REDUCTION OF VOLATILE HALOCARBONS IN ANOXIC SEAWATER, RESULTS FROM A STUDY IN THE BLACK-SEA

The Black Sea is characterised by an oxic surface water layer and anox ic deep water rich in both hydrogen sulphide and methane, and in betwe en a suboxic zone with very low concentrations of both oxygen and hydr ogen sulphide. This makes the Black Sea a useful site for the study of oxidation-reduction reactions in seawater. In this study, the distrib utions of tetrachloromethane, trichloromethane, 1,1,1,-trichloroethane , dibromomethane, dibromochloromethane and bromodichloromethane across the oxic-anoxic interlace were related to the oxygen/hydrogen sulphid e concentrations and thereby the redox potential gradient. All of the investigated halocarbons decrease rapidly in concentration as the oxyg en decreases, most likely due to reduction. The results show that the higher the redox potential of a halocarbon, the higher up in the water column, i.e. at a higher oxygen level, reduction occurs. Most of the tetrachloromethane is transformed to trichloromethane as an intermedia te product.