ESTIMATES OF LAST GLACIAL MAXIMUM CLIMATE OF THE SNOWY RANGE, SOUTHERN WYOMING, USING NUMERICALLY MODELED PALEOGLACIER RECONSTRUCTIONS

In this study we use field mapping and numerical modeling data to assess possible Last Glacial Maximum (LGM) climate conditions in the Snowy Range of southern Wyoming. Geomorphic evidence suggests that the range supported a small mountain icecap/outlet glacier complex at the LGM. Ours and previous field mapping constrain the paleoglacier margins and in some places the thickness of the ice. A dual-component model of glacier energy/mass balance and flow developed by Plummer and Phillips (2003) produced two-dimensional reconstructions of ice over a DEM of the study area utilizing modern input climate and hypothetical LGM combinations of precipitation and temperature relative to modern. Field-mapped estimates of paleoglacier margins from three drainages in the range were then compared with modeled margins to determine which input combinations of temperature and precipitation changes from modern climate would have been sufficient to maintain the glacier complex in mass-balance equilibrium at its LGM extent. Repeated simulations indicate that if LGM precipitation remained unchanged from modern, a 6.2°C temperature depression would have been necessary to sustain the glacier complex. A 4.05°C temperature depression would have corresponded with double modern precipitation, a 7.0°C depression with .75x modern precipitation, and a 8.0°C depression with .5x modern precipitation. However, existing interpretations of regional LGM climate patterns suggest that colder/drier is the more likely interpretation of LGM climate conditions in the Snowy Range as opposed to cool/ moist. These results place the range climatically between the Tetons and the Colorado Front Range. This is consistent with the range’s geographical position and with GCM simulations for the region, which indicate that a possible southerly migration of the winter Pacific Jet Stream during the LGM would have led to a increasing-northward reduction in precipitation for the northern and central Rockies.