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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.
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PALEOCLIMATE RECORDS FROM EVAPORITES IN THE EOCENE GREEN RIVER FORMATION
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.
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- Award ID(s):
- 1812741
- PAR ID:
- 10554274
- Publisher / Repository:
- Geological Society of America
- Date Published:
- Format(s):
- Medium: X
- Location:
- Denver
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/10.1130/abs/2022AM-383105
