Ocean Sediments
Importance of Sediments
• Economic Value
– Oil, fossil fuels
– Salt & Phosphorus deposits
• Determine shape & structure of Ocean
bottom
• Strongly affect distribution of Benthic
Organisms
• Chronological record of Earth’s history
– Tectonic history
– Climate history
– Evolutionary history
Sediment Thickness
Topographic profiles
Law of Superposition
Younger sediments over Old sediments
YOUNG
---------------------OLD
Sediment Classification
• By Grain Size
• By Origin
Sediment Classification
• Grain Size
–
–
–
–
Clay
Silt
Sand
Gravel
<4 μm
4-62 μm
62-2000 μm
>2000 μm
Table 3.1
Basic Sediment Transport
(READ CC4)
Sediment Sorting
Well-sorted sediments are those of
similar size class
– Beach: well sorted (far from source)
– Glacier: not sorted (close to source)
Sediment Angularity
Sediment weathering during transport
induces loss in angularity
– Angular grains (close to source)
– Rounded grains (far from source)
Sediment Classification
• Origin
–
–
–
–
Lithogenous or Terrigenous (~75%)
Biogenous
(~20%)
Hydrogenous
Cosmogenous
Lithogenous Sediments
• Fragments of rocks broken, weathered and
eroded form lithogenous sediments
Frost Wedging
http://images.google.com
Wind & Rain erosion
www.naturalphotos.com
Lithogenous Sediments
• Transport of sediments by:
–
–
–
–
–
–
Rivers
Glaciers
Waves
Wind
Landslides
Humans
www.southalabama.edu
http://earthobservatory.nasa.gov
Sediment Discharge by Rivers
• Ganges:
1700 million Tm/year
• Amazon:
900 million Tm/year
• Mississippi 260 million Tm/year
(Figure 6-2)
•http://www.pbs.org/harriman/images/
•http://www.pbs.org/harriman/images/
walrus.wr.usgs.gov/elnino/coastal/ images/
http://earthobservatory.nasa.gov
Aerial
dust
transport
•Winter
•Summer
St Helens
•http://geohazards.cr.usgs.gov/
•http://web.umr.edu/~rogersda
•http://www.hihwnms.nos.noaa.gov/graphics/
Biogenous Sediments
• Composed of planktonic organism remains
– Calcareous skeletons (CaCO3)
– Siliceous skeletons (SiO2)
• Accumulation rate controlled by:
– Primary productivity
– Rate of dissolution
(Importance of fecal pellets)
Figure 3.21a
Diatoms (siliceous high latitudes)
Coccololithospheres (calcareous – mid latitides)
Figure 3.21b
Radiolarians (siliceous – low latitudes)
Foraminifera (calcareous – all latitides)
Pteropods (calcareous – all latitudes)
•http://www.mbari.org/expeditions/
Dissolution Biogenous Particles
• Silica
– Ocean is UNDERSATURATED with silica
– Dissolution highest in surface waters
• Low Pressure
• High Temperature
Accumulation in sediments occurs in:
- Areas of very high productivity
- Poles and upwelling zones (diatoms)
- Tropics (Radiolarians)
Dissolution Biogenous Particles
• Carbonates
• Foraminifera (Calcite) – less soluble
• Pteropods (Aragonite) – More soluble
– Dissolution is highest in Deep Waters
• High pressure
• Low temperatures
• Low pH (high C02)
Carbonate Compensation Depth (CCD)
Carbonate Compensation Depth
•CCD varies with Latitude
•CCD varies between Oceans
•North Pacific:
1000m
•South Pacific:
2500m
•Atlantic:
4000m
Carbonate Compensation Depth
• New Deep Waters have low CO2 conc.
• Old Deep Waters have high CO2 conc.
– Animal respiration
– Decomposer activities
Pacific Deep Waters are older than
Atlantic Deep Waters
Global Thermohaline Circulation
Carbonate Compensation Depth
& Greenhouse Effect?
• CO2 atmosphere, seawater & sediments are interrelated!
• Will increase in atmospheric CO2 cause increase in dissolved
seawater CO2?
• Consequences of a shallow CCD?
• Release into atmosphere of dissolved carbonate sediments?
Hydrogenous Sediments
• Lower concentrations than Lithogenous and
Biogenous sediments
• Ocean water usually is UNDERSATURATED, but..
– Hydrothermal Vent Minerals (metal rich sedim.)
– Manganese Nodules (areas of low sedimentation)
– Carbonate banks - CaCO3 precipitates at:
• High Temperature
• Low Pressure
• High pH (low CO2)
– Caused by high productivity - photosynthesis
Bahamian Bank
Carbonate Sediments
Figure 3.23
Chicxulub crater
End