Background (Encyclopedia of Global Resources)
Water is an excellent solvent, so seawater contains more than sixty dissolved elements or their salts. The major constituent percentages of seawater are water (H2O), 96.5 percent; table salt, or sodium chloride (NaCl), 2.3 percent; magnesium chloride (MgCl2), 0.5 percent; sodium sulfate (Na2SO4), 0.4 percent; and calcium chloride (CaCl2), 0.1 percent. This slightly alkaline broth was probably the first home to life on Earth. Chemically, human blood is essentially seawater contained in the body for carrying nutrients to, and wastes away from, individual cells.
Table salt has been evaporated from seawater since antiquity, with sunlight and wind supplying the energy. In the twentieth century, additional processes began producing magnesium, bromine, and iodine. Extracting other minerals from seawater is generally not profitable because the potential resource is highly dilute, requiring more pumping and a processing cost that is not worth the return. Some plants and animals are able to do such extractions, and eventually genetic engineering may harness such organic processes. Water is the prime constituent of seawater, and commercial desalination (removal of salt from seawater or other salt solutions) began in the 1960’s. desalination is expensive, however, and competing natural sources of fresh water are cheaper except in desert regions.
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Water and the Cycles of Climate (Encyclopedia of Global Resources)
Nature desalinates on a global scale through the hydrologic cycle of evaporation and resulting moisture. This cycle not only waters land plants but also affects climate in two ways. First, evaporation transfers heat from the oceans to places where the moisture condenses. Second, water flow off the land carries minerals containing a large percentage of calcium oxides that are part of the carbon cycle. Seawater has a smaller percentage of calcium ions than the runoff water because various sea plants and animals extract calcium from seawater and fix carbon dioxide from the air to grow (accrete) calcium carbonate (CaCO3) shells. Much of the calcium carbonate goes to the seafloor. This process helps balance the other half of the carbon cycle, carbon dioxide entering the air from animal respiration and the burning of fossil fuels. Because atmospheric carbon dioxide is an insulator for Earth (the greenhouse effect), more oceanic life absorbing more carbon dioxide from the air could decrease Earth’s temperatures. It has been suggested that airborne dust from the Himalayan highlands may have fertilized blooms of sea plants, triggering ice ages.
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Currents, Climate, and Energy Sources (Encyclopedia of Global Resources)
The ocean waters redistribute heat from sunlight. The flows of this heat engine control the climate of Earth and hold the potential for energy production many times that used by humankind.
Water near the poles loses heat through evaporation, conduction, and radiation. As it cools, its density increases, and it sinks toward the ocean floor. From there it flows toward the equator, displacing warmer water as it goes. Meanwhile, water near the equator is warmed, becoming less dense. It tends to flow along the surface toward the higher latitudes to replace the sinking denser water.
The Gulf Stream is such a current. Warm water from the equatorial Atlantic and the Gulf of Mexico flows generally northward, parallel to the coast of North America, and bends gradually to the right (northeast) due to the rotation of Earth. This tendency to curve (right in the Northern Hemisphere, left in the Southern Hemisphere) is called the Coriolis effect, and it eventually bends the flow northeast past Western Europe, warming and moistening air that, in turn, moderates the climate in the region. The cooled water bends south and west back to the start.
Similar circular patterns (gyres) occur in all the world’s oceans. The gyre in the North Pacific warms East Asia and cools California. Along the way, the gyres help determine fertile areas in the oceans. Sinking water off Antarctica pushes other nutrient-rich water...
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Continental Shelves and Slopes (Encyclopedia of Global Resources)
The continents are essentially blocks of lighter rock, such as granite, floating on heavier rock, such as basalt. The oceans fill the low spots between and lap at the edges of continents. These edges, the continental shelves, usually slope gently for some distance before the continental slopes plunge into oceanic depths. Globally, the continental shelves, which extend down to roughly 200 meters, represent an area equivalent to that of Africa. They include areas such as most of the Baltic Sea, wide areas off eastern North America, and narrower areas off western North America. Because the shelves are close to land nutrients, they usually have the richest marine life but are also most vulnerable to pollution from land.
Land minerals continue out onto the continental shelves. Shelf areas in the South China Sea and the Gulf of Mexico have major petroleum deposits. In addition, water-sorted deposits called placers extend along ancient beaches now covered below sea level. (Sea levels have been several hundred meters higher and lower in different geologic times.)
Some minerals are obtained through tunnel mining. Tunnel mines extend from shore to reach particularly desired ores. Dredging, however, is the most common method of mining shallow ocean deposits. More than one hundred million metric tons of sand, gravel, and shells are dredged yearly worldwide. Smaller tonnages are mined of more valuable minerals, such...
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The Abyssal Zone (Encyclopedia of Global Resources)
The abyssal zone represents more than three-quarters of the ocean floor. It is an area with water consistently just above freezing. It starts at a depth of 1 to 3 kilometers and extends to roughly 6 kilometers. Because the abyssal zone has no light and depends on scraps falling from above, the biomass per unit volume can be a hundredth or even a thousandth that of surface waters. The life-forms are some of the most alien on the planet—usually small, often luminescent. Animals may have jaws capable of swallowing something twice their size. Abyssal topography is often low rolling hills. However, areas with heavy sediment inflow, such as much of the Atlantic, have underlying topography buried under abyssal plains composed of fine ooze; these slope less than 1 part in 1,000.
These differences have mining implications. Some abyssal plains have sediments several kilometers deep. Under pressure and heat, organic material in these sediments decomposes into hydrocarbons, particularly methane (CH4) and other hydrocarbons that make up petroleum. Meanwhile, the ocean bottom is only slightly above freezing and is under high pressure. With those conditions, a combination of methane and water called methane hydrate freezes, forming a layer that holds methane and acts as a cap rock to block the escape of other hydrocarbons. Therefore, it is possible that much of the sedimented ocean floor may be underlain by oil and gas deposits, perhaps...
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Abyssal Mineral Resources (Encyclopedia of Global Resources)
The waters without heavy land sediments, such as much of the Pacific, have other major potential resources. In tectonically active areas, water seeping down into the seafloor containing magma is heated and eventually expelled back into the ocean. The hydrothermal (water plus heat) vents, or marine vents, that exist where this occurs carry dissolved minerals, especially sulfides of zinc, lead, copper, and silver, along with lesser but still significant amounts of lead, cadmium, cobalt, and gold. Such deposits have been test mined in the Red Sea, where underwater valleys keep rich muds enclosed. In the deep ocean, such deposits make chimneys of metal sulfides that might eventually be mined.
Other, more soluble minerals may be carried hundreds or even thousands of kilometers before precipitating as potato-shaped ferromanganese nodules on the ocean floor. These nodules—commonly called simply manganese nodules—contain mostly oxides of iron and manganese, plus potentially profitable small amounts of copper, nickel, and cobalt. They cover millions of square kilometers and contain billions of metric tons of metal. The nodules accrete slowly and could be easily buried by land sediments (as exist in the Atlantic), so they are more commonly observed in the deep Pacific far from land.
Mining of ferromanganese nodules has been considered but not done for several reasons. (A ship that was once thought to be involved in...
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Oceanic Ridges and Trenches (Encyclopedia of Global Resources)
Another feature of the deep ocean—and perhaps the largest geographic feature on Earth—is the Mid-Oceanic Ridge (also called the Mid-Atlantic Ridge). This 56,000-kilometer mountain range is the area where new seafloor is spreading the ocean apart. It extends from the Arctic Ocean, through the Norwegian Sea, through the North and South Atlantic; it continues around South Africa through the Indian, Antarctic, and South Pacific Oceans. This is an area of intense hydrothermal activity, and it contains hydrothermal deposits similar to those in the Red Sea.
Areas of seafloor spreading are balanced by other areas where tectonic plates are driven under other plates. This process leads either to rising mountains on land (such as the Himalayas and Andes) or trenches at sea, such as the Marianas Trench, often with an arc of volcanic islands beside the trenches. At 11 kilometers, the Marianas Trench contains the deepest known spot on Earth.
Suggestions have been made that toxic chemicals and radioactive wastes be placed into sediments in deep sea trenches for disposal. The suggestion is based on the idea that trenches are areas where plates are submerged into Earth’s mantle, so the wastes would be entombed. However, an unexpected volcano tens of thousands of years in the future might punch through that heated layer of diving sediments and belch toxic material into the stratosphere, and hence around the globe....
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Commerce (Encyclopedia of Global Resources)
From Carthaginian traders through clipper ships to steam and container ships, ocean commerce has become ever more important in the world economy. Since the onset of steam power in the nineteenth century, power plants and ship structures have steadily improved.
However, until the 1960’s, even slower freighters spent a majority of time in port loading and unloading.Containerization—the use of standard-sized large cargo containers—allows one crane to do in an hour what a crew of laborers might need a day to do. Furthermore, the containers can be placed on rail cars or trucks for quick movement without tedious hand operations. This advance allows factories on opposite sides of the planet to compete directly, increasing world competition and decreasing wages in developed countries. There are dangers to this expanded commerce. Giant (and underpowered) supertankers have had spectacular oil spills. Those involving the Torrey Canyon and the Exxon Valdez are among the most famous, but they were not the largest.
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Sea Life and Food from the Sea (Encyclopedia of Global Resources)
During most of the time since life on Earth began, the majority of life has existed in the ocean. A majority of living tonnage is still there—perhaps as much as 100 billion metric tons. That oceanic life supports high-protein food consumption that approaches 100 million metric tons. Eventually that figure will be much larger. The oceans, covering an area three and one-half times larger than all the land and never limited by lack of water, have the potential to produce many times the amount of food produced on land when they are used for carefully planned sustainable production.
However, it has been predicted that before that happens, the existing fishing industry will collapse. This dire prospect is based on significant differences between food production from the sea and agriculture on land. Except for nearshore plants, such as eel grass and kelp, oceanic plants are drifting algae, most barely large enough to see without magnification. These phytoplankton support most of the animal life in the oceans and live in the top few hundred meters, where sunlight penetrates. All oceanic photosynthesis (that is, the use of light and nutrients to make food) occurs in this euphotic (lit) zone, and most life depends directly or indirectly on this zone.
The tiny plants of the phytoplankton are eaten by tiny animals (zooplankton), which are food for small shoaling fish such as sardines and anchovies. These fish are...
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Politics (Encyclopedia of Global Resources)
In 1608, Hugo Grotius defined “freedom of the seas” for the Dutch when they had a powerful fleet to defend their boats fishing in waters near Great Britain. The British did not share the Dutch view, however, and drove the Dutch boats away in a bloody war. Later the British fleet became the most powerful in the world, and Britain embraced freedom of the seas.
The concept held that territorial waters extended about 5.6 kilometers from shore, which was the farthest range of cannons. Beyond territorial waters were international waters where a ship could fish or dump anything. In the twentieth century, many countries proclaimed territorial waters 22.2 kilometers out and often farther where the continental shelf was wide.
In 1982, negotiations concluded on the International Law of the Sea Treaty. It includes the concept of a 370-kilometer exclusive economic zone (EEZ) within which the coastal country has exclusive control of all resources. The United States and many other countries adopted the EEZ but not the treaty itself. Following the doctrine of the EEZ, ownership of the tiniest spit of land confers control of a wide circle of ocean. Where circles overlap, claims conflict and are resolved in various ways. European countries have carefully negotiated boundaries in the North Sea. In the South China Sea, the Chinese and Vietnamese have negotiated with naval gunfire. The Philippines, Indonesia, and Malaysia also have...
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Further Reading (Encyclopedia of Global Resources)
Ballard, Robert D. Explorations: My Quest for Adventure and Discovery Under the Sea. New York: Hyperion, 1995.
Carson, Rachel. The Sea Around Us. Drawings by Katherine L. Howe. New York: Oxford University Press, 1951.
Clarke, Arthur C. The Challenge of the Sea. New York: Holt, Rinehart and Winston, 1960.
Earle, Sylvia Alice. Sea Change: A Message of the Oceans. New York: Putnam, 1995.
Goldin, Augusta. Oceans of Energy: Reservoir of Power for the Future. New York: Harcourt Brace Jovanovich, 1980.
Messier, Vartan P., and Nandita Batra, eds. This Watery World: Humans and the Sea. Rev. 2d ed. Newcastle upon Tyne, England: Cambridge Scholars, 2008.
Oceans: A “Scientific American” Reader. Chicago: University of Chicago Press, 2007.
Pinet, Paul R. Invitation to Oceanography. 5th ed. Sudbury, Mass.: Jones and Bartlett, 2009.
Rose, Paul, and Anne Laking. Oceans: Exploring the Hidden Depths of the Underwater World. Berkeley: University of California Press, 2008.
Smith, Hance D., ed. The Oceans: Key Issues in Marine Affairs. Dordrecht, the Netherlands: Kluwer Academic, 2004.
United Nations. U.N. Atlas of the Oceans. http://www.oceansatlas.org/index.jsp
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Oceans (Science Experiments)
Currents: Water behavior in density-driven currents
If you were to look down at Earth from space you would see a planet that was covered in blue. That is because oceans cover almost three-quarters of the Earth's surface and contain about 97 percent of the planet's water supply. Life on Earth began in the ocean almost three-and-a-half million years ago and life could not exist without a healthy ocean environment. Today, the oceans are home to an incredibly variety of creatures, from the largest animal that ever lived, the blue whale, to microscopic organisms that can live in boiling waters.
People depend on the oceans in many ways. Oceans have an important effect on weather patterns. They are essential for transportation, for both economic and military purposes. Many people throughout the world rely on the ocean for food and their livelihood. People also use the oil and minerals that come from beneath the ocean floor.
The first voyage planned specifically to study the oceans was a British expedition that set out in 1872. In the twentieth century, interest in the oceans grew enormously. A new field evolved for oceanographersA person who...
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