Modeling the Migration Characteristics and Identifying the Sources of ²³⁷Np and ^239+240Pu in Seawater and Sediments in the South China Sea and the Indian Ocean

Sediments from the South China Sea and the Indian Ocean were analyzed using sector field inductively coupled plasma mass spectrometry (SF-ICP-MS) to determine the activity of ²³⁷Np and ^239+240Pu, as well as their atomic ratios ²³⁷Np/²³⁹Pu and ²⁴⁰Pu/²³⁹Pu. These measurements served as tracers to investigate the transport pathways from the Pacific Proving Grounds (PPG) to both oceans. This study provides the first comprehensive report on the distribution of ²³⁷Np activity and the ²³⁷Np/²³⁹Pu atomic ratio in South China Sea sediments. The inventory of ²³⁷Np and ^239+240Pu in sediment cores shows a consistent decreasing trend from shallow to deep sea. Simulation analysis of core samples using the ADE model found that Np migrates faster than Pu, which may be related to their differences in solubility. The S-ADE model was employed to simulate the concentration of Np and Pu in seawater. The distribution of these two nuclides in seawater follows a similar trend to that in sediments, but there are significant differences in vertical distribution. Np concentration decreased exponentially with depth from the surface layer, while Pu concentration displayed a single-peak distribution, peaking at approximately 500 m depth. This difference is likely due to variations in their solubility and adsorption behavior onto sediment particles.