The Control of Calcite Dissolution Kinetics by the Major Ion Composition of Seawater

John Naviaux, California Institute of Technology, Pasadena, CA, United States, Jess F Adkins, California Institute of Technology, Geological and Planetary Sciences, Pasadena, CA, United States, Haoyu Li, California Institute of Technology, United States, Nick Rollins, University of Southern California, Los Angeles, CA, United States and William Berelson, University of Southern California, Department of Earth Sciences, Los Angeles, CA, United States
Calcite dissolution kinetics have historically been fit using the empirical equation: R = k(1-omega)^n. Here, k is the rate constant, omega is the saturation state, and n is a reaction order that varies from ~1 in low ionic strength to ~3-4.5 in seawater. The simplicity of this equation has led to its widespread use in oceanography, but this simplicity comes at the cost of mechanistic interpretability. Calcite dissolution kinetics may be broadly broken down into four interrelated pieces: 1) the thermodynamic driving force of the solution (omega), 2) the chemical speciation of the solution, 3) the chemical speciation of the mineral surface, and 4) the active surface dissolution mechanism (i.e. whether dissolution is dominated by the retreat of pre-existing steps, the formation of etch pits at defects, or the formation of etch pits homogeneously across the mineral surface). We distinguish between each aspect in our interpretation, but their effects on the overall dissolution rate are intertwined.

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.