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

In order to apply Sr/Ca and Ca-44/Ca-40 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 degrees C. Results show that the discrimination of Sr2+ versus Ca2+ during calcite formation is mainly controlled by precipitation rate (R in mu mol/m(2)/h) and is weakly influenced by ionic strength/salinity. In analogy to Sr incorporation, Ca-44/Ca-40 fractionation during precipitation of calcite is weakly influenced by ionic strength/salinity too. At 25 degrees C the calcium isotope fractionation between calcite and aqueous calcium ions (Delta Ca-44/40(calcite-aq) = delta Ca-44/40(calcite) = delta Ca-44/40(aq)) correlates inversely to log R values for all experiments. In addition, an inverse relationship between Delta Ca-44/40(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 Delta Ca-44/40(calcite-aq) relationship, Sr/Ca and delta Ca-44/40(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. (C) 2011 Elsevier Ltd. All rights reserved.