Direct observation of ¹³⁴Cs and ¹³⁷Cs in surface seawater in the western and central North Pacific after the Fukushima Dai-ichi nuclear power plant accident

Abstract. The horizontal distribution of radioactive cesium (Cs) derived from the Fukushima Dai-ichi nuclear power plant (FNPP) in the North Pacific is still unclear due to the limitation of direct measurement of the seawater in the open ocean. We present the result of direct observation of radioactive Cs in surface seawater collected from a broad area in the western and central North Pacific in July 2011, October 2011 and July 2012. We also conducted a simple particle tracking experiment to estimate the qualitative spatial distribution of radioactive Cs in the North Pacific. ¹³⁴Cs was detected at 94 stations out of 123 stations, and ¹³⁷Cs was detected at all stations. High ¹³⁴Cs and ¹³⁷Cs concentrations more than 10 m Bq kg⁻¹ were observed in the area of the northern part of Kuroshio Extension at 144° E and 155° E in July 2011, in the area 147–175° E around 40° N in October 2011, and the northern part of Kuroshio Extension at 155° E and 175°30´ E in July 2012. Combining the result of direct observations and particle tracking experiment, the radioactive Cs derived from the FNPP had been dispersed eastward to the central North Pacific during 2011. It was considered from the horizontal distribution that radioactive Cs was dispersed not only eastward but also north- and southward in the central North Pacific. Pronounced dilution process of radioactive Cs from the FNPP during study period is suggested from temporal change in the activity ratio of ¹³⁴Cs / ¹³⁷Cs, which was decay-corrected on 6 April 2011, and relationships between radioactive Cs and temperature.