Thiosulfate in the Upper Part of the Anoxic Water Column in the Black Sea

Thiosulfate is a sulfur species in an intermediate oxidation state between sulfate and sulfide. It appears during bacterial and chemical sulfide oxidation processes. Thiosulfate plays a prominent role in sulfide oxidation and its presence may clarify the mechanism of redox reactions in the biogeochemical cycles of sulfur and carbon. This study presents the thiosulfate distribution, determined by the derivatization technique with the reagent 2,2'-Dithiobis(5-nitropyridine), in the upper part of the anoxic water column of Black Sea in 2018–2021. The studies were carried out annually at the station located seaward of Gelendzhik in the area of continental slope. Independently of observation time, the thiosulfate concentration increases with depth simultaneously with increasing sulfide. Thiosulfate concentration above the detection limit of 0.01 µM is found at water depths with a density (σ θ ) of 16.3 kg/m 3 and sulfide concentration of 8–11 µM. Thiosulfate reaches its maxima of 0.30 µM at the maximal studied depth of 600 m. We suggest that in the absence of oxidants in anoxic waters, the appearance of thiosulfate is attributed to the sulfate reduction process in the reductive part of the sulfur cycle. Thiosulfate disappears in the upper part of anoxic water column before sulfide.