Evidence for a 55–50 ka (early Wisconsin) glaciation of the Cordilleran ice sheet, Yukon Territory, Canada

Abstract

Cosmogenic 10Be ages on boulders of 54–51 ka (n=4) on a penultimate Cordilleran ice sheet (CIS) drift confirm that Marine Oxygen Isotope Stage (MIS) 4 (early Wisconsin) glaciation was extensive in parts of Yukon Territory, the first confirmed evidence in the Canadian Cordillera. We name the glaciation inferred from the mapped and dated drift the Gladstone. These results are in apparent contrast to the MIS 6 (Illinoian) age of the penultimate Reid glaciation to the east in central Yukon but are equivalent to exposure ages on MIS 4 drift in Alaska. Contrasting penultimate ice extents in Yukon requires that different source areas of the northern CIS in Yukon responded differently to climatic forcing during glaciations. The variation in glacier extent for different source areas likely relates to variation in precipitation during glaciation, as the northern CIS was a precipitation-limited system. Causes for a variation in precipitation remain unclear but likely involve the style of precipitation delivery over the St. Elias Mountains possibly related to variations in the Aleutian low.

Investigating the glacial history of the northern sector of the Cordilleran Ice Sheet with cosmogenic 10Be concentrations in quartz

“…However, a recent reevaluation of the age of Sheep Creek and Old Crow tephras (Ward et al, 2008;Westgate et al, 2008) now indicates that the Reid glaciation in central Yukon probably dates to OIS 6. Cosmogenic nuclide concentrations in erratic boulders in the Aishihik Lake area of southwest Yukon, however, indicate an OIS 4 (Early Wisconsin) age for the penultimate glaciation in that area (Ward et al, 2007). The McConnell glaciation is firmly dated to OIS 2 (Late Wisconsin).…”

“…The paucity of absolute age control for Yukon glacial events and the disparity in age between what had previously been thought to be the same penultimate glacial surface point to the need for critical re-mapping combined with robust chronologic control, not just for areas affected by different ice discharge routes of the CIS, but also within the same ice discharge route (Ward et al, 2007). Here we present 43 new cosmogenic 10 Be surface exposure durations of samples from several areas within central Yukon Territory and western Northwest Territories (Fig.…”

“…If, for example, Reid ice removed all inherited cosmogenic nuclide concentrations and the sampled bedrock became exposed at the OIS 6 termination (~130 ka), the nuclide concentration in the two summit samples record steady-state bedrock erosion rates of 2.3±0.2 mm.ka -1 (YK46) and 3.4±0.3 mm.ka -1 (YK45) since exposure. This hypothesis indicates that the CIS during the OIS 4 Gladstone glaciation (Ward et al, 2007) did not reach this site, or did not erode sufficient bedrock to remove inherited cosmogenic nuclides.…”

“…The exposure duration range from the outer Lyon Creek moraine is consistent with the results from the Mountain River moraine in the Mackenzie Mountains, which has a minimum age of 32 ka (Duk-Rodkin et al, 1996). Both moraines may date to the early Wisconsin (55-50 ka) Gladstone glaciation of Ward et al (2007).…”

“…For the case where there is inheritance in bedrock samples, measurement of a second cosmogenic nuclide with a different half-life in the same mineral phase may provide further information about potential complex histories. (Modified from Duk-Rodkin et al, 1996;Duk-Rodkin, 1999;Ward et al, 2007. ) YK1,2,5,7,23,24,25) and Tintina Trench (YK8, 17, 18, 19) samples (Fig.…”