Cosmogenic 10 Be surface-exposure dating and numerical glacier modeling are used to reconstruct glacial chronology and climate in the Colorado Sangre de Cristo Mountains during the local last glacial maximum (LLGM) and the subsequent deglaciation. Twenty-two surface-exposure ages on moraine boulders and polishedbedrock outcrops in the Willow Creek valley and ten in two adjacent valleys indicate that glaciers were at or near their maxima from ~21ka until 17-16 ka, and then retreated rapidly, nearly deglaciating the Willow Creek valley entirely by ~14 ka. Coupled energy/mass-balance and flow modeling of two of the glaciers indicates that, if changing ice extent was driven only by temperature and insolation changes, temperature depressions of 5.0 and 5.1°C from modern conditions, with an uncertainty ofapproximately +1.5 −1.0°C , would have sustained the glaciers in massbalance equilibrium at their LLGM extents. Doubling or halving of modern precipitation during the LLGM would have been associated with 2.7-3.0°C and 6.9-7.0°C temperature depression respectively. Approximately half of the subsequent LLGM-to-modern climate change was accomplished by ~14 ka. If the rapid main phase of deglaciation betweenabout 16 kaand 14 ka was driven solely by temperature and insolation changes, it would have been associated with a temperature rise of about 2.5°C, at a mean rate of approximately 1.1°C/ky. This new chronology of the last glaciation is generally consistent with others developed recently in the Colorado Rocky Mountains. The numerical modeling, 2 however,suggests a lesserLLGM temperature depression from modern conditions than have most previous studies in Colorado.

Equilibrium-line altitudes (ELAs) were determined from reconstructions of 22 paleoglaciers at their extent during the local last glacial maximum (LGM) using the accumulation-area method. LGM ELAs thus derived ranged from 2980 to 3560 m and follow a statistically significant regional trend of rising ,4.5 m km 21 to the east. Two approaches using a degree-day model were used to infer LGM climate by finding plausible combinations of temperature and precipitation change that (1) would be required to lower ELAs to their mean LGM values in both the Taylor Park/eastern Elk Mountains region and western Elk Mountains, and (2) provide steady-state mass balances to maintain individual glaciers. The results of these two approaches are convergent and suggest that in the absence of significant changes in precipitation, mean summer (or mean annual) temperatures within the study area during the LGM were on the order of about 7 uC cooler than at present. The model also suggests that even allowing for modest changes in LGM precipitation (625%), the required mean summer temperature depressions are within ,0.5 uC of these values. Furthermore, there appears to be no significant dependence on small potential changes in temperature seasonality (i.e., winter temperatures). The inferred magnitude of LGM temperature change in the study area is consistent with other estimates from the broader Southern and Central Rocky Mountain region.

No abstract

Ice surface topography of a late Pleistocene glacier complex, herein named the Taylor River Glacier Complex (TRGC), was reconstructed on the basis of detailed mapping of glacial landforms combined with analyses of aerial photos and topographic maps. During the last glacial maximum (LGM), the TRGC covered an area of 215 km 2 and consisted of five valley or outlet glaciers that were nourished by accumulation in cirques basins and/or upland ice fields.Equilibrium-line altitudes (ELAs) for the glaciers of the TRGC were estimated using the accumulation-area ratio method, assuming that ratio to be 0.65 F 0.05. ELAs thus derived ranged from about 3275 to 3400 m, with a mean of 3340 F 60 m. A degree-day model (DDM) was used to infer the climatic significance of the LGM ELA. With no appreciable differences in precipitation with respect to modern climate, the ELA implies that mean summer temperatures during the LGM were~7.6 8C cooler than today. The DDM was also used to determine the temperatures required to maintain steady-state mass balances for each of the reconstructed glaciers. The required reductions in summer temperature vary little about a mean of 7.1 8C. The sensitivity of these results to slight (F 25%) changes assumed for LGM precipitation are less than F 0.5 8C. Even under an LGM climate in which precipitation is assumed to be substantially different (F 50%) than the present, mean summer temperatures must be on the order of 7.0 to 8.5 8C lower to depress equilibrium lines to LGM altitudes. The greater sensitivity of the ELA to changes in temperature suggests that glaciation in the region was driven more by decreases in summer temperature rather than increases in precipitation. D

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.