CaCO3 Dissolution Kinetics at the Sediment-Water Interface

Thursday, 18 December 2014
Claire Lix and Alfonso Mucci, McGill University, Montreal, QC, Canada
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+][CO32-]/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.