The plutonium isotopes, 239Pu and 240Pu, have been added to the surface oceans mainly as a consequence of global fallout from atmospheric nuclear weapons testing. There were two major sources of Pu isotope in seawater in the western North Pacific Ocean: global (stratospheric) fallout which occurred mainly in the late 1950s and early 1960s, and local fallout from the Pacific Proving Grounds (PPG) in the Marshall Islands which occurred mainly in the early 1950s. Since the 240Pu/239Pu atom ratio is characteristic for the Pu emission source, information on Pu isotopic signature is very useful to better understand the transport process in the oceans and to identify the sources of Pu. The mean atom ratio of 240Pu/239Pu from the global stratospheric fallout is 0.180 ± 0.014, whereas that from close-in tropospheric fallout from the PPG is 0.33 – 0.36 . The 239Pu and 240Pu inventories and 240Pu/239Pu atom ratios in seawater will provide important and useful data for understanding the process controlling Pu transport. The objectives of this study were to measure the 239Pu and 240Pu concentrations and 240Pu/239Pu atom ratios in seawater from the Bay of Bengal and to discuss the transport processes of plutonium in the ocean.
Seawater samples were collected from the surface to the bottom in the the Bay of Bengal with acoustically triggered quadruple PVC sampling bottles during the R/V Hakuho-Maru cruise. The Bay of Bengal station is located on the eastern end of the broad Bengal Fan. The 239Pu and 240Pu concentrations and 240Pu/239Pu atom ratios were measured with a sector field inductively coupled plasma mass spectrometer (SF-ICP-MS), which was equipped with a guard electrode to eliminate secondary discharge in the plasma and to enhance overall sensitivity.
The 239Pu, 240Pu and 239+240Pu concentrations in the surface water were 0.71, 0.56 and 1.27 mBq/m3, respectively, and they increased with depth; a sharp subsurface maximum over the depth interval 200 - 400 m was identified, and they decreased with depth. The 239Pu, 240Pu and 239+240Pu inventories in the entire water column were 6.9, 5.8 and 12.7 Bq/m2, respectively. The atom ratios of 240Pu/239Pu ranged from 0.22 to 0.24. The atom ratios in water column from the Bay of Bengal were significantly higher than the mean global fallout ratio of 0.180 ± 0.014. These high atom ratios proved the existence of close-in fallout plutonium originating from the Pacific Proving Grounds.