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 the processes of hydrogen sulfide oxidation in anoxic basins. Having low solubility, elemental sulfur is mainly represented by suspended forms. However, in sulfide waters it forms highly soluble polysulfides. This work is devoted to the study of elemental sulfur and polysulfides in the upper part of the Black Sea anoxic zone in 2017–2019 and 2022 at stations located on the continental shelf off the coast of the Caucasus and Crimea. Sampling, filtering and determination of sulfur were carried out under strictly anaerobic conditions in an argon atmosphere. The concentration of elemental sulfur (together with polysulfides) increases with depth and with an increase in the content of hydrogen sulfide from 0.01 in the area of the redox interface to 0.67 µmol/kg at a depth of 600 m. The fraction of elemental sulfur in the composition of ZVS is 23 ± 5%. The calculation of the polysulfides concentration in equilibrium with suspended sulfur based on thermodynamic data 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 by about 3 times. The predominance of elemental sulfur over sulfide sulfur in the composition of polysulfides in the anoxic zone at depths of 450 and 600 m can be the reason for the fractionation of its isotopic composition by +2.2‰ relative to the sulfur of dissolved sulfide (-41.0‰ VCDT).