LATE PLEISTOCENE GROUND-WATER DISCHARGE AT VALLEY WELLS, EASTERN MOJAVE DESERT, USA

Ground-water discharge deposits (GWDs), also called spring or wetland deposits, form in arid environments as water tables rise close to or breach the ground surface during periods of enhanced effective precipitation. Where preserved, these deposits contain information on the timing and hydrology of past ground-water “highstands”. This information, in turn, can be used to evaluate conceptual and numerical models of past climate conditions, such as the position of the polar jet stream and associated storm tracks, through time. Although GWDs have been identified in all four of the major deserts of North America (Chihuahuan, Great Basin, Mojave and Sonoran), they have been studied in detail at only a handful of localities. In the Mojave Desert, recent surficial mapping conducted by the U.S. Geological Survey has identified more than 130 localities containing GWDs. We have chosen several of these sites for detailed investigation, including a long-lived sequence of deposits at Valley Wells on the western flank of the Clark Mountains, ~50 km east of Baker, CA. Local field mapping, stratigraphic relationships, and preliminary ¹⁴C results from organic-rich black mats, terrestrial gastropod shells, and resistant ground-water carbonate units suggest that high-water table conditions prevailed at Valley Wells during at least three distinct time intervals: the Younger Dryas cold event, full glacial times (OIS 2, including the Last Glacial Maximum), and an older period that is beyond the limits of ¹⁴C dating, which we tentatively assign to the penultimate glaciation (OIS 6). We are exploring other dating techniques, including U-series and luminescence dating, to refine the GWD chronology, particularly for the older deposits. We are also integrating faunal, geochemical, and isotopic techniques to better understand water chemistry, seasonality, and air temperatures during wet periods at Valley Wells. Preliminary results for each of these techniques will be presented.