Radium-226 (²²⁶Ra) is a long-lived radionuclide (half-life: 1602 years) and a powerful tracer of large-scale ocean circulation. Its distribution in the ocean reflects the combined influences of benthic sediment diffusion, coastal inputs including riverine input and submarine groundwater discharge (SGD), hydrothermal input, and particle scavenging, but the relative importance of these processes remains poorly constrained and may vary among ocean basins. Here, we test the hypothesis that the dominant controls on dissolved ²²⁶Ra vary systematically across ocean basins by developing a data-constrained inverse model that captures both top-down and bottom-up forcing in the global ocean. The results show that sedimentary diffusion is the dominant source of dissolved ²²⁶Ra (56%), followed by hydrothermal inputs (33%) and coastal inputs (11%). Dissolved ²²⁶Ra is removed primarily by particle scavenging (59%) and radioactive decay (~41%), while ocean circulation redistributes it throughout the global ocean. Our results further reveal a shift in the dominant control from coastal inputs in the Atlantic and Arctic Oceans to sedimentary fluxes in the Southern, Indian, and Pacific basins, suggesting a transition from top-down to bottom-up control. These findings provide new insights into the mechanisms governing the global ²²⁶Ra cycle and have broader implications for understanding trace metal cycling in the ocean.

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