The Iron Biogeochemical Cycle Past and Present: A Geochemical Perspective

Our understanding of the complex aqueous chemistry of iron in natural systems has advanced significantly over recent years. Here we will highlight how new contributions from nanogeoscience combine with conventional aqueous geochemistry to provide a novel view of kinetics in the modern iron biogeochemical cycle, and the evolution of the iron cycle through geologic time. We start by considering the properties of nanoparticulate iron (oxyhydr)oxides, and the influence of aggregation and mineral transformations on the iron chemistry of seawater. Emerging evidence indicates that the well-established picture of aqueous iron complexing in seawater needs to be amended to include a role for nanoparticulate Fe (oxyhydr)oxide-organic matter aggregates. Evaluating the different contributions, and different behaviour, of aqueous organic complexes versus nanoparticulate (oxyhydr)oxide-organic aggregates will be a major challenge. We next adopt a historical viewpoint in considering iron diagenesis from the perspective of the geochemical proxies based on pyrite formation; the C/S ratio for distinguishing marine and freshwater depositional environments, and the Degree of Pyritisation, the Anoxicity Indicator (Reactive Fe/Total Fe), and the ratio of Total Fe/Total Al as indicators of sulphidic marine depositional environments. This viewpoint will show how ideas on pyrite formation evolved from the early standpoint of an organic C control to the emergence of reactive iron as