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

U and U 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 U and U in the western North Pacific Subtropical Gyre (NPStG), showing the presence of a source of anthropogenic U featured by a low U/U ratio (∼1 × 10), which is an order of magnitude lower than the global fallout signature (∼2 × 10). 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 U and 48 g of U 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 U inventory in the western NPStG. This work's findings highlight the potential of U and U as paired transient tracers to investigate the advection and mixing processes in the western NPStG, which are absent for the boundary currents currently.