COMPARISON OF MIS 4 AND 6 GLACIATIONS YUKON TERRITORY, CANADA

Yukon Territory has been repeatedly affected by the northern Cordilleran Ice Sheet. This ice complex produced irregular, digitate, horseshoe-shaped glacial limits on the plateau area of central Yukon. Many of these ice flows had separate source areas and dynamics but three broad mappable chrono-geomorphic regions have been defined that were thought to represent regionally coherent advances of the Cordilleran Ice Sheet. However, the penultimate “Reid” glaciation actually represents two separate glaciations.

Cosmogenic ¹⁰Be ages on boulders of 54-51 ka (n=4) on penultimate drift in western Yukon Territory confirm that Marine Oxygen Isotope Stage (MIS) 4 (early Wisconsinan) glaciation was extensive in parts of Yukon Territory. We name this glaciation the Gladstone, the first confirmed MIS 4 glaciation in the Canadian Cordillera. These results are in contrast to the MIS 6 age of the penultimate Reid Glaciation to the east in central Yukon, recently confirmed by the presence of Old Crow tephra (124 ka) along the Pelly River. There is no evidence for a MIS 4 glaciation that is more extensive than MIS 2 in central Yukon. Thus, there is a dichotomy between MIS 4 and 6 glacial extents for at least two of the source areas for the northern portion of the Cordilleran Ice Sheet, indicating different glaciological responses to climatic forcing during glaciations. The situation is more complex as sections exposed at the penultimate limit along White River in SW Yukon indicate that the MIS 4 Gladstone has an almost identical extent to an older, likely MIS 6 glaciation.

The northern CIS was a precipitation-limited system and we propose that variation in regional precipitation, specifically how moisture penetrates the St. Elias and Coast mountains, is a possible cause of the differences between glacial advances. Causes for regional variation in precipitation remain unclear. However, it likely involves the style of delivery over the St. Elias Mountains as controlled by the extent of the Laurentide Ice Sheet and broad variations in position and intensity of the Aleutian low.