Reefs

Abstract

Reefs are accumulations of carbonate derived largely from the skeletal materials of marine organisms. The principal reef-building organisms are algae and corals. Algal reefs can develop in most oceans, but corals are restricted to those in which surface temperatures remain in the range 18°-36°C (optimally, 26°-28°C) and in which salinity is 3.3-3.6%. In terms of size and importance in global-change studies, coral reefs significantly outweigh other types of reef, and are thus the focus of this entry. Living coral reefs occupy over 600,000 square kilometers of the Earth's surface, confined mostly to tropical oceans. Exceptions occur when warm water is continuously moved into higher latitudes; examples include the northwestern Hawaiian islands, Lord Howe Island in the southwestern Pacific, the Ryukyu Islands of Japan, and Bermuda in the western Atlantic. Reefs are most simply classified into fringing reefs, barrier reefs, and atoll reefs (Figure 1). Fringing reefs tend to be young and surficial, rising from comparatively shallow depths on the submerged flanks of the land. Barrier reefs rise from greater depths and, because they rise almost vertically, they break the ocean surface at some distance from the coast. Atoll (or ring) reefs rise upward from a submerged edifice, typically a sunken volcanic island. In 1842, Charles Darwin was the first to recognize that fringing reefs could develop into barrier reefs, which could develop into atolls by the progressive subsidence of a reef-fringed volcanic island (Figure 1). This subsidence theory of atoll formation was largely corroborated when atoll-reef drilling confirmed that such reefs rose from submerged volcanic foundations. Yet to insist - as Darwin did not - that every atoll must have once been a barrier reef and that every barrier reef must have once been a fringing reef has proved unhelpful, especially in tectonically active parts of the coral seas in the Caribbean and southwestern Pacific. Islands exist on reefs in many places. These may be surficial and transient (cays), often created and removed in successive storm events, or they may be more enduring (motus) as the result of having developed beachrock or other armor. In places where slight emergence has taken place, reef islands will be even more enduring, a good example being the low emerged reef Aldabra in the western Indian Ocean. Although coral reefs have existed since the Ordovician (500-440 million years ago), most of those living in the world today have foundations of Neogene age (since 25 million years ago). These reefs have proved to be sensitive recorders of many environmental changes and have thus been the object of studies by many global-change researchers. Reefs have, for instance, been accurate recorders of tectonic changes. Most ancient midocean atolls, such as those in the northwestern Pacific, have been subsiding slowly for several million years. In response to this subsidence, the veneer of living reef that caps the dead reef below has been growing slowly upward. Hence dated cores through the reef can provide information about the long-term subsidence rate.