Distribution and Variations of Elemental Sulfur in the Upper Part of the Black Sea Anoxic Water Column

Abstract

Elemental sulfur and its derivatives, polysulfides, play a key role in hydrogen sulfide oxidation processes in anoxic basins. Having low solubility, elemental sulfur is mainly represented by suspended forms. However, in sulfide waters, it forms highly soluble polysulfides. This article studies elemental sulfur and polysulfides in the upper part of the Black Sea anoxic water column in 2017–2019 and 2022 at stations located on the continental slope off the coast of the Caucasus and Crimea. Sulfur was sampled, filtered, and determined under strictly anoxic conditions in an argon atmosphere. The concentration of elemental sulfur (zero-valent sulfur (ZVS)–elemental sulfur together with polysulfides) increases with depth and hydrogen sulfide content from 0.01 µmol/kg at the redox interface to 0.67 µmol/kg at a depth of 600 m. The elemental sulfur fraction in the composition of ZVS is 23 ± 5%. Based on thermodynamic data, calculation of the polysulfide concentration in equilibrium with suspended sulfur shows that deeper than 20–25 m of the upper boundary of the anoxic zone, their concentration was higher than ZVS and at a depth of 600 m they differed about threefold. The predominance of elemental sulfur over sulfide sulfur in the composition of polysulfides in the anoxic zone at depths of 450 and 600 m may be the reason for the fractionation of its isotopic composition by 2.2‰ with respect to dissolved sulfide sulfur (–41.0‰ VCDT).