Intrusion of Fukushima-derived radiocaesium into subsurface water due to formation of mode waters in the North Pacific

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.

Oceanic dispersion of Fukushima‐derived radioactive cesium: a review

“…Kaeriyama et al . () also reported the Fukushima‐derived radioactive Cs in the CMW at just north of KE along 155˚E during October–November 2012. Yoshida et al .…”

“…After the FNPP accident, subsurface intrusion of Fukushima-derived radioactive Cs associated with these mode waters has been reported (Kitamura et al, 2013;Kumamoto et al, 2013Kumamoto et al, , 2014Kaeriyama et al, 2014Kaeriyama et al, , 2016Aoyama et al, 2015b;Men et al, 2015;Yoshida et al, 2015). Kitamura et al (2013), Kumamoto et al (2013Kumamoto et al ( , 2014 reported the subsurface peak of Fukushima-derived radioactive Cs within STMW along 149˚E during December 2011-February 2012, in the subtropical region far south from the FNPP-site and just south of KE.…”

“…Kaeriyama et al . () reported the Fukushima‐derived radioactive Cs during October–November 2012 in the 147˚E and 155˚E transects, areas where STMW formation occurs. Most of the Fukushima‐derived radioactive Cs occurred in the STMW and CMW rather than in the transition area during June–November 2012 (Fig.…”

“…(a) Sampling locations for radioactive Cs measurements during October and November 2012 (Kaeriyama et al ., ), in September 2012 (around 135–137 ˚E; Kaeriyama et al ., ), and in June 2012 (along 165˚E; Aoyama et al ., ) are shown. The sea surface height data based on 1‐week average gridded data (1/4°×1/4°) for 31 October 2012 are superimposed.…”

Spreading of Fukushima-derived Radiocesium in the Western North Pacific Ocean by the End of 2014

“…These were drawn using Ocean Data View software 55) and redrawn from , 147°E observed in November 2012 (b) 50) , 155°E observed in October 2012 (c) 50) , and 165°E observed between June and July 2012 (d) 30) . Contour intervals of Fig.…”