CaCO3 Dissolution Kinetics at the Sediment-Water Interface

Abstract:

The oceans have absorbed approximately one-third of the anthropogenic CO₂ released to the atmosphere since the beginning of the industrial revolution. Consequently, over the last century, the pH of the ocean surface has decreased by an estimated 0.1 units. If atmospheric CO₂ emissions continue unabated, it is estimated that the pH of the surface ocean will decrease by an additional 0.3-0.4 pH units by the end of this century, increasing the ocean’s hydrogen ion (H⁺) concentration by about 150% relative to beginning of the industrial era.

The dissolution of CaCO₃-rich sediments (> 30% w/w) serves as the ultimate sink of the anthropogenic CO₂ delivered to the deep ocean. Nevertheless, factors that control the kinetics of the CaCO₃ dissolution reaction under conditions encountered at the sediment-water interface have not been properly parameterized. We investigated the dissolution kinetics of a simulated, organic-free, calcite-rich sediment in seawater at 25°C and one atmosphere total pressure under hydrodynamic conditions that might be representative of those at the seafloor. Dissolution rates were measured in a flow-through system at various undersaturations (1-Ω; where Ω = [Ca²⁺][CO₃^2-]/K*_cand K*_c is the stoichiometric solubility constant of calcite) under well-controlled hydrodynamic conditions.

Preliminary results reveal that interface area-normalized CaCO₃ dissolution rates of pure calcite powder and the simulated sediment in quiescent seawater are identical within our experimental uncertainties (15%), and are a non-linear function of the overlying seawater undersaturation (1-Ω). These results will be compared with those obtained when the simulated-sediment interface is subjected to increasing but uniform shear stresses.