Bioerosion of magmatic rocks in a coastal environment: The example of the Cap Vert peninsula (Western Senegal).

Magmatic rocks (mainly dolerites and basanites) and thick ferruginous crusts (ironpans) outcroping on the shore of the Cap Vert peninsula (Western Senegal) are not uniformely abraded by borers and scrapers. Bioabrasion and bioattack are faint in basanites and thick ferruginous crust. Conversely dolerites are penetrated by microorganisms, essentially fungi, in particular related to the genus Ostracoblabe and actively bored by the sea-urchin Echinometra lucunter. Fungi are rather numerous throughout the intertidal zone, particularly in its upper half-part, and persist in the lower part of the supralittoral zone actively moistened by sprays. E. lucunter inhabits the lower part of the intertidal zone and the upper part of the subtidal zone. The study of thin sections showed that fungi first behave like chasmoendoliths before to act as euendoliths. Pyroxenes and peridots can be intensively permeated, whereas plagioclases are generally poorly affected by fungal attacks. Although very scarce, the cyanobacteria Hyella sp. can invade plagioclases situated at the sample surface, at least up to the half level of low tide. Frequencies of borers and scrapers were determined in the different biological horizons recognized by Sourie (1954). Boring activity of E. lucunter was surveyed during 21 months. The deepening of their dwelling holes is about 1 mm/year in the lower half of the intertidal zone whereas it is only about 0.4 mm/year in the associated thick ferruginous crusts. The calculated rate of bioerosion can reach annually about 2 050 g/m(2) in the lower horizon of low tide where the population of E. lucunter is particularly important. The rate of biological erosion decreases rapidly in the overlying horizons. The bioerosion of the dolerites of the Cap Vert peninsula is an active process, contribution of which to the destruction of the rocky shore and to the coastal morphogenesis during the Quaternary is certainly significant.