COSMOGENIC 3HE EXPOSURE DATING OF GLACIAL DEPOSITS IN THE OLYMPUS RANGE, ANTARCTICA: INFLUENCE OF COLD-BASED DEPOSITION AND WEATHERING PROCESSES ON AGE SCATTER

The high-elevation western Olympus Range in Antarctica contains a terrestrial record of cold-based glaciation that extends to the mid-Miocene. Within adjacent, ~2-km wide cirque valleys, a suite of stratigraphically-correlated drifts and moraines record recent fluctuations of cold-based alpine glaciers. We measured helium isotopes from pyroxenes in Ferrar Dolerites from a sequence of four moraines in Dipboye Cirque. All deposits exhibit significant age scatter, with a preliminary uncorrected ³He chronology ranging from ~200 kyr to ~2.4 Myr ago. Drift and moraine ages generally increase away from the modern glacier, indicative of alpine glacier advance during the early- to mid-Pleistocene, likely due to increased snowfall during warmer-climate intervals. The modern cold-based alpine glacier contains a near-continuous ridge of debris exposed at the glacier/ice apron contact. Based on its ice-sediment morphology and its relationship to the underlying ice apron, this “moraine” is likely re-entrained, reworked drift; it yields preliminary exposure ages from ~200 to ~800 kyr. Erosion from salt pitting and flaking of crusts has little effect on exposure ages over this time interval since erosion rates are generally < 10 cm Myr^-1. Rather, the key parameters causing age scatter appear to be 1) complex depositional/reworking processes associated with cold-based glacier termini when ice aprons are present, 2) a relatively high radiogenic helium component, which has not been a major source of error in Ferrar Dolerites in the Quartermain Mountains, and 3) the general difficulty in differentiating and sampling thin (< 50-cm) stacked drifts of uniform lithology. Our study highlights the importance of cold-based glacier dynamics and debris entrainment processes in interpreting exposure ages. Future work focuses on: 1) analyzing modern moraines and supraglacial tills that are not subject to reworking and 2) analyzing cosmogenic ³⁶Cl from pyroxenes and feldspars in these dolerites.