INVESTIGATING LATE PLEISTOCENE STABLE ISOTOPE SYSTEMATICS OF LAKES IN THE WESTERN UNITED STATES

The stable isotopic composition of lake sediments and modern lake waters reflect the hydrologic conditions of a lake basin. Lake sediments and shoreline features in the western United States record significant Pleistocene hydroclimate variability on glacial-interglacial and stadial-interstadial timescales. However, interpreting such lake records as an integrator of precipitation minus evapotranspiration is complex. For example, hydrologic integration of large multi-basin watersheds requires a nuanced interpretation of lake level curves from the largest Pleistocene lakes, such as Lake Lahontan and Lake Bonneville; many small terminal lakes in the Great Basin are too small for continuous sedimentation over glacial and interglacial periods; and fill-and-spill lake systems such as Bear Lake and Lake Bonneville, the Mojave River system, and the Owens Valley-Searles Lake-Panamint-Death Valley system integrate processes such as variable water residence times and are influenced by important geomorphic controls on sill height as well as sedimentation controls on basin hypsometry.

To address such complex relationships we build on two case studies. First, we present new water stable isotope data measurements from Bear Lake (UT/ID) and the surrounding watersheds covering base flow (late summer) and peak flow (spring) conditions. We develop a new hydrologic mass balance model, validate it against independent evaporation estimates and mass balance constraints, and apply the model to better interpret the environmental conditions inferred from previously measured carbonate stable isotope records covering the last ~200 kyrs. Second, we present a new carbonate clumped isotope measurements from Owens Valley, CA covering MIS 5e to 4. We find that open-lake conditions correspond to warmer carbonate formation temperatures, which we propose record surface lake conditions during periods of fill-and-spill. During closed-basin conditions characterized by elevated trace elements and evaporatively enriched δ¹⁸O, we find cooler carbonate clumped isotope temperatures, suggesting that calcite saturation extended deeper into the water column. Observations from this work provide a more nuanced understanding of lake balance as recorded by lakes in the western United States and make such archives more robust to comparison with other comparable proxy time series.