[A-HW22] Hydrological Cycle and Water Environment

[AHW22-19] Anthropogenic iodine-129 in the Japan Sea Bottom Water and Dense Shelf Water of Okhotsk Sea

*Tetsuya Matsunaka¹, Seiya Nagao¹, Mutsuo Inoue¹, Shinya Ochiai¹, Takami Morita², Shizuho Miki², Takafumi Aramaki³, Isao Kudo⁴, Naoto Honda², Tetsutaro Takikawa⁵, Kimikazu Sasa⁶, Maki Honda⁶, Keisuke Sueki⁶ (1.Institute of Nature and Environmental Technology, Kanazawa University, 2.Japan Fisheries Research and Education Agency, 3.National Institute for Environmental Studies, 4.Hokkaido University, 5.Nagasaki University, 6.University of Tsukuba)

Keywords:Japan Sea, Okhotsk Sea, Bottom water, Dense shelf water , Iodine-129

Long-term oceanographic observation revealed that warming and oxygen decrease of the Japan Sea Bottom Water (JSBW) and Dense Shelf Water (DSW) of Okhotsk Sea are responding to air temperature raise in winter [1–2]. The investigation of water dynamics in the Japan Sea and Okhotsk Sea using radioactive tracer are essential for elucidating the interaction between climate change and these convection systems. Anthropogenic ¹²⁹I (T_1/2 = 15.7 million years) produced from the thermal neutron fission, is dominated by release from nuclear fuel reprocessing plant in the Europe. To illuminate the availability of ¹²⁹I as a tracer of surface circulations and vertical convections in the Japan Sea and Okhotsk Sea, we investigated the horizontal and vertical distributions of the ¹²⁹I at the large area of these seas in 2017. The dissolved ¹²⁹I in surface water varied from 17.3±0.7 to 23.1±1.0 nBq L^–1 at the area of 38–46°N and 135–141°E in the Japan Sea, and was negatively correlated with salinity (R² = 0.82, n = 9). This salinity-dependent distribution revealed that the dissolved ¹²⁹I in the area was controlled by mixing of water mass from the Liman Current to the Tsushima Current. Meanwhile, the dissolved ¹²⁹I in the JSBW observed at the layer of 2480–3500 m of the Japan Basin in 2017 was 4.1±0.5 nBq L^–1, which increased by 1.1 nBq L^–1 in comparison to that in 2007 [4]. The estimated turnover time for the JSBW of 205±25 years using the ¹²⁹I was within the range of 75–380 years evaluated by bomb-derived ¹⁴C [5–6]. The ¹²⁹I is considered to be a tracer for the surface subarctic front and deep JSBW in the Japan Sea.

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