MARINE OXYGEN ISOTOPE STAGE 4 AND 6 GLACIATIONS AND LATE PLEISTOCENE PALEOENVIRONMENTAL RECONSTRUCTIONS OF THE NORTHERN CORDILLERAN ICE SHEET, YUKON TERRITORY, CANADA

Yukon Territory has been repeatedly affected by the northern Cordilleran Ice Sheet. Although termed an ice sheet, it is better described as an ice complex, comprised of a series of coalescing valley glaciers and piedmont lobes whose ice flow was strongly controlled by topography. 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, reflecting differences in the regional accumulation zones and underlying topography. Despite these complexities, 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. This glaciation, named the Gladstone is the first confirmed MIS 4 glaciation in the Canadian Cordillera. These results are in contrast to the assumed MIS 6 age of the penultimate Reid Glaciation to the east in central Yukon, recently confirmed by the presence of Old Crow tephra (140 ka) along the Pelly River. 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 responses to climatic forcing during glaciations.

The northern Cordilleran Ice Sheet was a precipitation-limited system. We propose that variations in regional precipitation are a possible cause for the differences between MIS 4 and 6 glacial advances. The style of moisture delivery over the St. Elias Mountains could be controlled by the extent of the Laurentide Ice Sheet and broad variations in position and intensity of the Aleutian low. Although a difficult hypothesis to test, sections at the penultimate limit along White River in southwestern Yukon yielding paleoenvironmental data from MIS 5 and MIS 3 deposits will allow some paleoclimatic conditions to be inferred.