Sulfate/hydrogen sulfide, sulfur isotopic composition

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 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). 

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