A preliminary study on dissolved iodine in the northern South China Sea Shelf

We investigated dissolved iodine species in seawater from the northern South China Sea Shelf. Iodide concentrations were determined by cathodic stripping square wave voltammetry, and iodate was measured by spectrophotometry. Dissolved organic iodine(DOI) was measured with reference to reduced iodide. R-TDI(R-X or rationalized-X is the concentration of X normalized to a salinity of 35, TDI represents total dissolved iodine)was in the range of 0.43–0.46 μmol/L, showing a relatively conservative behavior, while iodate, iodide, and DOI showed non-conservative behaviors. Distribution characteristics in the surface waters showed R-iodate values in the 0.28–0.32 μmol/L range and an offshore>inshore trend, while R-iodide was in the 0.11–0.19 μmol/L range and R-DOI in the 0–0.07 μmol/L range, reflecting an inshore>offshore trend for both. The vertical distribution showed the highest R-iodide concentrations in the surface waters and decreased values with depth, reaching less than0.01 μmol/L at depths>200 m. R-iodate increased with depth with a measured peak value of 0.43 μmol/L.Seawater with high iodate/iodide ratio(up to 2.9) was found in the central upwelling region and gradually decreased to 2.0 far from this center. The relationship between R-iodide and R-iodate among all samples followed the 1:1 relationship with a slope slightly less than 1, indicating that the conversion between iodate and iodide species could not account for the observed changes. This finding also suggests that DOI may be an important participant in the mass balance. A box model was applied to calculate the input and output of iodine species, and the result showed that approximately 8% of iodate(1.50 × 10～8 mol/a) imported to the shelf sea was reduced.Concomitantly, the amount of iodide and DOI produced in the shelf amounted to 1.07 × 10～8 mol/a, roughly 14%higher than the input iodide.