Impact of Fukushima-derived radiocesium in the western North Pacific Ocean about ten months after the Fukushima Dai-ichi nuclear power plant accident

We measured vertical distributions of radiocesium (¹³⁴Cs and ¹³⁷Cs) at stations along the 149°E meridian in the western North Pacific during winter 2012, about ten months after the Fukushima Dai-ichi Nuclear Power Plant (FNPP1) accident. The Fukushima-derived ¹³⁴Cs activity concentration and water-column inventory were largest in the transition region between 35 and 40°N approximately due to the directed discharge of the contaminated water from the FNPP1. The bomb-derived ¹³⁷Cs activity concentration just before the FNPP1 accident was derived from the excess ¹³⁷Cs activity concentration relative to the ¹³⁴Cs activity concentration. The water-column inventory of the bomb-derived ¹³⁷Cs was largest in the subtropical region south of 35°N, which implies that the Fukushima-derived ¹³⁴Cs will also be transported from the transition region to the subtropical region in the coming decades. Mean values of the water-column inventories decay-corrected for the Fukushima-derived ¹³⁴Cs and the bomb-derived ¹³⁷Cs were estimated to be 1020 ± 80 and 820 ± 120 Bq m⁻², respectively, suggesting that in winter 2012 the impact of the FNPP1 accident in the western North Pacific Ocean was nearly the same as that of nuclear weapons testing. Relationship between the water-column inventory and the activity concentration in surface water for the radiocesium is essential information for future evaluation of the total amount of Fukushima-derived radiocesium released into the North Pacific Ocean. - Highlights: • Fukushima-derived ¹³⁴Cs inventory were largest in the transition region. • Bomb-derived ¹³⁷Cs just before the Fukushima accident was determined. • Bomb-derived ¹³⁷Cs inventory was largest in the subtropical region. • Fukushima-derived ¹³⁴Cs and bomb-derived ¹³⁷Cs were nearly equivalent in winter 2012