STABLE AND CLUMPED ISOTOPE COMPOSITIONS OF SPRING DISCHARGE CARBONATES OF THE LAS VEGAS FORMATION, SOUTHERN NEVADA: PRELIMINARY CONSTRAINTS ON PALEOHYDROLOGY AND PALEOCLIMATE

The middle Pleistocene-early Holocene Las Vegas Formation consists of extensive fine-grained sediments covering the valley floor near Las Vegas, Nevada. These groundwater discharge deposits record a detailed history of wetter-than-present hydrologic conditions in the Mojave Desert that coincide with colder global climate of the last glacial cycle. Many beds contain authigenic carbonates, including marls, cap carbonates, and microbial as well as physio-chemically precipitated tufas, whose stable isotope composition provides insight into the source and temperature of groundwater discharging onto the valley floor. We measured oxygen and carbon isotope compositions of cap carbonates and tufas in units ranging in age from ~35 ka to 8 ka (units D₁ to E_2c) and clumped isotope paleotemperatures for tufas in the E_2c unit (10.6-8.5 ka), which records the final episode of wetlands discharge at valley floor elevation (672 m). We compare these isotope values and temperatures to those from modern spring discharge across the region, and modern oncoid and phytoclast tufa forming at Cold Creek spring (1930 m) in the Spring Mountains, today. A clumped isotope temperature of 15±8 °C (1 SE) for modern Cold Creek tufa is consistent with summer water temperatures (12-16 °C) measured during one year of monitoring at the site. Using this temperature, the δ¹⁸O value for this tufa (-12.3‰ VPDB) is consistent with formation from the discharging water (avg -13.5‰ VSMOW). Two phytoclast tufas in the E_2c unit yield similar clumped isotope temperatures (10±4 °C and 11±4 °C), indicating cold spring water, which only discharges at mountain elevations today, flowed in point source streams on the valley floor during the early Holocene. δ¹⁸O values for all tufas are very similar throughout the record (ranging -11.7 to -12.7‰ VPDB) suggesting similar water source and temperature. In contrast, values for cap carbonates, interpreted as forming during Dansgaard-Oeschger abrupt warming events, are much higher (-10.6 to -5.7‰ VPDB). This indicates formation from evaporated water sources, supporting desiccation of the Las Vegas Valley wetlands in response to millennial-scale dry climate intervals.