The Stable Isotopic Composition of Sulphur Species in the Black Sea Water Column

Abstract

The Black Sea is the unique anoxic quasi-stationary basin characterised by a total hydrogen sulphide inventory of about 519 M m 2 In the Black Sea water column large isotopic fractionation factor between sulphate and hydrogen sulphide of about 60%0 is observed which is mainly due to microbial sulphate reduction. Until now such high fractionation factors have not been reported in culture experiments including sulphate reducing bacteria cultures isolated from the Black Sea water (Fry et al., 1991). Factors that determine isotopic fractionation in sulphur cycle of anoxic environments are poorly understood. Recent works give evidence for participation of reactions in the oxidative part of the sulphur cycle on the formation of very isotopically light sedimentary sulphides. It is still unclear, to what extent oxidation processes in the Black Sea could influence the isotopic composition of sulphur species in the deeper anoxic zone taking into account significant spatial separation of chemocline layer from entire sulphide water. In this paper we present new data on sulphur isotopic composition of sulphate and hydrogen sulphide with emphasis on chemocline layer and first data on isotopic composition of intermediates species (elemental sulphur and thiosulphate).