Triple oxygen isotope constraints on the hydroclimate that sustained Pleistocene Lake Bonneville (Utah, USA)

Modern lake triple oxygen isotopes (17O-excess [Δ′17O], δ18O) can be used to distinguish flow-through from closed-basin systems and to quantitatively model precipitation composition and evaporative loss. Extending this framework to paleolake carbonates enables new hydrologic and climate reconstructions. For example, regional hydrologic models suggest that cold temperatures and reduced evaporation, not increased precipitation, sustained large Last Glacial Maximum (LGM) lakes. But this has not been tested with proxies sensitive to lake hydrology. We quantitatively assessed changing evaporation and temperature conditions at Pleistocene Lake Bonneville (Utah, USA), the LGM counterpart of the Great Salt Lake, using Δ′17O and clumped isotope (Δ47) analyses of modern lake water and lacustrine carbonates. We measured the Δʹ17O, δ18O, and Δ47 of 26−18 ka Lake Bonneville gastropods, tufa, and deep-lake carbonate. Δ47 values consistently indicate paleolake waters 6−12 °C below modern. Δ′17O−δ18O data place Lake Bonneville with modern flow-through lakes and clearly separate it from evaporative systems like the Great Salt Lake. Combined Δ′17O and Δ47 analyses robustly characterize paleolake hydrology and provide new proxy-based support for the lower-temperature, lower-evaporation hypothesis. We find via Δ′17O−based reconstruction of meteoric δ18O that LGM precipitation had higher Δ′17O values than modern. One explanation is that regional LGM moisture was sourced from higher latitudes, as a southward westerlies shift would drive ice-adjacent air masses toward the lake.