Distal Cirque Contribution to the Northern Cordilleran Ice Sheet, Yukon, Canada

Yukon has been repeatedly affected by the northern Cordilleran Ice Sheet (NCIS). Although termed an ice sheet, it is better described as an ice complex, with quasi-independent lobes originating from mountainous areas around the border of Yukon. This ice complex produced irregular, digitate horseshoe-shaped glacial limits largely on the plateau area of central Yukon, at the eastern edge of unglaciated Beringia. These limits have broadly followed a pattern of progressively diminished extent. It is likely that variations in precipitation across the source areas of these lobes affected their extent and timing during various glacial cycles. The growth model of the NCIS is contingent on ice from numerous cirques and ice fields in the source areas eventually amalgamating into these large, coalescent ice lobes. What is unclear is the contribution of cirque and valley glaciers from the few mountainous areas near the limits of glaciation. This research describes the contribution of cirques and valley glaciers in two areas at the glacial limits from MIS 6-2.Central Ruby Range is in southwest Yukon and was affected by the Saint Elias lobe. It encompasses the limits of MIS 2, 4 and 6 glaciations. Stratigraphic analysis paired with 10Be surface exposure dating indicates extensive local ice production from cirques and plateau surfaces during MIS 2. During early MIS 2, local valley glaciers advance to the edge of the range but had retreated before inundation by the St. Elias lobe, likely due to local precipitation reduction. These alpine ice centres were responsive to deglacial climatic fluctuations and hosted significant re-advances during the Older Dryas, despite their location in the rain shadow of the St. Elias Mountains and during rapid retreat of the St. Elias Lobe. The MIS 4 limit is slightly more extensive than the MIS 6 limit here, likely because local ice contributed to this portion of the St. Elias Lobe. The record and limit of the MIS 6 glaciation is poorly constrained here but 150 km to the NW, MIS 6 is 4 km more extensive than 4.Granite Creek is in the Gustavus Range in central Yukon at the MIS 2 limit of the Selwyn lobe. During MIS 2 a tongue of the Selwyn lobe occupied the lower portion of Granite Creek, forming a lake. Cirque glaciers near the margin were overrun by the Selwyn lobe. Cirque glaciers terminating in the lake advanced due to floating ice margins, but these maximum limits are not reflected in the geomorphic record; their well-defined moraines are recessional from this maximum. Stratigraphic studies indicate extensive MIS 4 cirque glaciation but no evidence of a proximal Selwyn lobe. During MIS 6, cirque glaciers were extensive early enough that the Selwyn lobe did not inundate local cirque valleys even though the entire area was overrun.This research indicates peripheral ice accumulation could contribute to the NCIS. However, variations in precipitation imply that peripheral ice sources were largely out of sync with local ice sources.

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