Reassessing the dissolution of marine carbonates: II. Reaction kinetics

We studied dissolution kinetics of the carbonate fraction >150 μm of sediments sampled along two bathymetric transects in the eastern tropical Atlantic: the Sierra Leone Rise (SLR) and the Cape Verde Plateau (CVP). The reaction was followed by monitoring solution pH during freedrift experiments lasting between 46 and 50 h (20 °C, pCO2≈3100 ppm and 1 atm pressure). The alkalinity reached at the end of the dissolution experiments ranged between 2.444 and 2.798 meq/kgsw. The dissolution time series was extrapolated to equilibrium by fitting an empirical relation to the data. The estimated asymptotic concentration products ([Ca2+]∞×[CO32−]∞, for t→∞ and dAc/dt=0) range from 4.27×10−7 to 6.77×10−7 mol2/kgsw2. These asymptotic concentration products are comparable with the stoichiometric concentration product of aragonite (6.56×10−7 mol2/kgsw2) and calcite (4.37 (±0.22)×10−7 mol2/kgsw2) derived for the same sediment material during long-term equilibration experiments. They are indicative of the presence of trace amounts of a higher solubility carbonate phase in sediments of the shallow stations (SLR station A, 2637 m; CVP station M, 3104 m). While it is likely that this phase is aragonite, the presence of authigenic carbonate precipitated in contact with supersaturated bottom waters cannot be excluded. Calcite is the main dissolving carbonate mineral in sediments from deeper stations. The order of reaction is always greater than unity. It varies between 1.4 (SLR station C) and 2.8 (CVP station M2), with an average n=2.3±0.4. The higher order reaction is explained in terms of a multiphase system. Specific rate constants range from 0.09 to 0.53 meq/m2/d.