PRE-LGM ADVANCES OF ALPINE GLACIERS IN THE OHIO RANGE, SOUTHERN TRANSANTARCTIC MOUNTAINS, ANTARCTICA

The marine-based West Antarctic Ice Sheet (WAIS) is potentially unstable and could raise sea level up to 5 m in response to increasing global temperature. Therefore, it is important to understand the relationship between past climate and WAIS behavior. We present a preliminary ³He cosmogenic nuclide chronology from moraine boulders deposited by two adjacent alpine glaciers in the Ohio Range in the Southern Transantarctic Mountains (TAM) near the WAIS divide in the interior of the ice sheet (85°S, 114°W). We assume these alpine glaciers, flowing from opposing sides of a NE-SW trending ridge, are primarily sensitive to changes in regional precipitation. Their coalescent moraine systems suggest that despite opposing glacier aspect, the two glaciers advanced simultaneously. Local factors, such as changing prevailing wind direction, were unlikely to be the primary cause for glacial expansion. Four ³He surface-exposure ages from ice-cored moraines of the south-facing glacier range from 52-45 ka. Four similar ³He ages occur from correlative moraines on the north-facing glacier suggesting both glaciers reached their most recent maximum extent during Marine Isotope Stage (MIS) 3. At the north–facing glacier, an older moraine segment projects out from beneath this moraine. The four ages from this moraine are significantly older, suggestive of an advance during MIS 6 or earlier. Exposure ages of boulders from the present ablation areas of the glaciers show scatter ranging between 11 and 38 ka. These exposure ages should be regarded as maximum estimates since no prior exposure or erosion corrections have been applied. However, erosion rates in the TAM are very low and the tight cluster of ages from MIS 3 suggests significant prior exposure is unlikely to be greater than 10 ka on those moraines, which would not change our conclusions. These results suggest that accumulation increases in the interior WAIS during interstadials rather than during global glacial maximums.