371
11 Manganese:
Predominant Role of Nodules and Crusts
GEOFFREY P. GLASBY
11.1 Introduction
The importance of manganese in the marine
environment can be deduced from the fact that it is
the tenth most abundant element in the Earth’s crust
(av. conc. 0.093%) and is available in two valency states
whose stability boundary lies within the range of the
natural environment (Glasby 1984). Manganese oxides
also have a high adsorption capacity. δMnO 2 , for
example, has a surface area of about 260 m
2
g
-1
and a
pH zpc of 2.25 and can therefore adsorb cations such as
Ni
2+
, Cu
2+
and Zn
2+
from natural waters. By comparison,
iron is the fourth most abundant element in the Earth’s
crust (av. conc. 5.17%) giving an average Mn/Fe ratio
of 0.02. It also occurs in two valency states whose
stability boundary lies within the range of the natural
environment. Fe oxyhydroxides have a high adsorption capacity and large surface area. Goethite has
a pH zpc of 7.1 and can adsorb both cations (REE
3+
) and
anions (e.g. PO 4
3, MoO 4
2and WO 4
2).
Both elements can therefore migrate under the
influence of redox gradients. They can also fractionate
from each other, particularly under acid or reducing
conditions such as in lakes and shallow seas or in
marine sediments. FeS 2 plays a major role in separating
Mn and Fe in anoxic marine environments such as in
Baltic Sea sediments where SO 4
2ions are present. The
reduced form of manganese, Ca-rhodochrosite (CaMnCO 3 ), is much rarer than FeS 2 but it is found in
environments such as Gotland Deep sediments. Mn is
more mobile than Fe. Mn therefore migrates more
readily than Fe but Fe deposits more readily than Mn.
This leads to the fractionation of Mn from Fe as, for
example, in submarine hydrothermal systems or in
anoxic environments such as Baltic Sea deeps.
The formation of huge quantities of manganese
nodules and crusts on the deep-sea floor is a function
of the fact that manganese and iron are relatively
abundant in the Earth’s crust and migrate from less
oxidizing to more oxidizing environments. The
increased glaciation of Antarctica about 12 Ma led to
the initiation of Antarctic Bottom Water (AABW) flow
and the increased ventilation of the deep ocean. The
modern, well-oxygenated deep-sea has therefore
become the ultimate repository for manganese. Deepsea Mn nodules formed since the lower Miocene unconformity (12 Ma) hold about 10
11
t of Mn (about 16
times the total Mn in terrestrial deposits) reflecting
the importance of manganese nodules in the global
cycle of manganese (Glasby 1988). The high contents
of Co, Ni, Cu and Zn in manganese nodules and crusts
make these deposits an important potential economic
resource for these elements and are a function of the
sorption characteristics of the manganese and iron
oxyhydroxides.
11.2 Manganese, Iron and
Trace Elements in Seawater
Manganese occurs in seawater mainly as Mn
2+
or
MnCl
+
(Bruland 1983). The dissolved Mn concentration
in the open ocean is in range 0.2-3 nmol kg
-1
which is
above the equilibrium concentration with respect to
MnO 2 or MnOOH. This situation reflects the slow rate
of oxidation of Mn
2+
in solution.
Figure 11.1 shows the vertical distribution of Mn in
seawater at the VERTEX-IV site situated in the centre of
the central North Pacific subtropical gyre, a region of
low biological productivity (Bruland et al. 1994). The
high content of Mn in the surface waters reflects the
input of eolian material into this region from Asia.
Photoreduction of particulate MnO 2 to Mn(II) takes
place in the surface waters resulting in 99% of the Mn
in the surface waters (0-100 m) being in the dissolved
form. By contrast, only 80% of the Mn in the deep water
(500-4,000 m) is in the dissolved form. Mn therefore
11 Manganese:
Predominant Role of Nodules and Crusts
GEOFFREY P. GLASBY
11.1 Introduction
The importance of manganese in the marine
environment can be deduced from the fact that it is
the tenth most abundant element in the Earth’s crust
(av. conc. 0.093%) and is available in two valency states
whose stability boundary lies within the range of the
natural environment (Glasby 1984). Manganese oxides
also have a high adsorption capacity. δMnO 2 , for
example, has a surface area of about 260 m
2
g
-1
and a
pH zpc of 2.25 and can therefore adsorb cations such as
Ni
2+
, Cu
2+
and Zn
2+
from natural waters. By comparison,
iron is the fourth most abundant element in the Earth’s
crust (av. conc. 5.17%) giving an average Mn/Fe ratio
of 0.02. It also occurs in two valency states whose
stability boundary lies within the range of the natural
environment. Fe oxyhydroxides have a high adsorption capacity and large surface area. Goethite has
a pH zpc of 7.1 and can adsorb both cations (REE
3+
) and
anions (e.g. PO 4
3, MoO 4
2and WO 4
2).
Both elements can therefore migrate under the
influence of redox gradients. They can also fractionate
from each other, particularly under acid or reducing
conditions such as in lakes and shallow seas or in
marine sediments. FeS 2 plays a major role in separating
Mn and Fe in anoxic marine environments such as in
Baltic Sea sediments where SO 4
2ions are present. The
reduced form of manganese, Ca-rhodochrosite (CaMnCO 3 ), is much rarer than FeS 2 but it is found in
environments such as Gotland Deep sediments. Mn is
more mobile than Fe. Mn therefore migrates more
readily than Fe but Fe deposits more readily than Mn.
This leads to the fractionation of Mn from Fe as, for
example, in submarine hydrothermal systems or in
anoxic environments such as Baltic Sea deeps.
The formation of huge quantities of manganese
nodules and crusts on the deep-sea floor is a function
of the fact that manganese and iron are relatively
abundant in the Earth’s crust and migrate from less
oxidizing to more oxidizing environments. The
increased glaciation of Antarctica about 12 Ma led to
the initiation of Antarctic Bottom Water (AABW) flow
and the increased ventilation of the deep ocean. The
modern, well-oxygenated deep-sea has therefore
become the ultimate repository for manganese. Deepsea Mn nodules formed since the lower Miocene unconformity (12 Ma) hold about 10
11
t of Mn (about 16
times the total Mn in terrestrial deposits) reflecting
the importance of manganese nodules in the global
cycle of manganese (Glasby 1988). The high contents
of Co, Ni, Cu and Zn in manganese nodules and crusts
make these deposits an important potential economic
resource for these elements and are a function of the
sorption characteristics of the manganese and iron
oxyhydroxides.
11.2 Manganese, Iron and
Trace Elements in Seawater
Manganese occurs in seawater mainly as Mn
2+
or
MnCl
+
(Bruland 1983). The dissolved Mn concentration
in the open ocean is in range 0.2-3 nmol kg
-1
which is
above the equilibrium concentration with respect to
MnO 2 or MnOOH. This situation reflects the slow rate
of oxidation of Mn
2+
in solution.
Figure 11.1 shows the vertical distribution of Mn in
seawater at the VERTEX-IV site situated in the centre of
the central North Pacific subtropical gyre, a region of
low biological productivity (Bruland et al. 1994). The
high content of Mn in the surface waters reflects the
input of eolian material into this region from Asia.
Photoreduction of particulate MnO 2 to Mn(II) takes
place in the surface waters resulting in 99% of the Mn
in the surface waters (0-100 m) being in the dissolved
form. By contrast, only 80% of the Mn in the deep water
(500-4,000 m) is in the dissolved form. Mn therefore