The flux of soluble organic-iron(III) complexes from sediments represents a source of stable iron(III) to estuarine waters and to the continental shelf

Iron speciation in the Satilla River Estuary was investigated using competitive ligand equilibration-adsorptive cathodic stripping voltammetry (CLE-ACSV) with the ligand 1-nitroso-2-naphthol (1N2N). The blackwater Satilla River contains high concentrations of dissolved organic matter and iron, suggesting that it could provide a source of dissolved iron to the continental shelf. Total dissolved iron in the water column decreases along the estuary, likely due to flocculation and precipitation processes associated with the increase in salinity. Simultaneously, the percentage of dissolved organic-Fe(III) complexes measured by CLE-ACSV in overlying waters of sediment cores increases with salinity. The speciation of dissolved iron in the pore waters indicates that these complexes originate in the underlying sediments. Diffusive fluxes calculated from depth profiles in the sediments indicate that only 8% of the sedimentary flux of Fe(III) is delivered to the continental shelf during low riverine discharge. During normal flow conditions, however, the sediment flux of Fe(III) represents 63% of the total riverine flux of Fe(III). These findings suggest that the flux of dissolved iron to the continental shelf is controlled by the sedimentation of iron in the estuary and the remobilization of soluble organic-Fe(III) complexes produced either during iron reduction deep in the sediment or oxidation of Fe(II) close to the sediment-water interface. The Satilla River Estuary generates five to eight times higher concentrations of dissolved Fe(III) than the average major world rivers, suggesting that it is, along with other small blackwater rivers, currently underrepresented in world average river flux calculations.