A resilient deep-water rhodolith bed off the Egadi Archipelago (Mediterranean sea) and its actuopaleontological significance

Rhodolith beds are priority marine benthic habitats for the European Community, because of their relevance as biodiversity hotspots and their role in the carbonate budget. The high-Mg calcite calcified thalli promote their preservation through time, resulting in their common occurrence in the fossil record, thus making rhodoliths a significant archive of past environmental and climate changes. The present temperate Mediterranean Sea is home to rhodolith-rich up to rhodolith-dominated habitats which have been targeted over the years for ecological research and management actions. Furthermore, the Mediterranean Basin hosts an important fossil legacy of rhodolith beds that formed under contrasting climatic scenarios since the Neogene. Most examples of Mediterranean rhodolith beds lay within the depth range (30-75 m), while far less is known about deeper occurrences. An outstanding rhodolith bed has been recently identified off the Egadi Archipelago (70-95 m, Central Mediterranean Sea) in the frame of the Italian Marine Strategy Framework Directive project. The rhodolith bed carpets an area >7 km2 between Marettimo and Favignana Islands (Egadi Archipelago), with a cover of live rhodoliths up to 95%. The dominant rhodolith morphotype is pralines, ellipsoidal to discoidal in shape (average size 11 mm, maximum size 31 mm) associated with biogenic gravelly-sand. They present both seafloor bedforms (megaripples and ripples) and frequent bioturbations by epifaunal and semi-infaunal sea urchins. Old algal nodules, biogenic sand/mud, or, less frequently, terrigenous particles serve as nucleus of individual rhodoliths. Living calcareous algae are Lithothamnion valens, L. cf. minervae, Phymatolithon lenormandii, and Lithothamnion spp. The latter characterize also the dead algal association close to the nucleus, with the subordinate presence of Lithophyllum sp. Benthic fauna corresponds to the circalittoral environment, with exclusive species of the Coastal Detritic biocoenosis or related to coarse sediments. Moreover, the general textural aspects and bedforms are in line with moderate water energy at the seafloor. Radiocarbon ages reveal that the cores of rhodoliths date back to 2334, 2159 and 1997 BP, which correspond to the Roman Warm Period. Calculated growth rate provides a very low figure (0.004 mm/yr). The inception of such rhodolith bed occurred under warmer climate conditions, but its development is continuing at present because of the persistence of adequate light conditions and bottom hydrological regime, with a very slow growth-rate. Therefore, the Egadi case history certifies a rare example of a deep-water resilient rhodolith bed, which should be taken into consideration for the potential recognition of fossil analogs in the Mediterranean Basin.