Transport processes of particulate radiocesium were investigated using a sediment trap deployed at about 100 km east of the Fukushima Daiichi Nuclear Power Plant. A sediment trap was installed at 873 m depth of the station (119 m above the bottom), and time-series sampling of sinking particles was carried out from August, 2011 to June, 2013. The accident-derived radiocesium was detected from sinking particles over two years after the accident. Observed 137Cs flux was highest in September 2011 (98 mBq m(-2) day(-1): decay-corrected to March 11, 2011), and decreased over time with seasonal fluctuations. Particulate fluxes of radiocesium were mainly affected by two principal processes. One was the rapid sinking of radiocesium-bound particles (moderate mode). This mode was dominant especially in the early postaccident stage, and was presumed to establish the distribution of radiocesium in the offshore seabed. Another mode was observed in winter, and secondary transport of particles attributed to turbulence near the seabed increased fluxes of particulate radiocesium (turbulence mode). Although the latter process would not drastically change the distribution of sedimentary radiocesium in the short term, attention should be paid as this key process redistributing the accident-derived radiocesium may cumulatively affect the long-term distribution.

Highlights  Concentration of 129 I in seabed sediment off Fukushima is reported for the first time  Deposition of the FDNPP accident-derived 129 I to the seafloor was 0.36 ± 0.13 GBq  Until October 2013, 129 I activity in sediment increased in the shelf-edge region  Remobilization of 129 I near the seafloor likely affected the sequential accumulation  The accident-derived 129 I is considered to negligibly affect the benthic ecosystem

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