David Rickard

Sedimentary Iron Biogeochemistry

Sulfur is mainly fixed in sediments as iron sulfides and the biogeochemistry of iron in aqueous solutions is important to understanding the processes involved and in interpreting sedimentary sulfides. Iron is unusual in that the riverine Fe supply to the oceans is mainly in the form of particulates. Equilibrium thermodynamic data for iron species confirm natural observations that ferrhydrites and goethite are the dominant inorganic Fe species in most non-sulfidic sedimentary environments although siderite and the complex Green Rusts may form in specific systems. Dissolved Fe is generally operationally defined and includes organic and nanoparticulate Fe. In modern oxidized ocean waters dissolved Fe is mainly present in organic complexes in which the dominant forms are saccharide complexes. However, the role of organic complexes in anoxic systems has not yet been fully characterized. Reactive iron - that is Fe that readily reacts with sulfide in sediments - is an operationally-defined concept which has proved useful in furthering understanding of sedimentary sulfides. The discovery of Fe-reducing bacteria and their widespread distribution in sediments has meant that more resistant Fe phases may contribute to the total reactive Fe load. A combination of chemical and micro- and macro- biological processes in shelf systems results in the reduction of Fe(III) phases and the efflux of Fe to the oceans. This reactive Fe shuttle may explain the relative abundance of Fe in deeper, basinal and sulfidic sediments.