Download The Iron Biogeochemical Cycle Past and Present: A Geochemical

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
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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
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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.