Labile Fe oxyhydroxides are important players in the benthic cycling of trace metals, especially in estuary sediments that experience frequent redox oscillations. To take a closer look into the correlations between labile Fe and trace elements, we collected surface sediments from Changjiang Estuary along a salinity gradient and investigated them with flow-through time-resolved analysis (FT-TRA) and microbial incubation. In the FT-TRA using diluted HNO₃, we found strong correlations between Fe and Cr, Co, Li, and rare earth elements (REEs), as well as between Mn and Ni in the labile fractions. Additionally, the enrichment of light REEs (LREE) in labile Fe oxyhydroxides such as ferrihydrite was also identified. The dominance of labile Fe in scavenging trace elements is likely due to its higher abundance and faster reoxidation and precipitation rates compared to Mn.
During microbial incubation of surface sediments, successively releases of Mn and trace elements were observed, while the release of Fe(II) was retarded due to the formation of authigenic Fe(II) minerals (e.g., siderite, vivianite). The delayed Fe(II) release facilitated trace elements to escape from the capture of Fe(II) oxidation and coprecipitation (so-called “Fe curtain”) under variable redox conditions. It potentially increases trace element fluxes from the estuarine sediments, and ultimately, the terrestrial inputs of trace metals and REEs to coastal oceans.

benthic process, trace elements, iron biogeochemistry, coastal ocean