Abyssal seafloor as a key driver of ocean trace-metal biogeochemical cycles
2025-06-11
Trace elements and isotopes (TEIs) are important to marine life and are essential tools for studying ocean processes 1 . Two different frameworks have arisen regarding marine TEI cycling: reversible scavenging favours water-column control on TEI distributions 2–5 , and seafloor boundary exchange emphasizes sedimentary imprints on water-column biogeochemistry 6,7 . These two views lead to disparate interpretations of TEI behaviours 8–10 . Here we use rare earth elements and neodymium isotopes as exemplar tracers of particle scavenging 11 and boundary exchange 6,7,12 . We integrate these data with models of particle cycling and sediment diagenesis to propose a general framework for marine TEI cycling. We show that, for elements with greater affinity for manganese oxide than biogenic particles, scavenging is a net sink throughout the water column, contrary to a common assumption for reversible scavenging 3,13 . In this case, a benthic flux supports increasing elemental concentrations with water depth. This sedimentary source consists of two components: one recycled from elements scavenged by water-column particles, and another newly introduced to the water column through marine silicate weathering inside sediment 8,14,15 . Abyssal oxic diagenesis drives this benthic source, and exerts a strong influence on water-column biogeochemistry through seafloor geometry and bottom-intensified turbulent mixing 16,17 . Our findings affirm the role of authigenic minerals, often overshadowed by biogenic particles, in water-column cycling 18 , and suggest that the abyssal seafloor, often regarded as inactive, is a focus of biogeochemical transformation 19,20 .