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. 

Abstract The growth and decay of the Laurentide ice sheet altered the hydrological cycle over southwestern North America. While it is well‐documented that the last glacial was wetter and had isotopically lighter precipitation, much less information is available for prior glacials. Increased proxy coverage is needed to test climate models' ability to reconstruct these changes and to assess their predictive power for water availability in response to future climate change. Here, we present parallel precipitation isotope records spanning the last two glacial cycles from two large, proximal lakes in Utah, USA: Great Salt Lake and Bear Lake. We use plant waxn‐alkane δD as a proxy for precipitation δD (δD_precip) and find coherent glacial‐interglacial fluctuations in δD_precip, with a ∼30‰ D‐depletion during glacial maxima relative to interglacials. We find similar δD_precipvalues between the Holocene and Eemian, but at the lower‐pCO₂MIS 7 interglacial, D‐enrichment is only weakly recorded at Great Salt Lake and absent at higher elevation Bear Lake. Comparison to regional proxy archives finds large‐scale coherence in regional hydroclimate change over the last two glacial cycles is best explained by thermodynamic processes, with increased rainout efficiency, isotopic fractionation, and snow in a colder atmosphere. Comparison of proxies to climate model experiments showed models considerably underestimate glacial lowering of precipitation isotopic values, but overestimate inland Rayleigh distillation. New and assembled proxy reconstructions provide greater temporal and spatial coverage as targets for model skill in capturing hydroclimate variations across the past two glacial cycles. 

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. 

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.