Multi-disciplinary investigation of fluid seepage on an unstable margin: The case of the Central Nile deep sea fan

We report on a multidisciplinary study of cold seeps explored in the Central Nile deep-sea fan of the Egyptian margin. Our approach combines in situ seafloor observation, geophysics, sedimentological data, measurement of bottom-water methane anomalies, pore-water and sediment geochemistry, and 230Th/U dating of authigenic carbonates. Two areas were investigated, which correspond to different sedimentary provinces. The lower slope, at ∼ 2100 m water depth, indicates deformation of sediments by gravitational processes, exhibiting slope-parallel elongated ridges and seafloor depressions. In contrast, the middle slope, at ∼ 1650 m water depth, exhibits a series of debris-flow deposits not remobilized by post-depositional gravity processes. Significant differences exist between fluid-escape structures from the two studied areas. At the lower slope, methane anomalies were detected in bottom-waters above the depressions, whereas the adjacent ridges show a frequent coverage of fractured carbonate pavements associated with chemosynthetic vent communities. Carbonate U/Th age dates (∼ 8 kyr BP), pore-water sulphate and solid phase sediment data suggest that seepage activity at those carbonate ridges has decreased over the recent past. In contrast, large (∼ 1 km2) carbonate-paved areas were discovered in the middle slope, with U/Th isotope evidence for ongoing carbonate precipitation during the Late Holocene (since ∼ 5 kyr BP at least). Our results suggest that fluid venting is closely related to sediment deformation in the Central Nile margin. It is proposed that slope instability leads to focused fluid flow in the lower slope and exposure of ‘fossil’ carbonate ridges, whereas pervasive diffuse flow prevails at the unfailed middle slope

Les suintements hydrothermaux froids et leur impact sur l'écosystème benthique

Les carbonates authigènes des croûtes et des concrétions associées aux volcans de boue et aux pockmarks de la Ride Méditerranéenne et du delta profond du Nil sont composés d aragonite et de calcite magnésienne. Le ciment carbonaté microcristallin inclut des lithoclastes, des bioclastes, des minéraux détritiques et d autres phases authigènes (pyrite, barytine, gypse). La plupart des valeurs de delta 18O des carbonates sont proches de 3 indiquant qu ils ont précipité en équilibre isotopique avec l eau de fond comparable à celle de l eau actuelle. Certains carbonates sont enrichis en 18O suite à leur précipitation dans des fluides riches en 18O. Les carbonates authigènes sont généralement appauvris en 13C, révélant que la source de carbone provient essentiellement de l oxydation du méthane. La présence de biomarqueurs archaéens et bactériens très appauvris en 13C indique que le méthane est oxydé de manière anaérobique, par couplage avec la réduction bactérienne des sulfates.PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Les suintements hydrothermaux froids et leur impact sur l'écosystème benthique

Les carbonates authigènes des croûtes et des concrétions associées aux volcans de boue et aux pockmarks de la Ride Méditerranéenne et du delta profond du Nil sont composés d aragonite et de calcite magnésienne. Le ciment carbonaté microcristallin inclut des lithoclastes, des bioclastes, des minéraux détritiques et d autres phases authigènes (pyrite, barytine, gypse). La plupart des valeurs de delta 18O des carbonates sont proches de 3 indiquant qu ils ont précipité en équilibre isotopique avec l eau de fond comparable à celle de l eau actuelle. Certains carbonates sont enrichis en 18O suite à leur précipitation dans des fluides riches en 18O. Les carbonates authigènes sont généralement appauvris en 13C, révélant que la source de carbone provient essentiellement de l oxydation du méthane. La présence de biomarqueurs archaéens et bactériens très appauvris en 13C indique que le méthane est oxydé de manière anaérobique, par couplage avec la réduction bactérienne des sulfates.PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Estimation of Soil Moisture and Sea Ice Concentration – A GNSS Reflectometry Concept

International audienc

Palaeo methane-seepage history traced by biomarker patterns in a carbonate crust, Nile deep-sea fan (Eastern Mediterranean Sea)

International audienc

Menes caldera, a highly active site of brine seepage in the Easetern Mediterranean sea : 'in situ" observations from the NAUTINIL expedition (2003)

International audienc

Estimation of Sea Ice Concentration and Soil Moisture: A GNSS Reflectometry Concept

International audienc

Physical properties of muddy sediments from French Guiana

International audienceThe North West migration of long and discontinuous mud banks along the French Guiana coast has been extensively studied during the past years, in particular with a large-scale vision, which consequently has integrated morpho and hydrodynamic data. The aims of the present paper were to use intrinsic sediment properties (grain-size, mineralogy, concentration, and cohesion) to (1) highlight the sedimentary conditions during the consolidation processes from fluid deposit to vegetation development, and (2) verify the apparent homogeneously derived sedimentary facies.Two intertidal transects, Macouria and Cayenne, were compared from the coast to offshore. Their altitude averages of 1 m and 2.8 m above mean sea level, respectively, were different enough to compare the influence of the hydrodynamic impact and emersion time on their sediment properties. The latter, i.e. grain size distribution, mineralogical content, mud concentration, and shear strength (cohesion), were determined from sampled surface sediments (first cm) and along sediment cores (20–30 cm depth) from each transect. A specific X-ray technique was applied to the whole core to differentiate clearly its thin layers.On both intertidal sites, the grain size dominated by the fine silt fraction (2–20 μm) and the bulk mineralogy characterized by five major minerals (quartz, feldspars, chlorite, illite, and kaolinite) appeared homogeneous along both transects and cores. In spite of this apparent uniformity of particle size and mineralogical parameters, as well as for visual observation along the core, high precision X-rays still showed a cyclic sedimentation at a micro-scale level. This cyclicity with intercalation of fine layers was related to distinct dynamic deposits marked by both tidal processes and hydrodynamic factors (swell propagation). The cohesion and concentration results were dependent on the topography, where high topography was characterized by sediments with high cohesion and concentration values, and vice versa. A comparison between these two parameters was done to define critical limits between soft and stiff muds, as well as unvegetated and colonized muds. The favorable intrinsic sedimentary properties for consolidation and colonization were also discussed according to the field observations and bibliographic dat

Diagenetic Carbonates Related to Hydrocarbon-rich Fluid Seepage in the Nile Deep Sea Fan (East Mediterranean Sea)

International audienceDuring the NAUTINIL (September-October 2003), MIMES (July 2004), BIONIL (October 2006) and MEDECO2 (November 2007) cruises, coring and submersible dives were realized in the Nile Deep Sea Fan (NDSF) area. Active fluid venting sites were identified by the presence of living benthic organisms and by methane plumes in the bottom waters above the seeping structures. At all sites, hard carbonate crusts cover irregularly the sea floor. The sediments from the venting areas are organic-rich, contain sometimes carbonate concretions and have a strong H2S smell indicative of active sulfate reduction. The mineralogy of carbonate crusts is dominated by aragonite and Mg-calcite; the mineralogy of concretions is more complex, with mixtures of Mg-calcite, dolomite and ankerite. The oxygen and carbon isotopic compositions of the carbonate from crusts and concretions exhibit large variations (-2.8< δ18O ‰ VPDB <+9.5; -42.6< δ13C ‰ VPDB <+22.4). The wide range of δ18O values reflects variable sources of fluids. Most of the authigenic carbonates from the NDSF were precipitated in isotopic equilibrium with the Mediterranean bottom water. The carbonate crusts and concretions from the brine seeps of the north-western NDSF are enriched in 18O indicating that a source of 18O-rich fluids originated from depth. Differently, a few crusts and concretions from the eastern NDSF exhibit relatively low δ18O values, which are due to precipitation at warm temperatures. The very low δ13C values of the diagenetic carbonates indicate that methane and possibly other heavier hydrocarbons were the major source of carbon that was oxidized as bicarbonate mostly through bacterial sulfate reduction coupled with anaerobic methane oxidation within the anoxic sediment. The very positive δ13C values of the diagenetic carbonates from many carbonate concretions are related to the production of 13C-rich CO2 during methanogenesis within the sub-seafloor sediments

Biofilm dynamics and production in a tropical intertidal mudflat in French Guiana

International audienceThe roles and functioning of the highly dynamic, changing and disturbed tropical intertidal mudflats of the north-eastern coast of South America located between the Amazon River and the delta of the Orinoco River, despite being considered the muddiest in the world, have been little investigated. Here we present a multidisciplinary study conducted in French Guiana during three consecutive days in November 2016 at the end of the dry season, aimed at describing the composition, primary production and photoregulation processes of a tropical mudflat microphytobenthic biofilm in relation to the sediment characteristics of this pristine environment. We selected three stations characterized by a marked compaction gradient due to mudflat topographic elevation and tidal cycles. The results showed that microphytobenthos are well adapted to extreme physical conditions with respect to light intensities. Indeed, the biomass of primary producers, as well as the rates of primary production, reached high levels, which could be higher than those measured in temperate intertidal mudflats. The thick biofilm of microphytobenthos that appeared on the sediment surface was dominated by common epipelic diatoms, endowed with photoregulatory capabilities, such as downward migration and efficient xanthophyll cycle, which was demonstrated for the first time for such an environment in this study via the de-epoxidation of diadinoxanthin to diatoxanthin. With a typical microbial mat/mudflat composition, prokaryotes also contributed together with diatoms to the essential biological function of the biofilm in mud consolidation, especially with the excretion of extracellular polymeric substances. Although these results are somewhat bitty for establishing general rules, they suggest that the surficial biofilm plays a key role in the functioning, consolidation and dynamics of tropical intertidal mudflats in French Guiana