Seafloor facies related to upward methane flux within a Giant Pockmark of the Lower Congo Basin

López, Michel; Ondréas, Hélène; Charlou, Jean-Luc; Sermondadaz, G; Cochonat, Pierre

doi:10.1016/j.margeo.2005.09.011

Cited by 98 publications

(76 citation statements)

References 76 publications

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“…The identification and delineation of seep sites on the ocean floor often arises from the detection of anomalously high acoustic backscatter on side scan sonar or multibeam echosounder recordings (Hovland, 1991;Hovland, 1992;Orange et al, 2002;Van Rensbergen et al, 2002;Johnson et al, 2003;Sager et al, 2003;Shoji et al, 2005;Gay et al, 2006;Klaucke et al, 2006;Rollet et al, 2006). High-acoustic backscatter is caused by the enhanced acoustic impedance or roughness contrast between certain regions of the seafloor and their surroundings (Blondel and Murton, 1997).…”

Section: Introductionmentioning

confidence: 99%

Anomalous sea-floor backscatter patterns in methane venting areas, Dnepr paleo-delta, NW Black Sea

et al. 2008

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The relation between acoustic seafloor backscatter and seep distribution is examined by integrating multibeam backscatter data and seep locations detected by single-beam echosounder. This study is further supported by side scan sonar recordings, high-resolution 5 kHz seismic data, pore-water analysis, grain-size analysis and visual seafloor observations. The datasets were acquired during the 2003 and 2004 expeditions of the EC-funded CRIMEA project in the Dnepr paleo-delta area, northwestern Black Sea.More than 600 active methane seeps were hydro-acoustically detected within a small (3.96 km by 3.72 km) area on the continental shelf of the Dnepr paleo-delta in water depths ranging from -72 m to -156 m. Multibeam and side scan sonar recordings show backscatter patterns that are clearly associated with seepage or with a present dune area. Seeps generally occur within medium-to highbackscatter areas which often coincide with pockmarks.High-resolution seismic data reveal the presence of an undulating gas front, i.e. the top of the free gas in the subsurface, which domes up towards and intersects the seafloor at locations where gas seeps and medium-to high-backscatter values are detected. Pore-water analysis of 4 multi-cores, taken at different backscatter intensity sites, shows a clear correlation between backscatter intensity and dissolved methane fluxes. All analyzed chemical species indicate increasing anaerobic oxidation of methane (AOM) from medium-to high-backscatter locations. This is confirmed by visual seafloor observations, showing bacterial mats and authigenic carbonates formed by AOM. Grain-size analysis of the 4 multi-cores only reveals negligible variations between the different backscatter sites.Integration of all datasets leads to the conclusion that the observed backscatter patterns are the result of ongoing methane seepage and the precipitation of methane-derived authigenic carbonates (MDACs) caused by AOM. The carbonate formation also appears to lead to a gradual (self-)sealing of the seeps by cementing fluid pathways/horizons followed by a relocation of the bubble-releasing locations. KeywordsMethane seeps; acoustic seafloor backscatter; anaerobic oxidation of methane; bacterial mats; pockmarks; methane-derived authigenic carbonates

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Section: Introductionmentioning

confidence: 99%

Anomalous sea-floor backscatter patterns in methane venting areas, Dnepr paleo-delta, NW Black Sea

et al. 2008

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“…Chemosynthetic communities, authigenic carbonates, and gas hydrates have been described for the giant Regab pockmark (see Fig. 1), which is 800 m in diameter and located close to the Congo Canyon (e.g., Charlou et al 2004;Ondréas et al 2005;Gay et al 2006c), and for the Kouilou pockmarks located north of the Congo Canyon (Sahling et al 2008;Haas et al 2010;Nöthen and Kasten 2011).…”

Section: Geological Settingmentioning

confidence: 99%

Gas hydrate decomposition recorded by authigenic barite at pockmark sites of the northern Congo Fan

et al. 2012

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The geochemical cycling of barium was investigated in sediments of pockmarks of the northern Congo Fan, characterized by surface and subsurface gas hydrates, chemosynthetic fauna, and authigenic carbonates. Two gravity cores retrieved from the so-called Hydrate Hole and Worm Hole pockmarks were examined using high-resolution pore-water and solid-phase analyses. The results indicate that, although gas hydrates in the study area are stable with respect to pressure and temperature, they are and have been subject to dissolution due to methane-undersaturated pore waters. The process significantly driving dissolution is the anaerobic oxidation of methane (AOM) above the shallowest hydrate-bearing sediment layer. It is suggested that episodic seep events temporarily increase the upward flux of methane, and induce hydrate formation close to the sediment surface. AOM establishes at a sediment depth where the upward flux of methane from the uppermost hydrate layer counterbalances the downward flux of seawater sulfate. After seepage ceases, AOM continues to consume methane at the sulfate/methane transition (SMT) above the hydrates, thereby driving the progressive dissolution of the hydrates "from above". As a result the SMT migrates downward, leaving behind enrichments of authigenic barite and carbonates that typically precipitate at this biogeochemical reaction front. Calculation of the time needed to produce the observed solid-phase barium enrichments above the present-day depths of the SMT served to track the net downward migration of the SMT and to estimate the total time of hydrate dissolution in the recovered sediments. Methane fluxes were higher, and the SMT was located closer to the sediment surface in the past at both sites. Active seepage and hydrate formation are inferred to have occurred only a few thousands of years ago at the Hydrate Hole site. By contrast, AOM-driven hydrate dissolution as a consequence of an overall net decrease in upward methane flux seems to have persisted for a considerably longer time at the Worm Hole site, amounting to a few tens of thousands of years.

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“…On sea-floor maps, Gay et al (2006) noticed that mussels, clams, and tube worms were restricted to the central area of pockmarks with relatively high fluid flow, whereas burrows extended farther on the edge of the pockmarks. All known modern lucinids and Vestimentifera are chemosynthetic and strictly restricted to reduced environments (Kiel, 2010), whereas most crustaceans found in seeps are not endemics but opportunistic species that can take advantage of this environment, feeding directly or indirectly on chemosynthetic bacteria (Martin and Haney, 2005).…”

Section: Ecology Of Seep Faunamentioning

confidence: 99%

“…Furthermore, the abundance of carbonate decreases and carbonate depth below seabed increases away from the venting site. In particular, where the SMTZ is close to the seabed, it can support the establishment of a chemosynthesis-based megafauna (Gay et al, 2006;Judd and Hovland, 2007) dominated by mollusks, annelids, and crustaceans (Kiel, 2010). Gas seepage intensity varies over various time scales (Naudts et al, 2010;Ho et al, 2012) as a function of migration mechanisms and pathways, typically faults, hydrofractures, or sand injectites (Mazzini et al, 2003;Duranti and Mazzini, 2005;Jonk et al, 2005;Hurst and Cartwright, 2007;Ho et al, 2016).…”

Section: Introductionmentioning

confidence: 99%

Mechanisms of biogenic gas migration revealed by seep carbonate paragenesis, Panoche Hills, California

Blouet¹,

Imbert²,

Foubert³

2017

Bulletin

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A comprehensive study of seep carbonates at the top of the organic-rich Maastrichtian to Danian Moreno Formation in the Panoche Hills (California) reveals the mechanisms of generation, expulsion, and migration of biogenic methane that fed the seeps. Two selected outcrops show that seep carbonates developed at the tip of sand dykes intrude up into the Moreno Formation from deeper sandbodies. Precipitation of methane-derived cements occurred in a succession of up to 10 repeated elementary sequences, each starting with a corrosion surface followed by dendritic carbonates, botryoidal aragonite, aragonite fans, and finally laminated micrite. Each element of the sequence reflects three stages. First, a sudden methane pulse extended up into the oxic zone of the sediments, leading to aerobic oxidation of methane and carbonate dissolution. Second, after consumption of the oxygen, anaerobic oxidation of methane coupled with sulfate reduction triggered carbonate precipitation. Third, progressive diminishment of the methane seepage led to the deepening of the reaction front in the sediment and the lowering of precipitation rates. Carbonate isotopes, with d 13 C as low as -51‰ Peedee belemnite, indicate a biogenic origin for the methane, whereas a one-dimensional basin model suggests that the Moreno Formation was in optimal thermal conditions for bacterial methane generation at the time of seep carbonate precipitation. Methane pulses are interpreted to reflect drainage by successive episodes of sand injection into the gas-generating shale of the Moreno Formation. The seep carbonates of the Panoche Hills can thus be viewed as a record of methane production from a biogenic source rock by multiphase hydraulic fracturing. A U T H O R S

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Seafloor facies related to upward methane flux within a Giant Pockmark of the Lower Congo Basin

Cited by 98 publications

References 76 publications

Anomalous sea-floor backscatter patterns in methane venting areas, Dnepr paleo-delta, NW Black Sea

Anomalous sea-floor backscatter patterns in methane venting areas, Dnepr paleo-delta, NW Black Sea

Gas hydrate decomposition recorded by authigenic barite at pockmark sites of the northern Congo Fan

Mechanisms of biogenic gas migration revealed by seep carbonate paragenesis, Panoche Hills, California

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