Plant Wax Isotopic Reconstructions Reveal Thermodynamic Drivers of Hydroclimate Over the Last Two Glacial Cycles at Great Salt Lake and Bear Lake, Utah

Abstract The growth and decay of the Laurentide ice sheet altered the hydrological cycle over southwestern North America. While it is well‐documented that the last glacial was wetter and had isotopically lighter precipitation, much less information is available for prior glacials. Increased proxy coverage is needed to test climate models' ability to reconstruct these changes and to assess their predictive power for water availability in response to future climate change. Here, we present parallel precipitation isotope records spanning the last two glacial cycles from two large, proximal lakes in Utah, USA: Great Salt Lake and Bear Lake. We use plant wax n ‐alkane δD as a proxy for precipitation δD (δD precip ) and find coherent glacial‐interglacial fluctuations in δD precip , with a ∼30‰ D‐depletion during glacial maxima relative to interglacials. We find similar δD precip values between the Holocene and Eemian, but at the lower‐pCO 2 MIS 7 interglacial, D‐enrichment is only weakly recorded at Great Salt Lake and absent at higher elevation Bear Lake. Comparison to regional proxy archives finds large‐scale coherence in regional hydroclimate change over the last two glacial cycles is best explained by thermodynamic processes, with increased rainout efficiency, isotopic fractionation, and snow in a colder atmosphere. Comparison of proxies to climate model experiments showed models considerably underestimate glacial lowering of precipitation isotopic values, but overestimate inland Rayleigh distillation. New and assembled proxy reconstructions provide greater temporal and spatial coverage as targets for model skill in capturing hydroclimate variations across the past two glacial cycles.