Conditions necessary for reef development Large reefs are limited to the warm seawater areas of the tropics. Calcium carbonate is easier to precipitate in warm water than in cold water. Secretion of calcium carbonate is aided by microscopic cells of algae that live in the tissues of reef builders (the algae remove carbon dioxide from the tissues, decreasing the acidity of the water).

Conditions necessary for reef development Reefs also tend to preferentially form in areas where: Little clastic sediment occurs (such sediment particles smother reef builders). Nutrient levels are low. Water is shallow

Reef zones Reef builders are zoned in a reef according to their form (encrusting forms tend to dominate the reef crest where wave action is strongest, while more delicate branching forms are confined to deeper water zones where water action is more gentle) A lagoon can develop behind a reef, where it is protected from strong waves

A special kind of reef: atoll An atoll is a special kind of reef that is ring-shaped and has a central lagoon. It is likely that Gilligan’s Island was set in an partially formed atoll.

An atoll is formed first as a reef that fringes a volcanic island. How an Atoll Forms An atoll is formed first as a reef that fringes a volcanic island. As the island sinks (after volcanic activity has ceased and the crust has cooled, becoming denser), the reef continues to build upward, eventually ending up as a ring-shaped structure. Gilligan’s Island ?

Bikini Atoll Bikini atoll (central Pacific) is a famous nuclear testing site (the US tested atomic bombs here in the 1940s and 1950s)

Darwin-Dana-Daly Theory of Atoll Formation First proposed by Darwin during his voyage on HMS Beagle (1836) Then supported and modified by 2 geologists: James Daly and Reginald Dana

3D Theory of Atoll Formation An oceanic volcano, which emerges from the sea surface and forms and island, becomes colonized by reef building corals This growth of corals begins to form a fringing reef around the island. The island begins to sink slowly. Coral growth continues

3D Theory of Atoll Formation (cont) 3. The island continues to sink and a barrier reef is formed with a lagoon between the reef and the island 4. Eventually, the small island disappears below the sea surface, leaving an atoll consisting of a ring of small islands with a relatively shallow lagoon in the center

Evidence For 3D Atoll Formation Deep drilling on coral atolls (Bikini Atoll) As the depth of the corals increased, the age of the corals also increased Fossil corals from base of the drilling were approximately 50 myo Corals were found to have grown on underlying volcanic rock Fossil corals were found at 1200 meters These corals only grow in shallow water!

Relate the 3D Theory to the Physiology of Coral Darwin also produced the first map showing the distribution of corals throughout the world, identified 3 kinds Fringing barrier reefs Barrier reefs Atolls USEFULL SITE!!!!

Coral Distribution

A coral reef is created by carbonate-secreting organisms. It is a wave-resistant, rock-like structure. Most of the reef is composed of loose to well-cemented organic debris of carbonate shells and skeletons. The living part of the reef is just a thin layer on the surface.

Coral reefs consist of several distinct parts developed in response to their exposure to waves The algal ridge occurs on the windward side of the reef and endures the pounding waves. The buttress zone is the reef slope extending down from the algal ridge. The reef face extends downward from the buttress zone. Not many colonial corals live here because insufficient light reaches this depth. The reef terrace is located landward of the algal ridge and lies at mean water level.

The Morphology of Coral Reefs FIGURE 12.20a: Morphology of a Barrier Reef. FIGURE 12.20b: Reef edge. Courtesy of Sean Linehan/NGS/NOAA

Corals grow continuously upward towards the sunlight. As sea level rises and/or land subsides, coral reefs pass through three stages of development. Fringing Reef Barrier Reef Atoll

Fringing reefs form limestone shorelines around islands or along continents They are the earliest stage of reef development. As the land is progressively submerged and the coral grows upward, a shallow lagoon expands. When the lagoon begins to separate the fringing reef from the shoreline, a barrier reef is created. In the final stage, the land vanishes below the sea and the reef forms a ring of islands, called an atoll, surrounding a shallow lagoon.

Evolution of Coral Reefs FIGURE 12.24a: Types of Coral Reefs. FIGURE 12.24b: Evolution of coral reefs.

Distribution Explained Conditions required for growth of coral Suitable temperature range 16-35°C, 23-25°C best Water clarity Clear without silt Silt reduces light penetration which inhibits the photosynthesis of zooxanthellae Depth: rapid growth usually occurs within 20m of surface Salinity Suitable rock substrate The continuous vertical growth of corals, over many thousand of years, occurs if the sea bed subsides or there is a rise in sea levels

Role of Reefs in Dissipating Energy of Waves Corals absorb the energy of waves and protect the shoreline from erosion Protection of ecosystems Reduces the cost of providing breakwaters Provides safer anchorage for boats Significant economic advantages

Factors that lead from reef growth to reef erosion Healthy corals accumulate calcium carbonate (CaCO3) at rates between 3-15m every 1000 years Many factors affect reef erosion Predation Storm damage Exposure to air

Predation: Crown of Thorns Starfish (Acanthaster planci) found on Indo- Pacific Reefs. Exceptionally low tides: corals can be exposed to air (dry and overheat) Storms: hurricanes causes physical breakage and scouring effect of abrasive sediments Hurricane Hattie (61) destroyed 43 km of barrier reef of British Honduras, complete recovery will take 25-100 years

Impact of Reef Erosion Reef erosion leads to shore and coastal properties being more exposed to the damaging effect of waves Many different materials are used to construct artificial reefs Concrete Stoneblock Sacks filled with sand Even sunken ships (or old Hathaway Bridges)

Artificial Reefs Become colonized by a wide range of organisms Algae, corals, and lots of fishies Function as a submerged breakwater Dissipate wave energy, reducing coral erosion and protecting anchored boats.

Methods for reef reconstruction Geomorphology: study of landforms and the processes involved in shaping them 3 main categories of reefs (fringing, barrier, atolls) Growth and geological history of a reef can be investigated by drilling and carbon dating

Deep drilling provides cores of material which can be used to id coral and estimate growth As coral grow, they produce bands (like tree rings) Interpretation of bands provides evidence for changes in growth rate in response to changes in environmental conditions (ambient temperature or nutrient availability)

Carbon dating Aka radiocarbon dating Can be used to find the age of a sample As coral grow, carbon (from the CO2) is deposited in their skeleton in the form of CaCO3 (calcium carbonate) Some of the carbon in the form of 14C slowly decays to 12C. The proportion of 14C to 12C in a sample is used to age corals up to about 50,000 years old

Investigating the effects of sea level changes on coral reefs Growth of coral is dependent on a number of factors (including light) For this reason, growth is restricted to the top 20 m of water Fossil coral, found at depths of 1200 m are evidence of subsidence Over geologic time new corals slowly grow on top of the old and the reef gradually sinks

Investigating the effects of sea level changes on coral reefs Corals are also found above sea level In increase in sea level allows growth Decrease in sea levels leave them high and dry Recently exposed coral in Mexico have correlated the growth of corals with changes in sea level between ice ages

Reef$ Value Found in over 100 countries Major source of food Over 500 million people worldwide rely on coral reefs Major source of food Poorer countries Fish and invertebrates Conch, bivalves, octopus, squid…. Food for pelagic fish species Limestone Building materials, breakwaters, cement Jewelry Aquarium trade Tourists $6-10billion US tourist industry Beach protection Wave breaks