Seasonal redistribution of Pu and 237Np in the North Yellow Sea: Implications for sediment transport and hydrodynamic shifts.

This study employs plutonium isotopes (239,240Pu) and 237Np as complementary tracers to investigate seasonal sediment transport and hydrodynamic shifts in the North Yellow Sea (NYS). Surface-sediment analysis revealed a distinct seasonal reversal in spatial distributions. 239+240Pu activity ranged from 0.015 ± 0.014-0.443 ± 0.173 Bq/kg in summer and 0.124 ± 0.028-0.549 ± 0.125 Bq/kg in winter, while 237Np varied from 0.008 ± 0.002-0.542 ± 0.043 mBq/kg in summer and 0.086 ± 0.010-0.880 ± 0.104 mBq/kg in winter, with respective maxima shifting from the northwestern NYS in summer to the southeastern NYS in winter. 240Pu/239Pu atom ratios confirmed binary mixing between global fallout and Pacific Proving Grounds (PPG) sources (PPG contribution: 20.2 % summer, 13.7 % winter). Elevated 237Np/239Pu ratios (up to 1.099) along the Shandong Peninsula in winter highlighted intense cross-frontal transport. Sediment-core vertical profiles reveal the central NYS as the most significant sink for radionuclides (especially PPG-derived Pu), whereas 237Np exhibits a more uniform inventory distribution owing to its greater environmental mobility. These distribution patterns are attributed to seasonal variations in ocean currents and storm events, a conclusion supported by seawater advection-dispersion modeling, which linked summer deposition to a weak circulation system and winter redistribution to the intensified Yellow Sea Coastal Current. Mass balance analysis further identified riverine input as the dominant Pu source and quantified substantial sediment export from the Bohai Sea. Our findings demonstrate the efficacy of Pu and Np as sensitive tracers for quantifying seasonal hydrodynamic and sediment dynamics in marginal seas.