10BE-BASED CHRONOLOGIES OF LATE PLEISTOCENE AND HOLOCENE CIRQUE GLACIATION ACROSS WESTERN NORTH AMERICA (Invited Presentation)

Despite progress since the1960s toward developing a better understanding of late Pleistocene and Holocene glaciation, the timing of alpine glaciation in western North America remains an open question. Uncertainties in this glacial chronology largely reflect the various methods that have thus far been used for constraining moraine ages. To address this issue, we present ~150 ¹⁰Be ages on 25 cirque moraines just distal to Little Ice Age moraines from eleven mountain ranges in western North America. Our results indicate glacial retreat from these cirque moraines occurred at approximately 16.0±0.5, 13.5±0.5, and 11.5±0.5 ¹⁰Be ka, with one site dating to 2.5±0.5 ¹⁰Be ka. Many of our sampled moraines were previously interpreted to be mid- to late Holocene, but with the exception of one moraine in western British Columbia, our new ¹⁰Be chronology suggests that all of these moraines are latest Pleistocene, requiring a refined interpretation of Holocene glacial activity in western North America and the associated climate forcing. 

Alpine glaciers are sensitive recorders of climate change and while the majority of our ¹⁰Be moraine ages may coincide with the timing of rapid climate events from the North Atlantic (e.g., Younger Dryas, Bølling-Allerød), others do not, emphasizing the importance of local-to-regional climate variability in contributing to glacier mass balance changes. To address these issues, we are using a hierarchy of numerical models to simulate the primary climatic controls that may have caused fluctuations in western North America alpine glaciers. In particular, we are focusing on isolating and quantifying the controls that may have caused the alpine glacier fluctuations suggested from our new glacial chronologies. Our modeling strategy employs a fully coupled atmosphere-ocean general circulation model (GENMOM), our paleo-version of the RegCM4 regional model, and a distributed energy-mass-balance glacier model.