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Chapter 3 The Sea Floor and Its Sediments. Look For The Following Key Ideas In Chapter 3. 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.
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Look For The Following Key Ideas In Chapter 3 • 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 Particle 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 Source Sediment can also be classified according to its source.
Terrigenous Sediments Come from Land A simplified 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. The material is eventually made into mountains again.
Marine Sediments Are Usually Combinations of 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.
Marine Sediments Are Usually Combinations of Terrigenous and Biogenous Deposits What differences in the type and distribution of sediments do you note between the Atlantic Ocean and the Pacific Ocean? (above) The general pattern of sediments on the ocean floor.
Pelagic Sediments Vary in Composition 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 occurinthe 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
Pelagic Sediments Vary in Composition 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 occurinthe 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
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 Specialized Tools to Study Ocean Sediments How do scientists study sediments? Deep-water cameras Clamshell samplers Piston Corers Core libraries Seismic profilers
Scientists Use Specialized Tools to Study Ocean Sediments One method of studying sediments uses a clamshell sampler. The sampler can be used to obtain a relatively undisturbed sediment sample.
Stepped Art Fig. 5-17 (b-d), p. 115
Scientists Use Specialized 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.
Sediments Are Historical Records of Ocean Processes What can scientists learn by studying sediments? • Historical information • Location of natural resources, especially crude oil and natural gas • Methane Hydrates Marine sediments are important as historical records and a site of natural resources. Scientists study marine sediments using many different methods, including core samples.
Historical Information Paleooceanography Geologists use sediments to processes occurring in the ocean basins over the past 200 million years. Antarctic Circumpolar Current Controls temperature of Antarctica Date the separation of Antarctica and South America Use oxygen isotopes to tell temperature and amount of glaciation
Methane Hydrate One ft3 of gas hydrate melts, it releases 160 ft3 of methane May be an important energy source in the future. Fracking is easier right now.
Seabed Resources • Sand and Gravel—Most valuable CA resource • Phosphorite—Used as fertilizer. Nodules • Oil and Gas—Pre-salt oil in Brazil Marine sediments are important as historical records and a site of natural resources. Scientists study marine sediments using many different methods, including core samples.
Manganese Nodules Mostly in northeastern Pacific Ocean with red clays Also contain 1% Copper (Cu) and 0.25% Cobalt (Co)
Sulfide Mineral Deposits Usually associated with divergent plate boundaries First mining operation on island of Cyprus Ocean crust circulation over thousands of years
Chapter 3 Summary Layers of sediment cover most places of the ocean floor. The sediment is comprised 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 near them. 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.
