Sulfur bacterial reduction

Elemental sulfur in the caprock of salt domes w almost certainly produced 1 the anaerobic bacterial reduction of sedimentary sulfate deposits (mainly anhydrite or gypsum, p. 648). The strata are also associated with hydrocarbtms these are consumed as a source of energy by the anaerobic bacteria, which use sulfur instead of O2 as a t drogen acceptor to produce CaC03, H2O and H2S. The H2S... 

Sulfur exists naturally in several oxidation states, and its participation in oxidation/reduc-tion reactions has important geochemical consequences. For example, when an extremely insoluble material, FeS2, is precipitated from seawater under conditions of bacterial reduction, Fe and S may be sequestered in sediments for periods of hundreds of millions of years. Sulfur can be liberated biologically or volcanically with the release of H2S or SO2 as gases. 

Hydrogen sulfide enters natural waters from decay of organic matter (e.g., in swamps), bacterial reduction of sulfate ion, or underground sour natural gas deposits. It can be removed by aeration, anion exchange (Eq. 14.14), or oxidation by chlorine to elemental sulfur ... 

Little or no fractionation accompanies the uptake of sulfate in soils by plants during ASR (60.611. Chukhrov et al. ( Q) showed that in cases where atmospheric sulfate is not subject to bacterial reduction in the soil, the value of the plant sulfur was identical to rainfall sulfur. In soils subject to dissimilatory sulfate reduction, the 6 S value of plant sulfur differed from that of local rainfall. Additionally, Chukhrov et al. (60) found that plants from oceanic islands had a sulfur content with higher values than those from continental areas, which they attribute to the relative influence of marine sulfate to these areas. 

In the freshwater peat swamp, bacterial reduction of organic sulfur in plant tissues may be an important process in the formation of pyrite (93). Altschuler et al. (93) proposed that in the Everglades peat, pyrite precipitates directly by the reaction of HS or organic sulfide (produced by reduction of oxysulfur compounds in dissimilatory respiration) with ferrous iron in the degrading tissues. Pyrite formation in low-sulfur coal may be accounted for by this process. 

Sulfur isotopic measurements can shed light on the origin of sulfur in coal. The 34S/32S ratio depends on the source of sulfur and the geologic processes involved during coal formation. For example, isotopic compositions are different for the two principal sources of sulfur in coal 1) the sulfur preserved from the precursor plant material, and 2) the sulfur derived from the bacterial reduction of dissolved sulfate in ambient waters. Plant assimilation of sulfur results in a slight depletion of 34S (4-4.5%c) relative to the 834S in the dissolved sulfate source (102.103). In contrast, the dissimilatory bacterial reduction of sulfate results in a large isotopic fractionation sulfide sulfur can be depleted as much as 60%o in the heavy isotope (89.104-106). 

Figure 10.12 illustrates the carbon and sulfur isotopic compositions of a variety of materials. For both carbon and sulfur, there is an important fractionation that obtains when organic processes are involved. Organic material is depleted in 3c, and sulfide produced from bacterial reduction of sulfate is depleted in 34s. In the exogenic carbon cycle, there are two principal reservoirs of carbon the oxidized inorganic carbon reservoir, which is mostly carbonate... 

Nakai, N., and Jensen, M.L. (1964) The kinetic isotope effect in the bacterial reduction and oxidation of sulfur. Geochim. Cosmochim. Acta 28, 1893-1912. 

Most of the hydrogen sulfide in the atmosphere—approximately 90%— comes from natural sources through nonspecific and anaerobic bacterial reduction of sulfates and sulfur-containing organic compounds (ATSDR 1999). These sources include stagnant or polluted waters and manure or coal pits with low... 

A study by Berner et al. (2002) suggests that as the bacterial reduction of sulfate progresses, there is variable sensitivity in the sulfur and oxygen isotopic fractionation in S04. They found that, in the beginning stages of bacterial reduction in a groundwater system, both the and 5 0... 

Sulfur Isotopic Fractionation during Bacterial Reduction... 

The dominant dissolved form of sulfur in seawater is sulfate (including the free form, SO , plus ion pairs with the major cations of seawater). In pore water of near-surface marine sediments, sulfate also dominates, but it is progressively removed with increasing depth by bacterial reduction and supplanted by dissolved H2S. Pore-water SO and H2S profiles are discussed more fully below. 

Habicht K. S., Canfield D. E., and Rethmeier J. (1998) Sulfur isotope fractionation during bacterial reduction and disproportionation of thiosulfate and sulfite. Geochim. Cosmochim. Acta 62(15), 2585-2595. 

In contrast to strontium and osmium isotopes, the isotopes of sulfur are strongly fractionated by biological processes, particularly during the dis-similatory bacterial reduction of sulfate to sulfide. The laboratory results for this step are anywhere from +4 to —46%o (CDT), but even larger fractionations have been observed in natural systems (Harrison and Thode, 1958 Chambers and Trudinger, 1979 Habicht and Canfield, 1996). 

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