QUANTIFYING THE MAGNITUDE AND SPATIAL VARIABILITY OF BEDROCK EROSION BENEATH THE SISTERS GLACIER, WASHINGTON, USING COSMOGENIC 3HE CONCENTRATIONS

Cosmogenic ³He analyses provide a tool to infer spatial variation of cirque-glacier bedrock erosion. ³He accumulates in bedrock exposed at the surface as a result of cosmic ray bombardment; the concentration of cosmogenic ³He increases with exposure time, and decreases with depth below the surface. We measured cosmogenic ³He in dunite bedrock from the Twin Sisters range, North Cascades, WA to infer cirque-glacial erosion depths and rates, and used the detailed record of Holocene glaciation from the nearby Mount Baker to constrain exposure periods. We used field mapping, lidar data and aerial imagery to identify bedrock fractures, glacial fluting, and terminal moraines to establish the maximum Holocene extent of the Sisters Glacier. We collected bedrock samples along glacial flow-line transects in the forefield of the Sisters Glacier, as well as several samples outside the limit of Holocene glaciation. Standard crushing techniques did not remove mantle-sourced ³He from the samples. Ranges of cosmogenic ³He concentrations in each sample were calculated based on ³He/⁴He ratios measured in shielded samples from a nearby quarry, introducing a significant amount of uncertainty. Our analyses of forefield samples show decreasing model exposure ages (10,500-0 yrs) and increasing calculated depths of erosion (0.15-146 cm) with proximity to the modern glacier. Patterns in erosion rates are more difficult to discern due to uncertainty in period of glaciation, but suggest increasing rates of subglacial erosion (0.001-0.7 mm/yr) with proximity to the modern glacier. These rates are consistent with those proposed by previous studies, and the relatively low and consistent erosion rates suggest that abrasion and/or small quarrying events (centimeters to decimeters) are the dominant mechanisms of erosion underneath the Sisters Glacier.