Purpose. The purpose of the work is to analyze the features of vertical distribution of the dissolved oxygen and hydrogen sulfide in the deep part of the Black Sea in the modern period.
Methods and results. The data obtained in 11 expeditions of Marine Hydrophysical Institute (MHI) of RAS in the Black Sea within the economic zone of Russia in 2017–2019 were used. These surveys included more than 200 deep-sea stations at which the hydrochemical samples were taken at specific isopycnal surfaces by means of a cassette of 12 bathometers of the Sea-Bird 911 plus CTD Seabird-Electronics INC device; as a rule, it was a series of σt values equal to 16.30; 16.20; 16.10; 16.00; 15.95; 15.90; 15.80; 15.60; 15.40; 15.20; 15.00 and 14.60 kg/m3. Such a scheme made it possible to determine both general vertical distribution of oxygen in the oxycline and depth of hydrogen sulfide occurrence with an accuracy of up to 0.05 kg/m3 in the conventional density scale.
Conclusions. All surveys showed a decrease in oxygen content with depth (and occurrence of oxycline, respectively) starting below isopycnal surface σt = 14.5 kg/m3. The position of sub-oxygen zone upper boundary defined by the isoooxygen 10 µM was not strictly isopycnal, but fell on the range of isopycns σt = 15.7–15.85 kg/m3. However, it was not possible to identify a relation between the change in the position of the upper boundary and a certain hydrological season. For example, the deepest occurrence of the upper boundary below σt = 15.8 kg/m3 was observed both in November and December 2017 and August 2018. The lowering of isoooxygen 10 µM to σt = 15.9 kg/m3 in the Kerch shelf area is related to a more voluminous and colder intermediate layer above the shelf in December 2017. Only one of 11 surveys showed the position of hydrogen sulfide upper boundary determined by isosulfide 3 µM raised almost to σt = 16.0 kg/m3 in April 2017. In all other cases (including the one in August 2017, i.e. six months after its rising), it was invariably found within range σt = 16.10–16.15 kg/m3. The concentration of hydrogen sulfide at the depths of 1750–2000 m has remained unchanged at level 383 ± 2 µM over the past 25 years.