NORTHERN CORDILLERAN ICE SHEET DISPLAYS A DIACHRONOUS RESPONSE TO MIS 4 FORCING, YUKON TERRITORY,CANADA

Yukon Territory has been repeatedly affected by the northern Cordilleran Ice Sheet (CIS). This complex of coalescing ice lobes sourced from separate mountain ranges, produced three broad, mappable, irregular, digitate, horseshoe-shaped glacial limits on the plateau area of Yukon thought to represent regionally coherent advances of the CIS. The intermediate mapped limits were thought to represent a single glaciation, but our results show in central Yukon it is MIS 6 while in SW Yukon it consists of 4 and 6.

Central Yukon was affected by west-flowing ice from the continental divide. Old Crow tephra (124 ka) above penultimate glaciation deglacial outwash along the Pelly River confirm a MIS 6 age. There is no evidence for a MIS 4 glaciation that is more extensive than MIS 2.

SW Yukon was affected by north and eastward flowing ice from the Coast and St. Elias mountains, respectively. Near where the Coast Mountains and St. Elias ice coalesced, ¹⁰Be ages on boulders of 54-51 ka (n=4) on penultimate drift indicate this limit is MIS 4. This is the first confirmed MIS 4 glaciation in the Canadian Cordillera. Recent mapping in the area suggests that the MIS 4 advance was comparable in extent to the MIS 6 advance. This mapping also indicates that ice caps and cirques may have contributed substantially to the local extent of late Pleistocene limits and could account for the local all-time limit corresponding to MIS 4 advance.

Further west the chronology provided by tephras from sections exposed near the all-time limit along the White River, confirms that the MIS 4 limit is nearly identical to that of the MIS 6 glaciation.

Thus, there is a dichotomy between the extents of MIS 4 glaciation from different source areas for the northern CIS, indicating different glaciological responses to this climatic forcing. The northern CIS was a precipitation-limited system. We propose that variation in regional precipitation, specifically how moisture penetrates the coastal mountains, is a possible cause for the observed differences between glacial advances in different source areas. Causes for this regional variation in precipitation will be discussed.