EXPOSURE HISTORY OF INTERMITTENTLY-GLACIATED NUNATAKS IN THE FORD RANGES, WEST ANTARCTICA

Cosmogenic Be-10, Al-26 and Cl-36 (with half-lives of 1.5, 0.7 and 0.3 Myr, respectively) accumulate in bedrock surfaces exposed at times of deglaciation; when surfaces are ice-covered, these nuclides decay at different rates, leading to isotopic disequilibrium. In Antarctica, where glaciation by thin, cold ice protects underlying bedrock from erosion, concentrations of these nuclides provide million-year records of exposure and ice cover. Although we cannot recover the ages of specific glaciations, we can obtain (i) minimum limits on the total duration of past exposure and glaciation, and maximum limits on (ii) the relative proportion of time-exposed to time-covered by ice, and (iii) total exposure time during the last million years.

Results from Marie Byrd Land show that: (i) Bedrock surfaces retain a memory of exposure and ice cover ranging from 0.4 Myr to more than 4 Myr. (ii) Mountain summits have the longest minimum cumulative exposure times, and record the greatest proportion of time exposed. (iii) Nonetheless, summits have spent a greater proportion of time ice-covered than exposed; maximum limits on exposure range from <45% for coastal mountain tops, to <5% for inland peaks. (iv) Samples close to glacier level record histories totalling >1-4 Myr, in which the surfaces have been exposed for as little as 1% of the time. (v) These low-elevation surfaces cannot have been exposed for more than 90-2300 years if last exposed during marine isotope stage (MIS) 5e, or 170-3800 years if last exposed during MIS 11, believed to have been the warmest and most prolonged Late Pleistocene interglacial. Thus: (vi) Parts of Marie Byrd Land that are presently exposed have seldom outcropped in the past, and (vii) The West Antarctic Ice Sheet is smaller in the late Holocene than it has been in all but the most extreme interglacial periods of the past million years.

Supported by NSF awards OPP-9909778, OPP-0229915 and the LLNL-CAMS small grants scheme.