Sediment resuspension and reductive dissolution of terrigenous Fe-Mn oxides control dissolved neodymium and hafnium inputs from the Amazon shelf

The Amazon shelf and slope are key components of the tropical ocean margin systems, representing large reservoirs of terrestrial minerals and organic matter and important sources of trace elements to seawater. Radiogenic neodymium (Nd) and hafnium (Hf) isotopes trace continental weathering regimes and lithogenic inputs and are also used for the investigation of water mass mixing and the reconstruction of past changes in ocean circulation. However, the global oceanic Nd budget is not balanced and a substantial global Nd input component is missing. Boundary exchange along continental margins has been proposed as the dominant Nd source to the ocean, controlling its isotopic compositions of seawater. Yet, the detailed processes and magnitude of these inputs are not well constrained. By analyzing combined Nd and Hf isotope compositions and concentrations of rare earth elements and Hf along the Amazon margin, we demonstrate that the dissolution of terrigenous riverine iron-manganese (Fe-Mn) oxyhydroxides in resuspended sediments and porewaters together supply a shelf area weighted-mean net Nd flux of 8.3 × 10 8 g yr −1 to the near-bottom shelf waters. Our study underscores the dominant role of interactions between resuspended sediments and seawater, in particular via Fe and Mn cycling, and porewater-seawater mixing, in controlling the geochemical behavior of rare earth elements and Hf, and by inference other trace elements on the Amazon shelf. Terrigenous riverine Fe-Mn oxyhydroxide fractions of sediments on the shelves are shown to be an important source in the global oceanic Nd and Hf budgets.