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 ²³⁹Pu and ²⁴⁰Pu 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 ²⁴⁰Pu/²³⁹Pu atom ratios showed a similar trend as that of the ^{239, 240}Pu 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 ^{239, 240}Pu 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 ^239+240Pu activity concentration is close to the estimated value of the suspended material from the Yangtze River catchment (0.18 Bq kg⁻¹), and the ²⁴⁰Pu/²³⁹Pu 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 ^239+240Pu was estimated to be 2.4×10¹⁰ Bq, which is small compared to the total amount of ^239+240Pu buried, 3.1×10¹³ 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 ^239+240Pu scavenged from the total ^239+240Pu transported by the Kuroshio Current (KC) and Taiwan Warm Current (TWC) into ECS sediments is estimated to be ∼10%. Our study shows that the ²⁴⁰Pu/²³⁹Pu 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 ²¹⁰Pb agree within uncertainties, which suggests that ^239+240Pu can potentially be used as a chronostratigraphic time marker in the marine environment.