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Chapter 15
Oceanography is the study of the Earth’s oceans
The depth of the oceans is measured by sonar
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The British Challenger Expedition
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The German Meteor (1920’s) Expedition
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Topex / Poseidon Satellite Expedition
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Side-Scan Sonar Expedition
◦ The first research ship to study oceans
◦ Investigated ocean currents, water temperature, and
chemical composition of the oceans
◦ Used sonar to map the seafloor features of the South
Atlantic Ocean (including the Mid-Atlantic Ridge)
◦ Continually monitors ocean surface temperatures,
currents, and wave conditions
◦ Technique that directs sound waves to the seafloor at an
angle so the sides of underwater features can be
mapped
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Scientists hypothesize Earth’s water could
have come from two places:
◦ Comets – when they collide with Earth, the impact
releases water – possibly enough to fill ocean
basins over geologic time
 Meteorites – contain up to 0.5% water. If early Earth
contained same percentage of water, it could have
easily filled up the oceans
◦ Volcanism – in early Earth history, violent volcanism
occurred over several hundred million years. These
volcanoes created/released much water vapor,
carbon dioxide, and other gases. Gases gradually
condense into oceans (over hundreds of years) as
the Earth’s crust cooled
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Oceans contain 97% of water
found on Earth
Another 3% of water is found
in polar ice caps, rivers, lakes,
and underground sources
Sea level has risen and fallen
by hundreds of meters
throughout Earth’s history
Approximately 71% of Earth’s
surface is covered by oceans
(Blue Planet). Most of the
oceans are found in the
southern hemisphere
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There are three major oceans
in the world:
◦ Pacific Ocean
 Contains roughly half of all
seawater
 Larger than all landmasses
combined
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◦ Atlantic Ocean
 Extends more than 20,000km
from Antarctica to the Arctic Circle
 North of the Arctic Circle, it is
referred to as the Arctic Ocean
(but is still the Atlantic)
 Surrounding Antarctica, referred
to as the Antarctic Ocean. It is
severely storm-lashed
◦ Indian Ocean
 Third largest ocean
 Located mainly in the southern
hemisphere
OCEANS
Smaller than Oceans in size
Land-locked
Examples…
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Mediterranean Sea
Gulf of Mexico
The Caribbean Sea
The Bering Sea
All seas belong to one global
ocean, whose waters are
mixed
That said, these waters
contain nearly identical
proportions of dissolved salts
SEAS
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The Arctic and Antarctic Oceans are covered by sea
ice, particularly during the winter. In the summer, the
ice “kind of” breaks up.
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Ice is less dense than water, so it floats.
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Formation:
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1. Ice crystals form
2. Slush develops at the surface of the water
3. Thickening of ice solidifies into round pieces called
pancake ice
4. Pancake ice thickens and freezes into a cover, called pack
ice
Salt ions concentrate in the water under the sea ice,
as the ions cannot continuously move into the
blocks of ice.
Have you ever swallowed a gulp
of seawater? Did you notice its
salty taste?
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Seawaters differ from
freshwater due to various
salts
Salt ions are heavy, increasing
density of water
Variations in salinity cause
the freezing point of seawater
to be lower than that of
freshwater
Oceans are generally dark,
light absorption occurs only
in the upper 100m of
seawater
Density varies depending on
temperature
◦ Cold water – more dense
◦ Warm water – less dense
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So, the most dense water
would be cold with high
salinity levels
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Temperatures range – with
average surface temperature
of the ocean being 15oC
◦ Polar waters average -2oC
◦ Equatorial waters average 30oC
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Temperatures decrease
significantly with depth.
Based on these variations, the
ocean is divided into three
parts:
1. Surface area - relatively warm,
sunlit (100m thick)
2. Transitional layer (thermocline)
- rapidly decreasing in
temperature with depth. This
layer is absent in polar seas
3. Bottom layer – cold and dark,
near freezing
Water comes from polar
seas. Cold water is
more dense and ice
does not incorporate
salt, this accumulates
below the ice causing
water to be more salty.
The salty water sinks
because it is more
dense.
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Cold, dense masses of salty water from the polar
seas migrates toward the equator as a water
mass along the ocean floor.
These masses are replaced as surface currents
bring salty, subtropical waters into polar
regions… which become colder and denser than
surrounding waters as winter hits.
There are three main deep water masses within
the oceans. The Atlantic Ocean is the only mass
of water that contains all three.
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1. Antarctic Bottom Water
◦ Forms when Antarctic seas freeze during the winter
◦ Coldest and densest in all of the oceans
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2. North Atlantic Deep Water
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3. Antarctic Intermediate Water
◦ Forms in similar pattern off of Greenland
◦ Warmer and less dense than Antarctic water that
overrides it
◦ Forms when relatively salty water of Antarctic
decreases temperature in winter and sinks
◦ Overrides other two masses
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Seawater is a solution of about 96.5% water and
3.5% dissolved salts.
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The most abundant salt is sodium chloride
(NaCl).
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Seawater also contains dissolved gases, such as
oxygen, nitrogen, and carbon dioxide
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Seawater contains nutrients, such as nitrates,
phosphates, and silicates
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Salinity is the measure of the amount of dissolved salts
in seawater.
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Levels of salinity have not changed much throughout history
Sources of sea salts have remained similar throughout history
Salinity is expressed at parts per thousand (ppt)
Salinity affects the density of seawater
Average salinity of seawater is 35ppt, but this can vary
based on location:
 Subtropical regions – may be as high as 37ppt due to rates of
evaporation exceeding precipitation
 Equatorial regions – slightly lower salinity because rainfall is
abundant
 Polar water – even lower salinity (32-33ppt) because water is
diluted by melting sea ice
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The lowest salinity levels are found where large rivers
empty into oceans because freshwater is mixing with
saltwater
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Sea salts are removed at the same rate they are
added, which is why salinity levels do not rise.
Process of removal:
1. Precipitate from seawater near arid, coastal regions
(removes sodium chloride, calcium sulfate & other sea
salts)
2. Spray droplets from breaking waves are picked up by
winds and deposited inland
3. Marine organisms remove ions from seawater to build
their shells, bones, and teeth. As they die, they sink to
the seafloor and mix with bottom sediments
Oceans are never completely
motionless. Their most obvious
movement is the constant motion
of waves.
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A wave is the rhythmic movement that carries
energy through space or matter.
◦ Waves are generated by wind flowing over the water
surface
◦ As a wave passes, water moves up and down in a
circular pattern – returning to its original position
◦ Only energy moves steadily forward
◦ Water moves only in circles but does NOT move
forward
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A typical ocean wave contains:
◦ Crest
 Highest point of the wave
◦ Trough
 Lowest point of the wave
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Wave-height is the vertical distance between the
crest and trough
◦ Depends on three factors:
1. Wind Speed
2. Wind Duration
3. Fetch
 Area of water the wind blows across
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Ocean waves lose energy due
to friction at the bottom of
the ocean
Breaking waves are common
along shallow waters and
shorelines
As water becomes more
shallow, incoming wave
crests gradually catch up to
wave crests ahead
Wave length then decreases,
making incoming waves
higher, steeper, and more
unstable
Crests then collapse forward
and create “breakers”
◦ Formation of breakers influence
the motion of wave crests
◦ High speed breakers affect
shorelines
Tides are the periodic rise and
fall of the sea level
Tide levels differ based on
geographic location:
Highest Level – High Tide
Lowest level – Low Tide
Generally, the daily cycle of
high and low tides takes 24
hours and 50 minutes
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Gravitational attraction
among the Earth, Moon,
and Sun
Lunar tides are more
than twice as high as
those caused by the Sun
– because the Moon is
closer to the Earth
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Spring Tides
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Neap Tides
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Spring Tides alternate every
two weeks with Neap Tides
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On average, Spring Tides are
3x higher than Neap Tides
Tidal bulges are always
aligned to the Moon
Solar Tides can either
enhance or diminish
lunar tides
Causes of Tides
◦ Large tidal ranges
◦ Occur with a Full or a New Moon
◦ High tides are higher than
normal
◦ Low tides are lower than normal
◦ Small tidal ranges
◦ Occur within the First Quarter or
Third Quarter Moons
◦ High Tides are lower than
normal
◦ Low tides are higher than
normal
Types of Tides
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Density currents
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Surface Currents
◦ Currents caused by
differences in the
temperature and salinity of
ocean water
◦ Move slowly in deep ocean
water
◦ Not noticeable due to their
depth
◦ Currents that are wind
driven on the surface of
the ocean
◦ Affect the upper few
hundred meters of the
ocean
◦ Can move as fast as
100km per day
◦ Move in predictable patters
◦ Noticeable to humans
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Closed circular currents of ocean water
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Caused by continents deflecting ocean currents to the north and south
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Follow circular motions based on the Coriolis Effect
◦ Clockwise in Northern Hemisphere
◦ Counter-clockwise in Southern Hemisphere
◦ Parts closest to equator move towards the west as equatorial currents
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Deflect towards poles as they encounter land
Poleward flowing waters carry warm, tropical water into higher/colder latitudes
Once warm waters reach polar regions, they gradually cool and move back toward the
equator (as they’re deflected by landmasses surrounding the poles)
Once they “leave the poles,” they carry cooler water from the polar regions back down
toward the equator
Five types:
1.
2.
3.
4.
5.
North Pacific
North Atlantic
South Pacific
South Atlantic
Indian Ocean
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Upwelling is the upward motion of ocean
water
Upwelling waters originate from the bottom
of the ocean and are cold
Areas of upwelling exist off the western
coasts of continents and trade-wind belts
Upwelling waters are rich in nutrients
Upwelling waters support abundant
populations of marine life
Some of the world’s best fishing grounds are
found in these waters (The Grand Banks –
New England / Eastern Canada)
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How did the oceans form?
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What is the Challenger expedition?
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What affects the density of seawater?
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What steps are involved in the formation of sea ice?
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What are the three masses of Atlantic Deep-Water?
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What is the basic cause of tides?
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What is a collapsing wave?
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What is a transitional layer of the ocean characterized by rapidly
decreasing temperatures with depth?
◦ Volcanism reached Earth’s atmosphere and created /released water vapor,
carbon dioxide, and other gases. Gases gradually condensed into oceans
(over hundreds of years) as the Earth’s crust cooled
◦ The first expedition to use scientific measuring devices to study the ocean
◦ Salinity
◦ Ice crystals, slush, pancake ice, pack ice
◦ Antarctic Bottom Water, Antarctic Intermediate Water, Atlantic Bottom
Water
◦ Gravitation
◦ Breaker
◦ Thermocline