EQUILIBRIUM-LINE ALTITUDES, SNOWLINE GRADIENTS, AND LATE-PLEISTOCENE CLIMATE IN WEST-CENTRAL COLORADO

Continued mapping of glacial features in the mountains of west-central Colorado combined with analyses of aerial photographs, topographic maps, and digital elevation models has further elucidated late-Pleistocene climate and climatic trends within the region. Twenty-five reconstructed valley glaciers or glacier complexes across a ~110 km, east-west transect were used to determine equilibrium-line altitudes (ELAs) during the last glacial maximum (LGM; dated here by cosmogenic exposure ages as lasting from ~22.5 to ~16.5 ka). ELAs were estimated using the accumulation-area ratio method, assuming that ratio to be 0.65 ± 0.05. ELAs are as low as 3100 m in the Ruby Range to the west and rise to over 3500 m on the eastern flank of the Sawatch Range and adjacent Mosquito Range to the east. Individual LGM ELAs, inversely weighted according to the uncertainty in the associated reconstruction, define a highly significant (r² ~0.95, p < 0.001) paleosnowline gradient of ~3.4 m km^-1.

Climatic inferences were made using a degree-day model which determines the changes in temperature and/or precipitation, with respect to modern values, required to both lower ELAs to their respective LGM altitudes, and maintain steady-state mass balances for the reconstructed glaciers. Both conditions are satisfied by rather consistent reductions in mean summer temperature across the region of between ~6.5 and ~7.5 °C, and show little sensitivity to small (< 25 %) changes in precipitation. This implies that the observed gradient of paleo-ELAs was the result of a relatively uniform depression of regional temperatures during the LGM. Moreover, this gradient parallels that derived from estimates of modern snowline within the study area. The latter is a consequence of a systematic eastward trend of slightly warmer and substantially drier local climates. The close parallelism of the LGM ELA gradient and modern snowline suggests that regional variations in temperature and precipitation, moisture transport, and orographic effects on precipitation during the late-Pleistocene were similar to what they are today.