10BE DATING THE LAST DEGLACIATION OF THE CLEAR CREEK VALLEY, SAWATCH RANGE, CO

During the Last Glacial Maximum (LGM; ~26-19 ka), mountain glaciers and ice sheets reached their maximum extents. The ending of the LGM is marked by a significant deglaciation event, but the timing and pace of deglaciation is highly variable. Recently, the Colorado Rocky Mountains have been a focus for the development of deglaciation chronologies due to the abundance of well-preserved moraine boulders and glacially-eroded bedrock outcrops that are suitable for cosmogenic ¹⁰Be surface-exposure dating. Previous studies of deglaciation in this region have proposed that deglaciation occurred synchronously between ~16 and 13 ka in several adjacent valleys in the Sawatch Range, Colorado. However, this interpretation is based on a transect of ¹⁰Be ages from bedrock in a single valley. Here, we present new cosmogenic ¹⁰Be surface-exposure ages from a transect of glacially-sculpted bedrock samples located up-valley of LGM moraines in Clear Creek valley. ¹⁰Be ages from five bedrock sites spanning 16 km of the valley floor date to 15.9±0.2 ka, 15.6±0.2 ka, 15.5±0.2 ka, 15.3±0.3 ka, 15.1±0.2 ka, from down- to up-valley. Our results indicate that deglaciation occurred between ~16-15 ka, which is in agreement with previously published ¹⁰Be ages of valley-bottom bedrock in the adjacent Lake Creek valley. Our new ¹⁰Be chronology supports that deglaciation occurred synchronously within adjacent valley systems in this region of the CO Rocky Mountains. The causes for rapid deglaciation during this interval, which occurs prior to the Bølling-Allerød warming, may relate to an increase in global carbon dioxide concentrations, although alteration of atmospheric patterns related to the retreat of the Laurentide Ice Sheet is another factor.