Pelagic iodine cycling over the Western Indian margin: Impact of anoxic events and benthic flux
Abstract
The iodine cycling in marine environments plays a crucial role in oceanic biogeochemical processes and atmospheric chemistry. Excess iodine has been reported previously in both the perennial and seasonal oxygen minimum zones (OMZ) of the eastern Arabian Sea. To understand the factors behind the excess iodine and the pelagic iodine cycling in general, we carried out a comprehensive study across the Western Indian margin off Goa during 2021�2023, which involved water column and porewater measurements, and intact core incubation at an innershelf station. We observed the highest iodide concentrations (up to 730 nM) in the shelf bottom waters during the fall intermonsoon anoxic periods, compared to the normoxic period (130�390 nM). The intact core incubations revealed that during seasonal anoxia, the benthic release of iodide (?52 ?mol m?2d?1) from the innershelf sediments could potentially lead to an accumulation of excess iodide (240 nM) in the 13 m-thick suboxic/anoxic subsurface water. This suggested that the anoxic events over the shelf can trigger significant release of iodide from the sediments, which could in turn impact the pelagic iodine budget and cycling. Moreover, high porewater iodide concentration over the slope indicated that the diffusive benthic iodide fluxes could potentially result in the excess iodine reported previously in the perennial AS OMZ. While high porewater iodide over the slope indicates potential benthic contributions, the slope-to-OMZ transport mechanisms remain unresolved. This gap underscores the need for further investigation, particularly integrating physical oceanography and biogeochemistry, to refine our understanding of pelagic iodine cycling in anoxic systems.
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