Sulfate/hydrogen sulfide, sulfur isotopic

These examples convincingly demonstrate that specific OSC are formed during the early stages of diagenesis by reactions of reduced sulfur species with specific biogenic substrates. The reactive substrates are proposed to contain either carbon-carbon double bonds or other reactive functional groups that react with either hydrogen sulfide or polysulfides to form the OSC (88). These views are consistent with evidence from sulfur isotopes that H2S produced by microbial sulfate reduction is the major source of reduced sulfur in sediments... 

Sulfur Isotopic Composition of Sulfate and Hydrogen Sulfide... 

Hydrogen sulfide, H2S (J]H2S = [H2S] + [HS ] + [S2 ], where [HS ] represents ca. 80% at pH 7.5-7.65 in the Black Sea anoxic interior), is the key chemical compound that defines the direction and origin of many biogeo-chemical cycles in the anoxic zone of the Black Sea. The main goal of this review is to present the contemporary inventory of hydrogen sulfide and sulfur intermediate species, the results of recent physiochemical studies of the bottom convective layer, and to discuss the sulfur isotopic composition of dissolved sulfide and sulfate. This review concludes by presenting the sulfur budget of the Black Sea. 

The average isotope compositions of the sulfate sulfur in the oxic and anoxic zones are + 18.5%o and + 19.5%, respectively. The isotopic composition of sulfate in the Black Sea forms from two distinct sources. The sea receives annually about 2.82 x 106 tons of sulfate with river discharge with the average isotope composition of + 4.6% [71]. The annual input with Mediterranean waters of 540 x 106 tons of sulfates has an isotopic composition of about + 19.8%o [18]. The isotopic composition of dissolved sulfide averaged over all depths is - 39.6 1.3%o and varies between - 42.0%o and - 32.6%o for all stations [65] (Fig. 5). There is no indication that the sulfur isotopic composition of hydrogen sulfide changes spatially and/or seasonally. 

The isotope ratios of the sulfur isotopes are also affected by kinetic and equilibrium isotope effects. Kinetic isotope effects are marked in the reduction of sulfates to hydrogen sulfide by bacteria (enrichment of the lighter isotopes in H2S). The equilibrium isotope effect in the reaction... 

The principal sources of sulfate in formation waters are dissolved marine sulfate, sulfate derived from the dissolution of evaporites, and sulfate formed by the oxidation of sulfides. Sulfate is destroyed by reduction to hydrogen sulfide. The value of 5 " S in gypsum is only — 1.6%o heavier than sulfate in solutions from which it precipitates. The isotopic composition of sulfur in gypsum in Phanerozoic deposits precipitated from seawater during evaporation thus tracks the secular changes in the isotopic composition of sulfur in seawater ( 10% to 30%o). 

Details of sulfur isotope geochemistry are presented elsewhere in this volume (see Chapter 7.10) and are only highlighted here as related to paleo-environmental interpretations of finegrained siliciclastic sequences. Formation of sedimentary pyrite initiates with bacterial sulfate reduction (BSR) under conditions of anoxia within the water column or sediment pore fluids. The kinetic isotope effect associated with bacterial sulfate reduction results in hydrogen sulfide (and ultimately pyrite) that is depleted in relative to the ratios of residual sulfate (Goldhaber... 

There is an indirect way in which bacteria could be involved in the removal of uranium from solution in near-surface environments. Jensen (1958), on the basis of sulfur isotope measurements, suggested that sulfate waters in sedimentary rocks had been reduced by anaerobic bacteria to hydrogen sulfide. Zones rich in carbonaceous matter could have pro-... 

The chemical and isotopic yields, based on the conversion of sodium sulfate to sulfur, are 92%. Usually 2 to 3% of unreduced sulfate is recoverable. The remaining losses of about 5 % are mainly due to losses of colloidal sulfur and of hydrogen sulfide from the solid sodium sulfide. 

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