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Discussed are the features of the oxygen and sulfide vertical distribution resulted from the expedition data obtained by Marine Hydrophysical Institute in the northern part of the Black Sea (within the economic zone of Russia) in 1995-2015. In course of the last 20 years, the upper boundary of the sulfide zone in this area, defined by the sulfide isoline 3 M, did not change its position in the scale of relative density and is located on t = 16.10-16.15 kg/m 3 . Closer to the northwest shelf (in the area of the Sevastopol anticyclone), the hydrogen sulfide isoline 3 M is slightly lifted to t = 15.9-16.0 kg/m 3 ; there is a single case when its position was recorded at t = 15.85 kg/m 3 . Position of the suboxic zone upper boundary (the threshold oxygen isoline 10 M) is characterized by spatial and temporal variability. During four expeditions in 2009-2013, in the region of the northwestern shelf the suboxic zone upper boundary was located, regardless of the season, within t = 15.6-15.7 kg/m 3 . In November, 2013 in the deep sea part, the threshold oxygen isoline 10 M was located on t = 15.7 kg/m 3 , whereas in November, 2015 it was considerably higher: on t = 15.35 kg/m 3 . The change of location of the threshold oxygen isoline 10 M observed in 1995-2015 provides no possibility to assess unambiguously alteration of the suboxic zone thickness in the central part of the sea. Thus within the depths 1750-2000 m in the Black Sea, no significant increase of the sulfide content in 1995-2015 has been recorded.
Кондратьев et al. (Thu,) studied this question.