Stable isotope paleoclimatology of brine inclusions in halite: Modeling and application to Searles Lake, California

Abstract

The isotopic composition of brine inclusions in halite can be used as paleohydrologic and paleoclimatic indicators in arid environments. For this purpose, steady-state isotopic models of the brine inclusions in halite were developed for three hydrologic regimes: the “evaporation pan,” “terminal-lake,” and “through-flow lake” or “strait-exchange sea.” The model calculations predict that the steady-state isotopic compositions are sensitive to the hydrologic regime, to the normalized relative humidity, and to the isotopic compositions of atmospheric water vapor and inflow water. Short-term fluctuations of the isotopic parameters in the steady-state equations are not important, and in general only the long-term mean values are recorded in the brine inclusions in halite. Modern and old (up to 1.95 Ma) samples of nonmarine halite from Searles Lake (California) were analyzed for the chemical and isotopic compositions of the brine inclusions. From the modern surface halite, the isotopic composition of the atmospheric water (probably of the winter time) and the probable seasons when the halite samples were deposited were obtained. From brine inclusions of the 1.3 Ma halite, the annual-mean isotopic compositions of atmospheric water vapor and inflow water were calculated using the steady-state “terminal lake” model. The calculated isotopic composition of the 1.3 Ma inflow water shows a δD enrichment of about 60%. compared to the modern counterpart. The amount of this δD enrichment is similar to that found from the calcite veins at the Spring Mountain in Nevada ( Wionograd et al., 1985), which was attributed to the effect of the orogeny of the Sierra Nevada. The chemical composition of the brine inclusions indicates that the Owens River was more enriched in HCO 3 −(+CO 3 2−) 10,000 and 28,000 a ago than at present, probably due to larger contribution of Long Valley caldera hot spring waters.