Paleoglacier reconstruction and late Pleistocene equilibrium-line altitudes, southern Sawatch Range, Colorado

Equilibrium-line altitudes (ELAs) for six reconstructed, late Pleistocene glaciers in the southern Sawatch Range were calculated using the accumulation-area ratio (AAR) method. An AAR of 0.65 ± 0.05 was used for five paleoglaciers having typical area-altitude distributions at the late glacial maximum. An AAR of 0.55 was used for one glacier with a piedmont-type geometry. The ELAs thus determined varied little from an average of 3,375 m in a nonsystematic way over the study area. Additional estimates and checks of paleo-ELAs were made using toe-to-headwall altitude ratios (THARs) and the maximum elevations of lateral moraines. Both methods (using THAR = 0.45) suggest an elevation of 3,375 m for regional equilibrium line, which is remarkably consistent with that determined by the AAR method. The paleoclimatic significance of ELA depression in the study area is interpreted within the range of temperature and precipitation conditions found at equilibrium lines on modern glaciers. Mean summer temperatures were on the order of 7-9 °C cooler during the last glacial maximum if winter precipitation was not significantly different than it is today. Lesser reductions in late Pleistocene temperatures would have required substantial increases in precipitation.