Instruction: Marine Sediments

Sediment is any loose material which can be transported by a liquid that eventually settles at the bottom of the liquid. In the ocean, the sediment consists of organic and inorganic matter from the weathering and erosion of rocks, the activity of living organisms, from volcanic eruptions, space, or from the chemical processes within the ocean itself. Marine sediment has a broad range of sizes and types. Additionally, marine sediment often comes in different colors. Organic sediment is often white, cream colored, or gray. Deep sea sediment can range from red to chocolate brown due to the iron oxide found on the ocean floor. Other clays can be green or tan and nodular sediments are dark brown or black.

Sediment Size

Sediment is classified by particle size ranging from large boulders to fine clay. Most marine sediment is found at the smaller end of the spectrum as sand, silt, and clay. The smaller the sediment size, the easier it is for it to be carried by streams, waves, and currents.

Type of Particle Diameter
Boulder 256 mm or larger
Cobble 64 – 256 mm
Pebble 4 – 64 mm
Granule 2 – 4 mm
Sand 0.062 – 2 mm
Silt 0.004 – 0.062 mm
Clay 0.004 mm or smaller

Sediment Source

Marine sediment can also be classified by origin. Marine sediment can be classified into four categories: terrigenous, biogenous, hydrogenous, and cosmogenous.

Sediment Type Source Examples Percent of the Ocean Floor
Terrigenous Erosion of land and volcanic eruptions Quartz sand, clay, and estuarine mud Approximately 55%
Biogenous Accumulation of hard parts of marine organisms Calcareous and siliceous oozes Approximately 45%
Hydrogenous Precipitation of dissolved minerals from water Manganese nodules and phosphorite deposits Less than 1%
Cosmogenous Dust from space such as meteorite debris Tektite spheres and glassy nodules Less than 1%

Terrigenous sediments are the most abundant sediment found in the ocean. Their formation is actually a part of a cycle in which the sediment moves through different parts of the Earth. As mountains and other land masses erode, wind and water transport sediment to the oceans where they collect on the ocean floor. As the sediment collects, it becomes part of the ocean plate. As the continental plates move, the ocean floor may become mountains, or part of the land mass, and the cycle begins again.

One type of terrigenous sediment is clay. About 38% of the seafloor is covered in clay collected from dust and volcanic ash. These are the finest of the terrigenous sediments and accumulate only about 2 mm every thousand years.

Biogenous sediment is the next abundant marine sediment. This sediment is formed from the remains of marine organisms, particularly those organisms that contain silicon or calcium carbonate. Such organisms include plankton, coral, and mollusks. After these organisms die, the hard structures of their bodies fall to the bottom of the ocean and collect in layers. Biogenous sediment is most abundant where there is high biological activity and upwellings. Over millions of years, the collected sediment can form oil and natural gas.

Oozes are biogenous sediment found on the continental margin in the pelagic zone. Ooze contains at least 30% organic material and is named after the organisms that produced it. Organisms that have glasslike silica shells produce siliceous ooze and organisms that have calcium containing material produce calcareous ooze. Oozes accumulate slowly, at a rate of about 1 to 6 cm per thousand years.

Hydrogenous sediment is formed from minerals that have precipitated from dissolved minerals in the ocean. These minerals originate from submerged rock, sediment, hydrothermal vents, and river runoffs. Perhaps the most prominent hydrogenous sediments are manganese and phosphorus nodules. Marine scientists are not clear how these nodules are formed from, but they are found all along the continental margins.

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Evaporites are another example of hydrogenous sediment. Evaporites are deposits that include many salts that are important to humans. As water evaporates, salts such as carbonate, calcium sulfate, and sodium chloride precipitate and are available for collection.

Oolite sands are also hydrogenous sediments. In areas of shallow water, the sunlight causes water to heat rapidly and the marine plants use up the dissolved carbon dioxide, resulting in water with a slightly higher pH. As seawater becomes less acidic, small white rounded grains of calcium carbonate collect. These grains called oolite sands are very abundant in warm shallow waters such as the Bahamas.

Cosmogenous sediment originate from the cosmos or outer space and are therefore the least abundant class of marine sediment. Cosmogenous sediment comes from interplanetary dust or from the impact of asteroids or comets on the Earth. Although 15,000 to 30,000 metric tons of interplanetary dust enters the Earth’s atmosphere every year, most of the material is vaporized or dissolves in the ocean before reaching the ocean floor. On the rare occasions that the Earth is hit by a large asteroid or comet, microtektites are formed. Microtektites are small particles of glass that are formed from the melting of the Earth’s crust. These particles do not dissolve easily in the ocean and are typically no more than 1.5 mm long.

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In the study of paleoceanography, scientists take core samples of the ocean sediment to determine important facts about the history of the ocean. A number of different methods can be used to collect these samples. One such method is to send submersibles or remotely operated cameras to the ocean floor to gather information. Another method is to use sound waves to determine the distinct sediment layers. Core samples can also be obtained by punching a hollow tube into the bottom sediments and then retrieving the tube and the content above the surface. By examining the layers in their original configuration, scientists can gather significant information about the history of the earth and the World Ocean.