ON THE EXISTENCE OF FREE AND METAL COMPLEXED SULFIDE IN THE ARABIAN SEA AND ITS OXYGEN MINIMUM ZONE

Free hydrogen sulfide was not detected in the oxygen minimum zone (OMZ ) of the Arabian Sea during legs D1 (September 1992) and D3 (October-N ovember 1992) of the Netherlands Indian Ocean Programme (NIOP). Howeve r, sulfide complexed to metals was detected by cathodic stripping squa re wave voltammetry at 2 nM or less throughout the water column. A sli ght increase in sulfide was measured in the OMZ relative to the surfac e waters and may be related to sulfur release from organic matter duri ng decomposition. Sulfide complexes are of two general types at low co ncentrations of metal and sulfide. First, metals such as Mn, Fe, Co an d Ni form complexes with bisulfide ion (HS-) that are kinetically labi le to dissociation and are reactive. Second, metals such as Cu and Zn form multinuclear complexes with sulfide (S2-) that are kinetically in ert to dissociation; thus, they are less reactive than free (bi)sulfid e and the labile metal bisulfide complexes. Zinc and copper sulfide co mplexes are important in allowing hydrogen sulfide to persist in seawa ter which contains measurable oxygen. (C) 1997 Elsevier Science Ltd.