Vertical diffusion modulates molybdenum isotope variability in pelagic deep-sea sediments and global cycling implications

Molybdenum (Mo) is a key redox-sensitive and bioavailable element. Pelagic deep-sea sediments (PDSs), representing the primary oxic Mo sink in the ocean, exhibit Mo isotope compositions distinct from those of Fe-Mn crusts. However, the mechanisms underlying this discrepancy remain poorly constrained, introducing substantial uncertainty into global Mo cycle models. We present depth profiles of Mo concentrations and isotope compositions in both bulk PDSs and porewaters from the West Pacific Ocean. A consistent isotopic offset (Δ98Mo = 2.73‰ ± 0.16‰, 1SD) is observed between sediments and porewater. The authigenic Mo isotope compositions of these PDSs (δ98Mo = −0.21‰ ± 0.36‰; 1SD) are more variable and heavier than those of Fe-Mn crusts (−0.71‰ ± 0.14‰, 1SD). Porewater profiles reveal Mo release at both the sediment-water interface (SWI) and deep redox interfaces, where organic matter degradation and Mn oxide reductive dissolution establish opposing concentration gradients, respectively. Vertical diffusion, driven by these concentration gradients, modulates Mo concentration and isotope variations in both solid and dissolved phases. Flux calculations reveal that global Mo oxic sink accounts for 46%−51% of the riverine flux, and downward Mo diffusion at the SWI represents a previously unrecognized output pathway that contributes ∼21% of the oxic sink. These findings highlight the coupled roles of Mn-oxide scavenging and organic matter degradation in regulating marine Mo cycling, with broader implications for other bioavailable trace metals in PDSs.