Temperature dependence of calcite dissolution kinetics in seawater

Knowledge of the mechanism of calcite dissolution in seawater is a critical component of our understanding of the changing global carbon budget. Towards this goal, we provide the first measurements of the temperature dependence of seawater calcite dissolution kinetics. We measured the dissolution rates of 13C-labeled calcite in seawater at 5, 12, 21, and 37 °C across the full range of saturation states 0<Ω=Ca2+[CO32-]Ksp'<1. We show that the dissolution rate is non-linearly dependent on Ω and that the degree of non-linearity both increases with temperature, and changes abruptly at “critical” saturation states (Ωcrit). The traditional exponential rate law most often utilized in the oceanographic community, R = k(1 − Ω)n, requires different fits to k and n depending upon the degree of undersaturation. Though we calculate a similar activation energy to other studies far from equilibrium (25 ± 2 kJ/mol), the exponential rate law could not be used to mechanistically explain our near equilibrium results. We turn to an alternative framework, derived from crystal nucleation theory, and find that our results are consistent with calcite dissolution kinetics in seawater being set by the retreat of pre-existing edges/steps from Ω = 1–0.9, defect-assisted etch pit formation from Ω = 0.9–0.75, and finally homogenous etch pit formation from Ω = 0.75–0. The Ωcrits for each mechanism are shifted significantly closer to equilibrium than they occur in dilute solutions, such that ocean acidification may cause marine carbonates to enter faster dissolution regimes more readily than would be expected from previous studies. We use the observed temperature dependence for each dissolution mechanism to calculate step kinetic coefficients (β, cm/s), densities of active nucleation sites (ns, sites/m2), and step edge free energies (α, mJ/m2). Homogenous dissolution is well explained within the surface nucleation framework, but defect-assisted dissolution is not. Dissolution is initiated via step-propagation at all temperatures, but the defect-assisted mechanism is skipped over at 5 °C, potentially due to a lack of nucleation sites. The surface nucleation framework enhances our understanding of calcite dissolution in seawater, but our results suggest that a complete theory will also need to incorporate the role of solution/surface speciation and complexation.