لطفا منتظر باشید ...
Ocean and Oceanography
I INTRODUCTION
Ocean and Oceanography, great body of salt water comprising all the oceans and seas that cover nearly three-fourths of the surface of the earth, and the scientific study of the physical, chemical, and biological aspects of the so-called world ocean. The major goals of oceanography are to understand the geologic and geochemical processes involved in the evolution and alteration of the ocean and its basin, to evaluate the interaction of the ocean and the atmosphere so that greater knowledge of climatic variations can be attained, and to describe how the biological productivity in the sea is controlled.
II OCEAN BASIN STRUCTURE
The world ocean covers 71 percent of the earth's surface, or about 361 million sq km (140 million sq mi). Its average depth is 5,000 m (16,000 ft), and its total volume is about 1,347,000,000 cu km (322,300,000 cu mi). The three major subdivisions of the world ocean are the Atlantic Ocean, the Pacific Ocean, and the Indian Ocean, which are conventionally bounded by the continental masses (see Continent) or by ocean ridges or currents; they merge below 40 South latitude in the Antarctic Circumpolar Current, or West Wind Drift, where they are then often referred to as the Southern Ocean. In the north polar region the nearly circular Arctic Ocean, almost landlocked except between Greenland and Europe, is considered by some part of the Atlantic and by others a fourth ocean subdivision. From the shorelines of the continents a submerged part of the continental mass, called the continental shelf, extends sea ward an average distance of 75 km (43 mi); it varies in width from nearly zero to 1,500 km (930 mi). The shelf gives way abruptly at a depth of about 200 m (660 ft) to a steeper zone known as the continental slope, which descends about 3,500 m (12,000 ft). The continental rise, a gradually sloping zone of sediment that is considered part of the ocean bottom, extends about 600 km (370 mi) from the base of the continental slope to the flat abyssal plains of the deep-ocean floor. In the central parts of the oceans are the midocean ridges, which are extensive mountain chains with inner troughs that are heavily intersected by cracks, called fracture zones. The ridges are sections of a continuous system that winds for 60,000 km (40,000 mi) through all the oceans. The Mid-Atlantic Ridge extends from the Norwegian Sea through the volcanic islands of Iceland and the Azores to the South Atlantic, where it is equidistant from the African and South American coasts. The ridge continues into the Indian Ocean, with a branch that reaches into the Gulf of Aden and the Red Sea, then passes between Australia and Antarctica and into the eastern South Pacific. The East Pacific Rise extends north to the Gulf of California; Easter Island and the Gal?pagos are volcanic islands that are part of this submarine mountain chain. The ridge system seems to merge into the continents in several areas, such as the Red Sea and the Gulf of California, and such areas are regions of great geologic activity, characterized by volcanoes, or earthquakes and faults (see Earthquake; Fault; Volcano).
The midocean ridges play a key role in plate tectonics (movements in the earth's crust), for it is from the inner troughs of these ridges that molten rock upwells from the earth's mantle and spreads laterally on both sides, adding new material to the earth's rigid crustal plates. The plates are moving apart, currently at the rate of 1 to 10 cm (0.39 to 3.9 in) a year and are being forced against adjacent plates. From the Mid-Atlantic Ridge, the continents, which rest on the plates and which once were joined, have moved away from one another. In the Pacific Ocean, plates are also moving apart from the East Pacific Rise, but the bordering plates are overlapping them and forcing them under at the edges. At these places, along almost the entire rim of the Pacific, deep trenches are formed as crust is subducted and returned to the mantle. The Pacific trenches commonly reach depths of more than 7 km (4.3 mi); the deepest known point, in the Mariana Trench east of the Philippines, lies 11 km (6.9 mi) beneath the surface. Trench areas, or subduction zones, are characterized by volcanic and seismic activity, indicative of the motions and stresses of the earth's crustal plates (see Plate Tectonics; Seismology).
The structure and topography of the ocean floor are studied through the use of satellite mapping (see Remote Sensing), which measures the level of the ocean surface to estimate the shape of the ocean floor; sonar, which measures the depth of the oceans; and seismic techniques, which measure the thickness of sediments of the ocean floor. Depth measurements are made by sonar from ships that travel slowly, so only a small fraction of the ocean's floor has been mapped from depth measurements. Even using the latest sonar techniques, it would take about 125 years to map the ocean floor with depth measurements.
The National Oceanic and Atmospheric Administration (NOAA) Seasat satellite launched in 1978 and the United States Navy Geosat satellite launched in 1985 used frequent, short pulses of microwave radiation to measure the level of the surface of the sea with great accuracy (within 3 cm/1 in). Underwater mountains and valleys cause subtle variations in the earth's gravitational field. The stronger gravity near high, massive formations attracts more water molecules, raising the level of the ocean slightly but measurably (the water above a 2-km/1.3-mi tall undersea volcano will be about 2 m/7 ft higher than average sea level). Valleys on the ocean floor produce areas of weaker gravity, so the level of the ocean will be lower over valleys. Using this method, a complete survey of the ocean floor was accomplished in less than two years. Maps made from data on the level of the ocean surface have been compared with maps made with direct depth measurements and the two types have corresponded well.
Using sonar, depth measurements are made by measuring the time for a sound wave to travel from the surface of the ocean to the ocean floor, and to return (see Sounding). Often several returns are recorded, indicating that some sound waves are bouncing off of several layers of sediment below the ocean floor. More extensive studies of the sediment below the ocean floor are carried out by teams of two ships: one ship fires an explosive in the water and the other uses sensitive instruments to record the sound waves as they reach the second ship. Some waves travel directly to the second ship; others travel to the ocean floor, are refracted (bent) within the layers of sediment, and then travel to the second ship. By measuring the different times sound from the first boat takes to reach the second, the strength of the explosive, and the distance between the boats, the thickness of the sediments can be determined.
III COMPOSITION OF SEDIMENT
The ocean floor is covered by an average of 0.5 km (0.3 mi) of sediment, but the thickness varies up to about 7 km (4.3 mi) in the Argentine Basin in the South Atlantic. Some regions, particularly the central parts of the midocean ridges where new crust is formed, have little, if any, sediment on them. The sediments are studied by dredging and by deep-sea exploration projects such as the Ocean Drilling Program, which obtains core samples of seafloor sediment from all the world's oceans (see Deep-Sea Exploration).
The sediments are found to consist of rock particles and organic remains; the compositions depend on depth, distance from continents, and local variants such as submarine volcanoes or high biological productivity. Clay minerals, which are formed by the weathering of continental rocks and carried out to sea by rivers and wind, are usually abundant in the deep sea. Thick deposits of such detrital material are often found near mouths of rivers and on continental shelves; fine particles of clay are spread through the ocean and accumulate slowly on the deep-ocean floor. These sediments are stirred up and periodically redistributed by fierce current-generated disturbances that are called benthic storms because they occur in the sparsely populated deep-sea habitat known as the benthic zone (see Marine Life). Also accumulating as sediment in the benthic zone are the calcium carbonate shells of small organisms such as foraminifera and the siliceous shells of marine protozoans (see Diatom; Protozoa).
IV DATING TECHNIQUES
Vast quantities of microscopic plants and animals live near the ocean surface. When they die, their remains drift down to the ocean floor and accumulate in thick layers of sediment. When studied in sedimentary core samples, which can represent many millions of years of deposits, they provide a detailed and continuous history of the earth's environmental changes. The record is particularly informative for the most recent 2 million to 5 million years, during which major fluctuations in global climate have occurred. Successive ice ages can be traced by the relative scarcity or abundance of the shells of warm-water and cold-water diatoms in various layers of a sedimentary core, as the organisms migrated to more hospitable habitats. Geochemical records of these same fluctuations are revealed by determining the ratios of two isotopes of oxygen, oxygen-16 and oxygen-18, in the shells of foraminifera. The ratio of the two isotopes is proportional to the temperature of the water in which the organism grew; hence, a temperature record is preserved when each organism dies and its shell drifts to the ocean floor. The records of climatic fluctuation found in ocean-floor sediments are much more continuous than similar records on land; they also lend themselves to worldwide correlation. The absolute ages of climatic changes can be determined by correlating the evidence of temperature changes with radioactive-dating techniques (see Chronology; Dating Methods; Radioactivity).Thorium-230 dating is applicable to samples younger than 300,000 years, potassium-argon dating to samples in the range of 75,000 years, and carbon-14 dating to samples younger than 40,000 years. Several other radioactive dating techniques are available for samples of very recent age. A geophysical dating method is also commonly used; it determines the magnetic orientation of sediment particles, since it is now known that the earth's magnetic field has reversed its orientation several times in the past few million years (see Earth: The Core and Earth's Magnetism). Such dating techniques indicate that the ocean basins are no older than 200 million years.
V COMPOSITION OF SEAWATER
Seawater is a dilute solution of several salts derived from weathering and erosion of continental rocks. The salinity of seawater is expressed in terms of total dissolved salts in parts per thousand parts of water. Salinity varies from nearly zero in continental waters to about 41 parts per 1,000 in the Red Sea, a region of high evaporation, and more than 150 parts per 1,000 in the Great Salt Lake. In the main ocean, salinity averages about 35 parts per 1,000, varying between 34 and 36. The major cations, or positive ions, present, and their approximate abundance per 1,000 parts of water are as follows: sodium, 10.5; magnesium, 1.3; calcium, 0.4; and potassium, 0.4 parts. The major anions, or negative ions, are chloride, 19 parts per 1,000, and sulfate, 2.6 parts. These ions constitute a significant portion of the dissolved salts in seawater, with bromide ions, bicarbonate, silica, a variety of trace elements, and inorganic and organic nutrients making up the remainder. The ratios of the major ions vary little throughout the ocean, and only their total concentration changes. The major nutrients, although not abundant in comparison with the major ions, are extremely important in the biological productivity of the sea. Trace metals are of specific importance for certain organisms, but carbon, nitrogen, phosphorus, and oxygen are almost universally important to marine life. Carbon is found mainly as bicarbonate, HCO3-; nitrogen as nitrate, NO3-; and phosphorus as phosphate, PO4?.
VI TEMPERATURE
The temperature of surface ocean water ranges from 26 C (79 F) in tropical waters to -1.4 C (29.5 F), the freezing point of seawater, in polar regions. Surface temperatures generally decrease with increasing latitude, with seasonal variations far less extreme than on land. In the upper 100 m (330 ft) of the sea, the water is almost as warm as at the surface. From 100 m to approximately 1,000 m (3,300 ft), the temperature drops rapidly to about 5 C (41 F), and below this it drops gradually about another 4 to barely above freezing. The region of rapid change is known as the thermocline.
VII OCEAN CURRENTS
The surface currents of the ocean are characterized by large gyres, or currents that are kept in motion by prevailing winds, but the direction of which is altered by the rotation of the earth (see Coriolis Force). The best known of these currents is probably the Gulf Stream in the North Atlantic; the Kuroshio Current in the North Pacific is a similar current, and both serve to warm the climates of the eastern edges of the two oceans. In regions where the prevailing winds blow offshore, such as the west coast of Mexico and the coast of Peru and Chile, surface waters move away from the continents and they are replaced by colder, deeper water, a process known as upwelling, from as much as 300 m (1,000 ft) down. This deep water is rich in nutrients, and these regions have high biological productivity and provide excellent fishing. Deep water is rich in nutrients because decomposition of organic matter exceeds production in deeper water; plant growth occurs only where photosynthetic organisms have access to light (see Photosynthesis). When organisms die, their remains sink and are oxidized and consumed in the deeper water, thus returning the valuable nutrients to the cycle. The regions of high productivity are generally regions of strong vertical mixing in the upper regions of the ocean. In addition to the western edges of the continents, the entire region around Antarctica is one of high productivity because the surface water there sinks after being chilled, causing deeper water to replace it.
Although the surface circulation of the ocean is a function of winds and the rotation of the earth, the deeper circulation in the oceans is a function of density differences between adjacent water masses and is known as thermohaline circulation. Salinity and temperature determine density, and any process that changes the salinity or temperature affects the density. Evaporation increases the salinity, hence the density, and causes the water to become heavier than the water around it, so it will sink. Cooling of seawater also increases its density. Because ice discriminates against sea salts, partial freezing increases the salinity of the remaining cold water, forming a mass of very dense water. This process is occurring in the Weddell Sea, off Antarctica, and is responsible for forming a large part of the deep water of the oceans. Water sinks in the Weddell Sea to form what is known as the Antarctic Bottom water, which flows gradually northward into the Atlantic and eastward into the Indian and Pacific oceans. In the North Atlantic, saline water cools and sinks to a moderate level to form the North Atlantic Deep water, which flows slowly southward; this water mass is less dense than the Antarctic Bottom water, and hence flows at less depth. Whereas speeds of surface currents can reach as high as 250 cm/sec (98 in/sec, or 5.6 mph) a maximum for the Gulf Stream, speeds of deep currents vary from 2 to 10 cm/sec (0.8 to 4 in/sec) or less. Once a water mass sinks below the surface, it loses contact with the atmosphere, and can no longer exchange gases with it. Oxygen, dissolved in the water, is used up in the oxidization of dead organic matter, and it is slowly depleted as the water mass remains below the surface. Thus, the oxygen content gives the oceanographer a qualitative idea of the “age” of the water mass, that is, the time it has been away from the surface. Radioactive carbon-14 is produced in the atmosphere and enters the ocean in the form of carbon dioxide gas, which equilibrates, or keeps in balance, with the bicarbonate ion of seawater. Carbon-14 has a half-life of about 5,700 years and decays with time; so its activity in a deep-water mass is largely a measure of the time since that water mass was at the surface.
The general pattern of deep-ocean circulation that is apparent from these measurements is that the deep-water masses formed in the North Atlantic and off Antarctica mix and flow together through the Indian and Pacific oceans, and that the oldest water found is in the deep North Pacific, which has an age of up to 1,500 years.
VIII RESOURCES
The oceans are being looked to as a major source of food for the future. High productivity characterizes certain regions in the oceans, but larger regions of low productivity also exist. Production is the amount of organic matter fixed, or changed into stable compounds, by photosynthetic organisms in a given unit of time. Estimates of the yearly world ocean production of organic matter, fixed from inorganic carbon and nutrients, amount to about 130 billion metric tons. This process begins with phytoplankton, which are photosynthetic plants that turn carbon into organic matter with the aid of sunlight; zooplankton and fish feed on phytoplankton, and each member of this part of the food web has its own predator (see Plankton). Most of this organic matter is recycled and reused, so that the standing crop of organic material is only a small fraction of this annual total. The harvestable amount of organic matter is a function of technology, tastes, needs, and the ability of the system to sustain this harvest. Presently, yearly harvests amount to about 82 million metric tons of fish and about 500,000 metric tons of seaweed. Estimates of the maximum harvestable amounts on a sustained-yield basis amount to about 100 million metric tons per year. Thus, predictions are that the sea can yield only about 25 percent more than the present amount of organic food resources. The decline of the whaling industry in recent years is a strong case against rapid and unwise exploitation of oceanic food resources (see Whaling). Food from the sea will be a good source of protein, but cannot meet the total world demand for calories in the future. The present yield of about 83 million tons per year supplies about 5.6 percent of the protein needs of the world at the present time.
The mineral resources of the sea have only recently begun to be known. Several valuable metals are known to be abundant in the sea, but the area over which they are found is so vast that it is difficult to extract them. The sea is estimated, for example, to contain 10 billion tons of gold, but at such low concentrations that recovery is impossible. Today the major minerals being obtained from seawater are magnesium, bromine, and sodium chloride, or common salt. The ocean floor yields sand, gravel, and oyster shells for construction purposes, and small quantities of diamonds are found in some submarine gravel bars. Phosphorite is a phosphorus mineral known to be available on the seafloor that has potential use as an agricultural fertilizer. Much interest has been expressed recently in manganese nodules, which are spherical concretions on the seafloor containing about 20 percent manganese, 10 percent iron, 0.3 percent copper, 0.3 percent nickel, and 0.3 percent cobalt. These are all valuable minerals that have not been obtained yet to any great extent from the seafloor (see Mining: Ocean Metal Mining).
Offshore oil and gas wells at present supply about 17 percent of the world petroleum production. Most of these wells are in the shallow waters of the continental shelves, but deep-sea drilling techniques are expected to discover petroleum on the outer continental margins. Many geologic structures under the seafloor are reservoirs for petroleum and also contain some commercially exploitable deposits of sulfur. Sulfur-rich waters also arise from deep-sea hydrothermal vents (see Hydrothermal Vent).
Oceans also hold potential as an important alternate source of energy. The thermal energy of the oceans resulting from absorption of solar heat and from ocean currents can be converted into electricity—a process known as ocean thermal energy conversion (OTEC).
IX POLLUTION
Because the sea is expected to yield still larger quantities of valuable resources in the future, and because the water itself is now being used on a small scale through desalination, the concern for preserving the integrity of the ocean has grown. The contaminative effect of increasing technological development and industrialization has been known to disrupt and destroy the fragile coastal ecology by indiscriminate discharge of industrial and municipal waste into the sea. The pollution of the marine environment by petroleum and chemical spillage and sewage disposal has helped focus world attention on the need for controlled use of resources and planned disposal of waste products. Other pollution concerns are the effects of insecticides and pesticides on marine fish and birds, increasing levels of lead in the surface waters, and the disposal of hot water from power plants into the sea with untoward effects on marine life.
See also Water Pollution.
Contributed By:
Edwin B. Shykind
John F. Wehmiller
Ocean Shore and Tidal Pool
Marine life is plentiful along shores and in tidal pools, where the sun penetrates to the floor and the fluctuating tide continually circulates resources. This beach is along the shore of the Pacific Ocean, at the Point of Arches in the Olympic National Park in the state of Washington.
Pat O'Hara Photography
