GLACIATION AND CLIMATE DURING THE LAST GLACIAL MAXIMUM IN THE MOUNT MASSIVE REGION, NORTHERN SAWATCH RANGE, COLORADO

Recent cosmogenic surface exposure dating of moraine sequences in the Mount Massive region suggests that locally the Last Glacial Maximum (LGM) occurred ~22-21 ka. The LGM here was characterized by extensive valley glacier systems that were to a large degree interconnected by common upland ice fields and/or pervasive ice divides. Evidence for upland ice includes streamlined or other glacially-eroded bedrock forms, scoured basins, subdued topography, and in some cases breached interfluves. Steady-state mass balances of several reconstructed glaciers at their LGM extents were determined using a temperature-index model that assumes seasonal (winter, spring, summer, and fall) temperature lapse rates and vertical precipitation gradients were comparable to modern values. Mean annual precipitation and its seasonal distribution are also assumed to be similar to those observed today. Preliminary simulations yield equilibrium-line altitudes (ELAs) between ~3350 and 3470 m that further document a statistically significant first-order trend showing a rise of ELAs from the Elk Mountains to the west to the Mosquito Range in the east of ~4.7 m km^-1. Estimates of LGM temperature depression required to maintain steady-state mass balances of the glaciers range from ~6.2 to ~7.5°C. Allowing for modest changes in precipitation (e.g. 1.25, 0.75 times modern) during the LGM, either in mean annual or a shift to wetter/drier winters, implies ~±0.15°C deviation from these values. These estimates are consistent with similarly-derived inferences regarding LGM cooling in the West Elk Mountains, Sawatch, and Mosquito Ranges, but are generally 1-2°C greater than those suggested for the Sangre de Cristo Mountains and Front Range.