Pacific Proving Grounds-Derived 236U and 233U: Potential Tracers for Western North Pacific Ocean Dynamics

236U and 233U are proven to be useful tracers to investigate upper-ocean hydrodynamics due to their source-specific isotopic ratios and conservative behaviors in the open ocean. However, their application in the Pacific Ocean has been limited by scarce observations and unclear source-term information. Here, we present our observations of 236U and 233U in the western North Pacific Subtropical Gyre (NPStG), showing the presence of a source of anthropogenic U featured by a low 233U/236U ratio (∼1 × 10–3), which is an order of magnitude lower than the global fallout signature (∼2 × 10–2). The analyses of soil from the tropical Pacific atolls confirmed that this anthropogenic U originates from thermonuclear weapon tests conducted at the Pacific Proving Grounds (PPG). We further estimated that PPG tests released at least 50 kg of 236U and 48 g of 233U into the Equatorial North Pacific Ocean in the mid-to-late 1950s. After 70 years of redistribution, the PPG-derived U has become the predominant contributor (∼70%) to the 236U inventory in the western NPStG. This work's findings highlight the potential of 236U and 233U as paired transient tracers to investigate the advection and mixing processes in the western NPStG, which are absent for the boundary currents currently.