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Abstract The sediment mixed layer (SML) in the deep ocean is an important interface with a rich diversity of benthic organisms. With increasing ocean mineral exploration, and eventual mining, the effect of sediment mixing on deep ocean ecosystems has raised considerable concern. We evaluate the distribution patterns and driving factors of SML depth in deep ocean nodule fields using naturally occurring 210Pb–226Ra isotopes. Results show that average SML depth has increased in Mn‐nodule fields since the end of the last century. SML processes are associated with significant desorption of 226Ra from sediments, resulting in a departure from radioactive equilibrium. By estimating possible driving factors, we conclude that anthropogenic exploration activities, rather than natural physical and/or biological drivers, are the most likely mechanism for intensified sediment mixing. 210Pb–226Ra disequilibria may be a potential tracer for quantifying the impact of human exploration on deep‐ocean sediment mixing and associated biological and geochemical effects.