Biogeochemical cycling of iodine in the Bay of Bengal: A comparison with the Arabian Sea.

Iodine is a redox-sensitive element and a potential oxidant for the respiration of organic matter. Here we report the spatial variation of dissolved iodine in the Bay of Bengal (BoB) oxygen minimum zone (OMZ) and compare it with that of the Arabian Sea (AS). Subsurface iodide peaks were observed in the upper boundary of the OMZ, representing 20 to 70 % of the total iodine budget in the BoB. In contrast, iodide accounts for the entire iodine budget in the AS OMZ. Secondary nitrite maxima (SNM) were absent in most of the stations in BoB unlike the AS. Incubation experiments suggest a simultaneous build-up of nitrite and iodide under the suboxic condition, however, the absence of SNM in the BoB OMZ suggests re-oxidation of nitrite by the remaining iodate present in the water column. Hence our study reveals important role of iodine in the OMZ of BoB which remains non-denitrifying.