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Lacustrine chemical sediments of the Wilkins Peak Member, Eocene Green River Formation potentially preserve paleoclimate information relating to the conditions of their formation and preservation within the lake basin during the Early Eocene Climatic Optimum. The Green River Formation comprises the world’s largest sodium-carbonate evaporite deposit in the form of trona (Na2CO3⋅NaHCO3⋅2H2O) in the Bridger sub-basin and nahcolite (NaHCO3) in the neighboring Piceance Creek basin. Modern analogues suggest that these minerals necessitate the existence of an alkaline source water. Detrital provenance geochronometers suggest that the most likely source for volcanic waters to the Greater Green River Basin is the Colorado Mineral Belt, connected to the basin via the Aspen paleo-river. We tested the hypothesis that magmatic waters from the Colorado Mineral Belt could have supplied the Greater Green River Basin with the alkalinity needed to precipitate sodium-carbonate evaporites that are preserved in the Wilkins Peak Member by numerically simulating the evaporation of modern soda spring waters from northwestern Colorado at various temperature and atmospheric pCO2 conditions. We compare the resulting simulated evaporite sequences of the modern soda spring waters to the mineralogy preserved within the Wilkins Peak Member. Simulated evaporation of Steamboat Springs water produces the closest match to core observations and mineralogy. These simulations provide constraints on the salinities at which various minerals precipitated in the Wilkins Peak Member as well as insights into the regional temperature (>15ºC for gaylussite and trona; >27º for pirssonite and trona) and pCO2 conditions (<1200ppm for gaylussite and trona) during the EECO. 

Mineralogy, petrographic textures, and sedimentary structures from the world’s largest trona deposit, the Wilkins Peak Member (WPM) of the early Eocene Green River Formation (GRF), Bridger subbasin, Wyoming, provide key data about depositional conditions and paleoenvironments. The 250 m-long WPM interval in the Solvay S-34-1 drill core analyzed in this study contains a detailed record of sedimentation in the Bridger subbasin at the deepest area of a hydrologically-closed basin during peak Cenozoic atmospheric CO2 concentrations. Large accumulations of trona (Na3(HCO3)(CO3)·2H2O), shortite (Na2Ca2(CO3)3), northupite (Na3Mg (CO3)2Cl), and halite (NaCl; now replaced by trona), occur in the lower half of the WPM. Modern saline lake environments such as Lake Magadi, Kenya, and the Dead Sea, Israel-Jordan, are useful analogues for interpreting paleolake conditions associated with evaporite deposition in the Solvay S-34-1 core. Solvay saline lake deposits are organized into meter-scale shallowing-upward successions, beginning with (1) oil shale overlain by (2) trona, in places interbedded with oil shale, followed by (3) peloidal dolomite grainstone and/or silty dolomitic mudstone, and (4) massive mudstone with disruption features or desiccation cracks, and/or siliciclastic sandstone with ripple cross-stratification. Based on observations of modern hypersaline lake environments, WPM evaporite deposition at the basin depocenter is interpreted to be controlled by inflow water composition and volume, evaporative concentration, and seasonally-driven lake temperature fluctuations, resulting in recurrent patterns in evaporite mineralogies and textures. 

Lacustrine evaporites have potential to document ancient terrestrial climates, including temperatures and their seasonal variations, and atmospheric pCO2. The sodium carbonate mineral nahcolite (NaHCO3) in the early Eocene Parachute Creek Member, Green River Formation, Piceance subbasin, indicates elevated pCO2 concentrations (> 680 ppm) in the water column and in the atmosphere if in contact with brine. These data support a causal connection between elevated atmospheric pCO2 and global warmth during the early Eocene Climatic Optimum. Trona (Na2CO3⋅NaHCO3⋅2H2O), not nahcolite, is the dominant sodium carbonate mineral in the coeval Wilkins Peak Member in the Bridger subbasin, which may be explained by interbasin variations in (1) brine chemistry, (2) temperature, and (3) pCO2. These interpretations are based on equilibrium thermodynamics and simulations that evaporate lake water, but they ignore seasonal changes in water column temperature and pCO2. Winter cooling, rather than evaporative concentration, best explains the fine-scale alternations of nahcolite, halite (NaCl), and nahcolite + halite in the Parachute Creek Member. Simulated evaporation of alkaline source waters from the paleo Aspen River at temperatures between 15⁰ and 27⁰ C and pCO2 at or below 1200 ppm produces the observed mineral sequence in the Wilkins Peak Member: gaylussite (Na2CO3⋅CaCO3⋅5H2O) at temperatures < 27⁰ C and pirssonite (Na2CO3⋅CaCO3⋅2H2O) > 27⁰ C (both now replaced by shortite Na2CO3·2CaCO3), then northupite (Na3Mg(CO3)2Cl), trona, and halite. The challenge of determining paleo-lake temperatures in the Bridger and Piceance subbasins using microthermometry has now been solved using femtosecond lasers that promote nucleation of vapor bubbles in brine inclusions without deforming the halite host crystal. This method shows general agreement with thermodynamic-based calculations and will be used to document mean annual temperatures in the Greater Green River Basin. 

The Early Eocene Climatic Optimum (~53-50 Ma) represents the most recent episode of sustained greenhouse climate, during which the deep oceans were as much as 12°C warmer than today. The lacustrine Wilkins Peak Member of the Green River Formation (Wyoming, USA) is one of the premier locations to study this period of global warmth due to its rich terrestrial archive of climate dynamics, biology, and geomorphology. Using radioisotopic geochronology, cyclostratigraphy, sedimentology, and geochemistry, previous studies have leveraged this extensive record to evaluate the ancient lake system’s temporal evolution and response to climate. Much prior work on Green River Formation cyclostratigraphy, including that of Alfred G. Fischer, has focused on the evaluation of oil yield, a measure of organic richness. In this study, X-Ray fluorescence (XRF) core scanning of a basin center core, Solvay S-34-1, is used to produce a high resolution (5mm), continuous, multi-proxy elemental record of the complete Wilkins Peak Member, spanning 240 meters. This new geochemical assessment is a component of a larger multidisciplinary investigation that that is underway, including new magnetostratigraphic and radioisotopic geochronology. Elemental abundances for a range of measured elements, such as Si, S, Cl, K, Ca, Ti, Fe, Zn, Br, and Rb, are interpreted in terms of evaporitic, siliciclastic, and redox-sensitive sedimentation, and show variable responses at specific Milankovitch (eccentricity, obliquity, precession) and sub-Milankovitch time scales. Using this long high-resolution geochemical dataset of the Early Eocene Climatic Optimum, we consider potential linkages between Milankovitch forcing and sub-Milankovitch forcing, and plausible non-linear transfer functions that translate the astronomical insolation signal into the stratigraphic archive.