Sr(2+)/Ca(2+) and (44)Ca/(40)Ca fractionation during inorganic calcite formation: III. Impact of salinity/ionic strength

In order to apply Sr/Ca and (44)Ca/(40)Ca fractionation during calcium carbonate (CaCO(3)) formation as a proxy to reconstruct paleo-environments, it is essential to evaluate the impact of various environmental factors. In this study, a CO(2) diffusion technique was used to crystallize inorganic calcite from aqueous solutions at different ionic strength/salinity by the addition of NaCl at 25 °C. Results show that the discrimination of Sr(2+) versus Ca(2+) during calcite formation is mainly controlled by precipitation rate (R in μmol/m(2)/h) and is weakly influenced by ionic strength/salinity. In analogy to Sr incorporation, (44)Ca/(40)Ca fractionation during precipitation of calcite is weakly influenced by ionic strength/salinity too. At 25 °C the calcium isotope fractionation between calcite and aqueous calcium ions (Δ(44/40)Ca(calcite-aq) = δ(44/40)Ca(calcite) − δ(44/40)Ca(aq)) correlates inversely to log R values for all experiments. In addition, an inverse relationship between Δ(44/40)Ca(calcite-aq) and log D(Sr), which is independent of temperature, precipitation rate, and aqueous (Sr/Ca)(aq) ratio, is not affected by ionic strength/salinity either. Considering the log D(Sr) and Δ(44/40)Ca(calcite-aq) relationship, Sr/Ca and δ(44/40)Ca(calcite) values of precipitated calcite can be used as an excellent multi-proxy approach to reconstruct environmental conditions (e.g., temperature, precipitation rate) of calcite growth and diagenetic alteration.