title: Impact of Fukushima-derived radiocesium in the western North Pacific Ocean about ten months after the Fukushima Dai-ichi nuclear power plant accident.
abstract: We measured vertical distributions of radiocesium ((134)Cs and (137)Cs) at stations along the 149°E meridian in the western North Pacific during winter 2012, about ten months after the Fukushima Dai-ichi Nuclear Power Plant (FNPP1) accident. The Fukushima-derived (134)Cs activity concentration and water-column inventory were largest in the transition region between 35 and 40°N approximately due to the directed discharge of the contaminated water from the FNPP1. The bomb-derived (137)Cs activity concentration just before the FNPP1 accident was derived from the excess (137)Cs activity concentration relative to the (134)Cs activity concentration. The water-column inventory of the bomb-derived (137)Cs was largest in the subtropical region south of 35°N, which implies that the Fukushima-derived (134)Cs will also be transported from the transition region to the subtropical region in the coming decades. Mean values of the water-column inventories decay-corrected for the Fukushima-derived (134)Cs and the bomb-derived (137)Cs were estimated to be 1020 ± 80 and 820 ± 120 Bq m(-2), respectively, suggesting that in winter 2012 the impact of the FNPP1 accident in the western North Pacific Ocean was nearly the same as that of nuclear weapons testing. Relationship between the water-column inventory and the activity concentration in surface water for the radiocesium is essential information for future evaluation of the total amount of Fukushima-derived radiocesium released into the North Pacific Ocean.
authors.affiliation: Research and Development Center for Global Change, Japan Agency for Marine-Earth Science and Technology, 2-15 Natushima-cho, Yokosuka, Kanagawa 237-0061, Japan. Electronic address: kumamoto@jamstec.go.jp.
authors.forename: Yuichiro
authors.initials: Y
authors.lastname: Kumamoto
authors.affiliation: Institute of Environmental Radioactivity, Fukushima University, 1-1 Kanayagawa, Fukushima 960-1296, Japan.
authors.forename: Michio
authors.initials: M
authors.lastname: Aoyama
authors.affiliation: Low Level Radioactivity Laboratory, Kanazawa University, Wake, Nomi, Ishikawa 923-1224, Japan.
authors.forename: Yasunori
authors.initials: Y
authors.lastname: Hamajima
authors.affiliation: Research and Development Center for Global Change, Japan Agency for Marine-Earth Science and Technology, 2-15 Natushima-cho, Yokosuka, Kanagawa 237-0061, Japan.
authors.forename: Akihiko
authors.initials: A
authors.lastname: Murata
authors.affiliation: Research and Development Center for Global Change, Japan Agency for Marine-Earth Science and Technology, 2-15 Natushima-cho, Yokosuka, Kanagawa 237-0061, Japan.
authors.forename: Takeshi
authors.initials: T
authors.lastname: Kawano
mesh_terms: D002588:Cesium Radioisotopes / Q000032:analysis*; D062406:Fukushima Nuclear Accident; D010140:Pacific Ocean; D011834:Radiation Monitoring; D014874:Water Pollutants, Chemical / Q000032:analysis*
chemical_list: D002588:Cesium Radioisotopes; D014874:Water Pollutants, Chemical
keywords: Fukushima Dai-ichi nuclear power plant accident; Nuclear weapons testing; Radiocesium; Seawater; Western North Pacific Ocean