COSMOGENIC EXPOSURE DATED BOULDERS REVEAL MORAINE EROSION HISTORY, BISHOP CREEK, CA, USA

Twenty-six cosmogenic exposure dated boulders from the surface of the moraine on Bishop Creek, eastern Sierra Nevada, California show an age distribution incompatible with a moraine degradation model that assumes a constant rate of downslope soil motion, creep, represented by topographic diffusivity. When the topographic diffusivity in the model is allowed to vary in concert with independently determined local climate shifts, the modeled and observed boulder age distributions match closely. The observed boulder age distribution and Owens Lake fresh water diatom abundancies suggest a general three-stage erosion history. The timing of erosion stages is derived from Owens Lake climate record and the topographic diffusivity is allowed to vary. The general erosion history begins with low erosion rate between 93-85 kyr, decrease to minimum values from 85 to 55 kyr, then increase to high rates from 55 kyr to present. The resulting erosion rates agree qualitatively with the Owens Lake fresh water diatom counts. These results provide a glimpse of the wealth of yet to be explored paleoenvironmental information that is contained in the moraine boulder ages.