Abstract
In the Pacific Ocean there are two principal sources of 239Pu and 240Pu: global stratospheric fallout and close-in fallout from the North Pacific Proving Grounds in the Marshall Islands. The 240Pu/239Pu atom ratio is an excellent tracer to characterize the sources of Pu and to investigate the transport and scavenging processes in the oceans. Large-volume seawater samples (250 L each) were collected from the Sagami Bay and their 239+240Pu activities and 240Pu/239Pu atom ratios were determined by alpha-spectrometry and SF-ICP-MS, respectively. The 239+240Pu activity was 13 mBq/m3 in the surface water and increased with depth to a maximum value of 39 mBq/m3. The 239+240Pu inventory in the water column was 41 Bq/m2. The 239+240Pu inventory in the water column in the western Northwest Pacific Ocean was equivalent to that of the integrated deposition density of atmospheric global fallout at the same latitude of 30-40 N. The 240Pu/239Pu atom ratios showed no notable variation from the surface to the bottom layer and were significantly higher than the global stratospheric fallout ratio. These high ratios could be attributed to close-in fallout Pu delivered from the North Pacific Proving Grounds.
