Deep rhodolith beds in the Ustica island (Sicily, Southern Tyrrhenian Sea): a sedimentary and paleoecological approach

Proceedings of the International Conference “GEOSUB 2016″, Ustica, Italy 13 – 16 September, 2016 (Furlani S., Antonioli A., Scicchitano G. & Busetti M. Guest Editors)

Abstract

The results of a multidisciplinary study conducted on the circalittoral soft bottom assemblages at Secchitello (Sicchiteddu) and in the neighbouring areas along the southern coast of Ustica Island (Sicily, Southern Tyrrhenian Sea) are presented. In particular, the study provided useful data for a taxonomic, ecological and paleoecological revision of the Coastal Detritic facies with free-living calcareous algae and for a critical analysis of their relationship with the sedimentary dynamics. During a survey, carried out by the University of Catania (1991) with a R.O.V. (remotely operated vehicle), the presence of the Coastal Detritic (DC) biocoenosis, characterized by a surprisingly high biodiversity, and of the Platform Coralligenous (C) biocoenosis, dominated by rhodolith bio-concretions with large and branched Bryozoa was recorded. The DC of Secchitello, situated at a depth of between 50 and 90 m, was mainly characterized by: prâlines, boxwork, branches and coated grains growth forms; the dominance of the Lithothamnion minervae facies; coarse sediments belonging to the sand and gravel classes, with the last one mainly of biogenic nature. Where the hydrodynamism is reduced (at a depth of between 80 and 90 m), an ecotone community between the DC and the C biocoenoses, growing on boxworks, was found. The density and biodiversity of the free-living calcareous algae and of epiflora found at Secchitello are high, and the extension of the rhodolith beds are comparable to that of the C assemblage. The rough morphology of the bottom, increasing the current activity, together with the presence of biodisturbance phenomena mainly due to echinoderms such as Cidaris cidaris, Echinaster sepositus and “sail” epiphytes, such as Cystoseira spinosa v. compressa and Phyllophora crispa, were responsible for the increase of the rhodolith overturning. The analysis of thin sections of non-geniculate coralline red algae allowed to reconstruct the paleoenvironmental evolution of the Secchitello, pointing out three different phases of colonization: initial, intermediate and recent. The transition from one phase to another is probably due to the changes in intensity and direction of the currents in the last millennium.