Deflection of natural oil droplets through the water column in deep-water environments: The case of the Lower Congo Basin

International audienceNumerous recurrent seep sites were identified in the deep-water environment of the Lower Congo Basin from the analysis of an extensive dataset of satellite-based synthetic-aperture radar images. The integration of current data was used to link natural oil slicks with active seep-related seafloor features. Acoustic Doppler current profiler measurements across the water column provided an efficient means to evaluate the horizontal deflection of oil droplets rising through the water column. Eulerian propagation model based on a range of potential ascension velocities helped to approximate the path for rising oil plume through the water column using two complementary methods. The first method consisted in simulating the reversed trajectory of oil droplets between sea-surface oil slick locations observed during current measurements and seep-related seafloor features while considering a range of ascension velocities. The second method compared the spatial spreading of natural oil slicks from 21 years of satellite monitoring observations for water depths ranging from 1200 to 2700 m against the modeled deflections during the current measurement period. The mapped oil slick origins are restricted to a 2.5 km radius circle from associated seep-related seafloor features. The two methods converge towards a range of ascension velocities for oil droplets through the water column, estimated between 3 and 8 cm s-1. The low deflection values validate that the sub-vertical projection of the average surface area of oil slicks at the sea surface can be used to identify the origin of expelled hydrocarbon from the seafloor, which expresses as specific seafloor disturbances (i.e. pockmarks or mounds) known to expel fluids

Structure and evolution of the Demerara Plateau, offshore French Guiana : rifting, tectonic inversion and post-rift tilting at transform-divergent margins intersection

International audienceWe present the structure and evolution of the eastern part of the Demerara plateau, offshore French Guiana, from the analysis of geophysical data collected during GUYAPLAC cruise. This area is located at the intersection of a transform segment and a divergent segment of a continental margin related to the Early Cretaceous opening of the Equatorial Atlantic. The main structures are NNE-SSW to NNW-SSE trending normal faults on the eastern edge of the plateau, and WNW-ESE to NW-SE trending acoustic basement ridges on its northern edge. When replaced in their Albian position, these structures appear to be parallel to the coeval oceanic accretion axis and transform faults, respectively. The most striking structures are related to a post-rift but syn-transform tectonic inversion, producing E-W to WNW-ESE trending folds, sealed by a regional unconformity. This shortening can not be related to ridge push, but is probably related to a plate kinematic change 105 My ago, that modified the deformation in the vicinity of the transform fault. Late post-rift evolution also includes a significant Tertiary oceanward tilt of the edge of the Demerara plateau. The driving mechanism of this late tilt is unclear, but may be related to a lithospheric flexure resulting from the loading of the abyssal plain by the Orinoco and Amazon deep-sea fans

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

Ledelta profond du Nil (structure et évolution depuis le Messinien (Miocène terminal))

PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

Glissements de terrain, surpressions de fluide et incisions (caractérisation géologique et modélisation)

Les glissements de terrain sont un des principaux facteurs d érosion des chaînes de montagne. L étude des facteurs de déclenchement et des paramètres contrôlant l évolution des instabilités gravitaires est donc un enjeu très important dans la compréhension des processus de dénudation des reliefs et dans la prévention du risque naturel associé. De nombreuses instabilités gravitaires affectent la Chaîne Côtière de l île du Nord de Nouvelle-Zélande. L analyse détaillée du complexe glissé de Waitawhiti, situé au cœur du synclinal de Tawhero, a permis de mettre en évidence un mécanisme de glissement original dans ce domaine émergé de marge active. Les glissements du complexe sont bordés en bas de pente par des vallées profondément incisées, dont la direction est perpendiculaire à la direction de mouvement des glissements. Trois zones d échappement de fluides, mettant en évidence la migration vers la surface de gaz thermogéniques, ont été découvertes dans la région. Ces fluides peuvent se trouver confinés à la base de niveaux silteux peu perméables, et ainsi entrer en régime de surpression. De telles surpressions réduisent de façon critique la résistance au cisaillement de ces niveaux peu perméables et facilitent alors le glissement de la couverture sus-jacente. De plus, les fortes incisions fluviatiles érodent les butoirs distaux et éliminent ainsi toute force résistant au mouvement en bas de pente des glissements.Des modélisations analogiques ont montré que la combinaison de surpressions de fluide et d incision favorise le déclenchement de glissements, même le long de faibles pentes basales. Des failles normales se forment le long des flancs de la vallée, et se propagent ensuite de façon rétrogressive vers le haut de pente. L étendue de la déformation augmente avec l augmentation de la pression de fluide et/ou de l angle de base. Nous avons également développé un modèle analytique de glissement afin de détailler les forces mises en jeu et l impact de la pression de fluide sur l évolution des glissements en l absence d un butoir distal. Les résultats analytiques montrent, à l instar des modèles expérimentaux, que le mouvement peut être initié dans ce cas lorsque de faibles pressions sont appliquées à la base de la couverture potentiellement mobile. La longueur de glissement augmente quand la pression de fluide augmente.L incision fluviatile conduisant à la mise à l affleurement de niveaux soumis à des surpressions de fluide et dont la résistance au cisaillement est réduite, et à l absence de résistance en bas de pente, est identifiée ici comme un facteur important de déclenchement d instabilités gravitaires de grande ampleur.Landsliding is a major erosional factor in mountain ranges. The study of the triggering factors and of the parameters controlling the evolution of gravitational instabilities is therefore an important stake in the understanding of the processes leading to the denudation of reliefs and in the prevention of the associated natural hazard. Multiple gravitational instabilities affect the Coastal Ranges of the North Island of New Zealand. The detailed analysis of the Waitawhiti sliding complex, located in the core of the Tawhero syncline, permits to reveal an original sliding mechanism in this emerged active margin setting.In this complex, the landslides are bounded downslope by deep-incised valleys having directions perpendicular to the sliding directions. Three gas seeps, evidencing upward migration of thermogenic gases to the surface have been discovered in the area. These fluids might be confined at the base of low-permeability siltstone layers, and be overpressured. Such fluid overpressure critically decrease the shear strength of these low-permeability layers and facilitate sliding of the overlying cover. Furthermore, deep fluvial incisions erode downslope buttresses, therefore removing any force resisting sliding at the base of the slope.Analogue modelling show that combined fluid overpressure and incision promote sliding, even on low-angle basal slopes. Normal faults form on the valley flanks, and propagate retrogressively upslope. The extent of the deformation increases with increasing fluid pressure and/or basal slope angle. We have also developed an analytical model of gravity sliding in order to detail the forces in presence and the influence of fluid pressure on the evolution of landslides in the absence of any downslope buttress. The analytical results show, as observed in the experimental models, that sliding might be triggered in this case where low pressures are applied at the base of the potentially mobile cover. The sliding-sheet length increases with increasing fluid pressure.Fluvial incision, leading to the exposure of mechanically weak layers subjected to fluid overpressure and to the removal of the force resisting sliding at the base of the slope, is identified in this study as an important triggering factor for large-scale gravitational instabilities.LILLE1-Bib. Electronique (590099901) / SudocSudocFranceF

Neodymium Isotopes in Glauconite for Palaeoceanographic Reconstructions at Continental Margins: A Preliminary Investigation From Demerara Rise

Contourite sediment accumulations at continental margins are related to strong bottom water circulation, where intense winnowing can result in neoformation of authigenic grains of glauconite at the seafloor. In this study, we investigated whether such glauconite grains could faithfully record ambient bottom-water neodymium (Nd) isotopic compositions, and hence be used as paleoceanographic archives. To this purpose, we measured Nd isotopic compositions (εNd) in a series of glauconitic grains, foraminiferal assemblages, leached Fe-Mn oxyhydroxide phases, and detrital clays separated from a contourite sediment record at the Demerara slope off French Guiana (IG-KSF-11; 2370 m water depth), at a location where the present-day εNd distribution along the water column is well characterised. We show that the εNd composition of core-top glauconite grains (−12.0 ± 0.5) agrees with the expected NADW-like seawater signature at the same location and water depth (−11.6 ± 0.3), while departing from measured εNd values for corresponding detrital clays (−11.3 ± 0.2), foraminiferal (−10.9 ± 0.2), and Fe-Mn oxyhydroxide fractions (−9.2 ± 0.2). This finding indicates that glauconitic grains at this particular location are probably best suited for paleoceanographic reconstructions than foraminifera and leached Fe-oxyhydroxide fractions, which appear to be influenced by sediment redistribution and the presence of terrestrial continental Fe-oxides, respectively. Using rare earth elements (REE), we tentatively propose that the acquisition of seawater Nd isotopic signatures by glauconite is controlled by the presence of authigenic REE-bearing phosphate-rich phases intertwined within clay mineral sheets, while confirming previous findings that the process of glauconitisation results in the progressive loss of REE within glauconitic grains. Preliminary paleoceanographic implications suggest strengthened bottom-water circulation of the glacial analogue of NADW at this particular location and water depth, with a εNd signature (between −10.8 and −11.5) similar to that of modern NADW

Monitoring of natural oil seepage in the Lower Congo Basin using SAR observations.

International audienceSynthetic Aperture Radar (SAR) is a spaceborne tool allowing near real-time imagery over large ground footprints of areas hundreds of kilometres wide. SAR also allows persistent observations of the process of oil discharges, benefiting from (1) day and night observations, (2) independence of cloud cover and (3) high revisiting frequencies. The interpretation of 82 SAR scenes over the Lower Congo Basin for the 1994–2012 period allowed the recognition of 1400 natural seepage slicks associated with 102 individual seep sites. We acquired an additional SAR data set with a short revisit time over a selected prolific area. The data set consisted of 22 SAR scenes acquired over a 10-day period, meaning a maximum revisiting period of 12 h between subsequent SAR acquisitions. The short-term approach shows that seepage slicks were detected with wind speeds between 1.5 and 6.5 m/s (2.91 to 12.63 knots). Both long-term and short-term monitoring evidences that seepage events are intermittent, with the occurrence rate of oil release ranging from 5 to 80%. Short-term monitoring shows that the seepage pace is independent between seep sites, suggesting contrasted controlling factors. It also shows that the residence time of seepage slicks is systematically shorter than 12 h, which is far below reported values. Understanding the seepage slick residence time is essential to provide an accurate estimation of the fluid flow and to compute the volumetric outflow. The integration of in-situ mooring points measuring the current velocity 10 m below the sea surface, in addition to local wind field and slick length, allowed us to more accurately estimate the residence time of oil slicks at the sea surface before vanishing. The results show that the sea surface residence time of oil slicks is limited to a few hours, with a median value of 3 h 15 min. These new residence time estimations allowed us to propose a quantification of the regional oil output in the Lower Congo Basin, estimated at 4380 m³/year. This area may therefore be considered as the world's third biggest oil-supplying province from natural leakages

DRADEM campaign - 2016 July - Scientific report

The DRADEM campaign was performed from July 9th to 21th 2016 on board the Pourquoi Pas?, in the Exclusive Economic Zones of Suriname and French Guyana. This campaign belongs to a program dedicated to geological investigations of the Demerara plateau, following the GUYAPLAC (2003) and IGUANES (2013) campaigns, and before the MARGATS campaign (2016). The aims of the DRADEM campaign were to map the continental slope of the transform margin north of the Demerara plateau, and to dredge the rocks outcropping in the slope.We completed the bathymetric mapping of the continental slope, including part of the edged of the Demerara plateau. These new bathymetric data confirm the segmentation of the transform margin in three parts with very different morphologies. In addition, two circular structures were interpreted as mud volcanoes, one on the northern edge of the plateau, the other one in the distal part of the Orinoco deep sea fan.Twelve dredges were performed between 4700 and 3500 m depths. Four from these twelve did not recovered rocks. The eight others brought back variables amounts of rocks, often encrusted, but of various natures: sediments (breccia, coarse sandstones, sandstones with plants debris, sandstones with shells, clayey ooze), micro-granular rocks and metamorphic rocks (including mylonite). The nature of the rocks was determined from macroscopic observation of the rocks, which are currently altered. Of course, these determinations need to be validated and specified by onshore further studies. Anyway, most of these rocks were previously unknown in this area, and they will strongly influence our understanding of the structure and evolution of this margin. They evidence huge vertical displacements that brought back to the surface some of these rocks that formed in a deep setting

DRADEM campaign - 2016 July - Scientific report

The DRADEM campaign was performed from July 9th to 21th 2016 on board the Pourquoi Pas?, in the Exclusive Economic Zones of Suriname and French Guyana. This campaign belongs to a program dedicated to geological investigations of the Demerara plateau, following the GUYAPLAC (2003) and IGUANES (2013) campaigns, and before the MARGATS campaign (2016). The aims of the DRADEM campaign were to map the continental slope of the transform margin north of the Demerara plateau, and to dredge the rocks outcropping in the slope.We completed the bathymetric mapping of the continental slope, including part of the edged of the Demerara plateau. These new bathymetric data confirm the segmentation of the transform margin in three parts with very different morphologies. In addition, two circular structures were interpreted as mud volcanoes, one on the northern edge of the plateau, the other one in the distal part of the Orinoco deep sea fan.Twelve dredges were performed between 4700 and 3500 m depths. Four from these twelve did not recovered rocks. The eight others brought back variables amounts of rocks, often encrusted, but of various natures: sediments (breccia, coarse sandstones, sandstones with plants debris, sandstones with shells, clayey ooze), micro-granular rocks and metamorphic rocks (including mylonite). The nature of the rocks was determined from macroscopic observation of the rocks, which are currently altered. Of course, these determinations need to be validated and specified by onshore further studies. Anyway, most of these rocks were previously unknown in this area, and they will strongly influence our understanding of the structure and evolution of this margin. They evidence huge vertical displacements that brought back to the surface some of these rocks that formed in a deep setting