@misc{oai:repo.qst.go.jp:00070823,
 author = {Yamada, Masatoshi and Zheng, Jian and 山田 正俊 and 鄭 建},
 month = {Jun},
 note = {Introduction
Anthropogenic radionuclides such as 239Pu (half-life: 24,100 yr), 240Pu (half-life: 6,560 yr) and 241Pu (half-life: 14.325 yr) mainly have been released into the environment as the result of atmospheric nuclear weapons testing. In the North Pacific Ocean, two distinct sources of Pu isotopes can be identified; i.e., the global stratospheric fallout and close-in tropospheric fallout from nuclear weapons testing at the Pacific Proving Grounds in the Marshall Islands.[1] The objectives of this study are to measure the 240Pu/239Pu atom ratios in seawater from the northern North Pacific Ocean and to discuss the transport processes of Pu. 
\nMaterials and methods
      Seawater samples were collected at Stn. DR-10 in the northern North Pacific and Stn. DR-13 in the Bering Sea with a double barrel PVC large-volume sampler.[2] The 240Pu/239Pu atom ratios were measured with a double-focusing SF-ICP-MS, which was equipped with a guard electrode to eliminate secondary discharge in the plasma and to enhance overall sensitivity. [3]
\nResults and discussion
The total (water + sediment) inventory of 53.8 Bq m-2 at Stn. DR-10 in the northern North Pacific was mostly the same as that (58.1 Bq m-2) of the expected cumulative deposition density of atmospheric global fallout at the latitude of 40 – 50N. The atom ratio of 240Pu/239Pu showed no notable variation from subsurface water of 100 m depth to deep water of 2000 m depth, then increased with depth to 0.255 at the bottom layer. The atom ratios in water column of the northern North Pacific were significantly higher than the mean global fallout ratio of 0.18.[4]  High atom ratios of 240Pu/239Pu in the northern North Pacific prove the presence of close-in tropospheric fallout from nuclear weapons testing at the Pacific Proving Grounds.
\n[1] Yamada & Zheng (2010) Sci. Total Environ. 408, 5951-5957. [2] Nagaya & Nakamura (1993) Deep Ocean Circulation, Physics and Chemical Aspects. Elsevier, 157-167. [3] Zheng & Yamada (2007) Anal. Sci. 23, 611-615. [4] Kelley et al. (1999) Sci. Total Environ. 237/238, 483-500., 2012 Goldschmidt Conference},
 title = {Pu isotopes in water columns of the northern North Pacific},
 year = {2012}
}