MODELING EQUILIBRIUM LINE ALTITUDES OF MOUNTAIN GLACIERS IN THE WESTERN UNITED STATES DURING THE LAST GLACIAL MAXIMUM

Reconstructed equilibrium altitude lines (ELA) of paleo-mountain glaciers can provide important information about past climates, specifically the temperature and precipitation accompanying a glacier in equilibrium. Previous reconstructions of Late Pleistocene ELAs of mountain glaciers across the western United States have been used to infer the pattern of temperature and precipitation change across the region, although most of this work was done with limited age control on glacial deposits and landforms. Cosmogenic nuclide exposure dating of moraines across the western United States combined with new mapping aerial imagery afford an opportunity to revisit the pattern of regional ELAs during multiple episodes of the last Pleistocene glaciation. The goal of this study is to reconstruct ELAs in the same region based on glacial sediments with cosmogenic nuclide exposure ages. We focus on the large number of glacial valleys with moraines corresponding to the Last Glacial Maximum (LGM; 26.5-19.0 ka). Paleo-ELAs are estimated using toe-headwall altitude ratio and the accumulation area ratio determined from published ELAs and existing glacial mapping. Cosmogenic-exposure ages of moraines are compiled from the informal cosmogenic nuclide exposure age database for alpine glacial features (ICE-D Alpine) and represented in a geographic information system along with ELAs for each glacial valley. We produce a reconstructed ELA surface map spanning the southwestern United States using existing algorithms in ArcGIS. Preliminary results show ELAs generally lower than initially estimated and a different pattern in the range of ELAs across the region. ELAs increase going southeast along the Sierra Nevada, which is consistent with previous estimates. ELAs rise eastward across Nevada toward the western shore of the area covered by Lake Bonneville, and then decrease eastward toward the Wasatch Mountains. This pattern is inconsistent with previous estimates and may reflect a west-to-east precipitation pattern that does differs from modern climate. We discuss this pattern and broader features of the ELA surface of the LGM and later episodes of the last Pleistocene glaciation.