A COSMOGENIC 10BE CHRONOLOGY OF THE LAST PLEISTOCENE GLACIATION IN THE NORTHERNMOST U.S. ROCKY MOUNTAINS AND INFERENCES OF CLIMATE CHANGE

The northern sector of the U.S. Rocky Mountains hosted numerous mountain glacier systems during the last Pleistocene glaciation. Although many glaciers in the northernmost ranges of this region interacted with southern lobes of the North American ice sheets, some formed discrete valley and piedmont glaciers. The records of these glaciers provide an opportunity to understand climate change during the culmination of the last glaciation and subsequent deglaciation, especially in places where the timing of mountain glaciation can be precisely limited. In the Lewis Range of northwestern Montana, a discrete glacier occupied the east flowing Cut Bank Creek valley and deposited a broad, hummocky terminal moraine on the piedmont. Cosmogenic ¹⁰Be exposure ages of this moraine indicate that the glacier was at or near its maximum extent for as many as 4000 yr after the start of global deglaciation, similar to glaciers in other ranges of the northern Rocky Mountains. Temperature and precipitation at the time of terminal-moraine deposition were assessed using a 2-d, numerical model of steady state mass balance and ice flow. Model results indicate that, if precipitation was near modern at the time of moraine deposition, then a temperature depression of ca. 9° C sustained the glacier at its maximum extent. This result is in contrast to the >10° C temperature depression and strong reduction in precipitation during the Last Glacial Maximum yielded by regional and global-scale climate models. However, the regional signal of a late onset of deglaciation in the northern Rocky Mountains relative to North American ice sheets has been explained by increased precipitation in the region after the Last Glacial Maximum. We propose that moderate cooling compared to the Last Glacial Maximum and near-modern precipitation were sufficient to sustain mountain glaciers at their maximum extent until as late as 15 ka, and that retreat of mountain glaciers was initiated by regional warming.