Late Quaternary paleoecology and paleoclimatology ...

References

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Brady, H. T. In press. Miocene diatoms in sediments beneath the Ross Ice Shelf (Science). McCollum, D. 1975. Diatom stratigraphy of the southern ocean. Initial Reports of Deep Sea Drilling Project, Volume 28, U.S. Government Printing Office, Washington, D.C. pp. 515-538. Schrader, H. 1976. Cenozoic planktonic diatom biostratigraphy of the southern ocean. Initial Reports of the Deep Sea Drilling Project, Volume 35. U.S. Government Printing Office, Washington, D.C. pp. 605-672. Webb, P. N., T. E. Ronan, J. H Lipps, and T. E. Delaca. In press. Miocene glaciomarine sediments from beneath the southern Ross Ice Shelf, Antarctica (Science). Figure 1. Percentage fine-fraction (less than 62 micrometers) curves for three Ross Sea piston cores arranged In order of Increasing latitude from left to right. (Schematic lithologic columns showing boundaries of sedimentary units described In the text are shown to the right of each curve.)

Late Quaternary paleoecology and paleoclimatology inferred from Ross Sea sediments

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105 Pinecrest Road Durham, North Carolina 27705 Our recent findings pertain to grounded-ice extent in the Ross Sea during the Late Würm glacial maximum. Bentley's (1976) gravity data from the Ross Ice Shelf region suggested that grounded ice extended no farther north than the center of the present Ross Ice Shelf during Late Wiirm. Denton and others (1971, 1975) and Denton and Borns (1974) presented glacial-geologic evidence indicating that grounded ice covered most or all of the continental shelf during Late Wurm. We studied microfossils in core-top samples to determine oceanographic, environmental, and sedimentary conditions that control modern faunal and floral distributions (Osterman and Kellogg, in press; Truesdale and Kellogg, in press). Down-core faunal and floral assemblages are interpreted in the same terms (Kellogg and Osterman, in preparation; Truesdale and Kellogg, in preparation). 124

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Figure 2. Lithologic columns of piston, gravity, and drill cores from the Ross Sea continental shelf, showing the systematic variation In thickness of unit A. (The top of each column Is positioned at the core location on the superimposed base map; Dathymetry is after hayes, of al., 1975.) ANTARCTIC JOURNAL

Three sedimentary units were encountered in Ross Sea cores (figure 1). Unit A, consisting of diatom-rich, silty sediment (greater than 90 percent silt-size or smaller), occurs at the tops of most Ross Sea cores. This unit varies systematically in thickness throughout the Ross Sea (figure 2); it is thin or absent near the Ross Ice Shelf but it is about 150 centimeters thick near the continental shelf margin. Foraminifera, Radiolaria, and sponge spicules comprise coarse particles in unit A. Unit B underlies Unit A in cores collected from the continental shelf. Unit B, coarser than unit A, is poorly sorted and compact and contains striated clasts and abundant angular, conchoidally fractured quartz grains. Faunal and floral remains are rare in unit B and, when present, are fractured or fragmented. Unit C underlies unit A and occurs only in cores collected from north of the continental shelf. Unit C contains an abundant diatom flora and about 5 to 12 percent rock fragments and angular, conchoidally fractured quartz grains. Units A and B are separated by a Transition Zone that consists of a well-sorted, sandy sediment in which rare diatoms of the Eucampia balaustium assemblage predominate. We equate Fillon's (1972, 1974, 1975) "Brunhes"-aged (0.69 to 0.0 million years old) sediment with our unit A and correlate it with unit 1A in Deep Sea Drilling Project cores at sites 270 to 273, which Hayes, Frakes, et al. (1975) stated was identical to Fillon's Brunhes-aged sediment. Unit B, unit 1B, and Fillon's Gauss-aged (3.32 to 2.43 million years old) sediment are similarly equivalent. We disagree with Fillon (1972, 1974, 1975) and Hayes, Frakes, et al. (1975) concerning the ages of the sedimentary units. We think unit A is Holocene rather than Brunhes. We obtained a radiocarbon date of 7,360 (+3,700— —2,500) years (QL-1 125) on a composite sample from the base of unit A from the southern Ross Sea (M. Stuiver, written communication, 1978). We think that unit B is not Gauss, as suggested by Fillon (1972, 1974, 1975) but that it was deposited during the Late Pleistocene. This latter age is assigned, even though rare Radiolaria of Gauss age occur in unit B, because the diatom flora shows intense reworking (diatom species in unit B have stratigraphic ranges in the Miocene, Pliocene, and Quarternary). Radiolaria, Foraminifera, and sediments in unit B are similarly reworked. These age interpretations for units A and B negate Fillon's (1975) hypothesis of a major Matuyama-to-Brunhes aged disconformity in the Ross Sea. We also disagree with Fillon's (1972, 1974, 1975) interpretations of Ross Sea sediments. Unit B represents deposition and reworking beneath grounded ice during the Late Wurm, and perhaps previous, ice advances. This interpretation is supported by the reworked faunal and floral constituents, and by the sedimentology of unit B which Barrett (1975) showed was that of an unmodified till. Unit B is a till and not, as proposed by Fillon (1975), a deposit that formed beneath a floating ice shelf. The great thickness of unit B at DSDP sites 270 to 273 (more than 20 meters) suggests that this sediment represents more than one advance of grounded ice. Unit A represents postglacial sedimentation just north of and on the continental shelf under conditions similar to those that prevail today. The systematic variation in thickness of unit A is related to the length of time since grounded ice covered each core location and indicates that the base of unit A is time-transgressive. The upper portion of unit B is probably a temporal equivalent of unit C. Unit C was deposited under glacialmarine conditions north of the grounded Late Wurm ice October 1978

sheet. The Eucampia balaustium assemblage in well-sorted, sandy sediments of the Transition Zone indicates that strong bottom currents winnowed material in the grounding zone. The currents probably were caused by a combination of meltwater streaming from beneath grounded ice, tidal pumping in the grounding zone, and marine bottom waters. This revised paleoclimatic interpretation of Ross Sea sediments supports the contention of some glacial geologists (Denton, et al., 1971, 1975; Denton and Burns, 1974) that grounded ice extended to the continental shelf margin in the Ross Sea during the Late Wurm glacial maximum. It does not support the evidence from gravity anomalies that Late Wurm grounded ice was only slightly expanded beyond the present grounding line. These results are preliminary, and additional work is underway to provide supporting evidence for our conclusions. This work was supported by National Science Foundation grants OPP 75-15524 and DPP 75-15524-AOl.

References

Barrett, P. J . 1975. Textural characteristics of Cenozoic preglacial sediments at site 270, Ross Sea, Antarctica. In: Initial Reports of the Deep Sea Drilling Project, Vol. 28 (Hayes, D. E., et al., eds.). U.S. Government Printing Office, Washington, D.C. pp. 757-784. Bentley, C. R. 1976. Isostatic imbalance and the past extent of the grounded ice sheet in the Ross Ice Shelf region, Antarctica. Abstracts with Programs (Geological Society of America), 8(6): 773. Denton, G. H., R. 0. Armstrong, and M. Stuiver. 1971. The Late Cenozoic glacial history of Antarctica. In: Late Cenozoic Glacial Ages (K. K. Turekian, ed.). Yale University Press, New Haven, Connecticut. pp. 267-306. Denton, G. H., and H. W. Borns, Jr. 1974. Former grounded ice sheets in the Ross Sea. Antarctic Journal of the US., 9: 167. Denton, G. H., H. W. Borns,Jr., M. C. Grosswald, M. Stu iver, and R. L. Nichols. 1975. Glacial history of the Ross Sea. AntarcticJournal of the US., 10: 160-164. Fillon, R. H. 1972. Evidence from the Ross Sea for widespread submarine erosion. Nature, 238: 40-42. Fillon, R. H. 1974. Late Cenozoic foraminiferal paleoecology of the Ross Sea, Antarctica. Micropaleontology, 20: 129-151. Fillon, R. H. 1975. Late Cenozoic paleo -oceanography of the Ross Sea, Antarctica. Geological Society of America Bulletin, 86: 839-845. Hayes, D. E., L. A. Frakes, P. J. Barrett, D. A. Burns, P.-H. Chen, A.B. Ford, A. C. Kaneps, E. M. Kemp, D. W. McCollum, D. J. W. Piper, R. E. Wall, and P. N. Webb. 1975. Initial Reports of the Deep Sea Drilling Project, Vol. 28. U.S. Government Printing Office, Washington, D.C. Kellogg, T. B., and L. E. Osterman. In preparation. Late Quaternary sedimentology and benthzcforaminiferal paleoecology of the Ross Sea, Antarctica.

Osterman, L. E., and T. B. Kellogg. In press. Recent benthic foraminiferal distributions from the Ross Sea, Antarctica: Relation to ecologic and oceanographic conditions (Journal of Foraminiferal Research).

Truesdale, R. S., and T. B. Kellogg. In press. Ross Sea diatoms: Modern assemblage distributions and their relationship to ecologic, oceanographic, and sedimentary conditions (Marine Micropaleontology).

Truesdale, R. S., and T. B. Kellogg. In Preparation. Late Quaternary paleoecology and paleoclimatology of the Ross Sea: The diatom record. 125