PLIOCENE-PLEISTOCENE GLACIAL HISTORY OF MARIE BYRD LAND, WEST ANTARCTICA, FROM COSMOGENIC NUCLIDES IN BEDROCK

In situ cosmogenic Be-10 and Al-26 are produced at the Earth's surface at a specific ratio to one another, but decay at different rates when a sample is shielded from cosmogenic radiation. Disequilibrium in this ratio in a sample implies a complex history of exposure and burial.

Measurement of Be-10 and Al-26 in quartz from bedrock surfaces throughout the Ford Ranges, West Antarctica, reveals a history of exposure, burial beneath the West Antarctic Ice Sheet, and survival against sub-glacial erosion. Except for low-altitude surfaces heavily scoured by the ice sheet, all surfaces display disequilibrium in the ratio of Al-26 to Be-10 and have apparent exposure ages significantly older than Holocene deglaciation. The data show that: (1) Effective exposure time increases with altitude on each mountain, (2) The ratio of time-exposed to time covered by ice decreases with altitude on each mountain and roughly decreases inland, (3) Minimum overall burial times range from hundreds of thousands to ~3.5 million years, and (4) Maximum limits on the fraction of time-exposed (compared to the time covered by ice) for these samples are as low as 1-5%. These results are consistent with survival of the surfaces through repeated Plio-Pleistocene glaciations, and indicate that the present deglaciation of West Antarctica is among the most extensive in the mid- to late-Quaternary.

Bedrock in the Ford Ranges is variably weathered, from clean, glacially abraded surfaces at low altitude, to weathered surfaces at high altitude, showing surficial oxidation, salt and gruss accumulation, widened joints and rounded corners. Our data show that weathered features at high altitude are the result of repeated exposure and glaciation, rather than continuous sub-aerial weathering.

Supported by NSF grant OPP-0229915, and the LLNL Center for Accelerator Mass Spectrometry.