Abyssal seafloor as a key driver of ocean trace-metal biogeochemical cycles.

Abstract

Trace elements and isotopes (TEIs) are important to marine life and are essential tools for studying ocean processes¹. 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 scavenging¹¹ 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 cycling¹⁸, and suggest that the abyssal seafloor, often regarded as inactive, is a focus of biogeochemical transformation^19,20.