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Ocean Sediments


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Definition ocean Sediments

Ocean sediments are unconsolidated organic and

inorganic particles that accumulate on the ocean floor.

Ocean sediments originate from numerous sources

weathering and erosion of the continents(terrigenous/lithogenous)

volcanic eruptions (volcanogenous) included in

terrigenous sediments.

biological activity (biogenous) chemical processes within the oceanic crust and

seawater (Hydrogenous/autigenous)

impacts of extra-terrestrial objects (cosmogenous)


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Sediments

Existing rocks

Chemical

weatheringPhysical

Weathering

solution solid

Transport by water, ice and winds

Volcano Cosmogen

depositionprecipitation

Chemical Biogenous piroclastsTerrigenous Cosmogen

Evaporites

Anhydrite

Mn nodules

Limestone

Chert

Conglomerate

Sand

Silt

Clay

Tuffs

Tefra

pyroclasts

Cosmogen

ous dust

Tektites

spherules


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Deep-sea Sedimentation has two main sources ofsediment: external- terrigenous material from the land

and internal-biogenic and authigenic from the sea.

4-2Sedimentation in the Ocean

Sedimentation in the Deep Sea


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Grain size and current velocity affect the

deposition and erosion of sediment.

12 - 5

Smallest and largest particlesbehave similarly with respect to

transportation and erosion.

Sand in the middle of the graph

takes the least amount of energy to

erode. Larger particles require more

energy to erode because theyre

heavy. It takes a stronger current to

lift them off the bottom.

Particles smaller than sand alsotake more energy to erode. Smaller

particles (especially clay) tend to be

cohesive.

Hjulstroms diagram


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Classification

1. Clasification by origin a. Terrigenous - erosional products (also volcanics)

composed of fragments of pre-existing rock material

b. Biogenous - composed of hard remains of once-

living organisms. shells c. Hydrogenous - formed when dissolved materials

come out of solution (precipitate) (in situ

precipitation). Desolved materials form as a result

weathering

d. Cosmogenous - extraterrestrial (derived from outer

space)


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% of all oceanSediment type floor covered

Terrigenous 45%

Biogenous 55%

Hydrogenous


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Classified by size according to theWentworth scale

2. Clasification based on size

a. Gravel (pebbles, cobbles) = > 2mm

b. Sand = 62 m - 2 mm

c. Silt = 4 - 62 m

d. Clay = < 4 m

Grain sizes are classified by using formula:

= -log2d

phi is Wentworth scale

d = diametre of the grains


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Sediment Size Wentworth scale


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1. By Constituents

a. Pelagic sediments - open ocean, fine grained

clays & biogenic oozes

b. Hemipelagic - continental margin, coarser grained muds


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Major Sediment Input to the Oceans

Source Amount (109 tons/yr)

Rivers 18.3

Glaciers and ice sheets 2.0

Wind blown dust 0.6

Coastal erosion 0.25

Volcanic debris 0.15

Groundwater


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Terrigenous (or Lithogenous

Sediments):

Derived from weathering of rocks at orabove sea level (e.g., continents, islands)

Two distinct chemical compositions

ferromagnesian, or iron-magnesium

bearing minerals

non-ferromagnesian minerals e.g.,

quartz, feldspar, micas

Largest deposits on continental margins

(less than 40% reach abyssal plains)

Transported by water, wind, gravity, andice

Transported as dissolved and

suspended loads in rivers, waves,

longshore currents


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Sedimentation Processes on the

Continental Shelf

Tides, waves, and currents strongly affect

continental-shelf sedimentation.

Shoreline turbulence: waves are one of the most

notable influences because it keeps particles from

settling. Surf and waves carry small particles out to

sea. Their affect diminishes further from shore.


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Sediments are alsotransported to the open-ocean by gravity-driventurbidity currents.

Dense 'slurries' ofsuspended sedimentmoved as turbulentunderflows

Typically initiated by storm

activity or earthquakes Initial flow often confined

to submarine canyons ofthe continental shelf andslope

Form deep-sea fanswhere the mouth of thecanyon opens onto thecontinental rise


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River input of silt to ocean


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Sediment delivered to

the open-ocean by

wind activity as

particulate matter(dust)

Primary dust source is

deserts in Asia and

North Africa Comprise much of the

fine-grained deposits in

remote open-ocean

areas (red clays) Volcanic eruptions

contribute ash to the

atmosphere which

settles within the

oceans

Pinatubo

June

1991

Wind

Blown

SandWest

Africa


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Boulder to clay size particles

also eroded and transported

to oceans via glacial ice

Glacier termination in circum-

polar oceans results in calving

and iceberg formation

As ice (or icebergs) melt,

entrained material is

deposited on the ocean floor

Termed 'ice-rafted' debris or

diamictites.


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Pelagic lithogenous sediments

Sources of fine material:

Volcanic ash (volcanic

eruptions)

Wind-blown dust

Fine grained materialtransported by deep

ocean currents

- Abyssal clay (red clay)

Oxidized iron


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Composition of Red Clay

Clay minerals: montmorillonite, illite,

chlorite, kalonite, and mixed-layer

derivatives Lithogenous minerals: feldspar, pyroxene,

quartz

Hydrogenous (or authigenic) minerals:

zeolite and ferromanganese oxides andhydroxides.


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Fig.8.8 Clay mineral distribution on the ocean floor. The map shows the dominant mineral in thefraction less than 2. Mixture indicates that no one clay mineral exceeds 50% of the total.

Distribution of Clay Minerals

The clay mineral which are most abundant in deep sea clay

are montmorillonite and illite


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Hemipelagic Sediments

Characteristic of the

continental slope & rise Muds carried across

shelf by wave & tide

energy as slightly dense

plumes extend out from slope

at depth where denser

water is encountered

Relatively fastsedimentation rate

Hemipelagic mud is

generally gray or green

from the presence of

sulfides or magnetite


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Biogenous Sediments: composed primarily of marine microfossil remains

shells of one-celled plants and animals, skeletal fragments

median grain size typically less than 0.005 mm (i.e., silt orclay size particles)

characterized as CaCO3 (calcium carbonate) or SiO2 (silica)dominated systems

sediment with biogenic component less than 30% termedcalcareous, siliceous clay

calcareous or siliceous 'oozes' if biogenic component greater

than 30%


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Oozes consist of biogenous

minterals: shells of planktonic

foraminifera, radiolarians,coccolithophores, and diatoms.

About one half of the deep seafloor is covered by oozes.

The most important factorscontrolling the composition of

biogenous deep sea sediments

are fertility and depth.

Fertility controls the supply ofplankton remains, while depth

controls the dissolution of

carbonate (through pressure

and water mass chemistry).

planktonic

foraminifera

radiolarians

coccolithophores

diatoms


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Controlling Factors

Fig.8.4 Distribution of major facies in a depth-fertility frame, based on sediment patterns in theeastern central Pacific. Numbers are typical sedimentation rates in mm/1000 yr(which is thesame as m/million yr). [Source as for Fig.8.2]


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Distribution of calcareous material


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Calcareous oozes

Consist of foraminifera, coccolithophores and pteropodswhich cover ~50% of the ocean floor

distribution controlled largely by dissolution processes

cold, deep waters are undersaturated with respect to

CaCO3 deep water is slightly acidic as a result of elevated

CO2 concentrations

solubility of CaCO3 also increases in colder water

and at greater pressures CaCO3 therefore readily dissolved at depth

level below which no CaCO3 is preserved is thecarbonate compensation depth

typically occurs at a depth of 3000 to 4000 m


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Calcium carbonate dissolves better in colder water, in acidic

water, and at higher pressures. In the deep ocean, all three

of these conditions exist. Therefore, the dissolution rate of

calcium carbonate increases greatly below the thermocline.This change in dissolution rate is called the lysocline.

Below the lysocline, more and more calcium carbonate

dissolves, until eventually, there is none left. The depth

below which all calcium carbonate is dissolved is called thecarbonate compensation depth or CCD.


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calcareous ooze


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Patterson (1542) showed adrastic increase of dissolutionrates below 3500 m in the centralPacific.


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Dissolution patterns in the deep sea

The CCD is the particulardepth level at any one

place in the ocean where

the rate of supply of

calcium carbonate to the

sea floor is balanced by

the rate of dissolution, so

that there is no net

accumulation of carbonate.

ACD (Aragonite

Compensation Depth)

CCD (Calcite

Compensation Depth)

Generalized diagrams illustrating the

relative position of calcite and aragonitesolubility profiles in the modern tropicalocean and the variation in temperaturewith depth. The major zones ofdigenesis are plotted to the right.

Figure 5-17


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Calcium Carbonate in the ocean

Figure 5 17

L li


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Lysocline Another CCD-like level

which can be mapped to

describe dissolutionpatterns is the lysocline.

The concept of thelysocline was introducedto denote a contour-

following boundary zonebetween well-preservedand poorly-preservedforaminiferalassemblages on the floorof the central AtlanticOcean and on that of theSouth Pacific.

The lysocline marks the

top of the AntarcticBottom Water.

White Cliffs of Dover


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White Cliffs of Dover

http://en.wikipedia.org/wiki/White_cliffs_of_Dover

composed largely of foraminifera and

coccolithophores

Formation of

calcareous

deposits

Carbonate Shelves
http://upload.wikimedia.org/wikipedia/commons/7/7e/White_cliffs_of_dover_09_2004.jpghttp://upload.wikimedia.org/wikipedia/commons/7/7e/White_cliffs_of_dover_09_2004.jpghttp://upload.wikimedia.org/wikipedia/commons/7/7e/White_cliffs_of_dover_09_2004.jpghttp://upload.wikimedia.org/wikipedia/commons/7/7e/White_cliffs_of_dover_09_2004.jpghttp://upload.wikimedia.org/wikipedia/commons/2/2a/White_Cliffs_of_Dover_map.png


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Carbonate sediments and reefs form in warm shallow

water regions where the influx of terrigenous materials is

low.

Carbonate Shelves


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Plate stratigraphy

Developed at the mid-oceanic ridge

The axial rift valley is flank with hosts which covered by biogenic sediments

As the spreading continues the hosts subsides below the CCD the biogenicsediments are overlain by pelagic red clay.

The stratigraphy of the plate consists of Basalt at the bottom, and is overain by

biogenic sediments and finally red clay.


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Siliceous Ooze

Distribution, production, and

dissolution patterns of the siliceous

deposits

Remains of diatoms, silicoflagellatesand radiolarians, and sponge spicules,

all of which are made of opal, a

hydrated form of amorphous silicon

oxide. Diatom oozes are typical for high

latitudes, diatom muds for

pericontinental regions, and

radiolarian oozes for equatorial areas.


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Fig.8.15 Flux of siliceous fossil to the sea floor.[W. H. Berger, J. C. Herguera, in P.G. Falkowski. A. D. Woodhead. eds, 1992, Primary productivity and biogeochemicalcycles in the sea. Plenum Press, New York]

The siliceous deposits typically occur in areas of

high fertility; that is, in regions of surface water

with relatively high phosphate values.

Siliceous ooze


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Siliceous ooze

Seawater undersaturated with silica

Siliceous ooze commonly associated with high biologic

productivity in surface ocean

Di t ib ti f iti d l i i


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Distribution of neritic and pelagic marine

sediments


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Silica content in the ocean


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Controlling factors

The formation of siliceous rocks is controlled by

the rate of production of siliceous organisms inthe overlying waters

the degree of dilution by terrigenous, volcanic,and calcareous particles

the extent of dissolution of the siliceous skeletons


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There is a distinct negative correlation between silicaand calcite distributional patters.

increasing fertility leads to decreasing preservation of

calcite, but increasing accumulation of silica. A similarly opposing trend is indicated for depth

relationships, with silica corrosion being greatest inupper waters, that of carbonate being greatest at depth


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Deep sea cherts

Silicified sediments cemented by cryptocrystalline and

microcrystalline quartz

appears to proceed from mobilization and reprecipitation

of opal, generating a disordered cristobalite (=fibrousquartz) which eventually alters toward a quartzitic rock

with mostly quartz-replaced and quartz-filled fossils as

diagenesis progresses.


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Hydrogenous (or Authigenic) Sediments

produced by chemical processes in seawater

essentially solid chemical precipitates of several

common forms

Non-biogenous carbonates form in surface waters supersaturated with calcium

carbonate

common forms include short aragonite crystals and

oolites


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Phosphorites

phosphate crusts (containing greater than 30% P2O5)

occurring as nodules

formed as large quantities of organic phosphorous

settle to the ocean floor unoxidized material is transformed to phosphorite

deposits

found on continental shelf and upper slope in regions

of high productivity


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Manganese nodules

surficial deposits of

manganese, iron,

copper, cobalt, and

nickel

accumulate only in

areas of low

sedimentation rate

(e.g., the Pacific) develop extremely

slowly (1 to 10

mm/million years)

i (' l '


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evaporites ('salt'

deposits')

occur in regions of

enhanced evaporation

(e.g., Isolated seas, Red

Sea. Persian Gulf and

Dead Sea)

evaporative process

removes water and

leaves a salty brine

Consist of gypsum,

anhidrite, halite.

Dead SeaJordan,


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The term evaporites is used for all deposits, such as salt

deposits, mainly chemical sediments that are composed

of minerals that precipitated from saline solutions

concentrated by evaporation. Evaporite deposits are

composed dominantly of varying proportions of halite(rock salt) (NaCl), anhydrite (CaSO4) and gypsum

(CaSO4.2H2O). Evaporites may be classified as

chlorides, sulfates or carbonates on the basis of their

chemical composition (Tucker, 1991).


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C S di t


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Cosmogenous Sediments:

sediments derived from

extraterrestrial materials includes micrometeorites and

tektites

tektites result from collisions

with extraterrestrial materials fragments of earth's crust

melt and spray outward from

impact crater

crustal material re-melts as itfalls back through the

atmosphere

forms 'glassy' tektites

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