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Oceans – Part I
Introduction
Composition of Seawater
Features of the Sea Floor
Sediments
Introduction
• The “Blue Planet”
– Seventy-one percent of Earth’s surface is
represented by oceans and marginal seas
– Continents and islands comprise the remaining
29
The Oceans of Earth
About 7% of
the size of the
Pacific
The largest and
has the greatest
depth
About half the
size of the
Pacific and
not quite as
deep
Slightly smaller than the
Atlantic, largely a Southern
Hemisphere body
Earth’s Northern & Southern
Hemispheres
The Composition of Seawater
Composition of Seawater
• Salinity
– Salinity is the total
amount of solid
material dissolved
in water
– Typically expressed
as 0/00 or parts per
thousand (ppt)
– 350/00 is world
average
Composition of Seawater
• Salinity
– Anion + cation = a salt
• Salt = NaCl = Na+ + Cl• Na+ has a positive charge (cation)
• Cl- has a negative charge (anion)
Composition of Seawater
• Salinity
– What goes in must equal what goes out
• Addition of sea salts
– Chemical weathering of rocks on continents (cations)
– Volcanic eruptions / outgassing (anions)
Composition of Seawater
• Salinity
– What goes in must equal what goes out
• Addition of sea salts
• Removal of sea salts
–
–
–
–
Sea spray
Evaporites
Biologic processes
Magma at M.O.R.
Important Factors:
Precipitation
Evaporation
Freezing
Run-off
The Ocean’s Layered Structure
• Temperature and salinity change with depth
in the oceans
– A three-layered structure exists in the open
ocean
The Ocean’s Layered Structure
• Temperature and salinity change with depth
in the oceans
– Salinity variations with depth correspond to the
general three-layered structure described for
temperature
– Thermocline = zone of rapid temperature
change
– Halocline = zone of rapid salinity change
Features of the Ocean Floor
Mapping the Ocean Floor
• Bathymetry – Measurement of ocean depths
and the charting of the shape or topography
of the ocean floor
Mapping the Ocean Floor
• Echo sounder (also
called sonar)
– Invented in the 1920s
– Primary instrument for
measuring depth
– Reflects sound from
ocean floor
Mapping the Ocean Floor
• Multibeam sonar
– Employs and array of
sound sources and
listening devices
– Obtains a profile of a
narrow strip of seafloor
Mapping the Ocean Floor
• Measuring the shape
of the ocean surface
from space
– Employs satellites
equipped with radar
altimeters
Mapping the Ocean Floor
• Three major topographic units of the ocean
floor
Continental Margins
• Classifications of ocean floor
– Continental Margins – the submerged outer
edge of a continent
– Ocean Basin – the deep seafloor beyond the
continental margin
Continental Margins
• There are two types of continental margins:
– Passive margins
• Also called Atlantic-type margins
• Face the edges of diverging tectonic plates.
• Very little volcanic or earthquake activity
Continental Margins
• There are two types of continental margins:
– Active margins
• AKA as Pacific-type margins
• Located near the edges of converging plates.
• Active margins are the site of volcanic and
earthquake activity.
Passive Continental Margins
1. Continental Shelf
2. Shelf Break
3. Continental Slope
4. Submarine Canyons
5. Continental Rise
Passive Continental Margins
• Continental Shelf
–
–
–
–
Found at margins of continents
Relatively flat
Average 65-100 km in width
Formed by:
• Thick accumulations of shallow-water sediments
• Some areas are mantled by extensive glacial
deposits
– Ends at change in slope (Shelf break)
back
Passive Continental Margins
• Continental Slope
– Boundary between continental crust and
oceanic crust
– Extends from shelf break to rise
– Submarine canyons are major features
– Submarine canyons formed by:
• River erosion
• Turbidity currents
Submarine Canyons and
Turbidity Currents
Graded Beds
Each layer grades from coarse at its base to
fine at the top.
back
Passive Continental Margins
• Continental rise
– Found in regions where trenches are absent
– Located where the continental slope grades into
a more gradual incline as it meets the deep
ocean floor
– Deep-sea fans form here
back
Active Continental Margins
• Located primarily around the Pacific Ocean
• Continental slope descends abruptly into a
deep-ocean trench
• Accumulations of deformed sediment and
scraps of ocean crust form accretionary
wedges
• Some subduction zones have little or no
accumulation of sediments
Active Continental Margins
• Deep-ocean trenches
–
–
–
–
Long, relatively narrow features
Deepest parts of ocean
Most are located in the Pacific Ocean
Sites where moving lithospheric plates plunge
into the mantle
– Associated with volcanic activity
• Volcanic islands arcs
• Continental volcanic arcs
An Active Continental Margin
Deep-Ocean Basins
• Features of the deep-ocean floor
–
–
–
–
–
Oceanic Ridges
Hydrothermal Vents
Abyssal Plains and Abyssal Hills
Seamounts and Guyots
Trenches and Island Arcs
Deep-Ocean Basins
• Ridges and Rises
– Underwater volcanic
mountain chain
• extends for 65,000 km
• 1,000 km wide
• 1,000-2,000 m high
South West Indian Ridge
Deep-Ocean Basins
Deep-Ocean Basins
• Ridges and Rises
– Ridges = steep slopes
– Rises = gentle slopes
– Contain central rift
valley
• 15-50 km wide
• 500-1500m deep
– Offset by fractures
Deep-Ocean Basins
Hydrothermal vents are sites where superheated water containing dissolved
minerals and gases escapes through fissures, or vents.
Deep-Ocean Basins
• Abyssal hills
– Flat areas of sediment-covered ocean floor
found between the continental margins and
oceanic ridges.
– Also small, extinct volcanoes or rock intrusions
near the oceanic ridges.
• Seamounts
– Volcanic projections from the ocean floor that
do not rise above sea level.
– Flat-topped seamounts eroded by wave action
are called guyots.
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
Seamounts & Guyots
Deep-Ocean Basins:
Trenches
Mariana trench
the deepest
(11,020 m)
Peru-Chile trench the
longest ( 5900 km)
Ocean Sediments
Seafloor Sediments
• Particles entering the ocean
• Accumulate
– Rapidly on continental margin (neritic)
– Slowly in the deep ocean (pelagic)
• Reflect ocean history
Seafloor Sediments
• Thickness varies
– Thickest in trenches—Accumulations may
approach 10 kilometers
– Pacific Ocean—About 600 meters or less
– Atlantic Ocean—From 500 to 1000 meters
thick
• Mud is the most common sediment on the
deep-ocean floor
Seafloor Sediments
• Sediment can be classified by particle size.
– Waves and currents generally transport smaller
particles farther than larger particles.
Seafloor Sediments
• Types of seafloor sediments
– Lithogenous sediment
• Material weathered from continental rocks
– Sands and muds from continental margins
– Glacial deposits
– Clays
• Fine particles remain suspended for a long time
• Oxidation often produces red and brown colored
sediments
Seafloor Sediments
• Types of seafloor sediments
– Lithogenous sediment
– Biogenous sediment
• Ooze = greater than 30% biogenous sediment
• Distribution related to sediment supply, rate of
dissolution and sediment dilution
• Siliceous fossils at all depths, calcareous not below
Calcium Carbonate Compensation depth
• Diatoms and radiolarians = major siliceous
microfossils
• Calcareous = foraminifera, pteropods,
coccolithophores
Biogenous Sediment
Accumulation
Seafloor Sediments
• Siliceous Oozes
– Fine-grained pelagic deposit
– Composition:
• 30% siliceous (SiO2) material of organic origin
• Diatoms (phytoplankton) and Radiolaria
(zooplankton)
– Siliceous particles dissolve more slowly than
calcareous particles
Siliceous Oozes
Diatoms
• Composed of SiO2
• Phytoplankton
• Base of food chain
Seafloor Sediments
• Calcareous Oozes
– Wide-spread in relatively shallow areas of the
deep sea
– CaCO3 particles dissolve at “Carbonate
Compensation Depth” = (CCD)
• Atlantic: ~ 4,000 m
• Pacific: ~ 500 - 1,500 m
Biogenous Sediment
Accumulation
Radiolaria
• Composed of SiO2
• Zooplankton
• Base of food chain
Foraminifera
• Composed of calcium carbonate (CaCO3)
• Zooplankton
Seafloor Sediments
• Types of seafloor sediments
– Terrigenous sediment
– Biogenous sediment
– Hydrogenous sediment
• Minerals that crystallize directly from seawater
• Most common types include
–
–
–
–
Manganese nodules
Calcium carbonates
Metal sulfides
Evaporites
Seafloor Sediments
• Ferromanganese Nodules
– First sub-marine ferromanganese concretions
were discovered in 1868 on the Kara Sea
(Russia)
– Characteristics - small balls (lightly flattened),
dark-brown, and 5 - 10 cm in diameter
– Found at depths of 4,000 to
6,000 m
– Not clear how these nodules
form
Distribution Of Manganese
Nodules
Seafloor Sediments
• Evaporites are precipitates that form as
water evaporates or as the conditions in the
water change
– include many salts with economic importance.
– currently form in the Gulf of California, the
Red Sea, and the Persian Gulf.
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
Studying Sediments
• How do scientists study sediments?
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–
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Deep-water cameras
Clamshell samplers
Dredges
Piston Corers
Core libraries
Seismic profilers
Studying Sediments
• What can scientists learn by studying
sediments?
– Historical information
– Location of natural resources, especially crude
oil and natural gas
End