257
typically ranging between 70 and 150 °C under
oxic and anoxic conditions (Cole and Shaw 1983).
Yet, lowest temperatures of formation were
deduced to be ∼ 20 °C (McMurtry et al 1983;
Singer et al. 1984). Experimental studies by
Decarreau et al. (1987) demonstrated the synthesis
of dioctahedral smectite, containing Fe(III) within
the octahedral sheet, only under strictly oxic conditions.
Experimental results by Harder (1976, 1978)
gave evidence for the potential of iron bearing
clay mineral formation under low temperature
deep-sea floor conditions. Subsequent findings of
sediments rich in montmorillonites in the north
equatorial Pacific (Hein et al. 1979) and nontronite
in the Bauer Deep of the eastern equatorial Pacific
(Cole and Shaw 1983; Cole 1985) were attributed
to authigenic aluminosilicate formation. For the
formation of low-temperature iron-bearing aluminosilicates the deposition of skeletal opal (e.g.
radiolarian) and iron oxyhydroxide (e.g. precipitation products of hydrothermal activity) as well
as a low carbonate content are considered (Cole
and Shaw 1983). Enhanced opal dissolution due to
the presence of high iron oxide concentrations
were reported (Mayer et al. 1991), yet kinetic
reasoning of this observation remains unclear. As
the skeletal opal closely associated with the iron
oxyhydroxide becomes buried it dissolves and
forms an amorphous Fe(III)-silica complex at the
skeleton surface which subsequently recrystallizes to form nontronite on the surface of the
partially dissolved skeletons (Cole 1985). By this
analogy, Harder (1976,1978) also found an amorphous Fe(III)-silicate precipitate as a precursor
which developed during aging under suboxic
conditions into a crystalline iron-rich clay mineral.
The presence of Fe
2+
was a prerequisite for the
synthesis of clay minerals under experimental
conditions and therefore partial reduction of iron
oxyhydroxide within the microenvironment of an
opal skeleton must be assumed. The oxygen
isotopic composition (δ
18
O) of the authigenic
mineral can be used to reconstruct the prevailing
temperature during formation by applying the
geothermometric equation of Yeh and Savin
(1977). For the aluminosilicates from the north
equatorial Pacific and the Bauer Deep formation
temperatures of ∼ 3-4 °C were deduced representing authigenic formation under low-temperature conditions in deep-sea sediments. Similar to
the above described deep-sea conditions, the
Amazon delta represents an iron and silicate rich
depositional environment. Incubation experiments
with sediments from the Amazon delta revealed
substantial K-Fe-Mg-clay mineral formation within
1-3 years under low-temperature conditions
(Michalopoulos and Aller 1995) implying
significant elemental transfer into solid phase
within the estuarine mixing zone.
7.4.4
Iron and Manganese Redox Cycles
Processes of early diagenesis can only be
understood by integrating biogeochemical reactions and modes of transport in the sediment. With
respect to the quantification of iron and manganese
reactions molecular diffusion and bioirrigation need
to be considered for the dissolved phase whereas
bioturbation and advection are relevant for the
particulate transport (Haese 2002). Bioirrigation is
the term for solute exchange between the bottom
water and tubes in which macro-benthic organisms
actively pump water. For iron and manganese a
recent study (Hüttel et al. 1998) points out the
importance of solute transport in the sediment and
across the sediment / bottom water interface due to
pressure gradients induced by water flow over a
rough sediment topography. Advection in the context of particulate transport describes the downward transport of particles relative to the sediment
surface due to sedimentation. Strictly speaking,
bioturbation (sometimes more generally termed mixing) also induces a vertical transport of dissolved
phase. Yet, as molecular diffusive transport is usually much greater than dissolved transport by bioturbation the later is usually neglected.
The cycling of reduced and oxidized iron and
manganese species are discussed together in this
chapter since the driving processes are principally the same. In Fig. 7.18 the operating modes
of transport are shown schematically along with
a redox boundary. Above this boundary the reactive fraction of total solid phase iron or manganese is present as oxidized species whereas
below the reduced species occur. Note that at
this boundary a build-up in the pore water
occurs if no immediate precipitation (e.g. FeS) or
adsorption inhibits a release into ambient water.
The change in the redox state implies oxidation
above and reduction below by some electron
donor / acceptor. In case of dissimilatory iron /
manganese reduction the organic carbon serves
as electron donor, the other most important
oxidants and reductants are discussed in the
sections 7.4.3.1 and 7.4.3.2 .
7.4
The Early Diagenesis of Iron in Sediments
typically ranging between 70 and 150 °C under
oxic and anoxic conditions (Cole and Shaw 1983).
Yet, lowest temperatures of formation were
deduced to be ∼ 20 °C (McMurtry et al 1983;
Singer et al. 1984). Experimental studies by
Decarreau et al. (1987) demonstrated the synthesis
of dioctahedral smectite, containing Fe(III) within
the octahedral sheet, only under strictly oxic conditions.
Experimental results by Harder (1976, 1978)
gave evidence for the potential of iron bearing
clay mineral formation under low temperature
deep-sea floor conditions. Subsequent findings of
sediments rich in montmorillonites in the north
equatorial Pacific (Hein et al. 1979) and nontronite
in the Bauer Deep of the eastern equatorial Pacific
(Cole and Shaw 1983; Cole 1985) were attributed
to authigenic aluminosilicate formation. For the
formation of low-temperature iron-bearing aluminosilicates the deposition of skeletal opal (e.g.
radiolarian) and iron oxyhydroxide (e.g. precipitation products of hydrothermal activity) as well
as a low carbonate content are considered (Cole
and Shaw 1983). Enhanced opal dissolution due to
the presence of high iron oxide concentrations
were reported (Mayer et al. 1991), yet kinetic
reasoning of this observation remains unclear. As
the skeletal opal closely associated with the iron
oxyhydroxide becomes buried it dissolves and
forms an amorphous Fe(III)-silica complex at the
skeleton surface which subsequently recrystallizes to form nontronite on the surface of the
partially dissolved skeletons (Cole 1985). By this
analogy, Harder (1976,1978) also found an amorphous Fe(III)-silicate precipitate as a precursor
which developed during aging under suboxic
conditions into a crystalline iron-rich clay mineral.
The presence of Fe
2+
was a prerequisite for the
synthesis of clay minerals under experimental
conditions and therefore partial reduction of iron
oxyhydroxide within the microenvironment of an
opal skeleton must be assumed. The oxygen
isotopic composition (δ
18
O) of the authigenic
mineral can be used to reconstruct the prevailing
temperature during formation by applying the
geothermometric equation of Yeh and Savin
(1977). For the aluminosilicates from the north
equatorial Pacific and the Bauer Deep formation
temperatures of ∼ 3-4 °C were deduced representing authigenic formation under low-temperature conditions in deep-sea sediments. Similar to
the above described deep-sea conditions, the
Amazon delta represents an iron and silicate rich
depositional environment. Incubation experiments
with sediments from the Amazon delta revealed
substantial K-Fe-Mg-clay mineral formation within
1-3 years under low-temperature conditions
(Michalopoulos and Aller 1995) implying
significant elemental transfer into solid phase
within the estuarine mixing zone.
7.4.4
Iron and Manganese Redox Cycles
Processes of early diagenesis can only be
understood by integrating biogeochemical reactions and modes of transport in the sediment. With
respect to the quantification of iron and manganese
reactions molecular diffusion and bioirrigation need
to be considered for the dissolved phase whereas
bioturbation and advection are relevant for the
particulate transport (Haese 2002). Bioirrigation is
the term for solute exchange between the bottom
water and tubes in which macro-benthic organisms
actively pump water. For iron and manganese a
recent study (Hüttel et al. 1998) points out the
importance of solute transport in the sediment and
across the sediment / bottom water interface due to
pressure gradients induced by water flow over a
rough sediment topography. Advection in the context of particulate transport describes the downward transport of particles relative to the sediment
surface due to sedimentation. Strictly speaking,
bioturbation (sometimes more generally termed mixing) also induces a vertical transport of dissolved
phase. Yet, as molecular diffusive transport is usually much greater than dissolved transport by bioturbation the later is usually neglected.
The cycling of reduced and oxidized iron and
manganese species are discussed together in this
chapter since the driving processes are principally the same. In Fig. 7.18 the operating modes
of transport are shown schematically along with
a redox boundary. Above this boundary the reactive fraction of total solid phase iron or manganese is present as oxidized species whereas
below the reduced species occur. Note that at
this boundary a build-up in the pore water
occurs if no immediate precipitation (e.g. FeS) or
adsorption inhibits a release into ambient water.
The change in the redox state implies oxidation
above and reduction below by some electron
donor / acceptor. In case of dissimilatory iron /
manganese reduction the organic carbon serves
as electron donor, the other most important
oxidants and reductants are discussed in the
sections 7.4.3.1 and 7.4.3.2 .
7.4
The Early Diagenesis of Iron in Sediments