Nickel isotope fractionation as a function of carbonate growth rate during Ni coprecipitation with calcite

The fractionation of Ni isotopes during Ni coprecipitation with calcite was measured at pH = 6.2 and pCO2 = 1 atm as a function of calcite growth rate. Light Ni isotopes are preferentially incorporated into calcite during its coprecipitation, which is likely due to a longer NiO bond length in calcite compared to that of the Ni aquo complex. The extent of Ni isotope fractionation between Ni in the solid and the aqueous fluid phase increases from −0.3 to −0.9‰ as the calcite growth rate slows from 10−7.3 to 10−8..3 mol m−2 s−1. This behaviour can be attributed to the strong hydration of the Ni2+ aqueous ion. As mineral growth rates depend strongly on the degree of supersaturation of the fluid relative to the mineral, the results of this study suggest that the Ni isotopic composition of natural calcite can potentially provide insight into the saturation state of seawater with respect to calcite at the time that this mineral formed. In addition, calculations based on our results suggest that the incorporation of Ni into calcite could be a significant sink of light Ni in the ocean.