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A well-defined suboxic zone was observed at the oxic-anoxic interface in the Black Sea. The redox zones for many elements, well-separated within this suboxic zone, were sampled by using a high-resolution pump-profiling sampler. Although characteristic features in the profiles occur at distinctly different depths at different locations, they are associated with the same density layer. A single vertical density scale can therefore be used to describe the features of the oxic-anoxic interface. In terms of a simple one-dimensional vertical exchange model, the distributions suggest that the upward flux of sulfide is not oxidized by oxygen. Instead, it may be oxidized by settling particulate manganese and iron oxides, it may be oxidized anaerobically during phototrophic reduction of CO₂, or the vertical model may not be appropriate and sulfide may be oxidized by oxygen transported intermittently by horizontal ventilation events …