Description of methods

Dissolved oxygen.

Reagent preparation, standardization of thiosulfate solution, water sampling, analysis and calculations of the dissolved oxygen in sea water are based on Carpenter's modification of the Winkler technique (Carpenter, J.N., 1965, The Chesapeake Bay Institute technique for the Winkler oxygen method. Limnology and Oceanography, vol.10, 141-143). 
Dry customs made BOD flasks of 100 ml nominal capacities with ground glass stoppers and blown up with argon gas were used for sampling from suboxic zone. Metrohm 765 Dosimat burette with none diffusive tip was used for whole bottle titration of samples. Taking into account, that 0.0017 ml of oxygen are inserted into sea water sample with the pickling reagents (Murray C.N., Riley J.P and Wilson T.R.S. 1968. The solubility of oxygen in Winkler reagents used for the determination of dissolved oxygen. Deep-Sea Res., vol.15, 237-238) we have subtracted 0.7 uM from initial results (average volume of BOD flasks equals 117 ml). Analysis of 14 samples drawn from 14 Niskin bottlles closed at the same depth (79 m, sigma-t 15.6) gave mean value 4.9 M 0.17 M (standard deviation).

Hydrogen sulfide

Hydrogen sulfide(total amount of the reduced species of sulfur) determined by the iodometric titration method.
Water samples are drawn carefully up to the mark in 250 ml volumetric flasks blown up previously with argon and containing 1 ml of diluted hydrochloric acid (18 %, v/v) and aliquots of iodine solution (2, 5 or 10 ml) added by high accuracy dispenser or pipette. Titrate the rest of iodine using the same Metrohm 765 Dosimat and thiosulphate solution as in oxygen procedure. The blank value or relation between iodine solution pipetted into the volumetric flask and thiosulphate solution is established by using seawater samples from the layer located just above the upper boundary of anaerobic zone (sigma-t 15.8-16.0). But some uncertainty with the blank value exists. It is connected with variable content of substances in the bottom part of suboxic zone that are able to consume iodine or produce it. As a result in spite of quite sufficient reproducibility of the method (standard deviation is 1.3 M for 2 M; 0.8 M for 40 M and 2.4 M for 380 M) the true absolute value of hydrogen sulfide concentration lies in the range 3 M from the calculated value. The absolute values can be corrected later by comparison of the depth of hydrogen sulfide onset determined by both iodometric and more sensitive voltammetric methods.
Detale description of manual procedures used in Knorr-2001 cruise for dissolved oxygen and hydrogen sulfide is in the "Manual and automated methods for sampling, measurement, and evaluation of oceanographic physical, chemical and biological data of the Black Sea", 1996, TU Black Sea Project, Institute of Marine Sciences, Middle East Technical University, Erdemli, Turkey.