Reconstruction of anthropogenic ¹²⁹I temporal variation in the Japan Sea using a coral core sample

The anthropogenic long-lived radionuclide ¹²⁹I is receiving increased attraction as a new oceanic tracer in addition to usage as a fingerprint of radionuclide contamination of the marine environment. To demonstrate the robustness of ¹²⁹I as an oceanic tracer in the Northwest Pacific area, specifically in the Japan Sea, the input history of ¹²⁹I to surface seawater was reconstructed using a hermatypic coral core sample from Iki Island in the Tsushima strait. Iodine isotopes in each annual band were measured using AMS and ICP-MS after appropriate pre-treatments of small amounts of coral powder. The ¹²⁹I/¹²⁷I ratios in the 1940s were almost at background levels (<1 × 10^-11) and increased abruptly in the early 1950s. Thereafter, the ratios continuously increased with some fluctuations; the maximum ratio, 7.13 ± 0.72 × 10^-11, being found in the late 1990s. After that period, the ratios remained nearly constant until the present time (2011). The ¹²⁹I originated mainly from the nuclear weapons testings of the 1950s and the early 1960s, and from airborne releasing by nuclear reprocessing facilities. The dataset obtained here was used to construct a simple model to estimate the diffusion coefficient of ¹²⁹I in the Japan Sea. The ¹²⁹I/²³⁶U ratios over the observation period were also reconstructed to help constraining sources of ¹²⁹I to the marine environment. Based on the results, the ¹²⁹I/²³⁶U ratio obtained here could be an endmember of the water mass in the Kuroshio Current area of the Northwest Pacific Ocean.