Continental margins are a dynamic interface linking the terrestrial lithosphere and oceanic hydrosphere, thus controlling fluxes of terrigenous materials from land to ocean. In particular, estuaries are places channeling vast amounts of minerals, nutrients, organic matter and trace elements (TEs) in dissolved and particulate forms into the oceans. As many TEs are essential nutrients for phytoplankton growth, their concentrations together with macronutrients in surface waters influence marine primary productivity, thereby impacting ocean ecosystems and carbon cycling. Consequently, studying geochemical cycling of TEs at continental margins is crucial for discerning their sources and processes that determine TE fluxes to oceans. Radiogenic neodymium (Nd) and hafnium (Hf) isotopic compositions, expressed as ε_Nd and ε_Hf, respectively are sensitive tracers of the origin and mixing of water masses and seawater-particle interactions at the continent-ocean interface. In this study a detailed investigation of the distributions of radiogenic Nd and Hf isotopes as well as rare earth elements (REEs) is performed across the Amazon freshwater plume and the Amazon shelf and slope as part of the GEOTRACES GApr11 cruise. The objective is to investigate the sources and processes influencing the TE supplies, Nd and Hf isotope distributions and their fluxes to the western Atlantic Ocean.

Within the Amazon freshwater plume, suspended particles are found to be a minor contributor to the Nd and Hf concentrations and εNd and εHf signatures of estuarine waters in the Amazon estuary. Instead, the Pará River, characterized by exceptionally high dissolved Nd and Hf concentrations and the least radiogenic isotope signatures of waters in the study area, substantially impacts the geochemical signatures of estuarine waters in the southern and outer Amazon estuary. Above the Amazon shelf, dissolved Nd and Hf concentrations are significantly elevated (~300%) paired with more radiogenic isotopic signatures in near-bottom. Notably, the observed extremely high Nd concentrations (5000-40000 pmol kg^-1) are noted in several porewater profiles. Such high the Nd and Hf concentrations in near-bottom waters and porewaters across the Amazon shelf are fed by the reductive dissolution of preformed riverine Fe-Mn oxyhydroxides, which simultaneously supply more radiogenic ε_Nd to the bottom water and porewater and thus control their geochemical signatures. This study underscores the pivotal role of sediment-seawater interactions, especially through Fe and Mn cycling, in controlling the behavior, distribution and fluxes of REE, Hf, and potentially other TEs on the Amazon shelf.