LATE PLEISTOCENE GLACIATION IN THE MOSQUITO RANGE, COLORADO: CHRONOLOGY AND CLIMATE

New cosmogenic ¹⁰Be surface exposure ages from seventeen moraine boulders in the Mosquito Range indicate that glaciers were at their maximum extent of the last Pleistocene (Pinedale) glaciation by ~21-20 ka. Exposure ages on recessional moraines in two valleys suggest that glaciers remained at or near their maximum extents, and significant ice recession did not commence until ~17 ka. These age limits suggest that the Pinedale Glaciation was synchronous but initial deglaciation was asynchronous 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, temperatures were ~8.1 and 7.5 °C cooler, 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.