LATE PLEISTOCENE EQUILIBRIUM–LINE ALTITUDES AND CLIMATE ON THE BLANCA MASSIF, SANGRE DE CRISTO RANGE, COLORADO

Field mapping and analyses of topographic maps, aerial photographs and digital elevation models were used to reconstruct the extent of glaciers during the local last glacial maximum (LGM) in seven valleys that radiate from the high (>4200 m) peaks of the Blanca Massif. On the western flank of the massif, glaciers were constrained to flow in relatively straight, narrow valleys and were characterized by relatively thin ice (maximum ice thicknesses ~100 m). Typically these glaciers were sourced from a single cirque basin. In contrast, glaciers on the eastern and northern flanks were thicker (maximum ice thicknesses up to ~250 m) and wider, often fed by the accumulation in multiple cirques. The largest glacier, flowing northward from Blanca Peak via the Huerfano Valley, was ~12 km in length and covered an area of ~1.8 x 10⁷ m².

LGM equilibrium-line altitudes (ELAs) for the reconstructed glaciers were determined using the accumulation-area ratio method (AAR = 0.65 ± 0.05). Six paleoglaciers were independently reconstructed by two or more of the authors, providing a unique measure of the subjectivity involved in delineating ice extents and recreating ice-surface topographies. In all but one case the differences in ELA estimates fell within the uncertainty associated with that in the AAR. ELAs thus determined vary between 3400 and 3570 m, with a mean value of 3490 ± 55 m. ELAs tend to be lower by ~100 m on the northern, eastern, and possibly the southern sides of the massif. This trend of ELAs being lower on the eastern side is also apparent from the limited data available for the entire Sangre de Cristo Range in Colorado.

Modern climate data are rather sparse, particularly those needed to define vertical precipitation gradients and temperature lapse rates. Nevertheless, ELAs are used for inferring climate during the LGM based on temperature and precipitation values found at the equilibrium lines of modern glaciers. This analysis suggests mean summer temperature in the region was depressed 7-8 ºC.