福島沖の堆積物コアに含まれるI。分布・発生源とその意味

I in a sediment core offshore Fukushima: Distribution, source and its implication.

• Yukun Fan
• Xiaolin Hou
• Miho Fukuda
• Jian Zheng
• Tatsuo Aono
• Ning Chen
• Luyuan Zhang
• Weijian Zhou

抄録

福島第一原子力発電所事故で放出されたIは、大気、陸域、海洋環境で観測され、また、日本周辺の海水移動や沈着による分散を経て海洋堆積物にも入り込んでいる。しかし、海水に関する研究が多いのに対し、海洋堆積物コアの研究はほとんど行われていない。その結果、この堆積物コア中のI/I原子比は、事故後の福島沖で採取した海水や堆積物中のI/I原子比と同程度であるが、事故前のこの地域の海水と比較して2桁高いことが観察され、かなりの量のIが沖合の浅い堆積物に移行して取り込まれていることが示唆された。堆積物の粒度分布と比較した堆積物コア内の深さ分布から、IとCsの環境挙動の違いを考察した。ヨウ素同位体のピーク濃度は放射性セシウムよりも比較的深い層に見られた。また、放射性セシウムは堆積物コア中の微細粒の分布に追従しており、微細粒との関連性が高いことが示唆された。

I released from the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident has been observed in the atmospheric, terrestrial and oceanic environments, and it also entered the marine sediments via dispersion by sea water movement and deposition around Japan. However, there have been few studies of marine sediment cores in contrast to the large number of studies on seawater. In this work, a sediment core collected near FDNPP was analyzed for I. It is observed that the I/I atomic ratios in this sediment core are comparable to those in the seawater and sediments collected from offshore Fukushima after the accident, but 2 orders of magnitude higher than those in seawater in this region before the accident, suggesting the significant amount of I has been transferred and incorporated to the offshore shallow sediments. The difference in environmental behavior between I and Cs is discussed based on their depth distributions in the sediment core in comparison with the grain size distribution of sediments. The peak concentrations of iodine isotopes were found in a relatively deeper layer than radiocesium. Radiocesium follows the distribution of fine grains in the sediment core, implying its high association to fine grains.

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