Lecture 6

The World Ocean
The Blue Planet: Chapter 10

Outline
• Ocean Basins and Ocean Water
• Ocean Circulation
• Ocean Waves
• Ocean Tides
• Where Land and Ocean Meet
• Changing Sea Levels

Ocean Basins and Ocean Water
• Seawater covers 70.8% of Earth’s surface
• Land comprising the remaining 29.2% is
unevenly distributed between the northern
and southern hemispheres
• Most of the water is contained in three
interconnected basins; Pacific, Atlantic
and Indian Oceans that, along with the
Southern Ocean, make up the “world
ocean”

• Before the 20th century, not much was
known about the depth of the ocean
• Water depths were made with a
weighted line lowered from a ship
– Not very efficient!
• In the 1920s echo sounders began to
be used, generating a pulse of sound
that measured how long it took for the
echo bouncing off the seafloor to return
to the instrument

• Thanks to echo sounders, the
topography of the seafloor and depth of
overlying water are known in great detail
for most of the ocean basins
– The Mariana Trench near Guam is the
greatest ocean depth measured at 10,924
m
• Recent satellite measurements reveal
the average depth of the sea is 3970 m,
and the average height of land above
sea level is 840 m

• As far back as the oldest sedimentary rocks,
more than 4 Ga, Earth has had liquid water
on its surface
– Oxygen isotopes in zircons suggest that the ocean
may be 4.4 billion years old
• Where the water came from is still uncertain
– Carbonaceous chondrites contain water as
hydrous minerals, suggesting some water
originated from accretion and volcanic steam
– Icy comets bombarded early Earth, bringing with
them frozen water

• About 3.5% of average seawater consists of
dissolved salts
– If these precipitated, they would form a layer about
56 m thick on the seafloor
• Average salinity is 33 to 37 per mil
– Principally sodium, chlorine and six other ions
derived from
• Dissolved load in streams
• Volcanic volatiles carried by atmospheric water
• Submarine volcanism
• Aeolian dust
• Pollutants

• Salinity of ocean water is related to
latitude, but also
– Evaporation
– Precipitation of rain and snow
– Inflow of fresh river water
– Freezing of sea ice

• The ocean differs from land in the
amount of heat it can store, for a given
amount of heat, water has a lower rise
in temperature than land
– Has a high heat capacity
– Low total and seasonal temperature range
– Ocean temperatures affect the climate over
ocean and land, which in turn controls
distribution of plants and animals

• Physical properties of seawater vary
with depth: ocean is vertically stratified
– Seawater becomes denser as temperature
decreases and as salinity increases
– Gravity pulls denser water down
– Drives deep ocean circulation
– Three major depth zones
• Surface zone: 100-500 m (mixed layer)
• Thermocline, halocline or pynocline
• Deep zone: contains 80% of ocean water

• Biotic zones
– Plants and animals living in the uppermost
water occupy the pelagic zone
– Benthonic organisms live on or within the
bottom sediment
– Floating organisms are planktonic
– Plant life is restricted to the photic zone
– Free swimmers (reptiles, fish, squid,
marine mammals, etc.) are restricted only
by their own locomotion

• Oceanic sediment is dominated by
skeletal remains of single-celled
planktonic and benthonic animals, when
more than 30% of bottom sediment
consists of these remains it is called
– Calcareous ooze (carbonate-based)
– Siliceous ooze (silica-based)
• Lithic sediment, consisting of rock
fragments, mantles continental shelves
and slopes

Ocean Circulation
• Surface ocean currents are broad, slow
drifts of surface water caused by friction
between the ocean and air flowing over
it
– 50-100 m deep
– Solar radiation provides heat energy, non
uniform heating generates winds, which
drive the movement of surface ocean water
• Ocean current direction is also
influenced by the Coriolis force

Ocean Circulation
• Ekman transport
– The balance of the wind on surface water
and Coriolis force at depth
– Generates a spiraling current pattern
– Results in a net direction of water
movement about 90˚ to the wind direction
– Near coasts this leads to vertical
movement of ocean water
• Upwelling if net transport is away from land
• Downwelling if net transport is toward land

Ocean Circulation
• With geography, ocean current drivers
set up the major surface current
systems in the world ocean
• Each major ocean current is part of a
large subcircular current system called
a gyre
– 2 in the Pacific
– 2 in the Atlantic
– 2 in the Indian

Ocean Circulation
• On either side of the equator, ocean
regions are dominated by westward-
flowing North and South Equatorial
currents
• Along the equator is the eastward-
flowing Equatorical Countercurrent
• Near 60˚ latitude, the Antarctic
Circumpolar Current circles the globe

Ocean Circulation
• North Atlantic Deep Water (NADW)
originates at the surface of the north
Atlantic, flows downward, and spreads
southward to the south Atlantic
• Flowing beneath this is the colder,
denser Antarctic Bottom Water (AABW)
• The sinking of dense, cold, saline
surface water propels a global
thermohaline circulation system

Ocean Waves
• Surface waves receive their energy from wind
– The size of a wave is determined by wind speed,
duration, and fetch (distance)
– Important wave dimensions are the height (from
crest to trough) and wavelength (from crest to
crest or crest to trough)
– As waves move, each parcel of water revolves in
a loop, returning nearly to its former position once
the wave has passed
– At a depth of half the wavelength, water motion is
negligible, this is the wave base

Ocean Waves
• Toward land, as water depth becomes
less than L/2, the circular motion of the
deepest water parcels is restricted by the
shallow seafloor, flattening the loop
• As depth decreases, the wave’s shape is
distorted; height increases, wavelength
shortens, and the wave front grows
steeper, eventually collapsing (breaking)
– This is turbulent surf

Ocean Waves
• Approaching shore, waves become
refracted to parallel the bottom contours
• The path of an incoming wave can be
resolved in two directional components
– Parallel to the shore: longshore current
– Perpendicular to the shore: surf

Ocean Waves
• Tsunami
– Technically a seismic sea wave
– Generated by sudden movements on the
seafloor
• Earthquake
• Submarine or coastal landslide
• Large volcanic eruption
– Sudden seafloor displacement causes
displacement in overlying water, when this
water falls back down it splits into two
oppositely moving components

Ocean Waves
• Tsunami
– Travel at speeds up to 950 km/h
– Have wavelengths measured in kilometers
– Wave height is only 1-2 meters
• Typically not seen or felt in the open ocean
– Periodicity can be from 20 min to 1 hr
– As the crest moves on shore, water can
pile up rapidly to heights of 30 meters and
travel great distances inland, as the trough
moves on shore, it causes drawdown

Ocean Tides
• Rhythmic, twice-daily rise and fall of ocean water
along coastlines
• Caused by gravitational attraction between the
Earth and Moon, and to a lesser extent, the Sun
• Generates tidal bulges due to gravitational pull
and inertial force
– Highest and lowest tides when sun and moon are
aligned
– Least tidal range when sun and moon are not

Where Land and Ocean Meet
• Zones of dynamic activity marked by
erosion and the creation, transport, and
deposition of sediment
• At any given moment, the geometry of
the shoreline represents an
approximate equilibrium between
constructive and destructive forces

• Coastal deposits
– Beach: wave crashed sediment along a
coast, including the surf zone
– Spit: elongate ridges of sand or gravel that
project from land and end in open water
– Barrier island: long, narrow sandy islands
lying parallel to a coast

• Marine deltas
– Where surf and currents do not erode all
new sediment carried to sea by a stream
– Sediment builds outward in a fan shape,
size and shape dependent on the balance
between sedimentation and erosion
– Very fertile regions

• Estuaries
– Semienclosed marine embayment diluted
with fresh water entering by one or more
streams
– With associated coastal wetlands, they
offer important habitats for an array of
plants and animals

• Reefs
– Warm-water coastlines are characterized by
limestone reefs
• Colonies built by corals and other carbonate-
secreting organisms that generate intense
biologic productivity and diversity
– Three principal reef types
• Fringing reef: attached to or closely borders land
• Barrier reef: separated from land by a lagoon
• Atoll: roughly circular reef enclosing a lagoon
that forms when a volcanic island subsides

• Coastlines are classic examples of
systems in a state of dynamic equilibrium
• As soon as the local system becomes
unbalanced, a change in the shoreline
becomes apparent
• Coastal erosion tends to have significant
impacts on human interests

• Responses to coastal erosion are of
three main categories
1. Hard stabilization: structural responses
– Jetties, breakwaters, seawalls
– May accelerate erosion in some areas
1. Soft stabilization: nonstructural approach
– Plantings, beach replenishment
1. Retreat: idea to limit human interference
– abandonment

Changing Sea Level
• On most coasts the level of the sea is
changing with respect to the land
– Rapid changes due to tectonic or isostatic
movements of the crust
– Slow changes related to global warming
– Long time interval changes - eustatic - due
to waxing and waning of continental
glaciers and ocean-basin volume as
lithospheric plates shift position

Changing Sea Level
• Submergence is the rise of water level
relative to the land
– Nearly all coasts experienced this in the
last 10,000 years due to last ice age
• Emergence is a lowering of water level
relative to the land
• Cycles of emergence and submergence
are related to the buildup and decay of
vast ice-age glacier systems

Changing Sea Level
• Sea ice, land ice, and sea level
– With all the water locked up in the
cryosphere, it is estimated that global sea
level would be 65 to 80 m higher if the ice
sheets were to melt
– Melting of the West Antarctic ice sheet
alone could contribute 8 m sea level rise

Changing Sea Level
• The ocean and society
– The world marine fish catch is over 80
million tons per year
– Coastal sedimentary rocks host productive
oil-bearing deposits
– Travel and transport
– Energy
– Climate moderation

Lecture 6

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Lecture 6