PLIOCENE-PLEISTOCENE GLACIAL HISTORY OF MARIE BYRD LAND, WEST ANTARCTICA, FROM COSMOGENIC NUCLIDE MEASUREMENTS ON BEDROCK

Cosmogenic nuclides are produced at the Earth's surface in specific ratios to one another, but decay at different rates. Disequilibrium between nuclides measured in a single sample indicates a complex history of burial and exposure. Although the specific number, timing, and duration of burial and exposure events generally cannot be determined, minimum cumulative burial and exposure times can be calculated.

We have measured cosmogenic Al-26 and Be-10 in quartz from bedrock surfaces throughout the Ford Ranges, in West Antarctica. With the exception of heavily scoured surfaces at low altitude (where overriding ice was thickest), bedrock surfaces have apparent exposure ages much older than the last deglaciation and show disequilibrium between Al-26 and Be-10. We interpret these results in terms of exposure during interglacial periods and episodic overrriding by cold-based, weakly erosive ice, as follows: (1) Mountaintop surfaces show evidence of longer cumulative exposure and shorter cumulative coverage by ice than surfaces at lower altitude, as expected. (2) Samples from inland sites and higher altitudes, where overriding ice was thinnest and coldest, record the longest cumulative exposure and burial histories. (3) The oldest surface encountered so far records a minimum of 3.5 million years of cumulative burial by ice, compared to a minimum of 130,000 years of cumulative exposure. (4) The Al-26 exposure age of this surface is so short that it cannot have been exposed for more than 10,000 years within the past million years. (5) Observation (4) above suggests that the present deglaciation of West Antarctica is among the most extensive to have occurred in the mid- to late-Quaternary. (6) Bedrock weathering, including surficial oxidation, salt and gruss accumulation, is the result of repeated exposures separated by periods of non-erosive or weakly-erosive ice cover. Heavy weathering of bedrock cannot be taken as evidence of prolonged ice-free conditions. This research is supported by LLNL-CAMS and NSF award OPP-0229915.