Mechanisms controlling the suboxic layer of the black sea
The Black Sea is a classic anoxic ocean basin resembling the earth's ancient oceanic conditions. Its present structure has evolved during the last 2.5 billion years, and is characterized by a well-defined sub-oxic zone at the interface between the oxic and sulfidic layers where many important bacterially-mediated redox reactions occur. The sub-oxic zone involves extremely low oxygen and sulfide concentrations with no perceptible vertical gradients. This is due to very strong density stratification along the sub-oxic and anoxic layers, which markedly restricts the downward flux of oxygen and upward flux of chemical properties and therefore maintains a stable structure with some inter-annual variations. The upper layer biogeochemical structure above the deep anoxic pool involves three distinct layers (Fig. 1). The biologically productive, oxic layer extends to the depth of nearly 50 m. About 90% of the sinking particles are remineralized inside this layer as well as in the subsequent 20-30 m deep "upper nitracline" zone. There, nitrate attains maximum concentrations around 8 µM, and is re-supplied to the surface layer to refuel the biological pump. Only a small fraction (~10%) of particulate matter sinks to the deeper anoxic part of the sea. This loss is however compensated by lateral nitrogen input mainly from the River Danube, by wet deposition and nitrogen fixation.