HYDROCLIMATE AND THERMOMETRY OF AN EARLY HOLOCENE FLUVIAL TUFA SYSTEM IN THE LAS VEGAS VALLEY, NEVADA

The Las Vegas Formation, with its impressive sequence of paleo-spring deposits, records over 500 ka of climate driven hydrologic fluctuations in the Las Vegas Valley. The last significant pulse of groundwater discharge occurred during the early Holocene as outflow streams, creating an extensive tufa-bearing fluvial network across the valley floor. Radiocarbon ages from organic material intercalated with the tufa indicate this multi-channel braided system persisted between 10.87 and 8.52 ka (bed E_2c of the Las Vegas Formation). The microbial precipitated tufas contain phytoclasts, oncoids, and cyanoliths, and their stable isotope composition provides insight into the source and temperature of groundwater discharging onto the valley floor. We measured the oxygen and carbon isotope compositions and determined clumped isotope paleotemperatures for phytoclast tufas at 672 m in Tule Springs Fossil Beds National Monument and from modern oncoid and phytoclast tufa forming at Cold Creek Spring at 1930 m in the Spring Mountains today for comparison. Modern tufa yielded δ¹⁸O and δ¹³C values that are consistent with the fossil tufa, and predicted δ¹⁸O values for each of the host waters are consistent with measured δ¹⁸O values of at Cold Creek Spring, which implies that the source waters were similar and that tufa carbonate was precipitated under equilibrium conditions. For the modern Cold Creek tufas, clumped isotope temperatures range from 12±3 to 17±3 °C (n=4), which align with measured summer temperatures (12-16 °C) of the emergent groundwater. Phytoclast tufas in bed E_2c yielded similar clumped isotope temperatures ranging from 11±3 to 20±3 °C (n=5), indicating cold spring water flowed in point source streams on the valley floor during the early Holocene, and tufa was precipitated at summer discharge temperatures that occur only at mountain elevations today. These results incorporate a combination of a precise chronology with stable and clumped isotopic thermometry and provide a new source of high-resolution data for interpreting hydroclimate and paleoenvironmental conditions during the early Holocene in the southern Great Basin.