Developing a Cosmogenic Chronology of Late-Pleistocene Glaciation, Wallowa Mountains, Oregon

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9725554 Clark Glacial record are an important and, in many areas, the only archive of late-Pleistocene climate variability from the western U.S. Alpine glaciers are sensitive to relatively small climatic perturbations and can respond rapidly (on order of 100-101 km within 101-102yr) to relatively small climate change (on order of 1(-2( K). Developing a chronology of alpine glaciation thus represents a significant strategy for extracting paleoclimate information associated with late-Pleistocene climate change. Exposure-ages of large boulders from moraines using cosmogenic nuclides offer great promise for developing such chronologies, but questions remain as to the level of resolution that such an exposure-age chronology can achieve. Our primary objective is to directly date a well-preserved moraine sequence in the Wallowa Mountains, OR using cosmogenic nuclides in order to develop a numerical chronology of mountain glacier fluctuations. In addition, we will be able to address the issue of whether exposure ages of closely spaced; well-preserved moraines having similar weathering characteristics can resolve significant differences (i.e., 5-10 ka) in the timing of moraine formation. Regardless of the resolution issue, these data will contribute geographic coverage to existing glacial records for examining spatial and temporal late-Pleistocene mountain glacier variability. As a further contribution to evaluating an exposure-age chronology based on cosmogenic nuclides, we also propose sampling moraines deposited by the northern Yellowstone outlet glacier. The existing U-series and radiocarbon chronology indicate that the moraines represent 3 major advances of the outlet glacier between 14 and 30 ka. Our Yellowstone data thus will compare the age resolution of a cosmogenic chronology with an established (U-series) chronology.