Annually Resolved 239,240 Pu and 237 Np Records in a Coral (1947–2007): Transport Timescales and Current Modulation From the Pacific to the South China Sea

This study presents the first annually resolved record of 237 Np and 239+240 Pu in a coral core collected off eastern Hainan Island in the South China Sea (SCS), covering 1947–2007. Both the 239+240 Pu activity and 240 Pu/ 239 Pu ratio exhibited an initial increase followed by a gradual decreasing trend. The 239+240 Pu activity reached its peak in 1958, earlier than the 1963 global fallout maximum, indicating predominant close‐in fallout contributions from the Pacific Proving Grounds (PPG). The 240 Pu/ 239 Pu ratio peaked in 1955, corresponding to the 1952 Ivy Mike thermonuclear test, implying a transport timescale of ∼2–3 years for Pu isotopes to travel from PPG to the study site via Luzon Strait. Based on this timescale, it can be inferred that radionuclides take roughly 15–30 months to transport from the eastern Luzon Strait to the study site. This study also presents the first reported data for 237 Np in a coral core. In contrast to Pu isotopes, 237 Np activity displayed a distinct bimodal pattern, with the first peak occurring in 1958 and a secondary peak in 1960, while the 237 Np/ 239 Pu ratio increased steadily until 1996. After 1980, several elevated 237 Np activities were significantly correlated with El Niño‐Southern Oscillation (ENSO) events. During El Niño episodes, the bifurcation of the North Equatorial Current (NEC) shifts northward, enhancing Kuroshio intrusion into the SCS and thereby increasing the input of PPG‐derived radionuclides. Additionally, the characteristic peak timing of Pu‐Np can serve as a novel dating method for SCS coral cores.