Spatial and temporal variation of dissolved iodine in the eastern Arabian Sea

Abstract

Here we report the seasonal and spatial variation of iodide (I⁻) and total dissolved iodine (TI) along a transect covering the oxygen minimum zone (OMZ) in the Arabian Sea (AS). This data is generated after a gap of more than two decades since the US-Joint Global Ocean Flux Study (JGOFS) reported observations of iodine speciation in the region. Also, this is the first study to report on iodine speciation from the coastal region of the west coast of India. Here the water samples were collected along the meridional transect (68°E) during spring inter-monsoon (SIM), southwest monsoon (SWM), and fall inter-monsoon (FIM), and from the coastal Goa transect during late SWM and FIM. In addition, sediment porewater was also collected from the coastal Goa transect during late SWM. Iodide was found to be dominant in the OMZ, whereas TI was majorly represented by iodate below the OMZ. The I⁻ and TI concentrations were higher around the 15°N stations during all three seasons sampled. Seasonal variability was observed in the study, with high I⁻ and TI during the SWM and FIM as compared to SIM. The coastal region was observed to have high I⁻ values during seasonal anoxia in FIM as compared to the late SWM. This may be due to the release of I⁻ from the sediments which are extremely rich in I⁻. The porewater showed extremely high I⁻ (up to 4545 nM) at the shelf stations, which is 10 times higher than what is measured in the overlying waters. The US-JGOFS study reported iodide values as high as 953 nM in the AS OMZ with excess iodine (difference between the measured TI and the predicted TI) of 400 nM and attributed this to the input from the shelf sediments. In the present study TI values up to 811 nM were observed in the AS. To ascertain the source of excess iodine, porewater extracted from the sediments covering shelf and open ocean stations along the Goa transect was studied for iodine speciation. The diffusive flux of I⁻ calculated from the surface sediments ranged between 14 and 43 μmol I⁻ m⁻² d⁻¹ from the shelf stations off Goa, which is high enough to explain the excess iodine concentrations seen in the OMZ. Apart from the seasonality observed in iodine concentrations, our study infers that the shelf sediments along the west coast of India act as a source of iodine to the AS OMZ.