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Ward, Dylan J ¹ ; Anderson, Robert S ² ; Briner, Jason P ³ ; Guido, Zackry S ⁴

¹ University of Colorado, Dept. of Geological Sciences and INSTAAR
² University of Colorado, Dept. of Geological Sciences and INSTAAR
³ State University of New York at Buffalo, Dept. of Geology
⁴ University of Colorado, Dept. of Geological Sciences and INSTAAR

We use cosmogenic radionuclide (CRN) exposure ages to constrain numerical simulations of deglaciation histories in the Middle Boulder Creek drainage, Colorado Front Range, and the Animas River valley, San Juan Mountains, Colorado. We present 14 new ¹⁰Be exposure ages from glacially polished bedrock sampled in the Middle Boulder Creek valley. All of these ages are younger than a ~17 ka ¹⁰Be terminal moraine age reported by Schildgen and Dethier (2002). Ages appear to decrease monotonically with distance upvalley from the moraine, and the youngest ages in the uppermost valley are uniformly ~13 ka. We include 4 ¹⁰Be ages in a cross section across the mid-valley, which show a pattern of Last Glacial Maximum (LGM) ages (12-14 ka) within the glacial footprint, and older exposure ages (~40 ka) near the trim lines. A similar age trend is seen in the Animas River valley in southwestern Colorado, which was occupied by a lobe of the LGM ice sheet that capped the San Juan mountains. Deglaciation began here ca. 19.4 ka, based on a ¹⁰Be depth profile in a proglacial terrace. A longitudinal transect of exposure ages from glacially polished samples indicates that terminus retreat proceeded at ~15 m/yr until complete deglaciation ca. 12.3 ka. Neither valley has obvious recessional deposits within the LGM glacial footprint. The first-order trend in each valley is a monotonic glacial retreat, but there are other possible retreat scenarios. For instance, we would like to test whether the same trend in ¹⁰Be concentrations could be generated by episodic retreat punctuated by periods of readvance. To investigate these scenarios, we modified the GC2D numerical glacier simulation (see Kessler et al., 2006) to incorporate a CRN accumulation layer. This layer can contain any starting value of CRN concentration. Production over each timestep is scaled to DEM latitude and altitude. Production is taken to be zero in areas covered by more than 10 m of ice. The CRN inventory can also decline due to glacial erosion. We incorporate a selectable erosion rule based on basal sliding or total ice velocity, ice discharge, ice power, or basal shear stress, and calculate the reduction in CRN inventory by the depth stripped in each timestep. We then simulate a glacier responding to equilibrium line altitude (ELA) changes imposed stepwise, gradually, or scaled to the GRIP δ¹⁸O record. Each scenario generates a pattern of ages in the CRN layer that can be compared with the map pattern of measured ¹⁰Be ages. Initial results show that a step-function ELA rise to its present value causes a retreat that is too rapid to explain the range of ages observed in both valleys. A steady ELA rise can replicate the monotonic age trend, while an episodic retreat with readvances results in several "domains" of similar ages within the LGM glacial footprint.

Kessler, M.A., Anderson, R.S., Stock, G., 2006. Modeling topographic and climatic control of east-west asymmetry in Sierra Nevada Glacier length during the Last Glacial Maximum. Journal of Geophysical Research Vol. 111(F2, F02002).

Schildgen, T., Dethier, D.P., Bierman, P., Caffee, M., 2002. Cosmogenic Age Estimates for Pinedale and Bull Lake Moraines in Colorado. Unpublished manuscript.