SUBSURFACE IODIDE MAXIMA - EVIDENCE FOR BIOLOGICALLY CATALYZED REDOX CYCLING IN ARABIAN SEA OMZ DURING THE SW INTERMONSOON

Sub-surface I- maxima (200-600 nM) were found in five of the six profi les from the Somali and Arabian basins of the Northwest Indian Ocean. In addition to these maxima, dissolved I- exhibited normally high (100 nM or greater) values in the surface and values ranging from 3 to 40 nM at depth, which are higher than other open oceanic systems. Sulfide was generally found to be less than 2 nM in the water column, indicat ing that the chemical reduction of iodate by sulfide is not important in regulating iodine speciation in the Northwest Indian Ocean. These n ovel high iodide values below the euphotic zone do not appear to be re lated to other bulk chemical or hydrographic features (sigma(theta)) b ut may be the result of two distinct biologically mediated remineraliz ation processes: (1) the direct reduction of IO3- to I- as seen in the sub-surface maxima, and (2) release of I- from C-I bonds during the d ecomposition of organic matter. Iodine normalized to salinity, specifi c iodine, is not found to be conservative in this system. Overall, our specific iodine data support the incorporation of iodine into organic material in the surface. Iodide, when present below the euphotic zone , is a product of the decomposition of that exported organic material. These data and processes are consistent with those found previously i n the Black Sea and the Chesapeake Bay. However, in the Northwest Arab ian Sea, iodide and oxygen are measurable throughout the water column, indicating that the system is not at equilibrium with the prevailing redox condition and that traditional thermodynamic considerations of p E do not appear to be applicable. Porewater iodide in the top 150 cm i ncreased with depth to approximately 19 mu M as a result of the releas e of I- during decomposition of organic matter. (C) 1997 Elsevier Scie nce Ltd.