Query: Reef material is calcium carbonate, which contains 44 per cent carbon dioxide.

Relevant passages:
Limestone <=SEP=> Limestone is a sedimentary rock , composed mainly of skeletal fragments of marine organisms such as coral , forams and molluscs . Its major materials are the minerals calcite and aragonite , which are different crystal forms of calcium carbonate ( CaCO3 ) .   About 10 % of sedimentary rocks are limestones . The solubility of limestone in water and weak acid solutions leads to karst landscapes , in which water erodes the limestone over thousands to millions of years . Most cave systems are through limestone bedrock .   Limestone has numerous uses : as a building material , an essential component of concrete ( Portland cement ) , as aggregate for the base of roads , as white pigment or filler in products such as toothpaste or paints , as a chemical feedstock for the production of lime , as a soil conditioner , or as a popular decorative addition to rock gardens .   The first geologist to distinguish limestone from dolomite was Belsazar Hacquet in 1778 .

Calcium carbonate <=SEP=> Calcium carbonate is a chemical compound with the formula CaCO3 . It is a common substance found in rocks as the minerals calcite and aragonite ( most notably as limestone , which contains both of those minerals ) and is the main component of pearls and the shells of marine organisms , snails , and eggs . Calcium carbonate is the active ingredient in agricultural lime and is created when calcium ions in hard water react with carbonate ions to create limescale . It is medicinally used as a calcium supplement or as an antacid , but excessive consumption can be hazardous .

Carbon sink <=SEP=> A carbon sink is a natural or artificial reservoir that accumulates and stores some carbon-containing chemical compound for an indefinite period . The process by which carbon sinks remove carbon dioxide from the atmosphere is known as carbon sequestration . Public awareness of the significance of CO2 sinks has grown since passage of the Kyoto Protocol , which promotes their use as a form of carbon offset . There are also different strategies used to enhance this process .

Coral <=SEP=> Corals are marine invertebrates in the class Anthozoa of phylum Cnidaria . They typically live in compact colonies of many identical individual polyps . The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton .   A coral `` group '' is a colony of myriad genetically identical polyps . Each polyp is a sac-like animal typically only a few millimeters in diameter and a few centimeters in length . A set of tentacles surround a central mouth opening . An exoskeleton is excreted near the base . Over many generations , the colony thus creates a large skeleton that is characteristic of the species . Individual heads grow by asexual reproduction of polyps . Corals also breed sexually by spawning : polyps of the same species release gametes simultaneously over a period of one to several nights around a full moon .   Although some corals can catch small fish and plankton using stinging cells on their tentacles , most corals obtain the majority of their energy and nutrients from photosynthetic unicellular dinoflagellates in the genus Symbiodinium that live within their tissues . These are commonly known as zooxanthellae and the corals that contain them are zooxanthellate corals . Such corals require sunlight and grow in clear , shallow water , typically at depths shallower than 60 m. Corals are major contributors to the physical structure of the coral reefs that develop in tropical and subtropical waters , such as the enormous Great Barrier Reef off the coast of Queensland , Australia .   Other corals do not rely on zooxanthellae and can live in much deeper water , with the cold-water genus Lophelia surviving as deep as 3000 m . Some have been found on the Darwin Mounds , north-west of Cape Wrath , Scotland . Corals have also been found as far north as off the coast of Washington State and the Aleutian Islands .

Reef <=SEP=> A reef is a bar of rock , sand , coral or similar material , lying beneath the surface of water . Reefs may be up to 261 feet below the surface .   Many reefs result from abiotic processes -- deposition of sand , wave erosion planing down rock outcrops , and other natural processes -- but the best-known reefs are the coral reefs of tropical waters developed through biotic processes dominated by corals 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 .