Identifying sources and quantifying contributions of plutonium in sediments of the Taiwan Strait

The ^{239 + 240}Pu activity concentrations and ²⁴⁰Pu/²³⁹Pu atom ratios in surface sediments and sediment cores from the Taiwan Strait (TS) were measured to identify Pu sources and quantify their contributions. The ^239+240Pu activity concentrations ranged from 0.06 to 1.11 mBq g^-1 in surface sediment and 0.03-1.17 mBq g^-1 in four sediment cores, while ²⁴⁰Pu/²³⁹Pu atom ratios varied from 0.205 to 0.277 and 0.220-0.287, respectively. These ratios consistently exceeded the characteristic value for global fallout (0.180 ± 0.014), indicating an additional, non-global fallout source. The spatial analysis confirms that the excess Pu originates from close-in fallout from the Pacific Proving Grounds (PPG), characterized by higher ratios (0.30-0.36). Application of a two-end-member mixing model reveals that the PPG and global fallout contribute 58% ± 10% and 42% ± 10% to the Pu inventory in surface sediments of the TS. Correspondingly, their contributions in sediment cores deposited over the past 40-50 years are 57%-59% and 41%-43%, respectively. Furthermore, using a dual-method framework that combines the ^239+240Pu activity concentration and total organic carbon content relationship with a δ¹³C_org-based mixing model, the contributions of terrestrial input (Terr-global fallout) and direct atmospheric deposition (Mar-global fallout) were quantified. The results indicate that Terr- and Mar-global fallout contribute 17% ± 10% and 25% ± 7%, respectively, to surface sediment, and 15%-19% and 23%-25% to sediment cores. Additionally, elevated sedimentation fluxes (2.14-4.67 mBq cm⁻² yr⁻¹) and inventories (48.8-241.0 Bq m⁻²) of ^239+240Pu indicate that the TS functions as an effective sink for this particle-reactive radionuclide. Finally, this study fills a key gap in Pu isotope distribution between the South China Sea and the East China Sea, providing baseline data for future environmental risk assessments related to nuclear power plant operations along the Chinese coast.