Utah contains unique analogues of microbial hydrocarbon reservoirs in the modern Great Salt Lake and the lacustrine Eocene Green River Formation within the Uinta Basin. Characteristics of both lake environments include shallow-water, ramp margins that are susceptible to rapid widespread shoreline changes, as well as comparable water chemistry and temperature that were ideal for microbial growth and formation/deposition of associated carbonate grains.

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In light of a modern understanding of early Eocene greenhouse climate fluctuations and new highly seasonal fluvial system faces models, the role of climate in the evolution of one classically-cited continental, terminal lake system is re-examined. Detailed stratigraphic description and elemental abundance data from fifteen cores and seven outcrop regions of the Green River Formation were used to construct a ∼150 km cross section across the Uinta Basin, Utah, USA. Lake Uinta in the Uinta Basin is divided into five lake phases: (1) post-Paleocene Eocene Thermal Maximum, (2) peak Eocene hyperthermal, (3) waning hyperthermal, Early Eocene Climatic Optimum (EECO), (4) post-hyperthermal, and (5) post-EECO regimes, based primarily on climatically driven changes in fluvial style in combination with sedimentary indicators of lacustrine carbonate deposition, organic matter preservation, salinity, and lake depth. Basinwide siliciclastic dominated intervals were deposited by highly seasonal fluvial systems and record negative organic carbon isotope excursions associated with early Eocene abrupt, transient global warming (hyperthermal) events. Carbonate dominated or organic rich intervals record stable, less seasonal climate periods between hyperthermals, with lower siliciclastic sediment supply allowing the development of carbonate and organic matter preservation. The stratigraphic progression from alternating organic rich and lean zones to the overlying organic rich Mahogany and R8 zones represents the global transition out of the pulsed early Eocene hyperthermal climate regime to a time of sediment starvation and lake stratification, sequestering sedimentary organic carbon. This study provides a novel approach to terrestrial paleoclimate reconstruction that relies largely on unique sedimentary indicators and secondarily on isotopic proxy records within the context of a large basin-wide sedimentologic and stratigraphic data set, thus setting the stage for future detailed geochemical terrestrial paleoclimate proxy development.

Extensive lacustrine microbialite deposits exposed along the shores of Great Salt Lake (GSL), Utah preserve a rich continental paleoenvironmental record. Newly‐reported microbialite carbon and oxygen stable isotope ratios in carbonate, nitrogen isotope ratios in organic matter, and organic matter radiocarbon ages archive paleolake hydrological and biogeochemical changes from the late Pleistocene through the Holocene. Positive correlations between δ18O and δ13C in ∼15 – 7.6 cal ka microbialite carbonate are consistent with a hydrologically closed‐basin lake with fluctuations in volume, chemistry, and associated changes in lake primary production. The δ15N of microbialite bulk organic matter (5 – 18 ‰ versus AIR) shows that the balance between nitrogen fixation and assimilation of dissolved inorganic nitrogen has varied significantly. Inverse δ18O and δ13C correlations in combination with high δ15N in some carbonate deposits may imply periods of higher salinity and stable lake stratification similar to modern GSL conditions. We compare our C and O data sets with Pleistocene Lake Bonneville carbonate stable isotope records and demonstrate progressive development of spatially‐isolated hydrological basins during the shift to warmer and drier conditions in the Holocene.

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