In this study, seabed sediment was collected from 26 stations located within 160 km from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) during the 2 years which followed the FDNPP accident of March 2011 and the concentrations of I and Cs were measured. By comparing the distribution of these two radionuclides with respect to their different geochemical behaviors in the environment, the transport of accident-derived radionuclides near the seafloor is discussed. The concentration of I in seabed sediment recovered from offshore Fukushima in 2011 ranged between 0.02 and 0.45 mBq kg, with I/Cs activity ratios of (1.9 ± 0.5) × 10 Bq Bq. The initial deposition of I to the seafloor in the study area was 0.36 ± 0.13 GBq, and the general distribution of sedimentary I was established within 6 months after the accident. Although iodine is a biophilic element, the accident-derived I negligibly affects the benthic ecosystem. Until October 2013, a slight increase in activity of I in the surface sediment along the shelf-edge region (bottom depth: 200-400 m) was observed, despite that such a trend was not observed for Cs. The preferential increase of the I concentrations in the shelf-edge sediments was presumed to be affected by the re-deposition in the shelf-edge sediments of I desorbed from the contaminated coastal sediment. The results obtained from this study indicate that I/Cs in marine particles is a useful indicator for tracking the secondary transport of accident-derived materials, particularly biophilic radionuclides, from the coast to offshore areas.
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