WHAT HAS SURFACE EXPOSURE DATING TOLD US ABOUT GLACIER FLUCTUATIONS IN WESTERN CANADA? (Invited Presentation)

Alpine glacier fluctuations have been studied for over 60 years in the Canadian Cordillera. Workers have traditionally used radiocarbon dating, tephrochronology, and lake sediments impounded by or downvalley from glaciers to constrain times of ice expansion. In many cases these methods are neither direct (e.g. lake sediments) nor provide closely-limiting ages for a glacier advance. In addition, most moraines and glacier forefields lack organic material for radiocarbon dating, and when it is present, young deposits commonly yield radiocarbon ages that return non-unique calendar ages. Surface exposure dating provides an opportunity to constrain times when surfaces emerged from an ice sheet or to directly date landforms left by alpine glacier advances. Our combined work in British Columbia, Yukon and Northwest Territories over the past decade has allowed us to collect over one hundred and thirty ¹⁰Be ages from latest Pleistocene and Holocene moraines and surfaces previously covered by the Cordilleran Ice Sheet (CIS). Collectively, our data bolster the existing glacier chronology but our data also provide new information including: deglaciation by 18.1 ka along the southwest margin of the CIS, differences in the maxima of the CIS along its margins, and deglaciation of extensive alpine areas prior to 14 ka. We clearly show dozens of alpine glaciers throughout western Canada advanced during the older and younger Dryas. Our data supports our hypothesis that all alpine moraines in Yukon and Northwest Territories formed during the Pleistocene or during the Little Ice Age. Despite the advantages afforded by exposure dating, several challenges remain: moraine degradation limits ages on moraines older than the Last Glacier Maximum, and improved analytical precision will be required to differentiate these latest Holocene moraines at sub-century scales.