Methane release from warming-induced hydrate dissociation in the West Svalbard continental margin: timing, rates, and geological controls

Hundreds of plumes of methane bubbles, first observed in 2008, emanate from an area of the seabed off West Svalbard that has become 1°C warmer over the past 30 years. The distribution of the plumes, lying close to and upslope from the present upper limit of the methane hydrate stability zone, indicates that methane in the plumes could come from warming-induced hydrate dissociation, a process commonly invoked as contributing to rapid climate change. We used numerical modeling to investigate the response of hydrate beneath the seabed to changes in bottom-water temperature over periods of up to 1000 years B.P. The delay between the onset of warming and emission of gas, resulting from the time taken for thermal diffusion, hydrate dissociation, and gas migration, can be less than 30 years in water depths shallower than the present upper limit of the methane hydrate stability zone, where hydrate was initially several meters beneath the seabed and fractures increase the effective permeability of intrinsically low-permeability glacigenic sediment. At the rates of warming of the seabed that have occurred over the past two centuries, the enthalpy of hydrate dissociation limits the rate of gas release to moderate values. Cycles of warming and cooling can create and sustain hydrate close to the seabed where there is locally a supply of methane of tens of mol·m–2 yr–1. This rate of gas flow can be achieved where stratigraphic and structural heterogeneity focus gas migration, although the regional rate of methane supply could be much less

High-resolution 3D seismic investigations of hydrate-bearing fluid escape chimneys in the Nyegga region of the Voring Plateau, Norway

Hundreds of pockmarks and mounds, which seismic reflection sections show to be underlain by chimney-like structures, exist in southeast part of the Vøring plateau, Norwegian continental margin. These chimneys may be representative of a class of feature of global importance for the escape of methane from beneath continental margins and for the provision of a habitat for the communities of chemosynthetic biota. Thinning of the time intervals between reflectors in the flanks of chimneys, observed on several high-resolution seismic sections, could be caused by the presence of higher velocity material such as hydrate or authigenic carbonate, which is abundant at the seabed in pockmarks in this area. Evidence for the presence of hydrate was obtained from cores at five locations visited by the Professor Logachev during TTR Cruise 16, Leg 3 in 2006. Two of these pockmarks, each about 300-m wide with active seeps within them, were the sites of high-resolution seismic experiments employing arrays of 4-component OBS (Ocean-Bottom Seismic recorders) with approximately 100-m separation to investigate the 3D variation in their structure and properties. Shot lines at 50-m spacing, run with mini-GI guns fired at 8-m intervals, provided dense seismic coverage of the sub-seabed structure. These were supplemented by MAK deep-tow 5-kHz profiles to provide very high-resolution detail of features within the top 1-40 m sub-seabed. Travel-time tomography has been used to detail the variation in Vp and Vs within and around the chimneys. Locally high-amplitude reflectors of negative polarity in the flanks of chimneys and scattering and attenuation within the interiors of the chimneys may be caused by the presence of free gas within the hydrate stability field. A large zone of free gas beneath the hydrate stability field, apparently feeding several pockmarks, is indicated by attenuation and velocity pull-down of reflectors

Leaking Methane Reservoirs Offshore Svalbard

Methane hydrate—a solid substance in which methane is trapped within ice-like crystals—is stable at low temperatures and high pressures and may be destabilized by ocean warming on both geological and human time scales. Methane is a powerful greenhouse gas, and methane released from hydrate provides a potential positive feedback mechanism in global climate change [e.g., Archer and Buffett, 2005]—in theory, the more methane is released by the hydrates, the warmer the climate gets, causing the ocean to warm and release more methane. However, methane escaping from the seabed is oxidized and dissolved in the ocean, and insufficient methane may reach the atmosphere to affect the climate significantly. Its importance for climate change therefore depends on whether the flux from the seabed is great enough to overcome solution in the ocean and perturb atmospheric concentrations over sufficiently long time scales

Fine-scale gas distribution in marine sediments assessed from deep-towed seismic data

In the context of seismic imaging of gas/gas-hydrate systems, the fine-scale structure of subseabed gas-related reflections is assessed by taking advantage of the source signature of the deep-towed high-resolution SYSIF seismic device. We demonstrate the value of an original wavelet-based method and associated multiscale seismic attributes, applied to seismic data recently acquired on the western margin of the Arctic archipelago of Svalbard. From analysis in the wavelet domain, we recognize two types of gas-related reflections associated with submetre-scale distribution of gas. We identify a thin gas-charged layer associated with an apparent normal polarity reflection, and we detect gas patches associated with a reverse-polarity bright spot with frequency-dependent elastic properties at small seismic wavelengths. The results provide valuable information on the scale of features through which gas migrates and resolve ambiguities in the interpretation of the seismic data

Globalization, regionalization and the history of international relations

Offshore western Svalbard plumes of gas bubbles rise from the seafloor at the landward limit of the gas hydrate stability zone (LLGHSZ; ∼400 m water depth). It is hypothesized that this methane may, in part, come from dissociation of gas hydrate in the underlying sediments in response to recent warming of ocean bottom waters. To evaluate the potential role of gas hydrate in the supply of methane to the shallow subsurface sediments, and the role of anaerobic oxidation in regulating methane fluxes across the sediment–seawater interface, we have characterised the chemical and isotopic compositions of the gases and sediment pore waters. The molecular and isotopic signatures of gas in the bubble plumes (C1/C2+ = 1 × 104; δ13C-CH4 = −55 to −51‰; δD-CH4 = −187 to −184‰) are similar to gas hydrate recovered from within sediments ∼30 km away from the LLGHSZ. Modelling of pore water sulphate profiles indicates that subsurface methane fluxes are largely at steady state in the vicinity of the LLGHSZ, providing no evidence for any recent change in methane supply due to gas hydrate dissociation. However, at greater water depths, within the GHSZ, there is some evidence that the supply of methane to the shallow sediments has recently increased, which is consistent with downslope retreat of the GHSZ due to bottom water warming although other explanations are possible. We estimate that the upward diffusive methane flux into shallow subsurface sediments close to the LLGHSZ is 30,550 mmol m−2 yr−1, but it is <20 mmol m−2 yr−1 in sediments further away from the seafloor bubble plumes. While anaerobic oxidation within the sediments prevents significant transport of dissolved methane into ocean bottom waters this amounts to less than 10% of the total methane flux (dissolved + gas) into the shallow subsurface sediments, most of which escapes AOM as it is transported in the gas phase

Contribution of high-resolution 3D seismic near-seafloor imaging to reservoir-scale studies: application to the active North Anatolian Fault, Sea of Marmara

International audienceHigh Resolution (HR) marine seismic acquisition contributes to numerous research fields. The vertical resolution is of metric scale in order to study geological processes at a short time scale or to characterise small objects. 3D seismic imaging allows optimal resolution to be reached whereas 2D images are blurred mainly by side effects. Developed for the oil industry decades ago and tailored to the exploration for hydrocarbon reservoirs, 3D seismic, as applied to higher resolution targets, is more recent. Available technological advances in acquisition have allowed research institutes to develop innovative 3D high-resolution marine seismic systems tailored to these targets. The seismic survey carried out in 2009 on the Western High, Sea of Marmara, illustrates the value of HR3D imaging. Since the destructive İzmit earthquake in 1999, an intensive international research effort has demonstrated that the Western High is one of the key structures for assessing the processes of deformation related to the North Anatolian Fault (NAF). The 30-km² HR3D survey centred on the main NAF was acquired using a dual streamers-dual source-array configuration. In spite of the minimal 3D processing sequence that was applied to the data, the fine imaging of the seabed and of the sedimentary stratigraphy and structures is much better than HR2D seismic. Comparison with an autonomous underwater vehicle (AUV) multi-beam bathymetric survey carried out at the same location enables the limits of the vertical resolution of the seismic data to be assessed. The lateral resolution is between 13.5 and 25 metres at the seabed. The HR3D seismic data highlight the interplay between tectonic processes and stratigraphy. In particular, differential uplift leads to syntectonic deposition and submarine slides. The widespread occurrence of gas in the sedimentary sequence is clearly shown by anomalously high seismic amplitudes. 3D imaging of these high amplitudes enables the identification of the pathways through faults and permeable units that gas takes as it migrates to the seabed

The Cenozoic tectonostratigraphic evolution of the Barracuda Ridge and Tiburon Rise, at the western end of the North America-South America plate boundary zone

International audienceThe Barracuda Ridge and the Tiburon Rise, two major oceanic basement ridges, lie at the western end of the diffuse North America-South America plate-boundary zone, where they enter the subduction zone beneath the Lesser Antilles island arc. There is a large degree of uncertainty in the motion between the North American and South American plates predicted by kinematic models of plate motion for the region of these two ridges during the Cenozoic and Quaternary. From the analysis of new multibeam and seismic reflection profiles acquired in 2007, together with older geophysical and geological data, we provide new information on the geological history of this area, including the timing of the formation of the Barracuda Ridge and Tiburon Rise in their present-day configurations. The timing of the deformation in this region is now much better constrained through the correlation of several key seismic horizons with existing DSDP and ODP holes. The seafloor topography inherited from the process of formation of the crust at the mid-oceanic ridge, was buried by distal turbidites by the end of the Paleogene. Beginning in the Middle-Late Miocene and then the Pleistocene, the Tiburon Rise and Barracuda Ridge, respectively, were uplifted and acquired their present-day forms and elevation, which is much more recent than previously believed. In the Quaternary, the uplift was accompanied by the deposition of very large mass transport deposits. The causes of uplift and deformation of the ridges have been convergence between the North American and South American plates and the flexure of these plates as they enter the Lesser Antilles subduction zone, The zone of uplift and deformation migrated northward during the Neogene and Quaternary