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 (²³⁴Th_xs) 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 ²³⁴Th_xs, establishing a geochemical benchmark for plume extent and an operational tool for tracing recent sedimentation under future commercial-scale mining scenarios.