Spatial and vertical distribution of ¹²⁹I and ¹²⁷I in the East China Sea: Inventory, source and transportation

Iodirne-129 is useful for tracking water mass movement in the ocean. In this study, the concentration of iodine isotopes in seawater of the East China Sea (ECS) in October 2013 were analyzed to investigate the spatial and vertical distribution of ¹²⁹I and ¹²⁷I to understand water mass exchange. Results showed that the ¹²⁹I/¹²⁷I atomic ratios varied with the water mass, with higher values of (10-20) × 10-11 in the coastal regions and lower values of b8 × 10−11 offshore. Inventories of ¹²⁹I were estimated to be (0.23–1.7) × 10¹² atoms m−2 (n = 18) in upper 100 m waters, which is comparable to those of other regions without being contaminated by the nuclear accidents or nuclear reprocessing facilities. The total amount of ¹²⁹I in the ECS water column was estimated to be 88 g in which over 90% is attributed to the oceanic input (e.g., West Pacific) via the Kuroshio Current (KC). The contributions of ¹²⁹I from Changjiang (Yangtze River) terrestrial watershed (b7.5%) and atmospheric fallout (b2.7%) were small. Those from the Fukushima accident were negligible during this investigation. The ¹²⁹I/¹²⁷I ratios versus salinity distribution showed the range and stratification of the Changjiang, Yellow Sea, and KC waters in the ECS. Our study shows that the Changjiang fresh water could be transported to the North Jiangsu coast in October; the Taiwan Warm Current water could intrude to Northern part of the Changjiang Estuary (32°N). Besides, our results suggest that the ¹²⁹I/¹²⁷I profile is useful to indicate the seawater mixing process in ocean marginal systems.