GLACIAL HISTORY, SURFACE EXPOSURE AGES, AND PALEOELA’S OF THE YUKON TANANA UPLAND, EASTERN ALASKA

Cosmogenic surface-exposure dating and spatial analysis of Equilibrium Line Altitudes (ELA’s) clarify the Pleistocene glacier and climate history of the Yukon Tanana Upland (YTU). Four granodiorite boulders collected from each of four terminal moraines in the type locality yeilded sixteen 10Be and seven 26Al exposure ages, corrected for latitude, elevation, shielding, snow cover, and erosion. The older moraines exhibit some scatter, as expected with degradation. The Mt. Harper moraine, including a boulder age of 190 ka, is OIS 6 or older (Middle Pleistocene or older); the Eagle moraine is dated approximately 80-50 ka (Early Wisconsin, s.l., OIS 4 or 5; younger than some regional correlations suggest). Boulders from the Salcha moraine are 23 +/- 7 ka (mean +/- s.d.), confirming a Late Wisconsin age. And the Ramshorn moraine is dated 24 +/- 7 ka (late-glacial, older than previously believed). Glacier extents were mapped and digitized into a GIS. Using an AAR of 0.6, former ELA’s average 1250 +/- 110 m, 1290 +/- 110 m, and 1400 +/- 60 m for the Eagle, Salcha, and Ramshorn glaciations. Small changes in ELA created dramatic changes in glacier extent. Eagle and Salcha ELA’s are roughly 500 m below extrapolated modern positions, reflecting cold but dry conditions. ELA surfaces depict a slight shift from SW to W moisture sources, decreasing local variability, and steeper gradients (more maritime) through time. In sum, the first direct ages for YTU moraines add to a debate on the timing, spatial variability, and paleoclimate forcing of glaciations across Alaska.