Red clay, also known as abyssal clay however, is mostly located in the ocean and is formed from a combination of terrigenous material and volcanic ash. In terms of size, terrigenous particles are generally larger than abyssal clay particles so they sink faster. Biogenous sediments are formed from the remnants of organisms that refused to be dissolved. Good examples of these organisms include shellfish, clams, anything that has a shell. Other things that could avoid being dissolved include bones and teeth and other appendages.
In deeper waters, shells of plankton and other microscopic organisms form these kinds of sediments. Hydrogenous sediments are sediments solidified out of ocean water. As such, chemical reactions create these kinds of sediments. The precipitation of dissolved chemicals from seawater.
Figure 3. Examples of common microscopic silica-secreting organisms. The bars indicate scale; 1 micron equals 1 millionth of a meter or 0. Not all silica from siliceous microorganisms winds up as siliceous ooze.
In some cases, minor amounts of silica are deposited with calcareous ooze. Although the exact method of formation is unclear, during burial siliceous material combines to form hard rounded lumps or nodules called chert nodules.
For example, the white cliffs of Dover England are made of chalk and also contain abundant chert nodules. Chert , a microcrystalline form of silica, is so hard that it is often used as a whetstone to sharpen knives. Hydrogenous Sediment. Hydrogenous sediments are created from chemical reactions in seawater. Under special chemical conditions, dissolved materials in seawater precipitate form solids. Many types of hydrogenous sediments have economic value. Hydrogenous sediments include evaporites , meaning any type of sediment that forms from the evaporation of seawater.
As seawater evaporates, the ions that remain behind can become so concentrated that they will combine with one another to form crystals that precipitate.
The two most common types of evaporates are gypsum and halite. Halite is sodium chloride NaCl , which is common table salt. When you salt your food, you are eating the evaporated remains of ancient ocean water! Manganese nodules are another type of hydrogenous sediment. They form marble-size to tennis ball-size lumps of iron and manganese oxide that lie scattered across the deep sea floor where sedimentation rates are particularly low.
Although they contain large amounts of manganese, these nodules are economically most important for their cobalt, nickel, and chromium. Their formation is not well understood; however, we do know that they form as concentric layers like an onion , adding layers of iron and manganese minerals slowly over time.
The rates of formation are on the order of 1 to 10 millimeters 0. Figure 4. Manganese nodules, a type of hydrogenous sediment. Oolites are sand-sized grains made of calcium carbonate precipitated out of seawater in warm, tropical regions, such as in the Bahamas.
Oolites need to roll back-and-forth to form , so they form only in shallow areas where waves cause back-and-forth motion on the seabed. The back-and-forth motion causes the grains to accrete layer after layer, somewhat like a snowball, so that each oolite has a spherical shape with a layered, onion-like internal structure.
Describing Sediment Characteristics. Figure 5 shows a hypothetical distribution of sediment types across a passive continental margin and adjacent ocean basin. Note that those sediments found close to the continent along the continental shelf are known as neritic sediments and are largely lithogenous. Those sediments found further from the continent are known as pelagic sediments and are often dominated by biogenous particles.
Figure 5. Schematic view of the distribution of various sediment types across an idealized passive margin and adjacent ocean basin. Notice that with increasing distance from the continent that the grain size of lithogenous sediments decreases. Lithogenous sediments exhibit characteristics that reflect the processes involved in their transportation and deposition. These characteristics are described as the sediment's texture, which includes the size and shape of the particles. For example, grain size is the size of the individual particles Table 2 , whereas rounding describes how angular versus how smooth the particles are Table 3.
Grain Size. Silica sediments will therefore only accumulate in cooler regions of high productivity where they accumulate faster than they dissolve. This includes upwelling regions near the equator and at high latitudes where there are abundant nutrients and cooler water. Oozes formed near the equatorial regions are usually dominated by radiolarians , while diatoms are more common in the polar oozes. Once the silica tests have settled on the bottom and are covered by subsequent layers, they are no longer subject to dissolution and the sediment will accumulate.
Biogenous calcium carbonate sediments also require production to exceed dissolution for sediments to accumulate, but the processes involved are a little different than for silica. Calcium carbonate dissolves more readily in more acidic water. Cold seawater contains more dissolved CO 2 and is slightly more acidic than warmer water section 5.
Therefore calcium carbonate tests are more likely to dissolve in colder, deeper, polar water than in warmer, tropical, surface water. At the poles the water is uniformly cold, so calcium carbonate readily dissolves at all depths, and carbonate sediments do not accumulate.
In temperate and tropical regions calcium carbonate dissolves more readily as it sinks into deeper water. The depth at which calcium carbonate dissolves as fast as it accumulates is called the calcium carbonate compensation depth , or calcite compensation depth , or simply the CCD. The lysocline represents the depths where the rate of calcium carbonate dissolution increases dramatically similar to the thermocline and halocline. At depths shallower than the CCD carbonate accumulation will exceed the rate of dissolution, and carbonate sediments will be deposited.
In areas deeper than the CCD, the rate of dissolution will exceed production, and no carbonate sediments can accumulate Figure The CCD is usually found at depths of 4 — 4. Thus calcareous oozes will mostly be found in tropical or temperate waters less than about 4 km deep, such as along the mid-ocean ridge systems and atop seamounts and plateaus.
The CCD is deeper in the Atlantic than in the Pacific since the Pacific contains more CO 2 , making the water more acidic and calcium carbonate more soluble. This, along with the fact that the Pacific is deeper, means that the Atlantic contains more calcareous sediment than the Pacific.
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