ALLUVIAL FAN DEPOSITION IN THE DRY VALLEYS, ANTARCTICA: A PALEOCLIMATIC RECORD OF GLACIAL MELTWATER STREAMFLOW

The McMurdo Dry Valleys are the largest ice-free region in Antarctica, consisting of a series of west-east trending glacially-carved valleys separated by ~2,000-m elevation mountain ranges. Modern climate is a cold polar desert, however summer air temperatures can be above 0°C for many weeks in the summer, resulting in seasonal glacial melt and streamflow. In the northern Dry Valleys, we identified 45 alluvial fans and fan complexes, ranging from 0.1–4.2 km². Based on remote mapping of fan surface morphology, ~70% of the fan sites are active and ~30% are inactive. Given the tectonic stability of the Dry Valleys during the Pleistocene, variations in fan sedimentation are likely controlled by summer meltwater rather than tectonics. We used optically stimulated luminescence (OSL) to date quartz and feldspar sands from 20 samples from nine alluvial fans. At the field sites, we also dated 17 samples from stratified deposits not associated with fans. The dated alluvial fans record meltwater stream deposition from Marine Isotope Stage (MIS) 5 (~112 ka) to the late Holocene (~1 ka). Our initial OSL analyses suggest that there is higher stream activity during MIS 5 and the Holocene than during MIS 3 and the Last Glacial Maximum. Remote mapping and GIS analyses indicate that the presence of alluvial fans is controlled by microclimatic conditions at the moisture source regions, such as incoming solar radiation and elevation. Therefore, dating of alluvial fans has implications for past variations in summer climate as well as variations in snow and glacier cover. These data also provide a geological context for understanding modern streamflow. Future fieldwork will focus on expanding the geographic range of dated alluvial fans in the Dry Valleys and extraction of ice-cemented sediment cores to acquire longer depositional records.