EVIDENCE IN SUPPORT OF FIRST-ORDER DISSOLUTION KINETICS OF CALCITE INSEAWATER

Re-examination of an experimental determination of the dissolution rat e of reagent-grade calcite in seawater shows that the original conclus ion of a rate law with 4.5-order dependence on undersaturation was ver y sensitive to uncertainties in the saturation state of the seawater w ith respect to calcite, Ohm(C). In particular, use of an erroneously h igh value of the calcite stoichiometric solubility product generated c orrespondingly low values of the saturation state. Extrapolation of th e experimental measurements to a rate of zero dissolution indicates th at the calcite solubility was about 20% lower than that used in the or iginal study, similar to more recent estimates. If the lower solubilit y is used for recalculation of the experimental saturation states, the dissolution rate R-d,R-C (% day(-1)) is adequately described by the r ate expression: R-d,R-C = 38(1 - Ohm(C))(1) In situ measurements of pH in the pore waters of calcite-rich sea floor sediments are more consi stent with first-order kinetics than with 4.5-order kinetics, Interpre tation of pore water pH data using the 4.5-order rate expression requi res dissolution rate constants that are different by at least two orde rs of magnitude and stoichiometric calcite solubility products that ar e different by several percent between two otherwise similar sites, Ap plication of the first-order dissolution reduces the variability in th e rate constant to less than one order of magnitude, and all in situ o bservations are consistent with a single estimate of calcite solubilit y, First-order kinetics do not reduce the discrepancy between the labo ratory determined rate constants and those based on pore water measure ments. Dissolution rate constants constrained by the in situ pH measur ements are at least two orders of magnitude less than the laboratory r esults.