GEOLOGICAL INSIGHT INTO THE HOLOCENE GLACIAL HISTORY OF WEST ANTARCTICA BASED ON MORAINE STUDIES, PETROGRAPHY OF ERRATICS, AND COSMOGENIC DATING

Ice sheet thicknesses in coastal West Antarctica have been dramatically reduced since 7000 yr BP according to new cosmogenic surface-exposure dates from Marie Byrd Land (Stone, 2000). Linear Ranges and groups of nunataks with fresh exposed bedrock from 1200 m to 250 m in elevation comprise the Ford Ranges of Marie Byrd Land. Glacial erratics, ranging from isolated clasts to ground moraines, are present at all elevations. Granitic glacial erratics were measured for in situ cosmogenic 10Be and 26Al. This field area was optimally suited for dating of ice sheet thinning using the cosmogenic surface-exposure methods, due to the presence of abundant erratics at a range of altitudes. A higher degree of weathering and a surface-exposure date of ~105 kyr BP on the summits of Mt. Rea and the Billboard, near the present-day grounding line, constrain the Last Glacial Maximum ice surface elevation to a maximum of 600-650m above the current glacier surface. Summits further inland commonly preserve striated bedrock surfaces and were overridden by ice during the LGM. Surface-exposure ages at these sites are between 7000 and 4000 yr BP. These dates indicate that thinning of the West Antarctic Ice Sheet was extremely delayed relative to the Northern Hemisphere ice retreat. They are in agreement with the Ross Sea Embayment grounding line retreat history of Conway et al. (1999). Bedrock striations suggest a reorientation of glacial flow directions from the LGM to the present, which is consistent with thinning of an overtopping ice sheet. New petrographic work on erratics identified some exotic lithologies (welded tuffs, porphyrys and fault breccias) not known in outcrop. These may provide further information about paleo-ice flow patterns. Pyroclastic deposits have not been previously known in Marie Byrd Land. The Quaternary glacial history of the West Antarctic Ice Sheet is vitally important in any effort to anticipate future glacial climate-response trends and evaluate ice sheet stability.