Calcium isotope fractionation during the partial dissolution of artificial calcite

Abstract

We studied the calcium (Ca) isotope fractionation during the partial dissolution of artificial calcite in pure water and in artificial seawater at 20 °C. Calcite (1.00 g) was stirred in 500 ml solution while bubbling it with air rich in CO2 gas (400 and 500 ppm). Depending on the composition of the dissolving medium, the measured pH close to equilibrium decreased with increasing concentration of CO2 in air. The dissolution rate (R*; μmol/m2.h) increased with the concentration of CO2 in air. The Ca isotope composition (δ44/40Ca) of the dissolved Ca in pure water during different intervals of partial dissolution process was almost constant and very close to that of calcite (1.06‰ ± 0.11‰). This phenomenon indicated that partial dissolution occurred without Ca isotope fractionation. By contrast, the isotope composition of Ca in artificial seawater (0.92‰ ± 0.17‰) was almost constant during the partial dissolution of calcite (<10% of calcite dissolved) because the Ca isotope composition in both calcite and in the artificial seawater were close to each other and the change in [Ca2+] ions due to dissolution was not >9%.