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

In order to apply Sr/Ca and ⁴⁴Ca/ ⁴⁰Ca 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, 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 (Δ ^44/40Ca _calcite-aq=δ ^44/40Ca _calcite-δ ^44/40Ca _aq) correlates inversely to logR values for all experiments. In addition, an inverse relationship between Δ ^44/40Ca _calcite-aq and logD _Sr, which is independent of temperature, precipitation rate, and aqueous (Sr/Ca) _aq ratio, is not affected by ionic strength/salinity either. Considering the logD _Sr and Δ ^44/40Ca _calcite-aq relationship, Sr/Ca and δ ^44/40Ca _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.