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Volume 1, Issue 1 The Iron Biogeochemical Cycle Past and Present Robert Raiswell, School of Earth and Environment, University of Leeds, Leeds LS2 9JT, United Kingdom and Donald Canfield, Institute of Biology, University of Southern Denmark, Campusvej 55, 5230 Odense M, Denmark Abstract 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 Volume 1, Issue 1 the principal control. These early steps provided an important platform for new ideas on iron cycling in early Earth history. Global models of the iron cycle now acknowledge that there are multiple sources of iron to the open ocean. The magnitudes and dynamics of three sources are considered here from a nanogeoscience viewpoint: shelf sediments, atmospheric dust and icebergs. Shelf sediments contain iron-rich porewaters that can be mixed by re-working into the overlying seawater producing a source of Fe to seawater. This Fe precipitates as nanoparticulate ferrihydrite. Some ferrihydrite escapes deposition and is transported from the shelf by mid-depth currents. Transport times to depositional sites appear sufficient for most ferrihydrite to alter to more stable, less bioavailable mixtures of goethite/hematite that enable long distance transport but allow reaction with sulphide in euxinic basins. Wind-blown dust contains aged and crystalline goethite and haematite.These (oxyhydr)oxide minerals, long considered the main source of bioavailable iron to the Southern Ocean, are rejuvenated and altered by acid processing in clouds to nanoparticulate ferrihydrite and goethite. We present a kinetic model evaluating the rates at which ferrihydrite is converted to a bioavailable form in surface seawater by dissolution, photochemical reduction, siderophore-aided dissolution and ingestion, and lost to deeper waters through aggregation, sinking, and scavenging. Rate constants from the literature enable the model to be used to quantify the supply of bioavailable iron from atmospheric dust. This source is next compared to icebergs as a newly-discovered source of bioavailable iron to the Southern Ocean. Iceberg-hosted sediments released by melting contain nanoparticulate ferrihydrite that has been preserved by freezing. The kinetic model indicates that the rate delivery of bioavailable Fe supply from icebergs to the Southern Ocean is at least as large as that by wind-blown dust. These features of the modern Fe cycle have altered over geologic time scales. We base this discussion on the modern iron cycle, but as the story unfolds, we will find that the iron cycle is intimately coupled to the cycling of other important bioactive elements, in particular sulfur, oxygen and nitrogen. Understanding how iron interacts both abiotically and microbially with these bioactive elements in modern ecosystems is an essential first step. From this, we Volume 1, Issue 1 focus on the evolving structure of the ancient marine water column. Recent results have demonstrated that this structure may be complex, for example high-resolution studies of Proterozoic sediments have demonstrated that sulphidic oxygen-minimum zones at times extended into anoxic ferruginous ocean basins. Here, the challenge is to understand how such conditions were established. We will argue that the answer lies in considering how the Fe cycle has evolved in concert with the chemical evolution of the ocean and atmosphere. and in particular, how the interactions between Fe and the elemental cycles of oxygen, sulphur and nitrogen have evolved through time. In principle, all of the processes needed to explain the ancient record occur today in low-oxygen environments, but their magnitude has varied through time in response to changes in the Earth surface environment. Within this context, and informed by the proxy record, we will explore how the iron cycle has evolved in light of multiple interactions with other element cycles on the chemically evolving earth. Finally, we will return to our earlier discussion on Fe nanoparticles and consider how these might have been players in the cycling of iron on the ancient Earth. The intricate and exquisite web of biological, chemical and physical processes that make up the iron biogeochemical cycle reflect the expansion of geoscience into areas from which it was effectively isolated during the early parts of our careers. This expansion presents an increasingly difficult challenge; to integrate many different specialisations into a single coherent picture. Seeing the big picture - by looking at the nanoscale - presents exciting opportunities but has never been more difficult. We hope this perspective of the iron biogeochemical cycle will help the geochemical community to probe earth's deep time more effectively and to expand our horizons into planetary science.