GLACIAL LIMITS AND PALEOENVIRONMENTAL RECONSTRUCTIONS OF THE NORTHERN CORDILLERA OVER THE LAST 200 KA, YUKON TERRITORY, CANADA
Yukon Territory has been repeatedly affected by the northern Cordilleran Ice Sheet. This ice complex was comprised of separate, topographically controlled ice lobes that differed with regional accumulation and underlying topography. Despite the complexities of these lobes, three broad, mappable chrono-geomorphic limits were defined, representing regionally coherent advances. However, new data indicates dichotomy between presumably synchronous glacial limits.
Four cosmogenic 10Be ages (54-51 ka) from boulders on penultimate drift in southern Yukon confirm that Marine Oxygen Isotope Stage (MIS) 4 glaciation was extensive. This glaciation, named the Gladstone, is the first confirmed MIS 4 glaciation in the Canadian Cordillera. In southwest Yukon, the maximum and pentultimate limits are separated by less than 3 km. MIS 5 aged Woodchopper Creek tephra and the informally named Donjek tephra constrain the penultimate and maximum limits to MIS 4 and MIS 6, respectively. Between these till sheets are pollen and plant and insect macrofossils that indicate boreal forest and birch-shurb tundra environments existed during MIS 5 substages.
These dated limits are in contrast to the absence of MIS 4, and the MIS 6 age of the penultimate Reid Glaciation to the east in central Yukon. This age was recently confirmed by the presence of Old Crow tephra (124 ka) along the Pelly River. Beyond the MIS 6 extent is an older, composite pre-Reid limit, likely representing several glaciations. This limit is missing in southwest Yukon.
This dichotomy between Pleistocene glacial extents for three lobes of the northern Cordilleran Ice Sheet suggests different responses to climatic forcing during glaciations. This ice sheet was a precipitation-limited system, with variations in regional precipitation causing differences between MIS 2, 4, 6 and older glacial advances. The style of moisture delivery over the St. Elias Mountains could have been controlled by the extent of the Laurentide Ice Sheet, changes in sea level, sea ice volumes and broad variations in position and intensity of the Aleutian low.