Abstract
In 2015–2017, we measured activity concentration of radiocesium in the western subtropical gyre of the North Pacific Ocean and revealed the time evolution of radiocesium derived from the Fukushima Dai-ichi Nuclear Power Plant (FNPP1) accident between 2011 and 2017. The FNPP1-derived radiocesium deposited on the area south of the Kuroshio/Kuroshio Extension Currents in March 2011 was transported southward and westward through subsurface layers due to subduction of the subtropical mode water. In 2014, the radiocesium in the subsurface layers returned to the north and circulated within the Kuroshio recirculation area. Then in 2015–2017, the radiocesium re-circulated with the area.
Highlights
The massive Tohoku earthquake and consequent giant tsunamis on 11 March 2011 resulted in serious damage to the Fukushima Dai-ichi Nuclear Power Plant (FNPP1) in eastern Japan
In 2015–2017, we measured activity concentration of radiocesium in the western subtropical gyre of the North Pacific Ocean and revealed the time evolution of radiocesium derived from the Fukushima Dai-ichi Nuclear Power Plant (FNPP1) accident between 2011 and 2017
In 2015–2017, the subsurface maximum of 134Cs still remained in 200–600 m depth in the western subtropical gyre of the North Pacific, south of the Kuroshio and Kuroshio Extension Currents (Fig. 1a, b)
Summary
The massive Tohoku earthquake and consequent giant tsunamis on 11 March 2011 resulted in serious damage to the Fukushima Dai-ichi Nuclear Power Plant (FNPP1) in eastern Japan. Radiocesium (134Cs and 137Cs) released from the damaged FNPP1 caused radioactive contamination in the land of eastern Japan and the North Pacific Ocean mostly in March and April 2011 [1]. Before the FNPP1 accident, radiocesium was released into the North Pacific by atmospheric nuclear weapons testing mainly in the 1950s and 1960s [5]. The bomb-derived 137Cs deposited on the North Pacific remained in the ocean in March 2011 because of its long half-life (30.17 years). After March 2011, the FNPP1derived 137Cs was added to the bomb-derived 137Cs, which resulted in about 30% increase of 137Cs activity in the North Pacific [6]. The 134Cs released before the FNPP1 accident had disappeared, because its half-life is only 2.06 years.
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