Ch. 5 Marine Sediments Lecture Notes Page - Cccd

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22 févr. 2014 (il y a 3 années et 8 mois)

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LECTURE NOTES:


OCEANOGRAPHY (MARSC 100),


SNYDER, L.


CH. 5: Marine Sediments


Marine Sediments:
Loose organic or inorganic particulate material that accumulates,
unconsolidated, on the seafloor.


Why study Sediments?

1. Ocean’s & Earth’s “recent” hi
story

Crust constantly recycled ~ 180 million years

2. Catastrophic events (asteroid 65 mya)

3. Past, present & future

Global Climate

Ocean/Atmosphere Conditions

4. Ocean resources

Oil & natural gas

Sand & lime (cement)


What Creates Sediments?

Weathe
ring: Breakdown of rocks by chemical or mechanical processes

Erosion: Transport of loose weathered material from land to sea

Biologic remains

Volcanism

Chemical processes (water)

Space debris


Classifying Sediments

1.

Particle Size:

large grains deposite
d closest to shore, small grains (dust) carried by
currents & wind to ocean basins



Settling Time


2.

Sorting:

Organized by grain size



Well
-
sorted


sediment composed of same size particles



Poorly sorted


sediment made up of particles of various sizes


3.

Source:

Origin (4 categories)


a.

Terrigenous Sediments:

Inorganic minerals (Granite, quartz, clay)



Most abundant



Volcanic eruption, land erosion, blown dust (EX: Dust from Gobi Desert: Asia to
Pacific)



Land Erosion:



Rivers: main source of terrigenous
sed.

1. Mountains rise via plate convergence & subduction

2. Wind & rain erode mountains

3. River to sea

4. Sediment uplifts or subducts: made into mountain again

Humans alter the sediment cycle (Ex. Dams, levees, destruction of riparian habitat
& wetland
s). Results in flooding, severe damage from storms (Hurricanes,
Tsunami, etc.)


b.

Biogenous Sediments:

Remains of living organisms (fossilize)



Hard structures (Shells, exoskeleton)



Oozes (30% biogenous)

o

Siliceous (Silicon): EX. Diatoms (phytoplankton)

o

Calcareous (Calcium
-
carbonate): Ex. Foraminifera (zooplankton),
Coccolithophore (Phytoplankton)




Calcareous Ooze:

Deposits of Calcium
-
containing shells, phytoplankton



Not found below 4,500 m (14,800 ft.)



Calcium carbonate compensation depth (CCCD):

Calcium carbonate dissolves in
seawater below 4,500 m: as pressure increases & temperature decreases.




Cold, deep water is acidic & High in CO2




Siliceous Ooze:

Deposits of Silica
-
containing organisms



Common at greater depth & colder seas



Diato
m ooze


Antarctica (cold, nutrient
-
rich, upwelling)



Radiolarian ooze

-

Equatorial (Radiolarian = amoeba
-
like protozoans)



c.
Hydrogenous Sediments:

Dissolved minerals precipitated from seawater (fall out
of solution)



Mineral sources: rock, sediment,

hydrothermal vents, land erosion

o

Evaporites

-

form in isolated seas with high evaporation [Ex. Gypsum
(CaSO
4
), Salt (NaCl)]

o

Authigenic



formed in place



Manganese Nodules: form via slow chemical reactions (bacteria
may aid)



Grow 1
-
10 mm (0.04
-
0.4”) / mil
lion years!



Compositon: Manganese & iron (50%), cobalt, nickel, copper,
often have shark tooth, algae, nuclei



Form at depths:



Pacific seafloor 30
-
50 % cover of Manganese nodules



d.
Cosmongeous Sediments:

Interplanetary dust: debris (comets, collisions
) falls into
Earth’s atmosphere (constant)



Most dissolves in seawater



Minor sediment in ocean



Impacts by large asteroids & comets: Crust thrown into space, melts, falls
back to Earth


forms microtektites (Glass
-
like, 0.2
-
0.8mm)


Classifying Sediments (c
ont):


4. Distribution:

3 Categories


a.

Neritic sediments: Continental shelf



Terrigenous, Current or wave deposited , Coarse grain, Rapid accumulation



b.

Hemi
-
pelagic sediments: outer shelf & slopes



c.

Pelagic sediments: deep sea



Mixed, but greater
proportion is Biogenous, Deposited suspension, Fine
grained (finest on floor), Slow accumulation


Terrigenous sediment: Not carried far out to sea

Result: Pelagic sediment dominated by biogenous material