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, s ? = 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 s ? = 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. |
