COSMOGENIC 10BE AGE LIMITS ON THE LAST TWO PLEISTOCENE GLACIATIONS IN THE NORTHEASTERN GREAT BASIN, NEVADA AND UTAH, U.S.A

During Pleistocene glaciations, the Great Basin of the western United States hosted numerous large paleolakes and small mountain glaciers in several ranges. Despite excellent preservation of surficial deposits representing both lakes and glaciers, records of paleolakes, chiefly Lakes Bonneville and Lahontan, are more precisely documented and better dated compared to those of mountain glaciers. As a result, inferences of climate changes in this region during Pleistocene glaciations have been based almost exclusively on paleolake records. We have developed a chronology of glacial deposits in several mountain ranges of the northeastern Great Basin based on cosmogenic ¹⁰Be exposure dating of moraines, thereby establishing an important framework for using glacial records as proxies of climate change. In the Great Basin, the last two Pleistocene glaciations are known as the Lamoille (penultimate) and Angel Lake (last) Glaciations, referring to type localities in the Ruby and East Humboldt Mountains, respectively. The type-Lamoille moraine is a broad area of hummocky topography at the mouth of the Lamoille Canyon, which was occupied by one of the largest Pleistocene mountain glaciers in the northeastern Great Basin. Cosmogenic ¹⁰Be exposure ages indicate that this moraine was deposited during marine oxygen-isotope stage six, similar to moraines representing the penultimate glaciation elsewhere in the western U.S. Interestingly, the type-Lamoille moraine was deposited by a glacier that was seven km longer (about 13% larger) than the glacier that formed in the same canyon during the Angel Lake Glaciation, suggesting that the penultimate glaciation featured a colder and/or wetter climate compared to the last glaciation. The Angel Lake Glaciation culminated during marine oxygen-isotope stage two, near the time of the Last Glacial Maximum. Although some mountain glaciers occupied terminal moraines prior to the highstands of paleolakes in the Great Basin, the chronology reported here suggests that glaciers upwind and downwind of Lake Bonneville were at or near their maximum extent during lake highstands. This observation supports previous suggestions that, near the end of the last glaciation, glaciers in this region responded to the same climate changes that drove paleolakes to their maximum extent.