COSMOGENIC ¹⁰BE CHRONOLOGY OF THE WALLOWA ICECAP, OREGON, AND VARIABLE RESPONSES OF WESTERN USA GLACIERS DURING THE LAST DEGLACIATION

Cosmogenic ¹⁰Be exposure ages from an exceptionally well-preserved moraine sequence in the Wallowa Mountains, northeastern Oregon, identify two maximal late Pleistocene glaciations at 21.1 ± 0.4 ka and 17.0 ± 0.3 ka, and a minor glacial event at 10.2 ± 0.6 ka. Our new high-resolution ¹⁰Be chronology of the Wallowa icecap fills an important gap between well-dated glacier records to the west (Sierra Nevada and Olympic Mountains, Puget Lowlands) and east (Rocky Mountains), thereby providing adequate coverage for identifying synoptic responses to the large climate changes during the last deglaciation. The Wallowa record, integrated within the broader context of other western U.S. glacial records, demonstrates substantial differences in the spatial pattern of western U.S. glacier responses to climate forcing associated with the last glacial maximum (LGM) and subsequent millennial-scale events originating in the North Atlantic region. These variable responses of alpine glaciers identify large changes in the relative contributions of regional-to-global controls on the climate of the western U.S. that accompanied the deglaciation.