Éditeur(s) : Elsevier
Résumé : We report on a multidisciplinary study of cold seeps explored in the Central Nile deep-sea fan of the Egyptian margin. Our approach combines in situ seafloor observation, geophysics, sedimentological data, measurement of bottom-water methane anomalies, pore-water and sediment geochemistry, and Th-230/U dating of authigenic carbonates. Two areas were investigated, which correspond to different sedimentary provinces. The lower slope, at similar to 2100 m water depth, indicates deformation of sediments by gravitational processes, exhibiting slope-parallel elongated ridges and seafloor depressions. In contrast, the middle slope, at similar to 1650 m water depth, exhibits a series of debris-flow deposits not remobilized by post-depositional gravity processes. Significant differences exist between fluid-escape structures from the two studied areas. At the lower slope, methane anomalies were detected in bottom-waters above the depressions, whereas the adjacent ridges show a frequent coverage of fractured carbonate pavements associated with chemosynthetic vent communities. Carbonate U/Th age dates (similar to 8 kyr BP), pore-water sulphate and solid phase sediment data suggest that seepage activity at those carbonate ridges has decreased over the recent past. In contrast, large (similar to 1 km(2)) carbonate-paved areas were discovered in the middle slope, with U/Th isotope evidence for ongoing carbonate precipitation during the Late Holocene (since similar to 5 kyr BP at least). Our results suggest that fluid venting is closely related to sediment deformation in the Central Nile margin. It is proposed that slope instability leads to focused fluid flow in the lower slope and exposure of 'fossil' carbonate ridges, whereas pervasive diffuse flow prevails at the unfailed middle slope.
Marine Geology (0025-3227) (Elsevier), 2009-06 , Vol. 261 , N. 1-4 , P. 92-104

Droits : 2009 Elsevier B.V. All rights reserved.
http://archimer.ifremer.fr/doc/2009/publication-6586.pdf
DOI:10.1016/j.margeo.2008.10.008
http://archimer.ifremer.fr/doc/00000/6586/

Éditeur(s) : Vertigo
Résumé : Submersible exploration of the deep-sea floor during the last decades revealed new insights in this part of our planet with the discovery of an unsuspected diversity of ecosystems. Coral reefs, confined in our mind to warm and shallow tropical waters, have been observed at several hundreds meter depth along continental margins. Like their tropical counterparts, cold water corals are home for several invertebrate and fish species. The diversity and complexity of this rich ecosystem has just started to be studied. Documented and potential treats by human activities including bottom fishing and petroleum industry activities have to be considered and there is an urgent need to prevent further degradation of these vulnerable reefs. The objective of the CARACOLE cruise, lead by Ifremer and gathering geologists and biologists of several European countries, was to explore by robotic submersible several coral mounds off Ireland. This new approach revealed the real extent of the coral colonies, the diversity of the associated fauna and helped to test hypotheses to understand the mound formation.
L'exploration des fonds océaniques, notamment à l'aide des submersibles, a apporté ces dernières décennies un nouvel éclairage sur cette partie du globe, avec la découverte d'une diversité d'écosystèmes jusque là insoupçonnée. Des massifs de coraux, surtout connus des eaux chaudes et peu profondes des régions tropicales se développent à plusieurs centaines de mètres de profondeur le long des marges continentales. Ils servent de substrat, de refuge et de nourriture à de nombreux invertébrés et poissons, et sont à l'origine d'un écosystème riche dont la diversité et la complexité commencent tout juste à être étudiées. Malgré leur profondeur, ils sont soumis à l'impact des activités humaines, notamment la pêche par chalut qui a déjà détruit certains de ces "récifs" mais aussi la menace potentielle de l'exploration pétrolière. La campagne CARACOLE, menée par l'Ifremer et regroupant des géologues et biologistes européens spécialistes de ces milieux, avait pour objectif la prospection par submersible filoguidé de plusieurs monts de coraux au large de l'Irlande. Elle a permis d'évaluer l'étendue des colonies de coraux, de caractériser la faune associée et de tester différentes hypothèses pour expliquer leur formation.
Vertigo (Vertigo), 2004-12 , Vol. 5 , N. 3

Droits : 2004 Vertigo
http://archimer.ifremer.fr/doc/2004/publication-2364.pdf
http://archimer.ifremer.fr/doc/00000/2364/