Éditeur(s) : Elsevier
Résumé : A study of organic carbon mineralization from the Congo continental shelf to the abyssal plain through the Congo submarine channel and Angola Margin was undertaken using in situ measurements of sediment oxygen demand as a tracer of benthic carbon recycling. Two measurement techniques were coupled on a single autonomous platform: in situ benthic chambers and microelectrodes, which provided total and diffusive oxygen uptake as well as oxygen microdistributions in porewaters. In addition, sediment trap fluxes, sediment composition (Org-C, Tot-N, CaCO3, porosity) and radionuclide profiles provided measurements of, respectively input fluxes and burial rate of organic and inorganic compounds. The in situ results show that the oxygen consumption on this margin close to the Congo River is high with values of total oxygen uptake (TOU) of 4 +/- 0.6, 3.6 +/- 0.5 mmol m(-2) d(-1) at 1300 and 3100m depth, respectively, and between 1.9 +/- 0.3 and 2.4 +/- 0.2 mmol m(-2) d(-1) at 4000 m depth. Diffusive oxygen uptakes (DOU) were 2.8 +/- 1.1, 2.3 +/- 0.8, 0.8 +/- 0.3 and 1.2 +/- 0.1 mmol m(-2) d(-1), respectively at the same depths. The magnitude of the oxygen demands on the slope is correlated with water depth but is not correlated with the proximity of the submarine channel-levee system, which indicates that cross-slope transport processes are active over the entire margin. Comparison of the vertical flux of organic carbon with its mineralization and burial reveal that this lateral input is very important since the sum of recycling and burial in the sediments is 5-8 times larger than the vertical flux recorded in traps. Transfer of material from the Congo River occurs through turbidity currents channelled in the Congo valley, which are subsequently deposited in the Lobe zone in the Congo fan below 4800 m. Ship board measurements of oxygen profiles indicate large mineralization rates of organic carbon in this zone, which agrees with the high organic carbon content (3%) and the large sedimentation rate (19 mm y(-1)) found on this site. The Lobe region could receive as high as 19 mol C m(-2) y(-1), 1/3 being mineralized and 2/3 being buried and could constitute the largest depocenter of organic carbon in the South Atlantic.
Deep Sea Research Part II: Topical Studies in Oceanography (0967-0645) (Elsevier), 2009-11 , Vol. 56 , N. 23 , P. 2223-2238

Droits : 2009 Elsevier B.V. All rights reserved.
http://archimer.ifremer.fr/doc/2009/publication-7316.pdf
DOI:10.1016/j.dsr2.2009.04.005
http://archimer.ifremer.fr/doc/00000/7316/

Éditeur(s) : Elsevier
Résumé : The main feature of the Congo-Angola margin in the Gulf of Guinea is the Congo (ex-Zaire) deep-sea fan composed of a submarine canyon directly connected to the Congo River, a channel and a [sediment] lobe area. During the multi-disciplinary programme called BIOZAIRE conducted by Ifremer from 2000 to 2005, two CTD-O2 sections with discrete water column samples were performed (BIOZAIRE3 cruise: 2003-2004) to study the influence of the Congo River discharges, both in the surface layer and in the deep and near-bottom layers. The surface layer water is greatly diluted with river water that has a heavy particle load. The deep layer is affected by episodic turbidity currents that flow in the deep Congo channel and reach deep areas far from the coast. Previous studies revealed deep anomalies in oxygen (deficit) and nutrient (excess) concentrations at not, vert, similar4000 m depth and assumed that they resulted from mineralisation of the particulate organic matter from the Congo River. The BIOZAIRE3 sections were designed to explore these phenomena in more detail near the Congo channel. Oxygen and nutrients were measured as well as additional parameters, including stable isotopes of oxygen and carbon, dissolved inorganic carbon and pH. For the surface layer, the effect of the Congo River was studied with reference to salinity. Deviations from the theoretical dilution of various inorganic solutes suggested the occurrence of mineralisation and consumption processes. For the deep layer, the network of CTD-O2 stations gave a more detailed description of the deep anomalies than in previous studies. From the east-west section, anomalies appeared on the bottom at 4000 m depth and became slightly shallower when they spread to the west. They were also present north and south on the bottom along the 4000 m isobath. In these deep waters, the decrease in the o13C values of dissolved inorganic carbon confirmed that the mineralisation of organic matter plays a role in generating these anomalies. The location of the origin of this deep anomaly is debated. Here, arguments are given in favour of mineralisation of the particulate organic matter input that overflows from the Congo channel at not, vert, similar4000 m depth during turbidity current events. Other authors suggest that this input comes from downslope particle transport. Anomalies of the same origin, but weaker, also occurred deeper on the Congo lobe, where the Congo channel ends, but with a significant pH decrease on the bottom which was not seen at 4000 m depth.
Deep Sea Research Part II: Topical Studies in Oceanography (0967-0645) (Elsevier), 2009-11 , Vol. 56 , N. 23 , P. 2183-2196

Droits : 2009 Elsevier Ltd All rights reserved.
http://archimer.ifremer.fr/doc/2009/publication-6737.pdf
DOI:10.1016/j.dsr2.2009.04.002
http://archimer.ifremer.fr/doc/00000/6737/

Éditeur(s) : HAL CCSD
Résumé : Sand intrusions (or injectites) are most often the product of post-depositional remobilization of sand leading to its injection into the surrounding rocks. While injectites were recognized for the first time nearly 200 years ago, their emplacement process has been studied for a couple of decades only, since the concepts of deep sea depositional environments have allowed us to better understand their emplacement processes. However, these processes are still relatively poorly understood. Our approach is based on the study of injectites in the Lower Congo Basin from seismic and well data, which we compare to a fossil system in the SE France basin. We have shown that: (1) In buried turbidite channel systems draping deposits on the channel flanks and terraces of channels have the same geophysical signature as 'wing-like' injectites. Finally, the only criterion for identifying seismic injectites is the presence of bedding-discordant seismic reflections, and in the best case the associated uplift of the overlying seismic reflectors. (2) Seismic-scale conical and saucer-shaped sand injectites have been identified in the Lower Congo Basin. The remobilization is likely due to overpressuring induced by the buoyancy effect of hydrocarbons trapped in the margins of a lobe buried underneath 160 m of sediment, followed by the sudden injection of fluidized sand associated with fault reactivation of faults (with a possible role of nearby salt diapirs). (3) A network of injectites (dykes, sills/wings and laccoliths) was formed in the Vocontian basin during the late Albian and/or early Cenomanian, from a lower-middle Albian turbidite channel. The emplacement is probably due to the early compartmentalization of the channel during its burial and the increase of the sedimentation rate generating overpressure; and the subsequent large influx of deep fluids triggering injection. The injection of sand was polyphased: a first episode formed the sills and another emplaced the dykes. Sills/wings and dykes propagated about 2 km laterally away from the parent sand body and about 200 m up to the surface, revealing a much more extended lateral than vertical reach, contrary to the classically accepted idea from the interpretation of seismic data. (4) The emplacement of this large network of injectites was governed by hydrofracturing. Therefore, its morphology is dependent on the host rock heterogeneity (isotropy, fractures), the paleo-stress orientation (ó3 = NW-SE) and the burial depth of the source (300-600 m) at the time of injection. The study of this fossil network allows us to define the relationship between morphology of the injected network and stress state at the time of injection. This relationship can be extrapolated to constrain the morphology of subsurface networks beyond seismicvisibility. (5) Sands injected into low permeability lithologies bear evidence to a major event of fluid escape in the studied basin, but also channeled fluids long after their formation. In this way, injectites both attest to specific episodes of fluid migration in sedimentary basins and contribute to long-lived re-routing of migrating fluids once emplaced. The injection of sand is associated with the sudden escape of fluids, probably resulting from a significant tectonic and/or sedimentary event; in addition, the architecture of injectite networks is governed by the local paleo-stress and heterogeneity in the host rock. Consequently, characterizing injectite networks is an important step in understanding the plumbing systems of continental margins, i.e. the post-depositional evolution of sedimentary basins.
Les intrusions sableuses (ou injectites) sont le plus souvent le produit de la remobilisation post-dépositionnelle des sédiments et de l'injection du sable dans les roches environnantes. Bien que reconnues pour la première fois il y a près de 200 ans, elles ne sont réellement étudiées que depuis quelques dizaines d'années, depuis que les concepts sur les environnements de dépôt dans les domaines marins profonds nous permettent de mieux comprendre les processus de mise en place. Cependant, ces processus restent encore aujourd'hui relativement mal compris. Notre approche repose sur l'étude d'injectites dans le bassin du Bas-Congo a partir de données de sismique et de puits que nous comparerons a un système fossile dans le bassin du SE de la France. Nous avons montre que : (1) Dans des systèmes de chenaux turbiditiques enfouis, les dépôts de drapage sur les marges et terrasses de chenaux présentent la même signature géophysique que les injectites de type "wing". Finalement, le seul critère sismique d'identification des injectites est la présence de réflexions sismiques sécantes vis-a-vis de la stratigraphie associée dans le meilleur des cas au soulèvement des réflecteurs sismiques sus-jacents. (2) Des injectites d'échelle sismique en forme de cône et d'assiette ont été identifiées dans le bassin du Bas-Congo. La remobilisation résulte probablement des pressions anormales induites par l'effet de flottabilité des hydrocarbures piégés dans les marges d'un lobe enfoui sous 160 m de sédiment, puis de l'injection soudaine du sable fluidise associée a la réactivation de failles (possible rôle des diapirs de sel a proximité). (3) Un réseau d'injectites (dykes, sills/wings et laccolites) s'est formé dans le bassin Vocontien entre la fin de l'Albien supérieur et/ou le début du Cénomanien, depuis un chenal turbiditique de l'Albien inférieur-moyen. La mise en place résulte probablement de la compartimentalisation précoce du chenal au cours de son enfouissement et de l'augmentation du taux de sédimentation générant la surpression et de l'apport ulterieur d'importante quantité de fluides profonds déclenchant l'injection. L'injection du sable a été polyphasée : une première injection a formé des sills et une suivante des dykes. Les sills/wings et les dykes se sont propagés latéralement au chenal source sur environ 2 km et vers la surface sur environ 200 m, mettant en évidence une forte remobilisation latérale plutôt que verticale, contrairement a l'idée classiquement admise a partir de l'interprétation des données sismiques. (4) La formation de ce large réseau d'injectites a été gouvernée par des mécanismes d'hydrofracturation. Par conséquent, sa morphologie a été dépendante des hétérogenéités de la roche hôte (milieu isotrope, fracture), des directions de paléo-contraintes (ƒÐ3 = NWSE) et de la profondeur d'enfouissement de la source (300-600 m) au moment de l'injection. L'étude de ce réseau fossile permet de définir les relations entre morphologie du réseau injecté et état de contraintes au moment de l'injection. Cette relation peut être extrapolée de façon à contraindre la morphologie des réseaux de subsurface au-delà de la visibilité sismique. (5) Les sables injectés dans des lithologies de faible perméabilité témoignent d'un épisode d'échappement de fluide important dans les bassins étudiés mais ont aussi guide les fluides longtemps après leur formation. Les injectites contribuent ainsi a l'initiation épisodique et la pérennisation de migrations de fluides dans les bassins sedimentaires. Le processus d'injection est associé a l'échappement brutal de fluides, résultant vraisemblablement d'un évènement tectonique et/ou sédimentaire important, et l'architecture des réseaux d'injectites est gouverné par les paléo-contraintes locales et les hétérogénéités de la roche hôte. Par conséquent, la caractérisation des réseaux d'injectites est une étape importante dans la compréhension de la plomberie des marges, c'est-a-dire l'évolution post-dépôt des bassins sédimentaires.
https://tel.archives-ouvertes.fr/tel-01011486

tel-01011486
https://tel.archives-ouvertes.fr/tel-01011486
https://tel.archives-ouvertes.fr/tel-01011486/document
https://tel.archives-ouvertes.fr/tel-01011486/file/These_Monnier_2013_banalisee.pdf

Éditeur(s) : Pergamon-elsevier Science Ltd
Résumé : The Congo deep-sea fan is one of the largest fans in the world still affected by presently active turbidity currents. The present activity of deep-sea sedimentary processes is linked to the existence of a direct connection between the Congo River estuary and the Congo canyon head that allows relatively continuous sediment feeding of the deep-sea environment, in spite of a wide continental shelf (150 km). Because of this important activity in terms of sedimentary processes, the deep-sea environment of the Congo-Angola margin presents major interests concerning physical, chemical and biological studies near the seafloor. The main aim of this paper is to present the initial geological context of the BioZaire Program, showing a synthesis of the major results of the ZaiAngo Project including (1) the brief geological setting of the Congo-Angola margin, (2) the structure of the modern Congo deep-sea fan, (3) the sedimentary architecture of the recent Congo turbidite system (from the canyon to the distal lobes) and (4) the recent and present turbidite sedimentation. In order to provide useful information and advice relevant to biological and geochemical studies across the Congo sedimentary system, this article is particularly focused on the present sedimentary processes and the present activity of turbidity current along the Congo canyon and channel.
Deep Sea Research Part Ii Topical Studies In Oceanography (0967-0645) (Pergamon-elsevier Science Ltd), 2009-11 , Vol. 56 , N. 23 , P. 2169-2182

Droits : Crown Copyright 2009 Published by Elsevier Ltd. All rights reserved.
http://archimer.ifremer.fr/doc/00000/11128/7848.pdf
DOI:10.1016/j.dsr2.2009.04.001
http://archimer.ifremer.fr/doc/00000/11128/

Éditeur(s) : Elsevier
Résumé : The long-term BIOZAIRE multidisciplinary deep-sea environmental program on the West Equatorial African margin organized in partnership between Ifremer and TOTAL aimed at characterizing the benthic community structure in relation with physical and chemical processes in a region of oil and gas interest. The morphology of the deep Congo submarine channel and the sedimentological structures of the deep-sea fan were established during the geological ZAIANGO project and helped to select study sites ranging from 350 to 4800 m water depth inside or near the channel and away from its influence. Ifremer conducted eight deep-sea cruises on board research vessels between 2000 and 2005. Standardized methods of sampling together with new technologies such as the ROV Victor 6000 and its associated instrumentation were used to investigate this poorly known continental margin. In addition to the study of sedimentary environments more or less influenced by turbidity events, the discovery of one of the largest cold seeps near the Congo channel and deep coral reefs extends our knowledge of the different habitats of this margin. This paper presents the background, objectives and major results of the BIOZAIRE Program. It highlights the work achieved in the 16 papers in this special issue. This synthesis paper describes the knowledge acquired at a regional and local scale of the Equatorial East Atlantic margin, and tackles new interdisciplinary questions to be answered in the various domains of physics, chemistry, taxonomy and ecology to better understand the deep-sea environment in the Gulf of Guinea.
Deep Sea Research Part II: Topical Studies in Oceanography (0967-0645) (Elsevier), 2009-11 , Vol. 56 , N. 23 , P. 2156-2168

Droits : 2009 Elsevier Ltd All rights reserved.
http://archimer.ifremer.fr/doc/2009/publication-7349.pdf
DOI:10.1016/j.dsr2.2009.04.015
http://archimer.ifremer.fr/doc/00000/7349/

Éditeur(s) : Elsevier
Résumé : To elucidate the origin of the silicic acid (DSi) anomaly observed along the 4000 isobath on the Congo margin, we have established a benthic Si mass balance and performed direct measurements of biogenic silica (bSiO(2)) dissolution in the deep waters and in the sediments. Results strongly suggest that the anomaly originates from the sediments; the intensity of DSi recycling is consistent with the degradation of organic matter, as observed from Si:O-2 ratios in the benthic fluxes compared to that ratio observed in the anomalies. Strong imbalances, observed in both the Si and C mass balances, suggest that the biogenic matter that degrades and dissolves in these sediments near 4000 m does not come from pelagic sedimentation. It is probably not coming also from the deep channel, because observations were similar in the deep channel vicinity (site D) and further south, far from its influence (site C). The composition of the sediments, with an Si:C ratio close to that observed on continental shelves, suggests that this matter is coming from downslope transport. A first estimate of the magnitude of this flux at global scale, close to 12 T mol Si yr(-1), suggests that it may be an important path for transferring Si from land to ocean.
Deep Sea Research Part II: Topical Studies in Oceanography (0967-0645) (Elsevier), 2009-11 , Vol. 56 , N. 23 , P. 2197-2207

Droits : 2009 Elsevier Ltd All rights reserved.
http://archimer.ifremer.fr/doc/2009/publication-7410.pdf
DOI:10.1016/j.dsr2.2009.04.003
http://archimer.ifremer.fr/doc/00000/7410/

Éditeur(s) : HAL CCSD Elsevier
Résumé : A synthesis of backscatter imagery coupled with a large 3D seismic dataset in the Lower Congo Basin (LCB) reveals a patchy distribution of features interpreted to be associated with fluid seepage from 300 m to 2500 m water depth. With the exception of one region of anomalous backscatter positive-relief mounds, all inferred seep sites occur in negative-relief pockmarks. The extensive 3D seismic dataset in the LCB offers a unique opportunity to study the plumbing system that is feeding surface cold seep systems, and in general, to reconstruct the relationship between tectonics and fluid flow in continental margins. The fluid seeps in the LCB are associated with morphologically, stratigraphically or tectonically controlled focused fluid flow. The integration of the geophysical datasets, backscatter imagery coupled to 3D seismic, clearly indicates that fluid seeps are not randomly distributed, but their seabed organization reflects 1) the location of the underlying structure (reservoir or trap) where the fluids are coming from, 2) the geometry and morphology of the reservoir/trap, and 3) the discontinuities in the sedimentary column along which fluids have migrated. In the LCB seafloor pockmarks are always associated with underlying tectonic structures (fault zones, salt diapirs, polygonal faults) or buried sedimentary bodies (turbiditic channels, erosional surfaces), whereas they never occur above sub-horizontal parallel-stratified fine-grained sediments. Even if triggering processes can not be clearly defined here, we propose a model of seafloor fluid seep organization, which represents a new tool for identifying the geometry of flow pathways and the underlying buried bodies where the fluids are originating from. This qualitative 3D model provides insight into the geohydrologic processes of continental margins.
ISSN: 0025-3227

hal-00406640
https://hal.archives-ouvertes.fr/hal-00406640
DOI : 10.1016/j.margeo.2007.06.003

Éditeur(s) : Elsevier Sci Ltd
Résumé : The quantity and the source of organic matter preserved in the Recent turbiditic channel-levees systems around 4000 m-depth off the Congo River were determined using bulk geochemical approaches (Rock-Eval, elemental and isotopic analyses) as well as molecular and optical analyses on selected samples. These mud-rich sediments contain high amount of organic matter (3% Corg on average), the origin of which is a mixture of terrestrial higher-plant debris and deeply oxidized phytoplanktonic material. Although the relative contribution of continental source versus marine source of the organic matter cannot be precisely quantified, the continental fraction appears significant (at least 70-80%) especially for such depths and distances from the coast. The organic matter distribution appears very homogeneous at different scales, from the single turbiditic event to the entire levee, and changes in accumulation rates have a little impact on the quantity and quality of preserved organic matter. With a petroleum potential around 4.5 kg HC per t rock, the fine-grained turbiditic sediments in the Congo deep-sea system could be regarded as an analog of gas-prone source rocks for the deep offshore of the Atlantic margins. Finally, the Congo deep-sea turbiditic system is a major conveyor of organic carbon to the deep ocean. Further studies are needed to evaluate the efficiency of such systems for the storage of continental organic matter into the deep ocean in relation to sea-level and climatic changes. (C) 2010 Elsevier Ltd. All rights reserved.
Marine And Petroleum Geology (0264-8172) (Elsevier Sci Ltd), 2010-05 , Vol. 27 , N. 5 , P. 995-1010

Droits : 2010 Elsevier Ltd All rights reserved.
http://archimer.ifremer.fr/doc/00006/11700/9285.pdf
DOI:10.1016/j.marpetgeo.2010.02.006
http://archimer.ifremer.fr/doc/00006/11700/

Éditeur(s) : Geological Soc Amer, Inc
Résumé : Pockmarks are seafloor depressions commonly associated with fluid escape from the seabed and are believed to contribute noticeably to the transfer of methane into the ocean and ultimately into the atmosphere. They occur in many different areas and geological contexts, and vary greatly in size and shape. Nevertheless, the mechanisms of pockmark growth are still largely unclear. Still, seabed methane emissions contribute to the global carbon budget, and understanding such processes is critical to constrain future quantifications of seabed methane release at local and global scales. The giant Regab pockmark (9 degrees 42.6' E, 5 degrees 47.8' S), located at 3160 m water depth near the Congo deep-sea channel (offshore southwestern Africa), was investigated with state-of-the-art mapping devices mounted on IFREMER's (French Research Institute for Exploitation of the Sea) remotely operated vehicle (ROV) Victor 6000. ROV-borne micro-bathymetry and backscatter data of the entire structure, a high-resolution photo-mosaic covering 105,000 m(2) of the most active area, sidescan mapping of gas emissions, and maps of faunal distribution as well as of carbonate crust occurrence are combined to provide an unprecedented detailed view of a giant pockmark. All data sets suggest that the pockmark is composed of two very distinctive zones in terms of seepage intensity. We postulate that these zones are the surface expression of two fluid flow regimes in the subsurface: focused flow through a fractured medium and diffuse flow through a porous medium. We conclude that the growth of giant pockmarks is controlled by self-sealing processes and lateral spreading of rising fluids. In particular, partial redirection of fluids through fractures in the sediments can drive the pockmark growth in preferential directions.
Geology (0091-7613) (Geological Soc Amer, Inc), 2014-01 , Vol. 42 , N. 1 , P. 63-66

Droits : 2013 Geological Society of America
http://archimer.ifremer.fr/doc/00186/29731/28293.pdf
DOI:10.1130/G34801.1
http://archimer.ifremer.fr/doc/00186/29731/