Role of sulfide oxidation intermediates in the redox balance of the oxic–Anoxic interface of the gotland deep, baltic sea

Depth profiles of sulfur species, including sulfide oxidation intermediates (zero-valent sulfur and thiosulfate), nutrients, metals (Mn, Fe), oxygen, temperature, salinity, and turbidity, were measured in the Gotland Deep, at the eastern Gotland Basin, in July 2007. We found that the highest concentrations of more oxidized sulfide oxidation intermediate, thiosulfate, were located below the highest concentrations of zero-valent sulfur. We explain this paradox by bacterial nitrate reduction coupled with thiosulfate oxidation. The same process using zero-valent sulfur is less effective due to particulate form of the latter. Oxic water intrusions were traced in both the redox transition zone (RTZ) and deep water column by decrease in concentrations of reduced nitrogen and sulfur species (sulfide, zero-valent sulfur, and thiosulfate) as well as by increase in nitrate concentration. Two turbidity maxima were found in the RTZ. Turbidity maximum, which coincides with past oxic water intrusion, was found in the deep sulfide-rich water layer. Profiles of metals and nutrients in most of the profiles indicate as well unstability of the redoxcline and oxic water intrusions at and below the redoxcline.