SPATIO-TEMPORAL PATTERNS OF LATE-PLEISTOCENE MOUNTAIN GLACIATION IN THE WESTERN UNITED STATES

Patterns of Late Pleistocene mountain glaciation in the western United States are evident from sedimentary, geomorphic, and chronological data, and can be used to infer past climate change. Cosmogenic nuclide exposure ages are the most abundant numeric age limits on mountain glacial deposits in the region, affording an opportunity for temporal analysis of Pleistocene glaciation. Reconstructed glacier shapes, thicknesses, hypsometry, and equilibrium-line altitudes (ELAs) inferred from glacial geomorphology are also abundant and permit an analysis of spatial patterns of the last glaciation. This study takes advantage of cosmogenic nuclide exposure ages available from the ICE-D Alpine database and a new compilation of reconstructed paleoglaciers to evaluate the region-wide temporal pattern of the last glaciation in the western U.S. Signals of Late Pleistocene moraine deposition are abundant during the intervals of MIS 4, 3, and 2, with the greatest number of dated moraines deposited during the global Last Glacial Maximum (26.5-19.0 ka). Within the conterminous western U.S., mountain glacier ELAs during this interval generally increased from northwest-southeast, with minor variations likely related to accumulation-season precipitation. Moraines deposited later during the MIS 2 interval are also abundant, especially during the interval 19-16 ka. Glacier ELAs in many mountain ranges remained low during this interval, suggesting persistent cooling and/or increased accumulation-season precipitation after the Last Glacial Maximum. Moraines deposited after 16 ka are less abundant and in most mountains are associated with significantly diminished glacier lengths and higher ELAs, which is consistent with other regional proxy data indicating warming commenced by this time. Although much is known about the last Pleistocene maximum ice extent in most glaciated mountains, the temporal patterns observed here likely reflect a lack of chronological data for recessional moraines of the last glaciation. Developing this piece of the Pleistocene record can help to identify regional-scale changes in climate during the last glacial-interglacial transition.