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

Yukon Territory 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 the Yukon. This ice complex produced irregular, digitate 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 variable 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 distal mountainous areas near the limits of glaciation. This research describes the contribution of cirque and valley glaciers in two areas at or near the limit of glaciation from MIS 6-2.

 

Ruby Range in southwest Yukon 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 up valley during 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 during rapid retreat of the St. Elias Lobe despite their location in the rain shadow of the St. Elias Mountains. The MIS 4 limit is slightly more extensive than the MIS 6 limit, likely because local ice growth contributed significantly 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 Gustavus Range in central Yukon at the MIS 2 limit of the Selwyn lobe. It was completely overrun during MIS 6, but cirque glaciers were extensive early enough that the Selwyn lobe did not inundate local cirque valleys. Stratigraphic studies indicate extensive MIS 4 cirque glaciation but provide no evidence of a proximal Selwyn lobe. During MIS 2, cirque glaciers near the margin were partially overrun by the Selwyn lobe. A tongue of the Selwyn lobe blocked Granite Creek forming a lake, and cirque glaciers terminating in the lake advanced due to floating ice margins. These limits are not reflected in the geomorphic record; well-defined MIS 2 moraines are recessional from this maximum.

 

This research indicates peripheral ice accumulation could contribute to the NCIS. However, stratigraphic studies indicate that peripheral ice sources in many cases were asynchronous with advances from primary source areas, likely due to variations in precipitation caused by the expansion of the CIS.