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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. 

Abstract Ooids are a common type of carbonate sand grain that form through a combination of constructive and destructive mechanisms: growth via precipitation and diminution via physical abrasion. Because growth and abrasion obey distinct morphometric rules, we developed an approach to quantitatively constrain the history of growth and abrasion of individual ooid grains using the record of evolving particle shape preserved by their cortical layers. We designed a model to simulate >10⁶possible growth‐abrasion histories for each pair of cortical layer bounding surfaces in an individual ooid. Estimates for the durations of growth and abrasion of each cortical layer were obtained by identifying the simulated history that best fit the observed particle shape. We applied this approach to thin sections of “modern” lacustrine ooids collected from several locations in the Great Salt Lake (GSL), UT, to assess the spatial and temporal variability of environmental conditions from the perspective of individual grains within a single deposit. We found that GSL ooids do not all share the same histories: Clustering ooid histories by a Fréchet distance metric revealed commonalities between grains found together locally within a deposit but distinct differences between subpopulations shared among localities across the GSL. These results support the tacit view that carbonate sedimentary grains found together in the environment do reflect a common history of sediment transport. This general approach to invert ooid cortical stratigraphy can be applied to characterize environmental variability over <1,000 year timescales in both marine and lacustrine ooid grainstones of any geologic age. 

Abstract Organic and inorganic stable isotopes of lacustrine carbonate sediments are commonly used in reconstructions of ancient terrestrial ecosystems and environments. Microbial activity and local hydrological inputs can alter porewater chemistry (e.g., pH, alkalinity) and isotopic composition (e.g., δ¹⁸O_water, δ¹³C_DIC), which in turn has the potential to impact the stable isotopic compositions recorded and preserved in lithified carbonate. The fingerprint these syngenetic processes have on lacustrine carbonate facies is yet unknown, however, and thus, reconstructions based on stable isotopes may misinterpret diagenetic records as broader climate signals. Here, we characterize geochemical and stable isotopic variability of carbonate minerals, organic matter, and water within one modern lake that has known microbial influences (e.g., microbial mats and microbialite carbonate) and combine these data with the context provided by 16S rRNA amplicon sequencing community profiles. Specifically, we measure oxygen, carbon, and clumped isotopic compositions of carbonate sediments (δ¹⁸O_carb, δ¹³C_carb, ∆₄₇), as well as carbon isotopic compositions of bulk organic matter (δ¹³C_org) and dissolved inorganic carbon (DIC; δ¹³C_DIC) of lake and porewater in Great Salt Lake, Utah from five sites and three seasons. We find that facies equivalent to ooid grainstones provide time‐averaged records of lake chemistry that reflect minimal alteration by microbial activity, whereas microbialite, intraclasts, and carbonate mud show greater alteration by local microbial influence and hydrology. Further, we find at least one occurrence of ∆₄₇isotopic disequilibrium likely driven by local microbial metabolism during authigenic carbonate precipitation. The remainder of the carbonate materials (primarily ooids, grain coatings, mud, and intraclasts) yield clumped isotope temperatures (T(∆₄₇)), δ¹⁸O_carb, and calculated δ¹⁸O_waterin isotopic equilibrium with ambient water and temperature at the time and site of carbonate precipitation. Our findings suggest that it is possible and necessary to leverage diverse carbonate facies across one sedimentary horizon to reconstruct regional hydroclimate and evaporation–precipitation balance, as well as identify microbially mediated carbonate formation.