GEOCHEMISTRY
DEEP SEA RUSH
T
he pressures to provide new sources of minerals are driving what most people would consider to be a gold- rush in the deep sea, according to Mark Hannington of GEOMAR at the University of Kiel, Germany. Growing demand for minerals makes global shortages highly likely by the end of this century, he believes. Shortages of elements like copper
will be felt even sooner - by mid- century, according to Thomas Graedel of Yale University, US, who agrees we will have to look to the deep sea to make up the shortfall. The necessary machinery for mining
the two major resources, manganese nodules and polymetallic sulphide deposits, is already ‘sitting on the wharf’, Hannington says. But he also questions whether there are enough sub-sea resources to justify this effort.
Long-term future Manganese nodules have a future, he believes. ‘There is enough nickel and cobalt in the Clarion-Clipperton zone between Hawaii and Mexico to satisfy demands for decades into the future,’ he says. A literature survey suggests there are between 20 and 30bn t of economically ‘extractable material’ in that area alone. And manganese nodules don’t just provide nickel and copper; they are also important sources of cobalt, zinc and molybdenum, minerals vital to high-tech industries from smart phones to NMR machines. The planned mining operations will
be conducted by a series of remotely operated harvesters that will plough, scrape and vacuum the seafloor over large areas. One estimate is seabed mining operations will harvest 300-800km2
/year. However,
Mining of the sea bed has come into renewed focus as plans to extract the untapped mineral wealth in the oceans gather momentum, Richard Corfield reports
Hannington believes more than 100km2
/year is more realistic,
producing about 1mt of nodules. Crushed or whole nodules and
entrained sediments will be pulled up a riser pipe to the surface, where nodules will be offloaded to a production support vessel for transport to land. Sediment- containing water will be returned to the ocean at depth to minimise contamination.
Manganese nodules, however,
remain an enigma, says Rachel Mills of the University of Southampton, UK. They were discovered in the 1870s during the voyage of HMS Challenger, but it was not until the 1970s that the size of their fields was fully appreciated. They form around a nucleus like a fish tooth, but beyond that no one knows why they form or how fast they grow. All that is known is that they need undisturbed waters and many millions of years to form.
Massive deposits Another important resource is Seafloor Massive Sulphide Deposits (SMSD), which form where new crust is produced at the mid-ocean ridges. SMSDs are rich sources of copper and
zinc, usually associated with black smoker vents.
In an initial assessment,
Hannington and colleagues estimated such deposits would yield only 600m t of massive sulphide, which in turn would produce 30m t of copper and zinc – not enough to be economically viable. However, that estimate was restricted to the km- wide zone around the mid-ocean ridges, which are reachable using conventional deep sea submersibles. Since new material is
continually being formed at the mid- ocean ridges, it follows that older deposits will also be present further out from the ridge axes. However, with increasing distance
from the mid-ocean ridge, the sedimentary overburden increases and so therefore, does the cost of extraction. Hannington cautions:
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