DISTRIBUTION AND SOURCES OF SCATTER IN ANTARCTIC EXPOSURE AGE CHRONOLOGIES: A CASE STUDY FROM MCMURDO SOUND

Cosmogenic exposure chronologies of Antarctic glacial sediments often yield high scatter with some individual ages greatly exceeding the true depositional age. This offset and variance is likely due to nuclides that accumulated during periods of prior exposure but were not removed by subsequent cold-based glaciations. Both scatter and inaccuracy in exposure age datasets complicates reconstructions of former ice sheet volume through time. McMurdo Sound is an excellent site to evaluate this problem because there is extensive glacial drift deposited during the Last Glacial Maximum (LGM) dated using both in situ produced cosmogenic nuclides and a calibrated radiocarbon chronology of algae embedded in moraines.

We present a compiled dataset of all new and published radiocarbon (n=237) and exposure ages (n=48) of LGM drift in McMurdo Sound to investigate exposure age scatter. Calibrated radiocarbon ages bracket the local LGM between 19.6 and 12.3 cal. ka (primary mode: 15.8 cal. ka, skewness: 0.03). Cosmogenic exposure ages vary by lithology and nuclide, suggesting that bedrock source and entrainment processes explain scatter. Be-10 exposure ages of quartz in granite, sourced from the base of the stratigraphic section in the Transantarctic Mountains, are younger (range: 10.6 - 50.1 ka, primary mode: 13.2 ka, skewness: 1.3), which indicates that many clasts entrained by sub-glacial plucking generate reasonable apparent exposure ages albeit with scatter. He-3 exposure ages of pyroxene in dolerite, which outcrops above outlet glaciers in the Transantarctic Mountains, are generally older (range: 29.1 - 405.7 ka, primary mode: 46.7 ka) but similar skewness (1.5), which suggests clasts exposed on cliff faces and entrained by rock fall carry greater nuclide inheritance. He-3 exposure ages of olivine in basalt, sourced from local volcanic bedrock in McMurdo Sound, range from 6.8 to 211.6 ka with a skewness of 2.47, but have a bimodal distribution with primary and secondary modes at 9.9 ka and 27.4 ka, respectively. The bimodal distribution suggests clasts from local bedrock record changes in ice extent with exposure prior to the local LGM (27.4 ka) and after ice retreat (9.9 ka). Sampling lithologies sourced from different horizons of the stratigraphic section provides a way to link geologic process to exposure age scatter.