Evidence in support of first-order dissolution kinetics of calcite in seawater

Abstract

Re-examination of an experimental determination of the dissolution rate of reagent-grade calcite in seawater shows that the original conclusion of rate law with 4.5-order dependence on undersaturation was very sensitive to uncertainties in the saturation state of the seawater with respect to calcite,Ω C. In particular, use of an erroneously high value of the calcite stoichiometric solubility product generated correspondingly low values of the saturation state. Extrapolation of the experimental measurements to a rate of zero dissolution indicates that the calcite solubility was about 20% lower than that used in the original study, similar to more recent estimates. If the lower solubity is used for recalulation of the experimental saturation states, the dissolution rate R d,C (% day −1) is adequately described by the rate experssion: R d, C = 38(1−omega; C ) 1 In situ measurements of pH in the pore waters of calcite-rich sea floor sediments are more consistent with first-order kinetics than with 4.5-order kinetics. Interpretation of pore water pH data using the 4.5-order rate expression requires dissolution rate constants that are different by at least two orders of magnitude and stoichiometric calcite solubility products that are different by several percent between two otherwise similar sites. Application of the first-order dissolution reduces the variability in the rate constant to less than one order of magnitude, and all in situ observations are consistent with a single estimate of calcite solubility. First-order kinetics do not reduce the discrepancy between the laboratory determined rate constants and those based on pore water measurements. Dissolution rate constants constrained by the in situ pH measurements are at least two orders of magnitude less than the laboratory results.