CLIMATE IN THE MOSQUITO RANGE, COLORADO, DURING THE LAST GLACIAL MAXIMUM INFERRED FROM STEADY-STATE MASS BALANCES OF PALEOGLACIERS

Glaciation in the Mosquito Range during the Last Glacial Maximum (LGM) was characterized by extensive valley glacier systems. These systems were to a large degree interconnected either by virtue of common ice fields and/or pervasive ice divides. In some locations glaciers in adjacent valleys coalesced to form composite termini. Estimates of equilibrium-line altitudes (ELAs) obtained using the accumulation-area ratio (AAR) method for seven reconstructed glaciers during their LGM extents range from 3420 to 3570 m. The mean AAR-derived ELA over the range is 3500 ± 60 m. Collectively the ELAs are consistent with and further substantiate a previously recognized, statistically significant west-to-east trend of increasing ELAs from the Elk Mountains and continuing through the Sawatch Range to the Mosquito Range, apparently due to the predominance of eastward transport of moisture during the LGM. ELAs on the eastern side of the range are, however, consistently lower than those on the west, the difference of average ELAs being 110 m. This might reflect the importance of southeasterly-derived late(?) winter precipitation in enhancing snow accumulation and hence glacier mass balances. Steady-state mass balances of the reconstructed glaciers were determined using a temperature-index model with no consideration for potential differences in precipitation across the range. The mean ELA thus derived is 3555 ± 40 m, and east-west differences are less distinct. The mean temperature depression required for steady-state mass balances of paleoglaciers is 7.5 ± 0.3°C with slightly cooler temperatures (by ~0.5°C) suggested for those on the eastern side. Alternatively, assuming regional temperature depression was essentially that determined for paleoglaciers on the western slopes (7.3°C), a precipitation enhancement of ~25% is required to maintain glaciers on the eastern side at their LGM maxima. Climatological data suggest this difference is comparable to what is observed today in both mean annual and late winter (February through March) precipitation, furthermore supporting the possibility that glaciers on the eastern flank of the Mosquito Range were nourished by southeasterly, Gulf of Mexico-derived moisture.