THE EFFECTS OF SOLAR INDUCED THERMAL FATIGUE AND STRESS ON COSMOGENIC NUCLIDE PRODUCTION IN HIGH-ALPINE CIRQUES, OLYMPUS RANGE, ANTARCTICA

We investigated the effect of surface weathering processes on cosmogenic nuclide chronologies in the high-elevation McMurdo Dry Valleys of Antarctica. Within adjacent ~2-km wide cirque valleys (Dean, Dipboye and Liebert) in the Olympus Range, a suite of stratigraphically correlated drifts and moraines record Pleistocene fluctuations of cold-based alpine glaciers. Chronologic control comes from helium isotope measurements from pyroxenes in Ferrar Dolerites from four moraines in Diboye Cirque. This sequence provides a preliminary uncorrected ³He chronology ranging from ~200 kyr to ~2.4 Myr ago; ages increase away from the modern glacier, but all moraines exhibit age scatter. The goal of this study is to isolate the relative influence of 1) nuclide inheritance, 2) complex burial histories, and 3) erosional processes on this age scatter. Our weathering studies focused on thermal stress erosion and surface rind development on dolerites and sandstones. We measured temperature fluctuations in boulders at 2-cm depth increments during periods of full sun, shade, and episodic cloud cover to quantify the influence of solar radiation on thermal stress weathering. In order to investigate the role of local topography, we used ArcGIS to model solar incidence and intensity during the austral summer months of November and December. ArcGIS modeling tracked sun-shade cycles over 24 hour and 30 day intervals, highlighting surface areas subject to repeated solar stresses. Results show that intermittent cloud cover can cause rapid temperature fluctuations of >40°C and that dolerite surface temperatures can change by >2°C/min, causing repeated stress on the rock. We expect relative age scatter caused by thermal fatigue to be more pronounced on younger drifts as fresh rock cores are exposed and less pronounced in older drifts as total nuclide inventories increase and average clast sizes decrease.