Minerals Report - International Seabed Authority

(3) where bottom currents are effective in transporting metals in
solution and supplying oxygenated water conducive to the
precipitation of metals and in promoting the growth of nodules
by inhibiting sedimentation;
(4) in areas of high biological productivity, where metals are
collected from the seawater by organisms and contributed to the
pelagic sediments.
The size, shape, mineralogy, physical and chemical properties, and
distribution of nodules are highly variable, on both broad and local scales
reflecting the complex interplay of environmental factors and the
dominant role of one or two genetic processes (Hein and Morgan, 1999;
Hein et al., 1997; Frazer and Fisk, 1980; Halbach et al., 1989).
4.5.6 Distribution and composition of manganese nodules and crusts
Manganese nodules and crusts occur in many different
environments including freshwater lakes, fiords, continental shelves,
seamounts or abyssal plains and basins. The most extensive nodule fields
are on oceanic crust that is Mesozoic or younger in age. At the present
time, nodules are forming at a slow rate of a one to a few tens of
millimetres per million years (Ku,1977; Cronan, 1980; Calvert, 1978). Their
formation appears to be related to active tectonic belts such as spreading
ridges and deep-ocean trenches. They are found chiefly below the
carbonate compensation depth in areas with low clastic sedimentation and
high biological activity in overlying surface waters (Cronan, 1980, 2000).
Nodules with high nickel and copper contents are found in some of the
deep ocean basins at depths of 4000 to 5000 metres.
Figure 11 shows the distribution of all known manganese nodules
and crusts for which there are quantitative chemical analyses and where
estimates of their abundance have been mapped. These data have been
compiled from sites where dredges, cores, submersibles and bottom
photographs have been taken. Although the chemical analyses cover wide
regions, the same is not true for the abundance of nodules and/or
thickness of crusts which are less well documented. Therefore, the
analyses presented here is only preliminary and should not be taken as
anything other than a guide to this resource.
INTERNATIONAL SEABED AUTHORITY 700