CHANGES IN SUBGLACIAL WATER CHEMISTRY OF THE LEWIS CLIFF REGION OF THE EAST ANTARCTIC ICE SHEET FROM MIS 5 TO MIS 2

The age and composition of subglacial chemical precipitates that form from waters beneath the East Antarctic Ice Sheet (EAIS) can be used to construct a record of the bulk chemistry, isotopic composition, and oxidation state of the subglacial fluids on 10⁴-10⁵ year timescales. Using ²³⁴U-²³⁰Th geochronologic and compositional data, our team’s early research on Antarctic precipitates from the Wilkes Basin region reveal behaviors of subglacial fluids that link EAIS response to global climate. Here we report preliminary geochronologic and geochemical data for a mineralogically similar subglacial precipitate found in the Central Transantarctic Mountains at Mount Achernar – a peak located at the junction between the MacAlpine Hills and Lewis Cliff, with a blue ice moraine at its base that is fed by the Law Glacier. The sample is over 12.4 cm thick and consists of interbedded opal and calcite layers, containing a total of nine opal-calcite transitions – each of which reflect major changes in subglacial water chemistry. Often in association with the opal layers we observe discrete siliciclastic layers, which may reflect changes in the subglacial depositional environment or grounding on the subglacial cavity. Preliminary ²³⁴U-²³⁰Th data place the sample bottom at 92 ka and the sample top at 25 ka, revealing an archive of 67 kyr that spans from marine isotopic stages 5 to 2. To date, our efforts to geochemically characterize the opal and calcite precipitating waters utilize laser ablation to map elemental changes with time. Based on Ce* and Fe/Mn data, opal-calcite transitions coincide with changes in water redox, with opals precipitating from reduced waters and calcites precipitating from oxidized waters. Ongoing research is working to provide additional geochemical characterization (⁸⁷Sr/⁸⁶Sr, δ¹⁸O, δ¹³C) and improve the age model in order to compare possible relationships between the timing of opal-calcite-clastic transitions and forcing mechanisms such as global climate.