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3.3. Benthic currents
The benthic and deep waters of the Indian Ocean may be derived in
large part from the Atlantic Ocean, with some contributions from the South
Australian and Wharton basins 9 . The currents appear to be generally
southerly, with mean speeds of a few centimetres per second and maxima
>10 cm/sec. These means and maxima are consistent with the currents in
the CCFZ.
Three dynamic regimes are in evidence in the CCFZ: calm periods,
intermediate periods and benthic storms (described in section 5.3.1 below).
Local bottom-water advection at the scales of hours to months
appears to be controlled by bottom relief. In the central and western parts
of the CCFZ this relief, with some exceptions, is made up of elongated hills
separated by gentle valleys that are transversely oriented with respect to
the major fault zones. Available studies suggest that benthic currents in
this region are effective in transporting sediments along the seafloor
valleys. These currents are sufficient to suspend and transport significant
quantities of the very fine-grained, relatively low-density, uppermost pelagic
sediments.
These studies suggest that the seafloor in the CCFZ is subject to
episodic periods of erosion and deposition caused by benthic current
activity. If future work confirms this and provides more quantitative
information about the frequency and extent of bedload transport and
resedimentation, it will have important implications for the prediction of
impacts due to the suspension and resedimentation of sediments
anticipated from mining activities. If benthic communities are normally
subjected to significant sedimentation from benthic storms, they will be
conditioned to adapt successfully to the resedimentation disturbances that
would be caused by mining. Conversely, if they are not normally subject to
such periodic disturbances, they may be more susceptible to long-term
disruption caused by mining activities.
Furthermore, baseline characterisation for any particular mining
claim would have to include some evaluation of the frequency and extent of
natural resedimentation episodes near the planned mining area, to
distinguish them from the disturbances caused by the mining system.
Methods to assess these variables are not well developed but could be
critical to impact assessment.
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