Development of a sequential extraction procedure for iron: implications for iron partitioning in continentally derived particulates

The development of a sequential extraction procedure for iron in modern and ancient sediments is presented. The scheme recognizes seven operationally derived iron pools: (1) carbonate associated Fe (Fecarb), including siderite and ankerite; (2) easily reducible oxides (Feox1), including ferrihydrite and lepidocrocite; (3) reducible oxides (Feox2), including goethite, hematite and akaganéite; (4) magnetite (Femag); (5) poorly reactive sheet silicate Fe (FePRS); (6) pyrite Fe (Fepy); and (7) unreactive silicate Fe (FeU). As such, this is the first extraction scheme specifically developed to allow the separate identification of magnetite, and the first to allow a complete evaluation of Fe carbonate phases such as siderite and ankerite. The scheme was developed following tests on pure mineral phases to evaluate the minerals solubilized by each technique and to determine optimum extraction times. Further tests on mixtures of pure minerals and on grain-size separated sediments from two major US rivers and two glacial meltwaters validate the specificity of the scheme for different pools of iron minerals, and demonstrate a high degree of reproducibility for each analytical stage. The data obtained for the riverine and glacial sediments are additionally discussed in relation to the dominant modes of transport of different iron minerals in fine-grained continental sediments.