October 18, 2017, 4:23 pm

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Coral mounds

Cold-water corals are important in the formation of cold-water coral reefs or mounds, which are impressive features on the sea floor. These reefs and mounds are commonly found along the European continental margin and occur from 50 m depth down to more than 2000 m. Some of these mounds are only a few metres high, such as the “shallow” reefs of Mingulay. But, some are more than 350 m high and up to a few kilometres in diameter, such as the giant mounds at Rockall bank in the Atlantic Ocean. 

These mounds are formed by a complicated set of processes. First, a coral larva will settle on a hard surface, such as bedrock or a dead coral skeleton. Then, when environmental conditions are favourable (i.e. its warm enough, there’s enough food, the currents are the right speed), the corals start to grow and form complex reef structures. These reefs then attract more organisms, which use the reefs as a home and place to find food. The 3D coral skeleton will also attract sediment, which will accumulate around the framework and therefore the mounds will grow. Then, when conditions aren’t favourable for coral growth, the tops of the mounds may be eroded, as the corals can’t grow fast enough to keep up with the natural erosion of the currents and other organisms. 

The periods of favourable and unfavourable environmental conditions for mound and coral growth are linked to geological time periods known as glacial and interglacial periods, which occur within an ice age. These periods last thousands of years; interglacial periods have warmer global average temperatures while glacial periods have cooler than average temperatures.   

Interglacial periods, such as the last 11,000 years, have been favorable for coral growth and thus coral-mound development. However, to date, we haven’t found any evidence of the cold-water coral Lophelia pertusa from glacial periods in the Northeast continental margin. We therefore assume that cold-water corals could not grow in these environmental conditions, and so mound structures were not built like they were in interglacial periods.