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. This study investigated the spatio-temporal variations in activityconcentrations in the Sea of Japan (SOJ) of 137 Cs and these transportprocess from the North Pacific Ocean to the SOJ through the East China Sea(ECS) during 2012–2016. The 137 Cs activity concentrations in the SOJhave been increasing since 2012–2013 and reached a maximum in 2015–2016 ofapproximately 3.4 Bq m −3 , more than twice the pre-Fukushima accident 137 Cs activity concentration of ∼1.5  Bq m −3 . The 134 Cs  ∕ 137 Cs activity ratios ranged from 0.36 to 0.51 in 2016.After taking into account radioactive decay and ocean mixing, we concludedthat these 134 Cs  ∕ 137 Cs activity ratios were evidence that theFukushima accident caused the increase in the 137 Cs activityconcentrations. In the North Pacific south of Japan (NPSJ), the highest 137 Cs activities in 2012–2013 were observed in water from a depth of300 m, the potential water density anomaly ( σθ) of whichcorresponded to subtropical mode water (STMW). In the ECS, a clear increasein the 137 Cs activity concentration started at a depth of 140 m( σθ=  25.2 kg m −3) in April 2013, propagated to thesurface layers at depths of roughly 0–50 m, reached a maximum in 2015 anddecreased in subsequent years. In the ECS, the Fukushima-derived radiocaesiumactivity concentration in surface water reached a maximum in 2014–2015,whereas the concentration in the SOJ reached a maximum in 2015–2016. Thepropagation of Fukushima-derived radiocaesium in surface seawater from theECS into the SOJ therefore required approximately 1 year. These temporalchanges in 137 Cs activity concentrations and 134 Cs  ∕ 137 Csactivity ratios indicated that part of the 137 Cs and 134 Cs derivedfrom the Fukushima accident (FNPP1-derived 137 Cs and 134 Cs) wastransported within several years to the ECS and then to the SOJ via STMW fromthe NPSJ. The integrated amount of FNPP1-derived 137 Cs that entered theSOJ before 2016 was estimated to be 0.21±0.01  PBq, 5.0 % of theestimated total amount of FNPP1-derived 137 Cs in the STMW. Theintegrated amount of FNPP1-derived 137 Cs that returned to the NorthPacific Ocean through the Tsugaru Strait was estimated to be 0.09±0.01  Bq, 43 % of the total amount of FNPP1-derived 137 Cstransported to the SOJ and 2.1 % of the estimated total amount ofFNPP1-derived 137 Cs in the STMW.

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