Marine barite and celestite saturation in seawater

The stoichiometric solubility product, K-sp,K-T*, of barite and celestite in seawater has been calculated using thermodynamic constants, K-s0, and the activity coefficients for barium, strontium, and sulfate in seawater. An equation of the form: ln K-sp,K-T* = A + bln + C/T + DSn has been used. The constants A, B, C, D and n are derived from the calculated stoichiometric (or total) solubility product of barite and celestite in seawater as a function of temperature and salinity. T is the absolute temperature (K) and S is the salinity. The effect of pressure on K-sp,K-T* is also calculated. Comparing the solubility products determined from this equation and the pressure effect equation to the distribution of Ba, Sr and SO, in seawater, we conclude that the upper surface water of the Southern Ocean is likely supersaturated with respect to pure barite, in agreement with Jeandel et al. [Jeandel, C., Dupre, B., Lebaron, G., Monnin, C., Minster, J.F., 1996. Longitudinal distributions of dissolved barium, silica and alkalinity in the western and southern Indian Ocean. Deep-Sea Res. 43, 1-31.] and Monnin et al. [Monnin, C., Jeandel, C., Cattaldo, T., Dehairs, F., 1999. The marine barite saturation state of the world oceans. Mar. Chem. 65, 253-261.] and that the oceanic water column is typically < 30% saturated with respect to celestite. The model, which includes the thermodynamic solid-solution behavior of barite in seawater at 25 degrees C and 1 atm, suggests that this mineral may contain up to 13 mol% SrSO4 at equilibrium. Accordingly, we have determined the stoichiometric solubility products of strontian barite as a function of salinity and temperature: (InKsp)-K-Ba' = 247.88 - 38.333lnT- 15421/T + 1.27S(0.3) Using our model results for the total solubility product of the Sr-barite phase and seawater Ba and SO, concentration data, we conclude that the maximum saturation level of the oceans with respect to marine barite is 63% in the North Atlantic, 88% in the Indian Ocean, and 111% in the North Pacific. The depth of this maximum saturation level is shallower in the Atlantic Ocean (about 1000 m) than in the Pacific and Indian oceans (about 2000 m). (C) 2000 Elsevier Science B.V. All rights reserved.