Preferential accumulation of particulate radioiodine in shelf-edge sediment off Fukushima, Japan

After the accident of the Fukushima Dai-ichi Nuclear Power Plant (1FNPP), occurred in March 2011, significant amount of anthropogenic radionuclides were deposited to the seabed in the northeastern coast of Japan. Since then, many researchers have investigated the distributions of the radionuclides in the seabed. However, much interest has been focused on radiocesium and few data has been obtained for biophilic radionuclides, such as radioiodine. A short-lived radioiodine, ¹³¹I (half-life: 8 days) is known as a major radionuclide that increased air dose immediately after the 1FNPP accident, but it has not been detected from the marine environment since May 2011. On the other hand, a long-lived radioiodine ¹²⁹I (half-life 15.7 Myrs) can be detected from seabed sediments by accelerator mass spectrometry, a highly sensitive analytical technique. This study aims to track biophilic particulate radionuclide off Fukushima from distribution of ¹²⁹I in seabed sediment. Concentrations of ¹²⁹I in seabed sediment off Fukushima in 2011 ranged between 0.02 and 0.45 mBq/kg, and were generally higher in the coastal region. The general distribution of sedimentary ¹²⁹I was established within two months after the accident. In January 2013, about two years after the accident, concentrations of ¹²⁹I in the sediment surface slightly increased in the shelf-edge region (bottom depth: 200—400 m), and such a trend was not observed for radiocesium. The preferential accumulation of ¹²⁹I in the shelf-edge sediment was explained by lateral transport of sedimentary ¹²⁹I from the coastal to shelf-edge regions. Considering the geochemical characteristics of iodine, it can be considered that particulate organic matter near the seabed efficiently carried ¹²⁹I to the offshore.