137Cs, 239+240Pu and 240Pu/239Pu atom ratios in the surface waters of the western North Pacific Ocean, eastern Indian Ocean and their adjacent seas

Surface seawater samples were collected along the track of the R/V Hakuho-Maru cruise (KH-96-5) from Tokyo to the Southern Ocean. The 137Cs activities were determined for the surface waters in the western North Pacific Ocean, the Sulu and Indonesian Seas, the eastern Indian Ocean, the Bay of Bengal, the Andaman Sea, and the South China Sea. The 137Cs activities showed a wide variation with values ranging from 1.1 Bq m-3 in the Antarctic Circumpolar Region of the Southern Ocean to 3 Bq m-3 in the western North Pacific Ocean and the South China Sea. The latitudinal distributions of 137Cs activity were not reflective of that of the integrated deposition density of atmospheric global fallout. The removal rates of 137Cs from the surface waters were roughly estimated from the two data sets of Miyake et al. (1988) and this study to be 0.016 yr-1 in the Sulu and Indonesian Seas, 0.033 yr-1 in the Bay of Bengal and Andaman Sea, and 0.029 yr-1 in the South China Sea. These values were much lower than that in the coastal surface water of the western Northwest Pacific Ocean. This was likely due to less horizontal and vertical mixing of water masses and less scavenging. 239+240Pu activities and 240Pu/239Pu atom ratios were also determined for the surface waters in the western North Pacific Ocean, the Sulu and Indonesian Seas and the South China Sea. The 240Pu/239Pu atom ratios ranged from 0.199 +- 0.026 to 0.248 +- 0.027 on average, and were significantly higher than the global stratospheric fallout ratio of 0.18. The contributions of the North Pacific Proving Grounds close-in fallout Pu were estimated to be 20 % for the western North Pacific Ocean, 39 % for the Sulu and Indonesian Seas and 42 % for the South China Sea by using the two end-member mixing model. The higher 240Pu/239Pu atom ratios could be attributed to close-in fallout Pu delivered from the Enewetak and Bikini Atolls by ocean currents of branches of the North Equatorial Current to the Southeast Asian seas