Comparing Mediterranean and NE Atlantic cold-water coral mounds: spatial and temporal distribution of benthic carbonate associations

Cold-water coral (CWC) mounds located along the Atlantic and Mediterranean margins have been subject of intensive research during the last 15 years. Data collected in over a hundred of oceanographic cruises have highly increased our knowledge on CWC mound typologies, settings and on the environmental factors controlling their evolution. Nevertheless, there is still a poor knowledge about calcifying benthic organisms characterizing CWC communities, though their skeletal accumulation represents the dominant component of most mound facies and can provide important insights on the mound evolutionary stages. This study focuses on benthic carbonate associations (BCA) collected, through box- and gravity cores, from selected CWC mounds located in four key regions: 1. Santa Maria di Leuca CWC Province, Ionian Sea, Central Mediterranean; 2. Melilla Mound Field, Alboran Sea, Western Mediterranean; 3. Pen Duick scarpment, Gulf of Cadiz, NE Atlantic; 4. Moira Mounds, Porcupine Seabight, NE Atlantic. The main goals of this work are (1) to characterize modern BCA (bio- and thanatocoenoses) from the examined sites and to relate their spatial distribution to patterns in environmental variables and (2) to identify and interpret BCA variations through time in order to better understand the mound evolutionary processes. Our preliminary results show that the modern BCA from the four analyzed regions share major common taxonomic components. Their spatial distribution, either within a single region or a single mound, is mostly influenced by local environmental variables such as the seafloor topography, the substrate composition and the intensity of bottom currents. However, NE Atlantic and Mediterranean coral-dominated BCA can be easily distinguished due to the presence of characteristic taxa, among which the solitary scleractinian Carophyllia sarsiae and the gastropod Amphissa acutecostata, common to abundant in the NE Atlantic CWC sites and absent (or rarely present as fossil) in the Mediterranean ones. Moreover, modern coral-dominated BCA from the Pen Duick Escarpment (Gulf of Cadiz), with a prevalent presence of dendrophylliid corals and a peculiar associated fauna, are clearly distinct from the Mediterranean and the Porcupine Seabight fields, dominated by Madrepora oculata and Lophelia pertusa respectively. Interestingly, although the modern NE Atlantic communities are known to be more diversified than the Mediterranean ones, the BCA from the Moira Mounds (NE Atlantic) show the lowest number of taxa among the examined sites. On the contrary, the benthic calcifying organisms (in particular molluscs) from the Alboran Sea mounds seem to be the most diversified and larger in size. Regarding the temporal distribution of the examined BCA from CWC mounds, several variations observed along the gravity cores seem to be related to environmental changes at a local scale. However the appearance/disappearance of specific taxa and/or morphotypes (e.g. thick-walled Lophelia pertusa) as well as striking variations in the relative abundance of taxonomic groups hints at larger-scale oceanographic and sedimentary variations, probably related to climatic oscillations.