Simone Lawson
Biogenous sediments are the remains of living organisms that settle out as sediment when the organisms die. They are predominantly formed from the hard parts of organisms, such as shells, teeth, or skeletal elements, which are more resistant to decomposition than the fleshy soft parts. Sediments composed of microscopic tests are far more abundant than sediments from macroscopic particles. Whale bones and shark teeth may contribute to macroscopic biogenous sediments, but these are rarer in the ocean compared to the omnipresent microscopic particles. If the sediment layer consists of at least 30% microscopic biogenous material, it is classified as a biogenous ooze. The primary sources of microscopic biogenous sediments are unicellular algae and protozoans that secrete tests of either calcium carbonate or silica.
| Characteristics | Values |
|---|---|
| Composition | Skeletal remains of once-living organisms |
| Type of remains | Hard parts of organisms like shells, teeth, or skeletal elements |
| Size of remains | Microscopic |
| Minimum percentage of biogenous material for classification as biogenous ooze | 30% |
| Composition of microscopic biogenous sediments | Calcium carbonate (CaCO3) or silica (SiO2) |
| Sources of microscopic biogenous sediments | Unicellular algae and protozoans |
| Examples of sources of silica tests | Diatoms (algae) and radiolarians (protozoans) |
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Microscopic shells
Biogenous sediments are composed of the remains of marine organisms, including their shells, teeth, and skeletal elements. These "hard parts" of organisms contribute to the sediments as they are usually mineralized and more resistant to decomposition than the "soft parts" that rapidly deteriorate after death.
The algae and protozoans that produce these microscopic shells are abundant in the surface waters of the oceans. When they die, their shells sink to the bottom and contribute to the formation of biogenous sediments. These microscopic shells are far more abundant than sediments from macroscopic particles, and they create fine-grained, mushy sediment layers.
The calcium carbonate shells are formed by organisms incorporating dissolved oxygen into their shells. During cooler climates, the water vapor that condenses into rain and snow forms glacial ice with a high proportion of O16. This leaves the remaining seawater with a higher proportion of O18. Thus, the organisms' shells will have a higher proportion of the O18 isotope. By examining the oxygen isotope ratios in these shells, scientists can reconstruct past climate history.
Additionally, the presence of microscopic shells in biogenous sediments can provide valuable information about environmental conditions in ocean surface waters. Changes in seawater temperature, for example, can cause shifts in the types of planktonic organisms that accumulate on the seafloor, indicating the coming and going of ice ages.
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Skeletal remains
Biogenous sediments are composed of the remains of once-living organisms. They are the "'hard parts' of organisms that settle as sediment when the organisms die. These include shells, teeth, and skeletal elements. The skeletal remains that constitute biogenous sediments can be divided into two categories:
Macroscopic Sediments
Macroscopic sediments contain large skeletal remains, such as the skeletons, teeth, or shells of larger organisms. This type of sediment is rare in most parts of the ocean because larger organisms do not die in high enough concentrations for their remains to accumulate. One exception is around coral reefs, where the stony skeletons of corals make up a large percentage of tropical sand.
Microscopic Sediments
Microscopic sediments are composed of the shells or "'tests'" of microscopic organisms. These organisms, such as unicellular algae and protozoans, are highly abundant, dying by the billions every day. Their tests sink to the bottom and create fine-grained, mushy sediment layers. If the sediment layer consists of at least 30% microscopic biogenous material, it is classified as a biogenous ooze, with the remainder often being clay.
The skeletal remains of microscopic organisms can be composed of either calcium carbonate (CaCO3) or silica (SiO2). Silica tests come from two main groups: diatoms (algae) and radiolarians (protozoans).
Environmental Significance
Biogenous sediments can provide valuable information about past climate conditions. Marine organisms incorporate dissolved oxygen into their shells as calcium carbonate. By examining the oxygen isotope ratios in these shells, scientists can reconstruct past climate history. For example, during periods of cooler climate, water vapor forms glacial ice with a high proportion of O16, leaving the remaining seawater with a higher proportion of O18. Thus, shells formed during these periods will have a higher proportion of the O18 isotope.
Additionally, changes in surface seawater temperature can cause shifts in the types of planktonic organisms that accumulate on the seafloor, providing insights into the coming and going of ice ages.
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Calcium carbonate
Calcareous ooze, a type of biogenous ooze, is composed of calcium carbonate. This type of ooze forms when the particles in the sediment are composed of more than 30% skeletal material in the form of calcium carbonate. The skeletal material is often in the mineral form of calcite but can also be found as aragonite. Foraminiferans and coccoliths, microscopic organisms that coat certain species of marine algae and protozoa, are significant contributors to calcareous ooze.
The presence of calcium carbonate in marine sediments, including biogenous sediments, can provide valuable information about past climate conditions. Marine organisms that incorporate dissolved oxygen into their shells as calcium carbonate reflect the oxygen isotope ratios in the seawater. By analyzing the isotope ratios in the shells, scientists can reconstruct past climate history. During cooler climate periods, seawater evaporates, leading to a higher proportion of O16 isotopes in the water vapor. This results in glacial ice with a high proportion of O16, while the remaining seawater has a relatively higher proportion of O18. Consequently, the shells formed by organisms during these periods will exhibit higher proportions of the O18 isotope.
Additionally, calcium carbonate sediments can provide insights into the distribution of sediment types. For example, the input of carbonate through rivers and deep-sea hydrothermal vents influences the variation in productivity and dissolution rates, impacting the distribution of calcareous and siliceous oozes in the oceans.
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Silica
Biogenous sediments are formed from the "hard parts" of the remains of living organisms, such as shells, teeth, and skeletal elements. These parts are usually mineralized and more resistant to decomposition than the fleshy "soft parts" that break down rapidly after death.
In areas where diatoms are abundant, the underlying sediment is rich in silica diatom tests and is called diatomaceous earth. Diatomaceous earth has been used as a filtering agent, an abrasive in toothpaste and cleaning agents, and for stabilizing nitroglycerine in dynamite. It also has insecticidal properties.
Siliceous oozes are a type of biogenous sediment dominated by diatom and radiolarian tests. Siliceous sediments are composed of silica, while calcareous sediments are composed of calcium carbonate (CaCO3). Siliceous oozes accumulate at an average rate of 1 centimeter (0.4 inches) per 1000 years.
The solubility and dissolution rate of biogenic silica in marine sediments are influenced by various factors, including the presence of aluminum and the density of reactive-surface silanol groups. Biogenic silica is believed to be a potentially powerful proxy for reconstructing paleoproductivity.
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Macroscopic remains
Biogenous sediments are composed of the remains of marine organisms. They are the "hard parts" of organisms that settle out as sediment when the organisms die. These include shells, teeth, or skeletal elements, as these parts are usually mineralized and are more resistant to decomposition than the fleshy "soft parts" that rapidly deteriorate after death.
Macroscopic sediments contain large remains, such as the skeletons, teeth, or shells of larger organisms. This type of sediment is fairly rare over most of the ocean as large organisms do not die in high enough concentrations for their remains to accumulate. One exception is around coral reefs, where there is a great abundance of organisms that leave behind their remains, in particular, the fragments of the stony skeletons of corals that make up a large percentage of tropical sand.
Sediments composed of macroscopic particles are less common than those composed of microscopic particles. This is because the small size of microscopic particles creates fine-grained, mushy sediment layers. If the sediment layer consists of at least 30% microscopic biogenous material, it is classified as a biogenous ooze, whereas sediments composed of macroscopic particles do not meet this threshold.
Biogenous sediments can tell us about environmental conditions in ocean surface waters. For example, changes in surface seawater temperature may cause shifts in the types of planktonic organisms that accumulate on the sea floor, indicating the coming and going of ice ages.
It is important to note that caution should be used when applying characteristics of energy environments to biogenous sediments. For example, some biological shells may start out round and thus are not useful indicators of environmental energy.
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