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Laboratory experiments, CaCO3

The quantification of in-situ carbonate dissolution rates relies on an accurate parameterisation of carbonate dissolution kinetics, which implies an improved knowledge of the rate constant and the solubility product. Dissolution, both it in the water column and surface sediments, is driven by the departure from solubility with respect to the particular carbonate phase. The solubility product evolves with depth under the combined influence of pressure and temperature. Because biogenic marine carbonates are complex solid-solution mixtures, their solubility product and dissolution kinetics are poorly known.

Experimental Work: - Solubility: [Abstract reproduced from "Reassessing the dissolution of marine carbonates, part 1" by Gehlen et al., submitted to Deep Sea Research I]. We studied the solubility of the [63 - 150 microm] and the greater than 150 microm size fractions of sediments from two bathymetric transects in the eastern tropical Atlantic (Sierra Leone rise and Cape Verde Plateau). Both fractions are mainly made of foraminiferal shells and fragments. We determined the calcite crystallinity (width at half weight of XRD (104) peak) of the >150µm size fraction.

Equilibration experiments were carried out in artificial seawater (20 degrees Celsius, pCO2 = 3100ppm) for up to 57 days starting from under-saturation with respect to calcite and super-saturation with respect to aragonite. Experiments starting from super-saturation indicate the presence of trace amounts of aragonite in samples from the shallowest stations of both transects. Concentration products computed for the deeper stations are intermediate between aragonite and calcite solubility. Our results are compatible with the precipitation of a high-Mg coating.

The equilibration period was too short to allow the complete re-crystallization of these Mg-rich overgrowths. Experiments initiated from under-saturation yield concentration products similar to previously reported estimates for biogenic calcite (Keir, 1976). They are between 4 to 24% higher than the stoichiometric concentration product of synthetic calcite (Mucci, 1983). 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 foraminiferal calcite (104) XRD peak from 0.168°(2theta) to 0.148°(2theta) 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.

- Reaction Kinetics: [Abstract reproduced from "Reassessing the dissolution of marine carbonates, part 2" by Gehlen et al., submitted to Deep Sea Research I] We studied dissolution kinetics of the carbonate fraction > 150 microm 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 of pH during free-drift experiments lasting between 46 and 50 hours (20 degrees Celsius, pCO2; 3100ppm and 1atm pressure). The alkalinity reached at the end of the dissolution experiments ranged between 2.444 and 2.798meq/kgsw. The dissolution time series was extrapolated to equilibrium by fitting an empirical relation to the data. The estimated asymptotic concentration products range from 4.27 X 10-7 to 6.77 X 10-7 mol2kgsw-2.

These asymptotic concentration products are compatible with the stoichiometric concentration product of aragonite (6.56 X 10-7 mol2kgsw-2) and calcite (4.37 (+/- 0.22) X 10-7 mol2kgsw-2) derived for the same sediment material during long term equilibration experiments (Gehlen et al., submitted). They are indicative of the presence of trace amounts of aragonite in sediments of the shallow stations (SLR station A, 2637m; CVP station M, 3104m). In sediments from deeper stations calcite is the main dissolving carbonate mineral. 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 0.53meq/m2/d. They are comparable to earlier results.

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