ANTHROPOGENIC IMPACTS ON THE DEEP SEA
451
remained impossible to evaluate the large-scale impacts
to be expected from tailings discharge over a period of
15–20 years.
The lower economic limit for the mining of polymetallic nodules is estimated at 5000 metric tonnes
(dry) per day. This would involve the total disturbance
of 1 km
2 each day. The collector system, 6–15 m wide,
is expected to move in close meanders. It will weigh
several tens of tonnes and certainly strongly compress
and mix the mud in its track, from which the nodules
have been lifted up just before by the collector. The
system is expected to create a large plume from ooze
taken up with and separated from the nodules by the
collector system. Much of the sediment, aggregated in
lumps of various sizes, will resettle immediately but the
lighter material will drift away for uncertain distances
(Thiel et al., 1991; Zielke et al., 1995; Jankowski et al.,
1996). Conservatively calculated (based on a sediment
layer of only 2–3 cm), 20 000 metric tonnes would
be mobilized per day, but it remains unknown what
proportion of this would constitute the near-bottom
plume, and for what distance it would drift. The effects
of these near-bottom and seafloor impacts still cannot
be evaluated correctly, and this is also true for the
discharge plume (see also Thiel, ed., 2001).
Ozturgut (1981) estimated that 25 000 m
3 of bottom water containing 1600 tonnes of sediment and
250 tonnes of nodule fines would need to be discharged
from the mining ship or the nodule carrier per day.
These figures may change in the future. They are
based on a daily mining rate of 5000 tonnes which
may increase, and the amount of nodule fines and
sediment will depend on the sediment and nodule types.
Comparing North and South Pacific mining areas, the
small and rather smooth nodules of the North Pacific
may be mined with less sediment, and they may lose
less nodule fines during their transport to the surface. In
contrast, mining the larger nodules of the South Pacific,
lying partly buried in the stiff deeper sediments and
having a rather loose, gritty surface, would result in a
larger volume of particulate discharge into the water
column.
Industry should be aware of these problems, and
should minimize the different discharge volumes. But
the need for discharge will inevitably exist, and a
plume will certainly be created. For each mine site
the appropriate discharge depth needs to be determined
in relation to the local oceanographic conditions.
Ecological factors to be taken into account include the
depth zones of primary production, vertical migration
depth of zooplankton and fish, the prevailing currents
and vertical mixing of the water mass. Also, discharge
above or into the oxygen minimum zone must be
avoided, because manganese oxide and other metal
components may go into solution following chemical
reduction. Therefore, discharge into surface waters is
certainly not acceptable for commercial mining, even
though it was practised without detectable adverse
ecological effects during the pre-pilot mining test
(Ozturgut et al., 1981a; Hirota, 1981) in the North
Pacific, because of the limited volumes mined and
discharged. For commercial mining a discharge depth
of at least 1000 m is recommended.
These considerations are valid for closed transport
pipes between the mining system and the ship.
Open systems like moving buckets and trawls would
minimize the amount of nodule fines, because the
nodules will be closely packed. Based on scientific
trawling in the Peru Basin nodule area, sediment would
be washed out of the containers or the trawl all
the way to the surface, and much sediment caught
between the nodules would be released onto the ship.
No experimental results are available to estimate the
amounts of mud arriving on the mining ship, but it is
assumed that there would be too much for release into
the ocean surface.
Mining of polymetallic crusts, massive sulphide and
phosphorite are not further considered, since exploration and technique development have not proceded
far enough (Thiel et al., 1998). However, economic
pressures for a large-scale intrusion into the deep sea
would be the same as for polymetallic nodules or for
metalliferous muds. Large amounts of ore must be
excavated and transported to the ship, and overburden
material or rather fluid tailings must be discharged into
the sea. The general problems seem to be the same,
though in particular situations it would be necessary
to consider, for example, the specific fauna of a
seamount or of hydrothermal vent communities in the
neighbourhood.
In the mining of phosphorites the situation differs
because of the relatively shallow depths at which they
occur. Though mining would probably be technically
less complicated and costly, environmental care would
call for even more expenditure. Recolonization of the
area mined out would probably be faster, but the plumes
above the seafloor and from discharge in the water
column may be more disturbing to the ecosystem. All
the action would occur close to the euphotic zone, and
upwelling conditions would carry the turbidity up into
451
remained impossible to evaluate the large-scale impacts
to be expected from tailings discharge over a period of
15–20 years.
The lower economic limit for the mining of polymetallic nodules is estimated at 5000 metric tonnes
(dry) per day. This would involve the total disturbance
of 1 km
2 each day. The collector system, 6–15 m wide,
is expected to move in close meanders. It will weigh
several tens of tonnes and certainly strongly compress
and mix the mud in its track, from which the nodules
have been lifted up just before by the collector. The
system is expected to create a large plume from ooze
taken up with and separated from the nodules by the
collector system. Much of the sediment, aggregated in
lumps of various sizes, will resettle immediately but the
lighter material will drift away for uncertain distances
(Thiel et al., 1991; Zielke et al., 1995; Jankowski et al.,
1996). Conservatively calculated (based on a sediment
layer of only 2–3 cm), 20 000 metric tonnes would
be mobilized per day, but it remains unknown what
proportion of this would constitute the near-bottom
plume, and for what distance it would drift. The effects
of these near-bottom and seafloor impacts still cannot
be evaluated correctly, and this is also true for the
discharge plume (see also Thiel, ed., 2001).
Ozturgut (1981) estimated that 25 000 m
3 of bottom water containing 1600 tonnes of sediment and
250 tonnes of nodule fines would need to be discharged
from the mining ship or the nodule carrier per day.
These figures may change in the future. They are
based on a daily mining rate of 5000 tonnes which
may increase, and the amount of nodule fines and
sediment will depend on the sediment and nodule types.
Comparing North and South Pacific mining areas, the
small and rather smooth nodules of the North Pacific
may be mined with less sediment, and they may lose
less nodule fines during their transport to the surface. In
contrast, mining the larger nodules of the South Pacific,
lying partly buried in the stiff deeper sediments and
having a rather loose, gritty surface, would result in a
larger volume of particulate discharge into the water
column.
Industry should be aware of these problems, and
should minimize the different discharge volumes. But
the need for discharge will inevitably exist, and a
plume will certainly be created. For each mine site
the appropriate discharge depth needs to be determined
in relation to the local oceanographic conditions.
Ecological factors to be taken into account include the
depth zones of primary production, vertical migration
depth of zooplankton and fish, the prevailing currents
and vertical mixing of the water mass. Also, discharge
above or into the oxygen minimum zone must be
avoided, because manganese oxide and other metal
components may go into solution following chemical
reduction. Therefore, discharge into surface waters is
certainly not acceptable for commercial mining, even
though it was practised without detectable adverse
ecological effects during the pre-pilot mining test
(Ozturgut et al., 1981a; Hirota, 1981) in the North
Pacific, because of the limited volumes mined and
discharged. For commercial mining a discharge depth
of at least 1000 m is recommended.
These considerations are valid for closed transport
pipes between the mining system and the ship.
Open systems like moving buckets and trawls would
minimize the amount of nodule fines, because the
nodules will be closely packed. Based on scientific
trawling in the Peru Basin nodule area, sediment would
be washed out of the containers or the trawl all
the way to the surface, and much sediment caught
between the nodules would be released onto the ship.
No experimental results are available to estimate the
amounts of mud arriving on the mining ship, but it is
assumed that there would be too much for release into
the ocean surface.
Mining of polymetallic crusts, massive sulphide and
phosphorite are not further considered, since exploration and technique development have not proceded
far enough (Thiel et al., 1998). However, economic
pressures for a large-scale intrusion into the deep sea
would be the same as for polymetallic nodules or for
metalliferous muds. Large amounts of ore must be
excavated and transported to the ship, and overburden
material or rather fluid tailings must be discharged into
the sea. The general problems seem to be the same,
though in particular situations it would be necessary
to consider, for example, the specific fauna of a
seamount or of hydrothermal vent communities in the
neighbourhood.
In the mining of phosphorites the situation differs
because of the relatively shallow depths at which they
occur. Though mining would probably be technically
less complicated and costly, environmental care would
call for even more expenditure. Recolonization of the
area mined out would probably be faster, but the plumes
above the seafloor and from discharge in the water
column may be more disturbing to the ecosystem. All
the action would occur close to the euphotic zone, and
upwelling conditions would carry the turbidity up into