Surface distribution of iodine-129 in the southwestern Okhotsk Sea in 2018

Full text: The investigation of water dynamics change in the East Asian marginal seas such as the Okhotsk Sea and Sea of Japan caused by the recent global warming is essential for forecasting the response of ocean circulation with climate change. Anthropogenic ¹²⁹I (half-life: 15.7 million years) produced from the thermal neutron fission, is dominated by release from nuclear fuel reprocessing plants in the Europe and supplied at the Okhotsk Sea via atmospheric deposition and surface runoff. To illuminate the availability of ¹²⁹I as a tracer of surface circulation in the Okhotsk Sea, we investigated the horizontal distribution of ¹²⁹I in 2018. Surface seawater samples with a depth 0 m of 1 L were collected at 15 stations in the Okhotsk Sea and Sea of Japan using a bucket during the Wakataka Maru expedition in September 2018 (WK18-09). After adding 1 mg of iodine carrier (Woodward old iodine) with an ¹²⁹I/ ¹²⁷I ratio of 1.5 × 10^-4 to the filtered 500 ml seawater sample, the iodine was isolated by solvent extraction with CCl₄. The purified iodide was precipitated as AgI by adding AgNO₃. The AgI precipitate was then washed with NH₄OH and ultra-pure water, and was dried and loaded into an Al holder with Nb powder. The ¹²⁹I/ ¹²⁷I ratio of the AgI targets was measured using the accelerator mass spectrometry (AMS) system at the Tandem Accelerator Complex, University of Tsukuba. A terminal voltage of 5 MV and a charge state of 5+ were chosen for acceleration and detection. The measurement ratios were normalized against the S-Purdue (Z94-0597) reference material, which had an ¹²⁹I/ ¹²⁷I ratio of 8.378 × 10^-12 and was provided by the Purdue Rare Isotope Measurement Laboratory at Purdue University, USA. Stable iodine (¹²⁷I) in the seawater was measured by an inductively coupled plasma–mass spectrometry (ICP–MS). The original ¹²⁹I/ ¹²⁷I ratios and ¹²⁹I concentrations in the seawater were calculated using ¹²⁹I/ ¹²⁷I ratio from AMS and ¹²⁷I concentration from ICP-MS. The dissolved ¹²⁹I in surface water in the Okhotsk Sea varied from 15.0±0.7 to 26.6±0.8 nBq/L, and was negatively correlated with water temperature (R² = 0.67) and salinity (R² = 0.68). This water temperature and salinity-dependent distributions revealed that the dissolved ¹²⁹I in the area was controlled by mixing of water mass from the Okhotsk Sea surface seawater with cold and lower salinity and the Soya warm current from the Sea of Japan with warm and higher salinity. ¹²⁹I is considered to be a tracer for the front of the Okhotsk Sea surface water.