NUMERICAL MODELING OF VALLEY GLACIERS TO INFER PLEISTOCENE CLIMATE IN THE BIGHORN RANGE OF WYOMING

The Bighorn Range of the Rocky Mountains featured numerous large valley glaciers during the Pleistocene. While the glacial record of the mountains has been known for decades, the timing of the last glaciation in the range has remained poorly known and has limited the potential of using the pattern of glaciation to infer Pleistocene climate change. Cosmogenic ¹⁰Be exposure dating of terminal moraines indicates multiple Late Pleistocene glacial episodes in the valleys of Tensleep Canyon and North Clear Creek. The first occupancy of a terminal moraine in Tensleep Canyon occurred around 17.9 ± 0.8 ka and the second occurred at about 14.4 ± 0.5 ka. The exposure ages from the terminal moraine in North Clear Creek are older, with the first occupancy at about 22.8 ± 1.3 ka and the second around 18.0 ± 1.1 ka. We applied a 2-D numerical model of mass balance and ice-flow to gridded elevation models of the two glacial valleys. The modeling approach identifies the range of temperature and precipitation combinations that could have accompanied steady state snow ablation and accumulation during the last glaciation. This is combined with an ice physics model to simulate the known areal ice extent and thickness based on mapped terminal moraines. The range of possible temperature and precipitation combinations for the two glacial valleys are compared with other proxy records to determine the paleoclimate of the Bighorn Mountains during the last glaciation. Ongoing work includes efforts to determine the change in climate accompanying ice retreat by fitting modeled glaciers to recessional moraines within these valleys and additional cosmogenic exposure dating in order to determine the rate of ice retreat in this region.