Journal
JOURNAL OF OCEANOGRAPHY
Volume 72, Issue 1, Pages 53-65Publisher
SPRINGER
DOI: 10.1007/s10872-015-0335-z
Keywords
Fukushima Dai-ichi Nuclear Power Plant accident; Radiocaesium; Surface pathway; Subduction; Subtropical mode water; Central mode water; Inventory
Categories
Funding
- Radioactive Survey and Research Fund of Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan (Houshanou-chousa- kenkyuhi) [FY2011-2014]
- J-RAPID fund of the Japan Science and Technology Agency
- EC 7th Framework project COMET-FRAME (COordination and iMplementation of a pan-Europe instrumenT for radioecology) [604974]
- Marine Project of Institute of Environmental Radioactivity, Fukushima University, Japan
- Grants-in-Aid for Scientific Research [25287118, 24110001, 24110004] Funding Source: KAKEN
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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.
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