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Halite deposits have long been utilized for interrogating past climate conditions. Microthermometry on halite fluid inclusions has been used to determine ancient water temperatures. One notable obstacle in performing microthermometric measurements, however, is the lack of a vapor bubble in the single-phase liquid inclusions at room temperature. (Pseudo-) isochoric cooling of the inclusions to high negative pressures, far below the homogenization temperature, has commonly been needed to provoke spontaneous vapor bubble nucleation in the liquid. High internal tensile stress in soft host minerals like halite, however, may induce plastic deformation of the inclusion walls, resulting in a wide scatter of measured homogenization temperatures. Nucleation-assisted (NA) microthermometry, in contrast, employs single ultra-short laser pulses provided by a femtosecond laser to stimulate vapor bubble nucleation in metastable liquid inclusions slightly below the expected homogenization temperature. This technique allows for repeated vapor bubble nucleation in selected fluid inclusions without affecting the volumetric properties of the inclusions, and yields highly precise and accurate homogenization temperatures. In this study, we apply, for the first time, NA microthermometry to fluid inclusions in halite and we evaluatethe precision and accuracy of this thermometer utilizing (i) synthetic halite crystals precipitated under controlled laboratory conditions, (ii) modern natural halite that precipitated in the 1980s in the Dead Sea, and (iii) Late Pleistocene halite samples from a sediment core from Death Valley, CA. Our results demonstrate an unprecedented accuracy and precision of the method that provides a new opportunity to reconstruct reliable quantitative temperature records from evaporite archives. 

Lacustrine halite deposits have long been utilized for interrogating past climate conditions. In particular, microthermometry performed on fluid inclusions in halite crystals has been used to interpret lake water temperatures from ancient deposits. One notable obstacle in performing microthermometry in halite fluid inclusions is the lack of a vapour bubble in the single-phase liquid brine. Isochoric cooling of the inclusions to high negative pressures far below the homogenization temperature has commonly been used to provoke spontaneous vapor bubble nucleation in the metastable liquid. In a host minerals like halite, however, internal tensile stress may result in plastic deformation of the inclusion walls and typically a wide scatter of measured homogenization temperatures. Nucleation-assisted microthermometry, in contrast, employs single ultra-short laser pulses provided by a femtosecond laser to stimulate vapour bubble nucleation in metastable single-phase liquid inclusions slightly below the expected homogenization temperature. This technique allows for repeated vapour bubble nucleation in fluid inclusions without damaging the inclusion walls, yielding highly precise and accurate paleotemperatures from halite fluid inclusions. Moreover, the highly selective nature of nucleation-assisted microthermometry allows for a higher degree of quality control compared to the previous standard method. In this study, we tested the precision and accuracy of nucleation-assisted microthermometry for use in paleoclimate reconstruction utilizing modern halites precipitated in the laboratory under controlled and monitored conditions, Pleistocene halite samples from Death Valley, and varved halites precipitated in the 1980s in the Dead Sea. 

Femtosecond lasers, fired in short pulses, can induce bubble cavitation in single-phase liquid fluid inclusions at temperatures near the inclusion homogenization temperature. By coupling femtosecond laser-induced cavitation with microthermometry, paleotemperatures can be extracted from fluid inclusions in primary halite crystals. This technique minimizes plastic deformation of halite by not subjecting samples to extreme temperatures during vapor bubble nucleation. The resultant homogenization temperatures are precise and reproducible. We applied this technique to Eocene Green River Formation primary bottom-growth halites from the Piceance Creek Basin in Colorado. Samples from the basin-center Savage 24-1 core yield average bottom water temperatures of 27.0 ± 1.3 ºC and 20.1 ± 1.2 ºC for the Upper and Lower Salt intervals, respectively. Average bottom water temperatures from modern perennial hypersaline lakes have been shown to reflect the local mean annual air temperature. Therefore, homogenization temperatures from primary bottom-growth halite fluid inclusions are a proxy for mean annual air temperature. These results agree with regional Early Eocene mean annual air temperature estimate ranges from other mineralogical and biochemical proxies, bolstering the reliability of temperature estimates obtained using this technique. Additionally, the highly selective nature of laser induced cavitation microthermometry allows for a higher degree of quality control compared to standard microthermometry, yielding more reproducible and precise results. 

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. 

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.