Saline and warm Mediterranean water flowing through the Bosporus Strait maintains a permanent pycnocline with vertical separation of oxic (O 2), suboxic (absence of O2 and H2S), and anoxic (H2S) zones in the Black Sea. The stable suboxic zone implies restricted vertical mixing of the upper oxic and lower anoxic layers and limited vertical flux of oxygen that cannot balance the upward flux of sulfide. We report data that directly confirm massive lateral injections (>200 km from the Bosporus) of oxygen-enriched waters of the Bosporus plume, created by the mixing of shallow, cold, intermediate-layer Black Sea water with Mediterranean water. These plume waters are laterally injected into the oxic layer and, more importantly, into the suboxic and anoxic layers over several small vertical scales ("fingers" of ∼5 m) at water densities (σt) from 15.0 to 16.4. O2 injection oxidizes Mn(II) to Mn(III,IV), which then oxidizes H2S. The onset of H2S detection occurs in deeper waters in the southwest (>170 m; σ t ≈ 16.4) relative to the west central Black Sea (110 m; σt ≈ 16.2) and coincides with increased MnO2 and S8 formation in the southwest.