Shallow Gas Hydrate Accumulations at a Nigerian Deepwater Pockmark—Quantities and Dynamics

The evolution of submarine pockmarks is often related to the ascent of fluid from the subsurface. For pockmarks located within the gas hydrate stability zone, methane oversaturation can result in the formation of gas hydrates in the sediment. An ~600 m‐wide sea floor depression in deep waters offshore Nigeria, Pockmark A, was investigated for distributions and quantities of shallow gas hydrates, origins of hydrocarbons, and time elapsed since the last major fluid ascent event. For the first time, pressure coring of shallow sediments and drilling of more than 50 m‐long cores with the sea floor drill rig MARUM‐MeBo70 were conducted in this pockmark. Unusually, high hydrate saturations of up to 51% of pore volume in the uppermost 2.5 m of sediment in the pockmark center substantiate that deepwater pockmarks are a relevant methane reservoir. Molecular and stable C and H isotopic compositions suggest that thermogenic hydrocarbons and secondary microbial methane resulting from petroleum biodegradation are injected into shallower sediments and mixed with primary microbial hydrocarbons. Two independent pore water chloride and sulfate modeling approaches suggest that a major methane migration event occurred during the past one to three centuries. A rough sea floor topography within the pockmark most likely results from combined sediment removal through ascending gas bubbles, hydrate clogging and deflection of migration pathways, gas pressure build‐up, and hydrate sea floor detachment. This study shows for the first time the chronological interrelationship between gas migration events, hydrate formation, and sea floor shaping in a deep sea pockmark

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

Typology, phenomenology and assessment of submarine landslides triggering factors (application to the Nile and Niger deep-sea fans)

Cette thèse aborde, avec différentes approches, l analyse des mécanismes impliqués dans l initiation, le transport et le dépôt de glissements sous-marins au sein de deux grands domaines soumis à de forts apports sédimentaires : les deltas profonds du Nil et du Niger, construits l un et l autre dans des environnements géologiques et climatiques différents. Les travaux conduits sur le delta profond du Nil intègrent des données de géophysique et de sédimentologie, acquises entre 2002 et 2007 dans un cadre universitaire, (équipe Géomer de Géoazur à Villefranche-sur-Mer) ainsi que des données sismiques industrielles (BP-Egypt). En se fondant sur la mise en évidence de diverses relations entre caractéristiques morphologiques et structurales, localisation et probables facteurs de déclenchement ce travail aboutit à une caractérisation typologique de sept grandes masses glissées mises en place au cours des derniers 120000 ans. L impact potentiel de la déstabilisation d hydrates de gaz sur le déclenchement du plus volumineux de ces glissements est évalué au moyen de modèles numériques. L utilisation de modèles numériques est aussi appliquée à l évaluation de l aléa tsunamigénique associé à la mise en place une masse glissée afin d estimer le risque potentiel sur le littoral nord égyptien densément peuplé. L analyse des processus d instabilité est abordée de manière plus détaillées, sur deux secteurs du delta profond du Niger. Ce travail a été conduit grâce à la mise à disposition de données de sismique 3D (d origine industrielle, société Total), de sismique 2D de haut à très haute résolution et de mesures géotechniques in situ et sur carottes (données acquises au cours de la campagne ERIG3D conduite en 2008 par l Ifremer). L intégration de ces données permet de caractériser, à différentes échelles, l architecture d une masse glissée enfouie et son mode de mise en place, mais également la nature et la distribution de zones de faibles résistances, ainsi que le potentiel d instabilité associé. Enfin, l impact sur la stabilité de pente de mouvements structuraux liés à la surrection d un pli est évalué.This thesis addresses with distinct approaches the aspects pertaining to the initiation, the transport and the deposition of submarine landslides in two environments subjected to high sedimentation rates : the Nile and the Niger deep sea fans. The approach conducted on the Nile deep sea fan integrates geophysical and sedimentological data acquired during several surveys carried out between 2002 and 2007 by GéoAzur laboratory along with industrial seismic data provided by BP-Egypt. A typological characterization of seven mass transport deposits emplaced during the last 120 000 years ins proposed based on the recognition of their morphological and structural characteristics, their location and the factor suspected to have participated in their triggering. The potential role played by gas hydrates destabilization in triggering the most voluminous of these mass transport deposits is evaluated using numerical models. Numerical models are also used to evaluate the tsunamogenic potential of a landslide triggered from the shelf edge in order to assess the hazard it may poses on the densely populated north Egyptian coastline. Slope failure processes are analyzed in more details on two different sectors of the Niger deep sea fan. This has been achieved by using 3D seismic data provided by Total along with high to ultra-high resolution seismic data, cores and in situ geotechnical measurements acquired during the ERIG3D cruise conducted by Ifremer in 2008. Integration of these different types of data allows to characterize at different scale, the architecture of a buried mass transport deposit and to study the nature and distribution of weakened zones in order to evaluate their potential impact on slope stability. Eventually, the factors participating in triggering slope failures on a growing anticline are examined.NICE-BU Sciences (060882101) / SudocSudocFranceF

Les glissements de terrain sous-marins sous haute surveillance

National audienceLa mer ligure : une région de montagnes sous-marines Le relief sous-marin de la région niçoise est particulièrement pentu, creusés par des canyons, et certains évènements, tels que des séismes, peuvent le déstabiliser jusqu'à provoquer des glissements de terrain sous-marins. Glissement de terrain meurtrier et tsunami au large de Nice Le 16 octobre 1979, un glissement de terrain se produit sur le chantier de l'agrandissement de l'aéroport de Nice et provoque la mort de 10 ouvriers ainsi qu'une vague de 2 m qui inonde le quartier de la Salis à Antibes et provoque le décès d'une promeneuse

Les glissements de terrain sous-marins sous haute surveillance

National audienceLa mer ligure : une région de montagnes sous-marines Le relief sous-marin de la région niçoise est particulièrement pentu, creusés par des canyons, et certains évènements, tels que des séismes, peuvent le déstabiliser jusqu'à provoquer des glissements de terrain sous-marins. Glissement de terrain meurtrier et tsunami au large de Nice Le 16 octobre 1979, un glissement de terrain se produit sur le chantier de l'agrandissement de l'aéroport de Nice et provoque la mort de 10 ouvriers ainsi qu'une vague de 2 m qui inonde le quartier de la Salis à Antibes et provoque le décès d'une promeneuse

Integrated geophysical, sedimentological and geotechnical investigation of submarine landslides in the Gulf of Lions (Western Mediterranean)

18 pages, 9 figures, 1 tableThe Gulf of Lions presents recurring mass-transport deposits (MTDs) within the Plio-Quaternary sediments, suggesting a long history of mass movements. The two large, surficial MTDs are located on the eastern and western levee of the Rhone canyon over an area exceeding 6000 km2 and volumes exceeding 100 km3. Both MTDs were emplaced 21 ka ago (peak of the Last Glacial Maximum), suggesting a common trigger. Here, we present a multidisciplinary high-resolution geophysical, sedimentological and in-situ geotechnical study of the source and deposit areas of both MTDs to characterize distinct expressions of sediment deformation as well as their spatial and chronological distributions. We show the internal structure of mass movements and resulting MTDs with unprecedented details that were previously represented in the conventional seismic data as transparent and chaotic facies. The combination of multidisciplinary approaches shows new insights into the nature of basal surfaces of the slope failures. In particular, we show that the basal surfaces of the failures consist of clay-rich material contrasting with the overlying turbiditic deposits, suggesting that a strong lithological heterogeneity exists within the strata. We suggest that this change in lithology between clay-rich sediments and turbiditic sequences most likely controls the localization of weak layers and landslide basal surfacesThis work and the PhD of Shray Badhani are funded by the European Union's Horizon 2020 research and innovation programme under the Marie-Skłodowska-Curie grant via project ITN SLATE (grant agreement no. 721403)Peer reviewe

Control of the geomorphology and gas hydrate extent on widespread gas emissions offshore Romania

The Romanian sector of the Black Sea deserves attention because the Danube deep-sea fan is one of the largest sediment depositional systems worldwide and is considered the world's most isolated sea, the largest anoxic water body on the planet and a unique energy-rich sea. Due to the high sediment accumulation rate, presence of organic matter and anoxic conditions, the Black sea sediments offshore the Danube delta is rich in gas and thus shows Bottom Simulating Reflectors (BSR). The cartography of the BSR over the last 20 years, exhibits its widespread occurrence, indicative of extensive development of hydrate accumulations and a huge gas hydrate potential. By combining old and new datasets acquired in 2015 during the GHASS expedition, we performed a geomorphological analysis of the continental slope north-east of the Danube canyon compared with the spatial distribution of gas seeps in the water column and the predicted extent of the gas hydrate stability zone. This analysis provides new evidence of the role of geomorphological setting and gas hydrate extent in controlling the location of the observed gas expulsions and gas flares in the water column. Gas flares are today considered an important source of the carbon budget of the oceans and, potentially, of the atmosphere

Building multisite science services for the European Multidisciplinary Seafloor and water column Observatory (EMSO) Research Infrastructure

American Geophysical Union (AGU) Fall Meeting, 9-13 December 2019, San FranciscoEMSO is a European Research Infrastructure Consortium (ERIC) with 8 member countries. It is coordinated by a central management office and promotes monitoring services offered by 11 fixed-point deep-sea and water column observatories around Europe, from the Atlantic, through the Mediterranean, and to the anoxic Black Sea. EMSO aims are to advance marine science and technologies in the frame of sustainable development policies concerning ocean health, biodiversity, climate, and environmental changes. EMSO provides Science Services, in the form of data and/or access to the regional distributed observatory locations for experiments or testing of new equipment. EMSO ERIC is in its second year of operation, and its challenge is to turn the diversity of technical approaches and science priorities of each regional distributed observatory locations into a strength for building robust network-scale best practices, labels, and science services. This presentation reports on collaborative work initiated in recent months by EMSO, primarily within the Science Service Group, focusing on 3 practical goals: 1) offer online access, Quality Control (QC) and adequate data management tools for deep-water salinity and temperature data from five EMSO sites; 2) develop and promote a standard sampling and analytical procedure for microplastic pollution assessments to be performed at each EMSO site during maintenance cruises; and 3) explore the needs of the growing group of users of marine acoustics data (e.g. geohazards, marine mammals, sound pollution, marine transport) and design online access, QC and suitable scan-visualisation tools for acoustic data records. Our goal is, through and beyond EMSO Science Services, to help build smart ocean observation systems that meet the growing knowledge needs on natural and anthropogenic processes in the oce