Catalysis and chemical mechanisms of calcite dissolution in seawater

The experimental system described here provides constraints on the relative balance of gross dissolution and precipitation fluxes contributing to the observed net dissolution rate of calcite in seawater. We show that our dissolution rates fit well within a framework that accounts for the geometry of the dissolving mineral surface. We further show that carbonic anhydrase (CA) catalyzes calcite dissolution, which implicates the hydration of aqueous ${\mathrm{CO}}_2$ as a rate-limiting step for calcite dissolution in seawater. The presence of carbonic anhydrase in carbonate-rich environments such as coral reefs or sinking marine particles is poorly understood. However, our findings suggest that CA activity would significantly enhance the rate at which alkalinity is cycled between solids and seawater in these environments.

Near-equilibrium calcite dissolution in seawater contributes significantly to the regulation of atmospheric ${\mathrm{CO}}_2$ on 1,000-y timescales. Despite many studies on far-from-equilibrium dissolution, little is known about the detailed mechanisms responsible for calcite dissolution in seawater. In this paper, we dissolve ¹³C-labeled calcites in natural seawater. We show that the time-evolving enrichment of ${𝜹}^{13}\mathrm{C}$ in solution is a direct measure of both dissolution and precipitation reactions across a large range of saturation states. Secondary Ion Mass Spectrometer profiles into the ¹³C-labeled solids confirm the presence of precipitated material even in undersaturated conditions. The close balance of precipitation and dissolution near equilibrium can alter the chemical composition of calcite deeper than one monolayer into the crystal. This balance of dissolution–precipitation shifts significantly toward a dissolution-dominated mechanism below about $\mathrm{\Omega }=\mathrm{\hspace{0.17em}0.7}$. Finally, we show that the enzyme carbonic anhydrase (CA) increases the dissolution rate across all saturation states, and the effect is most pronounced close to equilibrium. This finding suggests that the rate of hydration of ${\mathrm{CO}}_2$ is a rate-limiting step for calcite dissolution in seawater. We then interpret our dissolution data in a framework that incorporates both solution chemistry and geometric constraints on the calcite solid. Near equilibrium, this framework demonstrates a lowered free energy barrier at the solid–solution interface in the presence of CA. This framework also indicates a significant change in dissolution mechanism at $\mathrm{\Omega }=\mathrm{\hspace{0.17em}0.7}$, which we interpret as the onset of homogeneous etch pit nucleation.