Methane-generated(?) pockmarks on young, thickly sedimented oceanic crust in the Arctic: Vestnesa ridge, Fram strait

Vogt, Peter R.; Crane, Kathleen; Sundvor, Eirik; Max, M. D.; Pfirman, Stephanie

doi:10.1130/0091-7613(1994)022<0255:mgpoyt>2.3.co;2

Cited by 99 publications

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“…Our study focuses on the active seeping segment of the Vestnesa Ridge, an approximately 100-km-long gas hydrate charged contourite drift developed over <20 Ma oceanic crust offshore west Svalbard (Figure 1) (Eiken and Hinz, 1993;Vogt et al, 1994;Bünz et al, 2012). The contourite drift is in close proximity to the Molloy and the Knipovich slow-spreading oceanic ridges, and it is located between the Molloy and the Spitsbergen transform faults (e.g., Ritzmann et al, 2004).…”

Section: Study Areamentioning

confidence: 99%

Gas hydrate and free gas detection using seismic quality factor estimates from high-resolution P-cable 3D seismic data

et al. 2016

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We have estimated the seismic attenuation in gas hydrate and free-gas-bearing sediments from high-resolution P-cable 3D seismic data from the Vestnesa Ridge on the Arctic continental margin of Svalbard. P-cable data have a broad bandwidth (20-300 Hz), which is extremely advantageous in estimating seismic attenuation in a medium. The seismic quality factor (Q), the inverse of seismic attenuation, is estimated from the seismic data set using the centroid frequency shift and spectral ratio (SR) methods. The centroid frequency shift method establishes a relationship between the change in the centroid frequency of an amplitude spectrum and the Q value of a medium. The SR method estimates the Q value of a medium by studying the differential decay of different frequencies. The broad bandwidth and short offset characteristics of the P-cable data set are useful to continuously map the Q for different layers throughout the 3D seismic volume. The centroid frequency shift method is found to be relatively more stable than the SR method. Q values estimated using these two methods are in concordance with each other. The Q data document attenuation anomalies in the layers in the gas hydrate stability zone above the bottom-simulating reflection (BSR) and in the free gas zone below. Changes in the attenuation anomalies correlate with small-scale fault systems in the Vestnesa Ridge suggesting a strong structural control on the distribution of free gas and gas hydrates in the region. We argued that high and spatially limited Q anomalies in the layer above the BSR indicate the presence of gas hydrates in marine sediments in this setting. Hence, our workflow to analyze Q using high-resolution P-cable 3D seismic data with a large bandwidth could be a potential technique to detect and directly map the distribution of gas hydrates in marine sediments.

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Section: Study Areamentioning

confidence: 99%

Gas hydrate and free gas detection using seismic quality factor estimates from high-resolution P-cable 3D seismic data

et al. 2016

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“…Currently, pockmarks have been described for various marine geological environments (Paull et al 2002;Christodoulou et al 2003;Judd and Hovland 2007). Due to their diverse locations and morphology, pockmarks are thought to be formed by a variety of mechanisms, such as the venting of interstitial gas (Solheim and Elverhøi 1993;Baraza and Ercilla 1996;Cathles et al 2010), pore-water release due to compaction (Harrington 1985;Soter 1999), groundwater seepage (Hovland et al 2002;Whiticar 2002), and the rising of buoyant clumps of gas hydrates (Vogt et al 1994;Paull et al 1995). In freshwater environments, only a few pockmark sites have been recognized, such as those in hydrothermally active regions or near active faults (Pickrill 1993;Duck and Herbert 2006).…”

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confidence: 99%

Active pockmarks in a large lake (Lake Constance, Germany): Effects on methane distribution and turnover in the sediment

Bussmann

Schlömer

Schlüter

et al. 2011

Limnology & Oceanography

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In the eastern part of Lake Constance, the second largest prealpine lake in Europe, about 500 pockmarks (morphological depressions on the lake floor) were recently discovered. The diameters of these pockmarks are as large as 16 m, and about 24% of them continuously release methane in the form of visible bubbles. The isotopic composition of the escaping gas indicated that the methane was of biogenic origin and was predominantly produced by the CO 2 -reduction pathway. In shallow-water pockmarks (9 m and 12 m), pore-water analysis revealed increased methane concentrations of . 1100 mmol L 21 in the sediment. Diffusive methane fluxes and oxidation rates within these shallow pockmarks were much higher than in sediments outside pockmarks. An estimated methane ebullition into the water column of about 40 L h 21 per pockmark showed that methane ebullition is by far the dominant pathway of methane release from these shallow pockmarks. Meiofauna abundance and organic carbon contents of the sediment were also higher in the shallow pockmarks, but there were no differences in macrofauna. In a deep pockmark (80 m), even though bubble release was observed, the sediment inside the pockmark had almost the same methane concentrations and isotopic compositions as a site outside of the pockmark. At both sites, the shallow and deep pockmark act mainly as sedimentation traps, the emanating gas has only minor influence on the methane inventory of the sediments and, therefore, gas bubbles enter the water column essentially unaffected.

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“…Within a GHSZ in a sediment ridge, gas may become concentrated in pseudo-anticlinal forms at the base GHSZ (Vogt et al, 1994). The degree of NGH displacement of pore water decreases with depth (Max & Dillon, 1998).…”

Section: Discussion and Conceptual Modelmentioning

confidence: 99%

“…Within a 30 m contour around the bathymetric high (about 2,600 km 2 ) in Blake Ridge, drilling has provided ground truth for wider estimates of NGH abundance, using seismic analysis, at about 37.7 trillion cubic feet (TCF) of gas with about 20 TCF gas trapped below the existing GHSZ (Dillon & Max, 2001). In seismic data, stronger amplitude reflection strength in the crest of Blake Ridge (and the apex of the base GHSZ) indicates that gas may have migrated from the flanks toward the apex of the structural ridge-crest trap (e.g., Vogt et al, 1994). The BSR (Fig.…”

Section: Low Salinity Water Fluxes From Natural Gas Hydratementioning

confidence: 99%

Submarine vortices derived from natural gas hydrate conversion: a mechanism for ocean mixing

Barnard

Max

Gualdesi³

2017

Medit. Mar. Sci.

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We propose that the source water for some abyssal undular vortices cored by cool, low-salinity water identified at depths in excess of 2,500 m in the deepwater region of the Eastern Mediterranean Basin may be related to conversion of natural gas hydrate (NGH) in abyssal marine sediments. The conditions for extensive formation of NGH in the gas hydrate stability zones (GHSZ) of the upper seafloor sediments existed in this region during previous glacial episodes when colder water supported a thicker GHSZ. Seafloor warming during the most recent interglacial caused thinning of the GHSZ at its base and has driven endothermic NGH dissociation that would have released large volumes of low-salinity water and gas that would tend to pond below the base GHSZ. Periodically, trapped low-salinity water and gas would be released into the sea through the overlying sediments. Buoyant low-salinity water masses, supersaturated with gas and locally containing free gas would ascend and introduce a dynamic element into an otherwise generally static environment. As a result of the interaction of the rise of this buoyant plume and Coriolis acceleration the ascending mass would begin to rotate and form a vortex tube in midwater. NGH conversion within the seafloor introduces large coherent masses of low-salinity, lower-temperature water containing a buoyant free gas fraction from near-surface reservoirs into the abyssal depths even where there may only be a weak natural gas petroleum system.

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Methane-generated(?) pockmarks on young, thickly sedimented oceanic crust in the Arctic: Vestnesa ridge, Fram strait

Cited by 99 publications

References 0 publications

Gas hydrate and free gas detection using seismic quality factor estimates from high-resolution P-cable 3D seismic data

Gas hydrate and free gas detection using seismic quality factor estimates from high-resolution P-cable 3D seismic data

Active pockmarks in a large lake (Lake Constance, Germany): Effects on methane distribution and turnover in the sediment

Submarine vortices derived from natural gas hydrate conversion: a mechanism for ocean mixing

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