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Understanding and quantifying the migration of free-gas in hydrate-bearing sediments through time is particularly compulsive along continental margins, where gas hydrate dissociation could have triggered some of the largest submarine landslides observed on Earth. Offshore Romania, high-resolution seismic profiles reveal low reflective or low-velocity zones, which are indicative of free gas, beneath vertical stacked Bottom Simulating Reflectors (BSRs). To further understand the occurrence of double BSRs in the area and the possible effect of gas hydrate dynamics on slope instability and free gas releases, we performed a numerical 2D transient modelling of the evolution of the thermodynamic stability of gas hydrates, integrating in-situ measured physical data and indirect assessments of sea-bottom temperature, thermal conductivity, salinity and sea-level variations. We found that the shallowest BSR matches well with the current Base of the Gas Hydrate Stability Zone (BGHSZ) and the deeper one with the Last Glacial Maximum (LGM) base of GHSZ. The reduction of the GHSZ extension subsequently led to widespread gas hydrate dissociation associated with warming conditions and an increase in Black Sea salinity. However, this dissociation is only responsible of some very superficial submarine landslides (< 30 mbsf and 3 m thick in average) that occurred during this same period. These new constraints improve our understanding of the sliding mechanisms on the Romanian slope that have been ongoing since the LGM and support less catastrophic scenarios than those suggested previously in the case of active gas hydrate dissociation. These results also allow solving the mystery of the double BSR, which here corresponds to a relic of the LGM BGHSZ.

The role of solid Earth tide in fault reactivation has significant implications for understanding earthquake triggering, carbon sequestration, and the global carbon budget. Despite extensive research on this topic over the years, the relationship between Earth tide and fault reactivation remains unclear. In this study, we investigate the potential influence of solid Earth tide on the reactivation of sub-seabed fractures and faults, which may lead to the release of methane from the seabed. For a period of two weeks, we monitored the sub-seabed temperature and pore-fluid pressure at two sites on an over-pressured fault system located in the Black Sea. Our observations revealed that, despite the ~790 m distance between the two sites, the response in terms of methane discharge was synchronous during the measurement period. Our analysis showed that the presence of over-pressured fluid promotes fault reactivation under solid Earth tide cycles, resulting in synchronized degassing events. We also showed that that these faults can be reactivated with relatively low stresses, no greater than 2 kPa, illustrating the fragile equilibrium of these greenhouse gas laden systems.

International audience; Seismic investigation in marine gas-bearing sediments fails to get information below the acoustic mask created by free gas. To circumvent this problem, we combined collocated multichannel ultra-high resolution seismic imaging, marine electrical resistivity tomography (MERT) and core sampling to study the physical properties of gas-bearing sediments in the Bay of Concarneau (France). We obtained sections of compression (P-) wave velocitvalues where free gas was identified in seismic data. We tested a joint processing workflow combining the 1D inversion of the MERT data with the 2D P-wave velocity through a structural coupling between resistivity and velocity. We obtained a series of 2D resistivity models fitting the data whilst in agreement with. The resulting models showed the continuity of the geological units below the acoustic gas fronts which is associated with paleo-valley sediment infilling. We were able to demonstrate relationships between resistivity and velocity differing from superficial to deeper sediments. We established these relationships at the geophysical scale then compared the results to data from core sampliand porosity). We inferred the porosity distribution from the MERT data. At the core locations, we observed a good agreement between this geophysical scale porosity and the core data both within and outside the gas-bearing sediments. This agreement demonstrated that resistivity could be used as a proxy for porosity where no was available below gas caps. In these regions, the observed low resistivity showed a high porosity (60-70%) down to about 10-20 m in depth in contrast with the surrounding medium with porosity less than 55%. These results support the hypothesis that failures inside the paleo-valley sediment could control the gas migration

The global ocean is a net source of CH4 to the atmosphere. Among the natural processes, marine emissions are significant contributors with large uncertainties that deserves effort to improve current estimates, and eventually predict their trajectories in a changing climate. Oceanic CH4 emissions to the atmosphere can either be transported from seafloor or in situ produced in surface waters. Seafloor emissions include both CH4 emanating from CH4 hydrate degradation and from free gas in the sediment. Ultimately, CH4 enters the atmosphere across the sea-air interface either from bubbles rising from the seafloor or by diffusion of dissolved gas. Estimates of global marine emissions diverge widely due to very large uncertainties linked to limited data coverage, seasonal and methodological differences and the difficulty to capture the environmental factors that lead to high variability of the emissions.As the world’s largest natural anoxic waterbody, the semi-enclosed Black Sea (BS) is very sensitive to human and climate perturbations. It is characterized by widespread seafloor CH4 emissions from the shallow coast to the deep basin. One of the major issues that arises on the BS methane dynamics is the determine to what extent and in which quantity part of the urge amount of dissolved methane stored in the anoxic bottom water layer is transferred to the atmosphere.During the GHASS2 (Gas Hydrates, fluid Activities and Sediment deformations in the black Sea) cruise in September 2021, CH4transfer to the atmosphere has been investigated in the Western sector of the BS at sites with water depth ranging from 60 m to 1200m. CH4 partial pressures were measured in the surface water and in the atmosphere using optical spectrometers, respectively the SubOcean membrane inlet laser spectrometer (Grilli et al., 2021, https://doi.org/10.3389/feart.2021.626372) and an ICOS-calibrated commercial analyzer (Picarro G2401). We have also developed an open-path setup dedicated to shipborne measurement composed by an open-path CH4 analyzer Li-7700, a H2O-CO2 analyzer 7200RS from LiCor, a Gill 3D sonic anemometer, and an inertial navigation sensor (Lord). An inox structure was specifically designed to protrude by 1m the front mast of the R/V Pourquoi Pas? to install the open-path sensor.We present preliminary flux estimates comparison obtained from partial pressure gradient by the diffusive method with the experimental eddy covariance set-up. We also discuss our preliminary results in comparison with previous reports for the area and conclude on the respective challenges and relative basin-scale representativity of the various measurement techniques.

Marine landslides, which are observed worldwide along continental slopes, constitute the most important processes reworking sedimentary deposits and a major geohazard for marine and coastal domains. They can generate potentially the destruction of marine infrastructures through the formation of turbidity currents and/or hazardous tsunamis. In the Romanian sector of the Black Sea, high amounts of methane are detected in the sediments and at the seafloor through the identification of gas seeps in the water column. They occur on the upper slope, mostly outside the large gas hydrates system occurring in the sediment below -660 m water depth, where methane is trapped in ice cages that act as a buffer zone hampering methane to reach the water column. New geophysical and geotechnical dataset acquired along the Romanian margin reveals that the active seepage zone is associated with numerous slope failures, which incised the continental upper slope. Is there a possible relationship between gas hydrate system and recent slope instabilities? Could intense free gas emissions and/or gas hydrates dissociation have triggered such geohazards? To answer these questions, we present (1) an high resolution mapping and, more important, dating of landslides since the last glacial maximum (35 ka), (2) results of numerical modelling showing the evolution of gas hydrates stability zone inside the sediments since the last 35 ka taking into account the environmental variations that occurred during this time lapse (geothermic gradient, temperature, salinity and sea level).The models highlight the major effect of environmental changes and particularly the glacial/interglacial cycles and salinity variations on the extent of the gas hydrates. The confrontation of those models with slide extensions, depths and ages allows to better discuss the respective influences of gas hydrates dissociation, stratigraphic overpressure and seepage on slope instability since the last glacial maximum.

Fluid migration within the sedimentary column contributes significantly to slope failure and pockmark formation and can be an effective triggering mechanism to generate submarine landslides. Pockmarks are thus commonly listed among geohazards. Contrary to these accepted notions, we propose here an alternative view of pockmarks with an example from the Eastern Niger Submarine Delta: Pockmarks and associated chimneys may increase or modify the shear strength of sedimentary layers and locally enhance seafloor stability. The analysis of two 3D seismic volumes shows that a landslide deposit divides into two branches around a cluster of three pockmark chimneys, interpreted to impede its further development. The morphological characteristics of a slide constrained by fluid seepage features show the potential role of fluid escape in marine sediment strengthening.

International audience; Natural Gas Hydrates (NGH) collected during the Ghass cruise 2015 in the Western Black Sea onboard the R/V Pourquoi pas? are characterized by a suite of techniques. Gas Chromatography and Raman spectroscopy are used for the identification of the nature of the gas source, the hydrate structure and spatial variability of cage occupancies. The nature and source of hydrate forming gases primarily reveal a high methane content (99.6 mol%) and small amount of nitrogen (>0.29 mol%) and CO 2 (0.056 mol%). Isotopic analyses from the hydrate-bound methane and recently published results from Pape et al. (2020) clearly indicate a microbial source of gas supplying the hydrate deposit generated by the reduction of carbon. For the first time, Raman imaging spectroscopy was applied on NGH recovered in the Western Black Sea. The results show a heterogeneous distribution of the encapsulated guest molecules (CH 4 , N 2 and H 2 S), which is associated with a spatial variability of the guest-gas composition at the micron-scale. Some portions of the 2D-Raman images clearly exhibit a relative N 2-enrichment (with a concentration exceeding 6 mol% N 2 at some positions), while H 2 S shows a rather minor contribution on all the spectral maps investigated. A correlation is then established between the composition of the gas in the NGH and its impact on the CH 4 cage occupancy, with a ratio of θ LC /θ SC (large cage/small cage) between ~ 0.5 and 1.26 depending on the positions analyzed. The departure from the expected ratio in pure methane hydrate is attributed to the preferential encasement of N 2 in the large cage of the NGH structure. In addition, the occurrence of carotenoids identified in sediment-rich zones show a minor impact on the CH 4 cage occupancies. The results are discussed within the context of natural gas resource estimates in NGH to emphasize how the measured cage occupancies may impact the volumetric conversion factor commonly used with other geologic parameters to determine the resource endowment and global volume of methane. The small-scale heterogeneities revealed by the 2D-Raman images point out the importance to better understand stages of hydrates formation in methanerich seafloor environment.

Highlights • Geochemical analyses highlight multiple diagenesis processes occurring in the sediment. • Intense methane seepages and organic matter degradation contribute to the sulfate reduction. • Chemical of dissolved and mineral iron species indicate that iron is associated with clay minerals. • In response to seawater intrusion, ion exchange, dissolution and reverse weathering reactions change the composition of clay constituting the sediment. Abstract Pore water and sediment geochemistry in the western Black Sea were investigated on long Calypso piston core samples. Using this type of coring device facilitates the recovery of the thick sediment record necessary to analyze transport-reaction processes in response to the postglacial sea-level rise and intrusion of Mediterranean salt water 9 ka ago, and thus, to better characterize key biogeochemical processes and process changes in response to the shift from lacustrine to marine bottom water composition. Complementary data indicate that organic matter degradation occurs in the upper 15 m of the sediment column. However, sulfate reduction coupled with Anaerobic Methane Oxidation (AOM) is the dominant electron-accepting process and characterized by a shallow Sulfate Methane Transition Zone (SMTZ). Net silica dissolution, total alkalinity (TA) maxima and carbonate peaks are found at shallow depths. Pore water profiles clearly show the uptake of K+, Mg2+ and Na + by, and release of Ca2+ and Sr2+ from the heterogeneous lacustrine sediments, which is likely controlled by chemical reactions of silicate minerals and changes in clay mineral composition. Iron (Fe2+) and manganese (Mn2+) maxima largely coincide with Ca2+ peaks and suggest a close link between Fe2+, Mn2+ and Ca2+ release. We hypothesize that the Fe2+ maxima below the SMTZ result from deep Fe3+ reduction linked to organic matter degradation, either driven by DOC escaping from the shallow sulfate reduction zone or slow degradation of recalcitrant POC. The chemical analysis of dissolved and solid iron species indicates that iron is essentially associated with clay minerals, which suggests that microbial iron reduction is influenced by clay mineral composition and bioavailability of clay mineral-bound Fe(III). Overall, our study suggests that postglacial seawater intrusion plays a major role in shaping redox zonation and geochemical profiles in the lacustrine sediments of the Late Quaternary.

Gas hydrate (GH) systems constitute methane sinks sensitive to environmental changes such as pressure, temperature, and salinity. It remains a matter of debate as to whether the large GH system of the Black Sea has reached a steady state since the last glacial maximum (LGM). We report on an irregular free gas distribution in specific sediment layers marking an irregular bottom‐simulating reflector (BSR). This anomalous free gas distribution revealed by very high resolution seismic images, acquired by a deep‐towed multichannel seismic system, might be evidence of an on‐going migration of the base of the GH stability zone (GHSZ). We show that the reequilibrium is not occurring homogeneously as overpressure from hydrate dissociation slows their decomposition in specific sedimentary layers. The Black Sea example highlights that dissociation and the associated methane release in the water column or even in the atmosphere could be largely delayed by overpressure accumulation. Plain Language Summary Methane hydrate is an ice‐like compound composed of a cage of water molecules enclosing a methane molecule. Hydrates can form where water and methane are present under high pressure and low temperatures, for example in deep‐sea sediments. As a result of climate change (e.g., seawater temperature increase), hydrates can melt and release free gas and water. Yet we observe hydrates present where they should have melted according to modeling. We explain this irregular melting by differing properties of the host sediments and different quantities of hydrate in the sediments. Methane in the Earth’s atmosphere is a strong greenhouse gas. The release of methane from hydrate melting has been proposed as a runaway process where the methane released increases global warming, which further increases hydrate melting and methane release, repeating the cycle. Our results show that the destabilization of a hydrate system is actually a slow process, spanning several millennia. As such, a catastrophic destabilization of a gas hydrate system is unlikely. Key Points Very high resolution (< 1m) deep‐towed seismic imaging of the bottom‐simulating reflector allows characterizing gas hydrates dynamics Irregular bottom‐simulating reflector is an indicator the transient state of a gas hydrate system in the Black Sea Pore overpressure and hydrate recrystallization explain the long process of the readjustment of the base of the gas hydrate stability zone

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Offshore archives retrieved from marine/lacustrine environments receiving sediment from large river systems are valuable Quaternary continental records. In the present study, we reconstruct the Danube River activity at the end of the last glacial period based on sedimentological, mineralogical and geochemical analyses performed on long-piston cores from the north-west Black Sea margin. Our data suggest that the Danube River produced hyperpycnal floods throughout the ca. 33-17 ka period. Four main periods of enhanced Danube flood frequency, each of 1.5-3 kyr duration, are recorded at ca. 32.5 30.5 ka (equivalent to the first part of Heinrich Stadial HS 3), at ca. 29-27.5 ka (equivalent to Greenland Stadial 4), at ca. 25.3-23.8 ka (equivalent to HS 2) and at ca. 22.3-19 ka. Based on mineralogical and geochemical data, we relate these events to enhanced surface melting of the Alpine Ice Sheet (AIS) that covered -50,000 km2 of the Danube watershed at the Last Glacial Maximum (LGM). Our results suggest that (i) the AIS growth from the inner Alps to its LGM position in the northern Alpine foreland started from ca. 30.5 ka, ended no later than ca. 25.3 ka, and was interrupted by a melting episode ca. 29 27.5 ka; (ii) the AIS volume drastically decreased from ca. 22.3 to 19 ka, as soon as summer insolation energy at the AIS latitude increased; and (iii) HSs strongly impacted the AIS mass balance through enhanced summer surface melt. This, together with evidence of severely cool winters and the rapid expansion of sea ice in the North Atlantic, implies strong seasonality in continental Europe during stadials. (C) 2019 Elsevier Ltd. All rights reserved.

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. A ca. 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 meters of sediment in the pockmark center substantiate that deep‐water 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 two 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 Niger Delta is one of the largest hydrocarbon basin offshore Africa and it is well known for the presence of active pockmarks on the seabed. During the Guineco-MeBo cruise in 2011, long cores were taken from a pockmark cluster in order to investigate the state of its current activity. Gas hydrates, oil and pore-water were sampled for geochemical studies. The resulting dataset combined with seismic data reveal that shallow hydrocarbon migration in the upper sedimentary section was focused exclusively within the pockmarks. There is a clear tendency for gas migration within the hydrate-bearing pockmarks, and oil migration within the carbonate-rich one. This trend is interpreted as a consequence of hydrate dissolution followed by carbonate precipitation in the course of the evolution of these pockmarks. We also demonstrate that Anaerobic Oxidation of Methane (AOM) is the main process responsible for the depletion of pore-water sulfate, with depths of the Sulfate-Methane Transition Zone (SMTZ) ranging between 1.8 and 33.4 m. In addition, a numerical transport-reaction model was used to estimate the age of hydrate-layer formation from the present-day sulfate profiles. The results show that the sampled hydrate-layers were formed between 21 and 3750 years before present. Overall, this work shows the importance of fluid flow on the dynamics of pockmarks, and the investigated cluster offers new opportunities for future cross-site comparison studies. Our results imply that sudden discharges of gas can create hydrate layers within the upper sedimentary column which can affect the seafloor morphology over few decades. This article is protected by copyright. All rights reserved.

International audience; A comprehensive characterization of the gas hydrate system offshore the western Black Sea was performed through an integrated analysis of geophysical data. We detected the Bottom Simulating Reflector (BSR), which marks, in this area, the base of gas hydrate stability. The observed BSR depth does not fit the theoretical steady-state base of gas hydrate stability zone (BGHSZ). We show that the disparity between the BSR and predicted BGHSZ is the result of a transient state of the hydrate system due to the ongoing re-equilibrium since the Last Glacial Maximum. When gas hydrates are brought outside the stability zone due to changes in temperature and sea level, their dissociation generates an increase in interstitial pore pressure. This process is favorable to the re-crystallization of gas hydrates and delays the upward migration of the hydrate stability zone explaining the anomalously deep BSR. The BSR depth, which is commonly used to derive geothermal gradient values by assuming steady state conditions, is used here to derive the maximum excess pore pressure at the base of the gas hydrate stability zone. Derived excess pore pressure values of 1-2 MPa are probably the result of the low permeability of hydrate-bearing sediments. Higher pore pressure values derived at the location of a fault system could cause hydro-fracturing enabling the free gas to cross the gas hydrate stability zone and emerge at the seafloor, forming the flares observed in close vicinity to where the shallow gas hydrates were sampled.

International audience; The Sea of Marmara (SoM) is a marine portion of the North Anatolian Fault (NAF) and a portion of this fault that did not break during its 20th century earthquake sequence. The NAF in the SoM is characterized by both significant seismic activity and widespread fluid manifestations. These fluids have both shallow and deep origins in different parts of the SoM and are often associated with the trace of the NAF which seems to act as a conduit. On July 25 th , 2011, a M w 5 strike-slip earthquake occurred at a depth of about 11.5 km, triggering clusters of seismicity mostly located at depths shallower than 5 km, from less than a few minutes up to more than 6 days after the mainshock. To investigate the triggering of these clusters we first employ a match filter algorithm to increase the number of event located and hence better identify potential spatio-temporal patterns. This leads to a 2-fold increase in number of events relocated, coming mostly from the shallow seismic clusters. The newly detected events confirm that most of the aftershocks are shallow, with a large number of events located in the first few km below the SoM seafloor. Pore pressure diffusion from the position of the deep mainshock to the position of the shallow events is incompatible with the short time interval observed between them. We therefore investigate static and dynamic stress triggering processes. The shallow clusters fall into either positive or negative lobes with static stress variations of about ±5 kPa. Dynamic stresses may reach values of about ±40 kPa depending on the rise time and the fault orientation considered, but cannot last longer than the perturbations associated with the seismic waves from the mainshock. We then propose a mechanism of fluid pressure increase involving local fluid transfers driven by the transient opening of gas-filled fractures due to earthquake shaking, to explain the triggering of the shallow events of the clusters.

International audience; The geotechnical properties of clay sediments were investigated using laboratory and in-situ measurements as part of the geohazard assessment in the Romanian sector of the Black Sea affected by landslides and seafloor deformation features. The sediments were characterized as predominantly high plastic silty clay of high compressibility, low undrained shear strength, low cohesion and moderate sensitivity. A shallow increase in shear strength exceeding the general trend could have been reconciled with evidence for the precipitation of iron sulfides and calcium carbonates related to early diagenetic reactions of sulfate reduction (e.g. cryptic sulfur cycle) and anaerobic oxidation of methane. Comparison of the compression behaviour of natural and reconstituted samples indicated that precipitation of these materials at particle contacts produced cementation. The analysis of deeper sediments sharing similar geochemical properties illustrated that the early precipitation of even low quantity of sulfur and carbonate cements allowed clays to develop sufficient strength to preserve an abnormal high water content even under tens of meters burial depth. Because this cementation process was observed to be associated with an enhanced sensitivity and a strain softening response to undrained shearing it was concluded that such early diagenesis should be considered as a key environmental factor, leading to weak layer formation that may influence the susceptibility of seafloor to failure.

A review of recent literature shows that two geomorphologically different types of pockmarks, contribute to gas seepage at the seafloor. Type-1 pockmarks are defined as seafloor craters associated to fluid seepage and are the most classical type referred to as “pockmarks” in the literature. In contrast, Type-2 pockmarks reveal a complex seafloor morphology that may result from the formation/decomposition of gas hydrates in underlying sedimentary layers. Interpretation of very-high-resolution seismic data, sedimentological analyses and geotechnical measurements acquired from the Eastern Niger Submarine Delta reveal that Type-2 pockmarks are associated to the presence at depth of a conical body of massive gas hydrates. Based on acquired data, theoretical analysis and numerical modelling, it was possible to propose a novel geo-mechanical mechanism controlling the irregular seafloor deformations associated to Type-2 pockmark and to show that pockmark shapes and sizes are directly linked to the initial growth and distribution of sub-seafloor gas hydrates. The study illustrates the role of gas hydrates formation in the fracturation, deformation of the subsurface sediment and the formation of Type-2 pockmarks.

A detailed study, based on ocean-bottom seismometers (OBSs) recordings from two recording periods (3.5 months in 2011 and 2 months in 2014) and on a high-resolution, 3D velocity model, is presented here, which provides an alternative view of the microseismicity along the submerged section of the North Anatolian fault (NAF) within the western Sea of Marmara (SoM). The nonlinear probabilistic software packages of NonLinLoc and NLDiffLoc were used for locating earthquakes. Only earthquakes that comply with the following location criteria (e.g., representing 20% of the total amount of events) were considered for analysis: (1) number of stations >= 5; (2) number of phases >= 6, including both P and S; (3) root mean square (rms) location error 8 km) along the axis of the Main Marmara fault (MMF). In contrast, the following features were observed: (1) a significant number of earthquakes occurred off-axis (e.g., 24%), with predominantly normal focal mechanisms, at depths between 2 and 6 km, along tectonically active, structural trends oriented east-west or southwest-northeast, and (2) a great number of earthquakes was also found to occur within the upper sediment layers (at depths < 2 km), particularly in the areas where free gas is suspected to exist, based on high-resolution 3D seismics (e.g., 28%). Part of this ultra-shallow seismicity appears to occur in response to deep earthquakes of intermediate (M-L similar to 4-5) magnitude. Resolving the depth of the shallow seismicity requires adequate experimental design ensuring source-receiver distances of the same order as hypocentral depths. To reach this objective, deep-seafloor observatories with a sufficient number of geophone sensors near the fault trace are needed.

Gas seeping to the seafloor through structures such as pockmarks may contribute significantly to the enrichment of atmospheric greenhouse gases and global warming. Gas seeps in the Gulf of Lions, Western Mediterranean, are cyclical, and pockmark “life” is governed both by sediment accumulation on the continental margin and Quaternary climate changes. Three-dimensional seismic data, correlated to multi-proxy analysis of a deep borehole, have shown that these pockmarks are associated with oblique chimneys. The prograding chimney geometry demonstrates the syn-sedimentary and long-lasting functioning of the gas seeps. Gas chimneys have reworked chronologically constrained stratigraphic units and have functioned episodically, with maximum activity around sea level lowstands. Therefore, we argue that one of the main driving mechanisms responsible for their formation is the variation in hydrostatic pressure driven by relative sea level changes.

A joint industrial project with IFREMER and Total provides insights into the tectonic setting and the geochemistry of a large active pockmark offshore W-Africa in the Niger Delta. The study combines both geophysical (seismic) and geochemical data to infer the dynamics of a 300 m wide pockmark located at 667 m water depth. Two Calypso cores, one at the centre and one outside the pockmark, along with three box-cores, two inside the pockmark and one outside, were collected to study the geochemistry of gas hydrates, carbonates and pore waters. Two seismic lines, a 3D high resolution random line and a 2D high resolution seismic line, provide a good description of the plumbing of the system. The integrated results depict a very active pockmark characterized by several gas charged-bodies, a well identified BSR marking a hydrate accumulation zone, charged from deeper sources via a network of faults. Hydrates and carbonate concretions have been detected inside the pockmark while only the latter has been found outside. The hydrate analyses show that the hydrate-forming gases are of thermogenic origin, albeit overwhelmingly composed of methane. Pore-water analyses reveal the occurrence of anaerobic oxidation of methane coupled with sulphate reduction at shallow depth (∼200 cm). The chloride concentration profile exhibits both values lower than that of seawater which is inherent to hydrate dissociation after core recovery and positive anomalies (values 7% higher than that of seawater background) at the lowermost part of the core. The latter evidence suggests current or recent hydrate formation at this area. The carbonate dating provides additional temporal information and indicates that this pockmark has been active since ∼21 kyr.

On seismic profiles, the Eastern Niger Submarine Delta displays spectacular prograding wedges separated by discontinuities on the continental shelf and correlative conformities seawards. In spite of the numerous studies focused on deep oil reservoirs in this area, and because of the presence of pronounced syndepositional deformation, the detailed geometry and chronology of the Quaternary succession has been neglected, while it contains relevant information about sedimentary processes and the factors controlling sediment depocenters, presence of fluids and sediment deformation. This study provides the first comprehensive view of the Late Quaternary stratigraphic architecture of the Eastern Submarine Niger Delta. The stratigraphic interpretation is based on: (1) a seismic dataset composed of 2D seismic profiles with variable resolution (Sparker, Chirp) and four 3D seismic blocks (short offset processing); and (2) in situ measurements and samples (3 Calypso and 2 Stacor long piston cores). The correlation of seismic reflectors with chronological information from long piston cores (δ 18 O values and XRF-derived Ca profiles), shows that the upper-most 5 depositional sequences formed during the last ca. 500 kyr BP, in response to glacial/interglacial fluctuations driven by 100-kyr Milankovitch cycles and the subsidence rate for the late Quaternary period are around 300 m/Myr. The prograding wedges at the shelf break have distinct seismic facies with high-angle clinoforms building regional scale depositional sequences locally disrupted by growth faults. Although highly influenced by syndepositional deformation with thick accumulation in the hanging walls of growth faults, the main depositional sequence pattern is tuned with glacio-eustatic oscillations and the high-angle clinoforms near the shelf edge correspond to upper shorefaces/delta fronts or subaqueous deltas formed during glacial lowstand periods. The largest Mass Transport Complex in this zone is interpreted as resulting from oversteepening of the shelf edge during MIS 10.

The circulation of methane-rich fluids at cold seeps often leads to precipitation of authigenic carbonates mediated by microbial activity, which can be used as archives of fluid seepage on continental margins. In this study, we have investigated the potential of using rare earth element (REE) concentrations in authigenic carbonates for tracing fluid sources and providing information on associated biogeochemical processes at cold seeps. We have examined various chemical procedures for the analysis of REE in authigenic carbonates, and present a robust method using dilute acetic acid solution that allows the extraction of REE from carbonate phases with reduced contamination from Fe–Mn oxides and detrital minerals. Using this method, we analysed a series of carbonate samples collected at various deep-sea sites of active fluid seepage on the Niger Delta for their REE and trace element concentrations, δ13C and, δ18O values and 87Sr/86Sr isotope compositions. We show that the REE composition of cold seep carbonates is controlled primarily by that of the fluid from which they have precipitated. Our results also suggest that significant REE fractionation can occur during carbonate precipitation, which can be related to various degrees of REE complexation by organic versus carbonate ligands in response to changing pore water chemical composition for parameters such as carbonate alkalinity.

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.

The Corsica Trough between the island of Corsica and the mainland of Italy is dominated on its western side by turbidite channel-lobe systems fed by high-gradient rivers during glacial epochs, while the eastern side is markedly different. It is flanked by the Pianosa Ridge, a prominent tectonic structure confining the distal reaches of turbidite lobes and it is characterized by the development of contourite drifts with evident seafloor expression. The southern part of the Pianosa Ridge hosts a submarine landslide called ‘Pianosa Slump’ (PS, 6 km long, 14 km wide). Multichannel Sparker and Chirp profiles reveal the internal geometry of the PS, formed by two sediment bodies of up to 0.85 and 0.34 km3. A buried submarine landslide below the PS shows that mass wasting is a recurrent process in this area. Preliminary results suggest that submarine landslides have volumes and ages comparable with those of turbidite lobes from the Golo turbidite system and contribute actively to their confinement and to basin filling. Relatively steep gradients, rapid contourite drift accumulation during times of sea level lowstands, and fluid escape from distal turbidite sandy lobes are the main factors conducive to seafloor instability.

Abstract To characterize the link between faults and fluid release and to identify therole of fluid flow in gas hydrate occurrences, four piezometers were deployedat the border of a shale-cored anticline in the eastern Niger delta. Two ofthose piezometers were deployed along a major fault linked to a shallowhydrocarbon reservoir while the two others were deployed within two pockmarkfields. Pore pressures were measured for periods ranging from 370 and 435 days. The two piezometers deployed along the fault simultaneously registered aprolonged fluid flow event lasting 90 days. By combining and analyzing acquired data, we show that the fluid-fault systemoperates according to the following three stages:upward pore fluid migration through existing conduits and free gascirculation within several shallow sandy layers intersecting the majorfault,gas accumulation and pore pressure increase within sandy-silty layersandhydro-fracturing and fluid pressure dissipation through sporadic degassingevents, causing pore fluid circulation through shallow sandy layers andattracting seawater into the sediment. The present work clearly demonstrates how an integrated approach based onseafloor observations, in situ measurements, and monitoring is essential forunderstanding fault-fluid-hydrate systems. Introduction Published research papers concerning gas hydrates have exponentially increasedthose last three decades and mainly since natural gas hydrates have beenrecovered worldwide from polar regions to deep-water shelves (Kvenvolden,1994). Generally, most of those research papers focus on one of the followingthree main topics: role of gas hydrates in global climate change (see forinstance Kvenvolden, 1994);gas hydrates as a potential cause of sedimentdeformations and submarine slope instabilities (see for instance Kayen and Lee,1991);gas hydrates as a an unconventional energy resource (see forinstance Collett, 2002). Detecting gas hydrates and understanding the dynamic of any gas hydrates systemassociated to climate changes, geohazards, and/or energy resources leads toinvolve integrated approaches combining geophysical data, geochemical analysis, geotechnical testing and in situ monitoring. The present work aims to highlightthe importance of pore pressure monitoring in such integrated studies and todemonstrate the importance of such measurements in the comprehension of acomplex active fault-fluid hydrate system in deepwater Nigeria.

[1] Gas hydrates were recovered by coring at the eastern border of a shale-cored anticline in the eastern Niger Delta. To characterize the link between faults and fluid release and to identify the role of fluid flow in the gas hydrate dynamics, three piezometers were deployed for periods ranging from 387 to 435 days. Two of them were deployed along a major fault linked to a shallow hydrocarbon reservoir while the third monitored the fluid pressure in a pockmark aligned above the same major fault. In addition, 10 ocean-bottom seismometers (OBS) were deployed for around 60 days. The piezometers simultaneously registered a prolonged fluid flow event lasting 90 days. During this time, OBS measurements record several episodic fluid release events. By combining and analyzing existing and newly acquired data, we show that the fluid-fault system operates according to the following three stages: (1) upward pore fluid migration through existing conduits and free gas circulation within several shallow sandy layers intersecting the major fault, (2) gas accumulation and pore pressure increases within sandy-silty layers, and (3) hydrofracturing and fluid pressure dissipation through sporadic degassing events, causing pore fluid circulation through shallow sandy layers and drawing overlying seawater into the sediment. This paper clearly demonstrates how an integrated approach based on seafloor observations, in situ measurements, and monitoring is essential for understanding fault-fluid-hydrate systems.

Abstract Pockmarks are crater-like depressions resulting from fluid flows at the seabed. Fluids may be of different types (water, gas, oil) and origins (dewatering, biogenic, thermogenic). In the Eastern Niger Submarine Delta (ENSD), pockmarks are generally circular depressions of variable size and shape which are well expressed on bathymetry obtained from AUV / deep-tow seabed surveys or from 3D seismic data. On bathymetric dip maps, circular features interpreted as pockmarks can be separated into 2 types with different shapes. The first group "Type-1 Pockmarks" is composed by pockmarks whose regular shape with a depression forms a circular crater and it is similar to many occurrencies worldwide well described in the literature. The second group "Type-2 Pockmarks" is composed of "irregular" pockmarks with a distorted shape and boundaries less pronounced than "regular" pockmarks. Both types are well documented in the ENSD between 200 and 800 m water depth. An integrated study including AUV bathymetry, 2D VHR and 3D HR seismic profiles, in situ samples and geotechnical data revealed that different pockmark morphologies may be associated to specific sedimentary processes. This paper proposes a geomorphological characterisation of distinct pockmark types and infers their originating processes only based on 3D seismic and VHR bathymetric data interpretation. Introduction The Eastern Submarine Niger Delta (ENSD) is a region of intense oil exploration and exploitation. Many targets for exploration and seafloor installations (e.g. pipelines) are located in areas with irregular seafloor morphology whose potential implications in terms of geohazard must be assessed. Among other seafloor features whose presence constitute a potential geohazard are fluid-flow related features such as pockmarks, requiring often dedicated surveys because of their relatively small size (Kvalstad, 2007). Fluid flow seepage has different expressions when studied in detail, and its impact on the seafloor has to be evaluated before any offshore field development. In general, the identification of the presence and areal distribution of pockmarks influences the choice of anchor/structure locations and the routing of pipelines in order to avoid the pockmarks as much as possible. In addition, it is important to estimate the possible evolution of pockmarks, their state of activity, and their association with other potentially hazardous geological features/processes. Pockmarks are commonly defined as crater-like depressions resulting from fluid flows at the seabed (Hovland, 1984). We named here these pockmarks as Type-1. Recent studies showed that the morphologies of pockmarks may be the result of fluid seepage contained in buried structures (Pilcher et al., 2007). The analysis of buried structures (fault systems, channels, mass transport deposits, mud volcanoes) is important to detect pockmarks and to understand their origin. It exists another type of pockmarks related to the presence of gas hydrates (Sultan et al., in press), named here Type-2. Type-2 pockmarks have a distorted shape and boundaries less pronounced than "regular" pockmarks. They might present a ring depression or circular moat at the perimeter of the pockmark, but other expressions at the seafloor reflect different stages of evolution linked to the dissolution/dissociation of gas hydrates (Sultan et al., in press). It is crucial to identify Type-2 pockmarks, because they are associated with the presence of potentially hazardous features such as gas hydrates, methane-derived authigenic carbonates and gassy soils, and their seafloor expression is more subtle than Type-1 pockmarks.

In a site investigation of the eastern part of the offshore Niger delta, cone penetration tests (CPTU) showed significant drops in tip resistance, associated with decreases in sleeve friction and induced excess pore pressures at the interface between superficial sediments and the underlying deposits of a mass-transport complex (MTC) called NG1. Such signature characteristics of weakened zones are clearly expressed at three sites where the drop in tip resistance reaches more than 40% over 2-3 m-thick intervals. Correlations between CPTU profiles and both 3D and ultrahigh-resolution 2D seismic data suggest that the weakened zones surround syndepositional the within the frontal part of NG1. Hence, weakening appears associated with the remobilization of thrust faults, inducing localized plastic shear. Relatively recent, deep-seated structural movements affecting NG1 are suspected to have remobilized these thrusts faults. When considering the sole influence of gravity, the fact that shear strength is mobilized within scattered, limited zones along steeply dipping syndepositional faults is not favorable for the further development of a continuous slope-parallel failure surface above NC1.