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. Study Area and Main ObjectiveThe investigated area is located in deep water of Nigeria. Bathymetry in the area ranges from 1100 to 1250 m ( Figure 1). This area was previously shown to host a field of (sub) circular pockmarks [Georges and Cauquil, 2007]. These range in shape from a slightly depressed, hummocky seafloor to a much more pronounced depression and each of them is several tens to a few hundreds of meters wide (Figure 1). The various morphologies of the pockmarks suggest either distinct modes of formation or different evolutionary stages [Sultan et al., 2010]. Most of the pockmarks are located in an area bounded by two NW-SE trending deeprooted normal faults, which delineate a graben linked to the axis of anticline in the subsurface. Several deep and shallow faults and three N-S trending buried channels were recognized with high-resolution 3-D seismic data (Figure 1). The buried channels, which are situated between 80 ms and 180 ms (two-way travel time, TWTT) below the seabed, may have the potential of accumulating amounts of free gas and play therefore an important role for the gas hydrate distributions.Based on geophysical and sedimentological data, and in situ piezocone measurements, Sultan et al. [2007] have shown that pockmark-associated gas hydrate accumulated within a few meters thick sediment layers at shallow depth. In addition, Sultan et al. [2010] proposed that the formation of a circular depression around the gas hydrate occurrence zone (GHOZ) is related to multiple steps in the pockmark evolution. The sequence is starting with hydrate formation induced by upward migration of fluids oversaturated in gas through fracture systems followed by decrease of fluid flow resulting in gas undersaturation, hydrate dissolution, generation of excess pore pressure, and by concurrent collapse of the gas hydrate-bearing sediment structures. Respective analyses were mainly based on subseabed approaches, using piston cores and in situ piezocone geotechnical measurements with a maximum penetration of 30 m below seafloor (mbsf). Howe...

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.

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. Key Points:Seismic surveys and geochemical analyses were combined to study a cluster of hydrate-bearing pockmarks The pockmark dynamics is governed by fluid flow Sulfate-profile simulation allowed estimating the formation age of four selected hydrate layers Supporting Information:Supporting Information S1Correspondence to: L. Ruffine, livio.ruffine@ifremer.fr Citation:de Prunel e, A., et al. (2017), Focused hydrocarbon-migration in shallow sediments of a pockmark cluster in the

International audienceSeawater samples were collected by submersible above methane seeps in the Gulf of Guinea (Regab and Baboon pockmarks) in order to investigate the behaviour of iron (Fe), manganese (Mn) and rare earth elements (REE) during fluid seepage. Our aim was to determine whether cold seeps may represent potential sources of dissolved chemical species to the ocean. Dissolved (<0.45 mu m filtered samples) and total dissolvable (unfiltered samples) concentrations were determined over similar to 50 m long vertical transects above the seafloor and at various discrete locations within the pockmarks.We show that substantial amounts of Fe and Mn are released into seawater during seepage of methane-rich fluids. Mn is exported almost quantitatively in the dissolved form (more than 90% of total Mn; mean Mn-DISS similar to 12 +/- 11 nmol/kg). Although a significant fraction of Fe is bound to particulate phases, the dissolved iron pool still accounts on average for approximately 20 percent of total iron flux at vent sites (mean Fe-DISS similar to 22 +/- 11 nmol/kg). This dissolved Fe fraction also appears to remain stable in the water column. In contrast, there was no evidence for any significant benthic fluxes of pore water REE associated with fluid seepage at the studied sites.Overall, our results point towards distinct trace element behaviour during fluid seepage, with potential implications for the marine geochemical budget. The absence of any dissolved REE enrichments in bottom waters clearly indicates effective removal in sub-surface sediments. Most likely, precipitation of authigenic mineral phases at cold seeps (i.e. carbonates) represents a net sink for these elements. While Mn appears to behave near-conservatively during fluid seepage, the observed relative stability of dissolved Fe in the water column above seepage sites could be explained by complexation with strong organic ligands and/or the presence of Fe-bearing sulfide nanoparticles, as reported previously for submarine hydrothermal systems. Considering the ubiquitous occurrence of methane vents at ocean margins, cold seeps could represent a previously unsuspected source of dissolved Fe to the deep ocean