Ocean Sediments

coriandercultureMécanique

22 févr. 2014 (il y a 3 années et 4 mois)

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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


<4 μm


Silt


4
-
62 μm


Sand


62
-
2000 μm


Gravel


>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

www.naturalphotos.com

Wind & Rain erosion

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


Winter


Summer

Aerial

dust

transport

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 (CaCO
3
)


Siliceous skeletons (SiO
2
)



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 C0
2
)


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 CO
2
conc.


Old

Deep Waters have high CO
2

conc.


Animal respiration


Decomposer activities


Pacific Deep Waters are older than
Atlantic Deep Waters

Global Thermohaline Circulation

Carbonate Compensation Depth

& Greenhouse Effect?


CO
2

atmosphere, seawater & sediments are interrelated!


Will increase in atmospheric CO
2

cause increase in dissolved
seawater CO
2
?


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
-

CaCO
3

precipitates at:


High Temperature


Low Pressure


High pH (low CO
2
)


Caused by high productivity
-

photosynthesis

Bahamian Bank

Carbonate Sediments

Figure 3.23

Chicxulub crater

End