Sulfates in seawater

Lebel [224] has described an automated chelometric method for the determination of sulfate in seawater. This method utilises the potentiometric end-point method for back titration of excess barium against EDTA following precipitation of sulfate as barium sulfate. An amalgamated silver electrode was used in conjunction with a calomel reference electrode in an automatic titration assembly consisting of a 2.5 ml autoburette and a pH meter coupled to a recorder. Recovery of added sulfate was between 99 and 101%, and standard deviations of successive analyses were less than 0.5 of the mean. 

Similar data for sulfate have been reported in many studies. Figure 9.36, for example, shows overall average sulfate distributions measured in marine areas as well as at continental sites (Milford and Davidson, 1987). The marine data show two modes, a coarse mode associated with sea salt and a fine mode associated with gas-to-particle conversion. Sulfate in seawater, formed, for example, by the oxidation of sulfur-containing organics such as dimethyl sulfide, can be carried into the atmosphere during the formation of sea salt particles by processes described earlier and hence are found in larger particles. The continental data show only the fine particle mode, as expected for formation from the atmospheric oxidation of the S02 precursors. 

Fisher, F.H. (1967) Ion pairing of magnesium sulfate in seawater determined by ultrasonic absorption. Science, 157, 823. 

Thus, in an analogous fashion to carbon, if total exogenic sulfur has remained constant, then the 834S values of sulfate in seawater, and consequently in evaporite minerals, are functions of the ratio of oxidized sulfur to reduced sulfur in the sedimentary reservoirs of this element. If the size of the sulfate reservoir increases, then the sulfide reservoir shrinks, and the mean 34s/32s ratios of each reservoir changes. 

In an effort to systematize differences in the absolute magnitude of benthic phosphate efflux in freshwater versus marine systems, Caraco et al. (1989) argue that more efficient benthic P-release occurs in lake relative to marine sediments as a direct consequence of the presence of higher sulfate in seawater, and that redox conditions exert secondary control. This argument is overly simplistic, however, because redox conditions control production of sulfide from sulfate, and it is the removal of ferrous iron from solution into insoluble ferrous sulfides that decouples the iron and phosphoms cycles (e.g., Golterman, 1995a,b,c Rozen et al., 2002). Thus, the presence of sulfate is a necessary but not sufficient criterion to account for differences in benthic P-cycling in marine versus freshwater systems redox conditions are an equally crucial factor. 

One exception is acid sulfate soils formed on coastal sediments due to the influence of sulfate in seawater (see Chapter 7). 

The total mass of oxygen consumed by the reactions listed, added to that contained in the atmosphere, sums to about 3.1 x 1019 kg. The result provides a reasonable balance between production and losses only because we have made an effort to maximize the oxygen consumption. On the whole, the budget is subject to much uncertainty and is as yet unsatisfactory. But the data show that the major reservoirs of oxygen are sulfate in seawater and in evaporites, and Fe203 in sedimentary rocks. Only 4% of total oxygen resides in the atmosphere. One must appreciate the peculiarity of this distribution. Since oxidative weathering causes a steady drain on 02, we can understand its presence in the atmosphere only if it is continuously... 

As pointed out by Seal et al. (2000), many studies of ancient hydrothermal systems have utilized equilibrium sulfate-sulfide sulfur isotope fractionation models, but these should be applied with great caution. As shown in Figure 9, seafloor hydrothermal vent fluid 5" Sh2S values do not conform to simple equilibrium fractionation models. Shanks et al. (1981) first showed experimentally that sulfate in seawater-basalt systems is quantitatively reduced at temperatures above 250°C when ferrous minerals like the fayalitic olivine are present. When magnetite is the only ferrous iron-bearing mineral in the system, sulfate-reduction proceeds to sulfate-sulfide equilibrium, but natural basalts contain ferrous iron-bearing olivine, pyroxene, titanomagnetite, and iron-monosulfide solid-solution (mss) (approximately pyrrhotite). It is the anhydrite precipitation step... 

The primary causes of accelerated attack of copper alloys by polluted seawater are (1) the action of sulfate-reducing bacteria under anaerobic conditions (bottom muds and sediments) on the natural sulfates in seawater in coastal harbors and estuaries, and (2) the putrefaction of organic sulfur compounds fiom decaying plant turd animal matter within seawater systems during periods of extended shutdown. 

This is a process carried out by Desulfovibrio acting with other bacteria. Because of the presence of sulfate in seawater, this process for the formation of hydrogen sulfide is a significant source of atmospheric sulfur and a source of the pollutant H2S in coastal areas. In areas where this occurs, the sediment is often black in color due to the formation of FeS. In the presence of elemental oxygen, Thiobacillus thiooxidans and other bacteria may oxidize hydrogen sulfide to sulfate ion ... 

Sea spiay Near the ocean, some of the sulfate in precipitation comes from sea spray, not pollution. This sulfur is already fully oxidized and contributes no acidity to precipitation. (In fact, sea water is slightly alkaline.) A technique for distinguishing between the sulfate derived from sea spray and the sulfate from man-made pollution was developed in the middle of the nineteenth century. It is based on the fact that the ratio of chloride to sulfate in seawater is constant. Since the major source of chloride in precipitation near the coast is sea spray, measuring the amount of chloride in precipitation allows a determination of the amount of sulfate in precipitation contributed by sea-spray. In this book, the values given for sulfate in precipitation do not include the "excess" sulfate contributed by sea-spray. 

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