RECENT AND ABRUPT VARIATIONS IN DEPOSITIONAL ENVIRONMENT IN LAKE FRYXELL, DRY VALLEYS, ANTARCTICA

Dry Valleys lakes are extremely sensitive to hydrologic variations, and as such may provide high-resolution climate records for the south-polar region. To investigate paleoclimate and water-level fluctuations through late-Pleistocene and Holocene time, we recently collected sediment cores from nine sites on the perennially ice-covered Lake Fryxell (77º36'S, 163º08'E), which lies within a closed basin in eastern Taylor Valley. These sediments are in addition to twenty-two cores collected previously. To reconstruct paleoenvironment, we employ multiple proxies, including stratigraphy, grain size, magnetic susceptibility, diatom assemblages, and stable isotopes. Chronology comes from a combination of uranium-thorium, optically stimulated luminescence, and radiocarbon dating.

Currently being deposited are well-sorted sands with flakes of CaCO₃. Typically 30-35 cm thick and laminated with thin silt horizons, this unit has been accumulating for ~6000 years. Underlying this are finely laminated, calcium carbonate-rich sands and silts with d¹⁸O of -29.5 ± 1 ‰ and d¹³C of -3.6 to 0.2 ‰. These sediments have been dated by U/Th to be early-mid Holocene in age. Preliminary results indicate that species of Diadesmis are the dominant flora in the laminated sand horizon, whereas an unknown Naviculoid is most common within the carbonate-rich horizon. A thin (<15 cm) black sand unit commonly occurs in transition between the carbonate-rich horizon and underlying massive, silty clay. The clay unit grades downcore into basalt-rich sands as much as 60 cm thick. In all but one core, this sand forms the lowermost stratigraphic unit. In the longest core, the basalt sand overlies sorted brown sand.

The sedimentary record is principally a reflection of lake-level change within the basin over the last ~30,000 years. These fluctuations are due primarily to changes in climate, although superimposed on this may be some influence from the Ross Sea ice sheet. Sharp contacts between several stratigraphic units, shown by sediment type, magnetic susceptibility, and preliminary diatom and stable isotope results, point to periods of abrupt change in depositional environment and hydrology. These changes indicate substantial variation in Dry Valleys' and perhaps even regional climate during the late Pleistocene and Holocene.