Black sulfate reduction

Another factor that is of great importance for the observed sulfur isotope variations of natural sulfides is whether sulfate reduction takes place in an open or closed system. An open system has an infinite reservoir of sulfate in which continuous removal from the source produces no detectable loss of material. Typical examples are the Black Sea and local oceanic deeps. In such cases, H2S is extremely depleted in " S while consumption and change in " S remain negligible for the sulfate. In a closed system, the preferential loss of the lighter isotope from the reservoir has a feedback on the isotopic composition of the unreacted source material. The changes in the " S-content of residual sulfate and of the H2S are modeled in Fig. 2.21, which shows that 5 S-values of the residual sulfate steadily increase with sulfate consumption (a linear relationship on the log-normal plot). The curve for the derivative H2S is parallel to the sulfate curve at a distance which depends on the magnitude of... 

Abiotic oxidation of sulfide by oxygen cannot supply sulfate at rates comparable to rates of sulfate reduction. Unless high concentrations of sulfide develop and the zone of oxidation is much greater than 1 cm, rates of chemical oxidation of sulfide by oxygen will be much less than 1 mmol/m2 per day (calculated from rates laws found in refs. 115-118). Such conditions can exist in stratified water columns in the Black Sea water column chemical oxidation rates may be as high as 10 mmol/m2 per day (84). However, in lakes in which sulfide is undetectable in the water column and oxygen disappears within millimeters of the sediment-water interface (e.g., 113), chemical oxidation of sulfide by oxygen is unlikely to be important. 

Figure 6. An idealized scheme for a sedimentary porous medium with pore walls covered by a biofilm. High sulfate reduction rates are maintained even in depths to which sulfate cannot diffuse because of recycling of sulfate within the biofilm. Numbered points (in black circles) denote the following processes I, Respiration consumes oxygen. 2, Microbial reduction of reactive metal Oxides. Reduction of reactive ferric oxides is in equilibrium with reoxidation of ferrous iron by Os. Thus, no net loss of reactive iron takes place in these layers. 3, Microbial reduction of ferric oxides. 4, Sulfate reduction rate (denoted as SRR). 5, Sulfide oxidation, either microbiologically or chemically. 6, Sulfide builds up within the hiofilm, sulfate consumption increases, reactive iron pool decreases. 7, Formation of iron sulfides.

Tg year x. Lein and Ivanov [71] have estimated the total sulfide burial in the Black Sea of 2.4 Tgyear 1 including about 1 Tgyear-1 that is buried in the anoxic zone. Using these data and integrated over the upper 20 cm of sediment sulfate reduction rates, Neretin and co-authors [75] concluded that the annual sulfide flux into the water column from sediments of the anoxic zone is between 3 and 5 Tgyear x. The value is likely to be overestimated due to spatial differences in pyrite burial rates and possible sulfide diffusion downward into the deeper sediment layers. 

Weber A., Riess W., Wenzhoefer F., and Jorgensen B. B. (2001) Sulfate reduction in Black Sea sediments in situ and laboratory radiotracer measurements from the shelf to 2,000 m depth. Deep-Sea Res. I 48, 2073 -2096. 

The anaerobic oxidation of CH4 also occurs in anoxic water masses. In the Black Sea only —2% of the CH4 which escapes from sediments reaches the atmosphere. The remainder is largely lost by sulfate reduction in the anoxic parts of the water column (Reeburgh et al., 1991). In an ocean containing <1 mmolL the rate of CH4... 

Sorokin Y. I. (1962) Experimental investigations of bacterial sulfate reduction in the Black Sea using S. Mikrobiologiya 31, 329-335. 

Thamdrup B., Rosselo-Mora R., and Amman R. (2000) Microbial manganese and sulfate reduction in Black Sea shelf sediments. Appl. Environ. Microbiol. 66, 2888-2897. 

In the natural environment, large scale, and more complex sulfureta develop wherever sulfate reduction has become established (Fenchel and Riedl, 1970 Caldwell and Tiedje, 1975 Fenchel and Jorgensen, 1977). Numerous terrestrial, freshwater and marine exEunples have been reported in the literature but rarely has an attempt been made to analyse a single environment in terms of the biota and the chemical and biochemical processes associated with both the reductive and oxidative aspects of the sulfur cycle. One notable exception is the Black Sea where the work of Sorokin (1962, 1964b, 1970b) and Jannasch and his colleE es (Tuttle and Jannasch, 1973 ... 

It should be emphasised that most of the data in Table 6.1.3 are point measurements which are not necessarily representative of the overall rates of sulfate reduction in a particular environment. Nevertheless, Trudinger et al. (1972) and Rickard (1973) considered that the average rates in the Black Sea and other sediments may be typical of euxinic environments and concluded that they were of sufficient magnitude to account for synsedimentary sulfide ore deposition (see also. Temple, 1964). 

An indirect assessment of photosynthetic sulfide oxidation in Lake Repnoe, U.S.S.R., was made by Chebotarev et al. (1975). Using data from Gorlenko et al. (1974a) on COj fixation by phototrophic bacteria, they calculated HjS oxidation according to eqn (3), on the assumption that sulfur, not sulfate, was the oxidation product. The calculated rate of 22 mmol m d was considerably higher than that found in the Black Sea (see above) but agreed well with the rate of sulfate reduction (25 mmol m d" ) measured in the sediment plus water column in the same area of the lake. 

With abundant evidence for sulfate reduction in the hydrosphere, the question arises as to the actual site of the sulfate-reducing activity. This depends upon oxygen input to the system, organic matter concentration, and other factors. Suitable conditions are often encountered at, or just below, the water-sediment interface. Here, the population of sulfate-reducers is highest because of the availability of sulfate and organic matter. In some non-eutrophic lakes a secondary population maximum may arise at depths around 3 m in the sediments (Kuznetsov et al., 1963). Sulfate reduction occurs both within the water column and the sediments of the Black Sea and a number of the lakes examined by Ivanov (1964) and Sorokin (1970). 

Until recently it has been assumed that sulfate-reducing bacteria always required a strictly anaerobic environment. These environments are found in deep coastal-plain areas, oil-field brines, and in black (organic-rich), waterlogged soils and muds associated with rivers, lakes, and swamps. Sulfate reduction has also been observed in local microenvironments such as those created by the decay of a fish buried in otherwise oxidizing sediments (Berner 1971). Contrary to traditional belief, active sulfate reduction has also been observed in the presence of dissolved oxygen in the photosynthetic zone of microbial mats (Canfield and Des Marais 1991). 

Sulfur is treated in Chapter 13 and discussed only briefly here. The dominant reaction in the sedimentary sulfur cycle is microbial sulfate reduction. This gives rise to the formation of hydrogen sulfide which, by precipitating iron as "black unstable sulfide", will give the reduced sediment its characteristic blackish color ... 

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