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Résumé -Étude géochimique des émissions naturelles de CO 2 du Massif Central : origine et processus de migration du gaz -Cette étude présente les principaux résultats de campagnes de monitoring géochimique menées en 2006 et 2007 dans le cadre du projet Géocarbone-Monitoring, sur le site de Sainte-Marguerite, situé dans le Massif Central. Ce site constitue un « laboratoire naturel » pour l'étude des interactions CO 2 /fluides/roches et des mécanismes de migration du CO 2 vers la surface, à l'échelle des temps géologiques. Le caractère particulièrement émissif de cet « analogue » permet également de tester et valider des méthodes de mesure et de surveillance des futurs sites de stockage de CO 2 . Au cours des campagnes de terrain, nous avons analysé des flux de CO 2 entre le sol et l'atmosphère, et nous avons prélevé et analysé à la fois des gaz des sols, et du gaz provenant de sources carbo-gazeuses, présentes dans toute la région. Un dispositif de « monitoring continu » dans le temps a également été testé, afin d'enregistrer conjointement les teneurs en CO 2 de l'atmosphère et dans le sol en un point précis. Nous avons pu mettre au point un suivi géochimique basé sur la composition isotopique des gaz rares prélevés dans les sols. L'ensemble de nos résultats, confronté à la géologie de terrain, nous a permis de mettre en évidence l'origine mantellique du CO 2 . Ce CO 2 remonte rapidement à la surface à l'état gazeux, le long de failles normales et/ou décrochantes, actives actuellement. Les teneurs et flux de CO 2 dans le sol sont spatialement variables et élevés, et montrent également une origine mantellique. Les teneurs atmosphériques semblent faiblement augmenter par rapport à l'important dégazage observé dans la région. Abstract

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 toauthigenic carbonate crusts and 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. Highlights► Fluid seepages are associated with slope destabilization along the Zambezi continental slope ► Gas seepage has a microbial origin ► Carbonate diagenetic chimneys have been exhumed during the mass sliding ► The spatial organization is representative of the temporal evolution of slope destabilization J o u r n a l P r e -p r o o f Journal Pre-proof temperature along the SCAMPI track. In the study area, it was operated during two dives (SCB01 and SCB03) on two different areas where pockmarks are present (Fig. 2). Gas sampling and geochemical studyGas was sampled in three Calypso sediment piston cores (MOZ04-CS18 and MOZ04-CS19, and also MOZ04-CS17 made north of the study area; see supplementary material Fig. S1, S2, S3). The gas samples ...

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The origin of the Central European Volcanic Province, which includes the Massif Central and the Eifel regions, is currently debated. Several different causes have been proposed to account for the volcanism observed in the area. Namely, both the presence of one or more mantle plumes under Europe, and the upwelling and melting of upper mantle related to the formation of the Alps, have been suggested as possible drivers of volcanism. In order to distinguish between these possibilities, we have analysed noble gases in the Lignat Spring to constrain the nature of the mantle source below the Massif Central. The gas has a 3 He/ 4 He ratio of 5.51 Ra, whereas its neon isotopic signature is identical to that of MORB source. The gas has an 40 Ar/ 36 Ar ratio of 1113 ± 3, far in excess of the atmospheric ratio. The xenon isotopic pattern is explained by 95 % atmospheric contamination of a MORB-like gas. The noble gases clearly show that the mantle beneath Massif Central has a geochemical signature similar to MORB source mantle, with the exception of helium, which more closely corresponds to SCLM signatures, and thus removes the need for the presence of a mantle plume in the region.