Cold seep communities as indicators of fluid expulsion patterns through mud volcanoes seaward of the Barbados accretionary prism

Cold seep communities are sustained by massive methane-rich fluid expulsion through mud volcanoes located at about 5000 m in the Barbados Trench. These communities, dependent on chemosynthetic processes, are dominated by a vesicomyid bivalve assumed to be a new species related to the genus Calyptogena, and by large bushes of the sponge Cladorhizidae. Both are associated with symbiotic bacteria and are indicative of methane release in seawater and sulphide production in sediments. Non-symbiotic organisms, such as large fields of filter-feeding polychaetes and high densities of meiofauna are indicative of enhanced biological production in the sediments. The spatial distribution of the bivalve populations was mapped using video observations and a computer method based on a simple calculation of the area covered by a submersible camera. The observation of clam beds of Variable densities allows us to define two types of fine-scale fluid expulsion pattern: dense Calyptogena beds (up to 150 ind.m(-2)) are associated with ''vents'' with relatively high fluid discharge velocities of about 10 cm s(-1) and focused by high permeability conduits, whereas dispersed clams (1-10 ind.m(-2)) are probably sustained only by slow, diffusive ''seepages''. The distribution of the chemosynthetic fauna on a well-explored volcano indicates a biological zonation on different concentric zones from the centre to the edges of the volcano, highlighting the heterogeneity of the fluid expulsion pattern at the scale of the volcano. The spatial distribution of the chemosynthetic communities characterizes the fluid expulsion on several types of volcanoes: two mud volcanoes, identified as diatremes, named Atalante and Cyclope, are flat with a central lake of warm fluid mud that is devoid of life, whereas Calyptogena beds are located in the outer regions. On the two other structures, mounds shaped as cones, all activity is concentrated near the summit and seems to be related to higher flow Vents than on diatremes. However, the scarceness of bivalves on the flanks of these volcanoes indicates that low flow seepages are less developed than on diatremes. The high mean clam density and the presence of large Cladorhizidae bushes on the volcano Atalante show that larger quantities of methane are emitted through this structure than on the others, and suggest a more evolved surface, favouring chemosynthetic production, than on the other diatreme, named Cyclope. The level of colonization and the spatial patterns of the chemosynthetic communities, when compared on the different volcanoes, suggest that they are at different stages of activity. (C) 1997 Elsevier Science Ltd.