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1. PALEOCLIMATE RECORDS FROM EVAPORITES IN THE EOCENE GREEN RIVER FORMATION

   https://doi.org/10.1130/abs/2022AM-383105  

   Lowenstein, Tim; Arnuk, William; Klonowski, Elizabeth; Jagniecki, Elliot; Demicco, Robert V (January 2022, Geological Society of America)

   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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2. SIMULATING SOURCE WATERS FOR THE WILKINS PEAK MEMBER EVAPORITES WITH MODERN SODA SPRING ANALOGUES

   https://doi.org/10.1130/abs/2022AM-383424  

   Arnuk, William; Lowenstein, Tim K; Klonowski, Elizabeth; Carroll, Alan; Smith, Michael (October 2022, Geological Society of America)

   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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3. Searles Lake evaporite sequences: Indicators of late Pleistocene/Holocene lake temperatures, brine evolution, and p CO2

   https://doi.org/10.1130/B35857.1  

   Olson, Kristian J.; Lowenstein, Tim K. (March 2021, GSA Bulletin)

   null (Ed.)

   Searles Lake, California, was a saline-alkaline lake that deposited >25 non-clastic minerals that record the history of lake chemistry and regional climate. Here, the mineralogy and petrography from the late Pleistocene/Holocene (32−6 ka) portion of a new Searles Lake sediment core, SLAPP-SRLS17, is combined with thermodynamic models to determine the geochemical and paleoclimate conditions required to produce the observed mineral phases, sequences, and abundances. The models reveal that the primary precipitates formed by open system (i.e., fractional crystallization), whereas the early diagenetic salts formed by salinity-driven closed system back-reactions (i.e., equilibrium crystallization). For core SLAPP-SRLS17, the defining evaporite sequence trona → burkeite → halite indicates brine temperatures within a 20−29 °C range, implying thermally insulating lake depths >10 m during salt deposition. Evaporite phases reflect lake water pCO2 consistent with contemporaneous atmospheric values of ∼190−270 ppmv. However, anomalous layers of nahcolite and thenardite indicate pulses of pCO2 > 700−800 ppm, likely due to variable CO2 injection along faults. Core sedimentology indicates that Searles Lake was continuously perennial between 32 ka and 6 ka such that evaporite units reflect periods of net evaporation but never complete desiccation. Model simulations indicate that cycles of partial evaporation and dilution strongly influence long-term brine evolution by amassing certain species, particularly Cl−, that only occur in late-stage soluble salts. A model incorporating long-term brine dynamics corrects previous mass-balance anomalies and shows that the late Pleistocene/Holocene (32−6 ka) salts are partially inherited from the solutes introduced into earlier lakes going back at least 150 ka. 

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4. NOAA/WDS Paleoclimatology - Great Salt Lake GDGT Data from the Pleistocene to Present

   https://doi.org/10.25921/a4a7-8e40  

   So, R; Jagniecki, EA; Lowenstein, TK; Jost, A; Kinsley, CW; Olson, KJ; McGee, D; Tierney, JE; Feakins, SJ (January 2026, NOAA National Centers for Environmental Information)

   Great Salt Lake (GSL), Utah, USA, is a large hypersaline lake, but during short-lived (~10 – 20 kyr) pluvials, it had been an expansive freshwater lake. We update the age model of GLAD1-GSL00-4, a 120-m sediment core recovered by the Global Lakes Drilling (GLAD) project, with new U-series measurements, seeking to date evaporites representing the driest times in lake history and provide a new basal age of 236.3 ka (2s uncertainty = 6.7 kyr) during the penultimate interglacial. We measure abundances of microbial membrane lipids (glycerol dialkyl glycerol tetraethers; GDGTs) to reconstruct salinity, stratification, and aquatic productivity over the last two glacial cycles. High and variable salinity precludes use of GDGTs as a proxy for temperature. We detect freshening, increased productivity, and overturning during the “Bonneville” and “Little Valley” deep lake phases, here dated to 30.3 – 16.1 ka (2s = 0.6 – 2.6 kyr) and 140.4 – 134.6 ka (2s = 5.5 – 5.7 kyr). The salinity rise following the last glacial termination pluvial agrees with water balance reconstructions from well-dated lacustrine carbonates. However, the evaporite (thenardite) deposition dates to 12.2 – 16.1 (2s = 0.4 – 0.5 kyr), and contrasts with reports of Holocene deposition elsewhere in the basin. The same sequence of salinity increase and evaporite deposition (halite) occurs in the penultimate glacial termination. Despite disparate halite dates spanning 90 – 160 ka, age model assignment of the preceding Little Valley freshening agrees with U-series dating of shoreline tufa, highlighting cyclical lake expansion/contraction followed by evaporite deposition during glacial terminations, consistent with regional pluvials. 

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5. Fresh to Hypersaline Transitions Across Two Glacial Cycles at Great Salt Lake, Utah

   https://doi.org/10.1029/2026PA005536  

   So, Rachel T; Jagniecki, Elliot; Lowenstein, Tim K; Jost, Adam B; Kinsley, Christopher W; Olson, Kristian; McGee, David; Tierney, Jessica E; Feakins, Sarah J (August 2026, Paleoceanography and Paleoclimatology)

   Abstract Great Salt Lake (GSL), Utah, USA, is a large hypersaline lake, but during short‐lived (∼10–20 kyr) pluvials, it was an expansive freshwater lake. We update the age model of GLAD1‐GSL00‐4, a 120‐m sediment core recovered by the Global Lakes Drilling (GLAD) project, with new U‐series measurements dating evaporites representing the driest times in lake history and providing a new basal age of 236.3 ka (2σuncertainty = 6.7 kyr) during the penultimate interglacial. We measure microbial membrane lipid abundances, dialkyl glycerol diethers and glycerol dialkyl glycerol tetraethers (GDGT), to reconstruct salinity over the last two glacial cycles. High and variable salinity precludes use of GDGTs as a proxy for temperature. The freshwater, deep, expanded lake phases “Bonneville” and “Little Valley,” here dated to 30.3–16.1 ka (2σ = 1.0–1.4 kyr) and 140.4–134.6 ka (2σ = 5.5–5.7 kyr) respectively, occur during deglacial pluvials and briefly interrupt the hypersaline conditions which dominate the record. The salinity rise following the last pluvial agrees with water balance reconstructions from well‐dated lacustrine carbonates. However, evaporite (thenardite) deposition dated to 16.1–12.2 ka (2σ = 0.8–1.0 kyr) contrasts with Holocene timing elsewhere in the basin. After the Little Valley highstand, a salinity increase culminates in halite precipitation. Despite four disparate halite dates spanning 90–160 ka, age model assignment agrees with U‐series dating of Little Valley shoreline tufa. This continuous salinity record provides context for lake expansion/contraction followed by evaporite deposition across two glacial terminations, consistent with regional pluvials. 

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