Volcans d'ai de Turbaco Auteur(s) : Shelton Année de publication : Extrait de : Colombie et Guyanes (p.3) Siècle(s) traité(s) : 19 Droits : Domaine public Permalien : http://www.manioc.org/images/FRA112740141i1 FRA112740141i1 | Partager |
Volcan d'air de Turbaco Auteur(s) : Shelton Année de publication : Extrait de : L'Univers. Histoire et description de tous les peuples, de leurs religions, murs, coutumes, etc. (fin d'ouvrage, entre p. 22 et p. 23) Siècle(s) traité(s) : 19 Droits : Domaine public Permalien : http://www.manioc.org/images/GAD110440094i1 GAD110440094i1 | Partager |
Fish communities of Geyser and Zelee coral banks (Western Indian Ocean). Auteur(s) : Chabanet, P Tessier, E Durville, P Mulochau, T Rene, Francois Éditeur(s) : Soc Francaise D Ichtyologie Résumé : The ichtyofauna of Geyser and Zelee coral banks (north of Mozambique canal) was investigated in 1996, this work being the first quantitative study on this area. Quantitative data were collected on 11 stations (mean depth 18 m) by visual observations along 50 m transect, widened to 2 m for small sedentary species, and to 5 m for commercial species with a large action range. The main descriptors of the fish communities (species richness, abundance per species) and the trophic structure (omnivorous, herbivorous, carnivorous, browsers of sessile invertebrates) were described and analysed.A total of 294 fish species, belonging to 43 families were recorded. The species richness varied from 45 to 83 species / station, and the abundance, from 292 to 1322 individuals / 100 m(2), the lower values being recorded on the inner slope and the higher ones on the outer slope. In terms of number of individuals, diurnal planktivores were highly represented at all sites (mean 63.7%). Pomacentridae and Caesionidae were the most abundant families, damselfishes being mainly represented by Chromis ternatensis (mean 73 ind., max, 148 ind. 100 m(2)), C. dimidiata (mean 33 ind., max. 151 ind. / 100 m(2)) and Dascyllus carneus (mean 38 ind., max. 92 ind. 100 m(2)), and fusiliers by Pterocaesio chrysozona (mean 19 ind., max. 128 ind. / 100 m(2)), P. tile (mean 12 ind., max. 38 ind. / 100 m(2)) and Caesio teres (mean 6 ind., max. 32 ind. / 100 m(2)). The high values recorded showed that small and isolated coral banks can support relatively rich fish faunas and substantial populations of individuals. This result may be explained by different factors such as the oceanographic circulation favourable to a localised enrichment of nutrients from surface water, the low human impact, a good coral cover and a high degree of self-recruitment associated with regional recruitment. Nevertheless, the main commercial species such as Lutjanus bohar and Variola louti, observed at each station, were represented at a low density (mean 0.8 ind., max. 6 ind. / 100 m(2) and mean 0.2 ind., max. 0.8 ind. / 100 m(2) respectively). This result may be due to the exploitation of these coral banks between 1990 and 1992, which was interrupted following the fast decreasing output.Such an experience shows us the fragility of these small coral banks and the necessity to establish a rational management of the resource. Cybium (0399-0974) (Soc Francaise D Ichtyologie), 2002 , Vol. 26 , N. 1 , P. 11-26 Droits : info:eu-repo/semantics/openAccess http://archimer.ifremer.fr/doc/00000/10132/7687.pdf http://archimer.ifremer.fr/doc/00000/10132/ | Partager |
Geological context and vents morphology of the ultramafic-hosted Ashadze hydrothermal areas (Mid-Atlantic Ridge 13 degrees N) Auteur(s) : Ondreas, Helene Cannat, Mathilde Fouquet, Yves Normand, Alain Éditeur(s) : Amer Geophysical Union Résumé : Recent ROV dives and high-resolution bathymetric data acquired over the Ashadze fields on the Mid-Atlantic Ridge (13 degrees N) allow us to derive constraints on the regional and local geological setting of ultramafic-hosted hydrothermal fields. The active vent fields of Ashadze hydrothermal fields are located in the western axial valley wall, downslope from the termination of a prominent corrugated surface and in a transitional domain with respect to ridge segmentation. The study of the shipboard and ROV bathymetry shows that decameter (100 m by 60 m) to kilometer-scaled rockslides shape the axial valley wall slopes in this region. The Ashadze 1 vent field occurs on a coherent granular landslide rock mass that is elongated in an E-W direction. The Ashadze 1 vent field comprises hundreds of active and inactive sulfide chimneys. The Ashadze 2 vent field is located in a NNE-trending linear depression which separates outcrops of gabbros and serpentinized peridotites. Active black smokers in the Ashadze 2 field are located on ultramafic substratum in a 40-m diameter crater, 5-m deep. This crater recalls similar structures described at some vents of the Logatchev hydrothermal field (Mid-Atlantic Ridge 15 degrees N). We discuss the mode of formation for these craters, as well as that for a breadcrust-like array of radial fissures identified at Ashadze 1. We propose that hydrothermalism at Ashadze can be an explosive phenomena associated with geyser-like explosions. Our study also constrains the geological and geophysical context of the ultramafic-hosted Ashadze hydrothermal system that may use the oceanic detachment fault as a preferred permeability conduit. Geochemistry Geophysics Geosystems (1525-2027) (Amer Geophysical Union), 2012-11 , Vol. 13 , N. 1 , P. Q0AG14 Droits : 2012. American Geophysical Union. All Rights Reserved. http://archimer.ifremer.fr/doc/00111/22223/19895.pdf DOI:10.1029/2012GC004433 http://archimer.ifremer.fr/doc/00111/22223/ | Partager Voir aussi Ashadze Mid-Atlantic Ridge hydrothermal activity hydrothermal explosion crater ultramafic rocks Télécharger |