UNDERSTANDING SEASONAL PRECIPITATION IN THE UPPER LAS VEGAS WASH DURING THE LATE QUATERNARY USING STABLE CARBON AND OXYGEN ISOTOPES IN FOSSIL TEETH AND TUFA

During the late Quaternary, extensive paleowetlands covered much of the upper Las Vegas Wash (ULVW) of southern Nevada, an area recently designated as Tule Springs Fossil Beds National Monument. A comprehensive record of groundwater discharge (GWD) deposits shows hydrologic conditions in the ULVW responded in virtual lockstep with changes in climate recognized previously in the Greenland/North Atlantic climate record. Although we know that increased precipitation in arid regions raises local water tables, leading to expansion of wetland ecosystems, the source and seasonality of the late Quaternary moisture is unclear. Here we present carbon and oxygen isotope data of fossil teeth (Equus, Mammoth, Bison, and Camelops) and tufa (freshwater carbonate) to determine the seasonality of precipitation that supported desert wetlands in the southwestern U.S. during the late Pleistocene and early Holocene.

Today, summer δ¹⁸O_water values in southern Nevada springs range -12 to -14‰ (V-SMOW). From multiple GWD stratigraphic horizons that date to between ~40 and 16 ka, δ¹⁸O_water values calculated from teeth and tufa using standard isotopic transfer functions did not vary from a mean of -11.3±2‰ throughout the sequence. One exception is a single mammoth tooth dated to 14.59 ka which yielded δ¹⁸O_water=-17.3±2.5‰; tufa from the same deposit yielded δ¹⁸O_water=-9.9±1.2‰. Between 14.12 and 12.35 ka, δ¹⁸O_water values inferred from teeth and tufa increased to -8.6±3.5‰. Similarly, with the exception of the excursion at 14.59 ka (δ¹³C =-9.5±4.4‰) δ¹³C_tooth values from all units are variable but relatively high with a mean δ¹³C value of -5.8±4.1‰, indicative of dominantly C₄ vegetation. Together, relatively high δ¹⁸O_water (implying summer precipitation) and δ¹³C values (indicative of C₄ plants, which thrive in environments with summer rains) suggest that there was a greater summer/winter precipitation ratio during the latest Pleistocene than today.

Climate models and trends in the historical observational data indicate higher temperatures and lower water runoff in the American Southwest are likely in the future. Wetlands are key features in desert ecosystems and, given their sensitivity to climate change, this and future studies will provide insight into how modern wetlands may respond to rapid climate change.