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had to be mined each day. Assuming 300 days of operation a year, about
300 km 2 would be mined annually by a single mining operation. If 2.5 cm of
sediment were removed during mining, about 6 million m 3 of sediment per
year would be resuspended from a single mining operation. Modelling of
the deep-sea plume, including the area it would cover, was extremely
problematic without knowledge of how it would be released from the mining
head, flocculation, behaviour of particles and the settling behaviour of
deep-sea sediments. One model estimated that, if the tailings were
released at the surface, an instantaneous plume would cover an area of 85
by 20 km.
To give an idea of the scale of potential burial processes from
resedimentation, 6 million m 3 of sediment dispersed over a broad area
could bury about 6000 km 2 under 1 mm of sediment. According to current
thinking, a 1-mm deposition layer might have a significant impact on the
deposit-feeding biology of much of the community. Over the 20-year life of
a mining operation there would be the potential to bury something like
120,000 km 2 under 1 mm, assuming that the sediment cloud was relatively
widely dispersed. These numbers might be thought of as a worst-case
scenario rather than a real prediction, but the potential impacts from a
dispersed plume of sediment from a full-scale mining operation were large.
As test mining was likely to be about one-fifth the scale of actual mining and
might go on for something like three to six months, the predicted impacts
from test mining had to be scaled down, but there clearly remained a
chance that they would be large. Such tests would be useful from an
environmental perspective, because until disturbances approaching the
scale of a real mining operation were generated, i.e., until near-scale mining
operations took place, there could not be a good predictive understanding
of mining impacts.
What were some of the impacts that might occur from mining?
First, and perhaps most significant, there was a potential for interference
with surface-deposit feeding and suspension feeding, through dilution of
food materials by lower quality sediment resuspended from the seafloor.
Sediments 1-2 cm down in the sediment column had low food value, so that
when they were resuspended and mixed with the surface layer of food
material, the food resources of deposit feeders would be diluted –
potentially a widespread and severe problem in a food-poor environment
like the CCFZ. The second impact, intermediate in significance, was
entombment and burial of small animals associated with low bioturbation
rates. Disappearance rates of seafloor biogenic structures in the CCFZ
suggested that these animals might be very sensitive to physical burial. If
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