LATE PLEISTOCENE GLACIER DYNAMICS AND PALEOCLIMATE IN THE SANGRE DE CRISTO MOUNTAINS, COLORADO

Reconstructions of ice extent and glacier dynamics of last glacial maximum (LGM) valley glaciers in the northern Sangre de Cristo Mountains of Colorado provide insight into the late Pleistocene climate and morphological evolution of the range. The extent of LGM ice was mapped in two valleys on opposite sides of the range in the Cottonwood Peak area, based on preserved moraines, trimlines, and distribution of crystalline erratics. From this mapping, ice surface topography was reconstructed and reconstructions were checked and modified slightly on the basis of basal shear stress determinations. LGM equilibrium line altitudes (ELAs) were determined using the accumulation area ratio (AAR) method, with an assumed AAR value of .65 ± .05. LGM ELAs were 3440 and 3455 ± 30m on the west and east side of the range respectively. These ELAs are generally higher than those in the San Juan Mountains to the west, indicating that during the LGM, as at present, the San Juan Mountains generated a rain shadow, blocking westerly moisture flow to the northern Sangre de Cristo Mountains. Numerical coupled mass balance and flow models were created for the two glaciers to aid in understanding LGM climate and the magnitude of LGM-to-present climate change. With no significant changes from modern precipitation, a temperature depression of 7.5° to 8°C would have been needed to sustain the glaciers in mass balance at their LGM extents. If modern precipitation had been halved, a temperature depression of approximately 9°C would have been necessary to sustain the glaciers at their late Pleistocene maxima.