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 (T1/2 = 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 . 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.
 T. Gamo, Trends anal. chem., 30, 1308–1319 (2011)
 T. Nakanowatari et al., Geophys. Res. Lett., 34, 1955–2004 (2007)
 T. Suzuki et al., Nucl. Instr. Meth. B, 294, 563–567 (2013)
 T. Suzuki et al., Nucl. Instr. Meth. B, 268, 1229–1331 (2010)
 T. Gamo and Y. Horibe, J. Oceanogr. Soc. Japan, 39, 220–230 (1983)
 Y. Kumamoto et al., J. Oceanogr., 64, 429–441 (2008)