Stability of the oxic-anoxic interface in the Black Sea

We have examined the position of the oxic, suboxic and anoxic interfaces in the Black Sea between 1965–1992. It appears as if the suboxic zone (i.e. depths where DO and H 2S < 5 μM) has been a common feature throughout the Black Sea, and only recently been observed due to advances in sampling and analytical procedures. The upper boundary of the suboxic zone is identified based upon a minimum in the PO 4 profile at a density of 15.95 σ t . The lower boundary at 16.20 σ t is identified by the onset of H 2S. These densities show seasonal and interannual variations of the order of 0.05–0.10 σ t , but no directional trend over the past 2–3 decades. The position of the suboxic zone is consistent with T− S data. Large variations in T− S characteristics are seen between years and seasons above the suboxic layer, as ventilation is quite rapid. Below the DO < 5 μM horizon, entrainment processes bring surface water to depth, thus forming a suboxic layer as DO and H 2S are consumed. T− S characteristics vary slightly below 16.2 σ t , and are constant below 17.0 σ t , indicating that these waters are ventilated less frequently.