A LATEST PLEISTOCENE AND HOLOCENE CHRONOLOGY OF ALPINE GLACIATION FOR WESTERN NORTH AMERICA

We are developing a cosmogenic ¹⁰Be chronology of late-Pleistocene and Holocene alpine glaciation from sites across the western United States and southwestern Canada in order to address spatial and temporal glacier variability in response to postulated climate forcings. A number of studies have interpreted several Holocene glacial advances in western North America, but age control is based largely on relative dating techniques or limiting radiocarbon ages. Our ¹⁰Be chronology will provide new constraints on late-Pleistocene and Holocene climate change and their impact on the mass balance of western North American alpine glaciers. We present ~80 new ¹⁰Be ages from eight mountain ranges in the western United States and a single site in southwestern Canada. These initial results indicate that the majority of glacial deposits previously mapped as late Holocene are instead late Pleistocene or early Holocene. Our new ¹⁰Be ages imply retreat from moraine positions occurring at roughly 2.5, 9.0, 10.5, 11.5, 12.5, and 14.0 ka.