Late Pleistocene Equilibrium-Line Altitudes and Modern Snow Accumulation Patterns, San Juan Mountains, Colorado, U.S.A.

Abstract

Equilibrium-line altitudes (ELAs) are determined for 85 reconstructed late Pleistocene glaciers in the San Juan Mountains of Colorado. Paleo-ELA contours rise broadly from west to east across the range and wrap around the range's central core, where the highest ELA values are found. Detailed comparison of modern and paleosnowline variations within the range is not possible because there are too few modern snow fields. Snow survey data, however, allow comparison of modern snow accumulation patterns with paleo-ELAs. When the effect of elevation on accumulation is removed, modern accumulation patterns bear striking similarlity to paleo-ELA patterns, with lowest late Pleistocene ELAs in areas of highest modern snow accumulation and vice versa. Modern accumulation contours show the same west to east gradient and also wrap around the high, dry, central core of the range. Accumulation data suggest that “theoretical” modern ELAs would increase by approximately 600 m across the range, a figure closely comparable to cross-range Pleistocene ELA differences. These similarities suggest that accumulation-season general circulation patterns and moisture sources did not alter greatly between the time of the late Pleistocene glacial maximum and the present. They appear to rule out any strong increase in fall, winter, or spring southeasterly (Gulf of Mexico) flow as a mechanism for enhancement of precipitation in the range.