Late Pleistocene Paleohydrology of Willcox Basin, Southeastern Basin and Range from 14C-Chronostratigraphy, Sedimentology, Fauna, and Stable Isotopes of Wetland and Lake-Shoreline Deposits
Wednesday, 17 December 2014
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 (AMOC) 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 lake-level maxima 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 overlapped with many highstands in the Great Basin, forming a composite beach ridge in Willcox Basin from ca. 17 to ca. 13 Cal ka BP. However, recent 14C dating of shells and carbonates within a calcareous mudstone unit (Unit 5) buried beneath the ridge constrain emplacement of Unit 5 to ca. 19 Cal ka BP; evidence from the faunal and stable isotopic composition of ostracode and gastropod assemblages reveals the mudstone’s lacustrine origin. Given that similar mudstones in drained basins of the southern Great Basin are known to have formed in paleo-wetlands, our findings have large implications for detecting and constraining the timing of ancient lake cycles and wetland expansions in arid closed basins worldwide. With respect to paleoclimate, the Unit 5 lake cycle occurred in advance of H1 and formation of the beach ridge by ~2 millennia, whereas the beach ridge continued to form following the H1 stadial. This suggests that lake cycles associated with the terminal LGM, H1 stadial, Bolling-Allerod, and Younger Dryas climatic intervals occurred in response to different triggers, themselves effecting major changes in AMOC circulation and/or sea-surface temperatures in the North Atlantic.