Deciphering anthropogenic uranium sources in the equatorial northwest Pacific margin.

This work reports the first high-resolution deposition records of anthropogenic uranium (U and U) in a sediment core taken at the continental slope of the Philippine Sea off Mindanao Island in the equatorial northwest Pacific Ocean. Two notable peaks were observed in both profiles of U and U concentrations, with a narrower peak in 1951-1957 corresponding to close-in Pacific Proving Grounds (PPG) signal, and a broader peak in 1960s-1980s corresponding to the global fallout from nuclear weapons testing. U and U areal cumulative inventories in the studied sediment core are (2.79 ± 0.20) ∙ 10 atom ∙ m and (3.12 ± 0.41) ∙ 10 atom ∙ m, respectively, about 20-30% of reported U and U inventories from the direct global fallout deposition. The overall U/U atomic ratios obtained in this work vary within (0.3-3.5) ∙ 10, with an integrated U/U atomic ratio of (1.12 ± 0.17) ∙ 10. The contribution from global fallout and close-in PPG fallout to U in the sediment core is estimated to be about 69% and 31%, respectively. We believe the main driving process for anthropogenic uranium deposition in the Philippine sediment is continuous scavenging of dissolved U from the surface seawater by sinking particles.