Siliceous ooze
Encyclopedia
Siliceous ooze is a soft siliceous pelagic sediment that covers large areas of the deep ocean floor. Siliceous oozes consist predominately of either diatom
s or radiolarian
s. Sometimes, it contains lessor proportions of sponge spicules or silicoflagellates
. Diatoms are golden-brown algae that construct an opal
ine silica microscopic shell that is known as a “frustule
”. Silicoflagellates are a minor group of marine algae that construct microscopic shells composed of opaline silica. Radiolarians are marine protist
s that also construct microscopic shells composed of opaline silica and are distant relatives of the foraminifera
.
Siliceous ooze accumulates on the ocean floor where the bottom waters are close to saturation in respect to silica and the opaline remains of either diatoms, radiolarians, silicoflagellates, and sponge spicules, or combinations of these are rapidly buried. These conditions exist within areas of high biological productivity associated with volcanic islands and nutrient-rich upwelling
zones. As the least common type of pelagic sediment, it covers only 15% of the ocean floor. It accumulates at a slower rate than calcareous ooze: 0.2-1 cm / 1000 yr.
After burial, most siliceous oozes remain unconsolidated. However, a fraction siliceous oozes will dissolved and reprecipitate as a result of diagenesis
to form chert
beds or nodules. When siliceous oozes are incorporated into orogenic belts associated with subduction zones, they will also be altered by diagenesis and lithified to form chert, i.e. radiolarian chert.
Diatom
Diatoms are a major group of algae, and are one of the most common types of phytoplankton. Most diatoms are unicellular, although they can exist as colonies in the shape of filaments or ribbons , fans , zigzags , or stellate colonies . Diatoms are producers within the food chain...
s or radiolarian
Radiolarian
Radiolarians are amoeboid protozoa that produce intricate mineral skeletons, typically with a central capsule dividing the cell into inner and outer portions, called endoplasm and ectoplasm. They are found as zooplankton throughout the ocean, and their skeletal remains cover large portions of the...
s. Sometimes, it contains lessor proportions of sponge spicules or silicoflagellates
Dictyochales
Dictyochales are a small group of unicellular heterokont algae, found in marine environments.-Characteristics:In one stage of their life cycle, they produce a siliceous skeleton, composed of a network of bars and spikes arranged to form an internal basket...
. Diatoms are golden-brown algae that construct an opal
Opal
Opal is an amorphous form of silica related to quartz, a mineraloid form, not a mineral. 3% to 21% of the total weight is water, but the content is usually between 6% to 10%. It is deposited at a relatively low temperature and may occur in the fissures of almost any kind of rock, being most...
ine silica microscopic shell that is known as a “frustule
Frustule
A frustule is the hard and porous cell wall or external layer of diatoms. The frustule is composed almost purely of silica, made from silicic acid, and is coated with a layer of organic substance, which was referred to in the early literature on diatoms as pectin, a fiber most commonly found in...
”. Silicoflagellates are a minor group of marine algae that construct microscopic shells composed of opaline silica. Radiolarians are marine protist
Protist
Protists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy...
s that also construct microscopic shells composed of opaline silica and are distant relatives of the foraminifera
Foraminifera
The Foraminifera , or forams for short, are a large group of amoeboid protists which are among the commonest plankton species. They have reticulating pseudopods, fine strands of cytoplasm that branch and merge to form a dynamic net...
.
Siliceous ooze accumulates on the ocean floor where the bottom waters are close to saturation in respect to silica and the opaline remains of either diatoms, radiolarians, silicoflagellates, and sponge spicules, or combinations of these are rapidly buried. These conditions exist within areas of high biological productivity associated with volcanic islands and nutrient-rich upwelling
Upwelling
Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water towards the ocean surface, replacing the warmer, usually nutrient-depleted surface water. The increased availability in upwelling regions results in high levels of primary...
zones. As the least common type of pelagic sediment, it covers only 15% of the ocean floor. It accumulates at a slower rate than calcareous ooze: 0.2-1 cm / 1000 yr.
After burial, most siliceous oozes remain unconsolidated. However, a fraction siliceous oozes will dissolved and reprecipitate as a result of diagenesis
Diagenesis
In geology and oceanography, diagenesis is any chemical, physical, or biological change undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface alteration and metamorphism. These changes happen at relatively low temperatures and pressures...
to form chert
Chert
Chert is a fine-grained silica-rich microcrystalline, cryptocrystalline or microfibrous sedimentary rock that may contain small fossils. It varies greatly in color , but most often manifests as gray, brown, grayish brown and light green to rusty red; its color is an expression of trace elements...
beds or nodules. When siliceous oozes are incorporated into orogenic belts associated with subduction zones, they will also be altered by diagenesis and lithified to form chert, i.e. radiolarian chert.