Keywords:radiocaesium, Fukushima Dai-ichi Nuclear Power Plant accident , re-circulation of Sea of Japan, subduction, obduction, SubTropical Mode Water
The 137Cs derived from the Fukushima Nuclear Power Plant Accident (FNPP1-137Cs) rapidly transported to the Sea of Japan several years after its release to the environment in March 2011. The inflow of FNPP1-137Cs had started in 2012 and reached to the maximum in 2015/2016, and has been still continued in the coastal site of Sea of Japan in the year of 2016. In the south of the Japanese islands, the FNPP1-137Cs activity concentrations showed subsurface peak in the seawater of which density correspond to the Subtropical Mode Water (STMW). These suggests that FNPP1-137Cs injected into the western North Pacific Ocean at south of Kuroshio were subducted into the ocean interior just after the accident, then transported southward/southwestward. A part of FNPP1-137Cs in STMW reaches the western boundary at lower latitudes, and obducted from under the Kuroshio, and is transported to the west of Kyushu by Tsushima Warm Current bifurcated from the Kuroshio. This pathway might be new finding of transport process from the western North Pacific Ocean to the SOJ. Almost same value of the 134Cs/137Cs activity ratio in the coastal region of the Japanese islands (ECS, SOJ, and south of the Japanese islands in the western north Pacific Ocean) also support this circulation route. The integrated amount of FNPP1- 137Cs entered in the SOJ until 2016 was estimated to be 0.20±0.023 PBq, which corresponds to 4.8 % of the total amount of FNPP1-137Cs in the STMW. The integrated amount of FNPP1-137Cs back to the North Pacific Ocean through the Tsugaru Straight in the surface layer was 0.081±0.005 Bq, which corresponds to 1.9 % of the total amount of FNPP1-137Cs in the STMW.