Pleistocene glacial advances and exposure age scatter in the Olympus Range, Antarctica: A study of cosmogenic 36Cl/3He in dolerites and 10Be in sandstones

Abstract

In three cirques in the western Olympus Range of the McMurdo Dry Valleys, Antarctica, previous advances of cirque glaciers are recorded by a sequence of three drifts in each of the cirques. We dated drift limits and the deposits on modern glaciers in two of these cirques, Dean and Dipboye, via cosmogenic 3He in pyroxene from 41 dolerite boulders, 36Cl in pyroxene from 12 of those dolerites, and 10Be in quartz from 11 sandstone boulders. Exposure age scatter is high on all deposits. The 3He exposure ages across all deposits range from ∼35 to ∼2300 ka and 10Be exposure ages range from ∼7 to ∼435 ka. Coupled 36Cl/3He from dolerites support constant exposure with erosion for nine of the 12 samples, while the other three might have experienced complex exposure-burial histories. Due to the mesa-butte topography and slow bedrock erosion rates, nuclide inheritance is the primary cause of age scatter in dolerites, accounting for >1 Myr of exposure age error. Mean exposure ages from sandstones are 2–7 times younger than those from dolerites for the same deposits, indicating that inheritance is less common in sandstones in this region. Weathering analyses of sandstone boulders show an increase in average siliceous crust thickness and rock strength with deposit age, an example of case hardening. Based on both relative and exposure age dating, drift age increases with distance from the modern glaciers in both Dean and Dipboye cirques, with three advances during the past <700 ka. However, due to high exposure age scatter, it cannot be determined if the three drifts are temporally correlated across the two cirques and therefore the drifts might record different glacial advances in Dean Cirque vs. Dipboye Cirque despite the apparent stratigraphic correlation of the drifts. This study has implications for drift depositional processes of cold-based glaciers and the importance of source-bedrock lithology and geomorphology on nuclide inheritance in Antarctica.