Sr2+/Ca2+ and 44Ca/40Ca fractionation during inorganic calcite formation: III. Impact of salinity/ionic strength

In order to apply Sr/Ca and 44Ca/40Ca fractionation during calcium carbonate (CaCO3) formation as a proxy to reconstruct paleo-environments, it is essential to evaluate the impact of various environmental factors. In this study, a CO2 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 Sr2+ versus Ca2+ during calcite formation is mainly controlled by precipitation rate (R in μmol/m2/h) and is weakly influenced by ionic strength/salinity. In analogy to Sr incorporation, 44Ca/40Ca 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 (Δ44/40Cacalcite-aq = δ44/40Cacalcite − δ44/40Caaq) correlates inversely to log R values for all experiments. In addition, an inverse relationship between Δ44/40Cacalcite-aq and log DSr, which is independent of temperature, precipitation rate, and aqueous (Sr/Ca)aq ratio, is not affected by ionic strength/salinity either. Considering the log DSr and Δ44/40Cacalcite-aq relationship, Sr/Ca and δ44/40Cacalcite 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.