Ecological characterisation and paleo-evolution of two contrasting cold-water coral mound provinces of the Mediterranean Sea

Abstract

Scleractinian cold-water coral reefs are considered to be key hotspots of benthic biodiversity in the deep ocean. Due to their relevant ecological role and susceptibility to anthropogenic disturbances protection and conservation measures have been applied to these habitats, even though they are far from being completely understood. Throughout the last two decades several studies have quantitatively described the biodiversity of Atlantic cold-water coral reefs, finding considerable differences among biogeographic regions. In contrast, and probably owed to the scarcity of these habitats in the Mediterranean Sea, the knowledge related to coral reef biodiversity in this basin remains modest and almost purely qualitative. On a different note, when coral reefs are under persistent suitable environmental conditions and have a sufficient sediment input, they can develop and form large geomorphic structures known as coral mounds. The latter are sensitive to changes in climate and capable of recording such variations in the chemical composition of the coral skeletons. Numerous surveys in the Atlantic have associated coral mound development patterns to environmental variations caused by glacial-interglacial cycles. Within the Mediterranean, coral mound formation studies have been so far limited to the Alboran Sea and to the last 15 kyr, due to the lack of gravity cores encompassing longer periods of time. In this thesis a wide range of techniques, including ROV video-analysis, multivariate statistics, U-series dating, computed tomography and geochemical analyses were applied to acquire a better understanding of the spatiotemporal distribution of Mediterranean cold-water coral reefs and the processes controlling their evolution into mounds during the last 400 kyr. More precisely, the present study aimed to (1) quantify the density of uncommonly thriving coral reefs and accompanying megabenthic species within the Cabliers Coral Mound Province, and describe their distribution along it; and (2) explore which are the main environmental variables and paleoclimatic events that have controlled coral mound formation in Cabliers and in the newly discovered Tunisian Coral Mound Province. The research presented here revealed the densest and most flourishing cold-water coral reefs witnessed so far in the Mediterranean Sea and brought further insight into their distribution along the crests of ridge-like coral mounds. This thesis also contributed to increase our knowledge on the main species associated to Mediterranean coral reefs and their relative abundances, which showed considerable differences to those found in Atlantic reefs. In regards to coral mound formation, this work has expanded the current knowledge outside the Alboran Sea and back to 400 ka BP. Almost opposite development patterns were observed between the Cabliers and Tunisian coral mound provinces, with the former mainly developing throughout deglaciations and temperate interstadial periods and the latter during glacial periods. Nonetheless, both provinces seem to depend on a high surface productivity and an appropriate depth of the interface between Atlantic and Levantine Intermediate Waters for the coral mounds to develop. Lastly, the oceanographic alterations caused in the Eastern Mediterranean Basin during Sapropel events also seem to have had detrimental effects for coral mound formation in the Western basin.

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ORCID for Gavin Foster: orcid.org/0000-0003-3688-9668

ORCID for Veerle Huvenne: orcid.org/0000-0001-7135-6360

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