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the Clarion-Clipperton Fracture Zone (CCFZ). By summarizing the
characteristics of the sediment sequence, it revealed fields that had almost
no sediment coverage and others with varying thickness of young
sequences, associated with different nodule types and coverage and, most
likely, different benthic communities.
Sediment and nodule characteristics
Comparisons between potential mining regions of the Peru Basin
and the Sediperu area showed considerable differences in terms of surfacewater
productivity, as measured by phosphorus oxide, opal and barium
levels, with consequent differences in the characteristics of each area.
Another important parameter having a lot to do with the growth of
manganese nodules was Carbonate (or Calcite) Compensation Depth (CCD),
i.e., the water depth below which no carbonate was present. CCD was 150
m deeper in the Sediperu area, for example.
Wiedicke cited a comparison between two samples from the Peru
Basin, one from a lesser depth having many nodules about 4 centimetres in
diameter and another from a greater depth with fewer but larger nodules.
Such differences were closely related to CCD, which was thus a significant
factor both for the biota and for any mining operation. The overall pattern in
the Peru Basin showed a semi-liquid brown layer overlying a suboxic
yellowish to brown layer. The manganese nodules grew and lay atop the
brown layer. Manganese in the sediment was removed and remobilised in
the suboxic layer, moved upwards with the pore-water flux and redeposited
in the surface layer.
One of the most important parameters to define was the thickness
of the brown top layer, which was the target for mining as well as the area
where most of the animal life was concentrated. A box-core sample used
for illustration indicated a mixing, due to bioturbation, of dark sediment on
top and lighter-coloured sediment beneath. The density profile, from top to
bottom, showed an initial increase followed by a decrease, reflecting the
carbonate content, which increased to a certain depth and then decreased.
From studies of the Pacific Ocean it was known that carbonate production
during glacial periods was much higher than today, so the carbonate
maximum in the sediment column, in this case at a depth of 10 cm, was
considered to represent the glacial period of about 17,000 years ago. Opal
content and P wave velocity also dropped at first and then rose significantly.
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