Transcript INTRODUCTION - Geophile.net

Sediments
Geologic Oceanography
What are Sediments?
• Particles entering the ocean
• Accumulate
– Rapidly on continental margin (neritic)
– Slowly in the deep ocean (pelagic)
• Reflect ocean history
Types of Marine Sediments
• Sediment may be classified in two ways:
– By grain size
– By origin
Types of Marine Sediments
• Types of Marine Sediments by grain size:
Boulder
Cobble
Pebble
Granule
Sand
Silt
Clay
>256 mm
64 - 256 mm
4 - 64 mm
2 - 4 mm
1/16 - 2 mm
1/256 - 1/16 mm
<1/256
Types of Marine Sediments
• Types of Marine Sediments by grain size:
Move
Small
Large
Low –
moderate
High
Deposit Very quiet Low moderate
- Boulder
Energy needed to move particle
- Cobble
- Pebble
- Gravel
- Course sand
- Medium sand
- Fine sand
Low energy
Energy needed to deposit particle
High energy
- Boulder
- Cobble
- Pebble
- Gravel
- Course sand
- Medium sand
- Fine sand
- Silt
- Silt
- Clay
- Clay
Low energy
Still water
Types of Marine Sediments
• Types of Marine Sediments by grain size:
Types of Marine Sediments
• Sediment can also be classified according to
its source.
– Lithogenous
– Biogenous
– Hydrogenous / Chemical precipitates
– Cosmogenous
Types of Marine Sediments
• Lithogenous Sediments
– Sediments from terrestrial (land) sources
– Includes
• Sands and muds from continental margins
• Glacial deposits
• Clays
Types of Marine Sediments
• Lithogenous Sediments
– Red Clay
• Found in low productivity areas
• Low sedimentation rates
• Wind-blown sediment
Types of Marine Sediments
• Biogenous Sediments
– particles derived from hard parts and soft
tissues of organisms
– Ooze = greater than 30% biogenous sediment
– Distribution related to:
• sediment supply
• rate of dissolution
• and sediment dilution
Biogenous Sediment
Accumulation
Thurman Essentials of Oceanography 6/e fig 4.15
Types of Marine Sediments
• Biogenous 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
Radiolaria
• Composed of SiO2
• Zooplankton
• Base of food chain
Types of Marine Sediments
• Biogenous 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
Foraminifera
• Composed of calcium carbonate (CaCO3)
• Zooplankton
Types of Marine Sediments
• Hydrogenous sediments
– Minerals that crystallize directly from seawater
– Most common types include
• Manganese nodules
• Calcium carbonates
• Metal sulfides
• Evaporites
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
Types of Marine Sediments
• Hydrogenous sediments
– Ferromanganese Nodules
• First 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
Sulfide
Deposits
Types of Marine Sediments
• Hydrogenous sediments
– Evaporites
• Evaporation triggers
deposition of chemical
precipitates
• rock salt
• rock gypsum
Formation of
Evaporites
Astronaut photo of the southwestern edge
of the Zagros Mountains featuring salt
domes (the white section in the middle and
the bump on the left).
Source http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17245
Salt Ponds, S. San Francisco
Bay
(Earth Sciences and Image Analysis Laboratory at Johnson Space Center )
Death Valley,
California
• Once a deep
Pleistocene Lake
(100 miles long
600 feet deep)
• Change in
climate at end
of ice age
NASA images created by Jesse Allen, Earth
Observatory, using data obtained courtesy
of the MODIS Rapid Response team and the
Goddard Earth Sciences DAAC.
Wieliczka Salt Mine, Poland
Great cathedral, a large chamber carved
entirely within salt, including floor,
walls, ceiling, and decorations.
Chandeliers are made with salt crystals.
Photo date 9/97; © by J.S. Aber
http://academic.emporia.edu/aberjame/ice/lec14/wielicz.htm
Closeup view of the cathedral's altar.
Wieliczka Salt Mine
Virtual Tour
St. Anthony’s Chapel, built between 1690-1710
St. John Chapel built ~1859
Types of Marine Sediments
• Organic Matter
• Oxidized Sediments
– Slow burial, organic
matter will oxidize and
produce water +
carbon dioxide
• Find burrows = life =
oxygen
• Oxidized mud = olive
greenish gray
• Unoxidized Sediments
– Rapid burial, little time
for oxidation
• Unoxidized mud =
black
Types of Marine Sediments
• Organic Matter
– Weight of overlying sediment plus increase
temperature - cook organic matter = petroleum
• Usually if black have lots of organic matter
Sediment distribution
• Sand
– Closer to shore, on margins of continents
– Brought to margins by by rivers
– Sand requires high energy to transport
• Waves rework sand along coast to form our beaches
– Some sand makes it to deep ocean through
submarine canyons
Sediment distribution
• Clays & Silts
– Finer sediments are suspended in water
– Carried by the currents until they settle out of
the water
– Found in deep ocean, low energy environments
Sediment distribution
• Where are sediments the thickest?
– Thickest in trenches—Accumulations may
approach 10 kilometers
– Basins
• Pacific Ocean—About 600 meters or less
• Atlantic Ocean—From 500 to 1000 meters thick
– Thinnest along spreading centers
• Mud is the most common sediment on the
deep-ocean floor
Sediment distribution
• Types of deep-ocean basin sediments
– Turbidites – deposits made by turbidity
currents
– Oozes – deep-ocean sediment containing at
least 30% biogenous material
– Hydrogenous sediments - originate from
chemical reactions that occur in the existing
sediment
• Evaporites - salts that precipitate as evaporation
occurs
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
Sediment distribution
• Rates Of Deposition
– Deep ocean 0.5-1.0cm/1000 yrs
– slow rates yet can get thick because oldest crust
is 200 m.y.
– clays take 100 yrs to sink 3000 m
Studying Sediments
• How do scientists study sediments?
– Deep-water cameras
– Clamshell samplers
– Dredges
– Piston Corers
– Core libraries
– Seismic profilers
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
Studying Sediments
Dredges
Rock samples being removed
from a small rock dredge.
Scientists inspect a just recovered
dredge haul (below), which contains
samples of volcanic basalt (above left)
and a seafloor crust rich in manganese
(above right).
Studying Sediments
One method of studying sediments uses a clamshell sampler. The
sampler can be used to obtain a relatively undisturbed sediment sample.
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
Van – Veen Mud Grab
Studying Sediments
• Grab Sampler
– Good for collecting
soft sediment, sand or
perhaps gravel.
– Low tech - basically
just a weighted cage
that is dragged along
the sea floor.
Studying Sediments
• Piston Corer
– Allows a cylinder of
sediment to be taken for
analysis to determine the age
of the material, as well as the
density, strength, molecular
composition and radioactivity
of the sediment.
– Used by research vessels
such as the JOIDES
Resolution
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
Piston Corer
Drilling
Ocean
Cores
Corer
• This figure shows an
examination of deepocean sediment cores.
• Long cylinders of
sediment and rock
called cores are cut in
half and examined,
revealing interesting
aspects of Earth
history. APT photo
Studying Sediments
• What can scientists learn by studying
sediments?
– Historical information
– Location of natural resources, especially crude
oil and natural gas
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
Studying Sediments
Based on data
from core
samples,
scientists have
determined the
age of portions of
the Pacific floor,
measured in
mega-annums, or
millions of years.
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
Seabed Resources
• Depletion of onshore resources - need
alternatives
• Sand & Gravel
• Phosphorite
• Sulfur
• Coal
• Oil and Gas
How Much Do We Need?
Hydrocarbon Seeps
Gas Hydrate
• Formed from a mixture of water and natural
gas, usually methane.
• Occurs in the pore spaces of sediments
~ End ~