Evidence for a Cold, Dry Full-Glacial Climate in the American Southwest

Abstract

Analysis of the evidence concerning full-glacial (27,000–13,000 yr BP) time in the American Southwest has resulted in a reinterpretation of the past climate. After correction for previous errors, which include the use of (1) the July 0°C isotherm as an indicator of modern snowline and (2) relict cirques at the head of heavily glaciated valleys as indicators of past snowline, orographic snowline is estimated to have been about 1000 m lower. This, combined with similar depression estimates for timberline and for cryogenic deposits, results in an annual cooling estimate of 7–8°C. The cooler temperatures caused deposition of coarse “bouldery” alluvial-fan sediments by streams draining areas at high elevations, as well as hill-slope stability, soil development, and subsurface weathering in the lower desert ranges. Reliability of paleoclimatic inferences from water budgets of the extinct paleolakes is questioned on the basis that they exclude groundwater effects. However, recalculation of two water budgets following traditional methods indicates that lowered evaporation rates accompanying a 7–8°C annual cooling would have sufficed to maintain these lakes. Similarly, increased soil moisture would have allowed coniferous woodland to grow ∼700 m lower into the deserts, the maximum recorded by Pleistocene pack rat middens. Therefore, no significant increase in annual precipitation is inferred for full-glacial times.