Kaeriyama Hideki, Shimizu Yugo, Setou Takashi, Kumamoto Yuichiro, Okazaki Makoto, Ambe Daisuke, Ono Tsuneo
Research Center for Fisheries Oceanography and Marine Ecosystem, National Research Institute of Fisheries Sciences, Fisheries Research Agency, 2-12-4 Fukuura, Kanazawa, Yokohama, Kanagawa 236-8648, Japan.
Research and Development Center for Global Change, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsuhima-cho, Yokosuka, Kanagawa 237-0061, Japan.
Sci Rep. 2016 Feb 26;6:22010. doi: 10.1038/srep22010.
The Fukushima Dai-ichi Nuclear Power Plant accident in March 2011 released radiocaesium ((137)Cs and (134)Cs) into the North Pacific Ocean. Meridional transects of the vertical distribution of radiocaesium in seawater were measured along 147 °E and 155 °E in October-November 2012, 19 months after the accident. These measurements revealed subsurface peaks in radiocaesium concentrations at locations corresponding to two mode waters, Subtropical Mode Water and Central Mode Water. Mode water is a layer of almost vertically homogeneous water found over a large geographical area. Here we show that repeated formation of mode water during the two winter seasons after the Fukushima accident and subsequent outcropping into surface water transported radiocaesium downward and southward to subtropical regions of the North Pacific. The total amount of Fukushima-derived (134)Cs within Subtropical Mode Water, decay-corrected to April 2011, was estimated to be 4.2 ± 1.1 PBq in October-November 2012. This amount of (134)Cs corresponds to 22-28% of the total amount of (134)Cs released to the Pacific Ocean.
2011年3月发生的福岛第一核电站事故向北太平洋释放了放射性铯（¹³⁷Cs和¹³⁴Cs）。事故发生19个月后的2012年10月至11月，沿东经147°和东经155°测量了海水中放射性铯垂直分布的子午断面。这些测量揭示了在与两种模态水（亚热带模态水和中央模态水）相对应的位置，放射性铯浓度出现次表层峰值。模态水是在大面积地理区域发现的几乎垂直均匀的水层。在这里我们表明，福岛事故后两个冬季期间模态水的反复形成以及随后露升至地表水，将放射性铯向下和向南输送到北太平洋的亚热带地区。经衰变校正至自2011年4月起，2012年10月至11月期间亚热带模态水中源自福岛的¹³⁴Cs总量估计为4.2±1.1 PBq。该¹³⁴Cs量相当于释放到太平洋的¹³⁴Cs总量的22%-28%。
