CHRONOLOGY AND CLIMATE OF THE LAST PLEISTOCENE GLACIATION IN THE BIGHORN RANGE, WYOMING

During the last Pleistocene glaciation, numerous valley glaciers populated the Bighorn Range in northern Wyoming (Middle Rocky Mountains). The glacial record of this range has been known for decades; however, the pace and timing of deglaciation are poorly understood. This limits our ability to use the regional pattern of glaciation to infer Pleistocene climate change. Two episodes of Late Pleistocene moraine deposition are indicated by cosmogenic ¹⁰Be exposure ages of terminal moraines of both Tensleep Canyon and North Clear Creek. The first occupancy of a terminal moraine in Tensleep Canyon occurred at 17.9 ± 0.8 ka followed by a second occupancy at 14.4 ± 0.5 ka. A terminal moraine in North Clear Creek valley features bimodal exposure ages with an older mode of 22.8 ± 1.3 ka and a younger mode of 18.0 ± 1.1 ka. Through applying a 2-D numerical model of mass balance and ice-flow to gridded elevation models of these two valleys, we can identify the range of temperature and precipitation combinations that could have accompanied steady state glacial maximum during the dated episodes. The mass/energy balance model is paired with an ice physics model to simulate the known areal ice extent and thickness based on mapped geomorphological features. The range of possible temperature and precipitation combinations for the two valleys are compared to other proxy records to determine the paleoclimate of the Bighorns Mountains during the last glaciation. Preliminary results from the initial modeling of the glacier in North Clear Creek indicate that softer ice produces a glacier with a better fit to the geomorphic features than stiffer ice. Model output also shows that a temperature depression of at least 6˚C with modern precipitation was needed to support the glacier near its maximum length at ca. 22.8 and 18.0 ka, consistent with valley glacier simulations from elsewhere in the Middle Rocky Mountains in Wyoming, Utah, and Colorado. Additional modeling of maximum and recessional length of glaciers in the North Clear Creek valley and Tensleep Canyon will provide further limits on paleoclimate in the region. Cosmogenic exposure ages for the recessional moraine in North Clear Creek are anticipated and will be used to interpret the pace of deglaciation in this valley.