Neodymium is one of the rare earth elements (REE) essential in the geochemical studies of marine and earth surface processes. The radiogenic isotope composition of neodymium (the 143Nd/144Nd ratio, written in the form ofεNd) has been widely used as a tracer of ocean circulation. For a long time, it has been thought that external sources of Nd only occur at the surface ocean because of riverine and dust inputs, and no new sources of Nd are added to the water column below. According to this view, εNd should behave conservatively in the ocean interior. However, with increasing data coverage thanks to the GEOTRACES water column survey and recent sediment pore water studies, this traditional view has become untenable. Latest results show that the conservative behavior of εNd is likely an exception rather than a rule and that there are benthic inputs of Nd from seafloor areas on the margins and abyssal plains. However, it remains unclear what biogeochemical processes support the benthic fluxes and to what degree such fluxes represent new, instead of recycled, sources.
Here we investigate the sedimentary cycling of Nd and its impact on the water column distributions. We analyze the seawater εNd data and show that non-conservative behavior is widespread and linked to the addition of new lithogenic sources from the seafloor. We create a reactive-transport model to study the sediment diagenesis of Nd. We show that marine silicate weathering supports a new benthic flux, the isotope composition of which can be distinct from that of overlying bottom water. The impact of marine silicate weathering critically depends on the availability of reactive lithogenic detritus and the formation of authigenic clay. We then develop a water column cycle model and find that the new benthic sources of Nd can explain the observed non-conservative behavior of seawater εNd in the ocean interior. Our results thus show that sedimentary processes can imprint on water column biogeochemistry, and marine silicate weathering can play an important role in marine biogeochemical cycles.

marine silicate weathering, neodymium isotope, rare earth elements (REE), ocean biogeochemistry, sediment diagenesis