Paleoenvironmental studies of nonmarine diatoms in Quaternary ...
known. The framework modes and their sources suggest deposition in a foreland basin but do not preclude accumulation in a pull-apart basin. Resolution of basin type is hindered because sequences are fragmentary, folded, and sheared, thus obscuring stratigraphic relationships and thicknesses. In addition, basin type has not been clarified by lithofacies analysis because the lithofacies imply development of alluvial fans and fan deltas in lacustrine and marine environments (Rees and Rowell in press) and all such associations are common to both basin types. This work was supported in part by National Science Foundation grant DPP 85-18157 to the University of Nevada Las Vegas. References Burgess, C.J., and W. Lammerink. 1979. Geology of the Shackleton Limestone (Cambrian) in the Byrd Glacier area. New Zealand Antarctic Record, 2, 12-16.
Dickinson, W. R., and C. A. Suczek. 1979. Plate tectonics and sandstone compositions. American Association of Pet roleu in Geologists Bulletin, 63, 2164-2182.
Dickinson, W.R., L.S. Beard, G.R. Brakenridge, J.L. Erjavec, R.C. Ferguson, K.F. Inman, R.A. Knepp, F.A. Lindberg, and P.T. Ryberg. 1983. Provenance of North American Phanerozoic sandstones in relation to tectonic setting. Geological Society of America Bulletin, 94, 222-235.
Rees, M.N., and A.J. Rowell. 1987. The pre-Devonian Paleozoic Douglas Conglomerate of the Transantarctic Mountains: Origin and depo-
Paleoenvironmental studies of nonmarine diatoms in Quaternary antarctic sediments REED P. SCHERER
Department of Geology and Mineralogy
and Byrd Polar Research Center Ohio State University Columbus, Ohio 43210-1293
Nonmarine sediments from ice-free areas of southern Victoria Land contain a largely unexploited paleoclimatic resource. Diatom assemblages within lacustrine sediment sequences are being used to evaluate Quaternary paleoenvironments in the McMurdo Sound region. In the first phase of the study, begun during the 1986-1987 field season, lacustrine and associated deltaic sediments were sampled at outcrops in lower Taylor Valley, the Cape Chocolate region, Wright Valley, Brown Peninsula, East Dailey Island, and Ross Island. Results from two sequences near Cape Chocolate and one in lower Taylor Valley are reported here. These samples were processed for diatom analysis using a method (Scherer in preparation) which allows the establishment of absolute abundance (diatoms per gram of dry sediment). This relationship allows comparison of diatom productivity and sediment input. 1987
REVIEW
sitional setting. Fifth International Syinposiu in on Antarctic Earth Sciences Abstracts.
Rees, MN., and A.J. Rowell. In press. The pre-Devonian Paleozoic clastics of the central Transantarctic Mountains: Stratigraphy and depositional settings. Volume of the Fifth International Symposium on Antarctic Earth Sciences. Cambridge, U.K. Rees, MN., A.J. Rowell, B.R. Pratt, and P. Braddock. 1985. The Byrd
Group of the Holyoake Range, central Transantarctic Mountains.
Antarctic Journal of the U.S., 20(5), 3-5.
Rowell, A.J., and M.N. Rees. In press. Setting and significance of the Shackleton Limestone, central Transantarctic Mountains. Volume of the Fifth International Symposium on Antarctic Earth Sciences.
Cambridge, U.K. Rowell, A.J., M.N. Rees, and P. Braddock. 1986. Pre-Devonian Paleozoic rocks of the central Transantarctic Mountains. Antarctic Journal of the U.S., 21(5), 48-50.
Rowell, A.J., M.N. Rees, R.A. Cooper, and B.R. Pratt. In press. Early Paleozoic history of the central Transantarctic Mountains: Evidence from the Holyoake Range, Antarctica. New Zealand Journal of Geology and Geophysics.
Skinner, D.N.B. 1964. A summary of the geology of the region between Byrd and Starshot glaciers, south Victoria Land. In R.J. Adie (Ed.), Antarctic geology. Amsterdam: North Holland. Skinner, D.N.B. 1965. Petrographic criteria of the rock units between the Byrd and Starshot glaciers, south Victoria Land, Antarctica. New Zealand Journal of Geology and Geophysics. 8, 292-303.
Stump, E., M.R. Sheridan, S.G. Borg, P.H. Lowry, and P.V. Colbert. 1979. Geological investigations in the Scott Glacier and Byrd Glacier areas. Antarctic Journal of the U.S., 14(5), 39-40.
Diatoms within these sediments are compared with modern floras in sediment from Lake Vanda in Wright Valley and numerous meltwater streams and ponds distributed throughout Wright Valley, lower Taylor Valley, Cape Chocolate, Ross Island, Brown Peninsula, and on the McMurdo Ice Shelf around East Dailey Island. Modern diatom assemblages within these samples, as well as those reported in the literature, provide an analog for ecologic interpretation of fossil assemblages and their sedimentary environments. Older nonmarine diatom floras from upper Pliocene/Pleistocene sediments from DVDP-15 and CIROS2* drill cores in McMurdo Sound and Ferrar Fjord are also under study. A paleoecologic model is being constructed based in part on these results. Discontinuous outcrops of lacustrine and deltaic sediments are scattered along valley walls and dry basins of southern Victoria Land (Debenham 1921; Speden 1960; Péwé 1960; Kellogg et al. 1980). These generally represent only high stands of previous lake levels, and thus do not contain continuous stratigraphic successions. A high-resolution Late Quaternary paleoclimatic signal, based on diatom, sedimentologic and chemical analyses, may be extracted from the near-continuous sediment records underlying certain modern lakes. The only available record of this kind is the upper 4 meters of DVDP-4A in Lake Vanda. Although badly disturbed by drilling, this succession showed evidence of major changes in sedimentation, including periodic evaporite deposition (Brady 1981). Continuous sedimentary records underlying modern lakes need to be ex-
* "DVDP" denotes Dry Valley Drilling Project and "CIROS" denotes Cenozoic Investigations of the Ross Sea. 35
tracted using methods that preserve stratigraphy and sedimentary structures. Kellogg et al. (1980) studied diatom fossils from Late Wisconsin nonmarine perched deltas in Taylor Valley and defined six diatom assemblages. They used these assemblages to evaluate lake-level history but were unable to draw specific paleoecologic conclusions due to a paucity of modern ecologic data. Kellogg et al. (1980) reported nine diatom taxa as endemic to southern Victoria Land, including six species which they described as new. These findings conflict with the results of a recent algal monograph (Seaburg et al. 1979) which reports no nonmarine diatom taxa as endemic to southern Victoria Land. For this study, diatoms were identified using the taxonomic assignments of Kellogg et al. (1980). Some taxonomic revision to accomodate the above discrepancy will ultimately be necessary. Certain diatoms are cosmopolitan in antarctic nonmarine aqueous environments (e.g., Navicula multicopsis and varieties, Navicula quciternaria, Navicula shackleton i, Nitzschia westii), despite widely differing ionic concentrations between various bodies of water. However, it is the distribution of ecologically restricted taxa that permit assemblage differentiation and paleoecologic interpretation. Many extant species range back at least into the Pliocene (Brady 1981) suggesting that a paleoecologic model based on ecologically restricted nonmarine diatoms from modern environments has potential for application to pre-Quaternary sediments. The following is a descriptive analysis and comparison of sediments studied, with paleoecological notes. Salmon Stream, Cape Chocolate. Lacustrine sediments near Salmon Stream in the Cape Chocolate region were described by Speden (1960) and discussed by Miagkov, Nedeshava, and Riaboba (1976) and Kellogg et al. (1980). Our recent field studies brought to light some new observations. A thick succession of finely laminated, moderately calcareous sediments on the north shore of Salmon Stream, near the snout of Salmon Glacier, has been overridden by subsequent glacial activity. The lacustrine succession is overlain by a compact tillite and stratigraphy is disrupted by numerous small thrust faults. Five samples from this sequence were studied. Samples are rich in nonmarine diatoms, averaging 6.5 million valves per gram. Marine diatom fragments are rare in this sequence. The diatom assemblage is dominated (65-75 percent) by a small centric diatom, Melosira charcotii (figure a—c). This diatom is known from intertidal pools along the Antarctic Peninsula and associated islands (Peragallo 1921; Frenguelli and Orlando 1958) but has not been previously reported from the Ross Sea sector of the antarctic. Specimens of M. charcotii are common in meltpools on the McMurdo Ice Shelf today, especially near tidal cracks, but were not found in isolated inland waters or under exclusively marine conditions. The published ecology for M. charcotii and the distribution of this diatom in the McMurdo Sound region suggests an association with tidally influenced ice-shelf conditions and other marineproximal, brackish water environments. Melosira charcotii is also common in Pleistocene nonmarine intervals recovered from DVDP-15 and CIROS-2 drill cores in McMurdo Sound. These intervals, up to approximately 80 meters thick, contain an exclusively nonmarine diatom flora, as reported by Brady (1978, 1981, 1982) and Harwood (1986), respectively. The sediments of Salmon Stream and McMurdo Sound drill cores could reflect nonmarine conditions at the ice edge during advance or retreat of grounded Ross Sea ice in McMurdo Sound. Hobbs Glacier, Cape Chocolate. A 3-meter-thick outcrop of finely laminated, (average of 0.8 millimeters per laminade) highly calcareous sediments is located adjacent to the snout of Hobbs 36
a-c: Melosira charcotii Peragello, Salmon Creek, Cape Chocolate. (2000 x) d-f: Chrysomonad cyst, Ochromonas miniscula Conrad (?), Lake Vanda, Wright Valley. (2000 x)
Glacier. Small, discontinuous outcrops of similar material are scattered throughout the vicinity. Advances of the Hobbs Glacier since 1903 have apparently destroyed similar deltaic sediments (Denton and Hughes 1981). The sediments collected from the outcrop contain an average of 1.2 million diatoms per gram, based on six samples. The diverse nonmarine assemblage is dominated by Navicula ccntenta, Navicula gibbula,
and Hantzschia amphioxys. Marine fragments are uncommon. Melosira charcotii is absent from this sequence. Commonwealth Glacier, Taylor Valley. A 4-meter-thick exposure of lacustrine and associated deltaic sequences crops out at the snout of Commonwealth Glacier. The sequence has evidence of high sedimentation rates and rapid shifts in sedimentary regime, including several distinct units and a variety of soft sediment deformational structures. Nonmarine diatoms occur in low concentrations in this succession. Low nonmarine diatom abundance reflects high sediment input rather than low productivity. Despite the differing character of sediment units, the diatom assemblages in the samples studied are fairly consistent in absolute abundance and taxonomic compositions. Nonmarine diatom abundance in four samples from the lighter colored units averages only 200,000 per gram. Contrary to the observations of Kellogg et al. (1980), who report an absence of Nitzschia westii in Commonwealth Glacier lacustrine sediments, N. westii is common in all samples studied from this outcrop. Reworked nonmarine diatom-rich sediment clasts from older deposits are present in this succession. The clasts contain abundant Navicula shackletoni and common Melosira charcotii. The occurrence of these clasts and common marine diatom fragments indicates significant sediment transport and redeposition. Lake Vanda, Wright Valley. Recent sediment from the anoxic basin of Lake Vanda was evaluated using the same methods as above. This sample has about 3 million nonmarine diatoms per gram of sediment. Species composition is unlike the diatom assemblages present in outcrop or modern meltpool samples. While several cosmopolitan diatoms are present, most notably Navicula multicopsis and varieties, the dominant diatom is Pinnularia cymatopleura. Several forms are present which were not observed in outcrop samples or in modern shallow meltpools. These included Amphora delicatissima, which has been reported from Lake Miers (Baker 1967). ANTARCTIC JOURNAL
A small (5-8 micron) siliceous cyst formed by a planktonic chrysophyte (Ochromonas niiniscula?) (figure, d-f) makes the greatest contribution to the siliceous microflora in this sediment. The sample contains more than three times as many of these chrysomonad cysts as diatoms, amounting to 10 million cysts per gram, making them an important ecological marker. The distribution of this algal species has been previously described as abundant in Lake Vanda but rare in Taylor Valley lakes (Seaburg et al. 1979). The cysts are found in very low abundance in other nonmarine sediments, despite wide distribution. Chrysomonad cysts have been found forming under winter ice in Scandinavian lakes (Cronberg 1973), suggesting that cyst formation provides a survival strategy during winter and perhaps other times of environmental stress. Abundant occurrence of this fossil in a sediment sequence may suggest conditions similar to Lake Vanda today, i.e., thick, clear ice and deep, chemically and thermally stratified water. Oxygenated shallow antarctic lake floors are host to a complex community of mat-forming filimentous blue-green algae, which accumulate stromatolitic laminations (Parker et al. 1981). Benthic diatom floras are found in association with these algal mats. By contrast, the microfossil assemblage in sediments from the deep anoxic basin of Lake Vanda is dominated by planktonic forms, reflecting productivity in the water column under the ice but a relatively inactive benthic community. These assemblage differences may be useful in reconstructing past lake conditions. These results are very encouraging, but further work is needed. Detailed ecologic and distributional studies on the modern diatom floras will refine the paleoenvironmental model. To apply this model best, long, undisturbed cores should be extracted from the bottom of several modern dry valley lakes. Continuous sedimentary successions and the lack of bioturbation make these records attractive for high resolution paleoclimatic studies. The record spanning the last few 1,000 years maybe compared with the antarctic ice-core record. Longer time-scale lacustrine successions may contain trends for comparison with nearshore antarctic marine core records. These sedimentary successions have the potential for providing an excellent record of terrestrial Quaternary climate change in Antarctica. This research was supported by National Science Foundation grant DPP 85-17625 to Peter-Noel Webb. Lake Vanda sediment material was collected under National Science Foundation grant DPP 85-16465 to William Green and was generously provided for this study. Peter Webb and Dave Harwood made helpful comments regarding the manuscript.
1987 REVIEW
References Baker, A. N. 1967. Algae from Lake Miers, a solar-heated Antarctic lake. New Zealand Journal of Botany, 5, 453-468. Brady, H.T. 1977. Freshwater lakes in Pleistocene McMurdo Sound. Antarctic Journal of the U.S., 12(4), 117-118. Brady, H.T. 1981. The significance of fossil marine and non-marine diatoms in DVDP cores. in L.D. McGinnis (Ed.), Dry Valley Drilling Project. Antarctic Research Series, Vol. 33, Washington, D.C.: American Geophysical Union. Brady, H.T. 1982. Late Cenozoic history of Taylor and Wright Valleys and McMurdo Sound inferred from diatoms in Dry Valley Drilling Project cores. In C. Craddock (Ed.), Antarctic geoscience. Madison: University of Wisconsin Press. Cronberg, G. 1973. Development of cysts in Malloinonas eoa examined by scanning electron microscopy. Hydrohiologia, 43, 29-38. Debenham, F. 1921. Recent and local deposits of McMurdo region. British Antarctic Expedition "Terra Nova," 1910. Natural History Report, Geology, 1(3), 63-100. Denton, G. H., and T.J. Hughes. 1981. The last great ice sheets. New York: Wiley. Frenguelli, J., and H. Orlando. 1958. Diatoms and silicoflagellates of the South American sector of the Antarctic. Buenos Aires: Instituto Antartico Argentino. (In Spanish) Harwood, D.M. 1986. Diatom hiostratigraphy and paleoecology with a Cenozoic history of antarctic ice sheets. (Doctoral dissertation, Columbus:
Ohio State University.) Kellogg, D.E., M. Stuiver, T.B. Kellogg, and G.H. Denton. 1980. Nonmarine diatoms from Late Wisconsin perched deltas in Taylor Valley, Antarctica. Palaeogeography, Palaeoclimatology, Palaeoecology, 30, 157-189. Miagkov, SM., G.N. Nedeshava, and E.I. Riabova. 1976. McMurdo Sound sea level changes in the last 50,000 years. Antarctic Journal of the U.S., 11(5), 233-235. Parker, B.C., G.M. Simmons, Jr., F.G. Love, R.A. Wharton, Jr., and K.G. Seaburg. 1981. Modern stromatolites in Antarctic dry valley lakes. Bioscience, 31, 656-661. Peragallo, M. 1921. Freshwater and saltwater diatoms. In J . Charcot (ed.), Botanique. Paris: Second French Antarctic Expedition 1908-1910. (in French) Péwé, T. 1960. Multiple glaciation in the McMurdo Sound region, Antarctica: A progressive report. Journal of Geology, 68, 489-514. Scherer, R.P. In preparation. An absolute abundance method for quantitative analysis of diatoms or other fine sand and silt-sized sedimentary particles. Micropaleontology.
Seaburg, K.G., B.C. Parker, G.W. Prescott, and L.A. Witford. 1979. The algae of Southern Victorialand, Antarctica. Bihliothecza Phycologica, Vol. 34. Valduz: J. Cramer. Spederi, I. 1960. Post-glacial terraces near Cape Chocolate, McMurdo Sound, Antarctica. New Zealand Journal of Geology and Geophysics, 3, 203-217.
37
Dickinson, W. R., and C. A. Suczek. 1979. Plate tectonics and sandstone compositions. American Association of Pet roleu in Geologists Bulletin, 63, 2164-2182.
Dickinson, W.R., L.S. Beard, G.R. Brakenridge, J.L. Erjavec, R.C. Ferguson, K.F. Inman, R.A. Knepp, F.A. Lindberg, and P.T. Ryberg. 1983. Provenance of North American Phanerozoic sandstones in relation to tectonic setting. Geological Society of America Bulletin, 94, 222-235.
Rees, M.N., and A.J. Rowell. 1987. The pre-Devonian Paleozoic Douglas Conglomerate of the Transantarctic Mountains: Origin and depo-
Paleoenvironmental studies of nonmarine diatoms in Quaternary antarctic sediments REED P. SCHERER
Department of Geology and Mineralogy
and Byrd Polar Research Center Ohio State University Columbus, Ohio 43210-1293
Nonmarine sediments from ice-free areas of southern Victoria Land contain a largely unexploited paleoclimatic resource. Diatom assemblages within lacustrine sediment sequences are being used to evaluate Quaternary paleoenvironments in the McMurdo Sound region. In the first phase of the study, begun during the 1986-1987 field season, lacustrine and associated deltaic sediments were sampled at outcrops in lower Taylor Valley, the Cape Chocolate region, Wright Valley, Brown Peninsula, East Dailey Island, and Ross Island. Results from two sequences near Cape Chocolate and one in lower Taylor Valley are reported here. These samples were processed for diatom analysis using a method (Scherer in preparation) which allows the establishment of absolute abundance (diatoms per gram of dry sediment). This relationship allows comparison of diatom productivity and sediment input. 1987
REVIEW
sitional setting. Fifth International Syinposiu in on Antarctic Earth Sciences Abstracts.
Rees, MN., and A.J. Rowell. In press. The pre-Devonian Paleozoic clastics of the central Transantarctic Mountains: Stratigraphy and depositional settings. Volume of the Fifth International Symposium on Antarctic Earth Sciences. Cambridge, U.K. Rees, MN., A.J. Rowell, B.R. Pratt, and P. Braddock. 1985. The Byrd
Group of the Holyoake Range, central Transantarctic Mountains.
Antarctic Journal of the U.S., 20(5), 3-5.
Rowell, A.J., and M.N. Rees. In press. Setting and significance of the Shackleton Limestone, central Transantarctic Mountains. Volume of the Fifth International Symposium on Antarctic Earth Sciences.
Cambridge, U.K. Rowell, A.J., M.N. Rees, and P. Braddock. 1986. Pre-Devonian Paleozoic rocks of the central Transantarctic Mountains. Antarctic Journal of the U.S., 21(5), 48-50.
Rowell, A.J., M.N. Rees, R.A. Cooper, and B.R. Pratt. In press. Early Paleozoic history of the central Transantarctic Mountains: Evidence from the Holyoake Range, Antarctica. New Zealand Journal of Geology and Geophysics.
Skinner, D.N.B. 1964. A summary of the geology of the region between Byrd and Starshot glaciers, south Victoria Land. In R.J. Adie (Ed.), Antarctic geology. Amsterdam: North Holland. Skinner, D.N.B. 1965. Petrographic criteria of the rock units between the Byrd and Starshot glaciers, south Victoria Land, Antarctica. New Zealand Journal of Geology and Geophysics. 8, 292-303.
Stump, E., M.R. Sheridan, S.G. Borg, P.H. Lowry, and P.V. Colbert. 1979. Geological investigations in the Scott Glacier and Byrd Glacier areas. Antarctic Journal of the U.S., 14(5), 39-40.
Diatoms within these sediments are compared with modern floras in sediment from Lake Vanda in Wright Valley and numerous meltwater streams and ponds distributed throughout Wright Valley, lower Taylor Valley, Cape Chocolate, Ross Island, Brown Peninsula, and on the McMurdo Ice Shelf around East Dailey Island. Modern diatom assemblages within these samples, as well as those reported in the literature, provide an analog for ecologic interpretation of fossil assemblages and their sedimentary environments. Older nonmarine diatom floras from upper Pliocene/Pleistocene sediments from DVDP-15 and CIROS2* drill cores in McMurdo Sound and Ferrar Fjord are also under study. A paleoecologic model is being constructed based in part on these results. Discontinuous outcrops of lacustrine and deltaic sediments are scattered along valley walls and dry basins of southern Victoria Land (Debenham 1921; Speden 1960; Péwé 1960; Kellogg et al. 1980). These generally represent only high stands of previous lake levels, and thus do not contain continuous stratigraphic successions. A high-resolution Late Quaternary paleoclimatic signal, based on diatom, sedimentologic and chemical analyses, may be extracted from the near-continuous sediment records underlying certain modern lakes. The only available record of this kind is the upper 4 meters of DVDP-4A in Lake Vanda. Although badly disturbed by drilling, this succession showed evidence of major changes in sedimentation, including periodic evaporite deposition (Brady 1981). Continuous sedimentary records underlying modern lakes need to be ex-
* "DVDP" denotes Dry Valley Drilling Project and "CIROS" denotes Cenozoic Investigations of the Ross Sea. 35
tracted using methods that preserve stratigraphy and sedimentary structures. Kellogg et al. (1980) studied diatom fossils from Late Wisconsin nonmarine perched deltas in Taylor Valley and defined six diatom assemblages. They used these assemblages to evaluate lake-level history but were unable to draw specific paleoecologic conclusions due to a paucity of modern ecologic data. Kellogg et al. (1980) reported nine diatom taxa as endemic to southern Victoria Land, including six species which they described as new. These findings conflict with the results of a recent algal monograph (Seaburg et al. 1979) which reports no nonmarine diatom taxa as endemic to southern Victoria Land. For this study, diatoms were identified using the taxonomic assignments of Kellogg et al. (1980). Some taxonomic revision to accomodate the above discrepancy will ultimately be necessary. Certain diatoms are cosmopolitan in antarctic nonmarine aqueous environments (e.g., Navicula multicopsis and varieties, Navicula quciternaria, Navicula shackleton i, Nitzschia westii), despite widely differing ionic concentrations between various bodies of water. However, it is the distribution of ecologically restricted taxa that permit assemblage differentiation and paleoecologic interpretation. Many extant species range back at least into the Pliocene (Brady 1981) suggesting that a paleoecologic model based on ecologically restricted nonmarine diatoms from modern environments has potential for application to pre-Quaternary sediments. The following is a descriptive analysis and comparison of sediments studied, with paleoecological notes. Salmon Stream, Cape Chocolate. Lacustrine sediments near Salmon Stream in the Cape Chocolate region were described by Speden (1960) and discussed by Miagkov, Nedeshava, and Riaboba (1976) and Kellogg et al. (1980). Our recent field studies brought to light some new observations. A thick succession of finely laminated, moderately calcareous sediments on the north shore of Salmon Stream, near the snout of Salmon Glacier, has been overridden by subsequent glacial activity. The lacustrine succession is overlain by a compact tillite and stratigraphy is disrupted by numerous small thrust faults. Five samples from this sequence were studied. Samples are rich in nonmarine diatoms, averaging 6.5 million valves per gram. Marine diatom fragments are rare in this sequence. The diatom assemblage is dominated (65-75 percent) by a small centric diatom, Melosira charcotii (figure a—c). This diatom is known from intertidal pools along the Antarctic Peninsula and associated islands (Peragallo 1921; Frenguelli and Orlando 1958) but has not been previously reported from the Ross Sea sector of the antarctic. Specimens of M. charcotii are common in meltpools on the McMurdo Ice Shelf today, especially near tidal cracks, but were not found in isolated inland waters or under exclusively marine conditions. The published ecology for M. charcotii and the distribution of this diatom in the McMurdo Sound region suggests an association with tidally influenced ice-shelf conditions and other marineproximal, brackish water environments. Melosira charcotii is also common in Pleistocene nonmarine intervals recovered from DVDP-15 and CIROS-2 drill cores in McMurdo Sound. These intervals, up to approximately 80 meters thick, contain an exclusively nonmarine diatom flora, as reported by Brady (1978, 1981, 1982) and Harwood (1986), respectively. The sediments of Salmon Stream and McMurdo Sound drill cores could reflect nonmarine conditions at the ice edge during advance or retreat of grounded Ross Sea ice in McMurdo Sound. Hobbs Glacier, Cape Chocolate. A 3-meter-thick outcrop of finely laminated, (average of 0.8 millimeters per laminade) highly calcareous sediments is located adjacent to the snout of Hobbs 36
a-c: Melosira charcotii Peragello, Salmon Creek, Cape Chocolate. (2000 x) d-f: Chrysomonad cyst, Ochromonas miniscula Conrad (?), Lake Vanda, Wright Valley. (2000 x)
Glacier. Small, discontinuous outcrops of similar material are scattered throughout the vicinity. Advances of the Hobbs Glacier since 1903 have apparently destroyed similar deltaic sediments (Denton and Hughes 1981). The sediments collected from the outcrop contain an average of 1.2 million diatoms per gram, based on six samples. The diverse nonmarine assemblage is dominated by Navicula ccntenta, Navicula gibbula,
and Hantzschia amphioxys. Marine fragments are uncommon. Melosira charcotii is absent from this sequence. Commonwealth Glacier, Taylor Valley. A 4-meter-thick exposure of lacustrine and associated deltaic sequences crops out at the snout of Commonwealth Glacier. The sequence has evidence of high sedimentation rates and rapid shifts in sedimentary regime, including several distinct units and a variety of soft sediment deformational structures. Nonmarine diatoms occur in low concentrations in this succession. Low nonmarine diatom abundance reflects high sediment input rather than low productivity. Despite the differing character of sediment units, the diatom assemblages in the samples studied are fairly consistent in absolute abundance and taxonomic compositions. Nonmarine diatom abundance in four samples from the lighter colored units averages only 200,000 per gram. Contrary to the observations of Kellogg et al. (1980), who report an absence of Nitzschia westii in Commonwealth Glacier lacustrine sediments, N. westii is common in all samples studied from this outcrop. Reworked nonmarine diatom-rich sediment clasts from older deposits are present in this succession. The clasts contain abundant Navicula shackletoni and common Melosira charcotii. The occurrence of these clasts and common marine diatom fragments indicates significant sediment transport and redeposition. Lake Vanda, Wright Valley. Recent sediment from the anoxic basin of Lake Vanda was evaluated using the same methods as above. This sample has about 3 million nonmarine diatoms per gram of sediment. Species composition is unlike the diatom assemblages present in outcrop or modern meltpool samples. While several cosmopolitan diatoms are present, most notably Navicula multicopsis and varieties, the dominant diatom is Pinnularia cymatopleura. Several forms are present which were not observed in outcrop samples or in modern shallow meltpools. These included Amphora delicatissima, which has been reported from Lake Miers (Baker 1967). ANTARCTIC JOURNAL
A small (5-8 micron) siliceous cyst formed by a planktonic chrysophyte (Ochromonas niiniscula?) (figure, d-f) makes the greatest contribution to the siliceous microflora in this sediment. The sample contains more than three times as many of these chrysomonad cysts as diatoms, amounting to 10 million cysts per gram, making them an important ecological marker. The distribution of this algal species has been previously described as abundant in Lake Vanda but rare in Taylor Valley lakes (Seaburg et al. 1979). The cysts are found in very low abundance in other nonmarine sediments, despite wide distribution. Chrysomonad cysts have been found forming under winter ice in Scandinavian lakes (Cronberg 1973), suggesting that cyst formation provides a survival strategy during winter and perhaps other times of environmental stress. Abundant occurrence of this fossil in a sediment sequence may suggest conditions similar to Lake Vanda today, i.e., thick, clear ice and deep, chemically and thermally stratified water. Oxygenated shallow antarctic lake floors are host to a complex community of mat-forming filimentous blue-green algae, which accumulate stromatolitic laminations (Parker et al. 1981). Benthic diatom floras are found in association with these algal mats. By contrast, the microfossil assemblage in sediments from the deep anoxic basin of Lake Vanda is dominated by planktonic forms, reflecting productivity in the water column under the ice but a relatively inactive benthic community. These assemblage differences may be useful in reconstructing past lake conditions. These results are very encouraging, but further work is needed. Detailed ecologic and distributional studies on the modern diatom floras will refine the paleoenvironmental model. To apply this model best, long, undisturbed cores should be extracted from the bottom of several modern dry valley lakes. Continuous sedimentary successions and the lack of bioturbation make these records attractive for high resolution paleoclimatic studies. The record spanning the last few 1,000 years maybe compared with the antarctic ice-core record. Longer time-scale lacustrine successions may contain trends for comparison with nearshore antarctic marine core records. These sedimentary successions have the potential for providing an excellent record of terrestrial Quaternary climate change in Antarctica. This research was supported by National Science Foundation grant DPP 85-17625 to Peter-Noel Webb. Lake Vanda sediment material was collected under National Science Foundation grant DPP 85-16465 to William Green and was generously provided for this study. Peter Webb and Dave Harwood made helpful comments regarding the manuscript.
1987 REVIEW
References Baker, A. N. 1967. Algae from Lake Miers, a solar-heated Antarctic lake. New Zealand Journal of Botany, 5, 453-468. Brady, H.T. 1977. Freshwater lakes in Pleistocene McMurdo Sound. Antarctic Journal of the U.S., 12(4), 117-118. Brady, H.T. 1981. The significance of fossil marine and non-marine diatoms in DVDP cores. in L.D. McGinnis (Ed.), Dry Valley Drilling Project. Antarctic Research Series, Vol. 33, Washington, D.C.: American Geophysical Union. Brady, H.T. 1982. Late Cenozoic history of Taylor and Wright Valleys and McMurdo Sound inferred from diatoms in Dry Valley Drilling Project cores. In C. Craddock (Ed.), Antarctic geoscience. Madison: University of Wisconsin Press. Cronberg, G. 1973. Development of cysts in Malloinonas eoa examined by scanning electron microscopy. Hydrohiologia, 43, 29-38. Debenham, F. 1921. Recent and local deposits of McMurdo region. British Antarctic Expedition "Terra Nova," 1910. Natural History Report, Geology, 1(3), 63-100. Denton, G. H., and T.J. Hughes. 1981. The last great ice sheets. New York: Wiley. Frenguelli, J., and H. Orlando. 1958. Diatoms and silicoflagellates of the South American sector of the Antarctic. Buenos Aires: Instituto Antartico Argentino. (In Spanish) Harwood, D.M. 1986. Diatom hiostratigraphy and paleoecology with a Cenozoic history of antarctic ice sheets. (Doctoral dissertation, Columbus:
Ohio State University.) Kellogg, D.E., M. Stuiver, T.B. Kellogg, and G.H. Denton. 1980. Nonmarine diatoms from Late Wisconsin perched deltas in Taylor Valley, Antarctica. Palaeogeography, Palaeoclimatology, Palaeoecology, 30, 157-189. Miagkov, SM., G.N. Nedeshava, and E.I. Riabova. 1976. McMurdo Sound sea level changes in the last 50,000 years. Antarctic Journal of the U.S., 11(5), 233-235. Parker, B.C., G.M. Simmons, Jr., F.G. Love, R.A. Wharton, Jr., and K.G. Seaburg. 1981. Modern stromatolites in Antarctic dry valley lakes. Bioscience, 31, 656-661. Peragallo, M. 1921. Freshwater and saltwater diatoms. In J . Charcot (ed.), Botanique. Paris: Second French Antarctic Expedition 1908-1910. (in French) Péwé, T. 1960. Multiple glaciation in the McMurdo Sound region, Antarctica: A progressive report. Journal of Geology, 68, 489-514. Scherer, R.P. In preparation. An absolute abundance method for quantitative analysis of diatoms or other fine sand and silt-sized sedimentary particles. Micropaleontology.
Seaburg, K.G., B.C. Parker, G.W. Prescott, and L.A. Witford. 1979. The algae of Southern Victorialand, Antarctica. Bihliothecza Phycologica, Vol. 34. Valduz: J. Cramer. Spederi, I. 1960. Post-glacial terraces near Cape Chocolate, McMurdo Sound, Antarctica. New Zealand Journal of Geology and Geophysics, 3, 203-217.
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