Historical changes in 239PU and 240Pu sources in sedimentary records in the East China Sea: Implications for provenance and transportation

Concentrations and isotopic compositions of plutonium (Pu) are widely used for its source identification and to determine transport processes of Pu-associated particulate matter and water. We investigated the concentrations of (PU)-P-239 and Pu-240 and their ratios in a number of sediment samples from the East China Sea (ECS) collected in the summer of 2013 (August 6-28). The 239+240PU activity concentrations in surface sediment samples were found to range between 0.048 and 0.492 Bq kg(-1) and the Pu-240/Pu-239 atom ratios showed a similar trend as that of the 239,(PU)-P-240 activities; the Pu atom ratios ranged from 0.158 to 0.297 and were mostly higher than the mean global fallout value of 0.18. The Pu-239,Pu-240 inventories in the ECS varied widely, from 2 to 807 Bq m(-2), with the highest values commonly found in the coastal areas. In the Yangtze Estuary, the mean Pu239+240 activity concentration is close to the estimated value of the suspended material from the Yangtze River catchment (0.18 Bq kg-1), and the Pu-240/Pu-239 atom ratio was found to be 0.18, which indicates that the Yangtze River input is the dominant source of Pu for this area. The total annual Yangtze River input of Pu239+240 was estimated to be 2.4 x 10(10) Bq, which is small compared to the total amount of 239+240PU buried, 3.1 x 10(13) Bq in the whole ECS. The Pacific Proving Ground input appears to be the dominant source of Pu to the ECS, accounting for 45%-52% of the total inventory. The fractional amount of Pu239+240 scavenged from the total Pu239+240 transported by the Kuroshio Current (KC) and Taiwan Warm Current (TWC) into ECS sediments is estimated to be similar to 10%. Our study shows that the Pu-240/Pu-239 atom ratio is useful not only to obtain a better insight of the biogeochemistry influenced by the KC, but also to trace the long-range transport of other particle-reactive species. Besides, the sedimentation rates obtained based on the penetration depths of 239+240PU and vertical profiles of excess Pb-210 agree within uncertainties, which suggests that Pu239+240 can potentially be used as a chronostratigraphic time marker in the marine environment. (C) 2017 Elsevier B.V. All rights reserved.