240Pu/239Pu atom ratios in water columns from the North Pacific Oceanand Bering Sea: Transport of Pacific Proving Ground-derived Pu by Ocean Currents

• Main Authors: Yamada Masatoshi, Zheng Jian
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier 2020
• Subjects: Bering Sea; Pacific; Alaska
• Online Access: https://repo.qst.go.jp/?action=repository_uri&item_id=79710; http://id.nii.ac.jp/1657/00079692/

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.