INVESTIGATING THE GLACIAL HISTORY OF THE NORTHERN SECTOR OF THE CORDILLERAN ICE SHEET WITH COSMOGENIC 10Be CONCENTRATIONS IN QUARTZ

Exposure durations of glacial landforms in widely separated areas of central Yukon Territory affected by the northern sector of the Cordilleran Ice Sheet (CIS) and alpine glaciers have been determined using cosmogenic ¹⁰Be in quartz. The aim of our research is to test previous reconstructions of glacial history and to begin to address the paucity of chronological control for the lateral and vertical extent of the northern CIS. Chronological evidence for CIS expansion predating the Last Glacial Maximum comes from minimum surface exposure durations of c. 100 ka for two bedrock samples within the Reid glacial limit, indicating a possible marine oxygen isotope stage (OIS) 6 age for this event, and from minimum exposure durations of about 40 ka for boulders on moraines constructed by alpine glaciers on a nunatak within the McConnell glacial limit (OIS 2), indicating a possible OIS 4 age. High elevation minimum surface exposure durations within the McConnell limit indicate that some areas formerly mapped as nunataks were covered by cold-based ice prior to 30 ka. Montane glaciation in the Mackenzie Mountains, outside the McConnell glacial limit, was contemporaneous with nearby CIS advance at 17 ka, with CIS retreat by 15 ka. Deglaciation of the Tintina Trench, a major ice discharge route, was completed by 12 ka. At this time ice in an adjacent discharge route to the south was still entering higher-elevation valleys in the Pelly Mountains. A Late Glacial readvance may have peaked at ca. 10 ka in the Ogilvie Mountains. Considerable variation in ages from individual landforms, and possible complex histories, require additional cosmogenic nuclide measurements to confirm interpretations.