134Cs and 137Cs in the North Pacific Ocean derived from the March 2011 TEPCO Fukushima Dai-ichi Nuclear Power Plant accident, Japan. Part one: surface pathway and vertical distributions

Activities of radiocaesium released by the Fukushima Dai-ichi Nuclear Power Plant (FNPP1) accident were measured by surface sampling at 408 stations and in vertical profiles at 24 stations in the North Pacific Ocean, and time-series samples were collected at two coastal stations. After July 2012, Cs-137 activity in the surface water near FNPP1 remained around 1000 Bq m(-3), which corresponds to a discharge rate of about 10 GBq day(-1). FNPP1-derived radiocaesium spread eastward in surface water across the mid-latitude North Pacific with a speed of 7 km day(-1) (8 cm s(-1)) until March 2012, and of 3 km day(-1) (3.5 cm s(-1)) from March 2012 through August 2014. In June 2012, Cs-134 activity reached a maximum of 6.12 +/- A 0.50 Bq m(-3) at a 151-m depth (potential density, sigma (theta) = 25.3 kg m(-3)) at 29A degrees N, 165A degrees E. This subsurface maximum, which was also observed along 149A degrees E, might reflect the southward transport of FNPP1-derived radiocaesium in association with the formation and subduction of subtropical mode water (STMW). In June 2012 at 34A degrees N-39A degrees N along 165A degrees E, Cs-134 activity showed a maximum at around sigma (theta) = 26.3 kg m(-3), which corresponds to central mode water (CMW). Cs-134 activity was higher in CMW than in any of the surrounding waters, including STMW. These observations also indicate that the most effective pathway by which FNPP1-derived radiocaesium is introduced into the ocean interior on a 1-year time scale is CMW formation and subduction.