240Pu/239Pu atom ratios in seawater from Sagami Bay, western Northwest Pacific Ocean: sources and scavenging

Seawater samples were collected in Sagami Bay, western Northwest Pacific Ocean, and their 239+240Pu activities and 240Pu/239Pu atom ratios were determined by α-spectrometry and sector field high-resolution ICP-MS. A few samples also were analyzed for 137Cs activities. The 239+240Pu inventory of 41.1 Bq m−2 was equivalent to the expected cumulative deposition density of atmospheric global fallout at the same latitude and this inventory was considerably lower than inventories in the underlying sediment columns. This result indicated that a significant amount of 239+240Pu has been removed into the underlying sediments through enhanced scavenging from the water column by the high fluxes of particles in this region. The atom ratio of 240Pu/239Pu showed no notable variation from the surface to the bottom; the average value was 0.234 ± 0.004. This atom ratio was significantly higher than the mean global fallout ratio of 0.18, proving the existence of close-in fallout plutonium originating from the Pacific Proving Grounds (PPG). The relative contributions of the global stratospheric fallout and the PPG close-in fallout were evaluated by using the two end-member mixing model. The contribution of the PPG close-in fallout was estimated to be 15.2 Bq m−2, which corresponded to 37% of the 239+240Pu inventory in the water column. Thus 239Pu and 240Pu from the two sources of global fallout and close-in fallout have been homogenized in the water masses in the western Northwest Pacific margin during the past three decades.