DESERT WETLAND MOISTURE SOURCES DURING THE LATE PLEISTOCENE IN THE AMERICAN SOUTHWEST

During the Pleistocene-Holocene transition, increased groundwater discharge in southwestern Nevada due to climate forcing formed extensive regional paleowetlands. Conventional wetland paleoclimate proxy records (pluvial lakes, speleothems, wood rat middens) give limited insight into past ecological responses to climate change. Research at the San Bernardino County Museum (SBCM) has established a comprehensive record for groundwater discharge deposits in the upper Las Vegas Wash, Nevada, which strongly correlates with Greenland/North Atlantic climate proxy data. Paleowetlands develop during wet/cool cycles and subsequently collapse during dry/warm periods. It has been suggested that enhanced moisture transport increases precipitation during wet intervals; however, specific moisture sources are still disputed. Previous studies suggest a southward “shift of the westerlies” (SOW) driven by the Laurentide ice sheet would enhance winter precipitation over the mid-continent, while other studies propose moisture transported “out of the tropics” (OOT) would increase spring/summer precipitation δ¹⁸O.

Stable oxygen and carbon isotopes of fossil teeth are commonly used to investigate the paleoclimate of past environments and paleoecology of past fauna. Preserved δ¹⁸O and δ¹³C of interstitial carbonate in tooth enamel apatite depend on local moisture and food sources accessed during tooth formation. To identify the moisture source stabilizing paleowetland development, enamel samples were obtained from late Pleistocene Equus and Bison collected by the SBCM from the upper Las Vegas Wash, from geologic units corresponding with wet/cool periods. Tooth sections were subsampled prior to δ¹⁸O and δ¹³C analyses to assess seasonal zoning patterns.

δ¹⁸O values range from 19.71% to 28.86% V-SMOW, indicating an increase in spring/summer precipitation δ¹⁸O. Mean δ¹³C_diet values range from -16.53% to -21.73% V-PDB, implying these genera were mixed C₃/C₄-feeders. Preliminary data suggest OOT is the primary source of moisture establishing paleowetlands in the American Southwest deserts during the late Pleistocene. Past desert ecosystems relied heavily on wetlands and determining the moisture source responsible for increased groundwater discharge is crucial for assessing paleowetland development.