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

The last glacial maximum Balearic Abyssal Plain megabed revisited

Megabeds are thick sedimentary layers extending over thousands square kilometres in deep sea basins and are thought to result from large slope failures triggered by major external events. Such deposits have been found in at least three areas of the Mediterranean Sea. Although their discovery dates back to the early 1980s, many questions remain, concerning their initiation, source area, extent, and the nature of their emplacement. One of the largest previously documented megabeds was emplaced during the Last Glacial Maximum across the Balearic Abyssal Plain with a thickness of 8-10 m in water depths of up to 2800 m. New 3.5 kHz sub-bottom profiles and sediment cores provide greater constraint on the lateral variability of the megabed and allow to map it beyond previous estimates, with a revised areal extent up to 90,000-100,000 km2. Megabed terminations show gradual pinch-out to the West and an abrupt eastward termination against the Sardinia steep margin. The megabed presents both in seismic profiles and in sediment cores a tripartite subdivision likely corresponding to changes in flow regimes across the basin with a central area of sandy facies and erosional base oriented NNE-SSW allowing renewed discussions about sources and trigger of the megabed

Détections automatiques de structures géomorphologiques sous-marines en Deep Learning avec ArcGIS Pro ?

International audienceLes algorithmes d’apprentissage profond pour détecter automatiquement des objets à partir d’imagerie donnent, dans plusieurs domaines, des résultats prometteurs, et sont de plus en plus utilisés et accessibles sans expertise pointue. Nous souhaitons donc voir si leur utilisation sur de l’imagerie sous-marine comme la bathymétrie et ses dérivés, ou encore la réflectivité du fond sous-marin, pourrait nous permettre de réaliser une détection automatique de structures géomorphologiques comme des sorties de fluides, des glissements de terrain… En géosciences marines, nous disposons de jeux de données manuels de ces structures, et nous comptons les utiliser comme données d’apprentissage sur des composites d’imagerie adaptés

Les fluides géologiques dans les bassins sédimentaires

International audienceFor over 50 years, sedimentary basins have been considered as the lithosphere's surface film, belonging to the subsurface domain and containing the vast majority of accessible mineral and energy resources. Beyond their human use, sedimentary basins are more importantly the ultimate exchange interface between the earth's main reservoirs. Firstly, between the upper lithosphere and the atmosphere-hydrosphere reservoirs, exchanges are mainly vertical. Next, between the onshore reservoirs, i.e., on the continental part, and the offshore reservoirs, i.e., in the submerged part of the margins, exchanges are lateral and may take place over great distances. Unexpected low accumulations and/or dry..

Détections automatiques de structures géomorphologiques sous-marines en Deep Learning avec ArcGIS Pro ?

International audienceLes algorithmes d’apprentissage profond pour détecter automatiquement des objets à partir d’imagerie donnent, dans plusieurs domaines, des résultats prometteurs, et sont de plus en plus utilisés et accessibles sans expertise pointue. Nous souhaitons donc voir si leur utilisation sur de l’imagerie sous-marine comme la bathymétrie et ses dérivés, ou encore la réflectivité du fond sous-marin, pourrait nous permettre de réaliser une détection automatique de structures géomorphologiques comme des sorties de fluides, des glissements de terrain… En géosciences marines, nous disposons de jeux de données manuels de ces structures, et nous comptons les utiliser comme données d’apprentissage sur des composites d’imagerie adaptés

Chercher de la glace au fond de l’océan au large de l’Amazone

National audienc

Le bassin ligure : synthèse géomorphologique et géologique

International audienc

Analysis of Nighttime Radiances Measured by VIIRS Satellite Sensor (NASA/NOAA) Over Coastal Waters at Seasonal and Daily Time Scales. Application to the Observation of River Discharges During Flooding Events

International audienceThe observation of coastal waters using satellite remote sensing is of primary importance to gain understanding on the complex processes occurring in those highly dynamical ecosystems. The analysis of ocean color data relies on the interaction between the incident sunlight daytime radiation that propagates within the ocean–atmosphere system and the suspended material (hydrosols and aerosols) present in each medium. The goal of this study is to examine how relevant nighttime radiances as measured by the panchromatic Day–Night Band (DNB) of the Visible Infrared Imaging Radiometer Suite (VIIRS) satellite sensor could be for the detection and monitoring of river plumes in turbid waters. The spatial and temporal variation of the nighttime radiance is studied over areas that are subjected to intense river flooding events, namely, the regions of the delta of Mekong (Vietnam) and the delta of Rhône (France), at seasonal and daily scales and for various magnitudes of the lunar illumination. The VIIRS observations show that the variations of the radiance are closely related to seasonality of the water turbidity; higher radiances are observed during the wet season as a result of the intense rainfalls. The seasonal cycle of the radiance is noticeable near the full moon phase for both areas, thus demonstrating the high sensitivity of VIIRS sensor radiometry for detecting ocean changes at night. It was also noted that the radiance seasonal cycle remains distinguishable for the Rhône area for a lunar illumination that could be far from the full moon condition, namely, for a fraction value F of the full moonlight of 40%. The impact of the adjacency effects induced by the diffuse glow of city lights on the nighttime radiance was examined at seasonal scale. Such an impact could be as high as 40% over water targets, thus suggesting that aquatic areas located sufficiently far from coastal cities should be preferentially used to properly observe water turbidity at nigh

Influence of the Suspended Particulate Matter on the Satellite Radiance in the Sunglint Observation Geometry in Coastal Waters

International audienceThe analysis of satellite ocean color data that are acquired over coastal waters is highly relevant to gain understanding of the functioning of these complex ecosystems. In particular, the estimation of the suspended particulate matter (SPM) concentrations is of great interest for monitoring the coastal dynamics. However, a high number of pixels of satellite images could be affected by the surface-reflected solar radiation, so-called the sunglint. These pixels are either removed from the data processing, which results in a loss of information about the ocean optical properties, or they are subject to the application of glint correction techniques that may contribute to increase the uncertainties in the SPM retrieval. The objective of this study is to demonstrate the high potential of exploiting satellite observations acquired in the sunglint viewing geometry for determining the water leaving radiance for SPM dominated coastal waters. For that purpose, the contribution of the water leaving radiance L w to the satellite signal L TOA is quantified for the sunglint observation geometry using forward radiative transfer modelling. Some input parameters of the model were defined using in-situ bio-optical measurements performed in various coastal waters to make the simulations consistent with real-world observations. The results showed that the sunglint radiance is not sufficiently strong to mask the influence of the oceanic radiance at the satellite level, which oceanic radiance remains significant (e.g., 40% at 560 nm for a SPM concentration value of 9 g m −3). The influence of the sunglint radiance is even weaker for highly turbid waters and/or for strong wind conditions. In addition, the maximum radiance simulated in the sunglint region for highly turbid waters remains lower than the saturation radiances specified for the current ocean color sensors. The retrieval of L w and SPM should thus be feasible from radiances measured in the sunglint pattern by satellite sensors, thus increasing the number of exploitable pixels within a satellite image. The results obtained here could be used as a basis for the development of inverse ocean color algorithms that would interestingly use the radiance measured in sunglint observation geometry as it has been done for other topics than the field of ocean color research