to artificially view the modern world as some kind of
"end member," rather than as part of a larger continuum (Gould, 1991). For this reason, 15
it has become common to view the current configuration of the WAIS as an "interglacial"
configuration, rather than as, for example, a "transitional" configuration. Many
glaciologists now believe that the complex internal dynamics and current instability of the
WAIS can best be explained as a result of an on-going process of deglaciation that began
at the end of the Wisconsin glaciation (Alley and Whillans, 1991).
In order to more accurately predict the future behavior of the WAIS (under both
"greenhouse" and non "greenhouse" scenarios), robust geologic constraints on past ice
sheet variability must be established. These data will provide the fundamental boundary
conditions for realistic modelling. This requires greater understanding of ice sheet growth
and decay, especially for the Pleistocene cycles.
In addition to oceanographic and climatic forcing that can be modelled, the effects
of "wild cards" such as sub-ice volcanism must also be considered. The presence of
active volcanism and tectonism in West Antarctica is well known, though the extent,
history and potential effects on the ice sheet are not known (Behrendt and Cooper, 1991;
McIntosh and Gamble, 1991; LeMasurier and Thompson, 1990).
THE ROSS ICE STREAMS
The Ross Ice Shelf is fed by a series of ice streams that flow from the interior of
West Antarctica toward the Ross Sea (Fig. 4). The ice streams, which are fast-moving glaciers bounded by stagnant or slow-moving grounded ice, may be the key to marine ice sheet stability and its response to rapid climate change (Engelhardt et al., 1990; Lingle et al., 1991; Bindschadler, 1990; 1991). Ice Stream B in the Siple Coast region of the Figure 4. The Ross ice streams, Crary Ice Rise and Site J-9 of the Ross Ice Shelf Project
(from Shabtaie et al., 1987).
16 Figure 4 18 southern Ross Embayment is highly dynamic and currently flows at rates as high as 800 meters per year (Alley and Whillans, 1991).
It was recognized only recently that the sediments beneath grounded ice may themselves be a major controlling factor on rapid ice stream flow (Alley and Whillans,
1991; Alley, 1989; Alley et al., 1986, 1989a; Engelhardt et al., 1990). A thin layer of water-saturated till that lies between the ice and underlying in situ sediments may aid ice stream flow by acting as a lubricant. According to Alley et al. (1986, 1989a), the till deforms as the ice flows, eroding the sediments below and transporting particles toward the grounding line, where a "till delta" is formed (Fig. 5). A contrasting theory describes basal sliding over the water-saturated till layer as the main process responsible for rapid ice stream flow (Engelhardt et al., 1990; Kamb, 1990; 1991; 1987). The till layer has been identified by seismic reflection and appears to be widespread beneath ice stream B
(Blankenship et al., 1986; Rooney et al., 1987). Beneath this thin layer lies some 600 m of poorly consolidated sediments within a down-faulted basin (Rooney et al., 1991).
These sedimentary successions contain a fundamental archive of the Cenozoic history of
West Antarctica and the West Antarctic Ice Sheet. Seismic investigations on the Antarctic continental shelf have identified relict structures that have been interpreted as preserved
"till deltas," deposited during previous expansions of the WAIS (Anderson, 1991). Figure 5. The active (deforming) till/till delta model of Alley et al. (1989a)
19 20
COUPLING GROUNDING ICE LINE LINE FRONT
CE STREAM DELTA I ICE SHELF | SEA WATER
I I
'/•Ac T iV e :-.V-.T il l ’v T y V I .w ater
f ,l m . . . w .------^AFORESETS MAX/THICKNESS '\KL V v v \ \ \ OF ACTIVE TILL BOTTOMSET
BEDROCK
Figure 5 21
MICROPALEONTOLOGIC ANALYSES OF SUB-ICE SEDIMENTS
Break-up of the buttressing ice shelves and subsequent collapse of the marine ice sheet would create open marine conditions in the West Antarctic interior. The interior seaways would provide a fertile habitat for seasonal diatom productivity (Harwood, 1991), which would lead to rapid accumulation of diatom fossils on the floor of the interior basins
(Harwood, Scherer and Webb, 1989). As ice once again became grounded and advanced across the basin, the diatomaceous deposits would become eroded and mixed.
Consequently, a till sample collected from beneath the ice sheet may contain a diatom assemblage that includes fossils of widely differing ages. It is often possible to "unmix" these distinct assemblages by identifying associated taxa. Diatom-rich sediment clasts, within a diamicton matrix, provide unaltered views of distinct sedimentary units, and can be used to evaluate the ages and the paleoceanographic settings at the time of initial deposition of the clasts (prior to glacial erosion). Each distinct age represents at least one interval of biologic productivity over the basin. The youngest fossils present may represent marine productivity during the time following the most recent ice sheet collapse.
Utilizing a micropaleontologic approach, based on the premise outlined above, this dissertation addresses basic questions regarding the history of the West Antarctic Ice
Sheet, with particular regard to ice sheet minima. Sediments from beneath the West
Antarctic Ice Sheet/Ice Stream/Ice Shelf system are currently available from four regions south of the West Antarctic ice barrier. This study reports on quantitative and qualitative micropaleontologic analysis of all of these sites. From these microfossil data, it is possible to (1) identify the ages of certain ice minima events, based on biostratigraphic assignments, (2) evaluate the paleoenvironmental conditions that existed in West
Antarctica during those events, (3) evaluate sediment provenance and sediment transport mechanisms and (4) gain insights into modern and relict glacial depositional settings, as influenced by ice sheet configuration. Detailed analysis of diatoms in Holocene and
Recent sediment from northern Antarctic Peninsula basins provides a paleoceanographic model for the interpretation of paleoproductivity in West Antarctic interior basins during
Neogene de-glacial phases.
Although samples from beneath the WAIS are currently available only from very few regions, new drilling technologies have made the penetration of thick ice a fast and relatively simple endeavor. Thus, sample recovery from beneath the WAIS is expected to improve in the upcoming seasons. As more sites become available, a more complete picture of sub-ice-shelf sediments and West Antarctic glacial history will begin to emerge.
It is hoped that this work will stimulate productive dialogues between geologists and glaciologists regarding the history and stability of the West Antarctic Ice Sheet, by comparing geologic observations with ice stream and ice sheet models. CHAPTER II
METHODS: NEW APPROACHES TO ANTARCTIC
DIATOM ANALYSES
DIATOMS IN GLACIAL DIAMICTONS: SCIENCE FROM THE TRASH HEAP
A revolution in our understanding of Antarctic ice sheet history was spawned when
David Harwood (1986a) found diatoms of Pliocene age in sediments of the Sirius Group of diamictites along the Transantarctic Mountains of East Antarctica. Harwood recovered diatoms from samples of tillite that had been collected many years earlier by John Mercer of the Byrd Polar Research Center (then Institute of Polar Studies), The Ohio State
University. Recovering diatoms and other microfossils from this sediment was a challenge, due to their high clay content and low microfossil abundance. Harwood explored many different methods of concentrating diatoms for study. He was initially criticized by other diatom workers, who consistently use traditional methods in the study of diatom assemblages, because his methods "bias the assemblage." Harwood’s response to this criticism is that without these methods of enhanced diatom extraction, the assemblage recovered would consist only of rare fragments and very rare whole individuals. This would allow little or no statistical or biostratigraphic reliability. In fact, glacial reworking had already imparted a far greater bias on the assemblage than laboratory methods could (assuming no laboratory contamination). Harwood (1986a) successfully employed a variety of methods to extract diatoms from tills. These are
23 24 described by Harwood et al. (1986). Some of the methods used by Harwood (1986a, b) are adapted here for extracting microfossils from sediments from beneath the West
Antarctic Ice Sheet. The techniques used here are more standardized than the varied experimental approaches used by Harwood.
Stratigraphic integrity of the original diatomaceous deposits within the interior basins is destroyed by the erosion, mixing and transport caused by grounded glacial ice.
Therefore, microfossil assemblages in tills cannot provide the time-series paleoenvironmental data that are found in microfossil assemblages of in situ sediments.
Instead, microfossils in glacial sediments record episodes of biologic productivity that are punctuated by periods of no productivity. In glacial sediments, productivity events, as recorded by the occurrence of microfossils, represent times of reduced glaciation. In an ideal example, periods not represented by fossils indicate the presence of glacial conditions. Naturally the actual situation is considerably more complicated. The lack of microfossils of a certain age in a glacial sediment sample can be the result of a multitude of factors other than the presence of permanent ice, such as prior erosion, non preservation or low primary productivity.
Given the fact that the microfossil assemblages are stratigraphically mixed, age control from biostratigraphic zonations often must be based on total biostratigraphic range of each fossil, because the stratigraphic integrity of overlapping partial-range zones has been lost. Certain assumptions with regard to the application of zonal schemes are frequently necessary. Antarctic diatom biostratigraphy has recently been greatly improved as a result of Ocean Drilling Program (ODP) Legs 113,114,119 and 120 in the Southern
Ocean. Most of these drill core records are calibrated to an absolute time scale, allowing better definition of isochronous biotic events than that provided by previously available 25
records from Deep Sea Drilling Project (DSDP) and inshore and onshore Antarctic
stratigraphic drilling programs (Cenozoic Investigations of the Western Ross Sea (CIROS),
McMurdo Sound Sediment and Tectonic Studies (MSSTS) and the Dry Valley Drilling
Project (DVDP)). Given diatom biostratigraphic resolution on the order of a few hundred
thousand years (Harwood and Maruyama, in press) for Neogene Antarctic diatom
biostratigraphy, it is often possible to identify specific intervals when open marine
conditions existed in the Antarctic interior, based on recycled diatom assemblages in
glacial sediments.
Biostratigraphic resolution from reworked sediments can be greatly improved when
microfossil assemblages can be unequivocally identified. The best way to recognize co
occurring species from within glacially reworked sediments is to separate diatom-rich
sediment clasts from the diamicton matrix (Harwood, Scherer and Webb, 1989). These
sedimentary clasts represent a sample of a primary deposit that was eroded by glacial ice.
The clasts become entrained in the till and are transported by glacial processes.
Because diatomite is an easily erodible sediment, clasts would degrade quickly under the
rigors of wet-based ice stream basal processes. Therefore, the presence of poorly
indurated diatomite clasts in a till may suggest proximity to source beds (Chapter VIII). 26
DRAWING A DISTINCTION BETWEEN METHODS IN BIOSTRATIGRAPHY AND
PALEOCEANOGRAPHY
A clear distinction must be drawn between methods employed in biostratigraphic
studies and methods of paleoceanographic reconstruction. Quantitative methods
frequently used in paleoceanographic studies often do not provide sufficient information
to adequately assign a biostratigraphic zonation. For example, quantitative analysis of
diatoms typically includes, by convention, total assemblage counts of 300 to 500 valves.
However, the essence of biostratigraphy is the confirmed presence or absence of fossils
with weli-constrained biostratigraphic ranges, regardless of their specific abundances.
Acme zones are more susceptible to diachroneity and environmental factors than total
ranges (Barron, 1985). Furthermore, important biostratigraphic markers may be rare in
the sediments studied. This is particularly true for regions with physical oceanographic
boundaries and endemic plankton populations, such as the Antarctic. For these reasons,
assemblage counts of a limited number of individuals may not reflect the occurrence of
important taxa. Statistically speaking, these taxa contribute an insignificant component
to the population, unless data reduction techniques are sophisticated enough to "weight" biostratigraphically significant taxa within the data matrix. However, such a computer program would incorporate the bias of the programmer, and thus would lose much of the desired statistical objectivity. It is the opinion of this author that only under special circumstances can statistical analyses outweigh a priori interpretations of biochronostratigraphic data. 27
If biostratigraphy is the goal, the best approach is to view as many specimens as
possible in the available time, and compile a complete species list, including the rare taxa.
Qualitative (descriptive) relative abundances (e.g., abundant, common, few, rare, trace)
based on observation of, perhaps, thousands of specimens are more likely to provide
accurate ages than limiting the examination to "total" assemblage counts, based on an
arbitrary counting limit. Relative descriptive abundances should, however, be calibrated
to certain internal standards, such as number of individuals per microscope field of view
or transect, such as that described by Scherer (1991) for radiolarians. This qualitative
approach allows greater latitude in the laboratory methods employed. Enhancing the
recovery of diatoms from fossil-poor material by sieving or flotation methods (described
below) should be considered permissible when more standard methods produce less than
satisfactory results.
However, if the goal of the study is to trace oceanographic changes over short time
scales using diatoms, total assemblage counts provide the best approach. "Absolute'
abundance" counts (number of fossil specimens per unit mass or volume of sediment) in
conjunction with total assemblage counts show not only proportional relationships between
species in the assemblage, but also quantitatively assess the actual concentration of
fossils in the sediment, relative to other sedimentary constituents. Finally, interpretations
of oceanographic conditions from diatom assemblages in sediments are best constrained when modern analogs are available.
The following sections describe the two principal methods used during the course
of this study. A biostratigraphic approach was initially employed on sediments containing diatoms of unknown ages. A quantitative paleoceanographic approach was used to 28 identify subtle differences between diatom assemblages. These differences may be caused by biologic, taphonomic or sedimentologic effects.
METHODS FOR THE EXTRACTION OF DIATOMS FROM GLACIAL SEDIMENTS
BIOSTRATIGRAPHIC GOALS
In this section I discuss preparation and counting methods used for biostratigraphic analysis of fossil-poor material. The various methods employed to enhance the recovery of diatoms from glacial diamictons are described. Standard methods, not employing sieves, are used whenever possible. Samples from Ross Ice Shelf Project Site J-9 (RISP)
(Plate I) [Chapter IV] and Crary Ice Rise (CIR) [Chapter V] are relatively rich in diatoms.
However, the diatoms present tend to be highly fragmented and are diluted with terrigenous clays, silts, sands and gravel. Sediments from beneath ice stream B (UpB)
[Chapter VI] are very diatom-poor, with evidence of extensive mechanical degradation of most microfossils, as well as dilution by a large volume of terrigenous debris. These sediments were processed for biostratigraphic assessment as described below.
Washing
A sample, about 3cc in volume, is placed in a 1000ml beaker and disaggregated with 30% H20 2, which oxidizes organics. Reaction time can be reduced by gently PLATE I.
A comparison of standard smear slide preparations and diatom preparations using an enhanced diatom extraction method. The sediment used include a diatom-rich diamicton matrix (left figures 1,3) and lower Miocene diatomite clasts (right figures 2,4) from Ross
Ice Shelf Project (RISP) Site J-9. Uppermost micrographs (Figs. 1, 2) are smear slides of raw sediment preparations. Bottom micrographs (Figs. 3, 4) show preparation of the above sediments that were collected in a 25 micrometer sieve.
All figures are at magnification 250X.
1. Raw sediment preparation, matrix, Ross Ice Shelf Project (RISP) 78-12,
117-118 cm.
2. Raw sediment preparation, lower Miocene diatomite clast, RISP 78-16,
29-34 cm.
3. Sieved preparation ( >25^ size), matrix, RISP 78-16, 0-5 cm.
4. Sieved preparation ( >25p size), lower Miocene diatomite clast, RISP
78-16, 29-34 cm. 250.
29 30
PLATE I heating the beakers in a water bath. When the reaction is complete and the sample completely disaggregated, the beaker is filled with distilled water to dilute the peroxide.
After at least 1 hour, all coarse silt and sand-sized particles will have settled to the bottom.
The clay-rich suspension is poured into a 4 liter beaker and set aside for later analysis.
The beaker containing the coarser residue is refilled with distilled water and particles are allowed to settle for 30 seconds. Particles still suspended are transferred into another beaker, leaving gravel, sand and coarse silt behind. The coarse residue is then resuspended with a wash bottle and the finest particles are rinsed into the beaker containing the finer fraction. This second washing step insures that diatoms are not retained with the coarse fraction. The beaker containing the silt and clay fraction is allowed to stand for 1 hour and the clays are decanted. This step is repeated at least 5 times until particles in the size fraction ~10|nm to ~100|im strongly dominate. To prepare permanent slides, the residues described above are allowed to settle out completely.
Particles are drawn up from the bottom of the beaker using a disposable pipette and distributed on a cover slip and slowly dried on a hot plate. Diatoms are mounted in a high refractive index medium (e.g., Hyrax, Cumar, Naphrax, Norland adhesive, etc.). The residue on these slides will include many diatom fragments and fine sand-sized grains, as well as whole diatoms, but systematic bias with regard to the diatom assemblage will be minimal (Barron, 1985).
If slides prepared in this manner are still too diatom-poor to allow adequate documentation of the assemblage, further steps are necessary. The procedures described below undoubtedly impart a bias with regard to species relative abundances, but at least the major fossil components are documented. The recovery of a usable diatom assemblage from a diatom-poor sediment more than justifies the small bias associated 32
with these methods. Systematic bias is toward the larger centric diatoms, i.e., those with
a maximum of proportions and surface area. This bias does not preclude biostratigraphic
interpretation, because many of the key biostratigraphic marker taxa for Antarctic
sediments are large centric diatoms. Smaller diatoms are still present, though their actual
numbers may be somewhat lower than in the original sample. Slides prepared with the
decantation method described above will have a fair representation of the smaller taxa.
Enhanced extraction methods employ fine sieves and/or heavy liquid flotation. The
techniques described below provide a reasonable view of the diatom assemblages within
generally diatom-poor diamictons.
Sieving
Decanted residues described above may be sieved with a very fine sieve (e.g.,
20|i mesh), which will remove most clays, fine silt grains and unidentifiable diatom fragments, retaining most diatoms. A significant number of long, thin pennates will be trapped in the mesh, though many will be carried through the sieve. The fine fraction is
retained, and slides are prepared and evaluated for diatoms that were washed through the sieve. Much of the remaining sand can be removed by repeating the differential settling method described above. Harwood, Scherer and Webb (1989) used this method, employing a 25j i sieve, in a detailed biostratigraphic study of sediments from beneath the
Ross Ice Shelf (Ross Ice Shelf Project, Site J-9). Plate I shows a direct comparison of 33
slides prepared using standard "smear slide” methods (as employed by Kellogg and
Kellogg, 1981,1986 for RISP sediments) and the sieving method described here.
Sieves with mesh sizes as small as 5p.m are available, which would remove clay
and tiny diatom fragments without biasing the assemblage in a significant way.
Unfortunately these sieves are delicate and extremely expensive. Naturally, sieves, as
well as all glassware, must be carefully and thoroughly cleaned between samples to avoid
contamination. With care, contamination with foreign microfossils is easily avoided. Some
methods to avoid sample contamination include dissolution of diatom silica in a basic
solution (i.e., NaC03 with a pH >10) and staining of particles stuck in the sieves with
Methyl Blue, so that contaminating particles can be identified and discounted.
Heavy liquid flotation
Heavy liquid flotation using sodium polytungstate is an excellent method for
separating diatoms and microclasts of diatom-rich sediment, as well as foraminifera, plant
fragments and palynomorphs from diamictons. Diatom silica has a specific gravity of
about 2.0 - 2.25 gm/cm3 (depending on the degree of diagenetic crystallinity), compared
with 2.65 gm/cm3 for crystalline quartz. Sodium polytungstate is a non-toxic compound
that is highly soluble in water. It is available commercially and can be prepared with
specific gravities of as much as 2.9 gm/cm3 (Callahan, 1987). Diluted to a specific gravity
of 2.4 gm/cm3, the liquid will float diatoms, foraminifers, plant debris and low-density
sedimentary clasts. Quartz and other minerals will sink. Mica flakes tend to float due to their high surface area and air trapped between crystal faces. The light residues are
removed from the settling chamber (100 ml graduated cylinder) by pipette, washed with water and mounted for microscopy. As with the sieving method, small diatoms are often 34 not present in representative proportion. This may be caused by adherence of small diatoms to sinking particles.
Diatom Identification and Biostratiaraphv
Taxonomic identification of diatoms is based on evaluation of original references whenever possible, as well as the generally accepted species concepts of modern diatom studies, which include studies of DSDP and ODP material. Diatom biostratigraphic assignments are based on published biostratigraphic ranges from in situ reference sections of DSDP and ODP legs as well as other biostratigraphic studies.
Given their great abundance, very high diversity, excellent preservation and rapid evolution, fossil diatoms provide the best tool available for high-resolution biostratigraphy in in situ Neogene Antarctic stratigraphic successions. However, the biostratigraphic and paleobiogeographic distribution of Antarctic microfossil species is complex, frequently characterized by discontinuity, reworking and possible diachroneity. Given these potential difficulties, and the limited number of drillholes on the Antarctic continental shelf at this time, biostratigraphic interpretations must frequently be considered mutable.
For this study, in the absence of a complete local Ross Sea reference section, all available high southern latitude diatom records were used for biostratigraphic interpretations. Recent Southern Ocean drilling by the Ocean Drilling Program (Legs 113,
114,119 and 120) has greatly refined the diatom biostratigraphy established from earlier
Deep Sea Drilling Project Antarctic Legs 28 and 35. 35
Neogene Antarctic diatom biostratigraphy is very well established for nearly all of the Miocene and much of the Pliocene. The greatest difficulties remain for the late
Pliocene and early Pleistocene. These problems are dominantly stratigraphic. The intensive glacial cycles of the last 2.5 million years have led to extensive erosion of continental shelf sediments, and mixing of fossil populations. Furthermore, the intensification of climatic gradients associated with global cooling may have increased provincialism and endemism in Antarctic diatom populations, which could further complicate biostratigraphic interpretations (Harwood, 1991). Sites with a nearly continuous Upper Neogene marine sedimentary record probably do exist, such as in the deep Bransfield Basin, but these sites have yet to be drilled.
Diatom zones referred to are from Harwood and Maruyama (in press) from ODP
Leg 120 (Fig. 6). Numerical ages presented and discussed for diatom ranges are from
Harwood and Maruyama’s (in press) summary of all published Antarctic diatom ranges.
Other diatom biostratigraphic references are cited when appropriate. The numerical time scale of Leg 120 was tied to the chronostratigraphy of Berggren et al. (1985).
Correlations of diatom assemblages with the eustatic curve of Haq et al. (1988) are based on the numerical ages, although correlation between the Berggren et al. (1985) time scale and the Haq et al. (1988) curve is not exact for certain intervals. Chronostratigraphic and lithostratigraphic terminology follows Ocean Drilling Program editorial conventions. Figure 6. Diatom zonation of Harwood and Maruyama (in press), based on diatom biostratigraphy of ODP Leg 120, which incorporates data from Legs 113,114,119 and other Southern Ocean Antarctic drill holes. Also see Appendix B.
36 Ma DIATOM ZONES DATUMS LAO T.Jentgrea t.mugmosa" U D A. ngtm A ingens LAO H tm ri 2 — tl. keroueterisiiT U O T.UHi •^t.ntnsca LAO r.nM a UO T.inisia T. insigna - T. vuiniUca LAD At x m r i ~ N. inlertmidana~ FAO r . n M a N. barren KB ftS®"* FAO Htamti i.inura FAAO T.nn I. oestmvu FAO r.MStvpi n . reinnotdiT U O N.un&H.lrixvMs 6 — FAO N.mtiokM HovaHs
FAD F t. o r a tfs a. 8 - a. T. lorokina => FAD T, lorohm A kennettii FAO A kennetlil u. nusteatB IQ- IAAD o . dimoipha 0. tSmotpha LAD M denticuloides ?. praediwiorp/H -fl. denfcufoicfe Z 1 2 — U i ~"D: FAD 0 . dimcrpha O FAD 0. praedunorpha O N. dentKUkxdes FAD Ft. denticuloides D. hustedti-N. grossepunctata 14 — A mens vsr. nodus FAO D.hustedtS FAD A ingens vsr. nodus N. grossepunctata 16 . A moens-1). maccoiiumi FAD N. grossepunctata I), maccoauma FAAD A rn g e n s FAD D.maccaHumS C.kanayae 18 FAD C.kanayae LAD r. Frags a : T. Iraga u 20 LAD a symmetrica FAD r . Iraga T. spumelleroides LAD A gombosi 22 - FAD r . spumeieroides
24 - LAD Lomata R .g e lid a
26 - a . LAD R. vigilans a . 3 Fad R.gelid3 UJ 28 - z L omata U i o FAD Lomata o A gombosi o ■ 30 — FAD A gombosi LAD A vigilans (small) f t vigilans
3 2 - FAD A vigilans (small) d. rouseana FAD S.jouseana LAD A oligocaenca 34 - R. odgocaenica b LAD T. polymotphus
FAD A oligocaenca QA_ UNZQKD
Figure 6 38
A NEW METHOD FOR THE DETERMINATION OF ABSOLUTE DIATOM ABUNDANCE
IN SEDIMENTS FOR PALEOECOLOGIC STUDIES
Introduction
The advantages of estimating the number of microfossils per unit mass or volume of sediment in micropaleontologic studies has been recognized for many years. For example, CLIMAP project members recognized that "any paleoclimatic signal based on a sedimentary component not referenced to total sediment weight is susceptible to translational offsets and other complications" (CLIMAP, 1984, p. 133).
The establishment of "absolute abundances" can put a study of a particular microfossil group within a realistic sedimentologic context, which can greatly aid in the interpretation of microfossil results. Traditionally, absolute phytoplankton fossil abundance has been considered reflective of surface water productivity. While this is generally a reasonable interpretation, other factors, especially dilution by sedimentary particles and variations in fossil preservation, affect microfossil concentration and assemblage composition in the sediments. Biostratinometric or taphonomic effects result from processes such as sediment mixing due to bioturbation, bottom current activity, variable fluvial input, advection within the water column or preferential microfossil dissolution. Because microfossil assemblages are strongly influenced by the local sedimentary regime, evaluation of any microfossil record should include reference to potential sedimentary 39 effects. The determination of absolute abundances is an important step toward interpretation of corresponding taxonomic data. Furthermore, the use of absolute abundances allows a direct comparison of taxonomic and abundance data from different microfossil groups. As a result, the resolution of micropaleontologic data can be improved, refining paleoecologic and biostratigraphic interpretations.
Although numerous methods of establishing absolute abundances have been described for various microfossil groups and sediment types, absolute abundances are rarely employed routinely in micropaleontologic studies other than palynology. While larger, less abundant microfossils such as foraminifera or radiolaria may be separated from the remainder of the sediment by sieving or flotation techniques, small and abundant microfossils, such as diatoms or coccoliths, must be processed as bulk sediment. The use of separation methods in diatom processing is generally unacceptable for quantitative paleoecologic studies, due to the small mean size and the wide ranges of size and shape typical of diatom assemblages. High microfossil abundances in richly biogenic sediments make total aliquot counts of diatoms or coccoliths impractical, even when very small samples are examined.
Previous methods
Several methods to determine absolute abundances of diatoms have been described for both marine and lacustrine sediments. Most methods utilize subsampling of a larger volume of chemically disaggregated material, by pipetting a regulated volume from a "well mixed" suspension. These methods are based on the assumption that the aliquot accurately represents the total of the processed material, in terms of volume and species 40
composition. Absolute abundances are estimated from these aliquots by a variety of slide
preparation and counting techniques.
Schrader and Gersonde (1978) used standardized traverses across a pipetted strewn
slide, made from a 0.1-0.2 ml aliquot from a 50 ml suspension, to estimate the number
of marine diatoms in the sample. They acknowledged that the distribution of particles on
the slide, using the pipetting method, was non-random, occasionally resulting in significant
error. Laws (1983) confirmed the unreliability of this method. Pokrasand Molfino (1986)
extracted a 10 ml aliquot from a 50 ml suspension of washed marine diatoms and made
slides with a (presumably) random distribution of particles by a filtration method. The filter
was mounted on a microscope slide and a known area was counted. These methods
were used for diatom abundance-per-mass estimates. Using a 25 ml aliquot from a 2,000
ml suspension, Battarbee (1973) estimated diatom abundance-per-volume in lacustrine
sediments by settling suspended diatoms onto a cover slip in a shallow evaporation tray,
and counting a known area on the cover slip.
Subsampling from an aqueous suspension, as used in the methods described above,
can bias determinations of both total microfossil numbers and species composition. The wide range in size and shape of diatom populations makes consistent preparation of a
uniformly well mixed suspension unlikely.
Another method of establishing diatoms-per-volume adopts a technique frequently used in pollen studies (Benninghoff, 1962; Jorgensen, 1967; Maher, 1981). The diatom sample is "spiked" with a known concentration of foreign plant spores, offering a frame of reference for comparison with diatom counts (Stabell and Henningsmoen, 1981; Kaland and Stabell, 1981). Because plant spores are typically larger than most diatoms, and of different density, they have different hydrodynamic properties. This may amplify the 41
problems of aliquot extraction and slide preparation. In addition, spores obscure part of
the slide, which can adversely affect diatom counts. Finally, certain spore tablets are not
compatible with aqueous preparation techniques employed in diatom preparations.
New Method
Described here is a simple and accurate method of establishing absolute abundances
of microfossils in biogenic-rich sediments. The method can also be easily adapted for
absolute abundance analysis of diverse microfossil groups or abiogenic sedimentary
particles in the fine-sand to fine-silt size range. The method is best suited to
abundance-per-mass estimates, but abundance-per-volume estimates can be made if
controlled volumetric sampling is possible, and moisture content determined. The method
utilizes a random settling technique in slide preparation, similar to those described by
Laws (1983) and Moore (1973), but a known mass of material is used. The entire
weighed sample is prepared; no aqueous aliquots are removed. A known area is counted
on the slide, allowing an accurate estimate of particles in the sample. Identical permanent
light microscope slides and SEM stubs can be prepared simultaneously. The slide
preparation method makes slides with evenly distributed particles, so specimens are well
presented for identification and photography. This method may be adapted for use of
automatic particle counters for certain studies. The following description concentrates on the processing of diatoms in richly diatomaceous sediments; simple adjustments may be
made in chemical processing for other microfossil groups, sediment types and microfossil concentrations. 42
Step 1: weighing
In order to avoid pipetted subsampling, only the amount of material actually needed
is processed. In richly diatomaceous carbonate-free sediments, a very small sample is
processed, generally between 0.005 and 0.020 dry grams. Sample size will vary from one
sample to the next in order to optimize counting - this step takes some experience with
the samples. When an unknown is being processed, the raw sample should be evaluated
first by making a smear slide. Standard weighing procedures for small samples and a
high quality, analytical balance must be used; careful measurement is critical for obtaining
accurate and reproducible results. The sample may be placed directly from the scale into
a clean, labeled sample vial (20 ml), then immediately sealed and kept upright until
chemical processing.
Step 2: chemical attack
To minimize sample loss or contamination, the sample can be processed directly in
the glass sample vial. Only a very small volume of chemicals is needed. First, wet the
sample by adding a few drops of distilled water to the vial. Add one drop of dilute HCI.
If a reaction is noted, add just enough acid to remove the carbonate. After the reaction
is complete, add not more than 2ml 20%-30% H202 to oxidize organics and disaggregate
particles. The sample vials, loosely covered, should be left to react in a warm bath of
clean distilled water (to avoid contaminating the outside of the vials) until this reaction is complete (generally several hours). Excessive foaming should be avoided. After 43 oxidation, fill the remainder of the vial with a dispersing agent (e. g., Sodium hexametaphosphate or Calgon [TM] solution). Brief ultrasonic separation will aid dispersion of particles. Stubborn samples may be processed with more rigorous techniques (e. g., those described by Pokras and Molfino, 1986 or Roelofs and Pisias,
1986), but great care must be employed so that no material is lost during processing.
Removal of clays by decantation or centrifugation is generally not necessary and should be avoided. Most clay-sized material, as well as chemical residues, will be safely removed during slide preparation.
Step 3: slide preparation
The settling procedure used is modified from Moore (1973). A modified flat petri dish bottom is placed upside-down in a clean 1000ml cylindrical, flat-bottomed beaker, which is filled with 980ml of distilled water. A small hole has been drilled near the edge of the dish (Fig. 7a). A latex-tipped pipet (Fig. 7a) is suspended in the beaker with the end fitted into the hole in the upside-down dish (Fig. 7b). After particle settling, as described below, the beaker is drained very slowly by siphon from under the platform and along the sides of the beaker (Fig. 7b). A cleaned, oil-free cover slip and/or an SEM stub is placed on the petri dish platform. The cover slip should be temporarily fixed to a microscope slide, using a non-immiscible, non-hardening substance, such as rubber cement, which can be easily removed before permanently mounting the slide.
If the sample was successfully dispersed using the methods described, neutralization by centrifuge or other means is not necessary. The small volume of chemicals remaining Figure 7. a. Modified petri dish (note hole), latex-tipped pipet and perforate plunger; b. Pipet fitted through hole in inverted petri dish for slow beaker drainage by siphon.
44 Figure 7 46 in the vial is sufficiently diluted in the settling chamber. This method conserves chemicals, saves time and eliminates a significant potential source of sample bias. With pipets set in place, the settling chamber is mixed with a perforated plunger, using a gentle, vertical motion, as described by Moore (1973) and Laws (1983). Simultaneously, the mixed sample is transferred into the settling chamber with a wash bottle filled with distilled water.
A few drops of a wetting agent (e. g. Kodak Photo-Flo [TM]) added to the sample vial or wash bottle will prevent adherence due to surface tension. Inevitably, however, a minute amount of sample will be lost at this transfer.
The beakers should be covered during settling time. After at least four hours, the water may be removed by slowly siphoning water from under the petri dish through the pipet. Flow is regulated with an adjustable hose clamp or stopcock. Drainage must proceed very slowly in order to prevent redistribution of particles on the cover slip. Flow is restricted to less than 5 ml/minute, thus requiring at least three to four hours for complete drainage. Completely draining the beaker at this rate has no apparent effect on the distribution of microfossils on the cover slip, except possibly at the very edges. Most clays will not settle completely out of suspension during this time and they will be removed with the water column.
After drainage, the sample may be air dried, or dried more rapidly under a heat lamp.
The cover slip is removed from the bottom of the beaker and rubber cement removed from the underside. A permanent slide may then be made using an appropriate diatom mounting medium, as described earlier. Initial sample size was 0.024 grams. The procedure is very efficient if a single apparatus is constructed for processing multiple samples (Fig. 8). During settling or draining times, the samples may be left unattended. Figure 8. Apparatus for simultaneously preparing up to 12 samples. Pipets are suspended above beakers; after siphoning, heat lamps reduce drying time.
47 Figure 8 49
Step 4: counting and abundance calculation
Microfossil counts are performed at high power, using standardized counting techniques. For diatoms, the method of Schrader and Gersonde (1978) as modified by
Laws (1983) is recommended. The area of each microscope field of view or slide transect may be accurately determined using a slide micrometer. All "whole" fossils lying within each field of view are counted, and the number of fields of view counted are recorded.
Counting may be performed along any orientation on the slide, provided there is no overlap, but the edge of the cover slip is best avoided. High power should be used at all times because the number of small, whole diatoms may not be accurately documented at lower power. While processing and counting samples, it is advisable to frequently prepare and count duplicates and also include a sediment standard, in order to keep a record of consistency.
With these data, the number of microfossils within a known area can be determined.
Assuming the distribution of particles across the bottom of the beaker is random, the number of microfossils across the entire area of the base of the beaker, thus in the original sample, can be estimated. The number of microfossils per gram is then determined by the simple expression:
T = (NB/AF) / M (1) 50
where
T = number of microfossils per unit mass N = total number of microfossils counted B = area of bottom of beaker (mm2) A = area per field of view or transect (mm2) F = number of fields of view or transects counted M = mass of sample (gm)
Results
Reproducibility
To test the reproducibility of this technique, six replicates of each of five diatom-rich
sediment samples (Table 1) were processed and evaluated. Samples were processed out
of order to check for systematic biases in processing, and for sample contamination. The
sediments chosen for analysis each have a unique signature of diatom assemblages and
other sedimentary components. The sediment from the Kerguelan Plateau (SDO) consists
of pure diatoms, processed for stable isotopic analysis of diatom silica, according to the
method of Juillet-Leclerc (1986). This technique includes the loss of the smallest and
largest diatoms of the original assemblage through microsieving (10 pm) and differential
settling, and causes a reduction in the most delicate diatoms due to partial dissolution
under controlled conditions in the laboratory. The remainder of the test samples were prepared from raw sediment.
Sample mass used in preparations varies from 0.0007 grams (FWD) to 0.0640 grams
(NDO). Only whole diatoms were counted, according to the standardized TABLE 1. The sediments evaluated for reproducibility
1. FWD - Quaternary freshwater diatomite from outcrop near Bogota, Columbia, S. A.
2. DMD - Holocene diatomaceous mud from an Antarctic Peninsula fjord (DF82-111)
3. DNO - Eocene diatomaceous nannofossil ooze from the Falkland Plateau
(10-1678-44)
4. DPC - Quaternary diatomaceous pelagic day from the Argentine Basin
(10-1678-115)
5. SDO - Purified Pleistocene marine diatoms from the Kerguelan Plateau
(MD-73-026) 52 counting method of Schrader and Gersonde (1978; modified by Laws, 1983). Results are presented on Table 2 and Figure 9 (plotted on a log scale). Reproducibility was found to be excellent. Contamination between samples was insignificant; only one clearly foreign diatom fragment was encountered during the study. Sample size does not significantly affect results, provided the slide is thin enough to avoid overlap of particles.
Accuracy
Natural sediments are useful in testing reproducibility, but they cannot be used to test the specific accuracy of the method because the sediments contain an unknown number of particles. Tablets containing a known number of Lycopodium spores are available commercially for use in estimating absolute pollen abundance. Each tablet contains
11,500 spores (+ or - 500). Eight tablets were combined (92,000 spores) and slides were made using the methods described. Four slides were counted. Lycopodium spore abundance estimates range from 72,000 to 89,000 spores, demonstrating a loss of less than 10 percent of the original 92,000 particles. The methods described are much better suited to particles of the size, high abundance and hydrodynamic properties of diatoms, and therefore accuracy is probably higher for diatom estimates. Without use of any correction factor, absolute abundance estimates from this technique may be considered minimum estimates.
Quantitative counts of diatom assemblages in sediments, presented as cell number per gram (valves counted divided by two), show a still unknown proportion of diatom productivity, preservation and sediment flux. Interpreting paleoproductivity, at least in a TABLE 2. Replicate diatom abundance of five sediments
Diatoms Diatoms Log of SAMPLE Mass (gm) Counted per Gram Abundance
FWD-1 0.0010 545 5.88 e 9 9.769 FWD-2 0.0011 407 4.01 e 9 9.603 FWD-3 0.0007 600 4.64 e 9 9.667 FWD-4 0.0014 510 3.94 e 9 9.596 FWD-5 0.0025 1000 4.33 e 9 9.637 FWD-6 0.0013 564 4.70 e 9 9.672 Mean: 4.58 e 9 Standard Deviation: 6.46 e 8
DMD-1 0.0144 513 3.86 e 8 8.587 DMD-2 0.0141 483 3.71 e 8 8.577 DMD-3 0.0133 452 3.68 e 8 8.566 DMD-4 0.0238 624 3.19 e 8 8.504 DMD-5 0.0143 489 3.67 e 8 8.566 DMD-6 0.0210 796 3.59 e 8 8.555 Mean: 3.62 e 8 Standard Deviation: 2.07 e 7
SDO - 1 0.0070 320 2.48 e 8 8.395 SDO-2 0.0067 378 3.06 e 8 8.486 SDO-3 0.0151 630 2.36 e 8 8.373 SDO-4 0.0082 378 2.50 e 8 8.398 SDO-5 0.0083 477 3.09 e 8 8.490 SDO-6 0.0058 326 3.01 e 8 8.479 Mean: 2.75 e 8 Standard Deviation: 3.07 e 7
DNO- 1 0.0345 239 3.75 e 7 7.574 DNO-2 0.0640 467 3.95 e 7 7.597 DNO-3 0.0433 344 4.30 e 7 7.634 DNO-4 0.0388 250 3.79 e 7 7.579 DNO-5 0.0276 267 4.97 e 7 7.694 DNO-6 0.0473 422 4.83 e 7 7.684 Mean: 4.27 e 7 Standard Deviation: 4.84 e 6
DPC - 1 0.0149 315 7.78 e 6 6.891 DPC-2 0.0253 300 7.14 e 6 6.854 DPC-3 0.0213 300 7.06 e 6 6.849 DPC-4 0.0136 300 7.82 e 6 6.891 DPC-5 0.0181 300 8.45 e 6 6.927 DPC-6 0.0218 300 7.61 e 6 6.881 Mean: 7.64 e 6 Standard Deviation: 4.65 e 5 Figure 9. Diatom valves per gram (log scale) calculated for six observations on each of five different samples. See Table 1 for sample descriptions and Table 2 for raw data and standard error.
54 cells per gram (log scale)
+m m + m m o o +o + I 1 mm" 1-i.iin{£—i ■ III iffp 1. i iiiiiP
f i
a ® n 13 t t CD ^ I n •a c r l i 3 C t * !3 Q. 0) □ S o f o
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Ol Ol 56 relative sense, is possible with absolute microfossil numbers, assuming that taphonomic and sedimentary influences remained equivalent through the history of sedimentary succession under study. Estimates of organic carbon produced by these diatoms are possible with the plasma volume and organic carbon content equations of Strathmann
(1967). Using Strathmann’s equations, cell volume and initial organic carbon content of diatoms present in the sediment can be roughly estimated. In counting, diatoms can be separated into specific size classes to gauge the relative contribution to phytoplankton biomass. The unknown in micropaleontologic studies is always the degree of taphonomic loss. Paleoproductivity is most reliabily estimated in Holocene sediments, accompanied by phytoplankton and particle flux studies (Chapter III). CHAPTER III
DIATOM ABUNDANCE IN HOLOCENE SEDIMENTS OF THE
NORTHERN ANTARCTIC PENINSULA: A MODEL FOR THE
INTERPRETATION OF NEOGENE ANTARCTIC
PALEOPRODUCTIVITY
INTRODUCTION
This investigation of diatom abundances in late Holocene and Recent sediments from the northern Antarctic Peninsula serves largely as the basis of a paleoceanographic model for the interpretation of diatomaceous sediments from beneath the West Antarctic
Ice Sheet (WAIS) (Chapter VII). In addition, the potential of these records to provide a detailed paleoclimatic proxy for the Holocene is explored as a pilot study. A large amount of quantitative micropaleontologic data has been generated, but interpretation of these records as a high resolution paleoclimatic signal is only at a preliminary state, due to poor chronostratigraphic control downcore and possible loss of the upper few centimeters of sediment during the piston coring process. Paleoecological and paleoclimatic interpretations of these sedimentary records await independent dating from accelerator mass spectrometry (AMS) and conventional radiocarbon analyses, as well as comparison with regional records of terrestrial precipitation from ice cores along the Antarctic
Peninsula. A transect of ice cores from this region has been collected in a joint US/UK
57 58 program. Analyses of these ice cores are in progress (L. G. Thompson, pers. com.,
1991).
Setting
The Bransfield and Gerlache straits of the northern Antarctic Peninsula are part of a Cenozoic archipelago, and represent the northern-most regions of the Antarctic continent. The Bransfield Strait is situated over a tectonically active extensional basin
(Jeffers et al., 1991) that contains thick successions of Pliocene to Recent sediments
(Anderson et al., 1991). The structural, oceanographic and depositional setting of some of the marginal basins of the northern Antarctic Peninsula might be a reasonable analog for the Cenozoic West Antarctic rift system during warmer periods that lacked an ice sheet in West Antarctica. Active Ross Sea rifting during the last 50 million years created long, deep extensional troughs that are currently up to 2000m deep (Tessensohn and Worner,
1991). Earlier dimensions of these interior basins were probably comparable to those of the modern Bransfield Basin. I speculate that in the absence of the WAIS, the basin-wide oceanographic setting of those basins may also have been similar to that of the modern northern Antarctic Peninsula. West Antarctic sedimentary basins on the continental shelf are filled with thick successions of Cenozoic marine sediments (Rooney et al., 1991;
Cooper et al., 1991).
The climatic and oceanographic settings of the coastal regions of the northern
Antarctic Peninsula are more those of a sub-polar maritime province than the polar desert setting that characterizes the continent (Kennedy and Anderson, 1989). The Antarctic
Peninsula is a region of strong climatic gradients (Fig. 10), making it particularly sensitive 59 to regional or global variation in climate (Kennedy and Anderson, 1989; Ishman, 1990).
The Bransfield Strait exhibits primary productivity values seven times as great as those in the nearby Scotia Sea (1660 mg C m'2day'1 and 230 mg C m‘2day'1 respectively;
Bodungen et al., 1986). Rates of primary productivity twice that reported from the
Bransfield Strait have been measured in the northern part of the Gerlache Strait (3200 mg
C m’2day'1; El-Sayed, 1967). Surface waters flow into the Gerlache Strait from the
Bellingshausen Sea and exit from the northern end of the Gerlache Strait into the
Bransfield Strait (Fig. 11). Gerlache Strait surface waters are clearer and warmer (>2°C) than Bransfield Strait waters, due to protection from wind-induced surface instability. This allows greater absorbtion of solar energy in the form of luminosity and thermal heating
(Niiler et al., 1991). Gerlache Strait waters are also less saline (<33.6%o) than Bransfield
Strait waters, due to local glacier melt (Niiler et al., 1991). The oceanographic setting of the Gerlache Strait creates a shallow surface layer where primary productivity can be extremely high. Although macronutrients are not generally limiting in the Antarctic Ocean
(Priddle, 1990; Hayes et al., 1984), nitrogen limitation has been documented in the
Gerlache Strait during the austral summer, following phytoplankton blooms (Karl et al.,
1991; Holm-Hansen and Mitchell, 1991).
Diatomaceous sediments accumulate in deeper waters of overdeepened fjords and in deep basins on the Antarctic continental shelf. Diatoms are typically winnowed from current-swept bathymetric highs and transported into the deeps, which act as sediment sinks (Anderson et al., 1980). Thus, the high accumulation rates determined by 210Pb profiles in the Bransfield Strait (Harden, 1989; Nelson, 1988; DeMaster et al., Figure 10. Map of the Antarctic Peninsula showing the strong climatic gradients. Contours represent mean surface air temperatures (from Reynolds, 1981). Note strong climatic gradient between the Bellingshausen Sea/Drake Passage and the colder Weddell Sea.
60 61
DRAKE PASSAGE
WEDDELL SgSFOSSICXtf CT' bluff i SEA ‘ALEXANDER A. ISLAND
80T 75* 7crw65*: 60 50* _ / 75* S
Figure 10 Figure 11. General pattern of surface current flow along the northern Antarctic Peninsula
(compiled from Niiler et al., 1987).
62 63
l ° o k n \
Figure 11 64
1987) reflect basin-wide productivity and sedimentation from the surface waters above the site, with a contribution from up-current. Sediment accumulation rates average about
2mm/yr in the Bransfield Strait based on 210Pb and 14C profiles in sediment cores (Harden,
1989). Most fine-grained sediments are diatomaceous muds with biogenic opal contents of 16-22% (Singer, 1986). Nelson (1988) determined that approximately 71% of the biogenic silica fixed by plankton (strongly dominated by diatoms) is preserved in the sediments of the Bransfield Strait. By contrast, only 9% of the carbon produced in the water column is preserved in the sediments (Nelson, 1988). Preservation of more than
70% of the silica fixed in the water column implies that the Antarctic continental shelf is a major sink for particulate silica, accounting for as much as one fourth of the silica supplied to the oceans by rivers and hydrothermal vents (Nelson, 1988; Ledford-Hoffman et al., 1986). This high silica preservation compares with less than 10% silica preservation in sediments as a global average (Nelson and Gordon, 1982; Kozlova, 1964).
Most of the sea floor of the Antarctic Peninsula is well oxygenated (DeMaster et al., 1987), and has an active benthic community. The active mixed layer at the sediment- water interface is rarely deeper than 10cm in the Bransfield Strait (Harden, 1989). With a high sedimentation rate, shallow mixing by benthic organisms acts as a "low pass filter" that smoothes variation on periods shorter than a 10cm sampling interval (Cutler and
Flessa, 1990; CLIMAP, 1984). The Gerlache Strait experiences high accumulation rates of diatomaceous sediments and shallow sediment mixing (Harden, 1989), with a relatively flat bottom and relatively weak surface currents, so the criteria for an excellent stratigraphic record are met. While the Bransfield Strait experiences high sediment accumulation and shallow benthic mixing, the sedimentary record is complicated by variable sedimentation due to advection and episodic ash input. 65
The Diatom Assemblage
The diatom assemblage in sediments from the Antarctic Peninsula region is
strongly dominated by the resting spore of the neritic diatom of the genus Chaetoceros,
sub-genus Hyalochaete (Scherer, 1987b; Leventer, 1991) (Plate II). Chaetoceros spore
formation generally begins at the end of a bloom of vegetative Chaetoceros cells, and is
often associated with nitrogen limitation in the water column (Leventer, 1991; French and
Hargraves, 1985). Vegetative cells of Chaetoceros can be identified to the species level,
but they are poorly silicified and are almost never preserved in the sediments. Resting
spores of Chaetoceros are more heavily silicified, and are well preserved in sediments,
although their taxonomy is poorly documented (Stockwell et al., 1991).
The dominant diatom assemblage in Antarctic Peninsula sediments is radically
different from the assemblages that dominate much of the Antarctic continental shelf. The
modern Ross Sea sedimentary diatom assemblage, particularly that found in the western
Ross Sea, is dominated by the diatom Nitzschia curta (Leventer and Dunbar, 1988). A similar diatom assemblage dominates Prydz Bay of East Antarctica (Stockwell et al.,
1991). N. curta blooms as annual sea ice breaks out in the austral spring, and dominates the sediments in regions of heavy annual sea ice cover (Leventer and Dunbar, 1987).
The sedimentary diatom assemblage of open shelf areas, where primary productivity is
lower than in the nearshore zone, tends to be dominated by diatoms such as the winter form of Eucampia antarctica and members of the genus Thalassiosira, including T.
antarctica resting spores, T. lentiginosa, T. gracilis and T. gracilis var. expecta (Kellogg and Truesdale, 1979). This assemblage is typical of the central Ross Sea and Weddell
Sea. The pelagic diatom assemblage of the polar front zone is dominated by diatoms PLATE II.
Typical Chaetoceros spores (species unknown) from Holocene Antarctic
Peninsula sediments, DF82-60, 60cm. Scale bar = 10pm. PLATE II. 68
Nitzschia kerguelensis, Thalassiosira lentiginosa, Asteromphalus parvulus andAzpetia
tabularis (Fenner et al., 1976; Pichon et al., 1987; Defelice and Wise, 1981). Most of the
above mentioned diatoms, particularly T. antarctica and N. curta, can be found in
sediments of the Antarctic Peninsula, but they only occur in low abundance, relative to
Chaetoceros.
In the Bransfield Strait, a vertical array of time-series sediment traps revealed a
variable sediment flux through the water column (Gersonde and Wefer, 1987). At the
shallowest sediment trap, 323m water depth, a total mass flux of 2.8 g m'2day‘1 was
recorded, with a flux of biogenic silica of 2.0 g m"2day'1. However, through the water
column the total mass flux increased to 4.3 g m'2day'1 at a depth of 1835m, 45 meters
above the sediment surface, while the biogenic silica flux decreased to 0.5 g m'2day'1
(Gersonde and Wefer, 1987). This pattern reflects dissolution of lightly silicified diatoms
in the water column and an increase in terrigenous sedimentary debris carried by bottom currents. Gersonde and Wefer (1987) noted changes in species dominance of diatom tests with depth, in association with the mass flux changes. Most taxa decreased in abundance through the water column, due to dissolution and fragmentation by zooplankton grazing. However, the flux of Chaetoceros resting spores increased, paralleling the increase in terrigenous debris. This finding suggests that (1) Chaetoceros spores may be more resistant to dissolution and mechanical degradation than most other taxa, and (2) that much of the Chaetoceros deposited in the Bransfield Strait may have been carried into the region by currents, along with suspended terrigenous sediments.
The sediment trap data of Gersonde and Wefer (1987) are corroborated by the work of Leventer (1991) as part of the RACER (Research on Antarctic Coastal Ecosystem
Rates) program. This research program, which focuses on the northern part of the 69
Gerlache Strait, the southwestern Bransfield Strait and the Drake Passage near Livingston
Island, provides an integrated biological, chemical, physical and meteorological approach to mesoscale ecosystem research (Huntley et al., 1987). The program includes primary productivity measurements and sediment trap arrays, which allow a direct comparison between productivity, diatom assemblages, diatom flux and diatom taphonomy. RACER studies show the distribution of primary productivity (Holm-Hansen and Mitchell, 1991;
Holm-Hansen et al., 1987) and its link with physical oceanography (Niiler et al., 1991;
Amos, 1987), nutrient concentration (Karl et. al., 1987; 1991), diatom flux and assemblage
(Leventer, 1991) and zooplankton distribution (Huntley and Brinton, 1987).
Leventer (1991) found that the diatom flux peaks in December and trails off through January, February and March. This pattern, dominated by Chaetoceros, may be associated with local nutrient depletion after the spring blooms (Karl et al., 1991). The diatom flux studies of Leventer (1991) clearly demonstrate the link between high
Chaetoceros abundance in sediments and high primary productivity in the Gerlache Strait, despite complicating factors such as zooplankton grazing, advection and dissolution.
Given this interpretation, and the excellent preservation of Chaetoceros in the sediments, it seems fair to conclude that fossil Chaetoceros assemblages within sediment successions are likely to provide records of primary productivity in the surface waters, and of productivity changes through time.
It has been shown that primary productivity in the nearshore zone of the Antarctic
Peninsula during the austral summer is controlled largely by physical and chemical characteristics of the surface waters, as well as seasonal variation in solar insolation (Karl et al., 1991). In the Gerlache Strait, a warm, clear, low salinity lens develops, providing the conditions that allow such high primary productivity (Holm-Hansen et al., 1987; Holm- Hansen and Mitchell, 1991). The intense productivity in this region is reflected in the underlying sediments in the form of high Chaetoceros abundance. Using the modern relationship between primary productivity and the sedimentary record as a frame of reference, the downcore record of Chaetoceros abundance may be used to interpret past oceanographic conditions. Piston cores from high sedimentation rate Antarctic Peninsula basins contain a high-resolution record of Holocene sediment accumulation. These sedimentary successions may provide a detailed paleoenvironmental proxy record. Due to the paucity of chronostratigraphic and sedimentologic supporting evidence, these records have not yet been exploited for high-resolution paleoenvironmental interpretations.
However, preliminary correlations between cores are proposed. The validity of these correlations will be tested in the future as chronostratigraphic data become available.
Some of these records will, in the near future, be utilized in Holocene paleoenvironmental studies.
Diatom-rich Neogene sediments that have been collected from beneath the Ross
Ice Shelf contain high and variable abundance of Chaetoceros spores (Chapters IV, V and
VII). The similarity in diatom assemblage and similar basin geometries suggest that the neritic, maritime environment of the modern Antarctic Peninsula may provide a reasonable analog for the depositional setting of the West Antarctic interior basins during warmer periods, which lacked persistent ice cover. The ecologic and paleoecologic conclusions of this chapter provide the basis for interpretations of the Neogene West Antarctic diatomites (Chapter VII). METHODS AND MATERIALS
Sediment samples were processed for analysis of quantitative diatom abundances using the methods described earlier (Chapter II). Surface sediments from 36 sites (Table
3) were analyzed for absolute abundance of Chaetoceros spores and other diatoms.
Abundance of lithic fragments greater than 20jxm in size was determined for 25 surface sediment samples and organic carbon content was determined for 24 samples. Downcore variations in total diatom and Chaetoceros abundances were determined in 11 piston cores. Samples from the cores were taken every 10cm for the upper meter of sediment and every 20cm thereafter. Duplicate samples (>10%) were prepared and counted throughout the course of the study. Surface sediment samples include box core tops collected during the 1986 field season and Smith-Maclntire grab samples collected during the 1988 field seasons. Ishman (1990) describes the foraminiferal faunas of many of these samples. The piston cores studied were collected during the 1981 and 1982 field seasons. For the purposes of this study, piston core tops are treated as surface sediments, although piston coring frequently destroys the upper few centimeters (and occassionally much more) of the sediment column. Figure 12 shows piston core distribution and bathymetry along the Antarctic Peninsula. Downcore data are tabulated in Appendix II. TABLE 3: Surface samples from the Antarctic Peninsula
SAMPIF LATfSI LONG AM DEPTH oraanicC %CHAET CHAET/GM OTHERS/GM UTHICS/GM Strait/Bav Leoenc DK86^S 6214.4 5848.7 440 0.5 58.1 7.96E+06 5.73E+06 6.21 E+06 Maxwell Say "" X " DF86-30 6241.6 5855.2 1460 - 83.0 5.27E+07 1.08E+07 5.65E+06 Bransfield Str. B DF86-38 62 15.7 5848.4 440 0.42 65.7 6.15E+06 3.21 E+06 1.68E+07 Maxwell Bay X DF86-72 64 35. 61 34.2 471 1.17 90.9 8.00E+07 7.99E+06 4.81 E+06 Charlotte Bay C DF86-73 64 33.6 61 39.2 393 - 90.8 9.33E+07 9.45E+06 6.30E+06 Charlotte Bay C DF85-60 6449.5 6239.2 366 1.48 91.1 2.65E+07 2.58E+06 1.79E+06 Gerlache Strait G DF86-81 6445.2 6248.5 431 1.65 84.2 9.29E+07 1.75E+07 7.13E+06 Gerlache Str. G DF86-89 6819.7 7024.0 695 0.55 73.2 4.12E+07 1.51 E+07 5.37E+06 Marguerite Bay M DF86-92 68 26.7 7003.1 1370 0.61 85.6 4.69E+07 7.91 E+06 6.88E+05 Marguerite Bay M DF86-96 6819.1 69 17.9 723 - 89.4 2.08E+08 2.48E+07 2.15E+06 Marguerite Bay M USAP88-5 6206.0 58 23.5 285 0.58 65.6 1.64E+07 8.58E+06 9.88E+06 Admiralty Bay Ad USAP88-8 64 26.5 6320.3 370 0.35 85.2 1.57E+07 2.71 E+06 9.77E+06 LapeyrereBay L USAP88-10 64 21.0 6307.1 615 0.77 75.6 2.60E+07 8.42E+06 1.33E+07 LapeyrereBay L USAP88-12 6702.0 7233.1 410 0.51 37.4 3.91 E+06 6.55E+06 1.17E+07 Pacific Shelf S USAP88-13 64 45.7 63 28.0 280 0.33 62.8 1.07E+07 6.33E+06 1.38E+07 BoraenBay Bo USAP88-17 6452.9 6236.0 227 0.49 94.0 8.51 E+07 5.48E+06 1.06E+07 Anavord Bay A USAP88-19 64 54.4 6236.5 120 0.12 93.5 9.45E+07 6.56E+06 1.91 E+07 Andvord Bay A USAP88-21 64 53.5 6235.6 190 0.23 95.5 1.48E+07 6.89E+05 1.51 E+07 Gerlache Strait G USAP88-27 6504.2 6310.6 585 0.84 85.8 1.62E+08 2.69E+07 1.36E+07 Randres Bay F USAP88-30 6505.1 63 57.0 260 0.69 75.4 2.59E+07 8.44 E+06 1.51 E+07 Delconde Bay De USAP88-33 65 07.8 6359.2 358 0.72 69.3 3.21 E+07 1.42E+07 1.20E+07 Girard Bay Gi USAP88-36 6508.5 63 57.2 321 0.32 78.7 1.09E+07 2.96 E+06 2.99E+06 Girard Bay Gi USAP88-37 6508.1 6357.9 240 0.26 72.7 7.12E+06 2.67E+06 9.97E+06 BismarkStr. Bi USAP88-38 6507.6 6359.1 240 0.68 74.0 2.19E+07 7.70E+06 1.68E+07 BismarkStr. Bi USAP88-41 6513.8 6405.1 200 0.14 79.9 7.56E+06 1.91 E+06 3.51 E+06 BismarkStr. Bi DF81-13 63 34 6133 988 - 89.6 7.43E+07 8.59E+06 - Bransfield Str. B DF81-18 63 31 6134 1298 0.7 72.7 4.78E+07 1.80E+07 _ Bransfield Str. B DF82-34 62 17.7 5737.4 1979 0.98 87.5 8.56E+07 1.22E+07 - Bransfield Str. B DF82-47 6255.3 58 23.7 723 - 92.1 1.72E+08 1.49E+07 - Bransfield Str. B DF82-50 6237.1 58 28.0 1661 - 80.0 7.84E+07 1.96E+07 - Bransfield Str. B DF82-60 6323.4 5934.2 673 - 84.1 5.09E+07 9.60E+06 - Bransfield Str. B DF82-98 6408.2 6056.0 284 - 92.8 1.31 E+08 1.02E+07 . Hughes Bay H DF82-100 6409.5 61 16.8 526 - 93.1 2.03E+08 1.51 E+07 - GenacheStr. G DF82-151 6458.0 63 20.1 360 - 88.3 1.23E+08 1.63E+07 - Gerlache Str. G DFS2-182 .6351.3 57428 405 - 966 296E+08 103E+07 - -J-fioss Island J Figure 12. Map showing the generalized bathymetry of the northern Antarctic Peninsula with locations of piston cores used in this study (after Kaharoeddin et al., 1984).
73 74
•1S00 ■1000
Figure 12 75
RESULTS
Surface Sediments
Surface sediments of the western Antarctic Peninsula are dominated by
Chaetoceros spores in all samples counted. Relative abundance of Chaetoceros spores ranges from 37.4% on the outer continental shelf, seaward of Marguerite Bay (USAP88-
12), to greater than 90% in many of the samples from the Bransfield and Gerlache Straits and associated fjords. Absolute abundances ofChaetoceros spores in surface sediment range from a low of 3.9 million cells per gram on the continental shelf (USAP88-12) to a high of more than 200 million cells per gram at the northern mouth of the Gerlache Strait
(DF82-100). Organic carbon concentrations in sediments from the Antarctic Peninsula typically show a positive correlation with depth and diatom abundance (Ishman, 1990;
DomacK et al., 1989; Nelson, 1988). The data presented here are from several different settings (fjords, straits, open continental shelf, etc.) and show a weak correlation between organic carbon and depth (Fig. 13). Absolute counts of lithic grains (>20jim in diameter) show no apparent correlation with Chaetoceros abundance or depth (Fig. 14). Surface sediment data are summarized in Table 3.
The Gerlache Strait, which has a relatively flat bottom, has high Chaetoceros abundances throughout its length, with peak abundance at the northern mouth of the Strait
(Fig. 15). Leventer (1991) demonstrates that Chaetoceros abundance in the sediments of the Gerlache Strait directly mirrors the primary productivity trends recognized by Figure 13. Plot of organic carbon content vs. water depth for surface samples. See Table
3 for key to sample identification by region.
76 Depth (meters) 1000 1200 1400- 1600- 1800- 200 400- 600- 800- - - - Antarctic Peninsula Core Tops Core Peninsula Antarctic iue 13 Figure W ater Depth vs. Organic Carbon Organic vs. Depth ater W 0.2 . 0.6 0.4 Ad Organic Carbon (percent) Carbon Organic
0.8
1
1.2
1.4 1.6 1.8 77 Figure 14. Plot of absolute abundance of lithic grains greater than 20|im vs. water depth for surface samples. No correlation is apparent. See Table 3 for key to sample identification by region.
78 Depth (meters) Figure 14 1000 1600 200 400- 600- 800- - - - M Gi G M Antarctic Peninsula Core Tops Core Peninsula Antarctic Bi Lithics (>20um) vs Water Depth Water vs (>20um) Lithics C M B I G Lithic Gram Grains per 1 1 1 1 1 20 18 16 14 12 10 8 (Millions) Ad Gi s L i i A Bi
f l 0 De 00 G
Bi X A Figure 15. Absolute abundance of Chaetoceros spores vs. water depth for surface samples from the Gerlache and Bransfield Straits. Note that Chaetoceros abundance is
higher in the Gerlache Strait than the Bransfield Strait. Chaetoceros abundance in the
Gerlache is highly variable, despite little variation in water depth, whereas Bransfield Strait samples show a distinct positive correlation of Chaetoceros abundance and depth. One
Bransfield Strait sample (top of Piston Core DF82-47 - indicated by -*■) falls well outside the trend. Stratigraphic evidence suggests that the top of this core is missing (see Figure
17).
80 Figure 15 •CORE TOP MISSING Depth (meters) 2000 2500- 1000 1500- 500- - - Antarctic Peninsula Core Tops Core Peninsula Antarctic et s Chaetoceros vs. Depth Chaetoceros Abundance 100 (Millions) 15050 200 250 82
Holm-Hansen and Mitchell (1991) and the diatom flux. This result suggests that relative
and absolute abundances of Chaetoceros spores in sediments of the Gerlache Strait may
represent a fair proxy record of primary productivity.
Sedimentation patterns of the outer Bransfield Strait and of inner bays and fjords
appear to be more complex than patterns in the Gerlache Strait setting. Variable
background sedimentation complicates interpretation of the Chaetoceros record in terms
of paleoproductivity. Surface sediment samples from the Bransfield Strait show a strong
positive correlation of Chaetoceros spore abundance with depth (Fig. 15). Because it has
been established, based on sediment trap (Gersonde and Wefer, 1987; Leventer, 1991)
and oceanographic studies (Niiler et al., 1991) that a large percentage of the Chaetoceros
in sediments of the Bransfield Strait is advected in from the Gerlache Strait, the
depth/abundance correlation is interpreted as due, in part, to enhanced settling of spores
out of suspension as bottom waters lose vigor. The diatomaceous muds of Antarctic
fjords have relatively low diatom abundance (< 50 million cells per gram). This is probably
due to the high terrigenous input from sub-glacial outflow (Griffith and Anderson, 1989).
Chaetoceros Abundance as a Tracer of Basin-wide Paleoproductivity
A relative scale of paleoproductivity is proposed, based on concentration of
Chaetoceros spores in the sediments. High concentration of Chaetoceros spores in sediments reflects high regional productivity in the surface waters of the Antarctic continental shelf, although in areas of strong surface or bottom currents, this signal may 83 be displaced. This study suggests that absolute abundances of greater than 100 million cells per gram reflect little terrigenous input relative to a high diatom flux, such as in the
Gerlache Strait. Diatom assemblages consisting of greater than 80% Chaetoceros and between 50 and 100 million cells per gram may reflect high productivity basin-wide, with transport of spores and/or input of terrigenous material from suspension, such as in the
Bransfield Strait. Lower absolute abundance of Chaetoceros, with a high percentage of
Chaetoceros relative to other diatoms, may reflect high productivity with high terrigenous input, such as in the fjords. Low overall Chaetoceros abundance and a low ratio of
Chaetoceros to other diatoms reflects lower primary productivity, such as on the open shelf, or a different set of oceanographic conditions.
This interpretive scheme assumes "average" preservation of a diatom population and shallow mixing by benthic organisms. Unusually strong dissolution will affect the results, creating a bias toward diatoms that are more resistant to dissolution and fragmentation. Conversely, the spectacular preservation of rare monospecific oozes, which are believed to reflect instantaneous mass sedimentation events (Jordan, et al.,
1991; A. Leventer, pers. comm., 1990; 1991), may bias the assemblage toward brief and and anomolous productivity and/or sedimentation events. These monospecific oozes are generally dominated by diatoms that (1) have intense blooms and (2) have long spines or some other characteristic that makes them susceptible to flocculation or entanglement as clumps of "marine snow" (Smetacek, 1985). Monospecific oozes have been reported for Chaetoceros spores, Corethron criophilum, Thalassiothrix spp. and Rhizosolenia spp. 84
Late Holocene Paleoproductivity Record of the Northern Antarctic Peninsula
Absolute Chaetoceros abundance was determined downcore in 11 piston cores
(250 to 530 cm in length) from the northern Antarctic Peninsula region (Fig. 12). Five cores are from the main Bransfield Basin, and one is from the southwestern end of the
Bransfield Strait, near Low Island. Two cores are from the Gerlache Strait, one is from
Hughes Bay, a fjord basin of the northern Gerlache Strait. Finally, a core from the
Weddell Sea side of the Antarctic Peninsula, in the strait between James Ross Island and
Vega Island, was evaluated for comparison with the cores from the western side of the
Antarctic Peninsula. These cores contain high resolution records of Holocene sedimentation in the continental shelf basins. The high sediment accumulation rates
(Harden, 1989; Domack et al., 1989) provide a detailed record of diatom productivity and sedimentation for roughly the last few thousand years. At present, however, reliable interpretation of this record is limited by the lack of chronostratigraphic control.
Descriptive sedimentology and geochemistry of piston cores from the Bransfield
Strait revealed little downcore variability (Kaharoeddin et al., 1984; Singer, 1986), suggesting that regional oceanographic conditions have been stable over the last several thousand years (Singer, 1986). However, quantitative diatom studies have demonstrated clear patterns of downcore diatom abundance, particularly of Chaetoceros spores. This apparent contradiction suggests that the diatom record may be considerably more sensitive to subtle oceanographic changes than fluctuations in sedimentary characteristics. 85
Piston cores DF82-34 and DF82-50 are from the deep Bransfield Basin, greater
than 1,500 meters deep. Sedimentation here is partially influenced by primary volcanic
ashes and ash turbidites. Cores DF82-47, DF82-60 and DF82-71 are from the Bransfield
Basin at water depths between 500 and 1,500 meters. Volcanic grains in these cores
account for only a small contribution to the dominantly siliceous biogenic sediments.
Figure 16 shows the abundance of Chaetoceros spores with depth in cores DF82-
34 and DF82-50. Ash laminae are shown by narrow dashed lines, whereas ash turbidites
are shown by thick dashed lines. Prominent ash turbidites that are considered to
represent the same event are found in these cores at a depth of 250cm in core DF82-34
and at 180cm in core DF82-50. The descrepancy in downcore depth to the turbidite may
be a result of differing sedimentation rates between the cores, or may reflect a hiatus
associated with the turbidite. A third possibility is that there was significant loss of the top
of core DF82-50 during the coring procedure.
Thin ash laminae that are believed to be primary overlie the turbidite in DF82-34,
but these are not present in core DF82-50. Tentative correlation of ash and of
. Chaetoceros peaks (Fig. 16) suggests that sediment accumulation rates were faster in the
deeper site (DF82-34) than the shallower site (DF82-50). Using the 210Pb method, Harden
(1989) calculated apparent accumulation rates for the outer Bransfield Basin. His results
suggest accumulation rates of about 2.8 mm/yr for the deep basin. Given that
accumulation rate, the five meter long cores should represent approximately 1,800 years
of accumulation. This age is poorly constrained because it is based on several
assumptions, including, (1) the modern rates of sediment accumulation are accurate, (2)
accumulation rate has been constant with time, (3) piston coring recovered a Figure 16. Chaetoceros spore abundance (cells per gram) downcore in deep Bransfield
Basin Cores DF82-34 (1979 m water depth) and DF82-50 (1661 m water depth). Thin dashed lines represent the position of distinct lamina of volcanic ash. Thick lines represent ash-dominated turbidites. Chaetoceros abundance is displayed on a scale of
0 - 300 million cells per gram. A prominent ash turbidite is believed to be correlative and is used as a chronostratigraphic tie. Correlations of peaks a through d are tentative (see text). Data are presented in Appendix D.
86 (M a io n s )
• • z
aU i
DF82-34
DF82-50
Figure 16
00 ■ v i 88
complete section, including the top, and (4) no hiatuses are present in the sediment
column.
Overall, diatom abundance is higher in core DF82-50, although the correlation
suggests that accumulation rates are higher at DF82-34. This may be due to less intense
bottom current activity at the deeper site (DF82-34), with increased settling of fine silts and
clay. It is apparent that the diatom record of certain intervals of these Bransfield Basin
cores is strongly influenced by the variable volcanic ash component. This volcanic
overprint complicates the diatom abundance record of the deep Bransfield Basin, reducing
its potential for high-resolution paleoclimatic studies. Excellent age control on these cores
may allow the separation of these two signals, by the calculation of mass accumulation
rates for biogenic and abiogenic f ractions. However, at present, the uncertainty based on
these poorly constrained or unconstrained parameters places limits on potential
paleoenvironmental interpretation of the deep Bransfield Basin.
Piston cores from further south in the Bransfield Strait show greater promise for
providing a paleoenvironmetal signal. Figure 17 shows tentative correlation in
Chaetoceros abundance between cores DF82-60, DF82-47 and DF82-71 from the
Bransfield Strait. One shallow ash layer is recognized in all three cores and is considered
to represent one eruptive event. Despite being widely separated across the basin, the
Chaetoceros record shows a surprisingly similar pattern. This excellent correlation, in the
absence of a high ash content, suggests that accumulation of Chaetoceros spores is the
primary signal in these cores. Some contribution to the Chaetoceros record in these cores
may be a result of productivity in the Gerlache Strait. The signal shows high frequency abundance changes, with a trend toward higher Chaetoceros abundance down each core.
For the purposes of tentative correlation, Chaetoceros peaks are labeled A through E, Figure 17. Chaetoceros spore abundance in inner Bransfield Basin Cores DF82-47 (723m water depth), DF82-60 (673 m water depth) and DF82-71 (1350m water depth) and proposed correlations. Thin dashed lines represent the position of distinct lamina of volcanic ash. Chaetoceros abundance is displayed on a scale of 0 - 300 million cells per gram. Off scale point on DF82-71 is labeled with approximate value. Proposed correlation of Chaetoceros peaks are labelled a through e. Data are presented in
Appendix D.
89 (Milions) (Mibns)
DF82-60 DF82-71
Figure 17 91 with A being the most recent peak and E being the oldest. If these correlations are correct then they indicate coherence in the downcore signal from this region. Downcore variation in the Chaetoceros record may reflect productivity changes in the southern
Bransfield and northern Gerlache straits through time. If climate is the dominant control on the signal, then it may be possible to correlate the Chaetoceros signal with well-dated ice cores from the Antarctic Peninsula (L. Thompson, pers. comm., 1991).
The cores from the Gerlache Strait and associated bays (DF82-98, in Hughes Bay
DF82-151 from the southern Gerlache Strait and DF82-100 from the northern mouth of the Gerlache Strait) (Fig. 18) show downcore variation of the same order of magnitude as in the Bransfield Strait cores, but Chaetoceros abundance tends to be higher in the
Gerlache Strait cores. Also shown on Figure 18 is the Chaetoceros record for Core DF82-
182 from the eastern side of the Antarctic Peninsula. Highest Chaetoceros abundance in the Gerlache Strait is found in Piston Core DF82-100, which is located at the northern mouth of the strait where RACER and earlier studies found extremely high levels of primary productivity (Holm-Hansen and Mitchell, 1991; El-Sayed, 1967) and Chaetoceros flux (Leventer, 1991). Volcanic ash is not significant in these cores. It is believed that the downcore abundance of Chaetoceros in these cores reflects trends in paleoproductivity in the Gerlache Strait. The core from the James Ross Island region, on the eastern side of the Peninsula, was counted to provide a comparison with the records from the western side of the Peninsula. This core has very high Chaetoceros abundance throughout, averaging about 300 million per gram (Fig. 18), suggesting very high seasonal productivity in the surface waters in this region, though no local biological oceanographic data are available. Figure 18. Chaetoceros spore abundance in Gerlache Strait [DF82-151 (360m water depth)], DF82-100 (526m water depth), Hughes Bay [DF82-98 (284m water depth)] and
James Ross Island [DF82-182 (405m water depth)]. Abundance scale is 0 - 600 million cells per gram. Off scale point on DF82-100 is labeled with approximate value. Data are presented in Appendix D.
92 (U to ra ) (Mions) (MUcns) (Miens) JLJ-. I
8 DFB2-151 DF82-100 DF82-182
Figure 18 94
DF81-18, from the southwestern end of the Bransfield Strait has a very different
diatom record from the other cores. This core is from a small basin that is separated from
the main Bransfield Basin by a fairly shallow sill. Harden (1989) shows that sediment
accumulation in this region is lower than that of most of the Bransfield Basin (1.6 mm/yr).
Although the sediment surface has diatom abundance similar to the other cores, three
layers of unusually high Chaetoceros abundance are present. Two of these intervals
consist of distinct layers of pure white diatomite that contain more than 98% Chaetoceros
spores, amounting to as much as 3 billion Chaetoceros spores per gram (Fig. 19).
Organic carbon in the monospecific ooze layers is anomolously high, as much as 2%,
versus a background level of about 1% (Fig. 20), which are typical values for the
Bransfield Strait (Nelson, 1988). Regression analysis of Chaetoceros abundance versus
organic carbon content yielded an R2 of 0.92 (Fig. 20). These ooze layers probably
represent preservation of mass sedimentation events (Jordan et al., 1991) and are
considered anomolous. Discounting these peaks, the trend toward increasing
Chaetoceros abundance with depth downcore is apparent to a depth of 140 cm (peak E?).
Chaetoceros abundance drops off below this level.
DISCUSSION
The Holocene Record
Changes in abundance of Chaetoceros spores with depth downcore record variation in Chaetoceros spore production in the water column of the Gerlache Strait, the
Bransfield Strait and fjords on both sides of the northern Antarctic Peninsula. According Figure 19. Chaetoceros spore abundance from the southern basin of the Bransfield Strait, near Low Island [DF81-18 (1298m water depth)]. This region experiences complex oceanographic interactions between the Drake Passage, Bellingshausen Sea and
Bransfield Strait waters, which are quite distinct from typical Bransfield Strait oceanographic patterns. Note the Chaetoceros peaks at 60cm and 250cm. These are believed to reflect mass sedimentation events. Note change in scale to account for peaks.
Data are presented in Appendix D.
95 CHAETOCEROS CELLS PER GRAM
(Millions) (Times 10E9) in u o o * G a cn hs-i -O-
in o
O a ? o ^ o
o DF81-18
Figure 19 (O O Figure 20. Plot of Chaetoceros abundance vs. percent organic carbon for DF81-18. A strong positive correlation is evident from regression analysis, which yields an R2 value of 0.92.
97 Diatom Abundance (Times 10E9) ro
sD
00
(0 00 99
to the model proposed above, these trends are controlled predominantly by changes in
primary productivity through time. Variation in background sedimentation of terrigenous
and volcanigenic debris consitute an overprint on the paleoproductivity record and must
be evaluated fully in order to establish the validity of paleoenvironmental interpretations.
With stable background sedimentation, the Chaetoceros record is largely the result of
climatically influenced oceanographic variation. The speculative correlations proposed can
only be tested with good chronostratigraphic control, such as that available from AMS
(accelerator mass spectrometer) carbon dates on certain abundance peaks in several
cores. Tightly constrained age control will allow the calculation of mass accumulation rates
for ash-rich layers and undiluted biogenic intervals. If some of the correlations are shown
to be correct, then the applicability of this approach will be demonstrated. Analysis of ice
cores along the Antarctic Peninsula will provide an independent paleoclimate record that
may include a record of climatic or atmospheric conditions that correlate with the
paleoceanographic records of Antarctic Peninsula sediments. Efforts to interpret the
Holocene history recorded in these Antarctic Peninsula sediments are in progress.
Application to Neoaene Antarctic Paleoenvironments
This study documents the conditions under which sediments rich in Chaetoceros spores are deposited along the northern Antarctic Peninsula and quantitatively establishes abundance variations between regions and through time. These findings provide a model 100
for paleoenvironmental interpretations of Chaetoceros-hch Neogene sediments from
beneath the West Antarctic Ice Sheet.
Chaetoceros is an early diatom lineage. Chaetoceros and/or Chaetoceros-Uke
resting spores have been accumulating on the Antarctic continental shelf since the
Cretaceous (Harwood and Gersonde, 1990; Hargraves, 1986). Given the long, stable
history of this lineage, it may be fair to apply what is known of the modern ecological
constraints on Chaetoceros populations to fossil assemblages in Antarctic sediments.
This possibility is explored further in Chapter VII, where Chaetoceros abundance is used to estimate the relative intensity of primary productivity in the West Antarctic interior during periods of high biogenic sedimentation. CHAPTER IV
BIOSTRATIGRAPHIC STUDY OF SEDIMENTS FROM
THE ROSS ICE SHELF PROJECT (RISP), SITE J-9
INTRODUCTION
The Ross Ice Shelf Project (RISP) recovered fifty-eight short gravity cores (-1 m) from
Site J-9 on the Ross Ice Shelf (82° 22’S; 68° 38’W) (Fig. 21) during the 1977-78 and
1978-79 austral summers (Webb, 1978, 1979; Webb et al., 1979; Clough and Hansen,
1979). Coring strategy was to sample sub-shelf sediments along a transect as Site J-9 moved with the northward drift of ice stream B at a rate of ~1 m per day. At Site J-9, the sea floor is about 597 m below the ice shelf surface and the ice shelf is 420 m thick. Two distinct units are recognized in RISP sediment cores, distinguished by differences in color, foraminiferal assemblage composition, clay mineralogy, and organic carbon and carbonate contents (Webb, 1978,1979; Webb et al., 1979; Sackett, 1986; Raiswell and Tan, 1985;
Wrenn and Beckman, 1982). Both lower (Unit 1) and upper (Unit 2) sedimentary units have textural characteristics of a diamicton, with abundant rock and sediment clasts distributed throughout and no apparent bedding. Igneous rock clasts have been shown to be of West Antarctic provenance (Faure and Taylor, 1983). Unit 2, a thin (5-17 cm), oxidized, nearly carbonate-free and organic carbon-poor unit, is thought to represent
101 Figure 21. Location of RISP Site J-9 on the Ross Ice Shelf. Also shown are ice streams
A, B and C, Crary Ice Rise and stratigraphic drill sites 270,271 and 272 from DSDP Leg
28 and the MSSTS and CIROS drill sites in McMurdo Sound.
102 103
•180*
ANTARCTICA
Crary v‘ Ice Rise
ROSS ICE SHELF
270 272 ROSS 271 CIROS-I MSSTS-I SEA
20Q 273 k m
Figure 21 104
in situ submarine alteration of existing sediments following the most recent ice-shelf ungrounding, with a minor contribution from Recent sub-shelf sedimentation (Webb et al.,
1979).
Initial geologic interpretation of Unit 1 suggested deposition of glacimarine sediments from debris in floating ice and from coeval seasonal marine diatom productivity during the middle Miocene (Webb et al., 1979; Brady and Martin, 1979). Brady (1979a) and Brady and Martin (1979) identified lower middle Miocene diatoms Denticulopsis lauta,
D. maccollumii, Nitzschia grossepunctata and N. maleinterpretaria, but Brady and Martin
(1979) suggested a younger late middle Miocene age for RISP sediments due to the presence of the upper middle Miocene to lower Pliocene diatom Denticulopsis hustedtii.
Silicoflagellate assemblages in RISP cores support a Miocene age assignment (Ling and
White, 1979; White, 1980).
This interpretation was challenged by Kellogg and Kellogg (1981, 1983, 1986) who suggested the Miocene diatoms were reworked from older sediments into a basal till that was deposited during a late Pleistocene grounding of the Ross Ice Shelf. Kellogg and
Kellogg (1981, 1983, 1986) base the Pleistocene age assignment on their identification of diatoms they consider to be restricted to the Pleistocene, including Actinocyclus actinochilus, Asteromphalus pan/ulus, Coscinodiscus stellaris var. symbolophorus
(Grunow) Jorgensen [now Stellarima microtrias], Nitzschia curta, Thalassionema bacillaris, and Rouxia antarctica. Kellogg and Kellogg (1981, 1983, 1986) fail to separate the age of the fossils and the possible age of the deposit. They believe that the sediment is a late
Wisconsin age basal till and that "the youngest diatoms present indicate the time of most recent reworking" (Kellogg and Kellogg, 1986, p. 77). However, if the deposit was 105
emplaced by grounded ice then the youngest fossils must represent an earlier deglacial
episode, when open marine conditions and primary productivity existed deep within the
southern part of the Ross Embayment. Brady (1983) and Kellogg and Kellogg (1983)
debate their views on diatom identification and application of published biostratigraphic
ranges in a discussion/reply pair of papers to Nature. Subsequent papers by Kellogg and
Kellogg (1986, 1988) reaffirm their position. The first published report to include
photomicrographs of the diatoms identified from RISP sediments is the 1986 paper of
Kellogg and Kellogg.
Recently, this debate was re-opened, utilizing analysis of foraminifera (Greene,
1990) and a new approach to diatom analysis (Harwood, Scherer and Webb, 1989).
Harwood et al. (1989) found no evidence of post-Miocene input of diatoms into RISP sediments. Foraminiferal assemblages from matrix samples support the pre-Pliocene age suggested by diatoms (Greene, 1990; Harwood et al., 1989). The following discussion is an expansion of the report of Harwood et al. (1989), incorporating new taxonomic, biostratigraphic and sedimentologic data. Diatom data were developed using both quantitative and qualitative approaches. Depositional models for RISP sediments are discussed in Chapter VIII.
Diatomaceous sediments recovered in RISP cores can be differentiated into two distinct sediment types, including (1) a fine matrix containing 27% - 45% diatoms, and (2) distinct semi-lithified, subrounded clasts (Fig. 22) that range up to at least 5cm in size, are composed of 72% - 92% diatoms and contain little terrigenous material (percentages from smear slide analysis; Webb, 1978; 1979). The diatomite clasts are unequally distributed through the sediment column (Webb et al., 1979) and concentration of clasts in the matrix varies within and between cores (Webb, 1978; 1979; Harwood et al. 1989). Figure 22. Clasts (>500 micrometers) recovered from approximately 100 cc of sediment from RISP 78-16, 72-77 cm include igneous, metamorphic and soft sediment clasts
(white, sub-rounded). The sediment clasts contain abundant diatoms (up to 92%) and represent between 5% and 46% of all clasts greater than 2 mm in size (Webb, 1978;
1979). The majority of these clasts are from middle lower Miocene strata.
106 107
B 1:100 0
Figure 22 108
Prior to the studies of Harwood et al. (1989), biostratigraphic examination of RISP
sediments focused on sediment matrix samples, which contain a complex microfossil
assemblage of mixed ages. Although the reworked diatomaceous sediment clasts
received little attention in earlier studies, a wealth of biostratigraphic and paleoecologic
information can be found within these clasts. Analysis of clasts permits a separation of
mixed microfossil assemblages found in the matrix into distinct component assemblages.
Each discrete assemblage originates from a different primary sedimentary deposit.
The clasts indicate the presence of extensive diatomaceous units in West Antarctica
(Webb et al., 1979), from which sediment was eroded, transported an unknown distance,
and re-deposited by glacial processes at Site J-9. They provide essentially unaltered samples of sedimentary units that were originally deposited up-glacier (southeast) from
Site J-9. Although the geographic position of the source sediments is not known, diatom assemblages within RISP sediment clasts can be used to determine the age and depositional environment of ice-covered marine sedimentary successions. Incorporated into the sediment matrix are microfossils disassociated from the original diatomite deposits and from degrading clasts. Younger and older microfossils may also be reworked into the
RISP sediment matrix from stratigraphic levels above and below the diatomite.
METHODS AND MATERIALS
Diatom data from RISP sediments are used to evaluate (1) biostratigraphy, (2) paleoceanography and (3) glacial/glacial-marine sedimentology. This chapter reports the 109 biostratigraphic interpretations of RISP clasts and matrix, which provide the framework for paleoceanographic and sedimentologic interpretations discussed later. Biostratigraphy and diatom taxonomy are based on descriptive studies that include examination of a large number of samples and a large volume of sediment. Procedures employed in this part included enhanced extraction methods, which may have imparted some bias on the assemblage, although any bias in the laboratory is probably minor.in comparison with the natural bias imparted by glacial mixing and reworking (see Chapter II). The conclusions from this section are based in part on data that has been published (Harwood, Scherer and Webb, 1989). The data set of Harwood, Scherer and Webb (1989) has been augmented with numerous additional samples. Taxonomy and biostratigraphic interpretations are updated, based on new results from Ocean Drilling Program Southern
Ocean drilling Legs 113, 119 and 120.
For the biostratigraphic study, large sediment clasts (>500 micrometers) were removed from the matrix sediments by sieving. Smear slides of 80 sediment clasts (1 mm
- 20mm) and 10 matrix samples from various depths in 4 cores were examined for diatoms, ebridians and silicoflagellates. In addition, 30 matrix samples were prepared using standard methods (e.g., Barron, 1985), and 20 samples used enhanced diatom extraction methods described earlier (Chapter II).
Following the establishment of taxonomic and biostratigraphic assignments, a quantitative micropaleontologic approach was used (Chapter II), in order to better interpret paleoenvironmental significance of RISP matrix sediments and diatomaceous clasts. This quantitative approach aids in the evaluation of the influences on primary sedimentologic conditions, as reflected in sedimentary clasts (e.g., paleoproductivity, sea ice conditions, 110
ice rafting) (Chapter VII), and subsequent sediment mixing and transport due to glacial
influences, as reflected in the sediment matrix (Chapter VIII).
Four clasts from Unit 1 (lower sedimentary unit) and five from Unit 2 (upper unit)
were analyzed for carbonate and total organic carbon, using a Coulometrics carbon
analyzer, for comparison with published carbon results on RISP matrix samples (Wrenn
and Beckman, 1982; Sackett, 1986).
BIOSTRATIGRAPHIC RESULTS
Rare reworked Cretaceous and Paleogene microfossils present in RISP sediments suggest an extended marine history in the southern Ross Sea. Webb (1981) reported silicified casts of Cretaceous foraminifera, Wrenn and Beckman (1982) reported upper
Eocene dinocysts, and Brady (1980) reported the upper Oligocene diatom Rocella
vigilans. Harwood et al. (1989) encountered rare upper Eocene to lower Oligocene diatoms in RISP sedimentary matrix, including Pyxilla reticulata. One diamictite clast
recovered from Unit 2 (RISP 78-16, 0-5cm) contains a low diversity diatom assemblage that has an upper Oligocene character, but lacks age-diagnostic species (Harwood, et al.,
1989). This clast contains 4.17% carbonate but not more than 0.01% organic carbon
(Table 4). The carbonate appears to be a fine matrix of either diagenetic or detrital micrite. No calcareous microfossils are discernable in this clast.
Harwood, Scherer and Webb (1989) recognized three distinct Miocene microfossil assemblages in RISP sediments; two within diatomaceous sediment clasts and matrix samples and one assemblage restricted to the sediment matrix (Fig. 23, Table 5, Table 111
TABLE 4: Total Organic Carbon from RISP samples
RISP Clasts (this studv) RISP Matrix (Dublished means) Upper Unit % Wrenn & Beckman, 1982 % 78-16,0cm 0.3 79-5,0cm 0.29 Upper Unit 0.29 79-12,7cm 0.32 Lower Unit 0.41 Lower Unit 78-16,35cm 0.41 Sackett, 1986 78-16,47cm 0.42 78-16,53cm 0.43 Upper Unit 0.18 79-12.55cm 0.43 Lower Unit 0.43 TABLE 5. DISTRIBUTION OF SILICEOUS MICROFOSSILS IN SELECTED MATRIX AND CLAST SAMPLES FROM RISP SITE J-9
DIATOMS OTHER
11
t iMIMifMiU!!!*** 111!! 1 ! SAMPLES
(/) 79-19. O-Sem ft ft >4 FF ft ft F Aft Rft . . R ft ft Rft ft ft ft F ft ft F ft F A ft ft F F F Aft . F ft F F ft . . F ft rr ? 79-19. 17-22cm Aft ft F ft R . R ft ft FARRR . . Rft F F F ft F F RFRAAFFF F F ft AF , F RF Aft . . F A Ml 2 79-19. 29 34cm A ft ft ft ft ft ft Aft ft ft ft ft . ft ft R ft F AF ft ft . . Rft ft ft Rft F F ft FF AAft ft F AF F AF . F Rft AAft . . AF * S CO 7 9 - 1C, »*(fcsi ft ft R Aft F F ft ft R F Aft Rft . F Rft Rft AF ft AFF A Rft F A F . Aft . F ft F Aft . . AF < 7MI, ft ft ft ft ft A ft RH ft ft F F F R . . ft RRft F ft RF F FAFRF F Rft AF . F RF F ft . . F F _J 79-19, 72-77em ft ft A F F F ft ft RAF RF ft Rft ft ARFF AAFAF F F . Aft . F ft F F ft . . F F s O 79-19,29 94cm AD F h ft ft ft ft ft Aft RF ft ft ft ft ft F . ft ft ft F ft M. R RRF ft F RRft ft ft R RA ft ft ft F . F F . F ft F F F ft . . ft M'9 7M(, O -tc m F F F ft ft ft ft rt ft F ft F ft ft ft ft ft ft ft ft ft ft ft ft F F ft ft . . . ft ft F ft ft F F F F ft F Aft ft ft F Aft . F ft ft F ft ft 79-19, 17-22tm F ft ft ft ft F Rft F FRRRft ft ft ft . ft ft RRft ft ft ft RF ft R . RRFRft F Rft ft F ft AARRRRft ft RF ft AR ft ft FF F ft . F 79-19, 29 34cm F ft F ft ft R F ft F ft ft ft ft F ft R . ft ft ft F Rft ft ft ft F ft ft F FF FRFF ft F ARRRF R . F ft F ft . F QZ 79-19, 93-t9em F ft F F ft ft Rft ft ft ft ft F F Rft F ft R R ft F RR Rft ARft . ft ft Rft ft ft F F ft ft F ft F AAft , F F F r ft . F Ml Ml 79-19, 99-70tm F f ft ft ft ft F ft ft F F F ft F ft ft ft ft ft ft ft ft ft ft RRF Rft RF R ft F ft ft F ft F Aft ft F ft F MO < 79-19. 77-99CM F F ft ft ft ft F ft A ft FR ft . ft ft ft F Rft ft RAR . ft FF ft F ft ft F FRFA AR . F RF Aft . . AF r o 7 9 1 2 , 117'IIIm F F ft ft ft ft Rft F ft AF F F ft ft ft ft R 2? ft RRARft . Rft RFF ft ft ft ft AA RRFF ft ft F Aft . f ft F AR . AF 7 9 -1 4 . CC F ft ft FFAAF F ft R . ft ft ft ft ft . RRFF ft ft AF ft RFFF ft ft ft F ft F . Aft . F ft F F ft . . FA 112 TABLE 6: Quantitative counts of RISP matrix and clasts
MATRIX A MATRIX B MATRIX 0 CLAST A CLAST B CLAST C CLAST D JJLASTE DIATOMS, .------,— .----- , 79-12 17 % 79-12117 % 79-45 r . r . % 79-16 36 % 79-16 53 % 79-16 65 79.26 r.r. % ‘ "Acdnocyekts (tvxeSaa 0 0 _ 0 0 . 66 _ 0 0 _ 0 0 _ 0 0 0 0 _ . _ _ ’"Acdnbcvdus ingens 1 0 3 T 00 1 0 3 0 0 0 0 0 0 0 0 0 0 •"Ariinocyoliis nctnnariiis 1 0 3 T 0 0 1 0 3 0 0 „ 0 0 . 0 0 . 0 0 on ‘"Acdnocydus spo. . 0 0 T 00 0 0 0 0 _ 0 0 . 0 0 0 0 0 0 Actinoolicus senarians T 0,0 T 0,0 . 0,0 . 0 0 T 0 0 T 0 0 _ 0 0 0 0 _ _ . Asteromphaltts symmelricus on T 00 0 0 0 0 T 0 0 on T 0 0 0 0 Adacodiscus d brownei T 0 0 T 0 0 0 0 T 0 0 0 0 T 0 0 T 0 0 T 0 0 r.haatncems forma A 17B 56 3 Ififl 370 Ififl 500 26fl 70 7 229 55 0 2fiO 706 250 549 213 649 Ohaetoceros forma B 28 3 0 13 41 28 on 28 74 28 6 7 25 71 30 6 6 29 8 8 Chaetoceros forma C 9 2 6 21 33 18 37 8 21 14 34 9 25 0 0 4 1 2 Cocmneis spp 0 0 0 0 2 Ofi _ OO 1 0 2 no 4 0 9 1 0 3 Cosdnoritscus spp t 0 3 2 0 3 2 0 3 1 0 3 2 0 3 0 0 3 0 7 1 0 3 CymalnsifB hihamnsis a 0 9 1 0 3 5 i fi 1 0 3 13 3 1 P 0 6 3 0 7 4 1 2 •"Denticulopsis husteddi i 0 3 0 0 _ 0 0 00 0 0 0 0 _ 0 0 0 0 _ _ _ •“ Denliculopsls laula 0 0 _ 0 0 1 0 3 0 0 0 0 0 0 0 0 _ 0 0 •"nanticufopsls mrvrWltvna i 0 3 T 0 0 . 0 0 . 0 0 on _ on no no “ •Eucampla nntarcdca i 0 3 T 0 0 1 0 3 _ 0 0 _ 0 0 0 0 on on _ _ _ Navicula tromoeS 0 0 0 0 1 0 3 0 0 1 0? 0 0 0 0 0 0 _ _ Nitzschia mateintarprataria 1 0 3 1 03 1 0 3 0 0 0 0 3 0B 0 0 0 0 _ •"Nitzschia grossepundata 0 0 T 00 0 0 0 0 0 0 0 0 . 0 0 _ 0 0 Nitzschia sp A 15 4 7 8 23 12 3 8 4 1 1 18 4 3 6 1 7 45 9 9 14 4 3 Paratia sulcata 14 44 26 B? 30 95 6 1 6 30 72 12 34 24 S3 21 64 Reurosfoma sd. 1 0 3 1 0 3 2 0 6 0 0 0 0 1 0 3 3 0 7 1 0 3 Psetxfcipyxilla americana 0 0 1 03 1 0 3 _ 0 0 1 0 2 on 0 0 1 0 3 Rhaphoneis sp 1 0 3 _ 00 T 0 0 _ 0 0 0 0 0 0 0 0 OO Rhbnsnfania spp 11 3 5 14 44 9 2 ft 10 26 11 2 6 on 11 24 11 3 4 Rhizosotenia sp h 0 0 T 00 1 0 3 2 0 5 1 0 2 1 0 3 on 1 0 3 — Rouxia Spp. 1 03 T 0 0 2 0 6 0 0 0 0 0 0 _ 0 0 OO SteBarima miorolrias 5 13 2 03 3 on 1 0 3 _ on 1 0 3 2 04 . no _ Stephanopyxis spp. 4 13 1 03 1 0 3 0 0 0 0 0 0 0 0 1 0 3 _ . Steo tanoovxis sd. C . 1 0 3 1 03 2 0 6 1 0 3 0 0 . 0 0 0 0 1 0 3 Stephanopyxis turns 11 3 3 5 13 14 44 7 13 6 1 4 1 7 11 24 4 1 2 Svneda sp. A 3 on 2 06 4 13 19 5 0 . 19 4 6 6 1 7 30 66 10 3 0 Synerfra sp B 1 0 3 1 03 1 0 3 0 0 4 10 no 3 0 7 0 0 Thalassionema sp A 4 13 3 on 5 IB 5 13 2 OS 4 11 3 0 7 1 0 3 Thalassiosira fraaa 12 3 8 13 4,1 10 3,2 6 1,6 8 1,9 e 23 9 2 0 8 24 Thalass nfftn Irregtilata 7 2 2 B 2 3 6 19 4 11 16 3 8 3 on 14 3 1 2 06 **'T. olive rana & var. sparsa 0 0 0 0 T 0 0 . 0 0 0 0 0 0 oo- 0 0 Trinacria excavate 6 in 3 on 1 0 3 • _ 0 0 3 0 7 1 0 3 2 04 on _ Trinacria pileolus 2 0 3 1 0 3 1 03 0 0 0 0 0 0 0 0 0 0 "vase-like* diatom scores 10 3 2 6 19 7 22 8 21 9 22 __ 16 45 7 15 0 0 RilimRandlates A FhrirSans Distephanus spp 1 0 3 1 03 1 03 . 0 0 0 0 00 _ 0 0 0 0 Mesosena Daooii 1 0 3 T 0 0 0,0 . 00 . 00 , 0 0 1 0 2 0 0 Psettdnemmodochium snn 2 0 3 1 0 3 _ 0 0 _ 0 0 0 0 0 0 0 0 _ 0 0 lotal 338 316 332 379 416 3SJ 465 328 sample mass 00102 , 00109 00104 00104 00106 nmna 00109 00112 diatom cells per gm 2.24E+Q7 1 96E+07 1.57E+07 1.10E+08 1 18Er08 127E408 1 26F<06 106Fi08 Chaetoceros fonrta A oeram 1.18E+07 l,12Et97. 7.48Et96 _Z25Eifl7 6.50E+07 -8.96Ei07 . 6.9SE+OZ 6,97Ei07 *** - Middle and upper Miocene species present in sedment matrix but not in lower Miocene clasts T - Trace: Seen on scan of sBde but not encountered during counting or only fragments) seen 113 Figure 23. Biostratigraphic ranges of key diatom species in RISP sediments recovered from clast and matrix sediment samples. Three distinct assemblages are identified at right
- middle lower Miocene (21-18.6 Ma), lower middle Miocene (15.5-14.4 Ma), and middle upper Miocene (8.8-8.2 Ma). The upper limit of the upper Miocene assemblage (8.2 Ma) is constrained by the lack of Thalassiosira torokina. No diatoms of exclusively Pleistocene or Pliocene age have been encountered. Biostratigraphic ranges of diatoms and correlation to the absolute time scale are based on the recent compilation of Harwood and
Maruyama (in press), which incorporates the Antarctic diatom biostratigraphy from ODP
Legs 113, 119 and 120.
114 CO ro o o■ o■
TJ m: OLIGO r w ] MIOCENE 0 d 2 o ! 5 6 6 s S □ S' 9 3 £ m1 Thalassiosira affin. irregulata Cymatocyra biharensis Asteromphalus symmetricus Nitzschia sp. A Jtaphidodiscus m arylandicus Thaiassiosira fraga Nitzschia m aleinterpretaria Nitzschia sp. 17 (Schrader) Actinocyclus ingens Denticulopsis lauta Denticulopsis maccollumi Eucampia antarctica Nitzschia grossepunctata Denticulopsis hustedtii Actinocyclus fryxellae Thalassiosira oliverana var. sparsa 116
6). These assemblages are compared with those of existing in situ Antarctic reference sections and microfossil zones. A diatom assemblage of middle early Miocene age (21 -
~18.6 Ma) of the T. fraga Zone of Harwood and Maruyama (in press) was found in more than 90% of the clasts studied and makes up the dominant component of all matrix samples (Table 5, Table 6). The age of this assemblage is determined by the co-occurrence of Asteromphalus symmetricus, Cymatosira biharensis, Raphidodiscus marylandicus, Stephanopyxissp.C, Thalassiosiraaffin. irregulata, common Thalassiosira fraga [Coscinodiscus sp. 1 McCollum (1975)] and rare Nitzschia maleinterpretaria, in the absence of middle Miocene diatoms and upper Oligocene/lowermost Miocene diatoms, such asKisseleviella carina, which is known up to the lower lower Miocene in the Ross
Sea (Harwood, 1991; Steinhauff et al., 1987). The age of this assemblage correlates with part of the third order eustatic sea level cycle 2.1 (20 -17.5 Ma) of Haq et al. (1988).
A diatom assemblage of early middle Miocene age (15.5 - ~14.4 Ma), of the N. grossepunctata Zone (Harwood and Maruyama, in press) is found in one sediment clast
(RISP 78-16, 72-77cm, Clast AD). This assemblage is also represented in matrix sediments (Table 5; Fig. 23). The age of this clast is determined by the co-occurrence of diatoms Actinocyclus octonarius, A. ingens, Actinocyclus fryxellae and other
Actinocyclus species, Denticulopsis lauta, Denticulopsis maccollumii, Eucampia antarctica,
Nitzschia grossepunctata, Nitzschia maleinterpretaria, Nitzschia evenescens, and
Aulacodiscus brownei, in the absence of upper Miocene diatoms, such as Denticulopsis hustedtii and Thalassiosira oliverana var. sparsa [= Cosmiodiscus intersectus Jouse].
The silicoflagellate Mesocena pappii, a common element in the lower Miocene assemblage, is also absent from the lower middle Miocene assemblage of Clast AD.
Some early Miocene diatoms are present in this sample, due either to contamination from 117 the matrix on this small clast or to reworking of lower Miocene sediment during deposition in the middle Miocene. Rare reworked early Miocene species include Pseudopyxilla americana, Thalassiosira fraga and Thalassiosira affin. irregulata. Unlike the lower
Miocene clasts, Clast AD contains a minor terrigenous component that may represent an input from ice rafting. The age of this assemblage correlates with part of the third order eustatic cycle 2.4 (15 -13.8 Ma) of Haq et al. (1988).
The youngest diatoms in Unit 1 of RISP sediments are of middle late Miocene age
(~8.8 - 8.2 Ma), representing the upper part of the A. kennettii Zone (although
Asteromphalus kennetii has not been found) to the base of the T. torokina Zone of
Harwood and Maruyama (in press). Diatoms of this age are present in the matrix, along with older diatoms listed for the above two assemblages, but late Miocene diatoms are not found in any of the more than 100 clasts studied. This assemblage includes diatoms of the Actinocyclus octonarius group (including A. octonarius s.s., A. ehrenbergii var. asteriscus, A. fryxellae and transitional forms), plusDenticulopsis hustedtii, Thalassiosira oliverana and T. oliverana var. sparsa, among other diatoms listed in the previous assemblage that continue upwards from the middle Miocene. Fragmented specimens of
Rouxia spp. may belong to several of the Miocene assemblages. Rare examples of the late Miocene silicoflagellate Distephanus pseudofibula are also found in the matrix. Total assemblage counts and absolute abundances for RISP matrix and clasts are given in
Table 6. Note that total abundances in the matrix are about an order of magnitude lower than those in the diatomite clasts, due to the considerable terrigenous component present in the matrix. The diatom assemblage is strongly dominated by the lower Miocene diatom assemblage. The middle and upper Miocene diatoms contribute, cumulatively, not more 118
than 2% of the total diatom population. The age of this assemblage correlates with the
third order eustatic cycle 3.1 (9.2 - 8.2 Ma) of Haq et al. (1988).
The middle late Miocene planktonic diatoms and foraminifera are the youngest
microfossils present in RISP sediments, suggesting a maximum age of middle late
Miocene for the deposit. Mixing, transport and deposition of the sediments may have
occurred much later than mid-late Miocene, but for this to be the case, the eroded and
redeposited sediments would have to have incorporated only older sediments. This
implies that there is little source material available under this part of the Ross Ice Shelf
that is younger than upper Miocene. The implication of this finding is either (1) that no
sediments younger than mid-upper Miocene exist beneath the southern Ross Ice Shelf,
or (2) that younger sediments were previously removed by erosion. It is useful to note the
presence of abundant late Neogene diatoms in Quaternary sediments of the Ross Sea
(Barron and Burckle, 1987; Kellogg and Truesdale, 1979), where they are reworked into
an assemblage containing Quaternary diatoms, similar to the mixed assemblage expected
in the basal till model of Kellogg and Kellogg (1981,1986) for RISP sediments.
DISCUSSION OF DIATOM BIOSTRATIGRAPHIC OBSERVATIONS
Guided by the assumption that microfossils within clasts are primary and those in the sediment matrix are of mixed ages, it became possible to look past the confusion of
microfossil reworking and determine the age range and depositional environment of source sediment sequences represented by the clasts. The biostratigraphic interpretation presented here and in Harwood et al. (1989) for RISP diatomaceous sediment is in 119
general agreement with the late middle Miocene age suggested by Brady (1979a) and
Brady and Martin (1979), but it is apparent that a range of Miocene ages is present. The
results reported here and in Harwood, Scherer and Webb (1989) also differ from those
presented by the above authors in dating the youngest diatom assemblage as middle late
Miocene (~8.2 Ma). The youngest assemblage reflects either the age of the deposit if it
is in situ, or it sets a maximum age for the youngest phase of glacial mixing if it is
reworked (see Chapter VIII).
No exclusively uppermost Miocene or Plio-Pleistocene diatoms are identified in this
study, either as whole or fragmented specimens. These findings contradict the diatom-
based Pleistocene age suggested for units 1 and 2 by Kellogg and Kellogg (1981,1986).
They report the presence of late Pleistocene diatoms, but admit difficulty with
identifications due to the considerable diatom fragmentation (Chapter II, Plate I, {Fig. 1}),
a feature that is also reflected in their photomicrographs (Kellogg and Kellogg, 1986,
Plates 1, 2). The enhanced recovery of nearly whole diatoms using the techniques
described in Chapter II (Plate I, {Figs. 3, 4}) has permitted a more rigorous taxonomic
evaluation.
In addition to misidentification of key diatoms, the Pleistocene age assignment of
Kellogg and Kellogg (1981,1983,1986) is further flawed because it is based primarily on
reference to diatom biostratigraphy of incomplete stratigraphic sequences recovered in
Ross Sea DSDP sites of Leg 28 (McCollum, 1975). Kellogg and Kellogg (1981, 1986) tried to apply a local Ross Sea diatom zonation for comparison with material from Site
J-9, but they failed to recognize that age ranges reported by McCollum (1975) were biased towards a Pleistocene age, due to the absence of Pliocene and upper Miocene sediments in DSDP Leg 28 Ross Sea holes 270, 271, 272 and 273 (McCollum, 1975). This hiatus, which spans the Pliocene and uppermost Miocene across the Ross Sea, is a widespread erosional feature termed the Ross Sea Unconformity. This unconformity has been recognized by microfossil data (e.g., Fillon, 1975; Savage and Ciesielski 1983) and seismic surveys (e.g., Hayes and Frakes, 1975; Hinz and Block, 1983; Karl et al.,
1987). Results of studies from more complete Neogene sections of the high southern latitudes (Schrader, 1976; Gombos, 1977; Weaver and Gombos, 1981; Ciesielski, 1983;
Gersonde and Burckle, 1990; Baldauf and Barron, 1991; Harwood and Maruyama, in press) indicate longer stratigraphic ranges for the above diatoms than indicated in
McCollum (1975). Published stratigraphic ranges of key species are illustrated in Figure
23, where the three diatom assemblages identified in RISP are related to the high- resolution Southern Ocean diatom zonation of Harwood and Maruyama (in press). The results reported here are similar to those of Harwood, Scherer and Webb (1989), but more recent, well constrained biostratigraphic studies from ODP Legs 113 (Gersonde and
Burckle, 1990), 119 (Baldauf and Barron, in press) and 120 (Harwood and Maruyama, in press) provide more accurate calibration to the absolute time scale.
One species of Nitzschia, which is referred to as Nitzschia sp. A (Plate IV, {Figs.
12-14}), deserves special mention. This diatom, which was identified as Nitzschia curta in Kellogg and Kellogg (1981,1983, 1986) and named Nitzschia truncata n. sp. in Brady
(1979,1980), is present in lower and middle Miocene clasts and in the matrix sediments of RISP (Table 5, Table 6). N. truncata of Brady (1979) was not validly described or published, and is therefore nomen nudum, but nevertheless, this diatom is distinct from any published taxon, and warrants formal description. Kellogg and Kellogg (1986) illustrate several specimens of Nitzschia sp. A from RISP sediments on their Plate 2, and they also include a Recent specimen of N. curta (PI. 2, {Fig. 23} of Kellogg and Kellogg, 121
1986) for comparison. Distinct morphological differences separate Nitzschia sp. A in RISP from N. curta s.s., although it is likely that N. curta is a descendent of Nitzschia sp A of
RISP. The original description of N. curta (Ehrenberg, 1856) and subsequent studies
(e.g., Hasle and Medlin, 1990) describe N. curta as 10 to 42|xm in length, with a distinct heteropolarity. Nitzschia sp. A is larger (16-55pm), more heavily silicified and less heteropolar than N. curta. Furthermore, Nitzschia sp. A has straight striae and prominent fibulae (18 and 12 in 10 *im, respectively), whereas in N. curta, fibulae (9-12 in lOpm) are indistinct and striae are broad (9-12 in 10|xm) and tend to be curved. The possible paleoceanographic implications of Nitzschia sp. A in RISP clasts are discussed in Chapter
VII.
Interestingly, several key diatoms identified in RISP are also known from lower
Miocene sediments of DSDP Leg 38 in the Norwegian Sea (Schrader and Fenner, 1976;
Dzinoridze et al., 1978). For example, Thalassiosira fraga, Asteromphalus symmetricus and Thalassiosira aff. irregulata were originally described from Site 338 (Schrader and
Fenner, 1976). In addition, forms that appear to be akin to Nitzschia sp. A of RISP are reported from the Miocene of the Norwegian Sea by Schrader and Fenner (1976) as
Nitzschia sp. C., and by Dzinoridze et al. (1978) as Nitzschia aff. extincta Kozyrenko &
Sheshukova.
SUMMARY
Three distinct Miocene diatom assemblages, middle lower Miocene (21 -18.6 Ma), lower middle Miocene (15.5 -14.4 Ma) and middle upper Miocene (~8.2 Ma) are found in RISP sediment matrix, with a lesser record of Paleogene and Cretaceous microfossils.
All of these assemblages, with the possible exception of the middle upper Miocene assemblages, are reworked from older sediments into a younger deposit. These assemblages appear to correlate with 3rd order eustatic cycles 2.1 (21 -17.5 Ma), 2.4 (15
- 13.8 Ma) and 3.1 (9.2 - 8.2 Ma) of Haq et al. (1988). Exclusively Pliocene or
Pleistocene microfossils are not present in RISP sediments. If the sediments collected were deposited during a Pleistocene ice shelf grounding event, as Kellogg and Kellogg
(1986) suggest, then all of the diatoms present are reworked. If the deposit is a Miocene glacial-marine unit that was exhumed by later glacial erosion, as Brady (1979a) suggests, the youngest fossils present in Unit 1 (middle upper Miocene diatoms and foraminifera) were probably living in the water column at the time RISP sediments were deposited.
Harwood, Scherer and Webb (1989) discuss the issue of the depositional setting of RISP sediments. They consider the evidence equivocal, but argue in favor of deposition of
RISP sediments during the late Miocene in a glacial-marine setting. Harwood, Scherer and Webb (1989) base their conclusion on several lines of evidence including (1) stratigraphic inhomogeneities in the sediment successions, (2) good preservation of middle and upper Miocene foraminifera, (3) the occurrence of soft sediment clasts, (4) organic geochemical results and (5) the lack of Pliocene and Pleistocene diatoms. A third scenario for the deposition of RISP sediments, incorporating new concepts in glaciology and glacial geology, is proposed in Chapter VIII. PLATE III.
Marine diatoms from sediment clasts and matrix sediments recovered from RISP Site J-9.
1. Aulacodiscus brownei sensu McCollum (1975), RISP 78-14 core catcher (CC),
matrix; X 650.
2. Aulacodiscus brownei sensu McCollum (1975), (fragment), RISP 78-16,17-22 cm,
matrix; X 800.
3. Actinocyclus ingens, RISP 78-16,17-22 cm, matrix; X 1000.
4. Asteromphalus symmetricus, RISP 78-16, 29-34 cm, Clast AD;
X 1000.
5. Actinocyclus octonarius (A. ehrenbergii of some authors), RISP 78-16,
17-22 cm, matrix, arrow marks position of pseudonoduius; X 1000.
6. Stellarima microtrias, RISP 78-16, 0-5 cm, clast, high and low
focus; X 500.
7. Thalassiosira oliverana var. sparsa
RISP 78-16, 0-5 cm, matrix; X 1000.
123 124
PLATE III PLATE IV.
Marine diatoms from sediment clasts and matrix sediments recovered from RISP Site J-9.
1. Raphidodiscus marylandicus, RISP 78-16, 29-34 cm, matrix; X 1500.
2,3. Thalassiosira fraga (Coscinodiscus sp. 1 McCollum, 1975), RISP 78-16,29-34 cm,
Clast AD; X 2000.
4. Chaetoceros forma B, RISP 78-16, 29-34 cm, clast; x 750.
5. Denticulopsis maccollumii, RISP 78-16, 29-34 cm, Clast AD; X 1500.
6-11. Nitzschia grossepunctata, RISP 78-16, 29-34 cm, Clast AD; X 1500.
12-14. Nitzschia sp. A, RISP 78-16, 29-34 cm, clast; x2000, Nomarski illumination.
15. Chaetoceros forma C (Liradiscus sp. McCollum (1975)), RISP 78-12,117-118 cm,
matrix; x 1000, Nomarski illumination.
16. Denticulopsis lauta (Bailey) Simonsen, RISP 78-1 6, 29-34 cm, Clast AD; X 1500,
Nomarski illumination.
17. Paraiia sulcata (Ehrenberg) Cleve, RISP 78-16, 29-34 cm, clast;
X 1000, Nomarski illumination.
18. Rhizosolenia sp. 2 Schrader (1976), RISP 78-16, 17-22 cm, clast;
X 750, Nomarski illumination.
19. Rhizosolenia sp. A, RISP 78-16, 17-22 cm, clast; x750, Nomarski illumination.
20. Pseudopyxilla americana (Ehrenberg) Forti, RISP 78-16,29-34 cm, clast; X 750,
Nomarski illumination.
21. Synedra? sp. 2 Brady (1980), RISP 78-16, 17-22 cm, clast; x 1500, Nomarski
illumination.
22. Synedra? sp. 1 Brady (1980), RISP 78-16, 29-34 cm, clast; x 1500, Nomarski
illumination.
125 126
PLATE IV. PLATE V.
Marine diatoms, silicoflagellates and ebridians from sediment clasts and matrix sediments recovered from RISP Site J-9.
1,2. Stephanopyxis sp. A, RISP 78-16, 17-22 cm, clast; x 1000, Nomarski
illumination.
3. Asteromphalus sp. cf. A. symmetricus, RISP 78-16, 72-77 cm, clast; X 750,
Nomarski illumination.
4. Thalassiosira aff. irregulata, RISP 78-16, 53-58 cm, matrix; X 750, Nomarski
illumination.
5. Thalassiosira sp. A, RISP 78-16, 29-34 cm, clast; x 1000, Nomarski illumination.
6. Stephanopyxis sp. resting spore valve, RISP 78-12, 117-118 cm, matrix; x 750,
Nomarski illumination.
7. Stephanopyxis spinossisima, RISP 78-16, 29-34 cm, matrix; X 750, Nomarski
illumination.
8. Eucampia antarctica, RISP 78-16, 0-5 cm, matrix; X 750.
9. Chaetoceros forma C (Xanthiopyxis sp. of McCollum, 1975)), RISP 78-16, 29-34,
clast; X 750, Nomarski illumination.
10, 11. Cymatosira biharensis, RISP 78-16, 29-34, clast; 10, X 2000; 11,
X 1000; Nomarski illumination.
12. Silicoflagellate, Distephanus pseudofibula, RISP 78-16, 0-5 cm, matrix; X 750.
13. Ebridian statocyst, Pseudammodochium sp. cf. P. dictyoides of Ling (1983), RISP
78-16, 0-5 cm, matrix; X 750.
14. Silicoflagellate, Mesocena pappii, RISP 78-16, 17-22 cm, clast; X 500.
127 128
PLATE V.
lik lt u t ; .
Sf®§] W il^ W y if i i& iW S jk CHAPTER V
BIOSTRATIGRAPHIC STUDY OF SEDIMENTS FROM
CRARY ICE RISE (CIR), ROSS ICE SHELF
INTRODUCTION
Crary Ice Rise (CIR) (83°S, 175°W) (Fig. 21 {p.82}) is a north/west trending ice dome located on the southwest margin of the Ross Ice Shelf and is characterized by complex ice flow dynamics (Barrett, 1975; Jezek, 1984; MacAyeal et al. 1987; Bindschadler et al.,
1989; 1990). The ice rise lies about 100 km southwest of RISP Site J-9 and differs greatly in physiographic setting. The sea floor at RISP Site J-9 underlies 237 m of sea water and 420 m of fast-flowing shelf ice (Webb et al. 1979), whereas sediments collected from CIR are, at present, in contact with 480 m of nearly stationary ice (Bindschadler et al., 1988). The thermal profile of the ice at CIR suggests that it has been grounded for a few hundred years at most (Bindschadler et al., 1990). Ice grounding events from previous glacial advances or thickening are unknown, as are the age and general geology of the sub-ice-shelf topographic high. The effects of repeated grounding and ungrounding and "ice rumpling" on the surficia! sediments is also unknown. Almost certainly, though, the effects of these glaciological processes include extensive erosion, transport and redeposition. The erosional effects of a mobile ice grounding line are evidenced by the extensive unconformity across much of the Ross Sea, recognized in DSDP Leg 28 sediments (Hayes and Frakes, 1975) and in recent seismic data (Karl et al., 1987). Much of the in situ Pliocene and Pleistocene sediment record is absent, precluding detailed
129 130
reconstruction of the Ross Sea/Ross Ice Shelf/West Antarctic Ice Sheet system for that
time. In many cases reworked fossils in glacial sediments provide the only available
evidence of sedimentary deposits that have been eroded away or that now underlie vast
areas covered by ice sheets or ice shelves (Webb et alM 1984).
METHODS
During the 1987-1988 field season on the Siple Coast of the West Antarctic interior,
glaciologists led by Robert Bindschadler (NASA) drilled a hole through the ice at Crary Ice
Rise using a rapid hot water drill (PICO [Polar Ice Coring Office] design). Crary Ice Rise
is a topographic feature on which Ross Ice Shelf ice is grounded and nearly stagnant.
The floating portion of ice stream B bifurcates around this feature, creating a zone of
chaotic crevassing around the ice rise. At 480 m ice depth the drilling equipment
penetrated several meters of sediment, presumably the sediment-ice shelf interface.
Recovery of the drilling equipment at the surface revealed sediment adhered to the lower
10 m of the drill string (Bindschadler et al., 1988). A portion of the sediment collected was
made available for micropaleontologic analysis.
Sediments from CIR were processed for diatoms using standard methods, as
described earlier (Chapter II). The sediment matrix was carefully examined for small
sediment clasts. Diatom-rich clasts were picked from the matrix for analysis, using a
stereo microscope. Organic carbon and carbonate contents of matrix sediments were determined using a Coulometrics automated carbon analyzer. 131
It must be noted that the sediment may have been altered by the unorthodox collection methods (hot water drilling), making physical tests such as cohesive strength impossible. Results of analyses performed on CIR sediments are compared with similar studies of RISP sediments. Counts are presented as relative abundances. Absolute abundance counts are inappropriate, due to the potential sample bias imparted by hot water drilling. In addition to the total assemblage counts presented, more than 30 additional slides were scanned for qualitative biostratigraphic analyses.
RESULTS
The sediment matrix recovered from CIR is a light grey marine diamicton, containing a rich but highly fragmented diatom assemblage, with common silicoflagellates and sponge spicules. Textural characteristics compare favorably with RISP sediments, except for the fact that clays have been winnowed from CIR sediments, presumably due to the drilling technique. One large (3cm) diamicton clast was recovered (Clast A) as well as eight small, buff colored clasts. These clasts (Clast B - 1) are richly diatomaceous, but also contain lithic grains, likely of ice-rafted origin, suggesting deposition in a highly productive glacial-marine environment.
Organic carbon contents in CIR matrix sediments are less than 0.2%, similar to that of the thin (~10 cm), oxidized, uppermost sedimentary unit of RISP cores (Unit 2), but about one half that of the lower unit (Unit 1) of RISP matrix sediments (Harwood, Scherer and Webb, 1989; Wrenn and Beckman, 1982; Sackett, 1986) (Table 7). Carbonate is Table 7: Total Organic Carbon from CIR samples
CIR Matrix %
Matrix A 0.17 Matrix B 0.12 Matrix C 0.20
CIR Clasts
CL-A (diamict) 0.23 CL-C fdiatomite) 0.32 133 present in negligible quantities in CIR (Table 7). Diatom assemblages in CIR sediments differ from RISP, as noted below.
RISP sediments contain diatom assemblages of mixed Miocene ages, with the youngest unequivocal age of mid-late Miocene (~8.8 - 8.2 Ma) (Chapter IV). Two distinct diatom assemblages of Miocene age are identified in clasts from RISP sediments, plus a clast containing Late Paleogene diatoms. A third, younger Miocene assemblage is recognized in the sediment matrix by subtracting diatoms present in the older clasts
(Harwood, Scherer and Webb, 1989). The sediment clasts document supply from distinct sedimentary units. Studying microfossil assemblages in reworked sediment clasts allows a partial reconstruction of the stratigraphic succession prior to erosion.
Discussion of Biostratiaraphic Results
Eight richly diatomaceous sediment clasts (Clasts B-l), similar in texture to reworked lower Miocene clasts found at Site J-9, were recovered from the CIR sediment. These small clasts (<3mm) contain a low diversity fossil phytoplankton assemblage of late late
Miocene age (~7.5 - 6.3 Ma) (Fig. 24). This assemblage is defined biostratigraphically by the co-occurrence of diatom species Thalassiosira torokina Brady, Eucampia antarctica,
Denticulopsis hustedtii, Thalassiosira oliverana var. sparsa, Denticulopsis delicata
Yangisawa and Akiba, Nitzschia praecurta Burckle, Actinocyclus fryxellae Baldauf and
Barron, Rouxia spp. and silicoflagellates Distephanuspseudofibula and D. boliviensis and the ebridian Pseudoammodochium cf. dictyoides. Figure 24. Schematic range chart for selected diatoms in CIR sediments. Two distinct assemblages are identified at right - Upper upper Miocene (7.5 - 6.3 Ma), and middle
Miocene (15.5 -14.4 Ma). No diatoms of exclusively Pleistocene or Pliocene age have been encountered. Biostratigraphic ranges of diatoms and correlation to the absolute time scale are based on the recent compilation of Harwood and Maruyama (in press), which incorporates the Antarctic diatom biostratigraphy from ODP legs 113, 119 and 120 and other Southern Ocean drilling.
134 CO IV) o o II I ■D OLIGO r MIOCENE 0 5 c s 3 oO 3 3 Denticulopsis maccoilumi Nitzschia grossepunctata Eucampia antarctica Denticulopsis hustedtii Actinocyclus fryxellae Thalassiosira oliverana var. sparsa Thalassiosira torokina Denticulopsis delicata 136
The diatom Actinocyclus ingens, which is present in middle Miocene sediments
from RISP (Chapter IV) is absent. The unusual biostratigraphic range of this diatom in
Antarctic sediments is well established from Southern Ocean drilling (Gersonde and
Burckle, 1990; Baldauf and Barron, in press; Harwood and Maruyama, in press).
Actinocyclus ingens appeared in the late middle Miocene and often dominated the
thanatocoenose of upper middle Miocene Antarctic sediments. Then, in the early late
Miocene, A. ingens declined and disappeared in the Antarctic, only to reappear and
proliferate in the late Pliocene-early Pleistocene. A. ingens then dominated the early
Pleistocene assemblage and became extinct isochronously in the Southern Ocean,
600,000 years ago, in the late Pleistocene (Abelmann, et al., 1990; Harwood and
Maruyama, in press).
Crary Ice Rise sediments lack any diatoms restricted to the Pliocene or
Quaternary. The occurrence of T. torokina and T. oliverana var. sparsa and the lack of
A. ingens place a lower biostratigraphic limit of mid late Miocene. The lack of any of the numerous diatoms that appeared in the early Pliocene suggests that the assemblage is mid-late to late-late Miocene in age (7.5 - 6.3 Ma). This would correspond to late
Tortonian to Messinian time. The terminal Miocene is well known as a time of major global cooling and sea level fall. It is also generally regarded as the time when the West
Antarctic Ice Sheet first grew to a size roughly comparable to the modern configuration
(Kennett, 1977; Kennett and Barker, 1990). Coincident with this event was the desiccation of the Mediterranean Sea (Kennett, 1977). Given this paleoceanographic constraint, it seems likely that the diatomite deposited on-or-around the Crary Ice Rise represents deposition in the late late Miocene, possibly immediately prior to the Messinian event (6.3
Ma). 137
Clast A of CIR is a large diamicton clast. The age of the diatoms within this clast
is indistinguishable from that of diatoms in the matrix, including the presence of 2% middle
Miocene diatoms (Table 8). The only feature that distinguishes the assemblage in Clast
A from the matrix and the diatom-rich clasts is the relatively high abundance of Eucampia
antarctica (18%). This clast may represent a true example of the matrix sediments, prior
to alteration due to hot water drilling. However, the remainder of the clasts are,
unquestionably, unaltered examples of a richly diatomaceous marine deposit of late late
Miocene age. The in situ upper Miocene deposit probably is exposed to grounded ice on
Crary Ice Rise today, though current erosion rates are slow, due to minimal movement at
the bed.
CIR matrix sediments contain a more diverse diatom assemblage than that found in
reworked clasts. CIR matrix includes all of the diatoms found in Clasts A - H. In addition,
matrix sediments contain common sponge spicules and benthic diatoms that have long
stratigraphic ranges, Paralia sulcata in particular. These benthic diatoms are not found
in Clasts B - H, but are present in Clast A. The diatom assemblage in matrix sediments
also includes diatoms reworked from older Miocene sedimentary deposits, based on their
known stratigraphic ages. The older reworked diatoms include Denticulopsis maccollumii,
Nitzschiagrossepunctata and Denticulopsis lauta. RISP sediments contain these diatoms,
but lack the upper Miocene-Pliocene marker species Thalassiosira torokina. Although
many of the diatoms found in RISP and CIR sediments live in antarctic waters today, no diatoms that are known to be restricted to the Pliocene or Pleistocene have been identified
in CIR sediments. Therefore, the specific age of the CIR sedimentary deposit cannot be demonstrated based on diatoms, other than to say post-late Miocene. Judging from the TABLE 8: Quantitative counts of CIR matrix and clasts
DIATOMS MATRIX % MATRIX % DUST A % Cl AST E... % CLASTC % Actinocyclus spp. 32 10.67 w .. 14.24 72 2236 112 3696 52 1883 Actinncydus sp A 000 000 8 248 11 363 6 195 ChaatoCeros spores {forma A) 72 2400 76 24 05 38 1180 37 12 21 83 2695 Chaetoceros shores /forma Bi 9 3.00 10 3.16 11 3.42 3 0.99 0.00 Coscinodscus spp 3 1 00 2 063 3 0 93 3 099 1 0 32 Denticulopsis hustec tii 7 2.33 10 3.16 15 466 18 594 27 877 Denticulopsis delicate IS 5 00 22 696 19 5 90 39 1287 22 714 Eucampia antarctica 17 5 67 26 823 58 1801 10 330 14 455 Hvalodiscussd . 1 0.33 0.00 1 031 000 000 Nitzschia nraficnrta 9 067 1 032 1 0 31 7 2 31 14 4 55 Nitzschia aurica 1 033 000 1 031 000 2 0.65 Paralia sulcata 28 9.33 25 7.91 16 497 2 0.66 0.00 Rhiznsnlania spp 39 1300 35 1108 18 5 59 8 264 10 325 Rouxia sp. 3 1.00 6 1 90 1 031 2 066 7 227 Stellarima minmtrias 17 5 67 14 4 43 19 590 15 495 10 325 Stephanopvxis spp 2 0 67 4 1 27 3 093 000 000 "Svned'a" sb. a 3 1.00 2 063 1 0.31 5 1.65 14 4.55 “Synada" sp b 000 000 000 1 0 33 10 325 Thalasslonema s'p. b 2 067 3 095 1 031 2 066 14 455 Th nliverana var sparse 6 200 1 0 32 5 1 55 2 066 000 Thalassiosira torokina 7 233 9 285 8 248 15 495 14 4 55 Thalassiosirasd . A fcf. nordenskoldiil 3 1.00 9 2.85 8 248 2 0.66 3 0.97 Thalassiosira sp B ("praeantarctica") 10 333 4 1 27 2 0 62 000 000 Thalassiosira sp. C f'Porosira"! 9 300 000 000 2 066 oco "vase-tike" diatom spores 2 0.67 1 0.32 2 062 0.00 1 0.32 ■reworkerP Denticuloosis maccollumi 2 0.67 6 1.90 3 093 000 _ 000 Nitzschia nmssenunctate 3 1 00 2 0 63 4 1 24 . 000 _ 000 Silicoftaoellates. ebridians radiolarians . Disteohanus Dseudofibula 1 0.33 _ 0,00 _ 0.00 1 0.33 1 0.32 Distephanus spp _ 000 000 _ 000 4 1 32 3 0 97 Pseudoammododum cf. dctvoides 4 1.33 3 0.95 4 1.24 000 0.00 radiolarians _ 000 000 000 2 066 . 000 others /relative abundancel SDonae scicules F F C RR ...... lithic grains A A A F R TOTAL " 505 " — 3T6" 322 303 308 139 apparent youth of the grounding event at CIR (Bindschadler et al., 1990; Jezek, 1984), the deposit may have been mobile within the last few hundred years.
Based on the diatom and silicoflagellate results from Crary Ice Rise and RISP, a picture begins to emerge of episodic marine productivity in the Ross Sea Embayment during the Miocene, punctuated by sediment erosion and reworking. In a generalized (and almost certainly oversimplified) view, these sedimentary events coincide with ice shelf and/or ice sheet retreat and advance, respectively. Several episodes of extensive marine productivity during the early, middle and late Miocene, are recorded in RISP sediments.
Sediments from CIR define a younger phase of productivity and biogenic sedimentation near the end of the Miocene. The Crary Ice Rise deposit itself was apparently disturbed, and subsequently transported and emplaced at its present site. Because these sediments are known to be eroded and reworked by glacial-bed processes, the lack of unequivocally younger diatoms in these sediments does not necessarily imply that open marine conditions have not existed in the Ross Embayment at any time since the latest Miocene.
In fact, it is possible that younger diatoms underly the sediments that were sampled at
CIR. Repeated "ice rumpling" could have created a "reversed stratigraphy" by conveyor- belt style sediment transport and deposition of Miocene fossils over Pliocene and
Pleistocene sediments. Micropaleontologic results from the two sites beneath the Ross
Ice Shelf suggest that the Ross Embayment had a complex oceanographic and glacial history. CHAPTER VI
MICROPALEONTOLOGIC ANALYSES OF SEDIMENTS FROM
BENEATH THE WEST ANTARCTIC ICE SHEET
AT UPSTREAM B (UpB)
INTRODUCTION
Ross Ice Shelf Project (RISP) and Crary Ice Rise (CIR) are situated on the southern part of the Ross Ice Shelf, within 100 km of the grounding line of the West Antarctic Ice Sheet
(WAIS). RISP overlies floating shelf ice and CIR is a pinning point where the ice shelf is grounded. The sedimentary basins beneath the Ross Ice Shelf continue beneath the ice streams of the WAIS. At their origins, the Ross Ice Streams follow these structurally controlled basins (grabens) (Rooney et al., 1991). Unconsolidated sediments beneath the ice streams may provide the primary control on ice stream flow mechanics, and are a subject of controversy in the glaciological community (Alley et al., 1986; 1989a; Engelhardt et al., 1990).
In January, 1989, samples of sub-ice till were collected from beneath fast-flowing, grounded Ice Stream B at the Upstream B (UpB) camp (83°28’40H S, 138°05’49"W) by glaciologists from California Institute of Technology (Fig. 25). During the following field season (1989-1990), 4 piston cores (2-3m in length) were collected from different holes at Upstream B. The ice sheet at the drill site is 1,030 meters thick and the glacier
140 Figure 25. Location of Upstream B (UpB) relative to the Ross Ice Streams, RISP and CIR.
141 142
8 0 °S S o u th P o le *
West Antarctic Ice Sheet
East Antarctic ^'£?^n M’ce Sheet £ • A*
120°W
Ice Shelf
CScv.m *»,.
15 0 °W Ross Sea 180° W 200 km /
Figure 25 143 bed lies 644 meters below sea level. About 50cc of sediment from the 1988-1989 field season was made available for microfossil study. Samples from the 1989-1990 cores have not yet been made available for detailed study.
METHODS
The ice at Upstream B camp was penetrated by hot water drilling methods developed at California Institute of Technology. During the 1988-1989 field season, samples were collected by stirring up the sediment with hot water jets and collecting suspended material in cups. This sampling method biased textural characteristics (grain size) of the till by removing most large clasts and much of the clay-sized material, and conserving the fine silt to sand fractions. No stratigraphic information was preserved in the sediment samples recovered, and physical-property measurements on these sediments were not possible, due to disturbance. Although the sample collected is size- sorted, the dominant grain size includes most common microfossil groups. In addition to the size-sorted sample discussed above, a small volume of sediment was recovered by inserting a metal probe into the till and allowing the till to freeze onto the metal surface.
This sample is very clay-rich and, presumably, is more texturally representative of the deposit (Engelhardt et al., 1990).
The freeze-on sample was examined using standard diatom methods (Chapter II).
The larger, size-sorted sample was processed for diatoms using both standard and enhanced diatom-extraction methods. Eighty-eight microscope slides have been examined. Thirty-two slides were prepared using standard diatom methods for clay and mineral-rich, diatom-poor sediments, 30 slides from material floated in Sodium
Polytungstate (s.g. 2.5), and 26 slides of sieved material (20 pm mesh). Flotation methods were particularly useful for recovering large diatoms, pollen, plant and coal fragments and
micro-clasts of soft sediments. The sampling and laboratory preparation methods
undoubtedly created some bias toward larger diatoms, so the data presented in the
cumulative counts must be considered as qualitative. However, no size fraction was
overlooked and efforts to find smaller diatoms were considerable. To minimize the
possibility of contamination, the sieve and beakers used were purchased especially for this
study. Results of cumulative diatom counts are displayed in Table 9. Actual diatom
counts are shown for marine diatom species; counts of non-marine diatoms are
approximate.
MICROFOSSILS IN UPSTREAM B SEDIMENT
Fossils are rare in the UpB diamicton, compared to the diatom-rich RISP and CIR
samples. However, despite low abundances relative to non-biogenous mineral matter,
numerous fossil groups have been found in the UpB sediment. Planktonic non-marine
diatoms are numerically the most common fossil group in the sediment, and marine
planktonic diatoms are the second most common group. Other microfossils present
include sponge spicules and chrysophyte cysts (both marine and non-marine) plus rare
occurrences of foraminifera, radiolaria, silicofiagellates, marine dinocysts,
coccolithophorids, and terrestrial organic microfossils, which include pollen, spores, plant tissues, coal fragments and fungal and lichen spores. The diatom assemblage seen in the clay-rich freeze-on sample appears to be identical to that of the larger, size-sorted sample that was collected in cups. TABLE 9. Diatom species list and cumulative abundances in Upstream B sediment
Marine diatoms Cumulative abundance
Actinocyclus actinochilus (Ehrenberg) Simonsen 6 Actinocyclus actinochilus (earty form) Harwood 1 Actinocyclus fryxellae Barron 54 Actinocyclus octonarius (Ehrenberg) Efirenberg group 162. Actinocyclus sp. A (cf. achotensis) 10 Actinocyclus spp. (cf. octonarius) 97 Astemmphalus hookerii Ehrenberg 1 Astemmphalus parvulus Kansten 5 Azpetia oligocsnica (Jous£) Sims 2 Azpetia tabuiaris (Gamow) Fryxell and Sims (?) 2. Chaetocaros spp. resting spores 5 Cosdnodiscus spp. 3 Denticulopsis delicata Yanagisawa and Akiba 12 Denticulopsis hustedtii (Simonsen and Kanaya) Simonsen 4- Denticulopsis lauta (Bailey) Simonsen 1 Eucampia antarctica (Castracane) Manguin 17 Nitzschia separanda (Hustedt) Hasie 1 Nitzschia (?) sp. C 1 Pomsira pseudodenticulata (Hustedt) Jous6 4 Rhizosolenia barboi/praebarboi Schrader 2 Powda spp. (fragments) 5 Stetiarima microtrias Hasle, Sims and Syveitsen 134 Stictodiscus (?) sp. 1 Thalassionema nitzschiodes Grunow 4 Thalassiosira antarctica Comber spore, form a 8 Thalassiosira antarctica Comber spore, form b 14 Thalassiosira gradtis var. expecta (Van LancSngham) Fryxell and Hasie 2 Thalassiosira lentiginosa (Janiscb) Fryxell 8 Thalassiosira oliverana (O'Meara) Makarova and Nikolaev 1 Thalassiosira oliverana var. sparsa Harwood and Maiuyama 2 Thalassiosira ritscheri (Hustedt) Hasle 8 Thalassiosira striata var. "offcenter- sensu Harwood and Maruyama 1 Thalassiosira torokina Brady 115 Thalassiosira trifulta Fryxell 1 Thalassiosira tumida (Janisch) Hasle 9 Thalassiosira sp. A 1 Thalassiosira sp. B 3 Thalassiosira sp. 0 4 Thalassiothrix sp. (apices) 13 Trinacria excavata Heiberg 2
Non-marine diatoms and incertae sedls
Aulacoseira italica var. A >1000 Stephanodiscus? (Mesodictyon?) sp. A >500 Genus and spedes unidentified A 10 Genus and spedes unidentified B 7 Genus and spedes unidentified C 1 146
Non-marine Diatoms
A non-marine planktonic diatom, Aulacoseira italica var. A (a member of the
Aulacoseira italica (Ehrenberg) Simonsen group) is numerically the most abundant diatom
in the UpB sediments studied. This undescribed subspecies is found as individual fossils
in the matrix (Plate IX, {Figs. 6-9}) and is abundant in micro-clasts of diatomite (Plate X,
{Fig. 1}). The diatom assemblage in the clasts is monospecific, although other fossil
groups are present and include chrysophyte cysts and sponge spicules, as well as organic
walled microfossils such as pollen, spores, plant tissues and fungal remains. This rich
non-marine diatomite generally contains only trace clastic material, which suggests high
productivity and distance from sources of water-borne, windblown or ice-transported
sediment. The age of the non-marine deposit represented by the clasts cannot be well-
constrained based on the presence of this fossil, due to poor biostratigraphic control. The
Aulacoseira italica group is known from deposits as old as Eocene (Lohman and Andrews,
1968) and is cosmopolitan today.
The second most abundant diatom species in the UpB sample is a small (8-20 pm)
centric diatom with a shallow mantle and few large pores scattered across a largely
hyaline valve face (Plate IX, {Figs. 1-4}). One small strutted (?) process with internal but
little apparent external expression can be seen in the light microscope approximately
midway between the center and the margin (Plate IX, figs. 2,3). In a preliminary report, this diatom was identified as the Paleogene marine species Rocella praenitida (Fenner)
Fenner (Scherer, 1989), but although it closely resembles R. praenitida, it does not have the distinctive central labiate process "double pore" that is characteristic of Rocella.
Morphologically, this diatom resembles the marine, coastal (epipsammic) Psammodiscus 147
nitidus (Gregory) Round and Mann, but is an order of magnitude smaller than P. nitidus.
Despite the large size difference, affinity with P. nitidus cannot be ruled out at this time.
This diatom, which is tentatively referred to as Stephanodiscus? sp. A, may be more
closely related to a newly described species, Stephanodiscus dzhilindus Khursevich, which
is known from Miocene non-marine deposits near Lake Baikal, USSR (Khursevich, 1990;
Khursevich, pers. commun., 1990). Affinity with the late Miocene non-marine diatom
genus Mesodictyon Theriot and Bradbury may be more likely than Stephanodiscus for
both this species and S. dzhilindus. This species could even represent an undescribed
genus. Further taxonomic study of this unusual species is in progress.
Numerous microclasts of sediment rich in this diatom have been found (Plate X,
{Fig. 2}). These clasts generally contain a diatom assemblage limited to large numbers
of Stephanodiscus? sp. A. Like the Aulacoseira clasts, the Stephanodiscus? clasts also
contain chrysophyte cysts and, occasionally, plant fragments and sponge spicules, but few
mineral grains. Very rare clasts containing both Stephanodiscus? sp. A and Aulacoseira sp. A have been found (Plate X, {Fig. 3}), which further demonstrates that this diatom is
likely of non-marine origin. The occurrence of a freshwater diatom that apparently
possesses strutted processes (either Mesodictyon, Stephanodiscus or another genus of the Order Thalassiosirales) suggests that the lake sediments are probably Neogene in
age, because strutted processes are not known in freshwater diatoms from deposits older than upper Miocene (Theriot and Bradbury, 1989).
The occurrence of clasts of freshwater diatomite in the UpB till indicates the former presence of large lake systems in West Antarctica at some time in the Cenozoic. Living diatom assemblages in the small, seasonally or perennially frozen lakes of the modern antarctic environment are overwhelmingly dominated by benthic species (Scherer, 1987b; 148
Oppenheim, 1990). Early stratigraphic records of non-marine diatoms in West Antarctic sediments include Schrader (1976), who reported displaced non-marine benthic diatom species in middle Miocene sediments of the Bellingshausen Basin, and Harwood (1989) who reported non-marine benthic species in Oligocene sediments from McMurdo Sound.
In contrast to these reports, the freshwater diatoms in the UpB till are planktonic varieties.
The lack of benthic species, and the paucity of terrigenous components in the diatomite clasts of UpB may suggest that these sediments were originally deposited in a large and possibly deep lake that was not perennially frozen.
The sediment-ice interface at Upstream B camp is well below sea level (644 mbsl) today (Engelhardt et al., 1990) and still would be below sea level when corrected for isostasy (Drewry, 1984). It would at first seem likely, therefore, that the lake sediments were originally deposited far from the Upstream B camp and were later transported to that site at the glacier bed. Large lakes may have existed in the vicinity of the Whitmore
Mountains of West Antarctica, the Ohio or Wisconsin ranges of the Transantarctic
Mountains, or even the East Antarctic craton. However, it may not be necessary to invoke long-distance transport for these clasts. Subaerially exposed lakes may have existed over a large part of what is now the West Antarctic/Ross Embayment catchment area during earlier phases of tectonic rifting and subsidence, considering the unknown and presumably complex Cenozoic tectonic rifting history of West Antarctica. The lakes may have been part of an extensive rift valley lake system, similar to the modern East African system
(Tessensohn and Worner, 1991). 149
Calcareous Microfossils
Rare calcareous fossils have been found in UpB sediments. A particularly
surprising find is an assemblage of Late Paleogene calcareous nannofossils. The
assemblage is recognized based on the presence of a micro-clast of nannofossil ooze
(Plate IX, {Fig. 13}) and a few individual coccolith plates. The assemblage closely
resembles a lower Oligocene nannofossil assemblage recovered in Ocean Drilling
Program (ODP) Antarctic Legs 113,119, and 120, but a late Eocene age cannot be ruled
out (Wei and Wise, 1990; W. Wei, pers. com.). Coccoliths identified include
Reticulofenestra samodurovii, Reticulofenestra daviesii and Chiasmolithus alatus. R.
daviesii and C. alatus are characteristic of the Late Paleogene cool water Antarctic
nannofossil assemblage (Wei and Wise, 1990; W. Wei, pers. com.). Although this
assemblage represents cool water, it seems unlikely that a true nannofossil ooze would
be deposited under near-freezing conditions. This may suggest that the extreme latitudes
of the West Antarctic interior seaway experienced relatively warm periods or events in the
Late Paleogene, a time of climatic instability.
Despite an extensive search for foraminifera, only one foraminifer specimen has
been found in the sediments examined (Plate IX, {Fig. 12}). This single, broken specimen, which was found in a diatom preparation, is a biserial planktonic form, probably of the
Paleogene genus Chiloguembelina. Specific identification is not possible due to
incomplete preservation. The scarcity of foraminifera in UpB sediments may be due to
mechanical degradation in the till. 150
Marine Diatoms
Marine diatoms representing a wide variety of ages have been found in the UpB sediment studied. The dominant age of marine diatoms is late Miocene. Also present are rare diatom fossils that reflect marine sedimentation in the late Oligocene to early or middle Miocene and, of the greatest significance, during the Pleistocene. The presence of mid-Pliocene diatoms is somewhat equivocal. Figure 26 shows the published stratigraphic ranges of Late Neogene marine diatoms found in the Upstream B sample.
These ranges are based largely on recent results of Southern Ocean drilling (ODP Legs
113,114,119 and 120). Although this range chart appears to show a complete range in ages from late Miocene to Recent, the lack of any of the abundant marker species for the
Pliocene implies a late Miocene component mixed with a Pleistocene component.
Late Paleogene to middle Miocene sedimentation in West Antarctica is recognized by the very rare occurrences of diatoms Azpetia oligocenica, Trinacria excavata and
Denticulopsis lauta. Marine sedimentation in West Antarctica during the period of late
Miocene to early Pliocene is evident from the relatively common occurrence of marker species Thalassiosira torokina, Denticulopsis delicata, Denticulopsis hustedtii,
Thalassiosira sp. D [conspecific with T. nativa sensu Schrader, 1976 and Thalassiosira sp. 1 in Baldauf and Barron, in press], Thalassiosira oliverana var. sparsa [=
Cosmiodiscus intersectus Jous6 sensu McCollum, 1975; Gersonde, 1990; Baldauf and
Barron, in press], Actinocyclus octonarius and Actinocyclus fryxellae. The relative abundance of late Miocene diatoms in UpB as well as in RISP and CIR suggests that extensive late Miocene deposits exist in the southern Ross Embayment. The abundance Figure 26. Schematic range chart for selected Neogene diatoms in Upstream B sediment.
The late Miocene assemblage is well-documented (7.5 - 6.3 Ma - similar to CIR). The
Pliocene is poorly constrained due to few specimens and the lack of zonal markers. The
Pleistocene assemblage is constrained to younger than 600,000 years by the lack of
Actinocyclus ingens. Biostratigraphic ranges are from the results of recent antarctic stratigraphic drilling as part of ODP Legs 113,119 and 120 (summarized by Harwood and
Maruyama, in press).
151 p o s s o> (A A t . M N ^ ( > (B Jpstre; from Diatom sjeogei Late MIOCENE PLIOCENE o 2 R E W O L
R E P P U 8 m MIDDLE R E P P U f-8 m LL! ra» z z 0) m i 3 w ® m n CO
luajooojvau a icuugwuoa Thalassiosira tumida Astemmphalus parvulus ------A. actinochilus "early form" luaicuttiuzu a vuveraua Thalassiosira torokina T. oliverana var. sparse Denticulopsis delicata Denticulopsis hustedtii Thalassiosira sp. D Actinocyclus ffyxellae 153 of marine fossils of this age beneath the Ross Ice Shelf and WAIS (and the paucity of younger fossils) may support the contention of Kennett (1977), Ciesielski et al. (1982) and Kennett and Barker (1990), among others, that a dominantly glacial phase in West Antarctica was initiated near the end of the Miocene epoch (~6 Ma). However, this observation does not demonstrate that the late late Miocene marine ice sheet was the first to have formed in West Antarctica. Many diatoms that are common in modern antarctic waters have been found in the Upstream B till samples. Diatoms present in the UpB till that indicate middle or late Miocene to Recent ages include Thalassiosira oliverana and Eucampia antarctica. Due, in part, to a lack of undisturbed Plio-Pleistocene reference sections on the Antarctic continental shelf, biostratigraphic resolution for the upper Pliocene and Pleistocene of inshore Antarctica is not yet highly refined. However, the occurrence of Recent diatoms that are not known in Pliocene sediments, such as Thalassiosira antarctica ("cold water" form, typical of the southern Ross Sea; Villereal and Fryxell, 1983), Nitzschia separanda, Thalassiosira gracilis var. expecta and Thalassiosira trifulta, plus diatoms that appeared in the mid or late Pliocene, such as Asteromphalus hookeri, Thalassiosira lentiginosa, Thalassiosira tumida, Thalassiosira ritscheri, Actinocyclus actinochilus, Porosira pseudodenticulata and Asteromphalus parvulus argue for open marine conditions in the West Antarctic interior during certain Pleistocene interglacial periods. Further support for a Pleistocene age for this assemblage comes from the absence of any of the typical mid to late Pliocene marker species that are generally abundant in Pliocene antarctic deposits. These include Thalassiosira vulnifica, Thalassiosira inura, Thalassiosira kolbei, Thalassiosira insigna, Nitzschia interfrigidaria and Nitzschia barronii, among others. The only diatoms found at UpB that have been reported exclusively from 154 Pliocene strata are single specimens of the ancestral form of Actinocyclus actinochilus [sensu Harwood and Maruyama (in press)] and Thalassiosira striata Harwood "offcenter variety" [sensu Harwood and Maruyama (in press)]. These diatoms may, indeed, indicate a Pliocene component in the sediments studied; however these diatoms are very rare and their biostratigraphic ranges are not well constrained. DISCUSSION Climatic instability, culminating in a partial deglaciation of the East Antarctic Ice Sheet, has been documented for certain intervals in the Pliocene (Webb et al., 1984; Webb and Harwood, 1991) and possibly the early Pleistocene (Abelmann et al., 1990). During these warm periods, the West Antarctic Ice Sheet probably would have suffered a similar fate. Therefore, Pliocene and early Pleistocene diatoms would be expected in West Antarctic interior sediments. However, ice sheets, ice shelves and/or thick annual or multi-year sea ice were likely present in West Antarctica during much of post-Miocene time, so biogenic sedimentation in these basins would have been limited to deglacial episodes. Intermittent sedimentation, coupled with the erosional effects of successive ice shelf groundings during glacial maxima would allow relatively little net accumulation of biogenic sediments in the West Antarctic interior, relative to the dominantly oceanic Miocene epoch. Many of the Pliocene diatoms listed above, which have not been found at UpB, as well as common Miocene and older fossils, can be found reworked into Pleistocene and Recent antarctic continental shelf and slope deposits (Kellogg and Truesdale, 1979; Barron and Burckle, 1987). The occurrence of these older fossils within West Antarctic shelf and slope deposits suggests that a large volume of sediment has 155 been reworked on the shelf (Kellogg and Truesdale, 1979; Barron and Burckle, 1987). Much of the reworked material on the outer shelf may have been carried from the West Antarctic interior by glacial processes (Harwood et al.p 1989). Surprisingly, Actinocyclus ingens has not been found in the UpB sediment. This robust diatom appeared in the Miocene and strongly dominated the antarctic diatom assemblage during the early Pleistocene, notably in the interval of 1.6 - 0.7 million years ago, prior to its extinction in the Southern Ocean 0.6 million years ago (Abelmann et al., 1990). Because of its great abundance in lower Pleistocene sediments (up to 60% in the Weddell Sea sediments; Abelmann et al., 1990), the absence of this diatom in UpB suggests that there may be little or no contribution of lower Pleistocene sediments in the till sampled. As with Pliocene sediments, lower Pleistocene sediments may have been largely removed by advancing grounded ice sheets. The Pleistocene diatom assemblage in UpB sediments is interpreted as representing partial or complete deglaciation of the marine ice sheet in West Antarctica. Other possible explanations can be more easily discounted. No mechanism is known that could carry sediment, including diatom fossils, at the bed of a marine ice sheet from the open sea toward the center. It is also unlikely that the Pleistocene diatoms represent deposition on the surface of the ice sheet by aeolian transport, with subsequent translation through the ice sheet and accumulation at the bed through basal melting. The occurrence of diatoms transported by aeolian processes in Antarctic ice sheets has been documented by Burckle et al. (1988). Samples from the Dome C ice core revealed exclusively non marine benthic benthic diatoms that are common in melt pools and lakes along the coast of Antarctica. No marine diatoms were found in the ice core samples. Burckle et al. (1988) used the lack of marine diatoms in the East Antarctic Ice Sheet as an argument 156 against aeolian transport as a source of marine diatoms of Pliocene age in the Sirius group of tills. Similarly, if the Pleistocene marine diatoms beneath the WAIS were transported by aeolean processes, then non-marine diatoms should also be present in significant numbers in the sub-ice till. In fact, no non-marine benthic diatoms have been found in the UpB sample. From these data I conclude that most of the post-Miocene diatoms found at UpB represent the most recent period of open marine conditions in the West Antarctic interior. The lack of Pliocene marker species and the presence of diatoms typical of the modern antarctic assemblage (some known only from upper Pleistocene and Holocene sediments) suggest a Pleistocene age for the youngest diatoms in Upstream B. The absence of A. ingens implies a late Pleistocene age, younger than 600,000 years. The till sampled from beneath ice stream B includes upper Pleistocene biogenic sediments that have recently been eroded, and may be currently undergoing transport by glacier bed processes. During previous ice sheet advances, these processes eroded and transported Pleistocene and Pliocene diatoms from the West Antarctic interior toward the continental shelf edge. The presence of late Pleistocene diatoms in UpB sediments documents an interglacial event that must have included partial or complete collapse of the West Antarctic Ice Sheet. Biostratigraphic resolution is not refined enough to subdivide the last 600.000 years, but given the conclusion that the youngest diatoms in UpB are late Pleistocene in age, it is now reasonable to begin to interpret well-dated late Pleistocene proxy records of sea level or ice volume in terms of West Antarctic Ice Sheet history. Many proxy records suggest that the most sustained period of warmth during the last 600.000 years was oxygen isotope Stage 11, which spanned the time period of about 475.000 to 380,000 years ago (Fig. 27; Oppo, et al., 1990; Williams, et al., 1988). This 157 interval also experienced the highest eustatic sea levels of the last 3 million years (Fig. 28; Wornardt, 1990). Combined diatom evidence from UpB and independent proxy data provide a case for collapse of the West Antarctic Ice Sheet during the warm interglacial about 400,000 years ago. Based on sea level records as well as stable isotopic data from marine sediment cores, another likely candidate for Pleistocene-age WAIS collapse is the penultimate interglacial (oxygen isotope substage 5e), -120,000 years ago, as suggested by Mercer (1978) and Hughes (1977). Most oxygen isotope records show Stage 5e to be slightly more negative than either Stage 11 or modern values (Fig. 27; e.g., Mackensen et al., 1989; Oppo et al., 1990, Williams, et al., 1988). Furthermore, sea level records for this interval indicate a eustatic elevation up to 6 meters higher than today (Broecker et al., 1968; Moore, 1982; Matthews, 1990). The diatom data from UpB that are presented here are inconclusive with regard to the exact timing of WAIS collapse, but they are consistent with the possibility of WAIS collapse during the last peak interglacial. Figure 27. Benthic foraminiferal oxygen isotope signals that extend back over the last 1.7 million years, compiled from recent literature by Williams et al. (1988). Oxygen isotope Stages 11 and 5 are highlighted. 158 Figure27 isfefcjUlal t||sbarir)bk|||stslsH s|g|' = Ul-I = I = fcH OSOP504 »*o »*o rot OSOP 552 * e r « * t Figure 28. Coastal onlap curve with interpreted eustatic sea level for the Plio-Pleistocene (Wornardt, 1990). Onlap maximum dated at 400,000 years ago is highlighted. A high- resolution onlap curve for the last one million years is in preparation (W. Wornardt, pers. com., 1990). 160 Cd m 5 3 2 m 5 S ST *• B . O 2 O a o 2. O • « ® a 5 a n i i - 3 Age t Cycles « 9 « 9 «• o e• • : ► S a S ► :n (3rd Order) • * S * • • ?2 ia? JSJ •• If ; i; i i; ; llll ^ III i H TT j : n z > 3 TTf" GJ r Pleistocene m » - t ; - a e i : i i i : i f e O O 0 O O O 0 O O ?! ?! n r r i CJ CJ foro © © u y -j j: :j i * to Pliocene w b • !• • 1 , 1 j 11 j 1 , 1 I pi b r I 1 Mlo. \ 1 b pi Figure 28 191 162 CONCLUSIONS AND IMPLICATIONS The till sample recovered from beneath the West Antarctic Ice Sheet at Upstream B provides the first paleoenvironmental data for a vast marine basin that is presently obscured by thick, grounded ice and ice shelves. This sediment contains important clues regarding geologic questions of global significance. Fossils within this sediment provide direct evidence of deglacial and pre-glacial conditions in West Antarctica, and contain important paleoenvironmental and paleoclimatic data. The most significant paleoenvironmental observations based on microfossil analyses of this sediment can be summarized as follows. (1) The presence of freshwater diatomite clasts in the till provides evidence of large lake systems in West Antarctica during the Cenozoic, probably Miocene. (2) Nannofossil ooze was being deposited in the south polar region during the Late Paleogene, demonstrating that ocean temperatures and chemistry were, at times, favorable for deposition of calcareous ooze in the West Antarctic interior seaway. (3) The abundance of diatoms of late late Miocene age and the scarcity of younger diatoms in UpB sediments may support the contention of Kennett (1977) and others that the Late Neogene phase of glaciation in West Antarctica began near the end of the Miocene, although these data do not imply that this terminal Miocene marine ice sheet was necessarily the first or last to have formed in West Antarctica. (4) The scarcity of Pliocene and early Pleistocene diatoms beneath Ice Stream B is probably a result of low net biogenic accumulation during the Plio-Pleistocene, coupled with the erosional effects of repeated grounding line advances across the sea floor. (5) Of the greatest scientific significance is the presence of diatoms interpreted as late Pleistocene in age beneath the West Antarctic Ice Sheet. The occurrence of these fossils is interpreted as direct evidence that this marine ice sheet collapsed and later reformed during the late Pleistocene, and is the first such evidence available. Based on comparison with independent proxy data, it seems likely that open marine conditions were present in the southern Ross Embayment during the interglaciai period of oxygen isotope Stage 11 (400,000 years ago). Furthermore, microfossil evidence from UpB could reflect ice sheet collapse during the penultimate interglaciai (~120,000 years ago). These observations provide important controls on models of marine ice sheet development and evolution and their potential for collapse (see Elliot et al., in press). If the West Antarctic Ice Sheet collapsed in a pre-industrial Quaternary earth (possibly the last interglaciai), as is suggested here, the possibility of such an event in a "greenhouse earth” may be a genuine concern, as Mercer discussed in 1978. Plate VI. Marine diatoms from Upstream B (UpB) Scale bar = 10 pm. 1. Thalassiosira tumida (fine areolation) (800X). 2. Thalassiosira tumida (coarse areolation) (1000X). 3. Actinocyclus actinochiius (1000X). 4-6. Thalassiosira antarctica spore, forma A (1500X). 7-9. Thalassiosira antarctica spore, forma B (1500X). 10. Thalassiosira trifulta (1000X). 11. Eucampia antarctica (1000X). 12. Thalassiosira lentiginosa (1000X). 13. Thalassiosira ritscheri winter form (coarse areolation and weak fasciculation) (1000X). 164 165 PLATE VI. i I ^ « _ 1V>A . ''" -/A ZSffiBi ■•*5^ ...... ^ ' .>.l*cv- # f*«!»*«' 4 fts * f- »*«.''•■ - ' , • ,,,*,» j* ., nf f' -P *• 1\+V>£j£3fSk *» i. -jP^WT’ ,■*' 1 ?;.••••' * • *•*. t m Plate VII. Marine diatoms from Upstream B (UpB) Scale bar = 10 pm. 1-3. Actinocyclus sp. A (1000X). 4. Thalassiosira torokina (800X). 5, 6. Thalassiosira sp. D (7. nativa sensu Schrader, 1976; Thalassiosira sp. 1 sensu Baldauf and Barron, in press) (1500X). 7. Denticulopsis delicata fragment (1500X). 8. Denticulopsis hustedtii (1500X). 9. Thalassiosira oliverana var. sparsa (1000X). 10. Azpetia oligocenica (1000X). 11. Asteromphalus parvulus (1000X). 166 167 PLATE VII. ' V-)> Plate VIII. Marine diatoms from Upstream B (UpB) Scale bar = 10 pm. 1. Asteromphalus hookeri (1000X). 2. Actinocyclus actinochilus, early form sensu Harwood (1000X). 3. Thalassiosira striata "offcenter variety" sensu Harwood (1000X). 4. Thalassiosira sp. A (2000X). 5. Porosira pseudodenticulata (1000X). 6. Thalassiosira oliverana (1000X). 7. Thalassiosira sp. B (1000X). 8. Nitzschia? sp. C (1500X). 9. Actinocyclus fryxellae ( 1000X). 168 169 PLATE VIII. Plate IX Marine and non-marine diatoms and other microfossils from Upstream B (UpB) Scale bar = 10 (xm. 1-4. Stephanodiscus? (Mesodictyon?) sp. A; note small process off center (2000X). 5. Genus and species unidentified, A(1000X). 6-9. Aulacoseira italica var. A (2000X). 10. Genus and species unidentified, B (note heteropolarity) (1000X). 11. Genus and species unidentified, C (1000X). 12. Foraminifer, Chiloguembelina sp. (1000X). 13. Micro-clast of nannofossil ooze (800X). 14-16. Chrysophyte cysts (1500X). 170 171 PLATE IX. Plate X Microclasts of diatomaceous non-marine sediment from Upstream B Scale bar = 50 |xm. 1. Micro-clast rich in Aulacoseira italica var. A, with common chrysophyte cysts (400X). 2. Micro-clast rich in Stephanodiscus? sp. A, with common chrysophyte cysts (600X). 3. Large clast containing (a) A. italica var. A, (b) Stephanodiscus? sp. A, plus chrysophyte cysts and a large plant fragment (400X). 172 173 PLATE X. CHAPTER VII PALEOENVIRONMENTS OF THE WEST ANTARCTIC INTERIOR INTRODUCTION: THE CHRONOSTRATIGRAPHIC FRAMEWORK Microfossil data from sediments sampled beneath the West Antarctic Ice Sheet provide demonstrative evidence that the ice sheet has experienced dramatic changes in configuration, probably driven by climatic and oceanographic forcing. The global implications of these profound changes highlight the importance of investigations of Antarctic cryospheric evolution. The Ross Embayment contains key Cenozoic paleoclimatic and paleoenvironmental records of WAIS history and evolution. Due to mixing, glacial sediments from beneath the WAIS and Ross Ice Shelf do not provide stratigraphic time series data, as do drill cores, piston cores and outcrop sections. However, glacial diamictons contain evidence of sedimentary successions that are presently obscured by ice sheets (Harwood, Scherer and Webb, 1989; Webb et al. 1984). These discontinuous records include direct evidence of discrete events, such as periods of biologic productivity. Distinct depositional events, in the form of microfossil assemblages within sedimentary clasts, can be distinguished and dated by microfossil assemblages. Each distinct age represents one small part of the sedimentary column. In the case of sediments from beneath the WAIS, diatom assemblages representing at least six distinct phases of Neogene marine productivity have been 174 175 identified. Of these, three are represented by clasts of diatomite. Assemblages identified have been tied to the absolute time scale by correlation with ODP and DSDP drill holes in the Antarctic ocean. Antarctic diatom biostratigraphic resolution is now refined enough to reliably date assemblages to within one million years and often to less than one half million years (Gersonde and Burckle, 1990; Baldauf and Barron, 1991; Harwood and Maruyama, in press). Figure 29 shows a summary of diatom assemblages recovered from the three sites in the Ross Embayment. The specific ages of diatom assemblages identified appear to correlate with high stands of eustatic sea level described as 3rd order cycles of Haq et al. (1988). Third-order eustatic cycles are believed to be driven dominantly by glacio-eustacy. Figure 30 shows the eustatic curve of Haq et al. (1988). Biostratigraphic ages of diatom assemblages identified in sediments from beneath the WAIS are identified at the right. Note that the ages correspond to predicted high stands of sea level. I believe that the correlation of these assemblages to periods of elevated sea level is more than coincidence. Deglacial events opened pfanktonic habitats and may have invigorated the oceanic biotic systems. I expect that with further sampling and analysis of sediments from beneath the WAIS, additional assemblages, representing other high stands of sea level, will be identified. It is also likely that not all Neogene low stands of sea level were characterized by the presence of an ice sheet in West Antarctica, especially considering the unknown subsidence history of the West Antarctic basins. Therefore, fossils of these ages are likely to be present beneath the WAIS. If Neogene high sea level stands coincide with "warm" conditions, then biogenic sedimentation in the West Antarctic basins would have been higher during these intervals, due to factors such as sea ice conditions and metabolic rates. Figure 29. Summary of Cenozoic diatom biostratigraphic assemblages identified in RISP, CIR and UpB sediments. Shaded areas represent diatom assemblages. Letters refer to relative abundance of each assemblage in the diamicton matrix [A = abundant, C = common, F = few, R = rare, VR = very rare). Shaded areas represent the age-range of identified diatom assemblages. 176 DIATOM ASSEMBLAGES OF THE SOUTHERN ROSS EMBAYMENT Ma RISP CIR UpB o - PLEISTOCENE i R UPPER ...... , \ / n PLIOLOWER VR . . . AA- - - . - r* LUUPPER F 10- Z LLIMIDDLE o RR o r\ LOWER 20- A R 2 o UPPER o 30 -J VRVRVR o LOWER Figure 29 Figure 30. Eustatic sea level curve of Haq et al. (1988). Independently dated diatom assemblages recovered from Ross Embayment sediments are indicated to the right. Each Neogene diatom assemblage identified in Ross Embayment sediments coincides with a third-order high stand of sea level, but note that not all high stands are represented by an assemblage. 178 EUSTATIC CURVES SCRIES PlflSTO UpB (see Fig. 28) CENI ziizaKw iahcua* UpB (Cycle 3.4) u M fS S44 lN IA N UpB, CIR (Cycle 3.2) TO h TONIAH RISP (Cycle 3.1) ScRRAVALUAM LANGHIAH RISP (Cycle 2.4) i l l BUROKj AUAM iOnO 1111*4 RISP (Cycle 2.1) AQ Uff ANtAM QIArTlAW R U P tU A N - n I Figure 30 CO 180 MIOCENE PALEOENVIRONMENTS IN THE ROSS EMBAYMENT FROM RISP AND CIR CLASTS Paleoproductivitv: The Chaetoceros Record Significant paleoenvironmental information regarding certain early, middle and late Miocene intervals in the Ross Embayment can be drawn from the reworked diatomaceous sediment clasts of RISP and CIR. The diatomite clasts contain as much as 95% diatomaceous remains, with relatively high organic carbon content and a paucity of clastic sedimentary debris. Based on sedimentologic criteria it can be concluded that the sedimentary deposits represented by reworked diatomite clasts record extensive biosiliceous productivity in the West Antarctic interior basins (Webb, 1978; 1979; Harwood, Scherer and Webb, 1989). An absence of permanent ice cover is implied, with little input of coarse clastic debris by ice-rafting or other mechanisms. These features reflect minimal glacial ice at sea-level in West Antarctica, though seasonal sea ice cover may have been present. Quantitative analyses of these diatomaceous sediments provides more detailed information about the paleoenvironment at the time of their initial deposition. Reasonable interpretations are possible when the fossil assemblage can be compared with similar modern assemblages that have well documented environmental constraints. Quantitative analyses of Recent Antarctic continental shelf diatom assemblages from the northern Antarctic Peninsula (Chapter III) are compared with those of Miocene diatom assemblages from diatomite clasts recovered from RISP (Chapter VI) and CIR (Chapter V) glacial sediments. In Chapter III it was demonstrated that primary productivity in the surface waters of the northern Antarctic Peninsula is generally reflected in the relative and absolute abundance of Chaetoceros spores in the sediments. This paleoproductivity tracer is now utilized in analysis of Miocene Antarctic diatom assemblages from beneath the WAIS. Although many of the diatom taxa found in these Miocene sediments are extinct today, a similar community structure, at the generic level, exists in modern Antarctic diatom assemblages. Modern Antarctic diatom communities may, therefore, provide the basis for a credible paleoenvironmental analog. The Miocene diatom assemblages found in the clasts of RISP (Chapter IV) and CIR (Chapter V) are strongly dominated by resting spores of the neritic variety of the diatom genus Chaetoceros (subgenus Hyalochaete). Numerous distinct morphotypes of Chaetoceros spores are evident, but the taxonomy of this group is poorly known (Chapter III), particularly among fossil species. Although many of the Chaetoceros species present in these clasts are clearly different from any of the common Chaetoceros species living in ihe modern Antarctic ocean, the most abundant spore type appears virtually identical to the Chaetoceros spores that are common and often abundant in modern Antarctic continental shelf sediments. Although many of the Miocene Chaetoceros are likely to be extinct species, similar resting spores accumulate in great numbers in high productivity areas of the modern Antarctic Continental Shelf, including the Gerlache and Bransfield basins (Chapter III). The variable abundance of Chaetoceros in modern Antarctic sediments is used as an analog for the interpretation of the Miocene diatomaceous sediments of the Ross Embayment. Chaetoceros-ridn sediments of the modern Antarctic continental shelf accumulate at rates in excess of 2mm per year (Harden, 1989). Furthermore, extraordinary mass sedimentation events of Chaetoceros oozes are occasionally preserved in the sedimentary record (Jordan et al., 1991). These rare events are recognized as layers, occasionally several centimeters thick, composed of more than 98% Chaetoceros spores, and amounting to as much as 3 billion spores per gram of sediment (Chapter III). These layers may represent a single season’s bloom (Jordan et al., 1991). However, typical modern Antarctic sediments from a high productivity region, such as the Gerlache Strait on the Antarctic Peninsula, include Chaetoceros spore abundances on the order of 100 to 300 million spores per gram. The diatom assemblage is generally 80 - 90% Chaetoceros (Chapter III). This Chaetoceros- rich assemblage is distinct from the typical diatom assemblage of the open continental shelves of Antarctica, such as the central Ross Sea and Weddell Sea. In these regions, the diatom assemblage is dominated by Thalassiosira species in open water settings and Nitzschia curta and/or Nitzschia cylindrus in sea ice- dominated settings, such as McMurdo Sound (Leventer and Dunbar, 1987). Table 6 (Chapter IV) and Table 8 (Chapter V) show the quantitative results of diatom counts from the Miocene clasts that were removed from the matrix sediments of RISP and CIR, respectively. Calculated absolute abundances of diatoms and Chaetoceros spores are shown for lower Miocene RISP clasts. Absolute abundance estimates are not possible for middle Miocene clasts from RISP or upper Miocene clasts of CIR. For counting purposes, Chaetoceros species (and Chaetoceros-re lated genera) were counted in three distinct morphological groups. (1) Chaetoceros forma A is a group comprised of the small (10 - 15pm) and simple variety that is virtually identical to the modern Chaetoceros spores that accumulate on the Antarctic Continental Shelf today 183 (Plate II - Chapter III). (2) Chaetoceros forma B is a morphotype that possesses large, bifurcate setae (Plate IV, {Fig. 4}). This morphotype is found in modern Arctic settings, such as the Bering and Okhotsk seas (Sancetta, 1982), but it is no longer found in Antarctic waters. (3) Chaetoceros forma C includes large, heavily silicified varieties, all of which are extinct. This group includes some forms that are referred to in the literature as the fossil "genera" Liradiscus Greville (Plate IV, {Fig. 15}) and Xanthopyxis Ehrenberg (Plate V, {Fig. 9}). Although these have been described as genera, they are unquestionably resting spores, which have morphologies distinct from vegetative cells. As such, the validity of "spore genera" is in question. Hargraves (1986) and Harwood and Gersonde (1990) discuss this taxonomic dilema. A detailed taxonomic evaluation of all the varieties of Chaetoceros and related resting spore-forming diatoms is needed. Because little is known of the ecology of formas B and C, Chaetoceros abundance presented is based only on counts of Chaetoceros forma A. This may provide the most appropriate comparison with Chaetoceros abundance in modern antarctic sediments. Chaetoceros forma A dominates the diatom assemblages of all of the Miocene clasts removed from RISP and CIR matrix sediments. Chaetoceros forma A in the lower Miocene clasts of RISP ranges from 55% to 71% of the diatom flora. Other Chaetoceros morphotypes contribute an additional 9% to the diatom flora of these clasts. Absolute abundance of Chaetoceros forma A in these clasts ranges from 65 million cells per gram to 90 million per gram. These values compare favorably with modern Chaetoceros abundance in the moderately high primary productivity regions of the inner continental shelf and outer Bransfield Strait along the Antarctic Peninsula (Chapter III). These clasts appear to represent higher productivity than exists today on the central and outer continental shelf, but lower productivity than the Recent Gerlache Strait (Chapter III). 184 Although very little is known of the tectonic configuration of this basin within the Ross Embayment during the early Miocene, seismic evidence suggests that it is a graben structure (Rooney et al., 1991; Tessensohn and Worner, 1991), possibly similar in geometry to the modern Bransfield Basin. The high abundance of Chaetoceros in lower Miocene RISP clasts suggests that the oceanographic setting of this part of the Ross Embayment during the early Miocene may have been similar to the sub-polar conditions of the modern Bransfield Strait. RISP Clast AD, which contains the middle Miocene assemblage (Chapter IV), is also dominated by Chaetoceros spores. Chaetoceros forma A comprises 50 percent of the diatom flora in this clast. Chaetoceros forma B and forma C together contribute an additional 11%. This suggests that primary productivity levels during this middle Miocene interval may have been roughly similar to that represented by the lower Miocene clasts. Absolute abundance estimates are not possible for this assemblage due to the lack of sufficient material for processing. Due to glacial mixing in the matrix sediments, it is not possible to assess the contribution ofChaetoceros to the upper Miocene assemblage of RISP sediments. Total assemblage counts demonstrate that the matrix sediments are very strongly dominated by reworked lower Miocene diatoms (Chapter IV). Chaetoceros forma A is less dominant in upper Miocene sediments from CIR, comprising between 12% and 27% of the diatom flora. This assemblage includes a large concentration of the centric diatom genus Actinocyclus (20 - 40%). The genus Actinocyclus generally reflects open marine conditions and may reflect lower overall primary productivity than an assemblage dominated by Chaetoceros spores. High concentration of large Actinocyclus, Thalassiosira or Eucampia may suggest relatively strong dissolution of diatom silica (Shemesh, et al., 1989), relative to the lower Miocene 185 clasts. The diatom assemblage of CIR clasts compares well with modern sediments of the open Antarctic continental shelf. Chaetoceros forma B contributes between 1% and 3.5% of the diatom flora of CIR clasts. The group referred to as Chaetoceros forma C had become extinct by late Miocene time and is not found in CIR clasts. As with Clast AD of RISP, absolute abundance estimates of diatoms in CIR clasts has not been possible, due to a paucity of material. Inferences Regarding Sea Ice Extent in the Miocene Sea ice extent and thickness place an environmental constraint on Chaetoceros abundance. Chaetoceros is an open water neritic diatom that blooms after the annual sea ice breaks out. Many diatoms thrive where sea ice is dominant (Horner, 1990) and many of these forms are preserved in the sediments. These sea ice-related diatoms provide a tracer of annual sea ice extent and conditions (Leventer and Dunbar, 1987; 1988). The diatom assemblage beneath a modern Antarctic sea ice-dominated environment, such as McMurdo Sound, typically consists of abundantNitzschia curta or Nitzschia cylindrus, with diatoms Pleurosigma sp., Navicuia sp., Amphiprora sp., and Pinnuiaria sp. (Leventer and Dunbar, 1987). Other sea ice diatoms are very poorly silicified and are not preserved in the sediments. Nitzschia sp. A, a common component of the lower Miocene clast assemblage of RISP and present in middle Miocene Clast AD of RISP, may be the ancestor of the modern sea ice-related species Nitzschia curta (Harwood, Scherer and Webb, 1989). N. curta dominates the diatom flora of the modern western Ross Sea and reflects ice edge 186 diatom blooms coincident with seasonal sea ice break-up (Leventer and Dunbar, 1988). Crary Ice Rise clasts contain the direct ancestor of N. curta, Nitzschia praecurta. Relative abundance of this Nitzschia lineage never exceeds 10% in any of the Miocene clasts, and only rarely exceeds 5% (Tables 6, 8). This concentration of N. curta is typical of sediments of the northern Antarctic Peninsula continental shelf (Leventer, 1991), but is much lower than that in sediments of the modern Ross Sea (Leventer and Dunbar, 1988; Truesdale and Kellogg, 1979) or Weddell Sea (Gersonde, 1986). If N. curta can be used as an environmental analog for these morphologically similar taxa, then the lower and middle Miocene diatom assemblages from RISP and the upper Miocene assemblage of CIR reflect seasonal sea ice formation in the Ross Embayment, at least during certain intervals, throughout most of the Miocene. Other taxa that may be indicative of sea ice conditions are present in the lower Miocene clasts of RISP, notably Pleurosigma sp. and Navicula trompeii. However, the low overall abundance of these taxa suggests that primary productivity was not dominantly ice-edge influenced, as is the case in the modern McMurdo Sound region (Leventer and Dunbar, 1987; 1988). DESCRIPTIVE ANALYSES AND PALEOENVIRONMENTAL INFERENCES FROM UpB DIATOM ASSEMBLAGES The diatom assemblages from Upstream B (1988-1989 sample) (Chapter VI) are sparse and highly fragmented, due to glacial processes. Delicate diatoms, especially 187 pennates, and small centric diatoms such as Chaetoceros spores are extremely rare. Furthermore, Neogene marine diatomite clasts have not yet been found. The general paucity of diatoms greatly limits the potential for paleoenvironmental reconstructions based on the diatom data from Upstream B. Under these circumstances, the lack of delicate sea ice-related diatoms, such as Pleurosigma and the Nitzschia curta lineage does not provide useful constraints on past states of sea ice cover. Likewise, the paucity of Chaetoceros spores says nothing about paleoproductivity. The late Miocene diatom assemblage from UpB (1988-’89) is comprised of well silicified pelagic diatoms (e.g., Stellarima microtrias, Thalassiosira torokina and Actinocyclus species). Similarly, the Pleistocene assemblage is made up of larger, well silicified species that are found in open water settings. Neogene benthic diatoms, which are often large and very heavily silicified, have not been found, though siliceous sponge spicules are relatively common. The only diatom-rich clasts recovered from UpB sediments are indicative of non marine habitats. The Aulacosiera clasts suggest high productivity in a lacustrine setting. Aulacosiera italica s.s. is a planktonic diatom that proliferates in large, moderately oligotrophic (low nutrients) to slightly eutrophic (high nutrients) lakes (Germain, 1981; Hustedt, 1958). The lack of freshwater benthic species, abundance of diatoms and paucity of terrigenous material suggests that the lake was probably large, deep and clear. Such a diatom-rich lacustrine setting can be found in the modern rift valley system of East Africa (Gasse and Street, 1978). As West Antarctica is an active rift zone, the non-marine diatomite clasts found at UpB might be a remnant of Tertiary rift lakes in West Antarctica. Pliocene or Pleistocene diatoms have not been found in RISP or CIR sediments. The lack of post-Miocene diatoms in these regions is indicative of a more dominantly 188 glacial regime in West Antarctica since the end of the Miocene. It is important to note, however, that the lack of Pliocene and Pleistocene diatoms in RISP and CIR sediments does not preclude the possibility of Late Neogene marine productivity, with reduced ice conditions in the West Antarctic interior. The extensive erosion caused by grounded ice processes during periods of glacial expansion in the Ross Embayment (Cooper et al., in press; Alley et al., 1989a; Bartek et al., 1991; Edwards et al., 1987) could easily have removed these young fossils, and transported them toward the continental shelf edge (see Chapter VIII). Evidence of post-Miocene marine productivity, in the form of biostratigraphically significant diatoms, has been found in matrix sediment samples from beneath the WAIS at Upstream B (Chapter VII). This diatom assemblage is comprised of heavily silicified centric planktonic taxa that are characteristic of cold water, open ocean conditions. The direct association of specimens cannot be assured, because the assemblage is compiled from a diamicton matrix, rather than from diatomaceous clasts. The Pleistocene assemblage does not contain evidence of unusually high primary productivity in neritic (e.g., Chaetoceros spores) or sea ice (e.g., Nitzschia curta or Nitzschia cylindrus spp.) environments. This negative result may, however, be greatly influenced by taphonomic effects of dissolution (Shemesh et al., 1989) and mechanical degradation in the sub-ice environment (Murray and Dowdswell, in press). Diatom taxa such as Thaiassiosira * lentiginosa, Thaiassiosira oiiverana, Porosira pseudodenticulata, Eucampia antarctica, Asteromphalus parvulus and Asteromphalus hookeri are found in low abundance in modern Antarctic inshore assemblages, but these taxa are more common offshore. Furthermore, these heavily silicified diatoms tend to be disproportionately abundant in taphonomically altered assemblages (Shemesh et al., 1989). Despite the constraints outlined, this assemblage of diatoms is most easily interpreted as reflecting open marine sedimentation, rather than nearshore conditions. The diatom assemblage may reflect lower overall productivity than is found in inshore or ice-edge waters, though data are too sparse to test this hypothesis. Further sampling from beneath the WAIS is needed. ADVANTAGES AND LIMITATIONS OF PALEOENVIRONMENTAL INTERPRETATIONS OF GLACIAL SEDIMENTS The diatom assemblages recovered from beneath the WAIS only reflect moments in time. As such they say nothing about the environmental conditions that may have predominated through the Cenozoic. More continuous records can only come from stratigraphic drilling. Although discontinuous, the reworked diatomaceous clasts from beneath the WAIS provide fundamentally important documents of ice sheet history. Each sample contains new information regarding the oceanographic history of the West Antarctic basins, in the form of records of significant end-member events (i.e., deglaciation). It is likely that as more samples become available, additional deglacial/high productivity events will be identified. These events can be correlated with stratigraphic reference sections from offshore and continental shelf drilling. Eventually, a marriage between sub-ice and sub-marine studies will lead to a detailed reconstruction of the history of the Antarctic ice sheets, and a better understanding of the interactions between the atmosphere, the hydrosphere, and the cryosphere. CHAPTER VIII GLACIAL SEDIMENTATION AND SEDIMENT TRANSPORT MODELS: IMPLICATIONS FOR ICE STREAM BED PROCESSES AND WEST ANTARCTIC ICE SHEET RECONSTRUCTION INTRODUCTION In previous chapters, it was shown that the analysis of microfossil assemblages in sediments from beneath the West Antarctic Ice Sheet (WAIS) provides evidence regarding the marine history of West Antarctic basins and thus contributes to our knowledge of the history of the West Antarctic Ice Sheet (WAIS). In this chapter, microfossil data from these same sediments are used as criteria in the evaluation of models of marine ice sheet and ice stream bed processes, and their geologic records. There is first a synthesis, discussion and evaluation of West Antarctic ice stream bed processes, based on the works of Alley and others (1986, 1989a, b) and Kamb (1990; 1987; Engelhardt et al., 1990). A method of testing these hypotheses is proposed. Secondly, Provenance Envelopes (Harwood, Scherer and Webb, 1989; Scherer, 1989) are defined for the West Antarctic interior sediments. Finally, a model to explain the distribution of diatom assemblages in sub-ice till is put forth. This model takes the form 190 191 of a stratigraphic cross section through the Ross Embayment, from the Bentley Trough to the Ross Sea continental shelf. ICE STREAM FLOW MODELS: TILL DEFORMATION VS. BASAL SLIDING In contrast to high profile ice sheets, virtually all the movement of low profile ice streams takes place at the contact between the ice and the bed (Engelhardt et al., 1990). Alley (1989a, b) and Alley et al. (1986, 1989a, b) describe a potential mechanism to explain rapid ice stream flow, whereby the ice flows on a pervasive layer of unconsolidated, water-saturated sediment that deforms with advance of the ice, erodes the sediments beneath and is redeposited as a "till delta" at the grounding line (Fig. 4, Chapter I). Engelhardt et al. (1990) identified and sampled unconsolidated sediments beneath fast-flowing ice stream B that have the physical characteristics predicted by Alley et al. (1986,1989a, b) and Blankenship et al. (1986, 1987), but Engelhardt et al. (1990) were not able to determine whether this layer deforms with ice flow or whether the ice flows predominantly from sliding along the top of this layer. As part of my post-doctoral research I will be collaborating with Barclay Kamb and Hermann Engelhardt of the California Institute of Technology to perform sediment coring and in situ measurements of sediments beneath ice streams B and C in order to evaluate this problem. Figure 31 shows a simplified view of the basal sliding and till deformation models. The cartoon shows, for both models, the simplest form of shear that would be expressed within the till layer to explain the flow mechanics and how these processes might affect Figure 31. Simplified cartoon outlining the differences between the "Basal Sliding" model and the "Till Deformation” model of ice stream flow. Direction of dip is in accord with geophysical results from ice stream B, but the angle of dip is greatly exaggerated. Horizontal scale may be as much as a few hundred kilometers. Vertical scale is tens of meters. Arrows drawn within the till layer are hypothesized shear vectors. The Kamb model has decreasing shear through the till layer, whereas the Alley model has roughly equivalent shear throughout the layer. Particles shown in the till may be microfossils or clasts that were eroded from the underlying strata. Notice that the Kamb model includes few recently eroded particles from local strata whereas the till in the Alley model is dominated by locally derived particles that are actively being eroded. This figure is a conceptual model that predicts microstratigraphy and between-core variability in the till layer given each flow mechanism. See text for further discussion. 192 193 ICE STREAM FLOW MODELS A. Ice Stream Water Till MATRIX MATRIX MATRIX MATRIX “ *" MATRIX MATRIX _ n LJttlaActlve Erosion a _ f~l - P BASAL SLIDING MODEL (KAMB) lea Stream Waterfllm - • • A * ° \ ° .ctlve TILL DEFORMATION MODEL (ALLEY) Figure 31 194 erosion of underlying strata and mixing of particles within the till. This two-dimensional model predicts the kind of mixed microfossil assemblages that might be expected within a sub-ice stream till, given basal sliding and till deformation as mutually-exclusive end members. The angle of dip is exaggerated, but the direction is based on geophysical observations on ice stream B (Rooney et al., 1991). As the ice stream crosses geological contacts, particles (including diatoms) from each layer are incorporated in the till. The basal sliding mechanism allows for little erosion at the base of the till, so few particles are added and they do not become well mixed within the till. In contrast, the till deformation mechanism requires erosion of underlying strata and mixing within the till, so that many particles from the underlying strata are incorporated. If basal sliding is the dominant mechanism, then the till beneath the WAIS (Engelhardt et al., 1990) must be relict in nature, having been formed by a different glacial-erosional mechanism, prior to rapid streaming flow. The till deformation model allows for considerable active erosion of sub-till strata, constantly adding mass and volume to the till layer. Geologic and geophysical observations in the Ross Embayment and Ross Sea tend to support the till deformation model over the basal sliding model. This interpretation is founded largely on the abundant evidence of extensive erosion and the transport of massive amounts of sediment from the West Antarctic interior to the Ross Sea (e.g., Hayes and Frakes, 1975; Cooper et al., 1991) and Weddell Sea (Melles, 1991) continental shelf breaks. The basal sliding mechanism does not allow for such bed erosion and sediment transport along the length of most of the ice stream. Basal sliding likely becomes significant near the the grounding line, where the ice sheet becomes more loosely coupled to the bed, and erosion rates decrease. The work of Kamb (1991) and 195 Engelhardt et al. (1990) demonstrates that there is some kind of water conduit system at the bed. Borehole experiments at Upstream B (Kamb, 1991; Engelhardt, 1990) have shown that basal water can flow at a rate faster than the ice flow. This, however, does not preclude till deformation as a significant mechanism of flow. It seems likely that an element of basal sliding occurs over areas of free water, but that deformation of till may be significant over regions with less water, often called "sticky spots" (Alley and Whillans, 1991). The system can be visualized as similar to a sub-glacial braided stream, with water channels and islands. It is unknown whether the sticky spots migrate or are relatively immobile, as influenced by topographic features beneath the till. When evaluating a geologic record, such as this, one must always try to separate on-going modern processes from artifacts of past depositional mechanisms. This is particularly true in the case of sub-ice stream sediments, where active erosion and mixing (Alley model) are re-mobilizing relict deposits that represent previous glacial states. Working from the Alley model and assuming similar diatom abundances and similar erosion rates for the successive strata, the abundance of diatom assemblages of distinct ages or environments found in the till will be roughly proportional to the distance from each source bed (Fig. 31). Thus, a till sample from a point above Strata V will include abundant fossils from that layer (the oldest), proportionally fewer fossils from Strata IV, et cetera, with fossils from Stratum I (the youngest layer) as the most rare. If erosion rates are equivalent and the till layer is of roughly equal thickness throughout (Blankenship et al., 1986; 1987), then in order to keep the mass of material in balance, the rate of flow must increase down-glacier. Measured flow rates for ice stream B suggest that this is the case (Whillans et al., 1987). However, the situation is more complicated than this. As the grounding line is reached, the ice sheet becomes 196 more loosely coupled to the bed (Alley et al., 1989a, b). Erosion rates decrease as the ice sheet approaches flotation. At the grounding line, sediment is being deposited, rather than eroded. The sedimentary structure formed at the grounding line has been described as a "till delta" (Alley et al., 1986; 1989a). Large scale sedimentary structures that may be relict till deltas have been identified by high-resolution seismic surveys on the Ross Sea continental shelf (Anderson, 1991). The erosion caused by ice stream flow and till deformation creates an unconformity that is probably analogous to the Ross Sea Unconformity, which is recognized widely across the Ross Sea (Karl et al., 1987). The Ross Sea Diamicton (Edwards et al., 1987), a pervasive sedimentary layer that overlies the Ross Sea Unconformity across most of the Ross Sea floor, may be the remains of "active till" and "till deltas" that were stranded when the previously expanded WAIS de-coupled from the bed and retreated. This has probably occurred repeatedly through the Late Neogene, with the most recent retreat in the early Holocene (Anderson, 1991). Detailed sedimentology and micropaleontology of sediment cores of till from beneath the ice streams will provide important data for distinguishing between these flow mechanisms. Microstratigraphic analysis of the till layer (~6 -1 0m) will determine whether or not this sediment layer has vertical inhomogeneities with respect to particulate constituents. Future work will include quantitative study of microstratigraphic variability in diatom assemblages and abundances downcore, using methods described in Chapter II. I propose a working hypothesis for the interpretation of microstratigraphic results from sub-ice stream sediment cores. Basal sliding on a wet bed, with lateral shear in the upper part of the till [Kamb model] will likely create some (possibly very subtle) variation 197 in sedimentary components with depth down-core (Fig. 31a). This interpretation would be strengthened by the identification of a similar pattern in adjacent cores. Because the uppermost sediments would receive most of the shearing and net transport, this layer should have the greatest variability of particles of distinct and possibly distant provenance. The lower part of the till layer would be most influenced by the sedimentary deposit that lies immediately below the till, suggesting little net transport. In contrast, deformation of the entire 6 to 10m layer [Alley model] would more likely produce a nearly uniform distribution of constituents from the top to the bottom of the core (Fig. 31b). Stated simply, basal sliding will impart a recognizable stratigraphy whereas till deformation will tend to destroy any discernible stratigraphy within a core. Diatom assemblage data are compiled as an integral part of the absolute abundance counts and downcore variation in microfossil assemblages will provide an additional gauge of sedimentary fabric. Observations of this kind will provide significant data that could not be made by physical property or textural analyses alone. PROVENANCE ENVELOPES Regardless of the specific flow regime, it is clear that glacial tills contain evidence of the sub-ice "outcrop" geology present in the catchment area of the glacier that carried the material (Webb, 1990; Strobel and Faure, 1987). Glacially reworked microfossils have been used to evaluate sediment provenance and paleoenvironments in Antarctic sediments by Truswell and Drewry (1984), Webb et al. (1984), Harwood (1986a) and Greene (1990). This approach is fundamental to the "provenance envelope" concept of 198 Harwood, Scherer and Webb (1989) and Scherer (1989). A provenance envelope (PE) Is a generalized reconstruction of the potential source area from which sediments have been eroded and carried by glacial processes. A Provenance Envelope is defined by combining biostratigraphic, paleoenvironmental and sedimentologic data from tills and other glacial sediments with available geophysical and structural geologic information and models of regional glacial flow. PE definition will be greatly improved by a better understanding of glacial sediment transport processes. Although texturally well mixed, the stratigraphic distribution of particles in glacial sediments is not totally chaotic. Some coherence with regard to this mixing is expected given the assumption that assemblages of particles (microfossils or distinctive sediment grains) are incorporated into the glacial deposits in relative proportion to availability (and erodibility) of source material. For example, a glacial sediment sample containing a large percentage of lower Miocene diatoms and diatom-rich clasts (e.g., RISP) is likely to be situated close to an "outcrop" of lower Miocene strata. Because grounded ice can cut deeply into easily erodible strata (such as Cenozoic marine sediments), a complex set of sub-glacial "outcrops" may be present beneath the thin "active till" (Alley et al. , 1986; 1989a, b; Blankenship et al., 1986; Rooney et al., 1991). If this till is continuously eroding the sediments beneath, as Alley suggests, then samples of "active till" from different regions will be readily distinguishable, based on clay mineral, rock fragment and microfossil contents. Such an approach has been applied to Pleistocene till exposures in North America (Strobel and Faure, 1987). From careful analysis of tills, identification of distinct populations of microfossils or sediment grains is possible (Scherer, 1991; Harwood Scherer and Webb, 1989; Scherer et al., 1988). Distinctions of this kind are possible between regions sampled, but also within sediment cores from each region, even if stratigraphic variability is very slight. The "unmixed" populations provide evidence of the discrete sediment sources that were eroded by glacial processes. The degree of sediment mixing in the till, as defined by the presence or absence of fossils of widely differing biostratigraphic ages and a diverse suite of distinctive rock clasts, gives qualitative information about relative distance travelled as well as the ages of strata encountered. The presence of soft (easily degradable) clasts of diatom-rich sediment probably indicates an in situ deposit in contact with glacial ice nearby. Information of this kind, when considered in light of the regional glacial and sub ice topographic setting, is used in defining the boundaries of provenance envelopes. As more sites become available, PEs for each site will be compiled and mapped. Paleontologic and geologic data from multiple sites may eventually define overlapping PEs. These compiled interpretations will ultimately provide a first order geologic map of the ice-covered region. Although specific "outcrop" locations cannot be directly identified, geographic areas containing certain "exposed" strata will be known and significant trends will likely become apparent as more data become available. These compilations, in association with current and upcoming geophysical surveys (e.g., ANTOLITH1, ANTOSTRAT2, CASERTZ3), are fundamental steps toward surveying for ideal sites for future sub-ice stratigraphic drilling programs. It is possible to define a Provenance Envelope for virtually any microfossil-bearing glacial sediment, though the degree of certainty will vary widely. Although Provenance Envelopes are qualitative in nature, they do provide a useful approach for evaluating a large region with otherwise unknown SCAR Group of Specialists on the Antarctic lithosphere SCAR Group of Specialists on Cenozoic Palaeoenvironments of the Southern High Latitudes Corridor Aerogeophysics of the South Eastern Ross Transect Zone 200 geology. I foresee the eventual publication of folio-style Provenance Envelope maps of West Antarctica in association with geophysicsts (D. Blankenship, pers. com, 1991). Preliminary Provenance Envelope Definitions Ross Ice Shelf Project Site J-9 (RISP) The mixed Miocene sediments found in RISP cores suggest that the site is proximal to a sub-ice (or currently sub-marine) "outcrop" of lower Miocene sediment, including a lower Miocene marine diatomite, with younger Miocene sediments occurring further up-glacier from the site. Upper and middle Miocene sediments were transported at the glacier bed and incorporated into the lower Miocene-dominated matrix of the RISP diamicton. The very rare older fossils may have been recycled several times since their initial deposition. The microfossil assemblages of RISP suggest a Provenance Envelope that is moderate in size relative to CIR and UpB (see Fig. 32). Although the RISP camp was situated on ice that flowed out of ice stream B (at a rate of ~1 m/day), the configuration of the bed and what is known of the history of these ice streams suggests that the RISP sediments were more likely to have been deposited beneath ice stream C, rather than ice stream B. The site lies in a trough that continues beneath the grounding line of ice stream C. In marked contrast to fast-flowing ice stream B, ice stream C, which has a configuration generally similar to ice stream B, is currently stalled (Shabtaie et al., 1987; Whillans et al., 1987). Ice penetrating radar (Shabtaie et al., 1987) and microseismicity studies (Anandakrishnan, et al., 1989) of ice stream C Figure 32. Provenance Envelopes proposed for RISP and CIR sediments. At the current state of analysis, PE’s are strictly diagrammatic. The patterns outlining PE’s are considered roughly probabilistic with regard to location of source rocks. Dark shading represents the region of most likely provenance of particles in the till sample. Probability decreases with increasing distance from the sampling site, although no quantitative representation of provenance is implied. RISP sediments are interpreted as predominantly locally derived lower Miocene sediment, with younger sediments carried from up-glacier of ice stream C. An ice stream B component is possible, particularly for the upper sedimentary unit, as suggested by the question mark. CIR sediments are probably dominated by upper Miocene particles derived from the crest of the topographic high. 201 Ice Stream B \ |Ce 0%.-^ * u -r^A VStreams x&.-.-l' * . A ^<$<1, ' N v Jr " 5Vo V C Ridge \ x BC \ \ Stream \ Figure 32 203 suggest that it ceased rapid flow only about 250 years ago. Ice stream C is in positive mass balance, whereas ice stream B is in negative balance, suggesting that there may be complex interactions between the ice streams (Alley and Whillans, 1991; Whillans et al., 1987). Rooney et al. (1991) found that ice stream B lies in a graben. Ice stream C appears to follow a trough of similar geometry. For these reasons, I believe that RISP sediments represent a relict deposit that was deposited at the bed of ice stream C when it was active and its grounding line was advanced relative to today. This may have been as recently as late Pleistocene/early Holocene. Unlike the interpretations of Kellogg and Kellogg (1981; 1986), who identified RISP sediments as a Pleistocene till based on erroneous diatom identifications, I base the Pleistocene age on glaciologic criteria. Pliocene and Pleistocene diatoms were probably deposited at RISP Site J-9 during post- Miocene deglacial intervals, but these have been eroded by subsequent ice sheet advances across the Ross Embayment continental shelf. The possibility that very rare Pleistocene and Pliocene diatoms still exist in this sediment matrix is not discounted. Traces of post-Miocene diatoms (the first to be eroded) could be present, but if so they have been so diluted by abundant diatoms from locaily-derived and recently eroded Miocene sediments that, despite extensive searches, they have not been found. The Provenance Envelope defined for RISP sediments extends a moderate distance beneath ice stream C (Fig. 32). The sampling site probably overlies in situ lower Miocene sediments. The PE extends upglacier beneath ice stream C, crossing contacts between lower, middle and upper Miocene "outcrops" of sub-ice strata. A PE outline is drawn in the direction of ice stream B, with a question mark, to account for the possibility that some of the material, possibly the upper sedimentary unit, is composed of material carried by ice stream B. 204 Crary Ice Rise (CIR) It seems clear that the sediments collected from the northern flank of Crary Ice Rise were transported and deposited at the glacier bed, or at least very near the grounding line. It is known that the ice shelf was not in contact with the topographic high at this site as recently as 200 years ago (Bindschadler et al., 1989) and that the ice rise has a complex grounding line history (Bindschadler et al., 1989; Jezek, 1984; McAyeal and Thomas, 1980). As ice stream B ice thickened and made contact with CIR, the flow of the ice undoubtedly eroded soft sediments, carrying them to the local grounding line, at the northern flank of the ice rise. They were then deposited where the ice decoupled from the bed. Subsequently, as the ice thickened and slowed, erosion rates became greatly reduced. This sequence of events may have been repeated innumerable times over the last few to hundreds of millennia. Repeated cycles of erosion and deposition across CIR would explain the lack of Pliocene and Pleistocene fossils in the sediments studied. It is possible that these younger diatoms are buried beneath transported Miocene strata. Another possible control on deposition and erosion at CIR is the unknown tectonic history of this topographic feature. No data are available to determine whether the ice rise is a youthful volcanic feature or an older continental fragment. The depositional model described above suggests that CIR sediment has a small Provenance Envelope. The sediments are predominantly locally derived (Fig. 32). Specifically, upper upper Miocene sediments and rare middle Miocene and older sediments identified from the samples were stripped from the crest and deposited at the northern flank of the ice rise during the Quaternary, possibly the Holocene. 205 Upstream B (UpB) Sediments from Upstream B (UpB) contain a suite of rare but diverse microfossils, spanning a very wide range of ages and paleoenvironments (Chapter VI). This wide mix suggests a rather large PE, perhaps as large as the catchment area of ice stream B (Fig. 33). With the exception of older microfossils that may have been recycled several times, most of the microfossils (upper Miocene) are of local or regional origin, within the catchment area of ice stream B. This interpretation is based on the dominance of just a few ages, with an absence of many of the varied strata that undoubtedly lie beneath the WAIS. The outer limits of this zone are the region of high basal erosion rates, where streaming commences. The Upstream B camp lies approximately 200 km from the zone where streaming activity begins. This catchment region is greater than 100 km wide, at its widest part, though the ice stream at the camp site is less than 25 km wide. From the micropaleontologic and glaciologic data, I envision a PE as drawn on Figure 33. Byrd Station ice core (BYRD) basal debris The lower four meters of the ice core from Byrd Station contained sedimentary debris (Gow, 1979; Gow et al, 1968). Attempts to recover sediments beneath the ice failed. Geophysical evidence suggests that the ice at this region overlies rocks of high seismic velocity, interpreted as either pre-Cenozoic sediments or Cenozoic volcanics (Bentley and Clough, 1972). Clay mineralogy of the sediments collected includes a relatively high proportion of kaolinite (Gow, 1979; Turner and Krissek, 1991). This suggests source rocks of non-glacial origin. Harwood (1986d) searched for microfossils Figure 33. Provenance Envelopes proposed for UpB, Byrd, RISP and CIR. Notice that the PE for Upstream B sediments is much larger than the others. This interpretation is based on the wide mix of particles and the lack of a strongly dominant component. The rare Pleistocene component is derived from more distant sources than the upper Miocene fossils. The rare microfossils in the basal debris of the Byrd ice core are derived from older sediments that are being eroded by massive, slow-moving basal ice. At the current state of analysis, these PE’s are strictly diagrammatic, as previously described. Mountain peaks of the Ellsworth/Whitmore and Transantarctic ranges are indicated by X. 206 207 RONNE ICE. SHELF \X 90 CD BYRD UpB vnCIR , J-9 R O SS \ + rcE U SHELF L 180 Figure 33 208 in these sediments, but found none. As part of this study, I re-examined Harwood’s slides and prepared twenty new slides of sediments from the Byrd core. The only microfossils found include several very poorly preserved terrestrial palynomorphs that provide little biostratigraphic information (T. Taylor, pers. comm, 1990), two unknown siliceous cysts and one siliceous fragment of unknown affinity, possibly a radiolarian fragment (Plate XI). These fossils suggest a pre-Cenozoic age for source rocks. These results suggest that all younger sediments have been eroded and removed from this region, exposing older strata to the base of the glacier. The ice core overlies a relatively steeply sloping basal surface (Gow et al., 1968), which could explain the removal of younger strata in this region. A PE compiled for the BYRD sediment samples would be very small (Fig. 33), due to the apparent lack of recent glacial mixing. An Integrated Geoloalcal/Glacloloalcal Model for the Ross Sector of the WAIS Biostratigraphic analysis of sub-ice sediments provides a valuable aid for seismic and other geophysical investigations. Rooney et al. (1991) determined, based on seismic reflection methods, that ice stream B, and its associated unconsolidated (deforming?) till layer, overlies about 600m of sediments with low seismic velocities (<2.0 km/sec). The seismic results suggest that the ice stream overlies a sediment-filled, structurally controlled trough (graben). The sub-ice sediment column contains an important stratigraphic record of West Antarctic history. Based on comparison with DSDP Leg 28 drillcore and seismic results from the Ross Sea, Rooney et al. (1991) interpreted the sedimentary column beneath ice stream B as upper Oligocene and younger. Micropaleontologic analysis of PLATE XI. Microfossils in the basal debris of the Byrd Station ice core. All are probably pre- Cenozoic. a. - c. Palynomorphs. Scale bar = 50|xm d. Unknown siliceous microfossil, probably a radiolarian fragment. 209 210 PLATE XI. - 211 sediments from beneath Upstream B (Scherer, 1991) revealed that the most abundant marine microfossils are diatoms of late late Miocene age (similar in age to the diatoms found at Crary Ice Rise; Scherer et al., 1988). These fossils may represent the age of in situ sediments beneath the till layer. Pleistocene diatoms in the UpB till samples are much more rare and probably represent transport from farther up-glacier. A detailed view of sub-ice and submarine bathymetry of the Ross Embayment is presented in Figure 34. This diagram clearly shows the structurally controlled (Rooney et al., 1991) ridge-and-trough setting that delineate the boundaries of the modern Ross Ice Streams. Troughs beneath the northern Ross Ice Shelf and Ross Sea are more likely glacially carved (Cooper et al., 1991; Anderson, 1991). A new sediment erosion and transport model for the Ross sector of the West Antarctic Ice Sheet is shown in Figure 35. A cross section is presented, from the Bentley Subglacial Trough (A) to the southern Ross Sea, past DSDP Hole 270 ( A ’) (Fig. 34). The model is based on the micropaleontological analyses presented as well as glaciological, geophysical and geological observations from the WAIS and the Ross Sea. The purpose of this model is to portray the current configuration of the WAIS, relative to the uppermost strata beneath the ice. The geometry and structure of West Antarctic basinal stratigraphic successions are simplified and schematic. The thickess of upper Paleogene and Neogene sediments is constrained by DSDP drilling at Site 270 (Hayes and Frakes, 1975) and by seismic stratigraphy at Upstream B (Rooney et al., 1991). Stratigraphic thicknesses and sedimentary geometries in the Bentley Trough are inferred. Ice thicknesses are based on radio echo sounding (Drewry, 1983). Figure 34. Detailed bathymetry of the Ross Embayment (from Rooney et al, 1991). The transect line for Figure 35 is shown, as are UpB, RISP and DSDP Site 270. 212 213 000*1 10* 8* G* 4* 2* 0 Zm BYRD "GUI — EDWARD 'Sir / \P E N IN ./ Figure 34 Figure 35. Stylized stratigraphic cross section of the Ross Embayment with regard to the current configuration of the West Antarctic Ice Sheet (WAIS) and submarine and sub-ice strata. Detailed discussion of the glacial erosional/depositional zones can be found in the text. Geologic and geophysical constraints on the model and the assumptions used in the development of the model are also discussed in the text. This figure explains the distribution of microfossil ages beneath the Ross Embayment sector of the modern WAIS and predicts the general position of stratigraphic contacts relative to the unconformable contact between Neogene strata and the basal ice and till. See text for further discussion. 214 ROSS EMBAYMENT ZONE 1 ZONE ZONE 3 ZONE ZONE 5 RISPUpb DSDP Site 270 INLAND ICE ROSS ICE SHELF ICE STREAM SEA LEVEL ROSS SEA^ a c t iv e t i l l " " " " " " BENTLEY TROUGH RELICT TILL Coupling Grounding Line Line Onset of Streaming Figure 35 216 Although it is known that the troughs are fault-controlled (Rooney et al., 1991), this two-dimensional model assumes simple stratigraphy and geologic structure, because the age of the faulting is unconstrained. Other than the known unconformity at the base of the modern ice sheet, no stratigraphic discontinuities are presented, for the same reason that faulting is ignored. The purpose of this figure is not to describe West Antarctic stratigraphy, because too little is known at this time. Instead, it provides an explanation for the distribution of microfossils near the sediment-ice interface and illustrates the unconformable contact between the ice sheet, active till and the underlying strata. The cross section is divided into five zones, based on the modern glacial configuration and its relation to the local sedimentary regime. It helps to explain the extent of erosion and sediment transport driven by repeated advances and retreats of grounded marine ice sheets in West Antarctica. The model illustrates the lack of in situ Pliocene and Pleistocene strata at RISP and predicts the continued existence of undisturbed post-Miocene strata in the deep ice-filled basins of West Antarctica. Post- Miocene sediments were deposited episodically, during de-glacials in West Antarctica. Deep within the interior, in the Bentley Subglacial Trough (Zone 1), there is little ice movement at the bed, thus erosion is limited. Most of the ice flow is due to internal plasticity within the ice sheet. Under the West Antarctic Ice Sheet in this region there may be a thin layer of "lodgement till" overlying sediments deposited during de-glacials as recent as late Pleistocene. The Catchment Zone (Zone 2) is the region where basal ice movement begins. Erosion and transport of the basal sediments begins in this zone and intensifies near the region where ice streaming begins. The Ice Stream Zone (Zone 3) is a region of rapid erosion of sediments with the development of a layer of "active till" at the bed, as 217 described above. Erosion rates decrease near the coupling line, where the ice stream begins to go afloat. The Till Delta Zone (Zone 4) is the region where the ice is decoupled from the bed, where net deposition of till replaces erosional activity. The Marine Zone (Zone 5) lies beyond the grounding line, beneath the ice shelf and the open sea. This is a region currently lacking glacial erosion, with glacial sedimentation limited to minor glacial-marine deposition from the ice shelf and melting icebergs. The Ross Sea Diamicton is a vestigial deposit, referred to as a "relict till." This relict till probably represents an analog for "active till" and "till delta" sediments, collectively. The relict till layers represent glacial sedimentary processes from previous expanded grounding lines of the WAIS. In the open sea, north of the ice shelf, normal marine sedimentation occurs on top of this layer. Neogene glacial sediments thicken toward the continental shelf break, beyond Zone 5 (Cooper et al., 1991). Prograding successions of glacial sediment packages reach thicknesses as great as 6 km near the edge of the continental shelf (Cooper et al, 1991). The thick successions of glacial sediments are testament to the ability of marine ice sheets to move large volumes of sediment. The position of Cenozoic stratigraphic layers shown on Figure 35 (ie., "sub-ice outcrops"), relative to the ice, are derived from the microfossil evidence from the Ross Embayment, and from seismic evidence. UpB contains predominantly late Miocene fossils, with rare diatoms of Pleistocene and Pliocene age, suggesting that the underlying strata are upper Miocene and that Pliocene and Pleistocene strata are preserved further upglacier. RISP contains a dominance of early Miocene diatoms, with middle and late Miocene diatoms present but less common, suggesting that till from Site J-9 overlies lower Miocene strata, with middle and upper Miocene sediments lying further upglacier. The 218 youngest fossils were deposited all across the West Antarctic interior during de-glacial events of the Pliocene and Pleistocene, but these strata have been removed from much of the Ross Embayment by repeated erosion by till deformation at the bed of an active ice stream, or by an unknown glacial-erosional mechanism. Pleistocene and Pliocene fossils are believed to be preserved in situ in the deep basins (Zone 1), due to minimal erosion and transport. The strata dip to the south, toward the center of the ice sheet, due to isostatic depression and the tectonic influences on the basins. Repeated erosional events across RISP Site J-9 have stripped away all traces of Pleistocene, Pliocene and uppermost Miocene sediments. These sediments were carried across the continental shelf and were deposited at the grounding line at various stages of ice stream advance and/or retreat. The rare Pleistocene and Pliocene fossils found at UpB represent the last traces of young sediments that have been eroded from the edge of the basin at the onset of streaming flow. DSDP Sites 270 and 272 include several meters of a till-like diamicton, underlain by lower Miocene sediments. The overlying till contains a mix of diatoms, dominated by Miocene forms, but including Pliocene and Pleistocene diatoms. The model shown in Figure 35 helps describe the Provenance Envelopes for these sediments. SUMMARY AND CONCLUSION Sediments collected from beneath the Ross Embayment sector of the West Antarctic Ice Sheet (WAIS) and the Ross Ice Shelf provide a window into the complex history of the West Antarctic Ice Sheet and West Antarctic marine basins. The presence of fossil marine diatoms and other microfossils in sub-ice-shelf and sub-ice-sheet glacial sediments implies productivity in open marine conditions at the time of initial deposition of the fossils. An approximate chronology of ice minima (ie. open water) events in the Ross Embayment is possible by identifying age-diagnostic fossils in these sediments. Sediments recovered from beneath the southern Ross Ice Shelf at Ross Ice Shelf Project Site J-9 (RISP) contain abundant early Miocene fossils (21 - 18.6 Ma), an age that coincides with third order eustatic sea level cycle 2.1 of Haq et al. (1988). This assemblage is found in abundant clasts of lower Miocene diatomite, as well as in the matrix. A middle Miocene diatom assemblage (15.5 -14.4 Ma), coinciding with eustatic cycle 2.4, is found in rare clasts and the matrix. Finally, a late Miocene assemblage (~8.8 - 8.2 Ma), which coincides with cycle 3.1, is found in matrix sediments. Very rare older fossils are found in the matrix and in clasts, but diatom assemblages indicative of deposition more recently than 8.2 million years ago are not present. Sediments from Crary Ice Rise (CIR), 50km southwest of RISP, contain abundant late late Miocene (~7.5 - 6.3 Ma) diatoms. This assemblage coincides with cycle 3.2 of Haq et al. (1988). Diatoms of this age are found in diatomite clasts as well as in matrix sediments. Older diatoms, notably of middle Miocene age, are present, but no diatoms younger than Miocene have been found. The lack of Pliocene or Pleistocene diatoms in RISP and CIR sediments appears to suggest that sediments of this age are not present beneath this sector of the West Antarctic Ice Sheet. This would seem to imply that the WAIS has been in place continuously since the latest Miocene. However, diatom data from sediments collected in 1989 from beneath the WAIS at Upstream B (UpB), on ice stream B, suggest that the glaciologic history of West Antarctica is far more complicated, in addition to late Miocene diatoms, similar to those from Crary Ice Rise, and older fossils, late Pleistocene (<600,000 years) and rare Pliocene diatoms are present in sediments from Upstream B. The presence of younger diatoms at UpB, which is further up-glacier than RISP and CIR, suggests that the WAIS has undergone major ice sheet retreat and re-advance during the post-Miocene interval. These data provide the first and only direct evidence that the West Antarctic Ice Sheet does have a history of collapse. These results are in stark contrast with the views of many paleoceanographers who have interpreted oxygen isotope and other marine proxy data as indicating a stable history of the WAIS spanning the last 6 Ma. The lack of younger diatoms at RISP and CIR is likely to be the result of glacial erosion from repeated grounding line advances and retreats across the Ross Embayment. UpB lies closer to the deep interior basins, which may contain more extensive deposits of post- Miocene sediments. Microfossil data from these sediments indicate a wide variety of Cenozoic paleoenvironments in West Antarctica. Clasts of non-marine diatomite of probable Miocene age are found in the UpB sample, along with terrestrial palynomorphs and plant fragments. The presence of a clast of nannofossil ooze and a planktonic foraminifer in 221 UpB sediment indicates that calcareous ooze production extended to extreme southern latitudes during Paleogene intervals. Clasts of lower, middle and upper Miocene marine diatomite in RISP and CIR sediments provide evidence of high primary productivity in neritic environments, as indicated by the high absolute and relative abundances of Chaetoceros spores. Relative paleoproductivity levels indicated by these clasts are estimated based on comparison with similar analyses of Chaetoceros spores in modern sediments from the northern Antarctic Peninsula. The diatom assemblages in these clasts also suggest that seasonal sea ice was a significant feature of the Miocene Ross Sea, but that peak productivity was not sea ice-related during accumulation of these diatomites. Geologic and geophysical evidence from the West Antarctic interior and the Antarctic continental shelf demonstrates that there has been extensive erosion of Neogene stratigraphic sections. Most of this erosion was probably caused by grounded marine ice sheet processes. The mechanisms associated with rapid ice stream flow and erosion and transport of sub-glacial sediments in marine ice sheet systems are subjects of much debate. A novel approach for testing the various hypotheses is proposed, using quantitative micropaleontology. A schematic model of West Antarctic stratigraphy, as influenced by marine ice sheet processes, is presented. From these works it is concluded that microfossil studies of sub-ice sediments are providing significant breakthroughs that are leading toward a greatly improved understanding of climate history. In order to make reasonably accurate predictions of global climate change, coupled ocean-atmosphere-cryosphere models need to be greatly improved. The models must be able to recreate known past environmental changes, as identified by paleoenvironmental studies. It has now been demonstrated that the West Antarctic Ice Sheet has a history 222 of instability, including periodic collapse and reformation. 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APPENDICES APPENDIX A Comprehensive list of all diatom, silicoflagellate, ebridian and coccolithophorid species encountered in the study. Author references are included. To the right is a talley of occurrences of these taxa in sediments studied as part of this dissertation (AP=Antarctic Peninsula; RISP=Ross Ice Shelf Project; CIR=Crary Ice Rise; UpB=Upstream B). Diatoms AP RISP CIR UpB Achnanthes brevipes (Greville) Cleve, 1895 X Actinocyclus fryxellae Baldauf and Barron, 1991 X X X Actinocyclus ingens Rattray, 1890 X Actinocyclus octonarius (Ehrenberg) Ehrenberg group X X X (Hustedt, 1930) Actinocyclus sp. A (cf. A. ochotensis) Scherer, 1991a; X X Baldauf and Barron, 1991 Actinocyclus spp. X X X X Actinocyclus actinochilus (Ehrenberg) Simonsen, 1982 X X Actinocyclus sp. cf. A. actinochilus "early form" X sensu Harwood and Maruyama, in press Actinoptychus senarius (Ehrenberg) Ehrenberg (1843) X X X X Arachnoidiscus spp. X X Asteromphalus hookerii Ehrenberg, 1844 X X Asteromphaius parvuius Karsten, 1905 X X Asteromphalus symmetricus Schrader and Fenner, 1976 X Asteromphalus sp. cf. A. symmetricus X Harwood, Scherer and Webb, 1989 248 Aulacodiscus brownei Norman (sensu McCollum, 1975) Aulacoseira italica var. A Scherer, 1991a Auliscus sp. Azpetia oligocenica (Jous§) Sims (Fryxell Sims and Watkins, 1986) Azpetia tabularis (Grunow) Fryxell and Sims (Fryxell, Sims and Watkins, 1986) Biddulphia spp. Chaetoceros spp., forma A Chaetoceros spp., forma B Chaetoceros forma C {Xanthiopyxis spp.) Chaetoceros forma C (Liradiscus spp.) Chaetoceros spp. forma C Corethron criofilum Castracane, 1886 Cocconeis spp. Coscinodiscus sp. A Harwood, 1986b Coscinodiscus marginatus Ehrenberg (Hustedt, 1930) Coscinodiscus oculus-iridis Ehrenberg, 1939 Coscinodiscus radiatus Ehrenberg, 1839 Coscinodiscus spp. Cymatosira biharensis Pantocsek, 1889 Dactyliosolen antarcticus Castracane, 1886 Denticulopsis maccollumii (McCollum) Simonsen, 1979 Denticulopsis delicata Yangisawa and Akiba, 1990 Denticulopsis hustedtii (Simonsen & Kanaya) Simonsen, 1979 Denticulopsis lauta (Bailey) Simonsen, 1979 Diploneis spp. Drepanotheca bivittata (Grunow & Pantocsek) Schrader, 1976 Endictya hungarica Hajos, 1968 Eucampia antarctica Castracane, 1886 Grammatophora spp. Hyalodiscus spp. Isthmia sp. Navicula glaciei Van Heurck, 1909 Navicuia trompeii Cleve, 1900 Nitzschia angulata Hasle, 1972 Nitzschia curta (Ehrenberg) Hasle, 1972 Nitzschia cylindrus (Grunow) Hasle, 1972 Nitzschia evenescens Schrader, 1976 Nitzschia grossepunctata Schrader, 1976 Nitzschia kergulensis (O’Meara) Hasle, 1972 Nitzschia maleinterpretaria Schrader, 1976 Nitzschia praecurta Gersonde, 1990 Nitzschia obliquicostata (Van Heurck) Hasle, 1972 Nitzschia ritcheri (Hustedt) Hasle, 1972 Nitzschia separanda (Hustedt) Hasle, 1972 Nitzschia sublinearis (Van Heurck) Hasle, 1972 Nitzschia (?) sp. C Scherer, 1991a Nitzschia sp. A Harwood Scherer and Webb, 1989 Nitzschia sp. 17 Schrader, 1976 Odontella weisfloggii (Janish) Grunow, 1884 Paraiia sulcata (Ehrenberg) Cleve (Hustedt, 1930) Pieurosigma spp. Porosira pseudodenticulata (Hustedt) Jous§ (Kozlova, 1962) Pseudopyxilla americana (Ehrenberg) Forti, 1909 Pterotheca sp. B Harwood, 1989 Pyxilla reticulata Grove & Sturt, 1887 Raphidodiscus marylandicus Christian, 1887 Raphoneis spp. Rhabdonema spp. Rhizosolenia alata Brightwell, 1858 Rhizosolenia barboi Brun, 1894 Rhizosolenia hebetataBaWey, 1856 group Rhizosolenia praebarboi Schrader, 1976 Rhizosoienia styliformis Brightwell, 1858 Rhizosolenia sp. A Harwood, Scherer and Webb, 1989 Rhizosolenia sp. 2 Schrader, 1976 Rouxia spp. (fragments) Stellarima microtrias (Ehrenberg) Hasle and Sims, 1986 Stellarima stellaris (Roper) Hasle and Sims, 1986 Stephanodiscus? (Mesodictyon?) sp. A Scherer, 1991a Stephanopyxis spinosissima Grunow, 1884 Stephanopyxis turris (Greville & Arnott) Ralfs group (Pritchard, 1861) Stephanopyxis sp. A Harwood Scherer and Webb, 1989 Stephanopyxis sp. C Harwood, 1986b Stephanopyxis spp. Stictodiscus hardmanianus Greville, 1865 "Synedra" sp. A Harwood, Scherer and Webb, 1989 "Synedra" sp. B Harwood, Scherer and Webb, 1989 Thalassiosira antarctica Comber, 1896 Thalassiosira fraga Schrader (Schrader and Fenner, 1976) Thalassiosira frenguelopsis Johansen and Fryxell, 1985 Thalassiosira gracilis (Karsten) Hustedt, 1958 Thalassiosira gracilis var. expecta (Van Landingham) Fryxell and Hasle, 1977 Thalassiosira affin. irregulata Schrader and Fenner, 1976 Thalassionema nitzschiodes (Grunow) Peragallo, 1921 Thalassiosira lentiginosa (Janisch) Fryxell, 1977 Thalassiosira oiiverana (O’Meara) Makarova and Nikolaev, 1984 Thalassiosira oliverana var. sparsa Harwood and Maruyama, in press Thalassiosira poroseriata (Ramsfjell) Hasle, 1972 Thalassiosira ritscheri (Hustedt) Hasle, 1968 Thalassiosira striata var. "offcenter" sensu Harwood and Maruyama, in press Thalassiosira torokina Brady, 1977 Thalassiosira trifulta Fryxell (Fryxell and Hasle, 1979) Thalassiosira tumida (Janisch) Hasle (Hasle, Heimdal and Fryxell, 1971) Thalassiosira sp. A Scherer et al., 1988 Thalassiosira sp. B Scherer, 1991a Thalassiosira sp. C Scherer et al., 1988 Thalassiosira sp. D Scherer, 1991a Thalassiothrix sp. (apices) Triceratium oamaruense Grove & Sturt, 1887 Trinacria pileolus (Ehrenberg) Grunow, 1884 Trinacria excavata Heiberg, 1863 Trochosira spinosa Kitton, 1871 Tropidoneis spp. Genus and species unidentified A Scherer, 1991a Genus and species unidentified B Scherer, 1991a Genus and species unidentified C Scherer, 1991a Unidentified "vase-like" diatom spores ( Pterotheca ? spp.) 254 Ebridians Pseudammodochium sp. cf. P. dictyoides of Ling, 1983 X X Pseudammodochium sphaericum Hovasse, 1932 X Silicoflaqellates Dictyocha sp. X X Distephanus bolivensis Frenguelli, 1940 X X X Distephanus crux Ehrenberg, 1840 X Distephanus quinquangellus Bukry & Foster, 1973 X Distephanus pseudofibula (Schulz) Bukry, 1976 X X Distephanus speculum (Ehrenberg) Haeckel, 1886 X X X X Distephanus spp. X X Mesocena pappii Bachmann, 1962 X Calcareous Nannofossils Chiasmolithus alatus Bukry and Percival, 1971 X Reticulofenestra daviesii (Haq) Haq, 1971 X Reticulofenestra samodurovii (Hay, Moler and Wade) Roth, 1970 X Foraminifera Chiloguembelina sp. X APPENDIX B Biostratlgraphlc ranges of key diatoms beneath the Ross Ice Shelf/WAIS system* Diatoms FADLAD Reference* Actinocyclus fryxellae 11.7 6.1 Actinocyclus ingens 16.4 0.62 Actinocyclus actinochilus 3.0 extant Actinocyclus cf. actinochilus "early form" 3.0 2.3? Asteromphalus hookerii 2.5 extant Asteromphalus parvulus 4.2 extant Asteromphalus symmetricus 30.1 18.6 Cymatosira biharensis late Olig. early Mio. Fenner, 1984 Denticulopsis maccollumii 17.0 10.5 Denticulopsis delicata late Mio. late Mio. Akiba and Yangisawa, 1990 Denticulopsis hustedtii 14.2 5.7 Denticulopsis lauta 16.3 8.0 Eucampia antarctica 14.5 extant Gersonde and Burckle, 1990 Nitzschia grossepunctata 16.5 12.5 Nitzschia maleinterpretaria 20.2 14.1 Nitzschia praecurta 10.6 4.2 Gersonde and Burckle, 1990 Nitzschia separanda ? extant 255 256 Nitzschia sp. 17 (Schrader, 1976) 17.8 13.0 Porosira pseudodenticulata ? extant Raphidodiscus marylandicus 22.0 14.5 Thalassiosira antarctica ? extant Thalassiosira fraga 21.0 18.6 Thalassiosira gracilis var. expecta >2.2 extant Thalassiosira affin. irreguiata late Olig. mid early Mio. Barron, 1985 Thalassiosira lentiginosa 3.9 extant Thalassiosira oliverana var. sparsa 8.1 4.5 Thalassiosira ritscheri 3.5 extant Thalassiosira striata var. "offcenter" 4.1 ~3 Thalassiosira torokina 8.2 1.8 Thalassiosira trifulta ? extant Thalassiosira tumida ~2 extant Unless otherwise indicated, numerical ages are drawn from Tables 15 and 16 of Harwood and Maruyama (in press), which summarize published FADs (First Appearance Datum) and LADs (Last Appearance Datums) from Antarctic drill holes, up to and including ODP Leg 120. References for earlier published ranges can be found in Harwood and Maruyama (in press). First appearances marked with a question mark indicate extant Antarctic species that have never been reported from sediments older than Holocene/Pleistocene. APPENDIX C Flow charts for planning a field and laboratory program for sampling and analysis of sediments from beneath the West Antarctic Ice Sheet (WAIS) 257 Figure 36. Flow diagram outlining sediment recovery and handling procedures for sub-ice sediments. 258 WAIS sediment recovery and handling Pre-season planning Glaciological G e o lo g ic a l Geophysical G o a ls G o a ls G o a ls >f Site Selection Reid Work 3 Months Sample Recovery (Piston or Drill Cores) Field Analyses Physical Properties & Pore Waters and Field description Borehole experiments Other Geochemistry of Sediments Packing & Shipping Shipping cores •_ ...... _ _ jL. 3 Months ' ------— Laboratory Laboratory Analyses Analyses 12 Months + Cores Opened Detailed Description Sample Half Archival Half ...... v \ Sub-sampling -__ \ Ship to Antarctic Physical Property Core Facility, Analyses Florida State Univ. Paleontology Analyses Geochemical Geologic Analyses Analyses Bioqeochronology Paleoenvironments 12 Months after core description Curation, Storage and General Availability through FSU Repository Figure 36 Figure 37. Flow diagram outlining geological analyses that may be performed on glacial sediments (continuation of Figure 36). 260 WAIS sediment geological analyses Geological Analyses Sedimentary Paleontologic Analyses Analyses (Diatoms, Foramlnifers, Palynomorphs. others) Sediment Descriptive Mineralogy Paleoenvironments Biochronostratigraphy Texture Stratigraphy Coarse Non-Marine Marine Sedimentary Fine Grain Size Grain Fabric Fraction Structures Fraction (<63um) (>63um) De-glacial and Pre-glacial Chronology Lacustrine Terrestrial Bottom Surface Paleopro- Waters Waters ductivity Figure 37 APPENDIX D Antarctic Peninsula piston core data (Chapter III) Key to Row Headings: DEPTH (cm): Depth in centimeters down-core %Corg: Percent total organic carbon MASS (gm): Mass of sample in grams F/V: Microscope fields of view counted (0.0145mm2 per field of view) CHAET: Number of Chaetoceros valves counted OTHERS: Number of other diatom valves counted CH CEL/GM: Calculated number of Chaetoceros cells per gram of sediment DM CEL/GM: Calculated number of other diatoms per gram 262 TABLE 10: DF82-34 Diatom counts, cell and organic carbon abundance 1 cm 1 02 0 0151 75 358 51 875 8 56F+07 1 22F+07 10 CM 1.09 00128 60 554 55 910 1.95E+08 1 94F+07 20cm 1.03 0.0100 75 457 43 91.4 1.65E+08 1.55E+07 30nm 0.0192 50 557 67 893 1.57E+08 1 89E+07 40cm 1.35 0.0192 50 54S 69 888 1.54E+08 1.95E+07 SO cm 0.0206 75 425 35 924 745F+07 6 14F*Ofi 60cm ... 0.99 0.0177 75 472 51 90.2 1 04E+07 I - .... I I 80cm 1.11 0.022 50 329 54 85.9 8.10E+07 1 I 1 33E+07 I I I I 100cm - 0 0226 70 447 48 903 7S5E+07 ] L. 8.22E+06J I I I 120cm - 0.0189 60 376 47 88.9 8.98E+07 1I 1.12E+07 I I I I 140cm - .. 0.0200 50 368 34 915 9.97E+07 1 9 21E+06 I I I I 160cm 0.93 00187 50 478 43 91.7 138F+08 I 1 25E+07 I I I I 180cm - 00136 100 336 45 88 2 6 69F+07 I 8 96E+06 I I I I 200cm I 1.26 II 0.0137 IM Q Q l 373 II 33 I 91.9 I 7.37E+07 I 6.52E+06 I I I I 220cm I:. . - 1 0.0123 I 1601 314 I 37 I 89.5 I 4.32E+07 I 5.09E+06 I I I I 240cm I 0.02 1 0.0130 I 175 265 I 30 I___ 89.8 I 3.15E+07 I 3 57E+06 I I I I 260cm I - 1:.. 0 0166 I 50 I 630 I 29 I 95 6 I _2_06E+08 I 9 46E+06 I I I I 280cm I - 1 00123 I 125 I 365 I 65 I 849 I 6.43E+07 I 1 14E+07 I I I I 300cm I 1.14 I 0.0134 I 801 407 I 65 I 86.2 I 1 Q3E-t-08 I 1.64E+07 I I I I 320cm I - I 0.0127 I 120 336 I 61 I 84 .6 I 5.97E+07 i 1.08E+07 I I I I 340cm I 1.2 I 0.0137 I 501 379 I 27 I___ 93.31 1.5CE+08 1 1 07E+07 I I I I 360cm I - I 00123 I 50 1 _ 349 I 30 I __ 92 1 I 1 54F-U18 I 1 32F*07 I I ...... I I 380cm 1 0.48 I 0.0135 I 175 240 I 45 I___ 84.2 I 2.75E+07 l 5.16E+06 I 1 1 1 400cm 1 141 I 0.0125 I 50 I .... 357 I 56 I.... 86.4 I 1.55E+08 I .. 2.43&*QZ.J TABLE 11: DF82-47 Diatom counts and cell abundance DEPTH /cm) MASS (ami FA/CHAET OTHERS%CHAET ... £-G£L/GM._ 1 wn 0 0088 60 336 29 97 1 1.72E+08 3 cm 001 40 369 32 92 0 2.50E+08 10cm 0.011 60 354 31 91.9 1.45E-+08 20cm 0.0095 125 297 43 874 677Fa07 30cm 0.0097 100 303. 37 89.1 8.46E+07 40cm 0 0100 60 349 39 89 9 1 45F.U1R 47cm 0 0109 50 382 76 93 6 1.90F+08 60cm 0.0111 40 443 25 94.7 2.70E+08 7Dcm 0.0095 too 351 23 939 100F-08 80cm 0.0103 , 80 316 19 943 1.04E-08 90cm 0 01 OK 70 336.. 32 91 3 100cm 0.0103 100 340 48 . 87.6 8.94F-07 170cm OOOQfi 50 303 48 86 3 1.67E+08 140cm . 0.0098 60 340 30 91.9 1.13E+08 160cm 0.0104 60 320 43 88.2 1.16F+08 180cm 0.0105 70 376 25 93.8 1 39Fj08 700cm 00104 50 401 30 930 709F.J1fl 270cm 00095 150 761 87 90 3 145FJ18 240cm 0.0102 50 440 23 95.0 2.34E+08 260cm 0.0096 50 459 38 924 2.59E+08 280cm 0.0097 , 10Q ». .3.74 .. 32 92.1 1.04E+08 TABLE 12: DF82-50 Diatom counts and cell abundance DEPTH (cm) MASS (am) F/V CHAET CH CEL/GM DM CEL/GM 1 "■ 40CM 0 0 1 2 30 634 ... 260F+08 2 38E408 60CM 0 0172 25 932 3 08F*08 2 94F+Q8 80CM 0.0141 _3Q__ 644 2.34E+08 2.06E+08 100CM 0.0127 50 414 . 1.09F408 8.83E+07 120CM 0 015 25 300 2.31 E+08 2.17E+08 140CM 0 0137 50 202 525F+07 3.99E+07 160CM 00159 50 464 9.10E+07 7.90E+07 180CM 0.0166 50 384 7.70E+07 6.26E+07 200CM _ . _._ 990CM 0 0149 50 600 1 24E+08 1 Q9F+08 240CM 0 0153 55 514 9 75F+07 8 2 7 F J 1 7 260CM 0.0154 50 670 1.23E+08 1.18E+08 280CM 0.014 30 638 2.I 6 E4O8 2.06E+08 293CM 0 082 30 580 4.60E+07 4 22F+07 320CM 00149 60 554 9 35F+07 8 39F407 340CM 0.013 50 554 1.29E+08 1.15E+08 360CM 0.0133 45 ... 664 ... 1.60E+08 1.50E+08 380CM 0.0131 30 540 1.94E408 1.86E+08 398CM 00125 25 708 335F+08 3.07E+08 490CM 0 015 45 528 1 17F+na 1 06F+08 440CM 00127 70 238 .... 4.QQE+07 .. 3.63E+07 TABLE 13: DF82-60 Diatom counts and cell abundance DEPTH fcm) MASSfcrm) F/V CHAFT OTHERS %CHAET C CEL/GM 10M o o isa 100 297 56 84 1 6 05 10CM 0 0171 so 338 63 8 4 3 794F+ 07 20CM 0.0143 30 310 13 96.0 2.04E+08 40CM 0.0147 50 240 48 83.3 1.06E+08 60CM 0.014 55 182 36 8 3 5 767E+07 flOCM 0 015 25 300 19 9 4 0 230E+Q8 1000M 0 0135 85 168 30 848 611F+07 120CM 0.0138 70. 236 20 9 2 2 7 18E+07 140CM 0 0 1 2 4 60 159 16 9 0 9 6.37E+07 160CM 0 0 1 2 50 178 15 9 2 2 8 71E+Q7 180CM 0 013 50 364 32 91 9 1 65E+08 200CM 0.015 30 364 ... 32 91.9 2.38E+08 210CM 0 0189 30 347 26 9 3 0 178E+08 240CM 0 0 1 4 2 40 364 36 91.0 1.90E+08 2SOCM 0.0166__ .. 5 0 - .... 34Q 19 9 4 7 1.1ZE+08 267 TABLE 14: DF82-71 Diatom counts and cell abundance DEPTH (cml MASS (ami F/Y CHAETOTHERS %CHAET CH CEL/GM DM CEL/GM 1 cm 00100 — sen- ftrtft 90 Aft A 1 35F+08 9 OftFjJlA 10 cm 0.0098 80 259 37 _ 87.5 8.95E+07 1.28E+07 90 cm 0 0103 100 980 49 870 736F+07 1 10F+O7 30 cm 00102 80 345 90 94.5 1.15E+08 6.64E+06 40 cm 0.0098 50 358 22 94.2 1.98E+08 50 cm 0 0101 800 276 31 8 9 9 9.25E+06 1 04F+06 60 cm 0.0104 50 342 24 9 3 4 1.78E+08 1.25E+07 70 cm 0 0098 80 308 99 93 3 1 06F+08 7 ROFj/16 80 cm 0.0104 80 368 17 9 5 6 1.20E+08 5.53E+06 90 cm 0.0100 50 495 31 94.1 2.68E+08 1.68E+07 100cm 0 0 0 9 7 70 375 39 90.6 ___L5QE±Q8 __ L56E+07 I I I 120cm II 0.0097 II 100 JI 472 II 37 II 92.7 II 1.32E+08 II___ L 0 3 E + 0 7 ..I I I I 140cm I 0.0098 II 50 II 423 II 30 II 93.4 Ii 2.34E408 II 1.66E+07 I I I 1 160cm II 0.0096 II 60 II 376 II 26 II 93.5 II 1.77E+08 II 1.22E+07 I I I I 180cm I 00100 II 55 II 376 II 96 II 93 5 I 185F+08 I 1 28F+07 I I I I 900cm I 0 0100 II 38 I 471 I 98 I 9 4 4 I 3 36 F + 08 I 2 00F+07 I I I I 220cm I 0.0108 1I 70 I 338 II 15 I 95.8 Il ___1.21 E+08 JI___5.37E+06 I I ... I I 240cm I 0.0098 1I 50 I 455 I 16 I 96.6 I___2.51 £+ 08 J___8.84E+06 I I I I 260cm I 0.0106 I 55 I 430 I 19 I 95.8 I 900F-U18 J___8.83E+06 I I I 280cm 0 0098 60 815 99 9 6 6 450F+ 08 1 60F+O7 290cm 0.01.00 50 805 23 97.2 ... 4.36E+Q8 . 1.25E+07... 268 TABLE 15: DF82-98 Diatom counts and cell abundance C CEL/GM D CEL/GM . qiq 1cm 0 0113 326 29 1..41 E+OB 1 02F+07 12cm 0.0125 75 481 31 93.9 1 48F+08 8.96E+06 00121 10Q_ 282 33 89.5 705 E + 0 7 7.39E+06 40cm 0.0135 75 396 19 95.4 1.11E+08 5 08F+06 c c 3 50cm 75 368 16 9 5 8 1 12F+08 4 66F+06 fiflcm 00122 75_ 400 22 9 4 8 1 25F+0S 6.51 E+06 70cm 0.0120 75 291 34 89.5 9.78E+07 1 02E+07 fiOem 0 0126 100 381 20 9 5 0 8 62F+07 4 30F+06 90cm 0.0119 50 370 17 956 1.76E+08 7.74E406 100cm 0 0 1 1 6 75 352 36 907 1 19F+08 1 10F^O7 120cm 0.0087 100. 333 15.. 95.7 1.08E+08 4.67E+06 140cm 0.0122 100 340 21 94.2 8.01 E+07 4.66E+06 160cm 0 0 1 2 9 75.J 407 26 9 4 0 1 21E+OS 7 28F+06 160cm 0 0125 75 396 19 9 5 4 1 20E+08 5 49F+06 200cm 0.0123 50 340 18 95.0 1.58E+08 7.93E+06 220cm 0 0 1 3 0 75 382 _____ 16 96.0 1.11E+08 4.44E+06 240cm 0 0122 5 0 ! 512 28 9 4 8 2.40E+08 1.24E+07 260cm 0 0 1 1 8 SOLI - ..... 292 22 ...... 93.0 1 44F j0 8 1 01F+Q7 TABLE 16: DF82-100 Diatom counts and cell abundance DEPTH fcrrrt MASS faml FN CHAET OTHER %CHAET CH CEL/GM DM CEL/GM 10 CM 00111 50 416 31 93.1 218F + 08 1 51E+07 20 CM 0.0099 50 583 37 94.0 3.39E+08 202F+07 30 CM 001 OS 50 397 27 936 219F+08 1 39F+07 40 CM 00100 50 520 33 94.0 3 00 F + 08 1 79F+07 50 CM 0.0094 50 515 33 94.0 3.16E+08 1 90F4O7 60 CM 00122 40 485 34 9 3 4 2 88 F 4O8 1 89F+07 80 CM 0.0121 40 .....438 30 93.6 2.62E+08 1.68E+07 100 CM 0.0118 50 482 ... 34 93.4 2.37E+08 1.56E+07 120 CM 0.0119 50 376 20 94.9 1 80F+08 9.10F406 140 CM 00115! 60 346 26 93 0 1 46F+08 1 02F+07 1K1CM 00092 40 356 20 94.7 2 77F408 1 47F+07 180 CM 0.0096 50 367 22 94.3 2.19E+08 1.24E+07 200 CM 0.0152 25 370 10 97.4 2.71 E+08 7.13E+06 220 CM 0.0102 40 409 6 9 8 6 2.75E+08 3.98E+06 240 CM 0 01 S3 20 443 5 98 9 397F*08 4 43F+06 _ 260 CM 0.0151 .15 _.MQ .. 12 .... 7.32E*C8 , 270 TABLE 17: DF82-151 Diatom counts and cell abundance DEPTH (cm> MASS fnml F/V OTHERS %CHAET D H O F I/fiM DM CEL/GM 0.00&6 ^ 59 1.23h+08 1 S3F+07 9 r*n OO10O 150 .. 413 79 83 9 7 46F+07 1 43F+07 20 cm 0 0106 100 264 117 69.3 6 75F+07 2.99E+07 30 cm 0.0096 150... 955 104 90.2 1.80E+08 1.96E+07 40 cm 0 010(5 150 336 79 81 0 572E+G7 1 35F+07 50 cm 0 0 1 03 160 402 92 81.4 6.61 E+07 1.51 E+07 60 cm 0 0100 150 409 98 8 0 7 7.38E+07 1 77F+07 70 cm 0.0102 100 646 83 88.6 1 72F+08 2.20E+07 80 cm 0.0103 140 700 78 90.0 1.31 E+08 1.46E+07 100 cm 00103 110. 491 77 86.4 1.17E+08 1.84E+07 120 cm 0 0098 100 411 51 8 9 0 1 14F+08 1 41 E+07 140 cm 0 0100 100 902 79 91 9 2.44 E+08 214E +07 160cm 0.0108 90 ... 1130 75 93.8 3.15E+08 2.09E+07 UJ C) 9 180 cm 0.0103 175 400 79 83.5 6.01 E+07 200 cm 0.0100 75 479 37 928 1.73E+08 1.34E+07 220 cm 0 0100 100 518 40 92 8 1 40F+O8 1 08F+07 240 cm 0 0104 110 457 70 8 6 7 1 08E+08 1 66F+07 ___2SQ O T1- .....0.0103 _100,_ 839 64...... 92.9 221E±Q8 . .168E+Q2L- 271 TABLE 18: DF82-182 Diatom counts and abundance DEPTH ta n l M A S S ta n l FN CHAET.OTHERS %CHAEJ CH CEL/GM DMCFI/GM 1 cm o .o r W 5 4 T 19 ,lf..'96 .6 ' 2.96E+08 1.03E+07 3cm 0 0100 50 793 25 9 6 9 3 94F*OS 1.24E407 10 cm 00094 50 716 18 9 7 5 . 4.13E+08 104E+07 20cm 0 0107 100 395 28 93 4 1 nnoF*oa 7Q9F+06 30cm 0011 50 557 16 9 7 2 2.74E+08 788E+06 40cm 0 0 1 0 3 70 644 11 98.3 2.42E+08 4.13E+06 50cm 0 01 50 556 11 98 1 3 01F+08 5 96F+Q6 __ Q.QQ95 __ 100 330 23 9 3 5 6.56E+06 I I I 80cm II 00098 I 45 I I 504 II 9 II 98 2 I 3 10F4Q8 I 5.53E+06. I I . . . I I 100cm II 0.0097 II 45 II 821 II 13 II 984 I I___509E +08 !L 8.07E406 I I I I 120cm II 0.0103 I! 50 II 562 II 18 II 96.9 I L 2.96E+08 I ___9.47E+06 I I I I 140cm I 0 0098 I 45 I 601 I 10 II 98.4 I . 2.66E+08 I 4.43E+06 I I I I 160cm II 0.0107 I 45 II 715 II 16 II 97.8 I 3 44F-iQ8 I___7.70E+06 I I I I 180cm I 00091 I 45 I 888 I 16 I I 98.2 I__ 587F+08 I 106F+07 I I I I 200cm I 00104 I 45 I 722 I 11 I 98 5 I ___4.18E+08 I ___637F+ 06 I I I I 220cm I 0.0105 I 45 I 608 I 17 IL 97.3 I 3.48E+08 I 9.74E+06 I I I L .2.40cm, J -&Q 1Q 8. I 45 I 440 I 2 3 ...... I L ... 95.0... I 2.45E+08 I...... 128E-K2Z.J 272 TABLE 19: DF81 -18 Diatom and organic carbon abundance DFPTH fcml MASS Inml FN CHAETOTHERS %CHAFT C CEL/GM D C F I /OM 2cm 0.0128 ..100 85 "" 72.7 4 ./8 t+ 0 7 1.80h+07 10cm 0 0124 75 256 78 76 6 7.46E+07 2 27F+07 20cm . 0.014 75 343 64 84.3 8 85E+07 1 65F+07 .30cm 0 8 2 0 0141 75 253 60 ROB 6 48 F + 07 1 54F*07 40cm 0 0 1 1 7 50 383 57 8 7 0 1 77F+08 264F+07 50cm - 0.0126 50 386 50 8 8 5 1.66E+08 2.15E+07 60cm 2 0 9 0.0029 33 1063 19 9R 2 301E+09 5 38F+07 70cm 0.0029 30... 1000 27 97.4 3.11E+09 8.40E+07 80cm 1.03 0.0144 40 422 34 92.5 1.98E+08 1.60E+07 100cm - 0.0119 50 315 39 89.0 1.43E+08 1.78E+07 1?Ocm 1.18 0.0122 30 350 19 94 9 2.59E+08 1.41 E+07 140cm - 0 0136 50 1625 27 98 4 1.08E+07 160cm 1 16 0 0126 30 500 38 92 9 3 61F+OR 2 74E+07 180cm - 0.0141 33 652 17 97.5 3.79E+08 9.89E+06 200cm - 0.0124 50 646 36 94.7 2.82E+08 1.57E+07 220cm - 0 0 1 4 50 420 47 89.9 1.62E+08 182E+07 240cm 09Q 0 0135 50 420 34 02 5 1 R9F*Ofl 1 36F+07 260cm 1.67 0.0122 10 1060 16 98.5 2.35E+09 3.55E+07 270cm 0 87 0 0 1 4 3 100_ 304 46 86.9 5 76E+07 8 71F+06