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OceanographyOceanographyAn Invitation to Marine ScienceAn Invitation to Marine Science

66thth Edition Editionbyby

Tom GarrisonTom Garrison

http://oceanography.brookscole.com/garrison6e



Chapter 5 Sediments

© 2006 Brooks/Cole, a division of Thomson Learning, Inc.

Look For The Following Key Ideas Look For The Following Key Ideas In Chapter 5In Chapter 5

Sediment is particles of organic or inorganic matter that accumulate in a loose, unconsolidated form. Sediment may be classified by grain size of by the origin of the majority of the particles.

Marine sediments are broadly classified by origin into four categories. Terrigenous sediments are of geological origin and arise on the continents or islands near them; they are the most abundant. Biogenous sediments are of biological origin. Hydrogenous sediments are formed directly from seawater. Of less importance are cosmogenous sediments, which come from space.

Though there are exceptions, the sediments of continental margins tend to be mostly terrigenous, whereas the generally finer sediments of the deep-ocean floor contain a larger proportion of biogenous material.

Deep sea oozes-forms of biogenous sediment-contain the remains of some of the ocean's most abundant and important organisms.

Sediment deposited on a quiet seabed can provide a sequential record of events in the water column above. In a sense sediments act as the recent memory of the ocean. The memory does not extend past about 200 million years because seabeds are relatively young and recycled into Earth at subduction zones.


Sediments May Be Classified By Sediments May Be Classified By Particle SizeParticle Size

Sediment is particles of organic or inorganic matter that accumulate in a loose, unconsolidated form.

Sediment can be classified by particle size. Waves and currents generally transport smaller particles farther than larger particles.


Sediments May Be Classified By Sediments May Be Classified By Particle SizeParticle Size

The velocities of currents required for erosion, transportation, and deposition (sedimentation) of sediment particles of different sizes.

To dislodge and carry a particle of size A, the speed of a current must exceed 20 centimeters per second (8 inches per second). When the current falls below 1 centimeter per second (1/2 inch per second), the particle will be deposited.


Sediments May Be Classified by Sediments May Be Classified by SourceSource


Terrigenous Sediments Come from Terrigenous Sediments Come from LandLand

The Sediment Cycle.

Over geological time, mountains rise as lithospheric (crustal) plates collide, fuse, and subduct. Water and wind erode the mountains and transport resulting sediment to the sea. The sediments are deposited on the seafloor, where they travel with the plate and are either uplifted or subducted. Thus, the material is eventually made into mountains again.


Marine Sediments Are Usually Combinations Marine Sediments Are Usually Combinations of Terrigenous and Biogenous Depositsof Terrigenous and Biogenous Deposits

The sediment of continental shelves is called neritic sediment, and contains mostly terrigenous material.

Sediments of the slope, rise, and deep-ocean floors are pelagic sediments, and contain a greater proportion of biogenous material.

Insert Table 5.3



Marine Sediments Are Usually Marine Sediments Are Usually Combinations of Terrigenous and Combinations of Terrigenous and

Biogenous DepositsBiogenous Deposits

(above) The general pattern of sediments on the ocean floor. Note the dominance of diatom oozes at high latitudes.

What differences in the type and distribution of sediments do you note between the Atlantic Ocean and the Pacific Ocean?



Pelagic Sediments Vary in Pelagic Sediments Vary in Composition and ThicknessComposition and Thickness

How can the sediments of deep-ocean basins be classified?

• Turbidites – deposits made by turbidity currents

• Oozes – sediment containing at least 30% biogenous material• Siliceous ooze - is formed by organisms that contain silica in their shells.

Diatoms are one type of organism whose remains contribute to siliceous ooze.

• Calcareous ooze - is formed by organisms, such as foraminifera, which contain calcium carbonate in their shells or skeletons.

• Hydrogenous sediments - originate from chemical reactions that occur in the existing sediment. Hydrogenous sediments are often found in the form of nodules containing manganese and iron oxides

• Evaporites - salts that precipitate as evaporation occurs. Evaporites include many salts with economic importance. Evaporites currently form in the Gulf of California, the Red Sea, and the Persian Gulf



Turbidites Are Deposited on the Turbidites Are Deposited on the Seabed by Turbidity CurrentsSeabed by Turbidity Currents

The formation of turbidites.

The turbidity current moves quickly down the continental shelf and slope, sometimes encountering (and further eroding) a submarine canyon. When the material comes to rest, it sorts into layers with coarse sediment at the bottom and finer sediment above. Each graded layer is the result of one turbidity current event.

Fig. 5-12, p. 139Stepped Art

Key Turbidity currents

Submarine canyon

Deep-sea fans

Distance from shelf edge50 150 km

miles0 50 100Continental

slopeContinental

riseAbyssal

plain

Con

tinen

tal s

helf

Sediment slump masses

Graded beds of turbidites

Underlying basaltic crust

100

Storm winds Sea level

Wave base

Substrate liquefied by wave activity

Turbidity current



Oozes Form from the Rigid Oozes Form from the Rigid Remains of Living CreaturesRemains of Living Creatures

The dashed line shows the calcium carbonate (CaCO3) compensation depth (CCD). At this depth, usually about 4,500 meters (14,800 feet), the rate at which calcareous sediments accumulate equals the rate at which those sediments dissolve.


Scientists Use Sensitive Tools Scientists Use Sensitive Tools to Study Ocean Sedimentsto Study Ocean Sediments

How do scientists study sediments?

Deep-water cameras Clamshell samplers Piston Corers Core libraries Seismic profilers




One method of studying sediments uses a clamshell sampler. The sampler can be used to obtain a relatively undisturbed sediment sample.



One method of obtaining core samples by research vessels such as the JOIDES Resolution is by using a piston corer.

The 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.


Sediments Are Historical Records Sediments Are Historical Records of Ocean Processesof Ocean Processes

What can scientists learn by studying sediments?

• Historical information• Location of natural resources, especially crude oil and natural

gas

Marine sediments are important as historical records and a site of natural resources. Scientists study marine sediments using many different methods, including core samples.


Sediments Are Historical Records Sediments Are Historical Records of Ocean Processesof Ocean Processes

The ages of portions of the Pacific Ocean floor, based on core samples of sediments just above the basalt seabed, in millions of years ago (Ma, mega-annum). The youngest sediments are found near the East Pacific Rise; and the oldest, close to the eastern side of the trenches.


Chapter 5 SummaryChapter 5 Summary

In this chapter you learned that the sediemnts covering nearly all of the seafloor are parts of the great cycles of formation and destruction assured by Earth’s hot interior. Marine sediment is composed of particles from land, from biological activity in the ocean, from chemical processes within water, and even from space. The blanket of marine sediment is thickest at the continental margins and thinnest over the active oceanic ridges.

Sediments may be classified by particle size, source, location, or color. Terrigenous sediments, the most abundant, originate on continents or islands. Biogenous sediments are composed of the remains of once-living organisms. Hydrogenous sediments are precipitated directly from seawater. Cosmogenous sediments, the ocean's rarest, come to the seabed from space.

The position and nature of sediments provide important clues to Earth's recent history, and valuable resources can sometimes be recovered from them.

In the next chapter you will learn about our story’s main character – water itself. You know something about how water molecules were formed early in the history of the universe, something about the inner workings of our planet, and something about the nature of the ocean’s “container.” Now lets fill that container with water and see what happens.

End of Chapter 5

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