COSMOGENIC 3He EXPOSURE AGE DATING ATTEMPTS OF ROCK GLACIER BOULDERS, FISH LAKE PLATEAU, UTAH

Rock glaciers are found at high elevations in many mountain ranges in the Western US and are important water reservoirs and overlooked paleoclimatic proxies. The Fish Lake Plateau in south central Utah was repeatedly glaciated during the Pleistocene ice ages and has many relict to active rock glaciers in formerly glaciated cirques and valleys. Previously published 3He exposure ages on prominent terminal moraines around the Fish Lake Plateau range from 19-21 ka, and recessional moraines in select locations were deposited between 15 to 17 ka. Thus, the only age constraint on these rock glaciers is that they are post late glacial, or post ~15 ka. To determine the ages of these features we sampled multiple boulders for cosmogenic 3He exposure age dating from rock glaciers at Pelican Canyon, Tasha Canyon, Sevenmile cirques and Hilgard Mtn on the Fish Lake Plateau. The samples are currently being processed and are slated for mass spectrometry in early 2025. The andesite boulders we sampled for 3He exposure age dating require a correction for non-cosmogenic 3He that becomes more pronounced, and increases the internal age uncertainty (up to 14% added to the propagated analytical or ‘internal’ uncertainty), as the exposure duration decreases. So, the resulting ages will have increasing uncertainties if the exposure ages are close to modern, and much more typical uncertainties if they formed immediately after Heinrich Stadial 1 or the Younger Dryas. Since these landforms were likely actively moving downslope at some point, boulders on their surfaces may have shifted, thus the spread of exposure ages even from an individual ridge could be high. This geomorphic uncertainty will likely override the analytical uncertainty, even when considering younger exposure ages. Further study of rock glaciers will add to a growing database of similar features in Utah and the North American Cordillera and provide more data on the paleoclimate of the post glacial period through Holocene.