[Transport processes of Fukushima derived radioactivity in the Pacific Ocean]

Before the Fukushima Nuclear Power Plant 1 (FNPP1) accident, environmental (137)Cs was already detectable originating from nuclear weapon tests conducted in the late 1950s and early 1960s. In the western North Pacific Ocean, (90)Sr and (137)Cs activities in surface water were 10-100 Bqm(-3) in the late 1950s and early 1960s, then this parameter decreased gradually; (137)Cs activity in surface water subsequently decreased to around a few Bq m(-3). After the FNPP1 accident, (137)Cs and (134)Cs were released into the North Pacific Ocean by two pathways, direct discharge from the Fukushima NPP1 accident site and atmospheric deposition off Honshu Islands of Japan, east and northeast of the site. High-density observations of (137)Cs and (134)Cs in the surface water were carried out by 17 VOS cruises and several research vessel cruises between April 2011 and March 2012. The main body of radioactive surface plume of which activity exceeded 10 Bqm(-3) traveled along 40°N, and reached the International Date Line in March 2012, 1 year after the accident. The radioactive plume was confined along 40°N when the plume reached the International Date Line. Zonal speed of the radioactive plume was estimated to be about 8 cm s(-1), which is consistent with zonal speeds derived by Argo floats at the region.