240Pu/239Pu atom ratios in water columns from the North Pacific Ocean and Bering Sea: Transport of Pacific Proving Grounds-derived Pu by ocean currents

• Published in: The Science of the total environment Vol. 718; p. 137362
• Main Authors: Yamada, Masatoshi, Zheng, Jian
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 20.05.2020
• Subjects: Close-in tropospheric fallout; Global stratospheric fallout; Long-range ocean transport pathway; Nuclear weapons tests; Plutonium; Seawater; Long-range ocean transport pathway; Global stratospheric fallout; Close-in tropospheric fallout; Plutonium; Nuclear weapons tests; Seawater
• ISSN: 0048-9697; 1879-1026
• DOI: 10.1016/j.scitotenv.2020.137362

The 240Pu/239Pu atom ratios in seawater from the northern North Pacific Ocean and Bering Sea were determined to elucidate the temporal variability in 239+240Pu inventory, to identify their Pu sources and to propose the transport pathway of Pacific Proving Grounds (PPG)-derived Pu into the studied area. Water column 239+240Pu inventory in the Bering Sea was lower than that reported for a comparable station in the Geochemical Ocean Sections Study, indicating that the Bering Sea inventory has been decreasing at the average rate of 2.6 Bq m−2 yr−1. The 240Pu/239Pu atom ratios ranged from 0.222 to 0.255 with a mean value of 0.235 in the northern North Pacific Ocean and from 0.215 to 0.237 with a mean value of 0.224 in the Bering Sea. The obtained 240Pu/239Pu ratios were clearly higher than the mean global fallout ratio of 0.18. These high atom ratios proved the presence of close-in fallout Pu from nuclear weapons testing at the PPG. The relative contribution of the global fallout and the PPG fallout were evaluated by using the two-end-member mixing model. The 239+240Pu inventories originating from the PPG fallout were calculated as 17.9–25.4 Bq m−2 in the northern North Pacific Ocean and 11.6–16.6 Bq m−2 in the Bering Sea, which corresponded to 38–54% and 31–45% of the total 239+240Pu inventory in the water column in the northern North Pacific Ocean and Bering Sea, respectively. A significant amount of the PPG-derived Pu has been transported to the northern North Pacific Ocean and Bering Sea. The proposed transport pathway accounting for the high 240Pu/239Pu ratio in the Bering Sea water column is the transportation of PPG-derived Pu by the North Equatorial Current followed by the Kuroshio Current, Kuroshio Extension, North Pacific Current and subsequently the Alaska Current, and then the Alaskan Stream. [Display omitted] •Data on 240Pu/239Pu ratios in the North Pacific and Bering Sea are provided.•239+240Pu inventory decreased at the average rate of 2.6 Bq m−2 yr−1 in the Bering Sea.•240Pu/239Pu ratios were clearly higher than the mean global fallout ratio of 0.18.•The presence of Pacific Proving Grounds-derived Pu was observed in the Bering Sea.