Abstract
The Fukushima Dai-ichi Nuclear Power Plant accident in March 2011 released radiocaesium (137Cs and 134Cs) 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 134Cs 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 134Cs corresponds to 22–28% of the total amount of 134Cs released to the Pacific Ocean.
Highlights
The Fukushima Dai-ichi Nuclear Power Plant accident in March 2011 released radiocaesium (137Cs and 134Cs) into the North Pacific Ocean
Our observations showed that a large amount of FNPP-derived radiocaesium existed in the subtropical region south of the Kuroshio Extension (KE) 19 months after the FNPP accident
Subsurface peaks of FNPP-derived radiocaesium associated with Subtropical Mode Water (STMW) have been reported during December 2011 and February 2012 on a transect along 149 °E27, in December 2011 at 20–30 °N on a transect along 135 °E26, between August 2011 and March 2013 far south of the Japanese islands around 138 °E24, and in June 2012 off the coast of Taiwan[29]
Summary
The Fukushima Dai-ichi Nuclear Power Plant accident in March 2011 released radiocaesium (137Cs and 134Cs) into the North Pacific Ocean. The total amount of Fukushima-derived 134Cs 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 134Cs corresponds to 22–28% of the total amount of 134Cs released to the Pacific Ocean. Preliminary estimates of the total amount of radiocaesium trapped in the STMW are 6 PBq during March and April 2011 based on observations from December 2011 to February 201227, and an additional 5 PBq entered prior to April 2012 during the winter of 2011–2012 based on observations during September 201224 These estimates are based on limited data that were obtained from a single meridional transect along 149 °E27 or in the western part of the STMW (135 °E and 138 °E)[24], and spatial heterogeneity of radiocaesium was not considered.
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