Calcite dissolution on the sea floor: An in situ study

Calcite dissolution in seafloor sediments was studied with in situ measurements of oxygen, pH and CO;Models of the porewater pH data, using a dissolution rate law that depends on the degree of undersaturation to the 4.5 power, yielded conclusions that were mutually inconsistent between the two field study sites. The dissolution rate constant implied by the Ontong-Java data was at least a factor of 125, and probably more like 10,000, smaller than implied by the Ceara Rise data. The Ontong-Java data also implied calcite solubility that was at least 10% greater than the Ceara Rise data, for similar conditions. Re-examination of the laboratory study upon which the original rate law was based found that the 4.5-order result was extremely sensitive to the calcite solubility. If the lower limit of solubility was employed, the laboratory data were consistent with a first-order dissolution kinetics. Application of the first-order rate law to models of the in situ pH data decreased the variability in the rate constant to less than an order of magnitude, and all the data were consistent with the lower estimate of calcite solubility.