- Dona Paula, Goa, India.
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| Abstract Submission No. | ABS-2022-05-0168 |
| Title of Abstract | Iodine cycling in the oxygen minimum zones of the Northern Indian Ocean |
| Authors | Adnan Shaikh*, Siby Kurian, Damodar M. Shenoy, Anil K. Pratihary, Richita Naik |
| Organisation | CSIR-National Institute of Oceanography |
| Address | Chemical Oceanography Division, CSIR-National Institute of Oceanography Dona Paula, Goa, India Pincode: 403004 Mobile: 8007065523 E-mail: sadnan@nio.org |
| Country | India |
| Presentation | Oral |
| Abstract | Iodine is an important element in the metabolism of marine plants and animals and a potential oxidant for the respiration of organic matter. It exists in the ocean predominantly as dissolved iodate (IO 3 - ), iodide (I - ) and a small proportion of organic iodine. Oxygen minimum zones (OMZ) represent key regions of enhanced cycling of biologically active elements in the oceans. In the northern Indian Ocean, the Arabian Sea (AS) and the Bay of Bengal (BoB) house two unique low oxygen environments of the world⿿s ocean, which contribute to nearly 1/3 rd of the nitrogen loss from the ocean. While the AS OMZ is net denitrifying, the BoB OMZ is traditionally known to be non-denitrifying. Dissolved iodide and total iodine concentrations were measured over a latitudinal transect from 9° to 21°N (stations II-2 to II-14) along the 68°E in the AS during late spring inter monsoon (SIM)-early southwest monsoon (SWM) (May-June 2019) and peak SWM (July 2020), and at several stations spread between 15° to 20°N and 82° to 90°E during the SWM (September 2019) in the BoB. Analyses for dissolved iodide and total iodine (after reduction using ascorbic acid) were done using Cathodic Stripping Square Wave Voltammetry following standard protocols. Dissolved iodide concentrations up to 716 nM were measured within the AS OMZ in association with the secondary nitrite maxima (SNM). Higher concentrations of dissolved iodide were observed towards the north. Both iodide and total iodine were higher during peak SWM (up to 716 nM) as compared to the early SWM (up to 614 nM). The southern boundary of iodide and SNM shifted from II-06 (13°N) during early SWM to II-04 (11°N) during peak SWM suggesting seasonal variation and expansion of the AS OMZ during the peak SWM. While the secondary nitrite was absent in the BoB OMZ, iodide maxima (maximum of 578 nM) were present at several stations. Based on these results we speculate that iodate reduction may play an important role in the respiration of organic matter in the OMZ of the BoB. To understand the long-term changes in iodide concentrations in the AS, present data were compared with that of US JGOFS, sampled in 1995 (SIM, March-April). Dissolved iodide concentration up to 953 nM was reported within the OMZ during JGOFS and was attributed to the advection from the margin sediments off India. However, our data showed a significant decrease (~25%) in the dissolved iodide concentrations in the OMZ of the AS. The plausible sources of iodine in the OMZs of the northern Indian Ocean and the role of iodate in the respiration of organic matter will be addressed in the present study. |