MOUNTAIN GLACIER ACTIVITY IN THE WESTERN CORDILLERA OF THE UNITED STATES DURING THE PAST TWO GLACIAL MAXIMA

Accurate knowledge of the timing and magnitude of mountain snowline depression during glacial maxima and subsequent terminations at boreal middle latitudes can aid in deciphering the relative roles of orbital insolation, ice-sheet feedbacks, atmospheric circulation, and greenhouse gases in driving Northern Hemisphere ice ages. Here, we present reconstructions of mountain glacier activity in the Rocky Mountains of Wyoming and the Sierra Nevada of California. Our reconstructions are based on glacial geomorphologic maps, glaciological modeling, and ¹⁰Be surface-exposure chronologies targeting exceptionally well-preserved moraine sets deposited at the culminations of the past two ice ages. Our field sites straddle the Great Basin, which harbored extensive pluvial lakes during the Last Glacial Maximum (LGM) and Heinrich Stadial 1. More than 180 ¹⁰Be dates indicate coeval signatures of mountain glacier activity during the LGM and the last deglaciation in the Wind River Range and Sierra Nevada. These western cordilleran mountain glacier systems achieved maximum configurations concomitant with northern ice sheets. However, during the last deglaciation, snowline rise and mountain glacier recession from LGM to Holocene values led the disappearance of northern ice sheets by several millennia. Altogether, these results support a continent-wide climatic driver of glaciation in the western cordillera of the United States at the culmination of the last two ice ages, and provide insight into controls on ice age climate in western North America.