LATE PLEISTOCENE PALEOHYDROLOGY OF WILLCOX BASIN, SOUTHEASTERN ARIZONA FROM FOSSIL WETLAND AND LAKE-SHORELINE DEPOSITS AND IMPLICATIONS FOR SEDIMENTOLOGY AND REGIONAL ATMOSPHERIC CIRCULATION

Fossil shorelines record the areal extent of past lake expansions in closed basins of arid regions worldwide, reflecting hydrologic balance changes from one lake cycle to the next. Paleo-lake records from the northern Great Basin indicate low to intermediate lake levels there during the last glacial maximum (LGM: ca. 23-19 Cal ka BP) relative to those reached during the subsequent Deglacial interval, particularly in response to Heinrich events and attendant collapse of the Atlantic Meridional Overturning Circulation during the Heinrich 1 (H1) stadial (ca. 17.5-14.6 Cal ka BP). Competing hypotheses about the causes of these cycles imply that southern Basin and Range paleo-lakes experienced their highstands during the LGM, rendering shoreline chronologies from that region of paramount importance to understanding regional atmospheric dynamics during this critical climate transition. In southeastern Arizona, highstands of paleo-Lake Cochise formed a composite beach ridge in Willcox Basin from ca. 17 to ca. 13 Cal ka BP, overlapping with many highstands in the Great Basin. However, recent ¹⁴C dating of shells within the calcareous mudstones buried beneath the ridge constrain their emplacement to ca. 19 Cal ka BP, and microfaunal and geochemical evidence reveals their lacustrine origin. This lake cycle preceded Deglacial-age highstands of paleo-Lake Cochise, well in advance of H1, while several highstands post-dated the H1 stadial—altogether suggesting multiple triggers for the rise and fall of paleo-Lake Cochise during the latest Pleistocene. Finally, given that similar-looking mudstones in the southern Great Basin are known to have formed in paleo-wetlands, this study has large implications for detecting and constraining the timing of ancient lake cycles and wetland expansions in arid closed basins across the globe.