Marine barite and celestite saturation in seawater

Abstract

The stoichiometric solubility product , K sp,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,T *=A+B ln T+ C T +DS n 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,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 4 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°C and 1 atm, suggests that this mineral may contain up to 13 mol% SrSO 4 at equilibrium. Accordingly, we have determined the stoichiometric solubility products of strontian barite as a function of salinity and temperature: ln Ba K sp ′=247.88−38.333 ln T− 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 4 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).