Dissolved iodine species in the East China Sea—a complementary tracer for upwelling water on the shelf

Abstract

Samples were obtained at 26 stations in five cross-shelf transects in the East China Shelf-Sea for the determination of iodate and iodide. The upwelling water at the shelf-edge at 25.5°N was characterized as a dome of subsurface, cold (<18°C), nitrate-rich (>6μM), R-iodate-rich (>0.35μM) and R-iodide-poor (<0.05μM) water, with iodate/iodide of >5. (R-iodate and R-iodide were the concentrations of iodate and iodide normalized to a salinity of 35.) The upwelling water could be traced farthest away from its source region by the iodine species. They indicate that upwelling occurred at discrete upwelling centers at the shelf edge. Aside from the upwelling center at 25.5°N, a second center was found at 27°N as suggested in recent modeling exercises. Upwelling water was present along the entire length of the outer shelf in the study area from 25°N to 29°N. Its shoreward intrusion across the shelf was more extensive in the southern part of the Shelf-Sea where it reached the middle shelf. At the sea surface, the upwelling water could be detected as a continuous band of water, with low R-iodide (<0.12μM), high R-iodate (>0.3μM) and high iodate/iodide (>2.5), which traversed along the entire length of the middle/outer shelf. Thus, the influence of the upwelling water in the Shelf-Sea was more pervasive than what would be construed from the distributions of temperature and nitrate alone.High surface concentrations of dissolved organic iodine, DOI, were associated with the plume of Changjiang Diluted Water along the Chinese coast. While the changes in the concentrations of iodate in the surface waters could not be accounted for by the reduction of iodate to iodide alone, they could be largely explained when the conversion of iodate to DOI was also allowed.In box modeling exercises, the geochemical dynamics of the iodine system in the East China Sea Shelf system were qualitatively independent of the magnitude of the transport of the Taiwan Strait Warm Water used. Within the probable range of up to 1Sv, invariably, iodate was consumed and iodide was produced in the system. The accompanying net export of iodide to the adjoining Kuroshio and the Japan Sea was 1(±0.5)×109mol/year. Thus, marginal seas may act as a preferred site for the reduction of iodate to iodide and as a net source of iodide to the ocean interior. The production of iodide accounted for only 40(±10)% of the consumption of iodate, suggesting that iodate was converted not only to iodide but also to other forms of iodine in the Shelf system.