Insights into stable strontium isotope fractionation in marine 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 (δ^88/86Sr) 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 limited information on the isotope fractionation of Sr in gypsum. Here, we explore the behavior of strontium isotopes during gypsum precipitation and provide the first-order 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 of admixture of aragonite in the precipitate, surface-specific rate-controlled effects, and/or mineral occlusion (i.e., capturing of unfractionated Sr). Despite these complications, gypsum has the potential to effectively resolve significant variations in past seawater δ^88/86Sr following the suggested approach, which involves careful sample selection based on radiogenic Sr data, avoidance of aragonite admixtures, and ensuring a large dataset of coeval samples. Mass balance considerations indicate that the formation of giant evaporite deposits has the potential to induce only minimal, short-term, alterations in seawater δ^88/86Sr, 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 δ^88/86Sr and that estimated from the weathering of carbonate and silicate lithologies only.