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Objectives• Identify methods used by scientists to study
Earth’s oceans.
The Oceans
• Discuss the origin and composition of the oceans.
• Describe the distribution of oceans and major seas.
– oceanography
– side-scan sonar
– sea level
Vocabulary
• Oceanography is the scientific study of Earth’s oceans, which is usually considered to have started with the Challenger expedition.
The Oceans• In the late 1800s, the British Challenger
expedition became the first research ship to use relatively sophisticated measuring devices to study the oceans.
The Oceans
Modern Oceanography• In the 1920s, the German research ship Meteor
used sonar for the first time to map the seafloor features of the South Atlantic Ocean.
The Oceans
• Sonar, which stands for sound navigation and ranging, uses the return time of an echo and the known velocity of sound in water to determine water depth.
– The velocity of sound in water is 1500 m/s.
– To calculate the distance to the ocean floor, multiply the time by 1500 m/s, then divide by 2.
Modern Oceanography
Advanced Technology
The Oceans
– Satellites such as the Topex/Poseidon continually monitor the ocean’s surface temperatures, currents, and wave conditions.
– Submersibles, or underwater vessels, investigate the deepest ocean trenches.
– Large portions of the seafloor have been mapped using side-scan sonar.
– Side-scan sonar is a technique that directs sound waves to the seafloor at an angle, so that the sides of underwater hills and other topographic features can be mapped.
Origin of the Oceans• Studies of radioactive isotopes indicate that
Earth is about 4.6 billion years old.
The Oceans
• Radioactive studies and lava flows offer evidence that there has been abundant water throughout Earth’s geologic history.
Origin of the Oceans
Where did the water come from?
The Oceans
– Scientists hypothesize that Earth’s water could have originated from two sources.
• Comets occasionally collide with Earth and release water on impact–possibly enough to have filled the ocean basins over geologic time.
• Studies of meteorites indicate that they may contain up to 0.5 percent water.
• If the early Earth contained the same percentage of water, it would have been more than sufficient to form the early oceans.
Origin of the Oceans
Volcanism
The Oceans
– Shortly after the formation of Earth, violent volcanism released huge amounts of water vapor, carbon dioxide, and other gases, which combined to form Earth’s early atmosphere.
– As Earth’s crust cooled, the water vapor gradually condensed into oceans.
Origin of the Oceans
Volcanism
The Oceans
– Volcanism still adds water to the hydrosphere, but the process is balanced by the continuous destruction of some water molecules by ultraviolet radiation from the Sun.
Distribution of Earth’s Water• The oceans contain 97 percent of the water
found on Earth.
The Oceans
• The remaining 3 percent is freshwater located in the frozen ice caps of Greenland and Antarctica and in rivers, lakes, and underground sources.
• The percentage of ice has ranged from near zero to as much as 10 percent of the hydrosphere over geologic time.
Distribution of Earth’s Water• Global sea level is the level of the oceans’
surfaces.
The Oceans
– Sea level has risen and fallen by hundreds of meters in response to melting ice during warm periods and expanding glaciers during ice ages.
– Tectonic forces that lift or lower portions of the seafloor has also affected sea level.
Distribution of Earth’s Water
The Blue Planet
The Oceans
– Approximately 71 percent of Earth’s surface is covered by oceans that have an average depth of 3800 m.
– Because most landmasses are in the northern hemisphere, oceans only cover 61 percent of the surface.
– Water covers 81 percent of the southern hemisphere.
Distribution of Earth’s Water
Major Oceans
The Oceans
– There are three major oceans:
• The largest ocean, the Pacific, contains roughly half of Earth’s seawater and is larger than all of Earth’s landmasses combined.
• The second-largest ocean, the Atlantic, extends from Antarctica to the arctic circle, north of which it is often referred to as the Arctic Ocean.
• The third-largest ocean, the Indian, is located mainly in the southern hemisphere.
• The water surrounding Antarctica, south of 50° south latitude, is known as the Antarctic Ocean.
Distribution of Earth’s Water
Sea Ice
The Oceans
– The Arctic and Antarctic Oceans are covered by vast expanses of sea ice, particularly during the winter.
– An ice-crystal slush develops at the surface of the water and eventually solidifies into individual round pieces called pancake ice.
– The pieces of pancake ice thicken and freeze into a continuous ice cover called pack ice.
– Pack ice is generally several meters thick and may cover an area more than 1000 km wide.
Distribution of Earth’s Water
Seas
The Oceans
– Seas are smaller than oceans and are partly or mostly landlocked.
– The Mediterranean Sea is located between Africa and Europe and was the first sea to be explored and mapped by ancient peoples.
– Notable seas in the northern hemisphere include the Gulf of Mexico, the Caribbean Sea, and the Bering Sea, which is located between Alaska and Siberia.
– All seas and oceans belong to one global ocean whose waters are thoroughly mixed.
Objectives• Compare and contrast the physical and chemical
properties of seawater.
• Explain ocean layering.
• Describe the formation of deep-water masses.
– salinity
– temperature profile
– thermocline
Vocabulary
Seawater
Seawater• Seawater is a solution of about 96.5 percent
water and 3.5 percent dissolved salts.
Seawater
• The most abundant salt in seawater is sodium chloride (NaCl).
• Most elements on Earth are present in seawater.
• Because these substances are dissolved, they are in the form of ions.
Chemical Properties of Seawater• Salinity is a measure of the amount of
dissolved salts in seawater that is expressed as grams of salt per kilogram of water, or parts per thousand (ppt).
Seawater
• The total salt content of seawater is, on average, 35 ppt, or 3.5 percent.
• Seawater also contains dissolved gases and nutrients.
Chemical Properties of Seawater
Variations in Salinity
Seawater
– The actual salinities of the oceans vary from place to place.
• Salinities may be as high as 37 ppt in subtropical regions where rates of evaporation exceed those of precipitation.
• Salinities are lower in equatorial regions where precipitation is abundant.
• Salinities of 32 or 33 ppt occur in polar regions where seawater is diluted by melting sea ice.
• The lowest salinities often occur where large rivers empty into the oceans.
Chemical Properties of Seawater
Sources of Sea Salt
Seawater
– Geological evidence indicates that the salinity of ancient seas was not much different from that of today’s oceans.
– The proportion of magnesium in the calcium-carbonate shells of some marine organisms depends on the overall salinity of the water in which the shells form.
– Present-day shells contain about the same proportion of magnesium as similar shells throughout geologic time.
Chemical Properties of Seawater
Sources of Sea Salt
Seawater
– The sources of sea salts has remained the same over time.
• Chlorine and sulfur dioxide dissolve in water and form the chlorine and sulfate ions of seawater.
• The weathering of crustal rocks generates most of the other abundant ions in seawater.
• These ions are then flushed into rivers and transported to oceans.
Chemical Properties of Seawater
Removal of Sea Salts
Seawater
– Salts are removed from the ocean at the same rate as they are added.
– The removal of sea salts involves several processes.
• Some sea salts precipitate from seawater near arid, coastal regions.
• Salty spray droplets from breaking waves are picked up by winds and deposited inland.
• Marine organisms remove ions from seawater to build their shells, bones, and teeth.
Physical Properties of Seawater• Freshwater has a maximum density of 1.00 g/cm3.
Seawater
• Seawater is denser than freshwater because salt ions are heavier than water molecules.
• The density of seawater ranges from about 1.02 g/cm3 to 1.03 g/cm3 depending on its salinity and temperature.
• Because salt ions interfere with the formation of hydrogen bonds, the freezing point of seawater is –2°C.
Physical Properties of Seawater
Absorption of Light
Seawater
– Water absorbs light, which gives rise to another physical property of oceans—they are dark.
– In general, light penetrates only the upper 100 m of seawater.
– Red light penetrates less than blue light.
– Light sufficient for photosynthesis exists only in the top 100 m of the ocean.
Ocean Layering• Ocean surface temperatures range from –2°C in
polar waters to 30°C in equatorial regions, with the average surface temperature being 15°C.
Seawater
• Ocean water temperatures decrease significantly with depth.
Ocean Layering• A typical ocean
temperature profile plots changing water temperatures with depth.
Seawater
Ocean Layering• Based on temperature variations, the ocean can
be divided into three layers.
Seawater
– The first layer is a relatively warm, sunlit, surface layer some 100 m thick.
– The thermocline is a transitional layer which is characterized by rapidly decreasing temperatures with depth.
– The bottom layer is cold and dark with temperatures near freezing.
Ocean Layering• Both the thermocline and the warm surface layer
are absent in polar seas, where water temperatures are cold from top to bottom.
Seawater
• In general, ocean layering is caused by density differences of warm and cold water.
Water Masses• Cold water migrates toward the equator as a cold,
deep water mass along the ocean floor.
Seawater
– To start, sea ice that forms in the polar regions does not incorporate salt ions into growing ice crystals, causing them to accumulate beneath the ice.
– As the cold water beneath the ice becomes saltier and denser than the surrounding seawater, it sinks.
– Surface currents in the ocean also bring relatively salty midlatitude or subtropical waters into polar regions where they cool and sink.
– The dense, salty water then migrates toward the equator as a cold, deep water mass along the ocean floor.
Water Masses• Three water masses account for most of the deep
water in the Atlantic Ocean.
Seawater
1. Antarctic Bottom Water forms when antarctic seas freeze during the winter and water temperature drops below 0°C.
2. North Atlantic Deep Water forms in a similar manner offshore from Greenland.
3. Antarctic Intermediate Water forms when the relatively salty waters of the Antarctic Ocean decrease in temperature during winter and sink.
Water Masses
Seawater
• The Indian and Pacific Oceans contain only the two deep antarctic water masses.
– wave
– crest
– trough
– breaker
Objectives• Describe the physical properties of waves.
• Explain how tides form.
• Compare and contrast various ocean currents.
Vocabulary
Ocean Movements
– tide
– density current
– surface current
– upwelling
Ocean Movements• A wave is a rhythmic movement that carries
energy through space or matter, such as ocean water.
Ocean Movements
• As an ocean wave passes, the water moves up and down in a circular pattern and returns to its original position.
Wave Characteristics• A crest is the highest point of a wave.
Ocean Movements
• A trough is the lowest point of a wave.
• The vertical distance between crest and trough is the wave height; the horizontal crest-to-crest distance is the wavelength.
Wave Characteristics• The wavelength determines the wave base,
which is the depth to which the wave disturbs the water.
Ocean Movements
• Wave speed increases with wavelength.
Wave Characteristics
Wave Height
Ocean Movements
– Wave heights depend upon three factors: wind speed, wind duration, and fetch.
– Fetch refers to the expanse of water that the wind blows across.
– Large storm waves can be much higher than average.
Wave Characteristics
Breaking Waves
Ocean Movements
– Ocean waves begin to lose energy and slow down near the shore because of friction with the ocean bottom.
– As the water becomes shallower, incoming wave crests gradually catch up with the slower wave crests ahead.
– Breakers are waves where the crests collapse forward when the wave becomes higher, steeper, and unstable as it nears shore.
Tides• Tides are the periodic rise and fall of sea level.
Ocean Movements
• The highest level to which water rises is known as high tide, and the lowest level is called low tide.
• Because of differences in topography and latitude, the tidal range—the difference between high tide and low tide—varies from place to place.
• Generally, a daily cycle of high and low tides takes 24 hours and 50 minutes.
Tides
Ocean Movements
Differences in topography and latitude cause three different daily tide cycles. Areas with semidiurnal cycles experience two high tides per day. Areas with mixed cycles have one pronounced and one smaller tide each day. Areas with diurnal cycles have one high tide per day.
Causes of Tides• The basic causes of tides are the gravitational
attraction among Earth, the Moon and the Sun, as well as the fact that gravitational attraction decreases with distance.
Ocean Movements
• Both Earth and the Moon orbit around a common center of gravity.
• As a result, Earth and the Moon experience gravitational and centrifugal forces that generate tidal bulges on opposite sides of Earth.
Causes of Tides
The Sun’s Influence
Ocean Movements
– The gravitational attraction of the Sun and Earth’s orbital motion around the Sun also generate tides.
– Lunar tides are more than twice as high as those caused by the Sun because the Moon is much closer to Earth.
– Solar tides can either enhance or diminish lunar tides.
• Spring tides occur when the Sun, the Moon, and Earth are aligned, causing high tides to be higher than normal and low tides to be lower than normal.
• During neap tides, high tides are lower and low tides are higher than normal.
Ocean Currents• A density current moves slowly in the deep
ocean and is caused by differences in the temperature and salinity of ocean water, which in turn affect density.
Ocean Movements
• Surface currents are wind-driven currents that affect mainly the upper few hundred meters of the ocean and can move as fast as 100 km per day.
• Surface currents follow predictable patterns influenced by Earth’s global wind systems.
Ocean Currents
Gyres
Ocean Movements
– The continents deflect ocean currents to the north and south causing closed circular current systems, called gyres, to develop.
– There are five major gyres: the North Pacific, the North Atlantic, the South Pacific, the South Atlantic, and the Indian Ocean.
– The parts of all gyres closest to the equator move towards the west as equatorial currents until they are deflected toward the poles by a landmass.
– After cooling in the polar regions, the current, deflected by landmasses, moves back toward the equator.
Upwelling• Upwelling is the upward motion of ocean water.
Ocean Movements
• Areas of upwelling exist mainly off the western coasts of continents in the trade-wind belts.
• Upwelling waters are rich in nutrients, which support abundant populations of marine life.
