CLIMATE CHANGE DURING DEGLACIATION INFERRED FROM NUMERICAL GLACIER MODELING IN THE RUBY MOUNTAINS, NEVADA

During the last Pleistocene glaciation, glaciers were abundant in the Ruby Mountains of northeastern Nevada. East-flowing glaciers in these mountains were upstream of Paleolake Franklin, which has a known history of water level fluctuation during the time of mountain ice retreat. This setting presents an opportunity to limit the rate and magnitude of climate change during the Last Glacial Maximum and the subsequent interval of glacier retreat. In Overland Creek valley, cosmogenic ¹⁰Be exposure dating of glacial deposits and landforms limits the timing and pace of glacier retreat to a terminal age of ca. 20 ka, a pause or readvance at 17 ka coinciding with the highstand of Lake Franklin, and steady retreat thereafter until ca. 13 ka while Lake Franklin oscillated below its highstand and eventually vanished. To limit temperature and precipitation changes during the interval of ice retreat, we applied a widely used method of numerical mass balance and ice flow modeling to Overland Creek valley. The model computes surface mass and energy balance of an elevation grid and uses it as a distributed source term for an ice physics model to simulate glacier extent and thickness. Model solutions provide a range of estimates of temperature and precipitation changes relative to modern. Although precipitation changes through the interval of ice retreat are poorly known, model solutions can be used to infer the magnitude of warming from 20-13 ka. Results indicate that, if precipitation during the last glaciation was similar to modern, then temperature depression during the maximum ice advance at 20 ka was 9.2°C. The pause in ice retreat or readvance at 17 ka to 77% of the maximum glacier length, coinciding with a highstand of Lake Franklin, was likely accompanied by a similar temperature and precipitation combination as at 20 ka. Retreat between 17 ka and 15 ka resulted in 25% of the maximum glacier length and was accompanied by a temperature rise of 1.1°C. Retreat between 15 ka and 13 ka resulted in 3% of the maximum glacier length and was accompanied by a warming of 0.6°C. A small amount of warming between 20-13 ka is consistent with the apparent persistence of Lake Franklin and other pluvial lakes during the last deglaciation and suggests a slow shift from a glacial to interglacial climate.