GLACIAL EROSION RATE FOR THE CORDILLERAN ICE SHEET IN SOUTHEAST ALASKA USING COSMOGENIC 10BE CONCENTRATIONS IN BEDROCK AND PERCHED BOULDERS

We quantify the glacial erosion rate of the Cordilleran Ice Sheet in Southeast Alaska. Beryllium-10 ages of two perched boulders (17,100 ±330 and 16,900 ± 360 years old) represent the timing of the last deglaciation from Warren Island, a small island northwest of Prince of Wales Island. A ¹⁰Be age for an adjacent bedrock surface, however, is 24,800 ± 470 years old. We use the inherited ¹⁰Be in the bedrock surface to solve for glacial erosion depth and rate using several assumptions. Radiocarbon ages from bones found in a nearby, ice-sheet-overridden cave (Shuka Kaa) suggest the onset of glaciation in the region occurred at 19,800 cal yr BP. We assume that Warren Island was completely void of ¹⁰Be at the end of the major penultimate glaciation in Alaska 60,000 years ago. Using the local ¹⁰Be production rate, we calculate the depth of erosion to be 124 cm. Using the cave record and the ¹⁰Be ages from the two boulders to determine the duration of glacier overriding, we solve for a glacial erosion rate of 0.45mm/year. Because the sampled bedrock surface was glacially abraded, the total glacial erosion rate likely represents the glacial abrasion rate. This abrasion rate is consistent with other abrasion rates (0.1-1.0 mm/year) calculated for moderately thick temperate glaciers with low to moderate sliding velocities over resistant granite bedrock. Three additional erratic/bedrock pairs from other sites in the study area are being processed and may be available at the time of the presentation.