Search for: All records

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.