Geochemical dynamics of iodine in marginal seas: the southern East China Sea

Abstract

The distributions of iodate and iodide were determined in a transect across the southern East China Sea Shelf, the upwelling center at the outer shelf-upper slope, the adjoining Kuroshio to the Okinawa Trough. In the surface waters, the variations in the concentrations of iodate and iodide among the surface waters masses, namely, the Coastal Water, the Taiwan Current Warm Water and the Kuroshio Surface Water, were relatively small. The composition of the upwelling Kuroshio Subsurface Water was distinctly different and it could be readily distinguished from the surface water masses by its elevated concentration of iodate, depressed concentration of iodide, and the resulting elevated concentration ratio of iodate to iodide. In contrast, since the variations in salinity and temperature in the surface water masses and the upwelling water were less systematic, neither salinity nor temperature could be used effectively for distinguishing these two types of water from each other. Thus, the iodine system was a complementary, or even a superior, tracer for the upwelling water. In the subsurface, the upwelling water was depicted as a dome of cold, nitrate-rich, iodate-rich and iodide-poor water, with molar ratios of iodate to iodide exceeding 10 that extended from the outer shelf-upper slope to the middle shelf. The influence of the Coastal Water was manifested as a tongue of fresher, iodide-rich and iodate-poor surface water, with molar ratios of iodate to iodide of <2.5 that extended from the inner shelf into the middle shelf. The oligotrophic Kuroshio Surface Water was also iodide-rich and iodate-poor. A box model calculation revealed that iodate was imported into the East China Sea Shelf system by frontal exchanges with the adjoining Kuroshio and upwelling at the shelf edge. Within the Shelf system, iodate was consumed and iodide was produced. As a result, during frontal exchanges between the Shelf system and the Kuroshio, there was also a net export of iodide from the Shelf system to the Kuroshio. These behaviors of the iodine system in the East China Sea Shelf system are similar to those found in the South Atlantic Bight and suggest that the marginal seas may be a preferred site for the reduction of iodate to iodide so that exchanges between the marginal seas and the open oceans may constitute a significant net source of iodide to the ocean interior.