Catalysis and Chemical Mechanisms of Calcite Dissolution in Seawater
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, little is known about the detailed mechanisms responsible for dissolution in seawater. In this talk, I will show results from an isotope tracer-based approach to measure dissolution rates across a range of saturation states. The most surprising result from these experiments is that the enzyme carbonic anhydrase (CA) increases the dissolution rate by almost 2.5 orders of magnitude, and the effect is most pronounced close to equilibrium. CA can be a catalyst for carbonate dissolution through several different potential reaction mechanisms, not only equilibrating CO2 and H2 CO3, but also through its catalytic protolysis of water. Speciation modeling leads us to believe carbonate ion sites are significantly more complexed in seawater than freshwater, and could play a major role in the chemical dissolution mechanism in seawater. 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 or both 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 increases the ratio of gross dissolution to precipitation such that the net rate of dissolution is significantly enhanced. Furthermore, this result has major implications for natural biogeochemical cycling of calcite in the oceans, such as the influence of carbonic acid concentration on calcite dissolution in surface waters, day-night cycling of alkalinity on reef systems, and the saturation state distribution in the Pacific Ocean.