The solubility of calcite and aragonite in seawater at atmospheric pressure and 34.5 0 / 00 salinity

Abstract

Direct measurement of the solubility of calcite in seawater is difficult and irreproducible because of kinetic factors that inhibit the attainment of true, reversible equilibrium. To avoid this problem, the solubility product constant for calcite in seawater was determined in the present study by calculation from new measurements of the aragonite solubility product constant in seawater and the differential solubility of calcite and aragonite in carbonated distilled water. All measured equilibria were reversed, and redundant measurement of pH, P/sub CO/sub 2/, calcium concentration, and carbonate alkalinity was used as a check on the results. The resultant calculated values of K'/sub e/ = (Ca/sup + +/) (CO/sub 3//sup - -/) for 34.5 percent seawater and 1 atm total pressure are: K'/sub c/ = 5.94 +- 0.39 x 10/sup -7/ at 25/sup 0/C, K'/sub c/ = 7.98 +- 0.43 x 10/sup -7/ at 5/sup 0/C, where c refers to calcite, and units of K'/sub c/ are mol/sup 2/ l/sup -2/. The analogous measured values for aragonite in seawater are: K'/sub a/ = 8.76 +- 0.40 x 10/sup -7/ at 25/sup 0/C, K'/sub a/ = 11.4 +- 0.50 x 10/sup -7/ at 5/sup 0/C. The difference in solubility between calcite and aragonite determinedmore » is in good agreement with thermodynamic measurements of others. Recalculation, using the new K'/sub e/ data, of the state of saturation of the Atlantic and Pacific Oceans from the data of Takahashi (1975) shows that the lysocline, north of 30/sup 0/S, in both oceans is found well below the saturation depth for calcite and occurs near where accelerated dissolution, based on similarly recalculated laboratory results, is to be expected. However, additional indirect determinations of calcite solubility, especially for elevated pressures, are needed to check these conclusions.« less