LATEST PLEISTOCENE PALEOHYDROLOGY OF WILLCOX BASIN (32°N), SOUTHEASTERN ARIZONA, AND IMPLICATIONS FOR REGIONAL ATMOSPHERIC CIRCULATION DURING THE LAST DEGLACIATION

A succession of ancient nearshore and wetland deposits buried beneath the prominent 1,274 m shoreline berm in Willcox basin (~32^oN), in southeastern Arizona, previously dated to ~17-16 ka, records the occurrence of brief intervals characterized by wetter-than-modern conditions during MIS 2 and 3. Taxonomic and geochemical analyses of gastropod and ostracode assemblages, together with reliable ¹⁴C dating of semi-aquatic Succinid shells, indicate that paleo-Lake Cochise formed on several occasions during the last deglaciation (~19-11 ka), reaching elevations between ~1,271 and ≥1,275 m. The contemporaneity of lake expansions in southeastern Arizona and the Great Basin, with paleoceanographic and terrestrial climatic changes in the greater North Atlantic region, suggests that the latter drove synoptic-scale changes in atmospheric circulation over the western U.S. Furthermore, the irregular occurrence of anti-phasing between paleo-Lakes Cochise and Estancia (35^oN), in north-central New Mexico—with conditions shifting to wetter in the south and drier in the north—provides new insight into the nature of regional-scale circulation during the late Pleistocene. Such anti-phasing could be explained by drying of the Westerlies precipitation belt with simultaneous delivery of southerly-sourced precipitation to southeastern Arizona; although this is consistent with paleobotanical and stable isotopic evidence from packrat middens for periodic enhancement of the summer monsoon in the southeastern Basin and Range during MIS 2 and 3, it indicates that the influence of summer precipitation was at times much greater than previously recognized.