THE LAST GLACIAL MAXIMUM IN THE MOSQUITO RANGE, COLORADO: CHRONOLOGY AND CLIMATE

During the last glacial maximum (LGM) the Mosquito Range was characterized by valley glacier systems principally fed by cirques on the higher peaks but also nourished by extensive, shallow ice fields that developed on broad, gently sloping interfluves. Glacier systems were typically interconnected by common ice fields and/or pervasive ice divides. In some locations glaciers in adjacent valleys coalesced to form composite termini.

New cosmogenic ¹⁰Be surface-exposure ages indicate that glaciers attained their maximum LGM (Pinedale) extents between ~24 and 21ka. These ages are in excellent agreement with those from the adjacent northern Sawatch Range and elsewhere in the Colorado Rocky Mountains. In at least one valley a re-advance or recessional stillstand occurred ~17 ka. Two boulders from a moraine segment deposited during the penultimate (Bull Lake) glaciation yielded ages of ~126-130 ka.

LGM temperatures were estimated by determining steady-state mass balances of reconstructed glaciers using a temperature-index model that assumes seasonal (winter, spring, summer, and fall) vertical precipitation gradients and lapse rates were comparable to modern values. Model simulations account for climatic differences between the eastern and western sides of the range, also suggested by modern data. Specifically, the eastern slope (1) receives more winter and spring precipitation, particularly over elevations that would have served as accumulation areas of the paleoglaciers; and (2) experiences cooler summer temperatures, especially over elevations corresponding to ablation areas. Mean annual precipitation and its seasonal distribution are assumed to be similar to those observed today. Simulations yield equilibrium-line altitudes between ~3400 and 3500 m on the eastern, and 3500 and 3600 m on the western slope. LGM temperature depression required to maintain steady-state mass balances of glaciers on the east side of the range averages 8.1 ± 0.3 °C (n = 4) and 8.0 ± 0.2 °C (n = 3) on the west, assuming no significant changes in precipitation. Quantifiable uncertainties are ~0.5 °C. These results are relatively insensitive to modest changes in LGM precipitation. The consistency of LGM temperature depression over the range might suggest the climatic dissimilarities existing today also existed during the LGM.