DO GLACIAL EXTENTS IN YUKON AND EASTERN ALASKA REFLECT EITHER PERVASIVE TECTONIC OR SEA LEVEL INFLUENCE?

Yukon and Alaska contain some of the oldest and richest records of glacial response to Quaternary climate change in North America. In general, Middle and Late Pleistocene glaciations in Yukon were less extensive than Late Pliocene and Early Pleistocene glaciations. A prevailing model for this record is the long-term, pervasive influence of uplift of the Alaska Range and St. Elias Mountains, and the resulting rain shadow effect. This hypothesis stems from the correlation of three limits in central Yukon from the advance of the northern Cordilleran Ice Sheet in the Late Pliocene-Early Pleistocene (pre-Reid), marine oxygen isotope stage (MIS) 6 (Reid) and MIS 2 (McConnell). These limits are successively smaller, supposedly from continued uplift of the surrounding Wrangell, St. Elias, Selwyn and Mackenzie Mountains limiting moisture supply. In Alaska, two broadly correlated glacial extents are constrained to MIS 4 and MIS 2. The commonly cited control on these limits is sea level, with higher oceans in MIS 4 associated with more moisture availability and therefore larger glaciations than in MIS 2. However, recent work using tephrochronology and cosmogenic nuclide dating reveals that the MIS 6, 4 and 2 limits in southwest Yukon and eastern Alaska are tightly spaced, and there is no obvious record of more extensive Late Pliocene-Early Pleistocene glaciations. This suggests that the topographic rain shadow of the St. Elias and Wrangell Mountains restricted glaciations in this area equally throughout the Pleistocene. It also suggests sea level was not a primary factor on glacial limits in this area.