The collapse of the Cordilleran-Laurentide ice saddle and early opening of the Mackenzie Valley, Northwest Territories, Canada, constrained by .sup.10Be exposure dating

Deglaciation of the northwestern Laurentide Ice Sheet in the central Mackenzie Valley opened the northern portion of the deglacial Ice-Free Corridor between the Laurentide and Cordilleran ice sheets and a drainage route to the Arctic Ocean. In addition, ice sheet saddle collapse in this section of the Laurentide Ice Sheet has been implicated as a mechanism for delivering substantial freshwater influx into the Arctic Ocean on centennial timescales. However, there is little empirical data to constrain the deglaciation chronology in the central Mackenzie Valley where the northern slopes of the ice saddle were located. Here, we present 30 new .sup.10 Be cosmogenic nuclide exposure dates across six sites, including two elevation transects, which constrain the timing and rate of thinning and retreat of the Laurentide Ice Sheet in the area. Our new .sup.10 Be dates indicate that the initial deglaciation of the eastern summits of the central Mackenzie Mountains began at â¼15.8 ka (17.1-14.6 ka), â¼1000 years earlier than in previous reconstructions. The main phase of ice saddle collapse occurred between â¼14.9 and 13.6 ka, consistent with numerical modelling simulations, placing this event within the Bølling-Allerød interval (14.6-12.9 ka). Our new dates require a revision of ice margin retreat dynamics, with ice retreating more easterly rather than southward along the Mackenzie Valley. In addition, we quantify a total sea level rise contribution from the Cordilleran-Laurentide ice saddle region of â¼11.2 m between 16 and 13 ka.