New data concerning the coralligenous atolls of cap corse: an attempt to shed light on their origin

A new coralligenous morphotype named ‘atolls’ was discovered in 2011 in northern Cap Corse (Western Mediterranean Sea). With the aim of inventorying and characterizing these structures, and to attempt to shed light on their origins, two oceanographic campaigns (using e.g. side scan sonar, multibeam echosounder, sparker, ROV and submarine with 3D photogrammetry) were carried out in 2013 and 2014, in this sector, and in the south of the island in search of similar structures. Off Cap Corse, nearly one thousand atolls were identified between 105 and 130 m depth; the majority of them are situated between 110 and 125 m depth. These atolls are generally grouped (several tens to several hundred) and are to be found in two main sectors, situated between 22 and 31 km from the coast. They occur on subhorizontal bottoms, in rocky depressions occupied by coastal detritic bottom, and are 1 to 2 m thick. The average size of these atolls ranges from 20 to 25 m in diameter, but a few smaller or more extensive structures have been identified. The height of the central core ranges from 0.5 to 3.0 m, the intermediate zone, which surrounds this core, presents a width of about 10 m, and the exterior crown has a width of 1 to 5 m. No atoll was found along the south coast although the topography is similar (bathymetric patterns, presence of a seamount). Several typologies have been evidenced; these typologies could correspond to atolls at different stages of evolution in relation to phenomena of bioerosion or bioconstruction. Furthermore, the occurrence of ‘fossil coralligenous’ formations, between 138 and 140 m depth, with still living rhodoliths at the summit, might support the hypothesis of a biological origin, with the original formation occurring during a period when the sea level was lower, several thousand years ago.


Coralligenous, Atolls, Rhodoliths, Corsica

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Pergent-Martini Christine, Alami S, Bonacorsi Marina, Clabaut Philippe, Daniel Boris, Ruitton Sandrine, Sartoretto Stephane, Pergent Gerard (2015). New data concerning the coralligenous atolls of cap corse: an attempt to shed light on their origin. UNEP/MAP – RAC/SPA, 2015. Proceedings of the second Mediterranean Symposium on the conservation of Coralligenous and other Calcareous Bio-Concretions (Portorož, Slovenia, 29-30 October 2014). BOUAFIF C., LANGAR H., OUERGHI A., edits., RAC/SPA publ., Tunis: 247p. ISBN 978-9938-9574-2-6. pp.129-134.

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