The Holocene development of the Mediterranean algal reef Coralligenous

Several types of biogenic reefs, build-ups and mounds are represented in the Quaternary of the Mediterranean, responding to the complex interplay of geological and environmental controls with a diverse set of habitats, architectures and components. The most important reef of the Mediterranean is the Coralligène (Coralligenous = C), including several types of calcareous algal-invertebrate frameworks growing in normal open marine conditions. The Mediterranean C is an Open Frame Reef, characterized by the variable association of calcareous red algae, bryozoans, annelids, bivalves, and corals. Although the living surface of C have been subject of numerous studies, the processes and the components controlling its development are still poorly known. The Italian project FISR “CRESCIBLUREEF” provided an extraordinary opportunity to explore the development of the Mediterranean C, from inception to present-day morphology and distribution, off the SE coasts of Sicily. Our analyses and the comparison with previous studies on the Ligurian C yielded some results that are relevant both for our understanding of the Quaternary dynamics of the Mediterranean shelf environments and for the present-day management of this unique habitat. Below the depth limit of distribution of the Posidonia meadows, the spatial extension of the C hybrid banks over the studied Sicilian shelf controls the hydrodynamics at the seafloor, the carbonate production at the shelf scale, and the biodiversity. The analyzed C are Holocene in age and their development generated a significant change in the seafloor geomorphology. The most part of the framework is invariably built by calcareous red algae, despite the variable occurrence of scleractinians and octocorals on the living surface. The rapid dissolution of the cnidarian remains after death could explain their negligible contribution to the framework, that is very porous, with primary cavities derived from the mode of growth, shape and structure of the skeletonised builders and dwellers. An important secondary porosity derives from bioerosion and other taphonomic processes involving both skeletonised and soft-bodied organisms. A contribution to the construction of the framework is provided also by autochthonous and detrital micrite, with an important role of organic mediation played by sponges for the mineralization of the autochthonous fraction.