Provenance and sedimentation of Pu and Np in the northern Taiwan Strait suffering intensive land-ocean interaction.

The China Sea is faced with a heightened risk of anthropogenic radionuclide contamination, whose provenance, scavenging and migration are imperative to investigate to provide the background and nuclear safety emergency assessment. This study pioneers the measurement of anthropogenic plutonium and neptunium (Pu and Np) concentrations and atom ratios (Pu/Pu and Np/Pu) in sediment cores from the northern Taiwan Strait and the adjacent East China Sea using SF-ICP-MS, exploring their applications and characteristics. Typical vertical profiles confirm that Pu and Np serve as geochronological tools, with the Pu/Pu atom ratio as a fingerprint refining the chronology. Fallout history and sedimentary environments have been reconstructed by the comprehensive application of Pu, Np and Pb chronologies. The primary sources of Pu isotopes and Np are global fallout and close-in fallout from the Pacific Proving Grounds (PPG). Inventories of Pu ranged from 44 ± 3 Bq/m to 348 ± 11 Bq/m, with PPG contributions from 57% to 72%, while Np inventories varied from 58 ± 5 mBq/m to 137 ± 8 mBq/m. Differences in the distribution of Pu and Np are attributed to their distinct behaviors and sedimentary environments. Particle-reactive Pu isotopes are predominantly preserved in sediment, whereas conservative Np remains mostly dissolved in water, easily re-entering seawater from sediment through resuspension processes. Higher environmental mobility also makes more downward diffusion of Np than Pu isotopes.