STABILITY OF THE OXIC ANOXIC INTERFACE IN THE BLACK-SEA

We have examined the position of the oxic, suboxic and anoxic interfac es in the Black Sea between 1965-1992. It appears as if the suboxic zo ne (i.e. depths where DO and H2S <5 muM) has been a common feature thr oughout the Black Sea, and only recently been observed due to advances in sampling and analytical procedures. The upper boundary of the subo xic zone is identified based upon a minimum in the PO4 profile at a de nsity of 15.95 sigma(t). The lower boundary at 16.20 sigma(t) is ident ified by the onset of H2S. These densities show seasonal and interannu al variations of the order of 0.05-0. 10 sigma(t), but no directional trend over the past 2-3 decades. The position of the suboxic zone is c onsistent with T-S data. Large variations in T-S characteristics are s een between years and seasons above the suboxic layer, as ventilation is quite rapid. Below the DO <5 muM horizon, entrainment processes bri ng surface water to depth, thus forming a suboxic layer as DO and H2S are consumed. T-S characteristics vary slightly below 16.2 sigma(t), a nd are constant below 17.0 sigma(t), indicating that these waters are ventilated less frequently.