ORIGIN OF LAST-GLACIAL LOESS IN THE WESTERN YUKON-TANANA UPLAND, CENTRAL ALASKA, USA

Loess is widespread over Alaska and its accumulation has traditionally been associated with glacial periods. Surprisingly, however, loess deposits confidently dated to the last glacial period are sparse in Alaska. We report here a rare occurrence of loess deposits dating to the last glacial period, ~19 ka to ~12 ka, anchored by 50 radiocarbon ages from a dozen sites, in the western part of the Yukon-Tanana Upland. Loess in this area is very coarse grained, with particle size decreasing southward from the Yukon River, implying that this drainage basin was the source. Geochemical data (K/Ba vs. K/Rb; Cr/Sc vs. Th/Ta; As/Sb vs. Zr/Hf; and Eu/Eu* vs. LaN/YbN) show, however, that Yukon River silts were not the only source, as the Tanana River to the south is also a likely source of loess. The occurrence of last-glacial loess with sources to both the south and north is explained by both regional, synoptic-scale winds from the northeast, and opposing katabatic winds that could have developed from expanded glaciers in both the Brooks Range to the north and the Alaska Range to the south. Based on a comparison with recent climate modeling for the last glacial period, seasonality of dust transport may also have played a role in contributing loess from two distinct sources to this area.