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

Rates and mechanisms of cirque-glacial bedrock erosion are poorly constrained; ³He cosmogenic nuclide analyses, however, provide a powerful tool to infer erosion depths at a relatively high spatial resolution. Our study aims to estimate cirque-glacial erosion depths beneath the Sisters Glacier, North Cascades, WA, by measuring cosmogenic ³He concentrations in bedrock within the Holocene glacier limits. 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. ³He is well-suited for analyzing Holocene-aged surfaces because it has a relatively high production rate (~115-130 atoms g^-1 yr^-1), and can be detected at low concentrations (only 5,000-10,000 atoms). The Sisters Glacier lies atop ultramafic dunite bedrock (~95% olivine), in which diffusion loss of ³He is negligible on timescales less than 10 million years. Analyses of ³He concentrations in bedrock can be used to infer erosion depths if exposure time is known. Preliminary mapping of the field area using recently acquired Lidar data and aerial imagery allows identification of bedrock fractures, glacial fluting, terminal moraines, and degree of surface oxidation. As a result of these analyses, we have identified a likely Holocene maximum extent of the Sisters Glacier. Detailed Holocene glacial chronologies from nearby Mount Baker indicate that the Holocene forefield of the Sisters Glacier was likely exposed for ca. 10 kyr after Pleistocene deglaciation, then covered by Neoglacial ice for ca. 2 kyr until the last century. In August of 2017, we plan to collect bedrock samples along several glacial flow-line transects within the Holocene ice limits, as well as a suite of samples outside of this extent. Together, these results should allow us to constrain the magnitude and spatial variability of Holocene subglacial erosion.