241
7
The Biogeochemistry of Iron
RALF R. HAESE
7.1
Introduction
For our understanding of interactions between
living organisms and the solid earth it is fascinating to investigate the reactivity of iron at the
interface of the bio- and geosphere. Similar to
manganese (chapter 11) iron occurs in two valence
states as oxidized ferric iron, Fe(III), and reduced
ferrous iron, Fe(II). Two principal biological processes are of importance: Microorganisms such as
magnetotactic bacteria and phytoplankton (see
chapter 2 and section 7.3) depend on the uptake
of iron as a prerequisite for their cell growth (assimilation). Others conserve energy from the reduction of Fe(III) to maintain their metabolic activity
(dissimilation). In this case ferric iron serves as an
electron acceptor which is also termed oxidant.
Apart from biotic reactions manifold abiotic reactions occur depending on thermodynamic and kinetic conditions. Due to redox-reactions dissolution and precipitation of iron-bearing minerals may
result which has great influence on the sorption/
desorption and co-precipitation/release behavior
of various components such as phosphate and
trace metals. From a geologic point of view it is
striking to find discrete iron enriched layers such
as black shales or strata of the banded iron formation, which challenge geochemists to reconstruct
the environmental conditions of their formation.
7.2
Pathways of Iron Input
to Marine Sediments
Within the continental crust iron is the fourth
most abundant element with a concentration of
4.32 wt% (Wedepohl 1995). It is transported to
marine sediments by four major regimes: fluvial,
aeolian, submarine hydrothermal, and glacial
input. For the investigation of iron reactivity it is
important to differentiate regions of predominant
input regimes since characteristic reactions occur
at the interface of the transport regime and the
marine environment. For the chemistry of hydrothermal fluids and reactions during mixing with
seawater refer to chapter 13.
7.2.1
Fluvial Input
In Figure 7.1 averaged concentrations of dissolved and particulate iron, major fluxes and the
respective predominant reactions are shown.
Dissolved iron concentrations in averaged river
and marine water clearly show less solubility in
marine relative to river water. In contrast, the
concentration of particulate iron does not change
significantly and is similar to the average continental crust concentration. This is also reflected
in the conservative behavior of iron under chemical weathering conditions. Along with Al, Ti and
Mn, Fe belongs to the refractive elements
(Canfield 1997). Note that in Fig. 7.1 the given
value for particulate iron concentration in marine
sediments is derived from pelagic clay sediments.
Biogenic constituents such as carbonate and opal
may significantly dilute the terrigineous fraction
(chapter 1) and thus decrease the iron concentration.
The decrease of dissolved iron concentrations
can be traced within estuarine mixing zones. Within river water, dissolved iron is mainly present as
Fe(III)oxyhydroxide, which is stabilized in colloidal dispersion by high-molecular-weight humic
acids (Hunter 1983). Due to increasing salinity and
thus increasing ionic strength the colloidal dispersion becomes electrostatically and chemically
destabilized which results in the coagulation of
the fluvial colloids. This process is reflected in
Fig. 7.2 showing dissolved iron along a transect
off the mouth of the Congo (formerly: Zaire) river
7
The Biogeochemistry of Iron
RALF R. HAESE
7.1
Introduction
For our understanding of interactions between
living organisms and the solid earth it is fascinating to investigate the reactivity of iron at the
interface of the bio- and geosphere. Similar to
manganese (chapter 11) iron occurs in two valence
states as oxidized ferric iron, Fe(III), and reduced
ferrous iron, Fe(II). Two principal biological processes are of importance: Microorganisms such as
magnetotactic bacteria and phytoplankton (see
chapter 2 and section 7.3) depend on the uptake
of iron as a prerequisite for their cell growth (assimilation). Others conserve energy from the reduction of Fe(III) to maintain their metabolic activity
(dissimilation). In this case ferric iron serves as an
electron acceptor which is also termed oxidant.
Apart from biotic reactions manifold abiotic reactions occur depending on thermodynamic and kinetic conditions. Due to redox-reactions dissolution and precipitation of iron-bearing minerals may
result which has great influence on the sorption/
desorption and co-precipitation/release behavior
of various components such as phosphate and
trace metals. From a geologic point of view it is
striking to find discrete iron enriched layers such
as black shales or strata of the banded iron formation, which challenge geochemists to reconstruct
the environmental conditions of their formation.
7.2
Pathways of Iron Input
to Marine Sediments
Within the continental crust iron is the fourth
most abundant element with a concentration of
4.32 wt% (Wedepohl 1995). It is transported to
marine sediments by four major regimes: fluvial,
aeolian, submarine hydrothermal, and glacial
input. For the investigation of iron reactivity it is
important to differentiate regions of predominant
input regimes since characteristic reactions occur
at the interface of the transport regime and the
marine environment. For the chemistry of hydrothermal fluids and reactions during mixing with
seawater refer to chapter 13.
7.2.1
Fluvial Input
In Figure 7.1 averaged concentrations of dissolved and particulate iron, major fluxes and the
respective predominant reactions are shown.
Dissolved iron concentrations in averaged river
and marine water clearly show less solubility in
marine relative to river water. In contrast, the
concentration of particulate iron does not change
significantly and is similar to the average continental crust concentration. This is also reflected
in the conservative behavior of iron under chemical weathering conditions. Along with Al, Ti and
Mn, Fe belongs to the refractive elements
(Canfield 1997). Note that in Fig. 7.1 the given
value for particulate iron concentration in marine
sediments is derived from pelagic clay sediments.
Biogenic constituents such as carbonate and opal
may significantly dilute the terrigineous fraction
(chapter 1) and thus decrease the iron concentration.
The decrease of dissolved iron concentrations
can be traced within estuarine mixing zones. Within river water, dissolved iron is mainly present as
Fe(III)oxyhydroxide, which is stabilized in colloidal dispersion by high-molecular-weight humic
acids (Hunter 1983). Due to increasing salinity and
thus increasing ionic strength the colloidal dispersion becomes electrostatically and chemically
destabilized which results in the coagulation of
the fluvial colloids. This process is reflected in
Fig. 7.2 showing dissolved iron along a transect
off the mouth of the Congo (formerly: Zaire) river