AN EARLY HOLOCENE WET PERIOD IN THE SOUTHWESTERN UNITED STATES

During the early Holocene, multiple generations of spring-fed streams traversed ~800 km² of the Las Vegas Valley in southern Nevada, depositing an extensive tufa network that dates to between 10.87 and 8.52 ka. The scale of the network and diversity of tufa morphologies is novel in North America and offers an opportunity to obtain quantitative paleoclimate data for the region. We determined isotopic compositions and estimated past temperatures from clumped isotope data from early Holocene tufa from the valley floor (698 m) as well as tufa forming today at Cold Creek Spring (1856 m) in the nearby Spring Mountains. Modern and fossil tufa yielded comparable δ18O and δ13C values, implying similar source waters derived from high-elevation winter precipitation. Clumped isotope temperatures of modern tufa average 15.8±2.5°C, aligning with mean summer temperatures of the emergent spring water, and support equilibrium conditions of tufa formation. The early Holocene tufa yielded similar clumped isotope temperatures, averaging 15.2±3.9°C, meaning it precipitated at temperatures that occur during the summer at much higher elevations today. The Las Vegas record, combined with nearby and temporally correlative paleospring, lacustrine, and speleothem records, suggest cool/wet conditions prevailed throughout the Mojave Desert during the early Holocene. These records also demonstrate that spring ecosystems responded to millennial-scale hydroclimate variations that supersede climate change driven solely by insolation. The spatially widespread pattern of ecosystem response to hydroclimate documented here may assist in understanding climate drivers for the early Holocene and provide critical information for the fate of groundwater dependent ecosystems in the southwestern U.S.