Lake-Level Fluctuations in the Fryxell Basin, Eastern Taylor Valley, Antarctica

Thomas E. Whittaker

As of 2002, Degree of Master of Science (MS) Quaternary and Climate Studies published under the auspices of the Climate Change Institute.


Isotope records (δ180,δ13C, 14C), sedimentology, and diatom assemblages from Lake Fryxell (77°37'S, 163°06'E) sediment cores display evidence of significant changes in depositional environment from > 30 kyr B.P. to the present. These changes reflect variations in meltwater production related to regional climate and ice-sheet dynamics. My results are based on 18 cores, the chronology for which comes primarily from U/Th dates. Most of the 14C dates contain an unknown reservoir effect. I recognize eight sedimentary units (designated 1-8) within the upper eight meters of Lake Fryxell sediments. The earliest dated sediments are 27.3±2.8 kyr old and are from 478 cm depth. The lower 520 cm of the core (Units 4-8) record a shallow-lake environment and include sand and evaporite layers. Superceding these shallow lake sediments is a bluegray silt (Unit 3) indicative of a deep-lake environment. Unit 3 silt is contemporaneous with Glacial Lake Washburn highstands, ~11-22 kcal yr B.P., and is marked by elevated δ180 (-17 to -21 %o) and marine diatom fragment abundance, indicating that the lake at this time was fed by meltwater from the Ross Sea Ice Sheet. Unit 31 sediments, and nine sand layers in Unit 3, represent millennial-scale lake-level lowstands during the Glacial Lake Washburn phase. At 11,100 ± 1200 yr B.P. Lake Fryxell reached a lowstand at about 15 m elevation, and deposited a thick (21 cm) aragonite layer across the basin (Unit 2a). A plateau in apparent radiocarbon ages in the lower half of thise unit indicates that the lake was isolated from inflowing water. A reversal in apparent radiocarbon ages at the top of Unit 2a is a result of lowering lake level and erosion of the bottom waters. Holocene-age sediments in Lake Fryxell (Units 1a-1c1) are composed of aeolian sand, with algal and carbonate laminae. There is evidence of only minor lake-level changes (±5 m) in the Holocene. Water-column temperature, pH, salinity, and thickness of ice cover measured on 23rd January 2004 differ from those obtained by previous researchers. Ice thickness was the greatest observed in 40-45 years. The thick ice cover probably contributed to the observed low water temperatures and low biological productivity (indicated by pH). These results suggest that an observed decrease in air temperature from 1986-1999 may be continuing to the present day.