240Pu/239Pu atom ratios in water columns from the North Pacific Oceanand Bering Sea: Transport of Pacific Proving Ground-derived Pu by Ocean Currents
Nuclear weapons tests were conducted at the Bikini and Enewetak Atolls in the Pacific Proving Grounds (PPG) in the Marshall Islands between 1946 and 1958. The 240Pu/239Pu atom ratios in seawater samples from the northern North Pacific Ocean and Bering Sea were determined to elucidate the temporal va...
| Main Authors: | , |
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| Format: | Article in Journal/Newspaper |
| Language: | English |
| Published: |
Elsevier
2020
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| Subjects: | |
| Online Access: | https://repo.qst.go.jp/?action=repository_uri&item_id=79710 http://id.nii.ac.jp/1657/00079692/ |
| Summary: | Nuclear weapons tests were conducted at the Bikini and Enewetak Atolls in the Pacific Proving Grounds (PPG) in the Marshall Islands between 1946 and 1958. The 240Pu/239Pu atom ratios in seawater samples 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 pathways of 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 from 76.0 ± 3.0 Bq m-2 to 37.1 ± 0.9 Bq m-2 during the period from 1973 to 1988 at the average rate of 2.6 ± 0.2 Bq m-2 yr-1. The 240Pu/239Pu atom ratios in the water column ranged from 0.222 to 0.255 with a mean value of 0.235 ± 0.008 in the northern North Pacific Ocean and from 0.215 to 0.237 with a mean value of 0.224 ± 0.004 in the Bering Sea. The obtained 240Pu/239Pu atom ratios were clearly higher than the mean global stratospheric fallout ratio of 0.18. These high atom ratios proved the existence of close-in tropospheric fallout Pu from nuclear weapons testing at the PPG. The relative contribution of the global stratospheric fallout and the PPG close-in fallout were evaluated by using the two-end-member mixing model. The 239+240Pu inventories originating from the PPG close-in 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 close-in fallout Pu has been transported to the northern North Pacific Ocean and Bering Sea. The proposed transport pathway accounting for the high 240Pu/239Pu atom 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. |
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