Reassessing the dissolution of marine carbonates: I. Solubility

We studied the solubility of the [63-150 μm] and the greater than 150 μm size fractions of sediments from two bathymetric transects in the eastern tropical Atlantic (Sierra Leone rise and Cape Verde Plateau). Both fractions are made mainly of foraminiferal shells and fragments. We determined the calcite crystallinity (full width at half maximum of XRD (104) calcite peak) of the >150 μm size fraction. Equilibration experiments were carried out in artificial seawater (20 °C, pCO2=3100 ppm) for up to 57 days starting from undersaturation with respect to calcite and supersaturation with respect to aragonite. Experiments starting from supersaturation yielded concentration products close to aragonite solubility for sediments from the shallowest stations, suggesting the presence of trace levels of aragonite in these samples. Concentration products computed for the deeper stations were intermediate between aragonite and calcite solubility. Our results indicate the formation of a high-Mg coating. The equilibration period was too short to allow the complete recrystallization of these Mg-rich overgrowths. Experiments initiated from undersaturation yield concentration products that are between 4% and 24% higher than the reported stoichiometric concentration product of synthetic calcite. These differences between estimates of calcite stoichiometric solubility products are explained in terms of variations in experimental conditions (artificial versus natural seawater) and related choices of carbonic acid dissociation constants. They do not reflect a true difference in solubility between biogenic and synthetic calcite. The thinning of the foraminiferal calcite (104) XRD peak from 0.168°(2θ) to 0.148°(2θ) along the depth transects is interpreted as reflecting an improvement in calcite crystallinity. This and the change in specific surface area are consistent with the progressive change of the carbonate assemblage. The evolution of the bulk composition of the carbonate fraction is not paralleled by a significant change in its stoichiometric concentration product. It reflects ongoing differential dissolution due to kinetic effects.