Vertical distributions of 239+240Pu activities and 240Pu/239Pu atom ratios in sediment cores: implications for the sources of Pu in the Japan Sea

The vertical distributions of 239+240Pu-specific activities and 240Pu/239Pu atom ratios in sediment cores of the Japan Sea were investigated. For comparison, the 239+240Pu-specific activities and 240Pu/239Pu atom ratios in the surface layer of bottom sediments in the Sea of Okhotsk were also determined. The 239+240Pu-specific activities in the surface layer sediments varied from 0.43 to 2.65 mBq/g in the Japan Sea. The lowest value was seen in the eastern Japan Basin, while the highest value was found in a sample collected from a coastal area off Hokkaido in the northern Japan Sea. It was found that the 240Pu/239Pu atom ratios in the surface sediments for the Japanese side of the Japan Sea ranged from 0.15 to 0.22. The 240Pu/239Pu atom ratios in the surface sediments collected from the Yamato Basin and Japan Basin were typically close to the global fallout value of 0.18, but higher 240Pu/239Pu atom ratios (0.20–0.22) were seen in samples from the northern Japan Sea. Vertical profiles of 240Pu/239Pu atom ratios revealed that these atom ratios were higher than the global fallout value in the deeper layers in the sediment cores from the northern Japan Sea on the Hokkaido coast and the Yamato Basin. In addition, the sediment cores from the northern Japan Sea near the Soya Strait and in the Ishikari Estuary had a constant vertical distribution of 240Pu/239Pu atom ratios with a value around 0.20. The results obtained in this work support the hypothesis that the prevailing ocean currents transported Pu derived from the Pacific Proving Grounds to the Japan Sea, and this contributed to the general elevated Pu inventory in the Japan Sea