The Control of Calcite Dissolution Kinetics by the Major Ion Composition of Seawater
In this work we demonstrate that the effect of changing [SO4] in seawater is different near and far from equilibrium. Using purely labeled 13C of CaCO3 makes the evolution of 13/12 of total DIC a sensitive measure of dissolution rates in the lab. We create synthetic seawater solutions of 0, 14, and 28 mM in sulfate. Close to equilibrium the low sulfur solutions show decreased dissolution rates by two orders of magnitude, while dissolution rates in the same solutions far from equilibrium are enhanced. The conventional wisdom that SO4 inhibits dissolution only applies to far from equilibrium conditions. We explain this dichotomy using a speciation model of the calcite surface that more strongly bonds CO3 than SO4. Near equilibrium, with the correct surface activities of the key dissolution sites, our data are consistent with the chemical mechanism seen in fresh water by Arakaki and Mucci. Some preliminary work with changing [Mg] will also be discussed.