Reefs

Reefs are among the oldest known communities, existing at least 2 billion years ago. They exert considerable control on the surrounding physical environment, influencing turbulence levels and patterns of sedimentation. Ancient reefs are often important hydrocarbon reservoirs.

“True” Reefs Versus Reeflike Structures

Reefs or reeflike structures are among the oldest known communities, extending more than 2 billion years into Earth’s history. These earliest reefs were vastly different in their biotic composition and physical structure from modern reefs, which are among the most diverse of biotic communities and display amazingly high rates of biotic productivity (carbon fixation) and calcium carbonate deposition, despite their existence in a virtual nutrient “desert.” Reefs are among the few communities to rival the power of humankind as a shaper of the planet. The Great Barrier Reef of Australia, for example, forms a structure some 2,000 kilometers in length and up to 150 kilometers in width.

It is necessary to distinguish between “true,” or structural, reefs and reeflike structures or banks. Reefs are carbonate structures that possess an internal framework. The framework traps sediment and provides resistance to wave action. Thus, reefs can exist in very shallow water and may grow to the surface of the oceans. Banks are also biogenically produced but lack an internal framework. Thus, banks are often restricted to low-energy, deep-water settings. “Bioherm” refers to moundlike carbonate buildups, either reefs or banks, and “biostrome” to low, lens-shaped buildups.

Reef Classification

Modern reefs are classified into several geomorphic types: atoll, barrier, fringing, and patch. Many of these may be further subdivided into reef crest or flat, back-reef or lagoon, and fore-reef zones. Atoll reefs are circular structures with a central lagoon, thought to form on subsiding volcanic islands. Barrier reefs are elongate structures that parallel coastlines and possess a significant lagoon between the exposed reef crest and shore. These often occur on the edges of shelves that are uplifted by faulting. Fringing reefs are elongate structures paralleling and extending seaward from the coastline that lack a lagoon between shore and exposed reef crest. Patch reefs are typically small, moundlike structures, occurring isolated on shelves or in lagoons. The majority of fossil reefs would be classified as patch reefs, although many examples of extensive, linear, shelf-edge trends are also known from the geologic record.

Reefs form one of the most distinctive and easily recognized sedimentary facies (or environments). In addition to possessing a characteristic fauna consisting of corals, various algae, and stromatoporoids, they are distinguished by a massive (nonlayered) core that has abrupt contacts with adjacent facies. Associated facies include flat-lying lagoon and steeply inclined fore-reef talus, the latter often consisting of large angular blocks derived from the core. The reef core is typically a thick unit relative to adjacent deposits. The core also consists of relatively pure calcium carbonate with little contained terrigenous material.

Reef Environments

Modern reefs are restricted to certain environments. They occur abundantly only between 23 degrees north and south latitudes and tend to be restricted to the western side of ocean basins, which lack upwelling of cold bottom waters. This restriction is based on temperature, as reefs do not flourish where temperatures frequently fall below 18 degrees Celsius. Reef growth is largely restricted to depths less than 60 meters, as there is insufficient penetration of sunlight below this depth for symbiont-bearing corals to flourish. Reefs also require clear waters lacking suspended terrigenous materials, as these interfere with the feeding activity of many reef organisms and also reduce the penetration of sunlight. Finally, most reef organisms require salinities that are in the normal oceanic range. It appears that many fossil reefs were similarly limited in their environmental requirements.

Some of the most striking features of modern reefs include their pronounced zonation, great diversity, and high productivity and growth rates. Reefs demonstrate a strong bathymetric (depth-related) zonation. This zonation is largely mediated through depth-related changes in turbulence intensity and in the quantity and spectral characteristics (reds are absorbed first, blues last) of available light. Shallow (1- to 5-meter) fore-reef environments are characterized by strong turbulence and high light intensity and possess low-diversity assemblages of wave-resistant corals, such as the elk-horn coral, Acropora palmata, and crustose red algae.

With increasing depth (10–20 meters), turbulence levels decrease and coral species diversity increases, with the occurrence of mound and delicate branching colonies. At greater depths (30–60 meters), corals assume a flattened, platelike form in an attempt to maximize surface area for exposure to ambient light. Sponges and many green algae are also very important over this range. Finally, corals possessing zooxanthellae, which live in the coral tissues and provide food for the coral host, are rare or absent below 60 meters because of insufficient light. Surprisingly, green and red calcareous algae extend to much greater depths (100–200 meters), despite the very low light intensity (much less than 1 percent of surface irradiance). Sponges are also important members of these deep reef communities.

Reef Communities

Coral reefs are among the most diverse communities on Earth; however, there is no consensus on the mechanisms behind the maintenance of this great diversity. At one time, it was believed that reefs existed in a low-disturbance, highly stable environment, which allowed very fine subdivision of food and habitat resources and thus permitted the coexistence of a great number of different species. Upon closer inspection, however, many reef organisms appear to overlap greatly in food and habitat requirements. Also, it has become increasingly apparent that disturbance, in the form of disease, extreme temperatures, and hurricanes, is no stranger to reef communities.

Coral reefs exhibit very high rates of productivity (carbon fixation), which is a result of extremely tight recycling of existing nutrients. This is necessary, as coral reefs exist in virtual nutrient “deserts.” Modern corals exhibit high skeletal growth rates, up to 10 centimeters per year for some branching species. Such high rates of skeletal production are intimately related to the symbiosis existing between the hermatypic or reef-building scleractinian corals (also gorgonians and many sponges) and unicellular algae or zooxanthellae. Corals that, for some reason, have lost their zooxanthellae or that are kept in dark rooms exhibit greatly reduced rates of skeleton production.

In addition to high individual growth rates for component taxa, the carbonate mass of the reefs may grow at a rate of some 2 meters per 1,000 years, a rate that is much higher than that of most other sedimentary deposits. This reflects the high productivity or growth rates of the component organisms and the efficient trapping of derived sediment by the reef frame. Although the framework organisms, most notably corals, are perhaps the most striking components of the reef system, the framework represents only 10-20 percent of most fossil reef masses. The remainder of the reef mass consists of sedimentary fill derived from the reef community through a combination of biosynthesis (secretion) and bioerosion (breaking down) of calcium carbonate. An example of the relative contributions of reef organisms to sediment can be found in Jamaica, where shallow-water, back-reef sediment consists of 41 percent coral, 24 percent green calcareous algae, 13 percent red calcareous algae, 6 percent foraminifera, 4 percent mollusks, and 12 percent other grains. The most important bioeroders are boring sponges, bivalves, and various “worms,” which excavate living spaces within reef rock or skeletons, and parrot fish and sea urchins, which remove calcium carbonate as they feed upon surface films of algae.

Types of Reef Communities

A diversity of organisms has produced reef and reeflike structures throughout Earth’s history. Several distinct reef community types have been noted, as well as four major “collapses” of reef communities. The oldest reefs or reeflike structures existed more than 2 billion years ago during the Precambrian eon. These consisted of low-diversity communities dominated by soft, blue-green algae, which trapped sediment to produce layered, often columnar structures known as stromatolites, similar if not identical to those being formed today. During the Early Cambrian period, blue-green algae were joined by calcareous, conical, spongelike organisms known as archaeocyathids, which persisted until the end of the Middle Cambrian. Following the extinction of the archaeocyathids, reefs again consisted only of blue-green algae until the advent of more modern reef communities in the Middle Ordovician period. These reefs consisted of corals (predominantly tabulate and, to a much lesser extent, rugose corals), red calcareous algae, bryozoans (moss animals), and the spongelike stromatoporoids. This community type persisted through the Devonian period, at which time a global collapse of reef communities occurred. The succeeding Carboniferous period largely lacked reefs, although algal and crinoidal (sea lily) mounds were common. Reefs again occurred in the Permian period, consisting mainly of red and green calcareous algae, stromatolites, bryozoans, and chambered calcareous sponges known as sphinctozoans, which resembled strings of beads. These reefs were very different from those of the earlier Paleozoic era; in particular, the tabulates and stromatoporoids no longer played an important role. The famous El Capitan reef complex of West Texas formed during this interval. The Paleozoic era ended with a sweeping extinction event that involved not only reef inhabitants but also other marine organisms.

After the Paleozoic extinctions, reefs were largely absent during the early part of the Mesozoic era. The advent of modern-type reefs consisting of scleractinian corals and red and green algae occurred in the Late Triassic period. Stromatoporoids once again occurred abundantly on reefs during this interval; however, the role of the previously ubiquitous blue-green algal stromatolites in reefs declined. Late Cretaceous reefs were often dominated by conical, rudistid bivalves that developed the ability to form frameworks and may have possessed symbiotic relationships with algae, as do many modern corals. Rudists, however, became extinct during the sweeping extinctions that occurred at the end of the Cretaceous period. The reefs that were reestablished in the Cenozoic era lacked stromatoporoids and rudists and consisted of scleractinian corals and red and green calcareous algae. This reef type has persisted, with fluctuations, until the present.

Study of Modern Reefs

Modern reefs are typically studied by divers, which enables observation and sampling to a depth of approximately 50 meters. Deeper environments have been made accessible through the availability of manned submersibles and unmanned, remotely operated vehicles that carry mechanical samplers as well as still and video cameras. The biological compositions of reef communities are determined by census (counting) methods commonly employed by plant ecologists. Studies of symbioses, such as that between corals and their zooxanthellae, employ radioactive tracers to determine the transfer of products between symbiont and host. Growth rates are measured by staining the calcareous skeletons of living organisms with a dye, such as Alizarin red, and then later collecting and sectioning the specimen and measuring the amount of skeleton added since the time of staining. Another method for determining growth is to X-ray a thin slice of skeleton and then measure and count the yearly growth bands that are revealed on the radiograph. Variations in growth banding reflect, among other factors, fluctuations in ocean temperature.

Reef sediments, which will potentially be transformed into reef limestones, are examined through sieving, X-ray diffraction, and epoxy impregnation and thin-sectioning. Sieving enables the determination of sediment texture, the relationships of grain sizes and abundance (which will reflect environmental energy and the production), and erosion of grains through biotic processes. X-ray diffraction produces a pattern that is determined by the internal crystalline structure of the sediment grains. As each mineral possesses a unique structure, the mineralogical identity of the sediment may be determined. Thin sections of embedded sediment or lithified rock are examined with petrographic microscopes, which reveal the characteristic microstructures of the individual grains. Thus, even highly abraded fragments of coral or algae may be identified and their contributions to the reef sediment determined.

Study of Fossil Reefs

Because of their typically massive nature, fossil reefs are usually studied by thin-sectioning of lithified rock samples collected either from surface exposures or well cores. Reef limestones that have not undergone extensive alteration may be dated through carbon-14 dating, if relatively young, or through uranium-series radiometric dating methods.

Natural Laboratories and Economic Resources

Modern reefs serve as natural laboratories, enabling the geoscientist to witness and study phenomena, such as carbonate sediment production, bioerosion, and early cementation, that have been responsible for forming major carbonate rock bodies in the past. The study of cores extracted from centuries-old coral colonies shows promise for deciphering past climates and perhaps predicting future trends. This is made possible by the fact that the coral skeleton records variations in growth that are related to ocean temperature fluctuations. The highly diverse modern reefs also serve as ecological laboratories for testing models on the control of community structure. For example, the relative importance of stability versus disturbance and recruitment versus predation in determining community structure is being studied within the reef setting.

Modern reefs are economically significant resources, particularly for many developing nations in the tropics. Reefs and the associated lagoonal sea-grass beds serve as important nurseries and habitats for many fish and invertebrates. The standing crop of fish immediately over reefs is much higher than that of adjacent open shelf areas. Reef organisms may one day provide an important source of pharmaceutical compounds, such as prostaglandins, which may be extracted from gorgonians (octocorals). In addition, research has focused upon the antifouling properties exhibited by certain reef encrusters. Reefs also provide recreational opportunities for snorkelers and for scuba divers, a fact that many developing countries are utilizing to promote their tourist industries. Finally, reefs serve to protect shorelines from wave erosion.

Because of the highly restrictive environmental tolerances of reef organisms, the occurrence of reefs in ancient strata enables fairly confident estimation of paleolatitude, temperature, depth, salinity, and water clarity. In addition, depth- or turbulence-related variation in growth form (mounds in very shallow water, branches at intermediate depths, and plates at greater depths) enables even more precise estimation of paleobathymetry or turbulence levels. Finally, buried ancient reefs are often important reservoir rock structures containing hydrocarbons and thus represent an important economic resources.

Principal Terms

calcareous algae: green algae that secrete needles or plates of aragonite as an internal skeleton; very important contributors to reef sediment

carbonate rocks: sedimentary rocks such as limestone, composed of calcium carbonate minerals

coralline algae: red algae that secrete crusts or branching skeletons of high-magnesium calcite; important sediment contributors and binders on reefs

rugose corals: a Paleozoic coral group also known as “tetracorals,” sometimes colonial, but more often solitary and horn-shaped

scleractinian corals: modern corals or “hexacorals,” different from their more ancient counterparts in details of the skeleton and the presence of a symbiosis with unicellular algae in most shallow-water species

stromatolites: layered columnar or flattened structures in sedimentary rocks, produced by the binding of sediment by blue-green algal (cyanobacterial) mats

stromatoporoids: spongelike organisms that produced layered, mound-shaped, calcareous skeletons and were important reef builders during the Paleozoic era

tabulate “corals”: colonial organisms with calcareous skeletons that were important Paleozoic reef builders; considered to be more closely related to sponges than to corals

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