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The upper continental slope offshore Romania is a complex area hosting turbidite deposits, multiple types and ages of deep-seated faults, gas hydrates, gas-escape features, and numerous Mass Transport Deposits (MTDs). Multi-scale seismic data sets (2D-high-resolution and near-bottom very high-resolution) were used to study the interaction between such disparate geological features and determine their impact on slope stability. At least five main paleo-valleys have been identified in the north of the Viteaz (Danube) canyon/valley. The most recent channelized systems linked to these valleys formed over a basal layer of MTDs. These MTDs are associated with an unconformity corresponding to the Base Neoeuxinian Sequence Boundary formed during the last major sea-level fall. This erosional surface shows scarp alignments that coincide with underlying faults. We argue that gravity-driven fault reactivation, with possible upward gas/fluid migration along these faults, is a determinant factor controlling sedimentary instabilities. Numerous MTDs are also observed during channel-levees building and reveal local sediment instabilities related to localized erosional process in the canyon. Finally, MTDs recorded within the upper draping unit, suggest that sediment instability also occurred during recent sea level highstand. Sediment pulse, seismicity, and gas hydrate dynamics can also play a determinant role in sediment instability throughout the sediment record.

Off French Guiana and Surinam, NADW and AABW oceanic currents contour the Demerara marginal plateau, which promotes the formation of contourites. We have studied these contourites thanks to a new compilation of high-resolution sub-bottom profiles calibrated by sedimentary cores. The echo-facies and isopach maps we constructed highlight a sedimentary distribution parallel to the isobaths. The presence of moats along the slope is confirmed by the observation of parallel, elongated, sedimentary depleted zones and echo-facies strongly affected by diffraction hyperbola and transparent echo-facies. We interpret these features to be related to eroded slopes and Mass Transport Deposits. In contrast, the sedimentary drifts we mapped are characterized by elongated and thick slope-parallel depocentres displaying bedded echo-facies with wave-like bedforms. According to our interpretation, they result from interactions between currents and the seafloor. Seismic wipe-outs frequently affect those drifts, possibly resulting from high water or organic contents.

The Demerara Plateau, off the French Guiana and Surinam margin, corresponds to a prominent sub-horizontal marginal plateau. The interpretation of a large set of seismic data acquired along the Demerara Plateau and integration with previous seismic stratigraphy and drill studies enables a better understanding of the sedimentary evolution of the outer Demerara marginal plateau. Since the end of the transform activity (late Albian), three sedimentary evolutionary stages have been identified. 1) The Pre-contourite Stage (late Albian to early Miocene) is strongly controlled by the structure of the margin that influences the distribution pattern of post-transform deposits through differential thermal subsidence between transform and divergent segments. 2) The Transitional Stage (middle Miocene to early Pliocene) is expressed as a current-controlled erosive surface resulting from the onset of bottom-current interactions due to the establishment and strengthening of the modern thermohaline circulation that is coeval with the progressive closure of the Isthmus of Panama. 3) The Contourite Stage (middle Pliocene to Holocene) is characterized by the development of a contourite depositional system that is linked to the activity of the North Atlantic Deep Water circulation and by a complex interaction between alongslope and downslope processes. Our study highlights that marginal plateaus might be particularly prone to develop contourite features since they regionally influence ocean current dynamics in guiding and accelerating bottom contour currents. Marginal plateaus should thus be considered as a key target to study bottom current dynamics and paleocirculation patterns.

How and when sediment moves from shallow marine to deep-water environments is an important and poorly understood control on basin-scale sediment dispersal patterns, the evolution of continental margins, and hydrocarbon exploration in deep-water basins. The Golo River (Eastern Corsica, France), its delta, canyons, and fans provide a unique opportunity to study sediment routing from source to sink in a relatively compact depositional system. We studied this system using an array of high-frequency seismic data, multi-beam bathymetry, and five cores for lithology and age control. Movement of sediment to deep water was controlled by interactions between the Golo River, the Golo Delta, and shelf-penetrating submarine canyons. Sediment moved to deep water when lobes of the Golo Delta prograded to the heads of these canyons, or when the Golo River itself flowed directly into one of them. Sand accumulated in canyons, deep-water channels, and submarine fans during glacial periods of low sea level, while mud was deposited throughout the slope, in the relatively short reach of leveed-confined channels, and in the mud-rich fringes around the sandy fans. During interglacial periods of high sea level, the basin was blanketed by mud-rich deposits up to 10 m thick interbedded with distinctive carbonate-rich sediments. Deposition rates in the basin ranged from 0.07 m/ka to 0.59 m/ka over the last 450 ka. Mud deposition rates remained relatively constant at ¡«0.16 m/ka during all time periods, while sand deposition only happened during glacial periods of low sea level with an average rate of 0.24 m/ka. In addition to sea-level controls on sediment delivery, avulsions of the Golo River and its deltaic lobes preferentially routed sediment down either the North or South Golo canyons. Thus, while the larger, sequencescale architecture of the basin is controlled by allogenic sea level forcing, millennial-scale autogenic processes operating on the shelf and in deep water shaped the distribu

The seismic characteristics of focused fluid-related features such as pockmarks and carbonate build-ups on the Nigerian continental slope have been investigated using complementary seismic data sets (reprocessed 3D exploration data and 2D Very High Resolution hull-mounted and near-bottom seismic data) coupled with the results of previous studies (sedimentological, geotechnical and geochemical analyses). The results show different types of fluid-related features within the hemipelagic phase of long duration (∼1 Myr) Turbidite/Hemipelagic cycles. They are the product of the disturbance of the hosting sediments following two main distinct processes: (1) no or low sediment deposition caused by fluid escape associated with seafloor settlement (benthic fauna, carbonate precipitation, gas hydrate formation) and (2) post-depositional erosion, caused either by fluid release (dewatering from underlying features such as channels and upward migration from a deep source) or by collapse (destabilisation of gas hydrates). The temporal evolution reveals a decrease in fluid flow during the hemipelagic deposition as witnessed by the decrease in methanogenic activity and by gas hydrate destabilisation. The spatial distribution of fluid-related features seems to be controlled by the type of fault (small fault or structural lineament).

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., Le secteur roumain de la Mer Noire est dominé par la présence du canyon du Danube et d'un des plus grands systèmes de dépôts de sédiment du monde. La Mer Noire est considérée comme la plus grande mer isolée du monde, la plus grande masse d’eau anoxique de la planète et une mer riche en énergie fossile. En raison d’un taux de sédimentation élevé, de la présence d’une grande quantité de matière organique et des conditions anoxiques, les sédiments de Mer Noire situés au large du delta du Danube sont riches en gaz et l’étude de données de sismique réflexion montre la présence d’un réflecteur sismique particulier appelé communément « Bottom Simulating Reflector ou BSR » qui marque la base de stabilité des hydrates de gaz. La cartographie du BSR au cours des 20 dernières années montre que les hydrates de gaz se seraient accumulés sur de vastes zones géographiques et que le secteur roumain de la Mer Noire a un fort potentiel d'hydrate de gaz. En combinant les anciens et les nouveaux jeux de données acquis en 2015 lors de la campagne océanographique GHASS, nous avons réalisé (1) une analyse géomorphologique de la pente continentale au nord-est du canyon du Danube, (2) une cartographie des panaches de gaz acoustiquement détectés dans la colonne d'eau et (3) le calcul et la cartographie de la zone de stabilité des hydrates de gaz. La comparaison de ces résultats fournit de nouvelles preuves du rôle de la géomorphologie et de la présence des hydrates de gaz sur la migration du gaz libre et la localisation des panaches de gaz dans la colonne d'eau. L’expulsion de gaz dans la mer est aujourd'hui considérée comme une source importante alimentant le budget carbone des océans et, potentiellement, de l'atmosphère.

Changes in siliceous productivity in the eastern Equatorial Atlantic Ocean, off the Western African margin, over the last several glacial cycles have been either related to global-to-regional oceanographic changes (upwelling intensity) or climate changes (precipitation and river discharge). Based on diatom assemblages in core KZAI-02, located to the south of the mouth of the Congo River, integrated with a selection of geochemical proxies, we show that siliceous productivity in the southeastern Angola Basin responded to non-linear interactions between both oceanographic and climate changes over the last 190,000 years. High diatom accumulation rates were recorded in the middle part of MIS 6, in cold substage MIS 5d and in MIS 3-2. During these intervals, high diatom productivity was sustained essentially by nutrients, including dissolved silica, injected by the Congo River into the ocean. The highest productivity was observed during MIS 3, when nutrients were sourced both from the river and regional upwelling. Low diatom accumulation rates were recorded during early and late MIS 6, MIS 5e, early MIS 4 and during the Holocene. These resulted either from low river discharge and overall low nutrient stocks in the Angola Basin (despite evidence for upwelling) or from extremely high river discharge. In the case of the latter, the terrigenous load drastically increased the turbidity of the surface waters in the southeastern Angola Basin and lowered phytoplankton productivity despite the presence of sufficient dissolved silica

Based on a High-Resolution 3D seismic block acquired in the Gulf of Lion in 2004-2005 we investigated fluid pipes and pockmarks on the top of the interfluve between the Hérault canyon and the Bourcart canyon both created by turbidity currents and gravity flows from the shelf to the deep basin in the north-western Mediterranean Sea. Combining the geometry of the potential fluid pipes with the induced deformation of surrounding sediments leads then to the ability to differentiate between potential fluid sources (root vs source) and to better estimate the triggering mechanisms (allochtonous vs autochtonous cause). We linked together a set of derived attributes, such as Chaos and RMS amplitude, to a three dimensional description of pipes along which fluids may migrate. As previously shown in other basins, the induced deformation, creating cone in cone or V-shape structures, may develop in response to the fluid pipe propagation in unconsolidated sediments in the near surface. The level at the top of a cone structure is diachronous. It means that stratigraphic levels over this surface are deformed at the end of the migration. They collapse forming a depression called a pockmark. These pipes are the result of repeated cycles of fluid expulsion that might be correlated with rapid sea-level rise instead of sediment loading. The most recent event (MIS 2.2 stage) has led to the formation of a pockmark on the modern seafloor. It has been used as a reference for calculating the effect of a rapid sea-level rise on fluid expulsion. As all physical and geometrical parameters are constrained, we were able to define that a +34 m of sea level rise may account for triggering fluid expulsion from a very shallow silty-sandy layer at 9 m below seafloor since the last glacial stage. This value is consistent with a sea level rise of about 102 m during this period. This study shows that the episodic nature of fluid release resulted from hydromechanical processes during sea-level rise due to, À partir de données de sismique 3D Haute Résolution acquises dans le Golfe du Lion en 2004–2005, nous avons étudié des conduits de migration de fluides et des pockmarks à l'interfluve entre le canyon de l'Hérault et le canyon Bourcart, tous deux créés par des courants gravitaires et de turbidité actifs depuis la plateforme vers le bassin profond au Nord-Ouest de la Méditerranée. C'est en combinant la géométrie des conduits de migration de fluides avec les déformations induites des sédiments hôtes qu'il a été possible de discriminer l'origine de la déformation de l'origine des fluides et ainsi de mieux comprendre les paramètres déclencheurs (causes allochtones vs. autochtones). L'utilisation d'un panel d'attributs sismiques, comme le Chaos ou l'amplitude RMS, permet tout d'abord de définir en trois dimensions l'architecture des zones de migration de fluides. Comme cela a déjà été montré dans d'autres bassins à travers le monde, la propagation d'un conduit de fluides à travers des sédiments non consolidés dans les premières centaines de mètres sous le fond de la mer crée des structures verticales en V appelées déformations en cône dont la pointe représente le point d'initiation. Cette déformation conique est donc diachrone des niveaux stratigraphiques affectés et le dernier niveau, qui ensuite collapse et forme un pockmark, marque ainsi la fin de la propagation et non pas son initiation. Il est donc possible de dater le début des expulsions. Dans la zone d'étude ces conduits de migration sont donc le résultat d'expulsions répétées de fluides qui empilent plus ou moins verticalement des cônes. Un pockmark actuel, créé durant la dernière remontée du niveau marin (MIS 2.2) et dernier niveau d'un cône issu d'une série de sept cônes successifs, a été étudié en détail et utilisé comme référence. Nous avons ainsi établi qu'une remontée rapide du niveau marin de + 34 m est probablement la cause de la dernière expulsion de fluides dont l'origine se situe dans un niveau réservo

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 show 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.

A new 44 kyr long record of dinoflagellate (phytoplanktonic organisms) cysts (dinocysts) is presented from a marine sediment core collected on the Congolese margin with the aim of reconstructing past hydrological changes in the equatorial eastern Atlantic Ocean since Marine Isotopic Stage (MIS) 3. Our high-resolution dinocyst record indicates that significant temperature and moisture variations occurred across the glacial period, the last deglaciation and the Holocene. The use of specific dinocyst taxa, indicative of fluvial, upwelling and Benguela Current past environments for instance, provides insights into the main forcing mechanisms controlling palaeohydrological changes on orbital timescales. In particular, we are able, for the last 44 kyr, to correlate fluvial-sensitive taxa to monsoonal mechanisms related to precession minima–obliquity maxima combinations. While upwelling mechanisms appear as the main drivers for dinoflagellate productivity during MIS 2, dissolved nutrient-enriched Congo River inputs to the ocean also played a significant role in promoting dinoflagellate productivity between approximately 15.5 and 5 ka BP. Finally, this high-resolution dinocyst study permits us to precisely investigate the suborbital timing of the last glacial–interglacial termination, including an atypical warm and wet oceanic LGM signature, northern high-latitude abrupt climate change impacts in the equatorial eastern Atlantic, as well as a two-step decrease in moisture conditions during the Holocene at around 7–6 and 4–3.5 ka BP.

he IGUANES cruise took place in May 2013 on the R/V L'Atalante along the Demerara passive transform margin off French Guiana and Surinam. Seismic, multibeam and chirp acquisitions were made. Piston cores were collected for pore geochemistry and sedimentology. A mooring was deployed on the sea-bottom for 10 months (temperature, salinity, turbidity and current measurements). This new dataset highlights the lateral variability of the 350 km-long Guiana–Surinam transform margin due to the presence of a releasing bend between two transform segments. The adjacent Demerara Plateau is affected by a 350 km-long giant slide complex. This complex initiated in Cretaceous times and was regularly reactivated until recent times. Since the Miocene, contourite processes seem to be active due to the onset of the North Atlantic Deep Water (NADW) bottom current. A main NADW water vein flows towards SE, eroding slide headscarps and allowing the deposition of contourite drifts. Numerous depressions looking like comet tails or comet scours record this flow. Some of those were interpreted before the cruise as active pockmarks. Pore geochemistry and core analysis do not show any evidence of present-day gas seepage.

The role of internal and external forcing of sedimentation in turbidite systems remains a subject of debate. Here we propose new insights from the quantitative analysis of architectural parameters of the Congo Axial Fan. Fifty-two channel-levee-lobe systems, spanning the last ca. 200 ka, are visible on the seafloor, most of them having slightly elongated lobe complexes at their termination. Volumes of lobe complexes (usually 3 to 196 km3) are highly variable in time and space. The cumulative volume of the lobe complexes represents approximately 30% of the volume of the Axial Fan. The Axial Fan is sequentially divided into periods of increasing/decreasing channel lengths and basinward/landward migrations of avulsion points, representing successive prograding/retrograding architectural patterns called architectural cycles. These cycles are either symmetrical saw toothed and bell-shaped with progressive progradation and retrogradation phases, or asymmetrical, with long-lasting progradation phases and abrupt retrogradation phases that correspond to channel avulsions occurring high up on the fan. Our study points to the interplay between internal and external factors controlling the architecture of the Congo Axial Fan. The local topographic constraint is a major factor in the fan’s stacking pattern. However, cyclic evolution of the architecture reveals major shifts in the deposition site that are linked to very upfan avulsion events. These events are interpreted to be driven by external factors (e.g. climate and/or eustatic sea-level change) that were able to drastically increase and/or coarsen the sediment supply to the fan.

The Demerara plateau, located offshore French Guiana and Suriname, is part of a passive transform continental margin particularly prone to develop slope instabilities, probably in relation to the presence of a free distal border along its steep continental slope. Slope failure occurred at different periods (Cretaceous to Neogene) and shows an overall retrogressive evolution through time. Upslope these failure headscarp, an enigmatic regional MioPliocene unconformity has been discovered through the interpretation of new academic and industrial datasets. The aim of this work is to describe and understand the origin of this surface. Our analysis shows that this unconformity is made of a series of valleys that cross-cut sedimentary strata. Each one of these valleys has a short lateral extent and is closed along two perpendicular directions, which suggests that it could correspond to a highly meandering system, or to some sub-circular depressions. The infill of these features is equivalent to the regional stratigraphic strata found outside the structures, but in a subdued position. This seems to imply that the structures have originated by a local loss of sediments at their base or by sliding processes. Furthermore, these depressions intersect each other through time, while migrating progressively downslope. We discuss a series of hypotheses that try to explain the onset and evolution of these depressions forming the Mio-Pliocene unconformity (Canyons? Slope failures? Contourite moats? Hydrate pockmarks?). Having established that these structures are depressions formed by collapse, and have many similarities with structures recently described in the literature as pockmarks associated with gas hydrate dissolution, we favor this hypothesis. We propose that these hydrate pockmarks form with a mass failure that was triggered by fluid-overpressure development at the base of the hydrate stability zone.

A joint research expedition between the French IFREMER and the German MARUM was conducted in 2011 using the R/V Pourquoi pas? to study gas hydrate distributions in a pockmark field (1141 – 1199 meters below sea surface) at the continental margin of Nigeria. The sea floor drill rig MeBo of MARUM was used to recover sediments as deep as 56.74 meters below seafloor. The presence of gas hydrates in specific core sections was deduced from temperature anomalies recorded during continuous records of infrared thermal scanning and anomalies in pore water chloride concentrations. In situ sediment temperature measurements showed elevated geothermal gradients of up to 258 °C/km in the center of the so-called pockmark A which is up to 4.6 times higher than that in the background sediment (72 °C/km). The gas hydrate distribution and thermal regime in the pockmark are largely controlled by the intensity, periodicity and direction of fluid flow. The joint interaction between fluid flow, gas hydrate formation and dissolution, and the thermal regime governs pockmark formation and evolution on the Nigerian continental margin.

In previous works, it has been suggested that dissolution of gas hydrate can be responsible for pockmark formation and evolution in deep water Nigeria. It was shown that those pockmarks which are at different stages of maturation are characterized by a common internal architecture associated to gas hydrate dynamics. New results obtained by drilling into gas hydrate-bearing sediments with the MeBo seafloor drill rig in concert with geotechnical in situ measurements and pore water analyses indicate that pockmark formation and evolution in the study area are mainly controlled by rapid hydrate growth opposed to slow hydrate dissolution. On one hand, positive temperature anomalies, free gas trapped in shallow microfractures near the seafloor and coexistence of free gas and gas hydrate indicate rapid hydrate growth. On the other hand, slow hydrate dissolution is evident by low methane concentrations and almost constant sulfate values 2 m above the Gas Hydrate Occurrence Zone.

Geophysical data and sampling of the Golo Basin (East Corsica margin) provide the opportunity to study mass balance in a single drainage system over the last 130 kyr, by comparing deposited sediments in the sink and the maximum eroded volume in the source using total denudation proxies. Evaluation of the solid sediments deposited offshore and careful integration of uncertainties from the age model and physical properties allow us to constrain three periods of sedimentation during the last climatic cycle. The peak of sedimentation initiated during Marine Isotopic Stage (MIS) 3 (ca. 45 ka) and lasted until late in MIS 2 (ca. 18 ka). This correlates with Mediterranean Sea palaeoclimatic records and the glaciation in high altitude Corsica. The yield of solid sediment into the Golo Basin drops in the observed present day Mediterranean basins (gauging stations), whereas the palaeo-denudation estimate derived from the sediments over the last glacial period is one to ten times higher than that predicted using cosmogenic or thermochronometer estimates of exhumation. The catchment-wide denudation rate calculated from deposited solid sediment ranges from 47 to 219 mm kyr−1, which is higher than the estimate from palaeosurface ablation in the proximal part of the source (9–140 mm kyr−1) and lower than the distal, narrow, incised channel of the Golo River (160–475 mm kyr−1). This mismatch raises questions about the investigation of denudation at millennial-time scale (kyr) and at higher integrating times (Myr) as a reliable tool for determining the effect of climate change on mountain building and on sedimentary basin models.

Based on acquired geophysical, geological and geotechnical data and modeling, we suggest hydrate dissolution to cause sediment collapse and pockmark formation in the Niger delta. Very high-resolution bathymetry data acquired from the Niger delta reveal the morphology of pockmarks with different shapes and sizes going from a small ring depression surrounding an irregular floor to more typical pockmarks with uniform depression. Geophysical data, in situ piezocone measurements, piezometer measurements and sediment cores demonstrate the presence of a common internal architecture of the studied pockmarks: inner sediments rich in gas hydrates surrounded by overpressured sediments. The temperature, pressure and salinity conditions of the studied area have allowed us to exclude the process of gas-hydrate dissociation (gas hydrate turns into free gas/water mixture) as a trigger of the observed pockmarks. Based on numerical modeling, we demonstrate that gas-hydrate dissolution (gas hydrate becomes mixture of water and dissolved gas) under a local decrease of the gas concentration at the base of the gas-hydrate occurrence zone (GHOZ) can explain the excess pore pressure and fluid flow surrounding the central hydrated area and the sediment collapse at the border of the GHOZ. The different deformation (or development) stages of the detected pockmarks confirm that a local process such as the amount of gas flow through faults rather than a regional one is at the origin of those depressions.