Southwest Intrusion of ¹³⁴Cs and ¹³⁷Cs Derived from the Fukushima Dai-ichi Nuclear Power Plant Accident in the Western North Pacific

Enormous quantities of radionuclides were released into the ocean via both atmospheric deposition and direct release as a result of the Fukushima Dai-ichi Nuclear Power Plant (FNPP) accident. This study discusses the southward dispersion of FNPP-derived radioactive cesium (Cs) in subsurface waters. The southernmost point where we found the FNPP-derived ¹³⁴Cs (1.5–6.8 Bq m^–3) was 18°N, 135°E, in September 2012. The potential density at the subsurface peaks of ¹³⁴Cs (100–500 m) and the increased water column inventories of ¹³⁷Cs between 0 and 500 m after the winter of 2011–2012 suggested that the main water mass containing FNPP-derived radioactive Cs was the North Pacific Subtropical Mode Water (NPSTMW), formed as a result of winter convection. We estimated the amount of ¹³⁴Cs in core waters of the western part of the NPSTMW to be 0.99 PBq (decay-corrected on 11 March 2011). This accounts for 9.0% of the ¹³⁴Cs released from the FNPP, with our estimation revealing that a considerable amount of FNPP-derived radioactive Cs has been transported to the subtropical region by the formation and circulation of the mode water