HYDROCLIMATE AND THERMOMETRY OF A REGIONAL EARLY HOLOCENE WET PERIOD RECORDED IN A UNIQUE FLUVIAL TUFA SYSTEM FROM THE LAS VEGAS VALLEY, NEVADA

The Las Vegas Formation contains a sequence of paleospring deposits that records more than a half million years of hydrologic fluctuations in the Las Vegas Valley. The final significant groundwater discharge event occurred during the early Holocene when spring-fed streams created a tufa-bearing fluvial network that covered ~225 km² of the valley floor. Radiocarbon ages from organic material intercalated with the microbial precipitated tufa indicate this multi-channel braided system persisted between 10.87 and 8.52 ka. The scale of the system and the diversity of tufa fabrics represented is novel in North America and offers an excellent opportunity to gain insight into the source and temperature of groundwater discharging onto the valley floor during the early Holocene. We measured oxygen and carbon isotopic compositions and determined clumped isotope paleotemperatures for early Holocene tufa at 672 m and from tufa forming today at Cold Creek Spring at 1930 m in the Spring Mountains. Modern tufa yielded d¹⁸O and d¹³C values that are consistent with the fossil tufa. Predicted d¹⁸O values of the host waters are also consistent with measured d¹⁸O values at Cold Creek Spring, implying similar source waters as well as equilibrium conditions of the tufa carbonate precipitation. Clumped isotope temperatures of Cold Creek tufa range from 12±4 to 17±3 °C, which align with measured summer temperatures (12-16 °C) of the emergent groundwater. The early Holocene tufa yielded clumped isotope temperatures ranging from 11±3 to 20±3 °C, indicating cold spring water flowed in streams on the valley floor and tufa precipitated at temperatures that occur only at mountain elevations today. Calculated summer air temperatures in the Las Vegas Valley were on average nearly ten degrees cooler than summer air temperatures recorded thus far during the 21^st century. Clumped isotopic thermometry of precisely dated tufa provides a new source of high-resolution data to interpret hydroclimate and paleoenvironmental conditions during the early Holocene. Combined with nearby paleospring, pluvial lake and speleothem records, our results support an early Holocene wet period in the southern Great Basin and show that millennial-scale regional hydroclimate variations supersede those driven solely by insolation.