Elsevier

Marine Chemistry

Volume 65, Issues 3–4, June 1999, Pages 253-261
Marine Chemistry

The marine barite saturation state of the world's oceans

https://doi.org/10.1016/S0304-4203(99)00016-XGet rights and content

Abstract

This paper addresses the question of the eventual control of barium concentration in seawater by an equilibrium with barite. For this, we have used a new thermodynamic model to compute the barite saturation index of ocean waters, mainly from GEOSECS data. Our results show that equilibrium between barite and seawater is reached in a number of places: cold surface waters of the Southern Ocean, waters at intermediate depths (2000–3500 m) in the Pacific, deep waters (2000–3500 m) of the Gulf of Bengal. The only samples for which a slight barite supersaturation is found are the surface waters at GEOSECS station G89 in the Weddell Gyre. Besides these locations, the rest of the world's oceans is undersaturated, as was established by Church and Wolgemuth [Church, T.M., Wolgemuth, K., 1972. Marine barite saturation, Earth Planet. Sci. Lett. 15 35–44.]. There is a return to undersaturation of the water column at depths of about 3500 m in the Pacific and of about 2500 m in the Southern Ocean. The reverse is found for GEOSECS station 446 in the Gulf of Bengal for which the highest Ba concentrations can be found at depth: surface waters are undersaturated and equilibrium is reached below 2000 m. Finally, we briefly discuss the role of biogenic and inorganic processes on barite formation in the ocean as well as the influence of strontium substitution in marine barites.

Introduction

Barite particles are a universal component of suspended matter in the oceans and have a biogenic origin (Dehairs et al., 1980, Dehairs et al., 1990; Bishop, 1988). Correlations between alkalinity and dissolved Ba and Si concentrations in seawater also support a biogenic origin for these particles (see discussion and references in McManus et al., 1994; Paytan and Kastner, 1996). The question of the eventual limitation of the barium concentration of ocean waters by the inorganic precipitation of barium sulfate (barite) has been addressed by Chow and Goldberg (1960) and by Church and Wolgemuth (1972). From the limited data and the thermodynamic models available at that time, Chow and Goldberg (1960) concluded that “the saturation values of barium are not approached in surface ocean waters but may be reached at greater depth” while Church and Wolgemuth (1972) showed that the “Ba concentration of the entire seawater column in the Eastern Pacific and probably most of the world's oceans seems to fall below the barite saturation curve.” Such assumptions have been commonly accepted and referred to since then (see, e.g., Bertram and Cowen, 1997). More recently, Falkner Kenisson et al. (1993) have found that the anoxic Black Sea deep waters exceed saturation with respect to pure barite by at least a factor of two. Because of the potential use of barium as a tracer of oceanic circulation, it is important to know all the factors controlling its concentration in the oceans. The concentration of chemical elements likely to be used as geochemical tracers is the result of molecular diffusion, advection (transport) and chemical reactions. In this paper, we investigate the eventual limitation of Ba concentration in the ocean by an equilibrium with solid barium sulfate. Conversely, undersaturation of seawater with respect to barium sulfate indicates where barite can dissolve and where barium could be remobilized.

A large body of data on the distribution of Ba in the oceanic water column has been collected since Church and Wolgemuth's paper. In the same period, the development of Pitzer's ion interaction approach for the thermodynamic properties of electrolyte solutions has triggered the construction of a new generation of solubility models of great interest for the earth sciences (see Weare, 1987; Pitzer, 1991 for reviews). Above all, demonstrating equilibrium between a solid and an aqueous phase is a challenge to solution chemistry and to the way of calculating the thermodynamic properties of the aqueous phase. Using Pitzer's formalism, we have built a new solubility model for barite in electrolyte solutions from 0 to 200°C and to 1 kbar (Monnin, 1999). Here, we apply this model to barium concentrations in seawater samples collected during various oceanographic cruises (mainly GEOSECS) to calculate the saturation state of the world's oceans with respect to barium sulfate.

Section snippets

Calculation of the barium sulfate saturation index in seawater

The barite saturation index is the ratio of the barium sulfate ionic product to the barite solubility product:SI=Q/Ksp.

The expression of the barite solubility product as a function of temperature and pressure is given by Monnin (1999). The ionic product Q is defined as:Q=mBa2+(aq),F·mSO2−4(aq),F·γM2+(aq),F·γSO2−4(aq),Fwhere mBa2+(aq),F and mSO2−4(aq),F designate the molalities of the free barium and sulfate aqueous ions and γBa2+(aq),F and γSO2−4(aq),F, their activity coefficients. In this

The world ocean saturation state with respect to pure barium sulfate

As raw input data, we use in situ temperature, depth, salinity and total barium concentration as commonly reported in oceanographic tables. Depth is readily converted to pressure. Molalities of the major seawater components are calculated from the sample salinity and from the composition of standard 35‰ seawater taken from Clegg and Whitfield (1991). The Ba concentration (mol/kg seawater) is transformed into Ba molality (mol/kg H2O).

In Fig. 1Fig. 2Fig. 3Fig. 4Fig. 5Fig. 6Fig. 7Fig. 8Fig. 9Fig.

The energetics of barite formation in the oceans: the role of biogenic and inorganic processes

The picture of the saturation state of the world oceans with respect to pure barium sulfate emerging from our results challenges the earlier statements of Chow and Goldberg (1960) and of Church and Wolgemuth (1972). First of all, equilibrium is reached in a number of places: cold surface waters of the Southern Ocean, waters at intermediate (2000–3500 m) depths in the Pacific, deep waters (2000–3500 m) of the Gulf of Bengal. When supersaturation is found, the saturation indices are still very

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