Reactive-transport modeling of neodymium and its radiogenic isotope in deep-sea sediments: The roles of authigenesis, marine silicate weathering and reverse weathering

Dissolved Rare Earth Elements (REE) and radiogenic neodymium (Nd) isotope composition (εNd) of seawater are widely used geochemical tools in studying marine processes, but their modern ocean budgets are poorly understood. Recent discoveries of large benthic fluxes of REE with unique εNd signatures from marine sediments, particularly in the deep-sea, have led to a “bottom-up” hypothesis, which suggests that early diagenesis below the sediment-water interface (SWI) controls the ocean's REE and εNd budgets. To investigate such sedimentary processes, we created a reactive-transport model for the biogeochemical cycling of Nd and εNd in marine sediments. Here, we attempt to quantify the roles of authigenesis, marine silicate weathering and reverse weathering in the diagenetic cycling of Nd and εNd at a deep-sea (3000 m) site on the Oregon margin. Our model predicts that, at this site, Nd carried by Fe/Mn oxides into sediments eventually transforms to authigenic Nd-phosphate, during which ∼9% of the incoming solid Nd flux is released as a dissolved benthic flux back to the overlying bottom water. We also find that the classic reversible scavenging formulation applied to Nd co-cycling with Fe/Mn oxides is inconsistent with the data. Rather, a co-precipitation formulation, assuming Nd is structurally incorporated into Fe/Mn oxides, successfully simulates the data. The model also shows that authigenesis alone cannot explain the pore water and authigenic εNd, which are both more radiogenic than bottom water at this site. However, the weathering of volcanic silicates sourced from the local subduction zone can successfully explain εNd. We suggest that, because reverse weathering by authigenic clay formation maintains the under-saturation of primary silicates in pore water, marine silicate weathering can proceed. The processes we model likely affect the sedimentary cycling of many other trace elements and isotopes, with much broader implications for the understanding of ocean biogeochemistry. •We present a reactive-transport model of the early diagenesis of Nd and εNd.•Pore water Nd concentration is explained by Fe/Mn cycling and authigenic phosphate.•Reversible scavenging cannot explain pore water Nd concentration profile.•Weathering of volcanic silicates explains radiogenic pore water/authigenic εNd.•Reverse weathering is a driving force for marine silicate weathering.