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ABSTRACT Ancient terrestrial sediments provide critical information about the responses of continental environments to global scale climate and tectonic perturbations, which are vastly understudied relative to marine archives. The >1 km thick Sheep Pass Formation type section in east‐central Nevada preserves non‐marine carbonates, including microbialites, and lesser siliciclastics, deposited in a tectonically active, high‐elevation basin during the latest Cretaceous through to middle Eocene time, an interval spanning major global greenhouse climate states and warming events. This study combines outcrop and hand‐sample observations, thin section petrography and X‐ray diffraction mineralogical analyses to create a facies framework and interpreted environmental evolution for the Sheep Pass Basin. Together, these observations portray the Sheep Pass Formation type section as a dynamic and highly sensitive basin due to its small size. The dominance of thrombolite boundstones compared to metazoan fossils, which sets the Sheep Pass Formation type section apart from other Palaeogene‐aged lake basins in the western United States, reflects the resilience of microbial mats compared to metazoans in this dynamic setting. The major lacustrine phase of the Sheep Pass Basin records three intervals: a shallow lake with few microbialites, followed by abundant microbialites, before the transition to a marginal setting with evaporative conditions, marking the culmination of this major lacustrine phase. The transition to a microbialite‐dominated interval was likely driven by physicochemical conditions (for example, higher alkalinity), paired with lower competition from metazoan grazers. Although the Sheep Pass Formation type section preserves environmental change in response to both tectonics and climate, similar trends in facies, mineralogy and invertebrate abundance compared to other sedimentary basins from this time suggest that global climate influenced distinct environmental shifts in the Sheep Pass Basin. This work provides a detailed sedimentological framework for a new, high‐elevation palaeoclimate record during a pivotal geological climate interval. 

Abstract Lacustrine carbonates are a powerful archive of paleoenvironmental information but are susceptible to post‐depositional alteration. Microbial metabolisms can drive such alteration by changing carbonate saturationin situ, thereby driving dissolution or precipitation. The net impact these microbial processes have on the primary δ¹⁸O, δ¹³C, and Δ₄₇values of lacustrine carbonate is not fully known. We studied the evolution of microbial community structure and the porewater and sediment geochemistry in the upper ~30 cm of sediment from two shoreline sites at Green Lake, Fayetteville, NY over 2 years of seasonal sampling. We linked seasonal and depth‐based changes of porewater carbonate chemistry to microbial community composition, in situ carbon cycling (using δ¹³C values of carbonate, dissolved inorganic carbon (DIC), and organic matter), and dominant allochems and facies. We interpret that microbial processes are a dominant control on carbon cycling within the sediment, affecting porewater DIC, aqueous carbon chemistry, and carbonate carbon and clumped isotope geochemistry. Across all seasons and sites, microbial organic matter remineralization lowers the δ¹³C of the porewater DIC. Elevated carbonate saturation states in the sediment porewaters (Ω > 3) were attributed to microbes from groups capable of sulfate reduction, which were abundant in the sediment below 5 cm depth. The nearshore carbonate sediments at Green Lake are mainly composed of microbialite intraclasts/oncoids, charophytes, larger calcite crystals, and authigenic micrite—each with a different origin. Authigenic micrite is interpreted to have precipitated in situ from the supersaturated porewaters from microbial metabolism. The stable carbon isotope values (δ¹³C_carb) and clumped isotope values (Δ₄₇) of bulk carbonate sediments from the same depth horizons and site varied depending on both the sampling season and the specific location within a site, indicating localized (μm to mm) controls on carbon and clumped isotope values. Our results suggest that biological processes are a dominant control on carbon chemistry within the sedimentary subsurface of the shorelines of Green Lake, from actively forming microbialites to pore space organic matter remineralization and micrite authigenesis. A combination of biological activity, hydrologic balance, and allochem composition of the sediments set the stable carbon, oxygen, and clumped isotope signals preserved by the Green Lake carbonate sediments.