Anthropogenic iodine-129 tracks iodine cycling in the Arctic

Global warming has accelerated the iodine cycle in the Arctic Ocean, amplifying its significance in the global iodine cycle. However, some intricacies of the iodine cycle in the Arctic marine environment remain unclear. In this study, we present concentrations of stable- and radio-iodine (127I and 129I) in total iodine, iodide (I–), and iodate (IO3–) in seawater from the Beaufort, Chukchi, and Bering Seas. Results show a wide range of 129I/127I ratios of total iodine (3.2 × 10–12–9.3 × 10–9), indicating that the total 129I/127I ratio is a valuable fingerprint for identifying water masses and comprehending mixing processes. Iodine loss in surface seawater was quantitatively assessed using 129I, revealing that Arctic rivers substantially contribute to the emission of iodine into the atmosphere from the Arctic and subarctic seas. Our redox simulation in the Arctic intermediate layer shows the concentrations and 129I/127I ratios of I– and IO3– during its migration and provides the first evidence of an IO3– reduction process masked by the I– oxidation in the intermediate Arctic Ocean. Oxidation and reduction processes in the intermediate Arctic Ocean were characterized by extremely slow rates, at 0.95–2.41 and 0.26 ± 0.13 nM/year, respectively. Our findings demonstrate the effectiveness of anthropogenic 129I in tracking iodine cycling in the Arctic marine environment.