Calcite dissolution kinetics at the sediment-water interface in natural seawater

The absorption, or uptake, of anthropogenic CO2 by the oceans results in a decrease in pH and carbonate ion concentration, [CO32 −]; as a consequence, the saturation state of seawater with respect to CaCO3 minerals (calcite, aragonite) falls, leading to a shallowing of their saturation depths and triggering an increase in their dissolution at the seafloor. Nearly one-third of the seabed is composed of CaCO3-rich sediments, and their dissolution is the ultimate marine sink of anthropogenic CO2. Despite numerous past studies, much confusion and uncertainty still surround our understanding of the rates and kinetics of CaCO3 dissolution at the deep seafloor. Results from in situ studies disagree with laboratory studies, most of which have been carried out under conditions, e.g., mineral suspensions, that are not representative of processes at the seafloor. Herein, we report measurements of the dissolution rate of calcite, formed into synthetic sediment disks by mixing various amounts of this mineral with montmorillonite. These disks were placed in a stirred-flow reactor and exposed to a range of saturation states and shear stress conditions to simulate conditions at the sediment-water interface. The dissolution rates, normalized to the interfacial area of the sediment disks, were linearly dependent on the undersaturation state of the experimental seawater solution and displayed a square-root dependence on the calcite content, under both quiescent and stirred conditions. The rate of release of reaction products from the sediment increased with stirring rate, i.e., shear stress, until it became invariant at higher stirring rates. This latter result argues that calcite dissolution is transport (water-side) controlled for shear stress levels known to exist at the seafloor, which advises a simpler kinetic description of benthic calcite dissolution.