Transport of FNPP1-derived radiocaesium from subtropical mode water in the western North Pacific Ocean to the Sea of Japan

Abstract

Abstract. This study investigated the spatio-temporal variations in activity concentrations in the Sea of Japan (SOJ) of 137Cs and these transport process from the North Pacific Ocean to the SOJ through the East China Sea (ECS) during 2012–2016. The 137Cs activity concentrations in the SOJ have been increasing since 2012–2013 and reached a maximum in 2015–2016 of approximately 3.4 Bq m−3, more than twice the pre-Fukushima accident 137Cs activity concentration of ∼1.5 Bq m−3. The 134Cs ∕ 137Cs activity ratios ranged from 0.36 to 0.51 in 2016. After taking into account radioactive decay and ocean mixing, we concluded that these 134Cs ∕ 137Cs activity ratios were evidence that the Fukushima accident caused the increase in the 137Cs activity concentrations. In the North Pacific south of Japan (NPSJ), the highest 137Cs activities in 2012–2013 were observed in water from a depth of 300 m, the potential water density anomaly (σθ) of which corresponded to subtropical mode water (STMW). In the ECS, a clear increase in the 137Cs activity concentration started at a depth of 140 m (σθ= 25.2 kg m−3) in April 2013, propagated to the surface layers at depths of roughly 0–50 m, reached a maximum in 2015 and decreased in subsequent years. In the ECS, the Fukushima-derived radiocaesium activity concentration in surface water reached a maximum in 2014–2015, whereas the concentration in the SOJ reached a maximum in 2015–2016. The propagation of Fukushima-derived radiocaesium in surface seawater from the ECS into the SOJ therefore required approximately 1 year. These temporal changes in 137Cs activity concentrations and 134Cs ∕ 137Cs activity ratios indicated that part of the 137Cs and 134Cs derived from the Fukushima accident (FNPP1-derived 137Cs and134Cs) was transported within several years to the ECS and then to the SOJ via STMW from the NPSJ. The integrated amount of FNPP1-derived 137Cs that entered the SOJ before 2016 was estimated to be 0.21±0.01 PBq, 5.0 % of the estimated total amount of FNPP1-derived 137Cs in the STMW. The integrated amount of FNPP1-derived 137Cs that returned to the North Pacific Ocean through the Tsugaru Strait was estimated to be 0.09±0.01 Bq, 43 % of the total amount of FNPP1-derived 137Cs transported to the SOJ and 2.1 % of the estimated total amount of FNPP1-derived 137Cs in the STMW.

Evaluating the transport of surface seawater from 1956 to 2021 using 137Cs deposited in the global ocean as a chemical tracer

“…17b). The 137 Cs activity concentrations in the Eastern China Sea and Japan Sea increased following the processes elucidated in our previous studies (Inomata et al, 2018). The increase in 137 activity in the surface mixed layer (0-50 m) in the Eastern China Sea reached a maximum in 2014/2015 (Inomata et al, 2018).…”

“…The seasonal variation in the surface mixed layer depth might reflect the 0.5-yr average 137 Cs values in the western North Pacific Ocean. In addition, note that the behaviour, such as the transport pattern, of F1NPS-137 Cs is almost the same as that of 137 Cs derived from large-scale weapons tests in the 1950s-1960s (Aoyama et al, 2013(Aoyama et al, , 2016a(Aoyama et al, ,b, 2019(Aoyama et al, , 2020Inomata et al, 2012Inomata et al, , 2018.…”

“…After several years following the F1NPS accident, 137 Cs activity concentrations in surface seawater gradually increased in the Japan Sea, Eastern China Sea, subarctic North Pacific Ocean, and Bering Sea (Aoyama et al, 2016a(Aoyama et al, , 2017Inomata et al, 2018;Smith et al, 2017, Kumamoto et al, 2019. To estimate the contribution of the F1NPS accident, 137 Cs derived from the F1NPS accident ([F1NPS-137 Cs]) in 2011 was estimated using the following equation.…”

Estimate of Fukushima-derived radiocaesium in the North Pacific Ocean in summer 2012

“…According to the previous research based on the physical parameters in seawater, during the winter, the STMW is capped by the seasonal thermocline and then transported by the southwestward Kuroshio recirculation until approximately 20°N to south and east of Taiwan [ 50 , 51 ]. It was also found that the FNPP1 radiocaesium-labelled STMW was transported from NPSJ to the East China Sea (ECS), following then the Sea of Japan (SOJ) via the Tsushima Kaikyo, and transported into the North Pacific Ocean via the Tsugaru Strait within several years [ 52 ]. The integrated amount of FNPP1- 137 Cs that returned to the North Pacific Ocean through the Tsugaru Strait until 2016 was estimated to be 0.09 ± 0.01 Bq [ 52 ], which corresponds to 2.1% of the estimated total amount of FNPP1- 137 Cs in the STMW [ 23 ].…”

“…It was also found that the FNPP1 radiocaesium-labelled STMW was transported from NPSJ to the East China Sea (ECS), following then the Sea of Japan (SOJ) via the Tsushima Kaikyo, and transported into the North Pacific Ocean via the Tsugaru Strait within several years [ 52 ]. The integrated amount of FNPP1- 137 Cs that returned to the North Pacific Ocean through the Tsugaru Strait until 2016 was estimated to be 0.09 ± 0.01 Bq [ 52 ], which corresponds to 2.1% of the estimated total amount of FNPP1- 137 Cs in the STMW [ 23 ]. This indicates that a small amount of the FNPP1- 134 Cs went back to the surface layer in the North Pacific Ocean.…”