Distribution of Pu isotopes and ²¹⁰Pb in the Bohai Sea and Yellow Sea: Implications for provenance and transportation

Particle-reactive radionuclides are useful for tracing sediment dynamics in marginal seas. We collected a suite of surface sediment samples in May 2014 from the Bohai Sea (BS) and Northern Yellow Sea (NYS) to observe the spatial distribution of Plutonium (Pu) isotopes and ²¹⁰Pb activities. ^239+240Pu activities ranged from 0.001 to 0.288 and 0.040–0.269 Bq kg⁻¹ in BS and NYS surface sediments, respectively. ²¹⁰Pb_ex shows a significant correlation with ^239+240Pu (r = 0.84, p < 0.01) that suggested these two nuclides were scavenged to the same grade. ²⁴⁰Pu/²³⁹Pu atom ratios in BS (0.173–0.256) and NYS (0.196–0.275) were slightly higher than the global fallout value of 0.18 and lower than the Pacific Proving Ground (PPG) value of 0.36, indicating that some fraction of Pu originating from the PPG was capable of being transported to the BS and NYS. Mass balance results showed that 41% of ^239+240Pu (8.9 × 10⁹ Bq yr⁻¹) and 18% of ²¹⁰Pb (2.4 × 10¹² Bq yr⁻¹) in the NYS originated in the oceanic input. In the BS, 63% of ²¹⁰Pb originated from atmospheric deposition and 84% of ^239+240Pu originated from riverine input. Using Pu and ²¹⁰Pb as tracers, we estimate that (1.8–2.6) × 10⁸ t yr⁻¹ and (3.6–3.8) × 10⁸ t yr⁻¹ of sedimentary particles could be transported from the BS to the NYS and from the NYS to the Southern Yellow Sea, respectively. Furthermore, the ²²⁶Ra/²³⁸U activity ratio distribution suggested that sedimentary particles derived from the Yellow River could be transported to the middle of the BS and coastal areas of the NYS.