COSMOGENIC 10BE EXPOSURE AGE LIMITS OF PLEISTOCENE MORAINES IN GREAT BASIN NATIONAL PARK, NEVADA, USA

Much of the North American landscape has been shaped by environments of the last Pleistocene glaciation. The Great Basin region of southwestern North America was occupied by mountain glaciers and vast pluvial lakes during this time interval. Chronology of pluvial lake highstands in the Great Basin are largely known from radiocarbon dating, but chronologies of mountain glaciation are still being developed throughout the region and can help to bridge a data gap between the Sierra Nevada and Rocky Mountain ranges and provide a clearer context for inferring Late Pleistocene climate change. Recent cosmogenic ¹⁰Be exposure dating efforts of moraines in the Ruby, Santa Rosa, and Pine Forest Ranges have aimed to develop mountain glacier chronologies within the Great Basin region. These studies identify two distinct populations of moraine exposure ages: terminal moraines (approximately 21-20 ka) coinciding with the latter portion of the Last Glacial Maximum (LGM), and down-valley recessional moraines (approximately 18-15 ka) coinciding with the Heinrich Stadial 1 event. The South Snake range of eastern Nevada, which is home to Great Basin National Park, is centrally located in the Great Basin and was occupied by valley glaciers near the western edge of Lake Bonneville (the largest pluvial lake in the region). The largest glaciers in this range were located in the Lehman Creek and Baker Creek valleys. Samples were collected from terminal and recessional moraine complexes in the Baker Creek valley to limit the timing of the local glacier maximum and an apparent pause in ice retreat. The mean exposure age of the terminal moraine is 21.0 ± 0.7 ka, which is in agreement with terminal moraine ages from elsewhere in the Great Basin and corresponds to the latter part of the LGM. The recessional moraine, representing a ~50% decrease in glacier length, has a mean exposure age of 14.0 ± 1.0 ka. This exposure age is 1-4 kyr younger than other recessional moraines elsewhere in the Great Basin. The exposure age of the recessional moraine suggests that valley glaciers persisted in the mountains after Lake Bonneville began fall from its threshold and after the onset of hemisphere-scale warming that followed Heinrich Stadial 1.