Minerals Report - International Seabed Authority

depths. Up welling increases primary productivity, which in turn increases
the size and magnitude of the Oxygen Minimum Zone (OMZ), making
seamounts ideal fishing grounds.
Returning to the distribution of the crusts in the oceans, Dr. Hein
repeated that crusts are to be found essentially everywhere in the ocean basins
from the farthest northern parts of the pacific and the Atlantic down to the
Antarctic ridge. Dr Hein said that crusts occur in water depths of about 400 to
4000 m, but more commonly at 1000 to 3000 m. He also said that the most
cobalt-rich crusts occur between about 800 and 2200 m water depth largely
due to a phenomenon in the ocean called the Oxygen Minimum Zone.
Dr. Hein said that the Oxygen Minimum Zone is very important in the
formation of cobalt-rich ferromanganese crusts. He described the Oxygen
Minimum Zone as a layer of seawater where the oxygen content of the
seawater is low relative to the seawater above and below it. He pointed out
that an oxygen minimum zone is created because of primary productivity in
the surface waters of the oceans. He said that plankton created in the surface
waters drift through seawater when they die, and that their protoplasm is
oxidized through a reaction with seawater. This process, he also said results
in the removal of oxygen from seawater and affects the chemistry of what is
going on in seawater. Low oxygen seawater he pointed out acts as a reservoir
for a lot of the metals of interest, particularly manganese. Dr. Hein used a
slide showing a profile of manganese in the water column, that reveals a
dramatic increase in manganese in the oxygen minimum zone. He pointed
out that in anoxic conditions, where seawater has no oxygen, crusts would
never form. Dr. Hein said that in the central Pacific Ocean all the crusts are
composed of a mineral that is the most oxidized form of manganese
(Vernadite). He also said that the oxygen minimum zone varies from place to
place depending on the intensity of productivity in the area, currents and a
number of other factors.
Dr. Hein recalled that he had said that the metals in crusts precipitated
under cold, ambient seawater. He described two of the most important
factors concerned in this process. He said that the metals in crusts are not
floating around in seawater as pure metal but are dissolved in seawater as a
complex material combined either as a chloride or a hydroxide. He noted that
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