Spatial distribution of diverse cold seep communities living on various diapiric structures of the southern Barbados prism

Three sectors of the south Barbados prism between 1000 and 2000 m depth were explored by the French submersible Nautile. Chemosynthesis-based benthic communities were discovered on several structures affected by diapirism, including mud volcanoes, domes and an anticlinal ridge. The communities are associated with the expulsion of methane-rich fluids which is a wide-spread process in the area. These communities are dominated by large bivalves and vestimentiferans which harbour chemoautotrophic symbiotic bacteria. The symbiotic bivalves include two species of Mytilidae and one of Vesicomyidae, with dominance of a methanotrophic mussel. Cartography of the benthic communities, interpretation of thermal measurements and observation of sedimentary patterns have been used to define the life habits of each of the three species of symbiotic bivalves. Each species has a characteristic preference for different conditions of edaphic and fluid flow: the dominant methanotrophic mussel appears to require high velocity vents and hard substratum. The vesicomyids and the other species of mussel are able to take up sulfide from the sediments, and so are associated with low seepages, but also require soft sediment. The three bivalve species are assumed successively to colonize the top of a diapiric ridge, in a succession related to the temporal evolution of fluid flow and sedimentation. The composition of the bivalve assemblages, their densities and biomasses all differ between the several mud volcanoes and domes studied, and these parameters are thought to be related to the spatial and temporal variations of fluid expulsion through the structures, and the lithification processes linked to fluid expulsion. One very active dome is at present colonized by an exceptionally large and dense population of the methanotrophic mussel. In contrast, communities in another area, on the domes and volcanoes that are currently inactive, were colonized by only a few living vesicomyids and mussels, both associated with sulfur-oxydizing bacteria, and there were numerous empty shells. The densities and biomasses of symbiotic bivalves were far greater in the area studied than in a deeper mud volcano field on the same prism that had been studied previously. This is consistent with a report that methane production is greater in the southern region of this accretionary prism than in the northern. Numerous non- symbiotic organisms were observed in and around the areas of the seeps, some are endemic to the seep communities, including some gastropods and shrimps, others are either colonists or vagrants from the surrounding deep-sea floor. Filter feeders were very abundant, and some of these, like the serpulids and large sponges, may also be dependent on the chemosynthetic production. Faunistic composition of both symbiotic and non- symbiotic taxa, of the assemblages around these cold seeps, is closely related to that reported for communities living on hydrocarbon seeps in the Gulf of Mexico. (C) 1997 Elsevier Science Ltd