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, ¹³⁷Cs 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, ¹³⁴Cs activity reached a maximum of 6.12 ± 0.50 Bq m^-3 at a 151-m depth (potential density, σ_θ = 25.3 kg m^-3) at 29°N, 165°E. This subsurface maximum, which was also observed along 149°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 34°N–39°N along 165°E, ¹³⁴Cs activity showed a maximum at around σ_θ = 26.3 kg m^-3, which corresponds to central mode water (CMW). ¹³⁴Cs 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. (author)