Practically all of the mineral and energy resources found on land are present under the sea. Development, however, is limited by extraction costs that increase with depth of water, by the relative abundance of resources on land, and by political questions involving ownership of deep ocean resources.
Worth $80 billion, the most valuable undersea commodity is oil and natural gas , representing 90% of the resources obtained from the seafloor. Continental-margin deposits comprise about one-third of the world's estimated oil and gas reserves. Large deposits have been found on the continental shelves in the Gulf of Mexico, the Persian Gulf, the North Sea, and off the coasts of northern Australia , southern California, and the Arctic Ocean. Other sites are promising, and many other continental shelves remain relatively unexplored. The harsh conditions found in the North Sea have fostered huge investments in colossal offshore rigs. Extraction costs on the continental shelves are several times higher than on land, and projected costs in the Arctic Ocean are several times higher again. Such fields are profitable only because of their high flow rates and reserves. Oil exploration continues to push the limits of profitable extraction into deeper and deeper waters. One commercially promising well in the Gulf of Mexico was drilled in waters well over 1 mi (1.6 km).
Nearshore resources are much more likely to be mined than distant ones. Sand and gravel predominate in volume. Though in great demand for use in concrete and as fill material and beach sand, their low value restricts usage to local areas.
Delta sediments are often mined for valuable deposits washed from the land, including gold, platinum, tin, diamonds, iron, and uranium . Japan, an island country with limited land resources, is especially eager to exploit such deposits; coal beds are also being mined.
Phosphorite is the source rock for phosphate, a key fertilizer . Phosphorite deposits occur where upwelling brings dissolved phosphate to the surface, where, in turn, rising temperature and pH cause them to precipitate out. Their value comes from their high grade, since they are not diluted by land-derived sediments.
Manganese nodules were first discovered on the deep-ocean floor during the 1873–76 voyage of the British ship, Challenger. Although rich in valuable ferroalloys used in steelmaking, these potato-like ores have yet to be mined commercially. The deposits are immense, but the technology for profitable mining does not currently exist.
Another tantalizing resource is the sulfide ores precipitated out of ocean water around hydrothermal vents in deep-ocean rift systems. Commercial extraction is being attempted in the Red Sea, and experimental vehicles have been built for use in the deep ocean, but technological and economic limitations hinder commercial development.
Economic considerations are one of the greatest limitations to the growth of undersea mining, along with uncertain environmental impact. Only highly profitable ventures, as in oil and gas extraction, or readily accessible, near-shore, bulk resources, such as sand and gravel, are currently feasible. Undersea mining appears to hold great potential resources for the future , when supplies and extraction costs of land-based resources tip the scales in its favor.
Political considerations, especially questions of ownership and the sharing of profits, are also key factors here, with developing countries clamoring for their share. During the 1970s, the United Nations strove for a treaty to regulate deep-ocean exploitation. The 1982 Law of the Sea Treaty claimed manganese nodules as the heritage of all humankind, to be regulated by the United Nations' seabed authority and with profits to be shared. The United States chose not to sign the treaty, and has joined with other developed countries in a provisional understanding to award exploitation licenses.
These political and environmental issues are insignificant, so long as the economic realities favor resources on land. It will be beneficial if the time spent waiting for development of undersea mining creates the opportunity for more productive international discussions and agreements.
[Nathan Meleen ]
Duxbury, A. C., and A. B. Duxbury. An Introduction to the World's Oceans. 3rd ed. Dubuque, IA: Wm. C. Brown Publishers, 1991.
Ingmanson, D. E., and W. J. Wallace. Oceanograph. 5th ed. Belmont, CA: Wadsworth Publishing, 1995.