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Local topography exerts significant control over manganese nodule
distribution. The best deposits are found in gently sloping regions that are
partially sheltered from major inputs of bedload-derived sediments 13 .
5.2. Sediment properties
Manganese nodules are believed to differ morphologically from
manganese crusts or pavements primarily in that they form on sediments
rather than on hard substrates. Unconsolidated sediments are much more
susceptible than hard substrates to disturbance from tectonic activity,
benthic currents and biological mixing by burrowing organisms
(bioturbation). Extensive examinations of manganese nodules from many
environments show that the nodules experience repeated overturning
during their growth, resulting in their characteristic onion-like morphology.
(See also section 3.2 above.)
Sediments from the Indian Ocean site and the CCFZ consist mostly
of clays and siliceous biological casts. Sands and larger sediments are not
generally found so far from land, and the commonly formed carbonate
biological casts dissolve on the seabed in these deep-water regions faster
than they accumulate. In the Central Indian Basin, the sediments are
primarily siliceous and contain an average of 5.5% by weight of radiolarian
tests (shells) 14 .
The upper centimetre or so of the sediments is high in water
content and the water chemistry is very similar to that of overlying waters.
Most of the benthic fauna reside in this zone. Sediment permeability is
determined by grain size and shape, sorting, packing and other factors that
in turn determine suitability of the sediments as habitat for deep-sea
infauna. Biological disturbance of the sediments in turn affects pore-water
equilibria.
113 INTERNATIONAL SEABED AUTHORITY