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International audience; Evidences for active fluid seepages have been discovered along the Zambezi continental slope (offshore Southern Mozambique). These seepages are mostly associated with pockmarks which are aligned along a trend parallel to the slope and running closely upstream of the headwall scarp of a wide zone of slope destabilization. Fluid seepages are interpreted as a potential trigger for the slope destabilization. Acoustic anomalies within the water column have been interpreted as related to moderate bubble seepages mostly located outside and only punctually inside the destabilization zone. Exploration with the SCAMPI towed camera system in the widest pockmark (diameter 200 m wide) has shown fluid seepages associated to authigenic carbonate crusts and possibly bacterial mats. These fluid seepages are also associated to the presence of chemiosynthetic organisms (Vesicomyidae and Thyasiridae bivalves, Siboglinidae tubeworms). The sampled gas in the sediment corresponds mainly to CH4 of microbial origin, generated by hydrogenotrophic methanogenesis from a substrate of organic origin, i.e. a conventional process of genesis of microbial gas in the marine domain. No evidence for thermogenic gas was detected. Another type of pockmarks has been observed within the core of the slope destabilization zone. Most of these pockmarks are inactive in terms of fluid seepage at present time and are associated to carbonate buildups forming chimney geometries. They probably correspond to diagenetic chimneys of former fluid migration pathways that have been exhumed during the mass sliding and the surrounding depression are related to recurrent activity of strong lateral slope currents which have scoured the sediments around. The spatial organization of the slope destabilization features is considered as representative of the temporal evolution of the landslide giving information about the dynamics of slope instability processes. This proposed evolution started by scattered seepages of formation water with dissolved gas. Then free gas seepages appeared notably in the upper part of the slope. This was followed by progressive shallow deformation in the sediments downslope of the main gas seepages. Finally, the whole slope was destabilized forming imbricated landslides exhuming locally former diagenetic chimneys.

Seamounts in the Mozambique Channel host rich but potentially vulnerable ecosystems. A towed camera survey was used to assess the composition, vulnerability and resilience of four seamounts: Glorieuses, Sakalaves, Bassas da India, and Hall Bank. This six dive survey, between 300 and 1,000 m depth recorded > 40 hours of videos, > 6000 still images, covering > 200,000 m². Over 70,000 individuals were observed and 400 morphospecies identified. The main taxonomic groups were sponges, corals, crustaceans, echinoderms, and fish. Preliminary analysis based on morphotypes suggests that composition, densities and diversity of these groups varied significantly between seamounts. Variability has also been observed at a local scale, between the peak and upper slope of a seamount. Glorieuses is a muddy terrace dominated by three sponge morphotypes. Sakalaves’ plateau is dominated by brittle stars and corals, while the upper slopes are mainly dominated by fish and urchins, sponges and crabs. The contrary pattern was observed on Bassas da India and Hall Bank, where the peak is dominated by fish, and the upper slopes by sponges and corals. Relief and substrate appear to be the two main drivers of faunal community distribution. Ongoing work with taxonomists will verify image-based morphotypes with biological sample identifications.