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Coral Reef History

Corals are 500 million years old, and date back to the late Cambrian period, during the Paleozoic era (Fig. 1). Evidence suggests that they started as simple, solitary organisms but, in response to changes in their environment, later evolved into the coral reefs we know today. It is also known that over the 500 million years, during which corals are known to have existed, they have experienced a number of extinction events. These extinction events were largely the result of dramatic changes in their environment, such as we are seeing today.

Ordovician–Silurian Extinction Event

About 435 million years ago, as the supercontinent, Pangaea, was beginning to form, and the circulation of the world’s oceans was being affected by drifting land masses, an ice age began. Sea levels and ocean temperatures dropped significantly as glaciers grew. This period in Earth’s history has been termed “the Ordovician–Silurian Extinction Event”, which resulted in the disappearance of corals and as much as 60% of all marine life.

Devonian Period

Corals re-appeared during the Devonian period, around 410 million years ago and, for the first time, began to form extensive reef systems. These early coral reefs were predominantly composed of coral- like stromatoporoids (reef forming sponges), tabulate corals (mounds, branches and organ shapes), rugose corals (horn shaped), and predecessors of the modern-day coralline algae (encrusting multi-colored algae seen on rock surfaces). It was towards the end of this period that scleractinian or ‘stony’ corals first appeared that populate coral reefs today. Then, 350 million years ago corals again disappeared from the geological record. The reason for this is not clear but evidence points towards rapid fluctuations in sea levels and a rapid reduction in atmospheric carbon dioxide.

Permian-Triassic Extinction Event

It would take almost 100 million years before corals re-appeared around 260 million years ago, at the end of the Permian period (Fig. 1). This was followed by the Permian- Triassic Extinction Event of 251 million years ago. The Permian-Triassic Extinction was the greatest extinction event in Earth’s history, during which 96% of marine species were wiped out. Evidence suggests a period of hypoxia (reduced oxygen) and hypercapnia (elevated carbon dioxide) in the world’s oceans. These effects may have been caused by meteor impacts, dramatically reduced sea-levels, increased volcanic activity or gas hydrate venting from sedimented organic matter on the seabed. It is likely that a combination of some or all of these events triggered this extinction event.

Triassic, Jurassic and Cretaceous Periods

Corals made a comeback, 230 million years ago. In this Triassic period, modern-day scleractinian corals once again began to emerge. The supercontinent, Pangaea, was however, now breaking up and around 205 million years ago, possibly due to continental drift again impacting oceanic circulation, corals disappeared. They come and go in the geological record several times between 190 million and 75 million years ago throughout the Jurassic and Cretaceous periods, probably as a result of continuously changing oceanic circulation.

Cretaceous-Tertiary Mass Extinction Event

Around 65 million years ago, the Cretaceous-Tertiary Mass Extinction occurred, the last of the mass extinction events. Recent evidence seems to confirm that the cause was a large meteor impact which resulted in the formation of the Gulf of Mexico. It is likely that a short period (1-10 years) of significantly reduced sunlight occurred, during which time large numbers of organisms, including corals were wiped out. Corals rely, in part, on their ability to capture planktonic organisms, which are either photosynthetic or zooplankton, which themselves rely on photosynthetic prey as a food source. Photosynthetic organisms would have been severely impacted by the reduced levels of sunlight. To compound the problem for coral reefs, they are also reliant on energy, in the form of fixed carbon, from their endosymbiotic partners, zooxanthellae. These photosynthetic single-celled organisms would have suffered from the reduction in sunlight. Needless to say the fragile corals disappeared from the world’s oceans once again, following the Cretaceous-Tertiary Mass Extinction and only reappeared shortly before another minor extinction event, the Paleocene/Eocene Thermal Maximum, around 55 million years ago.

Paleocene/Eocene Thermal Maximum Event

The Paleocene/Eocene Thermal Maximum (PETM) is the most extreme global warming event (over 14oF or 6oC) in history. Rapid rises in atmospheric carbon dioxide are observed in the geological record, as well as rises in sea level and in the lysocline (Boxed text). The most likely cause of the PETM is the release of large quantities of gas hydrates as the earth gradually warmed during the preceding Paleocene period. These gas hydrates, including methane gas, are locked up in ice & sedimentary deposits and would have been released as the Earth warmed. They are potent greenhouse gases and would have rapidly accelerated global warming. Coral reefs suffered heavily during this ‘mini-extinction’ and most disappeared.

Mid-Eocene Period

Coral began to gain a foothold around 46 million years ago before disappearing for the last time in the Mid-Eocene period around 40 million years ago. Finally around 20 million years ago, the Great Barrier Reef, located off the west coast of Australia came into existence.

Modern Day

The climate of the globe is currently undergoing a rapid PETM-like event (a warming period), driven by greenhouse gases as in the PETM. Evidence now suggests that coral reefs will pass a point of no-return around 2040, and go into terminal decline, eventually disappearing at the end of this century. If so, based on past evidence, it is likely that many millions of years will pass before they return.

Examples of modern day Scleractinian or “Stony” corals from the Pacific.
• El Niño Southern Oscillation
• Global Impact of Carbon Dioxide
• Ocean Acidification

Coral Reef Graphic

The Lysocline
This is the depth below which the ocean is undersaturated with respect to calcium carbonate (CaCO3).

A saturated or super-saturated solution is a solution that cannot hold anymore of a particular salt. This salt will have a tendency to precipitate.

In an undersaturated solution, salts have a tendency to dissolve. So when the ocean is undersaturated with respect to calcium carbonate, the shells of organisms, made of CaCO3, have a tendency to dissolve.
The lower the level of calcium carbonate saturation, the quicker shells dissolve and the harder organisms have to work to maintain them.

As the lysocline in the ocean rises, organisms, at or near to the surface, that rely on calcium carbonate, start to struggle.

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