Chapter 5Sediments

OceanographyOceanographyAn Invitation to Marine Science, 7thAn Invitation to Marine Science, 7th

Tom GarrisonTom Garrison

Chapter 5 Study Plan

• Sediments Vary Greatly in Appearance• Sediments May Be Classified by Particle Size• Sediments May Be Classified by Source• Neritic Sediments Overlie Continental Margins• Pelagic Sediments Vary in Composition and

Thickness• Scientists Use Sensitive Tools to Study Ocean

Sediments• Sediments Are Historical Records of Ocean

Processes• Marine Sediments Are Economically Important

Chapter 5 Main Concepts

• Sediments are loose accumulations of particulate material. Their depth and composition tell us of relatively recent events in the ocean basin above.

• The most abundant sediments are terrigenous (from land) and biogenous (from once-living things). The volume of terrigenous sediment exceeds that of biogenous sediment, but biogenous material covers a greater area of seabed.

• Marine sediments have been uplifted and exposed on land. Arizona’s Grand Canyon is made of marine sediment.

• Because marine sediments are usually subducted along with the seabed on which they lie, the oldest sediments are relatively young—rarely older than 180 million years.

Sediments May Be Classified By Particle Size

Sediments are particles of organic or inorganic matter that accumulate in loose, unconsolidated forms.

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

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 Particle Size

Sediments May Be Classified by Source

Terrigenous Sediments Come from Land

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 Both 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

(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?

Marine Sediments Are Usually Both Terrigenous and Biogenous Deposits

Pelagic Sediments Vary in Composition and Thickness

• Turbidites – deposits made by turbidity currents• Oozes – sediment containing at least 30% biogenous material

– Siliceous oozes are formed by organisms that contain silica in their shells. Diatoms are one type of organism whose remains contribute to siliceous ooze.

– Calcareous oozes are formed by organisms, such as foraminifera, which contain calcium carbonate in their shells.

• Hydrogenous sediments - originate from chemical reactions that occur in the existing sediment.– Are often found in the form of nodules containing manganese

and iron oxides– Evaporites are salts that precipitate as evaporation occurs and

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 Seabed 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.

Oozes Form from the Rigid Remains 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 to 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.

Scientists Use Sensitive Tools to Study Ocean Sediments

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.

Scientists Use Sensitive Tools to Study Ocean Sediments

• 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 of Ocean Processes

Sediments Are Historical Records of 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 in Perspective

In this chapter you learned that the sediments 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 seafloor 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 let’s fill that container with water and see what happens.