R. P. Ackert, S. Mukhopadhyay, D. Pollard, R. M. DeConto, A. E. Putnam, and H. W. Borns
Earth and Planetary Science Letters (1 July 2011)
Knowledge of the timing and extent of past West Antarctic Ice Sheet (WAIS) elevation changes provides insight to the WAIS response to sea level and climate forcing, as well as constraints for ice sheet models. Trimlines and erratics indicate that maximum ice elevation at the Ohio Range, located near the WAIS divide, was 125 m above the present ice surface and was reached at 10 ka. However, 3He and 10Be exposure ages of most granitic erratics and supra-glacial boulders in adjacent blue ice areas are older than the last highstand and provide constraints on earlier interior WAIS elevation changes. Although diffusive loss of 3He occurs in quartz, an excellent correlation between 3He and 10Be concentrations in samples up to 400 ka is observed in the Ohio Range, demonstrating the utility of 3He measurements in Antarctic quartz. The correlation is used to calculate “equivalent 10Be exposure ages” from the larger 3He data set. The exposure ages occur in distinct clusters 10–20 ka, 70–80 ka, 130–140 ka and 180–190 ka that we relate to pulses in the emergence of englacial debris in localized blue ice ablation areas. A combined ice sheet/ice shelf model predicts that WAIS elevations near the Ohio Range have varied only 125 m over the last 200 kyr with highstands at 10 ka and 130 ka, in good agreement with observations of trimlines and peaks in the exposure age distribution at the Ohio Range. In the model, interior WAIS elevations are controlled by the interaction of accumulation rates and ice dynamics related to ice sheet/ocean interactions at the grounding line. WAIS highstands occur early in interglacial periods during intervals of warmer temperatures and high accumulation rates, prior to the arrival of a wave of thinning. We hypothesize that the correspondence of rising regional WAIS elevation with debris pulses relates to increased ice (debris) flux and ablation in the local blue ice areas resulting from the warmer temperatures. The occurrence of clasts with exposure ages up to 200 ka on the WAIS surface indicates elevation changes near the Ohio Range were limited in extent during stage 5e. The geologic observations, combined with model results, place constraints on WAIS geometry during stage 5e and limit the WAIS contribution to the higher sea levels observed during the last interglacial to 3 m.
keywords: WAIS; 3He; 10Be; exposure dating; marine ice sheet; ice sheet model