11:30 AM - 11:45 AM
[MAG34-09] Long-term variation of radiocaesium in the ocean interior in the western North Pacific
Keywords:Fukushima Dai-ichi Nuclear Power Station, radiocaesium, subtropical mode water (STMW)
This study summarizes the spatiotemporal variation in the distribution of radiocesium from the Fukushima Daiichi Nuclear Power Station (F1NPS) accident in the ocean interior of the North Pacific from 2011 to 2017. The area where radiocaesium derived from the F1NPS accident was deposited on the sea surface included the mode water formation area in the western North Pacific. The part of radiocaesium deposited on the sea surface immediately after the accident was intruded into the mode water and distributed in the ocean interior. The authors have conducted vertical cross-sectional observations in the North Pacific Ocean from 2011 to 2017.
The intrusion of radiocaesium from the F1NPS accident into STMW occurred during the winter when mode water was formed. Therefore, most of the intrusion was considered to have happened in March 2011 and winter of 2011/2012, when radiocaesium from the F1NPS accident was observed in the STMW formation area in the surface layer. After that, radiocaesium from the F1NPS in the STMW in the western North Pacific subtropical zone was confirmed until 2017, with a vertical distribution peak at around 300 m depth. The total amount of 137Cs originating from the F1NPS accident in STMW estimated based on the data from 2013 (4.3±1.5 PBq) was almost the same as that estimated in 2012 (4.2±1.1 PBq; Kaeriyama et al., 2016). A gradual decrease in the total amount of 137Cs from the F1NPS was observed in the southwestern part of the STMW distribution area. This decrease could be partially attributed to transport outside the STMW, such as the East China Sea and the Sea of Japan (Inomata et al., 2018). Alternatively, radiocesium from the F1NPS accident may have been diluted with radiocesium of global fallout origin or promoted circulation within the distribution area of STMW between 2013 and 2017.
This presentation is part of the forthcoming book "Radionuclides in the Marine Environment: Scientific view on the Fukushima Dai-ichi Nuclear Power Station accident by 7 Oceanographers.
References
Inomata et al. (2018) Ocean Sci., 14, 413-826.
Kaeriyama et al. (2016) Sci. Rep., 6, 22010.
The intrusion of radiocaesium from the F1NPS accident into STMW occurred during the winter when mode water was formed. Therefore, most of the intrusion was considered to have happened in March 2011 and winter of 2011/2012, when radiocaesium from the F1NPS accident was observed in the STMW formation area in the surface layer. After that, radiocaesium from the F1NPS in the STMW in the western North Pacific subtropical zone was confirmed until 2017, with a vertical distribution peak at around 300 m depth. The total amount of 137Cs originating from the F1NPS accident in STMW estimated based on the data from 2013 (4.3±1.5 PBq) was almost the same as that estimated in 2012 (4.2±1.1 PBq; Kaeriyama et al., 2016). A gradual decrease in the total amount of 137Cs from the F1NPS was observed in the southwestern part of the STMW distribution area. This decrease could be partially attributed to transport outside the STMW, such as the East China Sea and the Sea of Japan (Inomata et al., 2018). Alternatively, radiocesium from the F1NPS accident may have been diluted with radiocesium of global fallout origin or promoted circulation within the distribution area of STMW between 2013 and 2017.
This presentation is part of the forthcoming book "Radionuclides in the Marine Environment: Scientific view on the Fukushima Dai-ichi Nuclear Power Station accident by 7 Oceanographers.
References
Inomata et al. (2018) Ocean Sci., 14, 413-826.
Kaeriyama et al. (2016) Sci. Rep., 6, 22010.