ALLUVIAL FANS IN THE MCMURDO DRY VALLEYS: A PROXY FOR MELTING ALONG TERRESTRIAL MARGINS OF THE EAST ANTARCTIC ICE SHEET

We studied alluvial fans in the McMurdo Dry Valleys of the Transantarctic Mountains as a proxy for past surface melting on the East Antarctic Ice Sheet. These fans are adjacent to ice sheet margins and result from melting of alpine glaciers and snowbanks. Although fans that originate from low-elevation sources near the coast are seasonally active, inland fans are relict features suggesting they are active only during warmer-than-present climate episodes. Fans in the McMurdo Dry Valleys are a previously unutilized climate record that could help identify past climatic warmth when ice surface melting was a more significant contributor to ice sheet ablation. This is important because surface melting at Antarctic ice sheet margins is the most poorly understood input to models of future sea-level rise.

Results come from five representative fans in the western Dry Valleys with melt sources at elevations equal to ice sheet margins. We identified depositional processes of fan aggradation, established a chronologic framework of fan development, and identified factors controlling meltwater generation. Fans are composed of planar-bedded gravelly sands only centimeters thick, which resulted from low flow regime sheetfloods comparable to active streams near the coast. Well-developed desert pavements on fan surfaces indicate centuries to millennia have passed since fans last aggraded. Buried desert pavements, identical to those on fan surfaces, separate sets of alluvium at depth, which indicates similarly long hiatuses between past periods of aggradation. Fan deposits dated using OSL thus far are no older than early Holocene. Geospatial analysis suggests the distance of the meltwater source from the Ross Sea is the predominant control on fan activity. The ideal conditions for fan aggradation likely occur when regional climatic gradients shift inland. Interestingly, a dated sand wedge beneath one fan indicates no fan was present during the late Pleistocene. We question whether high-elevation fans are ephemeral features formed over the last 10,000 years. Fans formed during previous interglacial periods, such as isotope stage 5e, could have been completely eroded away during the most recent cold, dry glacial phases and have only begun to reform during the Holocene.