EVIDENCE FOR A 55 - 50 KA (EARLY WISCONSIN) GLACIATION OF THE CORDILLERAN ICE SHEET, YUKON TERRITORY, CANADA

Cosmogenic 10Be ages on boulders of 54-51 ka (n=4) on a penultimate Cordilleran Ice Sheet (CIS) 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 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 CIS indicating different responses to climatic forcing during glaciations.

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. When the Aleutian low is well developed and/or more easterly, meridional flow occurs. This results in strengthened moisture flux from the Pacific, and an increase in the size of the rain shadow. When the Aleutian low is weaker and/or located further west, atmospheric flow is more zonal, with flow westward over the St. Elias and Coast mountains. There is a reduced moisture flux, but the orographic affect is reduced as is the size and magnitude of the rain shadow. Zonal conditions were used to explain periods during the Holocene when O-isotopes indicated more effective precipitation in the present rain shadow area. A similar situation could have prevailed during MIS 4, allowing relatively extensive glacier growth on the leeward side of the St. Elias and Coast mountains. MIS 6 may have had more meridional flow, causing a well-developed rain shadow but still allowing subsequent growth of glaciers in the Selwyn Mountains.