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...

No abstract

Gas hydrates, a solid established by water and gas molecules, are widespread along the continental margins of the world. Their dynamics have mainly been regarded through the lens of temperature-pressure conditions. A fluctuation in one of these parameters may cause destabilization of gas hydrate-bearing sediments below the seafloor with implications in ocean acidification and eventually in global warming. Here we show throughout an example of the Black Sea, the world’s most isolated sea, evidence that extensive gas hydrate dissociation may occur in the future due to recent salinity changes of the sea water. Recent and forthcoming salt diffusion within the sediment will destabilize gas hydrates by reducing the extension and thickness of their thermodynamic stability zone in a region covering at least 2800 square kilometers which focus seepages at the observed sites. We suspect this process to occur in other world regions (e.g., Caspian Sea, Sea of Marmara).

A series of pockmarks observed at the seabed matches well the perimeter of a large submarine landslide, called NG1, located on the outer shelf and continental slope of the Eastern Gulf of Guinea. NG1 extends over 200 km2, is covered by a 120-m thick sedimentary layer which tapers downslope, and has an internal structure clearly identified in 3D seismic data consisting of three adjacent units on the upper continental slope. The pockmarks above NG1 have a diameter of several tens of meters and reveal distinct origins: (1) linked to >500 m deep fluid reservoirs, (2) rooted in NG1 internal discontinuities between NG1 units, and (3) well above NG1, superficially rooted in a regional conformity (D40), which marks the lowest sea level of the Marine Isotope Stage 6. The regional stratigraphic pattern of the study area is composed of muddy sedimentary sequences separated by correlative conformities and transgressive condensed units of coarser grain size. Mud-confined coarser-grained units constitute transient gas reservoirs favoring lateral gas migration and formation of pockmarks rooted in the condensed units. The buried NG1 landslide modifies the layered structure of the sedimentary column providing (1) overall, a barrier to fluid migration, and (2) localized pathways for fluid migration. The triggering factor for the formation of pockmarks above NG1 can be the variation of hydrostatic pressure driven by relative sea-level fall during Marine Isotopic Stages 6 and 2 and consequent gas exsolution and fluid flow. We anticipate our result to be a starting point for understanding the role of gas seeps on climate change worldwide. Furthermore, gas release intensifies during lowstands with relevant implication on global warming after ice ages. Highlights ►This is the first study linking the effect of a landslide on gas migration pathways. ►Pockmark formation is reconstructed with geophysical and geotechnical data. ►The landslide occurred during a sea-level fall period. ►The timing of pockmark formation is in part controlled by 100-kyr eustatic cycles. ►Once buried, the landslide controls the spatial organization of pockmarks.