Oceanography is the scientific study of the Earth's oceans and their boundaries. The interconnected world oceans, from which the continents rise like islands, cover 71 percent of the world's surface. Most human beings live on or near coastlines, and human history is closely linked to the oceans. They serve as a source of food, as the key to weather and climate, and as the highways for ships of commerce. Much of the history of the planet itself is recorded in the bottom topography, geophysical properties, and sediments of the oceans. The modern discoveries that have revolutionized geological thinking have in fact largely been the product of work in the ocean sciences.
Humans have observed the oceans since ancient times. The interest of early civilizations centered mainly on practical matters, such as gathering lore on the ways of sea life and on the tides and other ocean phenomena affecting the shores. With the development of ships, ocean studies were again principally concerned with practical matters such as the charting of seaways to aid navigation. Sailors also made soundings of ocean depths and gathered data on such phenomena as winds, ocean currents, water temperatures, and ice movements on the northern seas.
Oceanography as a science began in the 19th century with the work of such men as U.S. naval officer Matthew Fontaine Maury and his compilation of oceanographic data from ships'logs. The first major scientific expedition, and the one that firmly established the field of oceanography, was the around-the-world voyage of H.M.S. Challenger. Setting out from England in 1872, the Challenger Expedition returned three years and five months later with a wealth of information on the physical and chemical characteristics of seawater and bottom sediments, as well as the first comprehensive data on the distribution of organic life at all water depths and on the seafloor.
Following this voyage, oceanographic research was generally conducted either on short cruises that concentrated on small areas of the ocean, or on long cruises with limited objectives pursued in widely separated small areas. The South Atlantic voyage of the German ship Meteor in 1926 was the first to use an echo sounder to chart the ruggedness of the ocean bottom in a continuous manner, rather than making scattered and separate soundings.
Modern oceanography is a combination of several fields of science, and it is conventionally divided into the subdisciplines of physical, chemical, biological, and geological oceanography. Closely associated fields are those of marine technology, maritime law, and studies of the effects of ocean pollution.
Physical oceanographers study the physical processes underlying such phenomena as currents; tides; water waves; water transparency, density, and temperature; and underwater acoustics and sound transmission. (The latter subjects are also important for submarine technology.) Chemical oceanographers are concerned with the chemistry of seawater, its major salts, and its many trace elements. Marine biologists study life in the sea, marine ecology, and the total organic production in the oceans. Ocean life comprises the floating or weakly swimming forms called plankton and the rapidly swimming forms called nekton, as well as deep-sea life and various bottom dwellers. Marine geologists map the ocean floor, analyze shoreline problems, and study sediments of the ocean floor and rocks of the underlying crust.
As a whole, modern oceanography is pursued mainly at a few major centers around the world. The research goals within such centers tend to focus on intense studies of smaller ocean areas by teams representing each of the broad oceanographic disciplines. Among leading U.S. institutions are the Lamont-Doherty Earth Observatory in Palisades, N.Y.; the Scripps Institution of Oceanography in La Jolla, Calif.; and the Woods Hole scientific institutions on Cape Cod, Mass. Other important U.S. centers are at Oregon State, Texas A&M, and Washington universities. Federal responsibility for oceanography lies with the National Oceanic and Atmospheric Administration (NOAA).
In addition to work at such centers, a new era of oceanwide research was initiated when international scientific teams organized to tackle programs too vast in scope to be handled by individual institutions. One notable undertaking of this nature, the Deep-Sea Drilling Project, was conducted from 1968 to 1983 by a consortium of U.S. institutions and several European nations, the Soviet Union, and Japan. That program was succeeded in 1984 by a similar but more advanced program, the Ocean Drilling Program. International programs in the 1980s also included BIOMASS (Biological Investigations of Marine Antarctic Systems and Stocks) and the Global Ocean Flux Study, which investigated the processes that control biogeochemical cycles in the ocean.
The World Ocean Circulation Experiment, conducted from 1990 to 1997, studied the effects of the oceans on climate. Oceanwide mapping programs were also pursued in the 1990s, and ocean currents are being studied using remote-sensing data gathered by Earth satellites. International ocean-study programs of several kinds are likewise making use of such satellites.
Robert S. Dietz
The earliest technical devices used in the study of the oceans were weighted sounding lines, which obtained the local water depth, and rope-suspended scoops or dredges, which brought sediment and bottom-dwelling marine life to a ship's deck. By 1900, thermometers were devised that could be lowered to great depths and then "locked" on the temperatures recorded there as they were raised. Water samples were taken from all depths by Nansen bottles, metal tubular containers with ends that could be shut by "messenger" devices. By the time of World War I, echo-sounding gear was available that timed the passage of a sound pulse to the ocean bottom and back.
Thereafter a huge variety of specialized ocean instruments became available. Ocean currents are tracked offshore by constant-depth floats that report their position acoustically, while sea-bottom currents are measured by devices called inclinometers. In the nearshore area, currents are followed by drifting devices or measured by underwater current meters, while wave and tidal motions are usually followed by pressure cells or by stationary floats hooked to recorders. Some instruments operate on floating or underwater buoys. Oceanographers also collect and correlate biological data in order to determine the bioproductivity and health of a given ecosystem.
Physical Measurements. Current speed and direction meters collect data at different depths on tidal, nearshore, offshore, and deep-ocean currents. Wave meters provide researchers with data on wave height, length, and direction. Mechanical and visual tide gauges have long been used, and electronic sensors are now employed to telemeter tidal data to central processing centers with computer outputs. Nautical charts are updated by means of sensitive fathometers along with accurate navigational aids and satellite photographs, the data being computer-corrected to zero-tide values.
Thermoclines, or temperature differences with depth, for a long time were measured with mechanical bathythermographs, devices that could simultaneously record temperatures and depths down to about 300 m (980 ft). Disposable electronic instruments of this kind are now employed. Using thermistors and pressure sensors, these instruments are connected to the surface by a small wire that feeds off an internal spool, allowing readings to be made from ships while under way. Mechanical temperature meters are used for long-term monitoring of power-plant discharges.
Waterproofed photoelectric cells measure the amount of light that passes through water to different depths, relative to a surface reading. Secchi disks are used to measure visibility and provide a measure of water turbidity. These data are important, because the amount of solar energy that is present directly affects the bioproductivity of a given body of water. How sound is transmitted through water, along with the effects of different frequencies, is also a subject of oceanographic research. Low-frequency sound waves, for example, travel around the Earth in the deep-ocean trenches. The detection and identification of sounds made by deep-sea life is a study all its own.
Chemical Measurements. Modern instrumentation allows researchers to correlate vast and diverse quantities of data by means of computers so as to determine the factors that may affect a given ocean ecosystem. Historically, water samples were taken at different depths and analyzed through painstaking processes. Today instrumentation packages called "fish," which can be lowered on a cable, are capable of continuous sampling, with the data being fed directly to a computer. A typical "fish" may collect data on depth, temperature, salinity, turbidity, oxygen, carbon dioxide, and specific heavy metals and pollutants on a single lowering.
Meteorological Monitoring. NOAA uses so-called monster buoys doughnut-shaped, self-contained telemetering devices that provide meteorological information from remote, seldom-traveled ocean areas. They transmit data on barometric pressures, wind speeds and directions, and wave heights and directions to shore-based monitoring stations for analysis. When combined with satellite and nearshore sensory data, this information enables forecasters to predict weather, sea, and swell conditions. Such predictions are vital for shipping and other offshore operations.
Seabed Sampling. The composition of the seabed is usually examined by collecting samples. Mechanical devices called "grabs" snap shut upon touching the bottom, while dredges are towed across the bottom by ships. Instruments for collecting cores are lowered by winch and allowed to fall freely, driven by negative buoyancies of a ton or more. In softer sediments the sharpened lower end of a corer may penetrate the ocean bottom as deeply as 10 m (33 ft). Very deep coring samples are obtained by specifically designed drilling cranes. Two vessels equipped with such cranes have been the Glomar Challenger of the Deep-Sea Drilling Project and the JOIDES Resolution of the Ocean Drilling Program. Deeper probes are achieved by using strong acoustic pulses that can penetrate thousands of meters, sending back echoes that reveal underlying rock strata.
Submersibles and Habitats. Oceanographers can explore the ocean directly by using scuba diving equipment or more complex deep-sea diving systems. Deeper descents require some kind of a pressure vessel such as a bell or submarine. The first oceanographic device of this sort was the bathysphere, a hollow steel ball built in 1930, which had to be lowered and raised by a cable. In the late 1940s French explorer Auguste Piccard developed his first bathyscaphe, a vessel that could ascend and descend freely, and within a few years an advanced bathyscaphe had explored the world's deepest oceanic trench.
Since that time several true submersibles, or steerable underwater craft, have been built. One such is Alvin, designed by Allyn Vine of the Woods Hole Oceanographic Institution. In the mid-1980s Alvin was used to observe hydrothermal vents and also to visit the wreckage of the famous ocean liner Titanic. Another craft, Pisces, is a Canadian submersible that can carry a variety of packages for specimen, bottom, and water sampling. Flip is a semisubmersible ship that is towed into position, where it assumes a vertical position with most of its length underwater and then drifts with the offshore currents. It was designed as a platform and housing unit for long-term data collection and observation. Such craft, and attendant robot craft operated by remote control (ROVs, for Remotely Operated Vehicles), can be equipped with many instruments and cameras as well as with mechanical arms.
Underwater habitats offer scientists the opportunity to spend time living at a given location in relative comfort while doing their work. The U.S. Navy saturation experiments that Capt. George Bond initiated in 1957 led to the Sealab experiments that set the stage for these habitats. French oceanographer Jacques Yves Cousteau conducted several such programs in the early 1960s. One U.S. project, Hydrolab, started operating off Florida in 1966. It was moved to the Bahamas in 1971 and installed at a depth of 9 m (30 ft), moved to St. Croix in 1978, and retired in 1984 after nearly 200 missions. Tektite, originally designed as a space station, was used in the U.S. Virgin Islands in 1970 and 1971 for two missions involving physiological, oceanographic, and biological studies. Prinul, which operated off Puerto Rico from 1972 to 1975, enabled scientists to conduct two-week missions while living at depths of from 15 m (50 ft) to more than 30 m (100 ft) and, sometimes, working at depths of more than 70 m (230 ft). Aquarius, operated by NOAA, is designed for depths of up to 37 m (120 ft). Stationed off St. Croix in 1987 at a depth of 15 m, it was moved in 1993 to the Florida Keys National Marine Sanctuary off Key Largo at the same depth.
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hydrologic sciences; International Hydrological Decade.