Sub-surface 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 profiles 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, indicating that the chemical reduction of iodate by sulfide is not important in regulating iodine speciation in the Northwest Indian Ocean. These novel high iodide values below the euphotic zone do not appear to be related to other bulk chemical or hydrographic features (σθ) but may be the result of two distinct biologically mediated remineralization processes: (1) the direct reduction of I03− to I− as seen in the sub-surface maxima, and (2) release of I− from CI bonds during the decomposition of organic matter. Iodine normalized to salinity, specific 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 in the Black Sea and the Chesapeake Bay. However, in the Northwest Arabian 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 pE do not appear to be applicable. Porewater iodide in the top 150 cm increased with depth to approximately 19 μM as a result of the release of I− during decomposition of organic matter.