COSMOGENIC NUCLIDE EXPOSURE AGES AND GLACIAL HISTORY OF LATE QUATERNARY ROSS SEA DRIFT IN MCMURDO SOUND, ANTARCTICA

He-3, Be-10 and Al-26 surface exposure ages were determined for rock samples from late Quaternary drift sheets deposited on the coast of McMurdo Sound, Antarctica, by the Ross Sea Ice Sheet. The youngest drift, previously believed to be Late Wisconsin in age, has exposure ages of 8-106 kyr (19 samples). These results suggest that the apparent 'Late Wisconsin' ice margin may represent a number of ice sheet advances during the last glacial period. He-3 exposure ages from analogous, stratigraphically older drift range from 104 to 567 kyr (seven samples), and clearly distinguish the youngest drift from older deposits. Comparison of Be-10 and He-3 concentrations in Ross Sea Drift quartz samples indicates significant He-3 loss in only one sample, suggesting that cosmogenic He-3 is generally retained in young Antarctic quartz samples (exposure age < 100 kyr) of appropriate grain size. In two of the five quartz samples analyzed, significant excesses of Al-26 relative to Be-10 preclude meaningful Al-26 exposure age calculations. These excesses may be due to nucleogenic Al-26 produced as a byproduct of uranium and thorium decay.