The Amazon river-estuary–mangrove–shelf-ocean continuum represents one of Earth’s most dynamic land-ocean transition zones. It contributes approximately 20% of global freshwater discharge and 6% of global suspended sediment to the oceans and exerts substantial influence on trace element (TE) cycling in the western Atlantic. This Amazon continuum hosts a suite of biogeochemical processes including colloidal flocculation, mangrove groundwater input, reductive dissolution of metals in shelf sediments, and porewater–seawater exchange that collectively modulate dissolved TE cycling and fluxes to the Atlantic.
Radiogenic neodymium (Nd) isotopes (expressed as ε_Nd) are powerful tracers of water mass mixing and sediment–seawater interactions and are applied here together with rare earth element concentrations to investigate the processes controlling TE cycling across the Amazon continuum. Critically, prior studies have focused predominantly on wet-season conditions when freshwater discharge peaks but suspended sediment loads are comparatively low. During the dry season, suspended sediment concentrations in the estuary increase up to fourfold, tidal influence extends further upstream, and sediment–seawater interactions potentially intensify. However, the biogeochemical cycling responding to these changes to date remains largely unresolved. Here, we present dissolved and particulate ε_Nd data from a comprehensive dry-season field campaign conducted in November–December 2024, spanning Amazon estuary, mangroves, and continental shelf waters, collected during the GEOTRACES process study GApr21 (RV Meteor cruise M206). Preliminary data from the shelf reveal a distinct dry-season Nd cycling signature: bottom waters show progressively more radiogenic ε_Nd signatures toward the seafloor, an observation that weakens northward along the flow path of the Amazon plume and reflects exchange with riverine Fe-Mn oxyhydroxides at the sediment-water interface. Combined with wet-season surveys, these data provide the first systematic seasonal comparison of Nd isotope dynamics across the Amazon river-estuary–mangrove–shelf continuum, with implications for the quantification of Nd fluxes and corresponding advancement of our understanding of seasonal TE cycling in this system.

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