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). The two
minor subdivisions of the world ocean are the Southern Ocean, bounded by the
Antarctic Circumpolar Current to the north and Antarctica to the south, and the
Arctic Ocean, almost landlocked except between Greenland and Europe. 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 Galápagos 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,
PO43-.
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.
Scientists are increasingly concerned about
warming ocean temperatures due to increased amounts of greenhouse gases, such as
carbon dioxide and methane, in the atmosphere. The phenomenon known as global
warming could have a negative effect on many forms of marine life that are
sensitive to ocean temperature, such as coral and plankton, and also on ocean
currents due to the release of fresh water from melting polar ice caps and
glaciers.
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.
In the early 21st century, scientists drew
attention to another source of pollution that could have devastating effects on
the oceans and marine life. British scientists with the Royal Society reported
in 2005 that the release of more than 25 billion metric tons of carbon dioxide
into the air each year is turning the oceans gradually more acidic. This
dramatic change in ocean chemistry worldwide is reducing the availability of the
carbonates that a wide variety of marine animals, such as clams, coral, and
krill, need to produce limestone skeletons. The growing acidification of the
oceans may also be weakening their existing skeletons. Since these animals are
at the bottom of the food chain, any loss of population would negatively affect
the world’s fisheries.
In 2007 scientists reported that the oceans
have become 30 percent more acidic than they were at the beginning of the
Industrial Revolution when humankind began burning fossil fuels on a large
scale. Scientists have begun to monitor the oceans for acidity with sensors on
buoys that measure pH levels. The buoys provide real-time measurements by
transmitting their data via radio signals to satellites in space and then to
scientists onshore.
See also Water Pollution.
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