GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 122-2
Presentation Time: 9:00 AM-6:30 PM


BRUGGER, Keith A., Geology Discipline, University of Minnesota, Morris, 600 E. 4th Street, Morris, MN 56267, RULEMAN, Cal, US Geological Survey, Geosciences and Environmental Change, P.O.Box 25046, Denver Federal Center MS 980, Lakewood, CO 80225 and CAFFEE, Marc W., Department of Earth, Atmospheric and Planetary Sciences, Purdue University, West Lafayette, IN 47907

Glaciation in the northern Sawatch Range was characterized by extensive valley glacier systems that were interconnected by common upland ice fields and/or pervasive ice divides. Cosmogenic surface exposure dating of moraine sequences indicates that locally the Last Glacial Maximum (LGM) occurred ~22-21 ka, suggesting synchroneity of LGM glacial advances in the Colorado Rocky Mountains (i.e. elsewhere in the Sawatch Range, the Mosquito and Front Ranges, and the Elk, Sangre de Cristo, and San Juan Mountains). Temperature-index modeling is used to determine the magnitude of LGM temperature depression required to maintain steady-state mass balances of five reconstructed glaciers at their maximum extents. The model assumes modern lapse rates and seasonal variations in vertical precipitation gradients derived from the PRISM gridded climatology. Mean annual precipitation and its seasonal distribution are also assumed to be similar to those observed today. Derived values of temperature depression range from 7.7 to 8.9 °C, averaging 8.6 ± 0.5 °C. Quantifiable uncertainty, that also considers modest changes in LGM precipitation of ±10 cm, is estimated to be +0.6/–0.8 °C.

LGM temperature depression in the northern Sawatch Range was comparable, albeit 1-1.5 °C greater, than a regional mean of 7.5 ± 0.8 °C (n = 35) based on similarly determined values in other ranges in Colorado. Given uncertainties, variations in inferred LGM cooling may not be significant and could reflect the inability to account for potential changes in precipitation over the region. Uncertainties combined with sparse and clustered data yield regional trends in LGM temperature depression that are equivocal. In contrast, clear regional west-to-east gradients of ~2.5 to 5 m km-1 are evident in equilibrium-line altitudes (ELAs) reflecting general moisture transport during the LGM. Locally in the Mosquito Range and Sangre de Cristo Mountains, however, the gradient is reversed in that ELAs are lower on the eastern slopes presumably due to late winter/early spring upslope precipitation events. Nevertheless, if there was significant variation in LGM temperature depression over the region, this could imply local modulation of regional/global climate forcing, or other complexities in the interactions among regional and local climates and glacier dynamics.