What Causes Excess Deepening of the Sediment Mixed Layer in the Deep Ocean?

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. Plain Language Summary: The surface sediment mixed layer (SML) in the deep ocean is essential to benthic community structure and the biogeochemical cycling of many elements. The typical depth of mixed layers in the global deep ocean is <15 cm, which is mainly driven by natural biological and/or physical forces. By mapping sediment excess 210Pb profiles throughout the ocean, we found the depth of the SML in manganese nodule fields has increased significantly since the end of the twentieth century (reaching to >40 cm). The determine that the deepened mixed sediment layer is related to human exploration activities rather than other previously recognized natural drivers like bioturbation, internal solitary waves, and submarine storms. Keen attentions should be paid to the deepening and associated effects of sediment mixing in the deep ocean as human demand for critical materials continues to increase in the Anthropocene. Exploration approaches that involve low or negligible disturbance to deep‐ocean sediment should be encouraged. Key Points: Human exploration is the main reason for deepening of the sediment mixed layer in the manganese nodule areaSediment mixed processes are associated with significant desorption of 226Ra from sediments210Pb–226Ra disequilibria is a potential tracer for quantifying the impact of human drivers on deep‐ocean sediment mixing [ABSTRACT FROM AUTHOR]