Late Pleistocene glaciation in the Mosquito Range, Colorado, USA: chronology and climate

Abstract

New cosmogenic 10Be surface exposure ages from 17 moraine boulders in the Mosquito Range of Colorado suggest that glaciers were at their late Pleistocene (Pinedale) maximum extent at ∼21–20 ka, and that ice recession commenced before ∼17 ka. These age limits suggest that the Pinedale Glaciation was synchronous within the Colorado Rocky Mountain region. Locally, the previous (Bull Lake) glaciation appears to have occurred no later than 117 ka, possibly ∼130 ka allowing for reasonable rock weathering rates. Temperature‐index modeling is used to determine the magnitude of temperature depression required to maintain steady‐state mass balances of seven reconstructed glaciers at their maximum extent. Assuming no significant differences in precipitation compared to modern values, mean annual temperatures were ∼8.1 and 7.5 °C lower, respectively, on the eastern and western slopes of the range with quantifiable uncertainties of + 0.8/−0.9 °C. If an average temperature depression of 7.8 °C is assumed for the entire range, precipitation differences − that today are 15–30% greater on the eastern slope due to the influence of winter/early spring snowfall − might have been enhanced. The temperature depressions inferred here are consistent with similarly derived values elsewhere in the Colorado Rockies and those inferred from regional‐scale climate modeling.

Climate during the Last Glacial Maximum in the Northern Sawatch Range, Colorado, USA

“…Simulation of LGM climate in the northern Sawatch Range uses a temperature-index model (TM) to find the temperature and precipitation changes required to maintain steady-state mass-balances of the reconstructed glaciers. Details of the TM and the justification for its use were given in Brugger et al [7]. Here, we briefly review the approach and highlight some modifications necessary for the present application.…”

“…(1) Previously [7,33], H or its equivalent was treated as a constant; however, as alluded to above, PRISM data in the study area reveal a slight systematic decrease with increasing elevation. (2) Lacking a sufficient number of meteorological stations-especially at higher elevations-T jan (z) is determined using the modern lapse rate for January obtained from PRISM climate data (Table 1) sampled over the extent of glacier surfaces in conjunction with a value of T jan (z) at some reference elevation.…”

“…The development of glacial chronologies in the Rocky Mountains have constrained the timing of the Last Glacial Maximum (LGM, senso lato) in many of the individual ranges and provided valuable insights regarding late Pleistocene climate change [1][2][3][4][5][6][7]. Recent compilations [8,9] of available cosmogenic exposure ages (recalculated to facilitate comparison) of LGM terminal moraines in the Rocky Mountains suggest no apparent coherent geographic pattern of glacial behavior.…”

“…Many climate proxies (e.g., pollen spectra) are limited in that they often post-date the glacial maximum, or are hindered by the inability to provide quantitative measures of the relevant parameters. In contrast, studies using climate modeling in conjunction with LGM glacier extents have provided somewhat robust estimates of temperature depression and potential changes in precipitation in the Rocky Mountains [5][6][7][30][31][32][33][34][35]. Nevertheless, subregional discrepancies exist possibly owing to (1) actual variations in local temperature and/or precipitation, (2) the different methodologies used, and/or (3) glacier maxima that were time-transgressive and hence the inferred climates represent different times (see discussions in [5,7]).…”

“…In contrast, studies using climate modeling in conjunction with LGM glacier extents have provided somewhat robust estimates of temperature depression and potential changes in precipitation in the Rocky Mountains [5][6][7][30][31][32][33][34][35]. Nevertheless, subregional discrepancies exist possibly owing to (1) actual variations in local temperature and/or precipitation, (2) the different methodologies used, and/or (3) glacier maxima that were time-transgressive and hence the inferred climates represent different times (see discussions in [5,7]). Moreover, the geographic coverage of these estimates is sparse and only provides a relatively low-resolution regional picture of LGM climate.Based on the foregoing, it is clear that a better understanding of late Pleistocene climate change in the Rocky Mountains would be beneficial for both additional glacial chronologies and climate reconstructions.…”