First Insights Into Strontium Isotope Fractionation in Gypsum and Its Geochemical Implications
The geochemical cycle of strontium is intimately linked to the long-term cycle of carbon, for instance, through their mutual involvement in continental weathering and marine carbonate sedimentation. Stable strontium isotopes (δ88/86Sr) have recently emerged as a valuable tool, complementing radiogenic Sr isotope ratios (87Sr/86Sr), to further our understanding of the Sr cycle. Stable strontium isotopes are sensitive to both the sources of strontium (e.g., silicate versus carbonate weathering) and to earth surface processes (e.g., mineral precipitation from solution). Gypsum, a common evaporitic mineral, precipitates directly from seawater and holds the potential to serve as a marine archive for studying the elemental cycle of Sr. In addition, the impact of gypsum formation on the marine strontium isotope budget has been a matter of speculation due to the absence of information on the isotope fractionation of Sr in gypsum. Here, for the first time, we explore the behavior of stable strontium isotopes during gypsum precipitation and provide the first estimates of the associated isotope fractionation. Gypsum was produced through the evaporation of natural seawater in a series of experiments. Strikingly, in contrast to the fractionation of Ca isotopes into gypsum, as well as Sr isotopes into carbonate minerals, heavier isotopes of Sr are preferentially incorporated into gypsum with an estimated average isotope fractionation of ~ 0.20‰ (range between 0.14-0.27‰). The variability in the observed experimental isotope fractionation is suggested to be the result admixture of aragonite in the precipitate, surface-specific effects, and/or mineral occlusion (i.e., entrapment of unfractionated Sr). Despite these complications, gypsum has the potential to resolve large variations in past seawater δ88/86Sr, provided that there is a large set of samples available for the studied time interval, and that samples are carefully selected following the criteria established here. Furthermore, the study demonstrates that δ88/86Sr in gypsum can help to identify excess localized aragonite precipitation, serving as a proxy for increased alkalinity in the system. Mass balance considerations indicate that formation of giant evaporite deposits has the potential to induce only minimal, short-term, alterations in seawater δ88/86Sr, largely not discernible due to the uncertainties in the existing Sr isotope records. Finally, we find that weathering of 88Sr-enriched marine gypsum exposed on the continents is likely a significant source of Sr to the ocean. This Sr source can explain at least 25% of the apparent mismatch between the observed global riverine δ88/86Sr and that estimated from the distributions of carbonate and silicate lithologies only.