Fukushima-derived radiocesium in the subarctic and transition areas of the North Pacific in 2017.

The East-Japan Earthquake and the subsequent tsunami on 11 March 2011 damaged the Fukushima Dai-ichi Nuclear Power Plant (FNPP1). As a result, radioactive cesium isotopes (134Cs and 137Cs) and other radionuclides were released into the North Pacific Ocean. Over the past decade, previous studies have examined the transport processes and total inventory of the FNPP1-derived radiocesium in the North Pacific. In this study, we measured vertical profiles of dissolved radiocesium at stations along nominal 47°N zonal and 145°W meridional lines during the summer of 2017. With these data, we delineated the three-dimensional distribution of the FNPP1-derived radiocesium in the subarctic and transition areas of the North Pacific and discussed its transport process by 2017. Additionally, we estimated the total inventory of the FNPP1-derived radiocesium in this area in 2017 to be approximately 6 PBq. This inventory in 2017 was approximately half of that estimated in 2014 (12 PBq). The missing inventory of approximately 6 PBq had been partially transported northward into the Bering Sea (0.3-1.3 PBq). In contrast, the expected southward transport from the subarctic/transition to the subtropical areas has not yet been observed. We proposed that one of the possible export paths is subduction in the eastern subarctic/transition area, followed by southward transport through the subsurface layer to the eastern subtropical and then the tropical areas.