PALEOGLACIER RECONSTRUCTIONS AND LATE PLEISTOCENE EQULIBRIUM-LINE ALTITUDES IN THE SAWATCH RANGE, COLORADO

Ice-surface topography for three small paleoglaciers was reconstructed on the basis of the geometry of moraine segments, the upper limits of erratic boulders, and the upper limits of glacial erosion. The accuracy of each reconstruction was evaluated by calculating basal shear stresses along the length of the glacier. Shear stresses fell within, or close to, the commonly accepted range of 50-150 kPa. Equilibrium-line altitudes (ELA) for the glaciers were calculated using the accumulation-area ratio (AAR) method. An AAR value of 0.65±0.05 was used. LGM ELAs were fairly consistent, with a mean of ~3415 m. This value is slightly higher than the regional mean of ~3375 m, and may reflect the southerly aspect of the catchment areas which is very consistent with other estimates of ELA within the region.

Paleoclimate inferences were made on the basis of mean summer temperatures and annual precipitation values found at the ELAs of modern glaciers. Assuming no significant changes in precipitation (compared to modern values), mean summer temperatures during the LGM may have been about 6.5 °C cooler than today. A 50% increase in precipitation would require mean summer temperature to have been ~5 °C cooler; a 50% decrease in precipitation would require a cooling of ~8.5 °C. These estimates of LGM temperature change are consistent with others made for the Sawatch Range, the nearby Sawatch Range, and elsewhere in the Colorado Rocky Mountains.