Thorium-234 as a tracer for deep-sea mining sediment plume deposition

Deep-sea mining for polymetallic nodules is currently exploratory, but commercial-scale operations require indicators of environmental change to support regulatory thresholds and inform adaptive management. In the Clarion-Clipperton Zone, where background sedimentation rates are low, seafloor imagery has validated mining plume deposition but cannot resolve repeated sedimentation as nodules become buried. Thorium-234 (234Th), a naturally occurring radionuclide with a 24.1-day half-life and strong particle reactivity, serves as a high-resolution geochemical tracer. Here we apply sedimentary 234Th to identify the spatial extent of plume deposition following the NORI-D mining test. Excess 234Th (234Thxs) activity was low at baseline but elevated after mining and declined to background within 1–2 km of the directly mined area. Results suggest that mining plumes scavenge and redistribute 234Thxs, establishing a geochemical benchmark for plume extent and an operational tool for tracing recent sedimentation under future commercial-scale mining scenarios. Thorium-234 provides a time-sensitive geochemical tracer to identify the extent of deep-sea mining sediment plume deposition, enabling detection of recent impacts and supporting long-term environmental management.