388
11 Manganese: Predominant Role of Nodules and Crusts
migrates upwards towards the sediment surface where
it is either oxidized to Mn (IV) at the nodule surface or
substitutes in the phyllomanganate lattice as Mn
2+
.
This behavior explains both the very high Mn/Fe ratios
of suboxic nodules and their correspondingly low
Ni+Cu contents. The boundary between oxic and
suboxic diagenesis is marked by a Mn/Fe ratio in the
nodules of about 5. The nature of the diagenetic
processes occurring in the sediments depends directly
on the input of organic carbon and therefore on the
productivity of the overlying surface waters.
Based on a study of manganese nodules taken on
the Aitutaki-Jarvis Transect in the S.W. Pacific, Cronan
and Hodkinson (1994) also demonstrated the influence
of the CCD on nodule composition. Above the CCD,
accumulation of carbonate tests was thought to dilute
the organic carbon in the sediments, thus inhibiting
diagenesis, whereas, below the CCD, the decay of
organic material in the water column was thought to
reduce its effectiveness in driving diagenetic reactions.
Cronan and Hodkinson (1994) indeed found that Mn,
Ni, Cu and Zn were most concentrated in nodules taken
near the CCD at the north of the transect in an region
of high productivity and least concentrated in nodules
taken away the CCD at the south of the transect in a
region of low productivity. They therefore inferred that
diagenetic cycling of Mn, Ni, Cu and Zn to the nodules
is enhanced as a result of the decay of organic material
near the CCD.
On a more local scale, the influence of diagenetic
processes on nodule morphology and composition
has been related to variations in sedimentation rate in
areas of hilly topography within the C-C F.Z. (von
Stackelberg and Marchig 1987; von Stackelberg and
Beiersdorf 1991). On the flanks of hills and in parts of
basins, sedimentation rates are lower and dominantly
diagenetic nodules are formed whereas, in areas of
sediment drift, sedimentation rates are higher and
dominantly hydrogenous nodules are formed. These
processes are related to the degree of decomposition
of organic matter within the sediment column. Where
sedimentation rates are low, organic matter is rapidly
consumed within the sediment releasing Mn
2+
into the
pore water and resulting in the formation of dominantly
diagenetic nodules. In areas of sediment drift, on the
other hand, the organic carbon is buried within the
sediment stimulating bioturbation and the resultant
biogenic lifting of the nodules to the sediment surface.
Bioturbation is an essential requirement for benthic
lifting of manganese nodules and maintaining the
nodules at the sediment surface. In both the C-C F.Z.
and the Peru Basin, the biological productivity of the
oceanic surface waters is high enough that the amount
of organic matter reaching the sea floor is sufficient to
stimulate the activity of sediment-feeding organisms.
As a result, bioturbation appears to be the dominant
factor in maintaining nodules at the sediment surface
in the C-C F.Z. (von Stackelberg and Beiersdorf 1991;
Skornyakova and Murdmaa 1992) and the Peru Basin
(von Stackelberg 1997, 2000). Several mechanisms have
been proposed by which nodules could be maintained
at the sediment surface by bioturbation. These include
biological pumping, burrowing by crack propagation
as well as bioturbation by benthic fauna and megafauna
(Sanderson 1985; McCave 1988; Banerjee 2000 and
Dogan et al. 2005). The biological productivity of the
surface waters in the Southwestern Pacific Basin, on
the other hand, is much lower than that in the C-C F.Z.
and the Peru Basin and the abundance of benthic fauna
on the sea floor is therefore much reduced. This may
account for the greater influence of ocean bottom
currents in maintaining nodules at the sediment
surface there as proposed by Glasby et al. (1983). The
biological productivity of the oceanic surface waters
may therefore also influence the way in which nodules
are maintained at the sediment surface.
In addition to the above, Calvert and Piper (1984)
have proposed a diagenetic source of metals derived
from sediments far away for nodules occurring in an
erosional area with thin sediment cover in the C-C F.Z.
The metals were thought to be transported to the site
of deposition by oceanic bottom water, probably
AABW. Although this process is well known in
shallow-water continental margin areas such as the
Baltic Sea, only limited evidence to support this
hypothesis has been presented for the deep-sea
environment.
11.4.3 Rare Earth Elements (REE) as
Redox Indicators
A key parameter in understanding nodules formation
is the redox milieu of the environment at the time of
deposition. This can influence the mineralogy and
therefore composition of the nodules (see below). In
this regard, Glasby (1973) used the Ce/La ratio of
nodules as a redox indicator. He showed that deepsea nodules from the NW Indian Ocean had much
higher Ce/La ratios (4.4) and ΣREE contents (490 ppm)
than those from shallow-water continental margin
environments such as Loch Fyne, Scotland (1.9 and
29 ppm, respectively). The higher REE contents of the
deep-sea nodules were taken to reflect the more
oxidizing conditions in the deep sea which facilitated
the oxidation of the trivalent Ce
3+
in seawater to CeO 2
on the surface of the nodules. The lower ΣREE
11 Manganese: Predominant Role of Nodules and Crusts
migrates upwards towards the sediment surface where
it is either oxidized to Mn (IV) at the nodule surface or
substitutes in the phyllomanganate lattice as Mn
2+
.
This behavior explains both the very high Mn/Fe ratios
of suboxic nodules and their correspondingly low
Ni+Cu contents. The boundary between oxic and
suboxic diagenesis is marked by a Mn/Fe ratio in the
nodules of about 5. The nature of the diagenetic
processes occurring in the sediments depends directly
on the input of organic carbon and therefore on the
productivity of the overlying surface waters.
Based on a study of manganese nodules taken on
the Aitutaki-Jarvis Transect in the S.W. Pacific, Cronan
and Hodkinson (1994) also demonstrated the influence
of the CCD on nodule composition. Above the CCD,
accumulation of carbonate tests was thought to dilute
the organic carbon in the sediments, thus inhibiting
diagenesis, whereas, below the CCD, the decay of
organic material in the water column was thought to
reduce its effectiveness in driving diagenetic reactions.
Cronan and Hodkinson (1994) indeed found that Mn,
Ni, Cu and Zn were most concentrated in nodules taken
near the CCD at the north of the transect in an region
of high productivity and least concentrated in nodules
taken away the CCD at the south of the transect in a
region of low productivity. They therefore inferred that
diagenetic cycling of Mn, Ni, Cu and Zn to the nodules
is enhanced as a result of the decay of organic material
near the CCD.
On a more local scale, the influence of diagenetic
processes on nodule morphology and composition
has been related to variations in sedimentation rate in
areas of hilly topography within the C-C F.Z. (von
Stackelberg and Marchig 1987; von Stackelberg and
Beiersdorf 1991). On the flanks of hills and in parts of
basins, sedimentation rates are lower and dominantly
diagenetic nodules are formed whereas, in areas of
sediment drift, sedimentation rates are higher and
dominantly hydrogenous nodules are formed. These
processes are related to the degree of decomposition
of organic matter within the sediment column. Where
sedimentation rates are low, organic matter is rapidly
consumed within the sediment releasing Mn
2+
into the
pore water and resulting in the formation of dominantly
diagenetic nodules. In areas of sediment drift, on the
other hand, the organic carbon is buried within the
sediment stimulating bioturbation and the resultant
biogenic lifting of the nodules to the sediment surface.
Bioturbation is an essential requirement for benthic
lifting of manganese nodules and maintaining the
nodules at the sediment surface. In both the C-C F.Z.
and the Peru Basin, the biological productivity of the
oceanic surface waters is high enough that the amount
of organic matter reaching the sea floor is sufficient to
stimulate the activity of sediment-feeding organisms.
As a result, bioturbation appears to be the dominant
factor in maintaining nodules at the sediment surface
in the C-C F.Z. (von Stackelberg and Beiersdorf 1991;
Skornyakova and Murdmaa 1992) and the Peru Basin
(von Stackelberg 1997, 2000). Several mechanisms have
been proposed by which nodules could be maintained
at the sediment surface by bioturbation. These include
biological pumping, burrowing by crack propagation
as well as bioturbation by benthic fauna and megafauna
(Sanderson 1985; McCave 1988; Banerjee 2000 and
Dogan et al. 2005). The biological productivity of the
surface waters in the Southwestern Pacific Basin, on
the other hand, is much lower than that in the C-C F.Z.
and the Peru Basin and the abundance of benthic fauna
on the sea floor is therefore much reduced. This may
account for the greater influence of ocean bottom
currents in maintaining nodules at the sediment
surface there as proposed by Glasby et al. (1983). The
biological productivity of the oceanic surface waters
may therefore also influence the way in which nodules
are maintained at the sediment surface.
In addition to the above, Calvert and Piper (1984)
have proposed a diagenetic source of metals derived
from sediments far away for nodules occurring in an
erosional area with thin sediment cover in the C-C F.Z.
The metals were thought to be transported to the site
of deposition by oceanic bottom water, probably
AABW. Although this process is well known in
shallow-water continental margin areas such as the
Baltic Sea, only limited evidence to support this
hypothesis has been presented for the deep-sea
environment.
11.4.3 Rare Earth Elements (REE) as
Redox Indicators
A key parameter in understanding nodules formation
is the redox milieu of the environment at the time of
deposition. This can influence the mineralogy and
therefore composition of the nodules (see below). In
this regard, Glasby (1973) used the Ce/La ratio of
nodules as a redox indicator. He showed that deepsea nodules from the NW Indian Ocean had much
higher Ce/La ratios (4.4) and ΣREE contents (490 ppm)
than those from shallow-water continental margin
environments such as Loch Fyne, Scotland (1.9 and
29 ppm, respectively). The higher REE contents of the
deep-sea nodules were taken to reflect the more
oxidizing conditions in the deep sea which facilitated
the oxidation of the trivalent Ce
3+
in seawater to CeO 2
on the surface of the nodules. The lower ΣREE