Paleo-communities across the mesophotic to deep zones of the Terrible and Nameless Banks, The Sicily Channel, Central Mediterranean

Mesophotic (30–150 m) and deep reef (> 150 m) paleo-communities are significantly less studied in the Central Mediterranean, limiting our understanding of their development across this region. The Terrible and Nameless Banks in the Sicily Channel present a valuable opportunity to investigate these ancient ecosystems, offering insights into the processes that shaped deeper reef communities around these structures.

Here, we examined carbonate rocks collected from mesophotic to deeper settings on the Terrible and Nameless Banks. We aimed to describe the carbonate bioconstruction surrounding these structures and to understand the processes that led to their evolution. Carbonate rocks and fossil samples have been collected using rectangular chain bag dredges during the R/V Meteor expedition M191. A total of 50 carbonate samples, including well-lithified carbonate crusts and dead benthic organisms, have been described, of which 20 have been cut and prepared for thin-section analysis.

Samples recovered from mesophotic depths of both Terrible and Nameless Banks show that paleo-reefs associated with these depths are mainly associated with coralline algae-rich facies. Highly bioeroded coralline boundstone is associated with fragments of corals, bryozoans, and serpulids. The free-living form of coralline algae, rhodoliths, are also significant contributors to mesophotic ecosystems. Rhodoliths are 2 to 5 cm in size, showing spheroidal to ellipsoidal morphologies, with growth forms ranging from encrusting, warty to fruticose, lumpy. Coralline algae typically encrust various lithified rocks, including sedimentary and volcanic rocks, which can serve as nuclei for their growth. These encrusted rocks are often further covered by fenestrate bryozoans and encrusting foraminifers. Coral species at this depth are represented by Corallium rubrum (red coral) and Dendrophyllia sp. In the deeper regions of the banks, down to a depth of 655 meters, the carbonate samples consist primarily of lithified, hard rocks, and no coralline algae have been observed. The paleo-communities are mainly represented by cold-water coral species such as Lophelia pertusa, Desmophyllum dianthus, and Dendrophyllia alternata. Additionally, colonies of deep-water oyster-coral framestone, possibly Neopycnodonte zibrowii-Desmophyllum association, are present.

Our initial results show that the presence of sufficient light in the mesophotic zone is an important factor for the thriving of coralline algae-related facies. The specific topography of the Terrible and Nameless Banks creates moderate to strong currents around these structures, prevents sediment burial, and promotes the rolling and shaping of rhodoliths into rounded forms. These currents also produce local upwelling around banks, bringing cool, nutrient-rich water from deeper depths to the mesophotic zone and enhancing the development of corals and bryozoans. In deeper waters, internal waves can play a significant role in nutrient delivery, sustaining the biogenic communities such as coral and oyster assemblages.