Deep-water oyster cliffs at La Chapelle continental slope (Armorican margin)
Van Rooij, D.; De Mol, L.; Le Guilloux, E.; Bourillet, J.-F.; Staelens, P.; Foubert, A.; Wheeler, A.J.; Henriet, J.-P.; Huvenne, V. (2007). Deep-water oyster cliffs at La Chapelle continental slope (Armorican margin), in: ASF (Ed.) 11e Congrès Français de Sédimentologie. Caen, 23-24-25 octobre 2007: Livre des résumés. Publications ASF, 57: pp. 304
In: ASF (Ed.) (2007). 11e Congrès Français de Sédimentologie. Caen, 23-24-25 octobre 2007: Livre des résumés. Publications ASF, 57. Association des Sédimentologistes Français: Paris. ISBN 2-907205-56-0. 358 pp.
In: Publications ASF. Association des Sédimentologistes Français: Paris. ISSN 0990-3925
Within the framework of the EC projects FP6 IP HERMES and FP5 RTN EURODOM, as well as the ESF Euromargins project MoundForce, Ghent University organised a joint geophysical and biological research cruise to the La Chapelle continental slope with FUV Belgica from 13 to 20 June 2006. This area was targeted for its reported cold-water coral finds (Le Danois, 1948), rugged topography and hydrodynamics in a setting linking the shelf seas to the deep marine realm. The area was first surveyed using FUV Belgica's multibeam echosounder, imaging canyon heads and thalweg channels between prominent spurs where corals had been reported. High-resolution seismic sparker lines provided a geological context and linked into the existing seismic stratigraphy (Bourillet et al., 2003). Here, we present preliminary results of the La Chapelle area, including the geological and biological ROV GENESIS image analysis which revealed a hidden ecosystem of deep-water oyster banks down to 700 m.The ROV GENESIS dives were mainly concentrated along NW-SE ridges alongside the eastern slope of a newly mapped and yet unnamed spur. The hence called "Ostrea Spur" has a main NE SW orientation in water depths from 150 to 1100 m. The slopes of the spur are intersected by a 'herringbone' like pattern of WNW-ESE orientated gullies on the western slope and NNW-SSE orientated gullies on the eastern slope.The 2 successful dives along the "Ostrea" Spur revealed a sandy to muddy bioturbated seabed, curious bedforms and erosion exposing consolidated carbonate-like sedimentary sequences. These are often cut by vertical step-like escarpments or cliffs up to 10 m high. Ripple marks were also observed on the ROV images, indicating the presence of strong bottom currents, possibly originating from internal tides focused along the canyon axis (New, 1987; Pingree & Le Cann, 1989). At the base of the cliffs, fallen blocks provided settlement sites for sessile organisms whilst the cliffs and protruding banks revealed dense communities of giant oysters, forming a 3D assemblage with occasional cold-water corals (Lophelia pertusa). Though deep-water 'oyster banks' of Neopyncodonte cochlear had already been reported in the Bay of Biscay based on dredges, the larger and deeper Neopycnodonte observed during the ROV GENESIS dives likely belongs to a new species, which have remained hidden to the human eye up to now. Unfortunately the lack of living samples does not allow an exact, phylogenetic identification of the oysters involved in the formation of dense and rich 'hotspot' communities on the eastern slope of the "Ostrea" Spur.
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