of the equator. Few nodules have been recorded in
the Arabian Sea or the Bay of Bengal, most probably
because of the high rates of terrigenous sediment
input in these regions from the south Asian rivers.
The equatorial zone is also largely devoid of
nodules. High nodule concentrations have been
recorded in parts of the Crozet Basin, in the Central
Indian Ocean Basin and in the Wharton Basin.
Atlantic Ocean Nodule abundance in the Atlantic
Ocean appears to be more limited than in the Pacific
or Indian Oceans, probably as a result of its
relatively high sedimentation rates. Another feature
which inhibits nodule abundance in the Atlantic is
that much of the seafloor is above the calcium
carbonate compensation depth (CCD). The areas of
the Atlantic where nodules do occur in appreciable
amounts are those where sedimentation is inhibited.
The deep water basins on either side of the MidAtlantic Ridge which are below the CCD and which
accumulate only limited sediment contain nodules
in reasonable abundance, particularly in the
western Atlantic. Similarly, there is a widespread
occurrence of nodules and encrustations in the
Drake Passage–Scotia Sea area probably due to the
strong bottom currents under the Circum-Antarctic
current inhibiting sediment deposition in this
region. Abundant nodule deposits on the Blake
Plateau can also be related to high bottom currents.
Buried nodules Most workers on the subject agree
that the preferential concentration of nodules at the
sediment surface is due to the activity of benthic
organisms which can slightly move the nodules.
Buried nodules have, however, been found in all the
oceans of the world. Their abundance is highly
variable, but it is possible that it may not be entirely
random. Buried nodules recovered in large diameter
cores are sometimes concentrated in distinct layers.
These layers may represent ancient erosion surfaces
or surfaces of nondeposition on which manganese
nodules were concentrated in the past. By contrast,
in the Peru Basin large asymmetrical nodules get
buried when their bottoms get stuck in tenacious
suboxic sediment just below the surface layer.
Compositional Variability of
Manganese Nodules
Manganese nodules exhibit a continuous mixing
from diagenetic end members which contain the
mineral 10A ˚ manganite (todorokite) and are enriched in Mn, Ni and Cu, to hydrogenous end
members which contain the mineral d MnO 2
(vernadite) and are enriched in Fe and Co. The
diagenetic deposits derive their metals at least in part
from the recycling through the sediment interstitial
waters of elements originally contained in organic
phases on their decay and dissolution in the sediments, whereas the hydrogenous deposits receive
their metals from normal sea water or diagenetically
unenriched interstitial waters. Potentially ore-grade
manganese nodules of resource interest fall near the
diagenetic end member in composition. These are
nodules that are variably enriched in Ni and Cu, up
to a maximum of about 3.0% combined.
One of the most striking features shown by
chemical data on nodules are enrichments of many
elements over and above their normal crustal abundances (Table 1). Some elements such as Mn, Co,
Mo and Tl are concentrated about 100-fold or more;
Ni, Ag, Ir and Pb are concentrated from about 50- to
100-fold, B, Cu, Zn, Cd, Yb, W and Bi from about
10 to 50-fold and P, V, Fe, Sr, Y, Zr, Ba, La and Hg up
to about 10-fold above crustal abundances.
Regional Compositional Variability
Pacific Ocean In the Pacific, potentially ore-grade
nodules are generally confined to two zones running
roughly east–west in the tropical regions, which are
well separated in the eastern Pacific but which
converge at about 1701–1801W (Figure 5). They
follow the isolines of intermediate biological
productivity, strongly suggestive of a biological
control on their distribution. Within these zones,
the nodules preferentially occupy basin areas near
or below the CCD. Thus they are found in the Peru
Basin, Tiki Basin, Penrhyn Basin, Nova Canton
Trough area, Central Pacific Basin and Clarion–
Clipperton Zone (Figure 5). Nodules in all these
areas have features in common and are thought to
have attained their distinctive composition by
similar processes.
The potentially ore-grade manganese nodule field
in the Peru Basin, centered at about 71–81S and
901W (Figure 5), is situated under the southern flank
of the equatorial zone of high biological productivity
on a seafloor composed of pelagic brown mud with
variable amounts of siliceous and calcareous remains. Nodules from near the CCD at around
4250 m are characterized by diagenetic growth and
are enriched in Mn, Ni and Cu, whereas those from
shallower depth are characterized mainly by hydrogenous growth. The Mn/Fe ratio increases from
south to north as productivity increases, whereas the
Ni and Cu contents reach maximum values in the
middle of the area where Mn/Fe ratios are about 5.
370 MANGANESE NODULES
the Arabian Sea or the Bay of Bengal, most probably
because of the high rates of terrigenous sediment
input in these regions from the south Asian rivers.
The equatorial zone is also largely devoid of
nodules. High nodule concentrations have been
recorded in parts of the Crozet Basin, in the Central
Indian Ocean Basin and in the Wharton Basin.
Atlantic Ocean Nodule abundance in the Atlantic
Ocean appears to be more limited than in the Pacific
or Indian Oceans, probably as a result of its
relatively high sedimentation rates. Another feature
which inhibits nodule abundance in the Atlantic is
that much of the seafloor is above the calcium
carbonate compensation depth (CCD). The areas of
the Atlantic where nodules do occur in appreciable
amounts are those where sedimentation is inhibited.
The deep water basins on either side of the MidAtlantic Ridge which are below the CCD and which
accumulate only limited sediment contain nodules
in reasonable abundance, particularly in the
western Atlantic. Similarly, there is a widespread
occurrence of nodules and encrustations in the
Drake Passage–Scotia Sea area probably due to the
strong bottom currents under the Circum-Antarctic
current inhibiting sediment deposition in this
region. Abundant nodule deposits on the Blake
Plateau can also be related to high bottom currents.
Buried nodules Most workers on the subject agree
that the preferential concentration of nodules at the
sediment surface is due to the activity of benthic
organisms which can slightly move the nodules.
Buried nodules have, however, been found in all the
oceans of the world. Their abundance is highly
variable, but it is possible that it may not be entirely
random. Buried nodules recovered in large diameter
cores are sometimes concentrated in distinct layers.
These layers may represent ancient erosion surfaces
or surfaces of nondeposition on which manganese
nodules were concentrated in the past. By contrast,
in the Peru Basin large asymmetrical nodules get
buried when their bottoms get stuck in tenacious
suboxic sediment just below the surface layer.
Compositional Variability of
Manganese Nodules
Manganese nodules exhibit a continuous mixing
from diagenetic end members which contain the
mineral 10A ˚ manganite (todorokite) and are enriched in Mn, Ni and Cu, to hydrogenous end
members which contain the mineral d MnO 2
(vernadite) and are enriched in Fe and Co. The
diagenetic deposits derive their metals at least in part
from the recycling through the sediment interstitial
waters of elements originally contained in organic
phases on their decay and dissolution in the sediments, whereas the hydrogenous deposits receive
their metals from normal sea water or diagenetically
unenriched interstitial waters. Potentially ore-grade
manganese nodules of resource interest fall near the
diagenetic end member in composition. These are
nodules that are variably enriched in Ni and Cu, up
to a maximum of about 3.0% combined.
One of the most striking features shown by
chemical data on nodules are enrichments of many
elements over and above their normal crustal abundances (Table 1). Some elements such as Mn, Co,
Mo and Tl are concentrated about 100-fold or more;
Ni, Ag, Ir and Pb are concentrated from about 50- to
100-fold, B, Cu, Zn, Cd, Yb, W and Bi from about
10 to 50-fold and P, V, Fe, Sr, Y, Zr, Ba, La and Hg up
to about 10-fold above crustal abundances.
Regional Compositional Variability
Pacific Ocean In the Pacific, potentially ore-grade
nodules are generally confined to two zones running
roughly east–west in the tropical regions, which are
well separated in the eastern Pacific but which
converge at about 1701–1801W (Figure 5). They
follow the isolines of intermediate biological
productivity, strongly suggestive of a biological
control on their distribution. Within these zones,
the nodules preferentially occupy basin areas near
or below the CCD. Thus they are found in the Peru
Basin, Tiki Basin, Penrhyn Basin, Nova Canton
Trough area, Central Pacific Basin and Clarion–
Clipperton Zone (Figure 5). Nodules in all these
areas have features in common and are thought to
have attained their distinctive composition by
similar processes.
The potentially ore-grade manganese nodule field
in the Peru Basin, centered at about 71–81S and
901W (Figure 5), is situated under the southern flank
of the equatorial zone of high biological productivity
on a seafloor composed of pelagic brown mud with
variable amounts of siliceous and calcareous remains. Nodules from near the CCD at around
4250 m are characterized by diagenetic growth and
are enriched in Mn, Ni and Cu, whereas those from
shallower depth are characterized mainly by hydrogenous growth. The Mn/Fe ratio increases from
south to north as productivity increases, whereas the
Ni and Cu contents reach maximum values in the
middle of the area where Mn/Fe ratios are about 5.
370 MANGANESE NODULES