UNDERSTANDING SEASONAL PRECIPITATION IN THE UPPER LAS VEGAS WASH DURING THE LATE QUATERNARY USING STABLE CARBON AND OXYGEN ISOTOPES IN FOSSIL TEETH AND TUFA
Today, summer δ18Owater values in southern Nevada springs range -12 to -14‰ (V-SMOW). From multiple GWD stratigraphic horizons that date to between ~40 and 16 ka, δ18Owater 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 δ18Owater=-17.3±2.5‰; tufa from the same deposit yielded δ18Owater=-9.9±1.2‰. Between 14.12 and 12.35 ka, δ18Owater 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‰) δ13Ctooth 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 δ18Owater (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.