Late Pleistocene glacial chronologies and paleoclimate in the northern Rocky Mountains

Abstract

The geologic record of mountain glaciations is a robust indicator of terrestrial paleoclimate change. During the last glaciation, mountain ranges across the western US hosted glaciers while the Cordilleran and Laurentide ice sheets flowed to the west and east of the continental divide, respectively. Records detailing the chronologies and paleoclimate significance of these ice advances have been developed for many sites across North America. However, relatively few glacial records have been developed for mountain glaciers in the northern Rocky Mountains near former ice sheet margins. Here, we report cosmogenic beryllium-10 surface exposure ages and numerical glacier modeling results, which show that mountain glaciers in the northern Rockies abandoned terminal moraines after the end of the global Last Glacial Maximum around 17–18 ka and could have been sustained by −10 to −8.5 ∘C temperature depressions relative to modern assuming similar or less than modern precipitation. Additionally, we present a deglacial chronology from the northern Rocky Mountains that indicates while there is considerable variability in initial moraine abandonment ages across the Rocky Mountains, the pace of subsequent ice retreat through the late glacial exhibits some regional coherence. Our results provide insight on potential regional mechanisms driving the initiation of and sustained deglaciation in the western US, including rising atmospheric CO2 and ice sheet collapse.