251
2002). Kinetic experiments with synthetic iron
oxyhydroxides have shown that the initial microbial reduction rate increases with increasing initial
ferric iron concentration up to a given maximum
reduction rate (Bonneville et al. 2004). This
observation was explained by a saturation of
active membrane sites with Fe(III) centers. The
respective reaction was best described with a
Michaelis-Menten rate expression with the maximum reduction rate per cell positively correlating
with the solubility of the iron oxyhydroxides
(Bonneville et al. 2004). Kinetic studies involving
iron are not only inherently important to describe
reaction pathways and to derive rate constants,
which can be used in models. Kinetic studies also
increasingly focus on iron isotopic fractionation
to better understand the iron isotopic composition
of ancient sediments, which may assist in the
reconstruction of paleo-environments. Importantly, iron isotope fractionation occurs in abiotic and
biotic processes; the degree of isotopic fractionation depends on individual reaction rates and
the environmental conditions, e.g. whether reactions take place within an open or closed system
(Johnson et al. 2004).
7.4.2.3 The Bioavailability of Sheet Silicate
Bound Ferric Iron
As early as in 1972 Roth and Tullock published
results on the chemical reduction of smectites.
Subsequently, Rozenson and Heller-Kallai
(1976a,b) studied the potential of reduction and
reoxidation in various dioctahedral smectites
with the aid of different reducing and oxidizing
agents. Concurrent with the change of the intercrystalline redox state a change of the smectite
color was observed. Oxidized smectites showed a
white or yellowish color whereas the reduced
smectites revealed a greenish-grey or black color.
To balance the intercrystalline charge (de-)
protonation was postulated. Additionally, as a
consequence of smectite reduction a decrease of
the specific surface area and the swellability in
water, as well as an increase of nonexchangeable
Na
+
was found (Lear and Stucki 1989). A potential
importance of smectite redox reactivity in natural
sediments was first pointed out by Lyle (1983) for
sediments of the eastern equatorial Pacific where
a distinctive color transition from tan (above)
green-gray (below) was found in near-surface
sediments. König et al. (1997) conducted a high
resolution Mössbauer-spectroscopy study for
one core from the Peru Basin which revealed a
present-day reduction of 42 % of total iron which
can be differentiated into an immobile fraction (36 %)
consisting of smectite bound iron and a mobile
fraction (16 %) which diffuses back into the oxidized
upper sediment layer. Evidence for the bioavailability
of ferric iron bound to smectites was given by
Kostka et al. (1996). During culturing of an iron
reducing bacteria (Shewanella putrefaciens strain
MR-1) on smectite as a sole electron acceptor a
reduction of Fe
3+
by 15 % within the first 4 hours
and a total of 33 % after two weeks occurred.
7.4.3
Iron Reactivity towards S, O 2 , Mn,
NO 3
-
, P, HCO 3
-
, and Si-Al
The (microbial) dissimilatory iron reduction was
shown in the previous section. In this section the
reactions with major oxidants and reductants will
be introduced. Additionally, the interactions between
iron and phosphorus, as well as the formation of
siderite and iron-bearing sheet silicates will be
pointed out briefly to show to the variety of reactions in marine sediments coupled the reactivity of
iron.
7.4.3.1 Iron Reduction by HS - and Ligands
Apart from the dissimilatory iron reduction
(section 7.4.1) iron oxides can be dissolved by
protons, ligands and reductants. Dissolution reactions by protons and ligands are generally considered to be the rate-determining step for weathering processes. For iron bearing minerals in marine
sediments proton-promoted dissolution is of no
importance due to prevailing neutral or slightly
alkaline conditions. Ligands (e.g. oxalates and
citric acid) are by-products of biological decomposition and dissolve iron oxides by primary
surface complexation onto the iron oxide surface
resulting in a weakening of the Fe-O bond which
is followed by a detachment of Fe
3+
-ligand. Reductive dissolution (e.g. by HS
-
, ascorbate, and dithionite) is characterized by primary surface complexation followed by an electron transfer from the
reductant to ferric iron and detachment of Fe
2+
.
The three pathways of Fe(III) (hydr)oxide dissolution are shown in Fig. 7.14.
Experimental determination of reduction rates
(e.g. Pyzik and Sommer 1981; Dos Santos Afonso
and Stumm 1992; Peiffer et al. 1992) reveal rates
under well defined conditions and information on
the reaction kinetics. Under natural conditions the
7.4
The Early Diagenesis of Iron in Sediments
2002). Kinetic experiments with synthetic iron
oxyhydroxides have shown that the initial microbial reduction rate increases with increasing initial
ferric iron concentration up to a given maximum
reduction rate (Bonneville et al. 2004). This
observation was explained by a saturation of
active membrane sites with Fe(III) centers. The
respective reaction was best described with a
Michaelis-Menten rate expression with the maximum reduction rate per cell positively correlating
with the solubility of the iron oxyhydroxides
(Bonneville et al. 2004). Kinetic studies involving
iron are not only inherently important to describe
reaction pathways and to derive rate constants,
which can be used in models. Kinetic studies also
increasingly focus on iron isotopic fractionation
to better understand the iron isotopic composition
of ancient sediments, which may assist in the
reconstruction of paleo-environments. Importantly, iron isotope fractionation occurs in abiotic and
biotic processes; the degree of isotopic fractionation depends on individual reaction rates and
the environmental conditions, e.g. whether reactions take place within an open or closed system
(Johnson et al. 2004).
7.4.2.3 The Bioavailability of Sheet Silicate
Bound Ferric Iron
As early as in 1972 Roth and Tullock published
results on the chemical reduction of smectites.
Subsequently, Rozenson and Heller-Kallai
(1976a,b) studied the potential of reduction and
reoxidation in various dioctahedral smectites
with the aid of different reducing and oxidizing
agents. Concurrent with the change of the intercrystalline redox state a change of the smectite
color was observed. Oxidized smectites showed a
white or yellowish color whereas the reduced
smectites revealed a greenish-grey or black color.
To balance the intercrystalline charge (de-)
protonation was postulated. Additionally, as a
consequence of smectite reduction a decrease of
the specific surface area and the swellability in
water, as well as an increase of nonexchangeable
Na
+
was found (Lear and Stucki 1989). A potential
importance of smectite redox reactivity in natural
sediments was first pointed out by Lyle (1983) for
sediments of the eastern equatorial Pacific where
a distinctive color transition from tan (above)
green-gray (below) was found in near-surface
sediments. König et al. (1997) conducted a high
resolution Mössbauer-spectroscopy study for
one core from the Peru Basin which revealed a
present-day reduction of 42 % of total iron which
can be differentiated into an immobile fraction (36 %)
consisting of smectite bound iron and a mobile
fraction (16 %) which diffuses back into the oxidized
upper sediment layer. Evidence for the bioavailability
of ferric iron bound to smectites was given by
Kostka et al. (1996). During culturing of an iron
reducing bacteria (Shewanella putrefaciens strain
MR-1) on smectite as a sole electron acceptor a
reduction of Fe
3+
by 15 % within the first 4 hours
and a total of 33 % after two weeks occurred.
7.4.3
Iron Reactivity towards S, O 2 , Mn,
NO 3
-
, P, HCO 3
-
, and Si-Al
The (microbial) dissimilatory iron reduction was
shown in the previous section. In this section the
reactions with major oxidants and reductants will
be introduced. Additionally, the interactions between
iron and phosphorus, as well as the formation of
siderite and iron-bearing sheet silicates will be
pointed out briefly to show to the variety of reactions in marine sediments coupled the reactivity of
iron.
7.4.3.1 Iron Reduction by HS - and Ligands
Apart from the dissimilatory iron reduction
(section 7.4.1) iron oxides can be dissolved by
protons, ligands and reductants. Dissolution reactions by protons and ligands are generally considered to be the rate-determining step for weathering processes. For iron bearing minerals in marine
sediments proton-promoted dissolution is of no
importance due to prevailing neutral or slightly
alkaline conditions. Ligands (e.g. oxalates and
citric acid) are by-products of biological decomposition and dissolve iron oxides by primary
surface complexation onto the iron oxide surface
resulting in a weakening of the Fe-O bond which
is followed by a detachment of Fe
3+
-ligand. Reductive dissolution (e.g. by HS
-
, ascorbate, and dithionite) is characterized by primary surface complexation followed by an electron transfer from the
reductant to ferric iron and detachment of Fe
2+
.
The three pathways of Fe(III) (hydr)oxide dissolution are shown in Fig. 7.14.
Experimental determination of reduction rates
(e.g. Pyzik and Sommer 1981; Dos Santos Afonso
and Stumm 1992; Peiffer et al. 1992) reveal rates
under well defined conditions and information on
the reaction kinetics. Under natural conditions the
7.4
The Early Diagenesis of Iron in Sediments