CLIMATE CHANGE DURING DEGLACIATION OF THE OVERLAND CREEK VALLEY, RUBY MOUNTAINS, NEVADA, U.S.A

The Ruby Mountains of northeastern Nevada hosted numerous small valley glaciers during the last Pleistocene glaciation, some of which occupied the headwaters of Pluvial Lake Franklin. Together, the record of mountain glaciation and changes in the extent of Lake Franklin provide useful clues to the timing and magnitude of climate change in the northern Great Basin. The largest east-flowing glacier in the Ruby Mountains occupied Overland Creek Valley; here, the pattern of ice retreat at the end of the last glaciation is recorded by a succession of terminal and recessional moraines as well as glacially polished bedrock. Cosmogenic ¹⁰Be exposure dating was applied to the moraines and glacially polished bedrock to limit the time of ice retreat. Ice retreat from a terminal moraine commenced at ca. 20.8 ± 1.3 ka, and paused or readvanced to a recessional moraine at 16.9 ± 0.8 ka, near the time of the Lake Franklin highstand. Ice uncovered Overland Lake and retreated to a small cirque at the head of the valley by 12.8 ± 0.7 ka. Using the known ages and positions of recessional moraines, a 2-D numerical model of glacier mass balance and ice flow was applied to the valley in order to limit a range of possible temperature and precipitation changes accompanying deglaciation. Model results indicate that the shift in ice margin position between ca. 20.8-16.9 ka, which coincided with the expansion of Lake Franklin to its highstand, was caused by a rise in temperature of >1.4°C. Further ice retreat between 16.9-12.8 ka was accompanied by warming of at least 3.8°C. These results are discussed in the context of existing and new limits on Latest Pleistocene climate change in the northern Great Basin.