Recent trends of plutonium fallout observed in Japan: plutonium as a proxy for desertification
Plutonium in monthly deposition samples collected in Tsukuba (the Meteorological Research Institute), Japan from 1990 to end of 2001 is reported, together with monthly plutonium deposition in Nagasaki and Yonaguni in 2000. The annual deposition of 239,240Pu during the period from 1990 to 2001 shows no systematic inter-annual variation. However, monthly 239,240Pu depositions show a typical seasonal variation with a maximum in spring season (March to April), which corresponds to seasonal cycle of soil dusts originating from the East Asian arid area. Plutonium isotopic ratios in the deposition samples suggest that significant amounts of the recent 239,240Pu deposition observed in Japan are attributed to the resuspension of plutonium-bearing surface soil particles; resuspended plutonium originates from the East Asian arid areas. The recent increased tendency of 239,240Pu content in residues in deposition samples may reflect desertification in the East Asian continent.