71
4 Characteristics and Formation Process
Recent studies have shown that the formation of nodules is independent of active
volcanic activity. However, it is worth emphasising the importance of all levels of
life for nodule genesis. The quantity of elements required for their formation is easily made available by the dissolution of planktonic organisms which accumulate in
the sediment. There is a link between surface plankton productivity and nodule formation on the seabed. The richest nodules are associated with the dissolution of tests
of siliceous organisms. The quantity of available organic matter also determines the
amount of benthic life and therefore the degree of bioturbation. Fragments of organisms often act as a nucleus for nodule formation. Furthermore, through their involvement in redox processes, micro-organisms doubtlessly play an important role
in the test dissolution process then in the precipitation of the metals concentrated
in nodules. Ultimately, there are not several nodule growth phenomena, but rather
nodules with different geological histories. The question of the maintenance of surface nodules, raised by the fact that their growth rate is a thousand times lower than
the sedimentation rate (a few millimetres per 1000 years), is not entirely resolved.
It is estimated that the proportion of nodules buried in the top metre of sediment
is equal to that of surface nodules. Active bioturbation causes small nodules to be
relocated. Periodic variations in currents connected to climate variations can also
periodically erode the sediment around nodules. In short, current models involve
an exclusively sedimentary origin of nodules. Life, lithospheric plate dynamics and
global climate variations play a fundamental role in their genesis.
Composition Polymetallic nodules form dark-coloured balls, 5 to 10 cm in diameter, containing around 40 % water (Photo 13). They are mainly composed of manganese and iron hydroxides (Table 4.2). The most crystallised layers are the richest in
nickel and copper, which do not form specific minerals, but are incorporated in the
crystalline networks of manganese and iron oxides. In the nodules in the Pacific, the
average concentrations are 0.42 % copper, 0.63 % nickel, 0.24 % cobalt and 18.50 %
manganese. The Clarion-Clipperton Zone, for which many mining licenses have
been allocated, is particularly rich in copper (0.82 %), nickel (1.28 %) and manganese (25.40 %). Recent estimations for this zone show that, for a surface area of
around 9 million square kilometres (i.e. 15 % of the Pacific Ocean floor at depths of
between 4,000 and 5,000 m), nodules weigh an estimated 34 × 10
9
tonnes, including
7.5 billion tonnes of manganese, 340 million tonnes of nickel, 275 million tonnes of
copper and 78 million tonnes of cobalt. Recent data obtained by submersibles show
heterogeneous distributions which require rigorous mapping and sampling efforts
in order to accurately select the most favourable areas for exploitation. One of the
main problems posed by nodule mining is the environmental impact of their extraction over considerable areas.
Over and above base metals (Cu, Ni, Co), nodules contain rare earths whose
economic importance is growing for cutting edge technologies (electronics) and
technologies related to the development of green energy (electric motors, new generation photovoltaic cells …).
Two major morphological types are distinguished, smooth surface nodules of
purely hydrogenetic origin, i.e. generated from elements contained in seawater, and
4 Characteristics and Formation Process
Recent studies have shown that the formation of nodules is independent of active
volcanic activity. However, it is worth emphasising the importance of all levels of
life for nodule genesis. The quantity of elements required for their formation is easily made available by the dissolution of planktonic organisms which accumulate in
the sediment. There is a link between surface plankton productivity and nodule formation on the seabed. The richest nodules are associated with the dissolution of tests
of siliceous organisms. The quantity of available organic matter also determines the
amount of benthic life and therefore the degree of bioturbation. Fragments of organisms often act as a nucleus for nodule formation. Furthermore, through their involvement in redox processes, micro-organisms doubtlessly play an important role
in the test dissolution process then in the precipitation of the metals concentrated
in nodules. Ultimately, there are not several nodule growth phenomena, but rather
nodules with different geological histories. The question of the maintenance of surface nodules, raised by the fact that their growth rate is a thousand times lower than
the sedimentation rate (a few millimetres per 1000 years), is not entirely resolved.
It is estimated that the proportion of nodules buried in the top metre of sediment
is equal to that of surface nodules. Active bioturbation causes small nodules to be
relocated. Periodic variations in currents connected to climate variations can also
periodically erode the sediment around nodules. In short, current models involve
an exclusively sedimentary origin of nodules. Life, lithospheric plate dynamics and
global climate variations play a fundamental role in their genesis.
Composition Polymetallic nodules form dark-coloured balls, 5 to 10 cm in diameter, containing around 40 % water (Photo 13). They are mainly composed of manganese and iron hydroxides (Table 4.2). The most crystallised layers are the richest in
nickel and copper, which do not form specific minerals, but are incorporated in the
crystalline networks of manganese and iron oxides. In the nodules in the Pacific, the
average concentrations are 0.42 % copper, 0.63 % nickel, 0.24 % cobalt and 18.50 %
manganese. The Clarion-Clipperton Zone, for which many mining licenses have
been allocated, is particularly rich in copper (0.82 %), nickel (1.28 %) and manganese (25.40 %). Recent estimations for this zone show that, for a surface area of
around 9 million square kilometres (i.e. 15 % of the Pacific Ocean floor at depths of
between 4,000 and 5,000 m), nodules weigh an estimated 34 × 10
9
tonnes, including
7.5 billion tonnes of manganese, 340 million tonnes of nickel, 275 million tonnes of
copper and 78 million tonnes of cobalt. Recent data obtained by submersibles show
heterogeneous distributions which require rigorous mapping and sampling efforts
in order to accurately select the most favourable areas for exploitation. One of the
main problems posed by nodule mining is the environmental impact of their extraction over considerable areas.
Over and above base metals (Cu, Ni, Co), nodules contain rare earths whose
economic importance is growing for cutting edge technologies (electronics) and
technologies related to the development of green energy (electric motors, new generation photovoltaic cells …).
Two major morphological types are distinguished, smooth surface nodules of
purely hydrogenetic origin, i.e. generated from elements contained in seawater, and