Fine hydrochemical structure of the redox zone in the black sea according to the results of measurements with an open oxygen sensor and with bottle samplers

Abstract

This study is based on the data obtained on July 14–17, 2004 over a section across the coastal anticyclonic eddy near the town of Tuapse. The data on the oxygen distribution obtained by the Winkler procedure and with an open (membrane-free) sensor with a 15-cm spatial resolution were compared. The divergence of the results in the lower part of the oxycline was 3 μmol or lower; the sensor error was estimated as 1 μmol. The previous data on the absence of the layer of oxygen and hydrogen sulfide coexistence were confirmed. It was shown that the lower boundary of oxygen permeation (where its concentration appeared to be below 1.0 μmol) closely coincided with the upper boundary of the bivalent manganese occurrence, because their coexistence layer was 1 ± 1 m in thickness. Within the same layer or 1–2 m above it, the upper boundary of the deep-sea ammonium and the phosphate minimum were situated. These data may be treated in the following way: oxygen is completely utilized for the oxidation of ammonia and bivalent manganese, and the oxidized suspended manganese formed sinks and is utilized in the reactions of hydrogen sulfide oxidation. The role of the near-slope downwelling (the supply of oxygen-enriched waters to the layers close to the redox zone), which was also observed in the cross section, is discussed.