The 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 Pacific atolls confirmed that this anthropogenic U originates from the thermonuclear weapon tests conducted at the Pacific Proving Grounds (PPG). We further estimate that PPG tests released at least 50 kg of ²³⁶U into the Equatorial North Pacific Ocean in the mid-to-late 1950s. After 70 years' redistribution, the PPG-derived ²³⁶U has become the predominate contributor (~70%) to the ²³⁶U inventory in the western NPStG. This work's findings highlight the potential of ²³⁶U and ²³³U as transient tracers to investigate the advection and mixing processes in the western NPStG, which are absence for the boundary currents currently.

North Pacific,U-236,U-233,Pacific Proving Grounds