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Ocean Science An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/os-2017-90
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
29 Nov 2017
Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Ocean Science (OS).
Rapid recirculation of FNPP1 derived radiocaesium suggesting new pathway of subtropical mode water in the western North Pacific to the Sea of Japan
Yayoi Inomata1, Michio Aoyama2, Yasunori Hamajima1, and Masatoshi Yamada3 1Institute of Nature and Environmental Technology, Kanazawa University, Kanazawa, 920-1156, Japan
2Institute of Environmental Radioactivity, Fukushima University, Fukushima, 960-1296, Japan
3Institute of Radiation Emergency Medicine, Hirosaki University, Hirosaki, 036-8564, Japan
Abstract. The rapid recirculation of 137Cs derived from the Fukushima Nuclear Power Plant Accident (FNPP1-137Cs) occurred in the Sea of Japan (SOJ) in several year timescale after released to the environment in March 2011. The recirculation of FNPP1-137Cs had started in 2012 and reached to the maximum in 2015/2016 in the East China Sea (ECS) and the western North Pacific Ocean. The recirculation of FNPP1-137Cs has been still continued in the coastal site of Sea of Japan in the year of 2016. The re-circulated 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 ocean interior just after the accident, then transported southward/southwestward. A part of FNPP1-137Cs in STMW entered into the ECS between Kyushu Island and Okinawa Islands. Then it obducted in the region west of Kyushu Island, north of the ECS, following then entered into the SOJ associated with northward transport with Tsushima Warm Current in several year time scale. This rapid 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 Ogasawara) also support this re-circulation route. The integrated amount of FNPP1137Cs entered in the SOJ until 2016 was estimated to be 0.21&thins;± 0.03 PBq, which corresponds to 5.1 (3.4–8.0) % 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 0.1 ± 0.02 Bq, which corresponds to 0.6 (0.4–1.0) % of the total amount of FNPP1-137Cs in the STMW.

Citation: Inomata, Y., Aoyama, M., Hamajima, Y., and Yamada, M.: Rapid recirculation of FNPP1 derived radiocaesium suggesting new pathway of subtropical mode water in the western North Pacific to the Sea of Japan, Ocean Sci. Discuss., https://doi.org/10.5194/os-2017-90, in review, 2017.
Yayoi Inomata et al.
Yayoi Inomata et al.
Yayoi Inomata et al.

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Short summary
By analysis of increased 137Cs and 134Cs / 137Cs ratio derived from the Fukushima Nuclear Power Plant accident in the Sea of Japan and its marginal sea, we found rapid transport process associated with subduction in the Subtropical mode water formation region and obduction in the north East China Sea. The integrated amount of FNPP1-137Cs entered in the Sea of Japan until 2016 was estimated to be 0.21 ± 0.03 PBq, which corresponds to 5.1 (3.4–8.0) % of the total amount of FNPP1-137Cs in the STMW.
By analysis of increased 137Cs and 134Cs / 137Cs ratio derived from the Fukushima Nuclear Power...
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