Using inherited cosmogenic ³⁶Cl to constrain glacial erosion rates of the Cordilleran ice sheet

Cosmogenic ³⁶Cl/Cl ratios measured from glacially eroded bedrock provide the first quantitative constraints on the magnitude, rate, and spatial distribution of glacial erosion over the last glacial cycle. Of 23 ³⁶Cl/Cl ratios, 8 yield exposure ages that predate the well-constrained deglaciation of the Puget Lowland, Washington, and are inferred to result from ³⁶Cl inherited from prior exposure during the last interglaciation where ice did not erode enough rock (∼1.80-2.95 m) to reset ³⁶Cl/Cl ratios to background levels. Surfaces possessing inherited ³⁶Cl evidently were abraded only 0.25-1.06 m, corresponding to abrasion rates of 0.09-0.35 mm·yr^-1. These results indicate that in the absence of glacial quarrying, the Cordilleran ice sheet may have abraded as little as 1-2 m of bedrock near its equilibrium-line altitude over the last glacial cycle, equating to only tens of meters over the entire Quaternary.