Salinity, suboxic zone

The suboxic zone is defined as the region between where oxygen decreases to near zero (O2 < 10 xM) and where sulfide first appears (H2S > 1 iM) [16, 17]. Many important redox reactions involving Fe, Mn, N, and other intermediate redox elements occur in the suboxic zone. Similar redox reactions take place in sediments throughout the world s oceans, but they are easier to study in the Black Sea because they are spread out over a depth scale of tens of meters (rather than centimeter or millimeter scales as in sediments). The Black Sea suboxic layer hydrophysical structure is very stable compared with other ocean redox regions such as Cariaco Trench, which is influenced by mesoscale eddies, or the Baltic Sea that is influenced by inflows of the North Sea saline oxygenated waters in cold winters. 

Silicate has conservative characteristics in the suboxic zone. Its vertical distribution practically coincides with that of salinity and density because silicate is not involved in the processes connected with changes in redox conditions. Silicate is not consumed in the processes of chemosynthesis, and its distribution reflects the degradation of OM produced only in the euphotic zone. Silica concentrations are low in the surface and increase smoothly from 50 to 100 xM across the suboxic zone. 

These changes were thought to be either by natural climate-induced variability or by a decrease in river inflow from Europe (25). The new appearance of a suboxic zone and the increase in salinity were possibly related because the increase in salinity of the surface layer could change the rate and depth of ventilation of the pycnocline. Mixing events in other anoxic basins like Saanich Inlet, British Columbia, have been known to result in similar suboxic zones (26-28). 

Ventilation as Source of Suboxic Zone. Ventilation processes may be the ultimate explanation for the origin of the suboxic zone. Murray et al. (32), Buesseler et al. (42), and Ozsoy et al. (51) suggested that significant ventilation occurs, especially in the upper 500 m of the Black Sea. Oguz et al. (29) showed that horizontal variability in salinity and currents associated with eddies and filaments can be seen to almost 500 m. 

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