Temporal variation of 240Pu/239Pu atom ratio and 239+240Pu inventory in water columns of the Japan Sea

The 239+240Pu concentrations and 240Pu/239Pu atom ratios were determined by alpha spectrometry and double-focusing SF-ICP-MS for seawater samples obtained in 1984 and 1993 from the Yamato and Tsushima Basins of the Japan Sea in the western North Pacific margin. The total 239+240Pu inventories in the whole water columns were approximately doubled during the period from 1984 to 1993 in the two basins. The increasing rates were estimated to be 5.1 Bq m−2 yr−1 in the Yamato Basin and 4.2 Bq m−2 yr−1 in the Tsushima Basin and they corresponded to ~ 0.02% of the annual 239+240Pu inflow rate into the Japan Sea through the Tsushima Strait. The mean 240Pu/239Pu atom ratios were ~ 0.240 and significantly higher than the mean global fallout ratio of 0.18. Furthermore, there were no temporal or spatial variations of 240Pu/239Pu atom ratios during this period in the Japan Sea. The total 239+240Pu inventories originating from the close-in fallout increased from 17.6 Bq m−2 to 34.6 Bq m−2 in the Yamato Basin and from 20.1 Bq m−2 to 34.6 Bq m−2 in the Tsushima Basin; however, the relative percentage of ~ 40% from the close-in fallout was unchanged during this period. A likely mechanism for the increasing Pu inventory would be the continuous inflow of the Tsushima Current from the western North Pacific, and the removal of Pu from surface waters by scavenging onto the settling particles, followed by regeneration of Pu from the settling particles during the downward transport.