New insights into the kinetics of calcium carbonate dissolution in seawater and a way forward for CO₂ mitigation.

Near-equilibrium dissolution of calcite in seawater contributes significantly to the regulation of atmospheric CO₂ on thousand-year timescales. Despite the large number of studies on far-from equilibrium dissolution, and considerable work in dilute solutions, little is known about the detailed mechanisms responsible for dissolution in seawater. In this talk, we use an isotope tracer-based approach to measure dissolution rates across a range of saturation states. We show that the enzyme carbonic anhydrase increases the dissolution rate by almost 2.5 orders of magnitude, and the effect is most pronounced close to equilibrium. A model of calcite surface speciation indicates that carbonate ion sites on the calcite surface are responsible for most of the dissolution enhancement. Using isotopic information and a simple box model, we separate our measured net dissolution into gross dissolution and precipitation fluxes. We show that the net dissolution rate is actually the difference between two very large gross fluxes. Processes that can alter one of these large gross fluxes have the ability to significantly change the near-equilibrium reactivity of calcite in seawater. Our finding suggests that carbonic anhydrase might also be an important tool in forging a new way to store anthropogenic CO₂ in the ocean, by increasing the rate at which CO₂ is naturally buffered by reaction with CaCO₃.