The isotopic signature of plutonium in sediments from the western North Pacific margin

• Main Authors: Zheng Jian, Yamada Masatoshi
• Format: Conference Object
• Language: English
• Published: 2019
• Subjects: Pacific; ice core
• Online Access: https://repo.qst.go.jp/?action=repository_uri&item_id=60285; http://id.nii.ac.jp/1657/00060273/

Information on Pu isotopic compositions in sediments is very useful in understanding the source of Pu present there since the 240Pu/239Pu ratio has proved to be a powerful fingerprint to identify the Pu contamination. The atomic ratio of 240Pu/230Pu in fallout may vary, depending upon the specific weapon design and test yield. The global fallout average 240Pu/239Pu atom ratio is 0.18, based upon atmospheric aerosol sampling, soil samples and ice core data. It is also recognized that different test series can be characterized by either lower or higher ratios. For example, fallout from the Nevada test site has a lower 240Pu/239Pu ratio, averaging 0.035, while elevated 240Pu/239Pu ratios (0.21-0.36) in fallout from the 1950s from the Pacific Proving Grounds were reported. Recently, we have developed a highly sensitive method for the determination of 240Pu and 239Pu in marine sediment samples by means of ICP-MS (inductively coupled plasma mass spectrometry) with a shield torch system. In the present study, we have applied this method in order to determine 240Pu and 239Pu and their ratios in sediment core samples collected in Sagami Bay. Based on the 240Pu/239Pu ratio signature, we identified the additional Pu input in Sagami Bay was from the transported close-in fallout from Bikini and Enewetak nuclear weapon tests. We demonstrate that Pu activity and isotopic ratio profiles of marine sediment cores contain records of global fallout and close-in fallout Pu, which provide chronological information on the recent sedimentation. We propose that the transport route is through the oceanic currents, e.g. the North Equatorial Current and Kuroshio Current. Using a two fallout end-member model, we resolved the relative contribution of Pu between global fallout and close-in fallout in Sagami Bay sediments. International Workshop on Global Carbon Cycle and Related Mapping Based on Satellite Imagery and Climatic Change