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Other usesIn nautical terminology, a reef is a Rock (geology)|rock , bar (landform)|sandbar , or other feature lying beneath the surface of the water (six fathom s or less beneath low water).
Many reefs result from abiotic processes— Deposition (sediment)|deposition of sand , wave erosion planning down rock outcrops, and other natural processes—but the best-known reefs are the coral reefs of tropical waters developed through Biotic component|biotic processes dominated by coral s and calcareous algae . Artificial reefs such as shipwrecks are sometimes created to enhance physical complexity on generally featureless sand bottoms in order to attract a diverse assemblage of organisms, especially fish .
Biotic reef
See also|Formation of coral reefs|Reef habitat zonesThere are a variety of biotic reef types, including oyster reefs, but the most massive and widely distributed are tropical coral reef s. Although corals are major contributors to the framework and bulk material comprising a coral reef, the organisms most responsible for reef growth against the constant assault from ocean waves are calcareous algae, especially, although not entirely, species of coralline algae .
These biotic reef types take on additional names depending upon how the reef lies in relation to the land, if any. Reef types include fringing reef , barrier reefs , as well as atolls . A fringing reef is a reef that is attached to an island. A barrier reef forms a calcareous barrier around an island resulting in a lagoon between the shore and the reef. An atoll is a ring reef with no land present. The reef front (ocean side) is a high energy locale whereas the internal lagoon will be at a lower energy with fine grained sediments.
Geologic reef
Definition
Geologists define reefs and related terms (for example, bioherm , biostrome , carbonate mound ) using the factors of depositional relief, internal structure, and biotic composition. There is no consensus on one universally applicable definition. A useful definition distinguishes reefs from mounds as follows. Both are considered to be varieties of organosedimentary buildups: sedimentary features, built by the interaction of organisms and their environment, that have synoptic relief and whose biotic composition differs from that found on and beneath the surrounding sea floor. Reefs are held up by a macroscopic skeletal framework. Coral reefs are an excellent example of this kind. Corals and calcareous algae grow on top of one another and form a three-dimensional framework that is modified in various ways by other organisms and inorganic processes. By contrast, mounds lack a macroscopic skeletal framework. Mounds are built by microorganisms or by organisms that don't grow a skeletal framework. A microbial mound might be built exclusively or primarily by cyanobacteria . Excellent examples of biostromes formed by cyanobacteria occur in the Great Salt Lake of Utah (USA), and in Shark Bay , Western Australia .
Cyanobacteria do not have skeletons and individuals are microscopic. Cyanobacteria encourage the precipitation or accumulation of calcium carbonate and can produce distinct sediment bodies in composition that have relief on the seafloor. Cyanobacterial mounds were most abundant before the evolution of shelly macroscopic organisms, but they still exist today ( stromatolite s are microbial mounds with a laminated internal structure). Bryozoan s and crinoid s, common contributors to marine sediments during the Mississippian (for example), produced a very different kind of mound. Bryozoans are small and the skeletons of crinoids disintegrate. However, bryozoan and crinoid meadows can persist over time and produce compositionally distinct bodies of sediment with depositional relief.
The Proterozoic Belt Supergroup contains evidence of possible microbial mat and dome structures similar to stromatolite reef complexes. http://www.uta.edu/paleomap/homepage/Schieberweb/Publications/PDF/sedgeol120.pdf Jürgen Schieber, Possible indicators of microbial mat deposits in shales and sandstones: examples from the Mid-Proterozoic Belt Supergroup, Montana, U.S.A., Sedimentary Geology 120 (1998) p105–124
Geologic reef structures
Ancient reefs buried within Stratigraphy|stratigraphic sections are of considerable interest to geologist s because they provide paleo-environmental information about the location in History of Earth|Earth's history . In addition, reef structures within a sequence of sedimentary rock s provide a discontinuity which may serve as a trap or conduit for fossil fuel s or mineralizing fluids to form petroleum or ore deposits.
Corals, including some major extinct groups Rugosa and Tabulata , have been important reef builders through much of the Phanerozoic since the Ordovician Period. However, other organism groups, such as calcifying algae, especially members of the red algae Rhodophyta , and molluscs (especially the rudist bivalves during the Cretaceous Period) have created massive structures at various times. During the Cambrian Period, the conical or tubular skeletons of Archaeocyatha , an extinct group of uncertain affinities (possibly sponges), built reefs. Other groups, such as the Bryozoa have been important interstitial organisms, living between the framework builders. The corals which build reefs today, the Scleractinia , arose after the Permian–Triassic extinction event that wiped out the earlier rugose corals (as well as many other groups), and became increasingly important reef builders throughout the Mesozoic Era. They may have arisen from a rugose coral ancestor. Rugose corals built their skeletons of calcite and have a different symmetry from that of the scleractinian corals, whose skeletons are aragonite . However, there are some unusual examples of well preserved aragonitic rugose corals in the late Permian . In addition, calcite has been reported in the initial post-larval calcification in a few scleractinian corals. Nevertheless, scleractinian corals (which arose in the middle Triassic) may have arisen from a non-calcifying ancestor independent of the rugosan corals (which disappeared in the late Permian).
See also
Benjamin Kahn
Reef Hobbyist Magazine
References
Refimprove|date=July 2010Reflist
Shears N.T. (2007) Biogeography, community structure and biological habitat types of subtidal reefs on the South Island West Coast, New Zealand. Science for Conservation 281 . p 53. Department of Conservation, New Zealand. http://www.doc.govt.nz/upload/documents/science-and-technical/sfc281.pdf
