Quantifying the magnitude and spatial variability of bedrock erosion beneath the Sisters Glacier, Washington, using cosmogenic…

Cosmogenic 3He analyses provide a tool to infer spatial variation of cirque-glacial 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 as well as proximity to the surface. The Twin Sisters range, North Cascades, WA is an ideal location to use cosmogenic 3He to infer cirque-glacial erosion depths and rates, due to the dunite bedrock and the detailed record of Holocene glaciation from the nearby Mount Baker. 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. Concentrations of cosmogenic 3He in all samples were calculated based on 3He/4He ratios found in shielded samples from a nearby quarry because standard crushing techniques did not remove mantle-sourced 3He from the samples, introducing a significant amount of uncertainty. Our analyses of forefield samples show decreasing exposure ages (10,500-0 yrs) and increasing depths of erosion (0.15-146 cm) with proximity to the modern glacier. The patterns in erosion rates are more difficult to discern due to larger uncertainties, but potentially show 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; however, 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. This study helps to better understand how cirque glaciers shape mountain topography.