450
Hjalmar THIEL
Ellis, 1995; Garnett, 1996). Where seafloor excavation
is required for the mining of diamonds, two methods
are employed. The drill system consists of a disclike cutting wheel with hardened steel scrapers. The
seabed crawler, carrying suction boxes at its front, is
tracked along the seafloor, while water jets agitate loose
sandbars and obstructing boulders. Both techniques are
employed down to a water depth of 200 m, and their
use may be extended to 1000 m. They can penetrate
5 m into the sediment. Airlift systems transport the
sediment to the mining ship (Garnett, 1995; Environmental Evaluation Unit, 1996; P.K. Dixon-Savage, pers.
comm., 1998). Whereas these shallow-water methods
could be adapted to greater depths for mining of
phosphorite nodules, the better choice might be the
modification of approved deep-sea systems.
Environmental concerns
An essential characteristic of all mining activities
in the ocean is their large scale. Economic feasibility
determines all aspects: the amount of ore to be mined
per unit time, the area of seafloor disturbed by mining,
the extent of sediment plumes, and finally the scope of
research. Although the figures available were calculated
during the active exploration phases some 25 years ago,
they are still valid; if they need to be corrected in
the future, they seem more likely to increase than to
decrease.
Economic calculations for the mining of metalliferous muds from the Red Sea Atlantis II Deep were
based on the exploitation and ship-bound processing of
100 000 metric tonnes per day. Flotation of the strongly
diluted mud would separate up to 3% of valuable
material from inert overburden materials. All the “rest”,
97 000 t, at that stage diluted to nearly 400 000 t, needs
to be discharged back into the sea (Thiel et al.,
1986; Karbe, 1987). Environmental impacts are to
be expected as a result of mining and discharge.
Throughout the lifetime of the mine, 15–20 years, the
sediment in the Atlantis II Deep would be excavated.
A disturbed mass of mud and brine would remain in the
depression of the seafloor, deep water at a temperature
of 21.8ºC and 41 salinity, would be mixed into the
brine with a temperature of >60ºC and 257 salinity
(Karbe, 1987). The original sulphidic mud certainly
contains bacteria adapted to high temperatures and
salinity (Fiala et al., 1990), but their habitat would no
longer exist. Whether there may be hydrothermal vents
and communities above the brine, or whether special
sulphide biomes occur in some boundary layer, is not
known. The extent of the likely impacts is difficult
to evaluate. It is also an open question whether the
disturbance of this habitat of 60 km
2 will influence
adjacent regions of the seafloor. The transport of mined
material would have no effects on the environment
unless a pipe failure occurs. If this happened, though, it
would preclude further mud transport, and the impact
would remain limited.
The most serious impact, the extent of which is not
fully ascertained, would be from the discharge of the
highly diluted mud. The fate of this material is not well
understood. A gravity-flow model and a momentumjet-flow model were calculated, but limited to a few
hours (Thiel et al., 1986). The pre-pilot mining test
(PPMT)
4 or technique test provided an opportunity
for a small discharge experiment at 400 m depth as a
compromise between surface discharge (industry aim)
and a commercial discharge depth of 1000–1100 m
within the central graben (science aim), accepted after
initially severe resistance by industry. The results
of this experiment showed plume development at a
depth of 700–800 m, which falls between the two
models (Thiel et al., 1986). Characteristics of the
tailings generated during the pre-pilot mining test are:
density 1.1 g cm
−3 , temperature 35ºC, particulate matter
25 g °
−1 , salt content 150 g °
−1 and grain sizes 60%
<0.002 mm (Abu Gideiri, 1984; Thiel et al., 1986).
The original sulphidic mud will be oxidized at least
partly following the strong dilution with deep and
surface waters, resulting partially in altered chemical
bonds. This process is promoted during the flotation
process and after discharge into the deep water, the
prevailing oxygen concentrations being about 2 ml °
−1
(Karbe et al., 1981; Karbe, 1987).
Dissolved trace metals including antimony, arsenic,
cadmium, cobalt, lead, mercury, nickel, silver and
vanadium were released. First toxicological tests with
tailings demonstrated for standard test organisms a
threshold level of approximately 10 mg solids per litre.
If one applies a typical safety factor of 0.1% to
exclude chronic effects, figures for solid content above
0.01 mg °
−1 would be unacceptable (Karbe et al., 1981).
Although the environmental studies conducted during
the commercial exploration period gathered results
on the ecology of the central Red Sea (Thiel, 1979,
1987; Weikert, 1980, 1982, 1987; Karbe et al., 1981;
Beckmann, 1984; Thiel et al., 1986; Karbe, 1987) it
4 For a list of acronyms see the appendix on p. 471.
Hjalmar THIEL
Ellis, 1995; Garnett, 1996). Where seafloor excavation
is required for the mining of diamonds, two methods
are employed. The drill system consists of a disclike cutting wheel with hardened steel scrapers. The
seabed crawler, carrying suction boxes at its front, is
tracked along the seafloor, while water jets agitate loose
sandbars and obstructing boulders. Both techniques are
employed down to a water depth of 200 m, and their
use may be extended to 1000 m. They can penetrate
5 m into the sediment. Airlift systems transport the
sediment to the mining ship (Garnett, 1995; Environmental Evaluation Unit, 1996; P.K. Dixon-Savage, pers.
comm., 1998). Whereas these shallow-water methods
could be adapted to greater depths for mining of
phosphorite nodules, the better choice might be the
modification of approved deep-sea systems.
Environmental concerns
An essential characteristic of all mining activities
in the ocean is their large scale. Economic feasibility
determines all aspects: the amount of ore to be mined
per unit time, the area of seafloor disturbed by mining,
the extent of sediment plumes, and finally the scope of
research. Although the figures available were calculated
during the active exploration phases some 25 years ago,
they are still valid; if they need to be corrected in
the future, they seem more likely to increase than to
decrease.
Economic calculations for the mining of metalliferous muds from the Red Sea Atlantis II Deep were
based on the exploitation and ship-bound processing of
100 000 metric tonnes per day. Flotation of the strongly
diluted mud would separate up to 3% of valuable
material from inert overburden materials. All the “rest”,
97 000 t, at that stage diluted to nearly 400 000 t, needs
to be discharged back into the sea (Thiel et al.,
1986; Karbe, 1987). Environmental impacts are to
be expected as a result of mining and discharge.
Throughout the lifetime of the mine, 15–20 years, the
sediment in the Atlantis II Deep would be excavated.
A disturbed mass of mud and brine would remain in the
depression of the seafloor, deep water at a temperature
of 21.8ºC and 41 salinity, would be mixed into the
brine with a temperature of >60ºC and 257 salinity
(Karbe, 1987). The original sulphidic mud certainly
contains bacteria adapted to high temperatures and
salinity (Fiala et al., 1990), but their habitat would no
longer exist. Whether there may be hydrothermal vents
and communities above the brine, or whether special
sulphide biomes occur in some boundary layer, is not
known. The extent of the likely impacts is difficult
to evaluate. It is also an open question whether the
disturbance of this habitat of 60 km
2 will influence
adjacent regions of the seafloor. The transport of mined
material would have no effects on the environment
unless a pipe failure occurs. If this happened, though, it
would preclude further mud transport, and the impact
would remain limited.
The most serious impact, the extent of which is not
fully ascertained, would be from the discharge of the
highly diluted mud. The fate of this material is not well
understood. A gravity-flow model and a momentumjet-flow model were calculated, but limited to a few
hours (Thiel et al., 1986). The pre-pilot mining test
(PPMT)
4 or technique test provided an opportunity
for a small discharge experiment at 400 m depth as a
compromise between surface discharge (industry aim)
and a commercial discharge depth of 1000–1100 m
within the central graben (science aim), accepted after
initially severe resistance by industry. The results
of this experiment showed plume development at a
depth of 700–800 m, which falls between the two
models (Thiel et al., 1986). Characteristics of the
tailings generated during the pre-pilot mining test are:
density 1.1 g cm
−3 , temperature 35ºC, particulate matter
25 g °
−1 , salt content 150 g °
−1 and grain sizes 60%
<0.002 mm (Abu Gideiri, 1984; Thiel et al., 1986).
The original sulphidic mud will be oxidized at least
partly following the strong dilution with deep and
surface waters, resulting partially in altered chemical
bonds. This process is promoted during the flotation
process and after discharge into the deep water, the
prevailing oxygen concentrations being about 2 ml °
−1
(Karbe et al., 1981; Karbe, 1987).
Dissolved trace metals including antimony, arsenic,
cadmium, cobalt, lead, mercury, nickel, silver and
vanadium were released. First toxicological tests with
tailings demonstrated for standard test organisms a
threshold level of approximately 10 mg solids per litre.
If one applies a typical safety factor of 0.1% to
exclude chronic effects, figures for solid content above
0.01 mg °
−1 would be unacceptable (Karbe et al., 1981).
Although the environmental studies conducted during
the commercial exploration period gathered results
on the ecology of the central Red Sea (Thiel, 1979,
1987; Weikert, 1980, 1982, 1987; Karbe et al., 1981;
Beckmann, 1984; Thiel et al., 1986; Karbe, 1987) it
4 For a list of acronyms see the appendix on p. 471.