SEDCORAL · The sedimentary environment of deep-water Corals: characterisation of a fragile marine habitat in need of conservation

Final Activity Report Summary - SEDCORAL (The Sedimentary Environment of Deepwater Corals: characterisation of a fragile marine habitat in need of conservation)

Corals are generally associated with warm and shallow tropical or subtropical seas; however, they are not limited to such environments. Many species are adapted to a life in cold and dark waters and do not rely on sunlight and symbiosis with algae for their food supply. Some of those deep-water corals, also referred to as 'cold-water corals', are found to build impressive mound structures along the European continental margin. Typical species with such behaviour are lophelia pertusa and madrepora oculata and the reefs they create host rich ecosystems. However, several reefs are damaged due to human impact, and are in need of protection. In order to support conservation measures, a better insight into the working of cold-water coral reefs is necessary. Scientific research over the last 15 years showed that the surroundings in which the corals live are very important. The current regime and seabed sediment transport processes in particular are major factors. They determine whether the corals survive or not if they create large reefs or small mounds and patches. The aim of the SEDCORAL project was to provide a detailed characterisation of the sedimentary environment of some of those cold-water coral mounds in order to better understand its influence on the growth, building capacity and recolonisation capacity of the corals. The undertaken approach was to study three areas in detail, namely two sets of small mounds of circa 5 m height (Darwin and Moira mounds, in the north Rockall Trough and west of Ireland, respectively) and one large coral mound, of approximately 155 m height, which was drilled to its base during the Integrated ocean drilling programme (IODP) Expedition 307 (Challenger mound, also west of Ireland). In each case, the focus laid on the sedimentary environment of the corals and mounds. The techniques used included grain size analysis of samples obtained with a variety of coring equipment, the study of seabed maps acquired with acoustic techniques, the interpretation of seabed photographs and video data which were sometimes collected with Remotely operated vehicles (ROVs), current meter measurements and the study of foraminifera, which were microfossils that once lived in the water column of the open ocean and were buried in the sediments. The data were collected and analysed within a Geographical information system (GIS), which was a computer-based interactive map and database. The project resulted in major scientific advances in the understanding of cold-water corals and the functioning of deep sea sediment dynamics. It increased the insight in the processes and development history of the sedimentary environment of the Darwin and Moira mounds since the last ice age. We understood that the small Darwin and Moira mounds were not the best analogues for the initial stages of the large Challenger mound. They were created through sediment baffling, i.e. collecting and trapping between the coral branches, and were part of large sand sheets on the seabed. Those sands were sorted and shaped because of subtle variations in the seabed topography and bottom currents. As they slowed down, currents first deposited the coarser sediments, taking the finer grains a little further. The sediment baffled in the initial Challenger mound was much finer, probably because the currents were slower at the time or because it was deposited at a later stage, when the seabed conditions were quieter. All mounds studied though shared the common feature that they were initiated on a compacted or cohesive erosional surface, formed as the result of a large paleoclimatic change associated with a change in bottom current regime. This seemed to have created the ideal substrate for the initiation of cold-water coral mounds. A further achievement of the SEDCORAL project was to implement a new approach to the methods used for the investigation. Applying traditional techniques, e.g. the study of foraminifera, in unusual settings, i.e. in sandy rather than muddy sediments, allowed the project to come to surprising results that formed a major part of the final conclusions.

Data: CORDIS, © European Union