ABRUPT CHANGES IN ENVIRONMENTAL CONDITIONS AT LAKE FRYXELL, ANTARCTICA, FROM THE LGM TO THE PRESENT

Isotope records (δ¹⁸O, 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 the last glacial maximum (LGM) to the present. These changes reflect variations in meltwater production related to regional climate and ice-sheet dynamics. Our results are based on 18 cores, the chronology for which comes from U/Th and ¹⁴C dates, and OSL age estimates. We recognize eight sedimentary units within the upper 8 m of Lake Fryxell sediments. The earliest sediments yet dated are ~23 kyr old and are from 419 cm depth in basaltic sands. Overlying the sands is CaCO₃-rich silt (150 cm thick) containing 9 fine sand layers. δ¹⁸O values of the silt are –16.9 to –21.0 ‰, and reworked marine diatom fragments are common. Overlying the silt is dark gray CaCO₃-rich sand in which δ¹⁸O and δ¹³C become more negative (-21.2 to –27.1 ‰, -3.0 to –6.8 ‰ respectively), and the abundant diatoms are an unidentified naviculoid and Diadesmis contenta var. parallela. Above the sand is an aragonite layer (~30 cm thick) deposited 8-10 kyr B.P. Within this unit δ¹⁸O remains at ca. –27 ‰, while δ¹³C rises ~3 ‰ then declines ~1.5 ‰. A sustained and increasing reversal in ¹⁴C dates parallels the decline in δ¹³C. An unknown naviculoid and Navicula shackletoni are the dominant diatom species. The uppermost unit is medium sand with algal and CaCO₃ laminations. D. contenta var. parallela is dominant. Contacts between units commonly are sharp, indicating rapid transitions in depositional environment. The sedimentologic, isotopic and diatom data suggest that the silt unit was deposited in a large proglacial lake during the LGM when a marine-based ice sheet blocked the valley mouth. The interbedded fine sand layers may represent cyclical changes in meltwater input to the lake. Ice sheet influence ceased and water level had decreased by the time the aragonite unit was deposited. Sediments of the uppermost unit reflect water depths and environments similar to those at present. Lake-level data derived from the sediments complement and extend an existing deltaic record of surface-level change. The combined data describe a lake that responds dramatically on short timescales and can be used to address questions concerning the cause of millennial-scale climate change.