SALINITY AFFECTS OXYGEN ISOTOPE FRACTIONATION DURING INORGANIC CALCITE PRECIPITATION

During precipitation of carbonate minerals from aqueous solution, oxygen isotopes partition between the co-existing phases, CaCO₃ and H₂O. If crystals grow slowly, near equilibrium, then isotope partitioning depends primarily on temperature. In many cases, however, isotopic exchange between phases does not reach equilibrium, and mass-dependent transport and reaction rates contribute to the isotopic fractionations. For oxygen isotopes in carbonates, these kinetic isotope effects (KIE) manifest in a variety of ways, including a dependence of oxygen isotopic fractionation on crystal growth rate, speciation of the dissolved inorganic carbon (DIC) and solution pH. In this study, we test whether the presence of other ions in solution influence surface reaction-controlled KIE in calcite.

We synthesized inorganic calcite in the presence of dissolved carbonic anhydrase (CA) and variable salinity (NaCl). The CA promotes isotopic equilibrium between DIC and H₂O such that any KIE recorded by the crystal can reasonably be attributed to processes operating at or near the mineral-solution interface. Experiments were carried out at constant temperature (25°C), constant pH (8.3), and salinities ranging from 5 to 45 g/kg. During calcite growth, the degree of supersaturation (W = [Ca²⁺][CO₃^2-]/K_sp) is typically 5±2 in our experiments.

Oxygen isotope fractionation between calcite and water varies systematically with salinity, spanning a 3‰ range between freshwater (5 g/kg) and saline water (45 g/kg). Saltier solutions result in calcite that is isotopically lighter. Since the presence of other ions in solution can affect the mass-dependent desolvation kinetics of CO₃^2- ions, we postulate that increasing salinity (or ionic strength) increases the ion attachment rate more than the ion detachment rate, thereby pushing the system further from isotopic equilibrium. The implications of our experimental results for paleoclimate reconstructions will be discussed.