Numerical modeling of cosmogenic déglaciation records, Front Range and San Juan mountains, Colorado

We use cosmogenic radionuclide (CRN) exposure ages from polished, striated bedrock to constrain numerical simulations of déglaciation in the Middle Boulder Creek Valley, Colorado Front Range, and the Animas River Valley, San Juan Mountains, Colorado. In both valleys, the cosmogenic ages suggest initiation of déglaciation ~20 ka and ongoing retreat until 12-13 ka. While the first-order trend in CRN concentrations in each valley suggests a monotonie glacial retreat, we evaluate other retreat scenarios with different implications for post-Last Glacial Maximum regional climate. We use a 2-D numerical glacier simulation with a CRN layer to investigate how CRN-based déglaciation records are affected by retreat histories that are punctuated by periods of glacier readvance. The CRN layer simulates both production during periods of exposure and reduction by glacial erosion during readvances. We simulate glacial occupation of the valleys as they respond to equilibrium line altitude (ELA) histories characterized by stepwise change, gradual rise, or a rise punctuated by short periods of lowering. Each scenario generates a distinct spatial pattern of concentrations in the CRN layer. These results and the spatial pattern of measured concentrations in bedrock constrain the range of ELA histories that reproduce the CRN pattern in each valley. In the Animas River Valley, the exposure ages are well explained by a linear ELA rise from full glacial to deglacial conditions. Ages in Middle Boulder Creek Valley are best explained by a déglaciation history including a stillstand or partial readvance between 16 and 14 ka, followed by rapid retreat.