Reconstructing changes in seep activity by means of pore water and solid phase Sr/Ca and Mg/Ca ratios in pockmark sediments of the Northern Congo Fan

Nöthen, Kerstin; Kasten, Sabine

doi:10.1016/j.margeo.2011.06.008

Cited by 136 publications

(98 citation statements)

References 80 publications

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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%

“…Numerous investigations of pockmarks on the lower Congo Fan (e.g., Charlou et al 2004;Ondréas et al 2005;Gay et al 2006aGay et al ,b, 2007Pierre and Fouquet 2007;Sahling et al 2008;Haas et al 2010;Nöthen and Kasten 2011) have revealed their association with morphologically, stratigraphically, or tectonically controlled focused fluid flow. Chemosynthetic communities, authigenic carbonates, and gas hydrates have been described for the giant Regab pockmark (see Fig.…”

Section: Geological Settingmentioning

confidence: 99%

“…Sediment porosity was calculated from measurements of electrical conductivity performed at 2-cm intervals (cf. Niewöhner et al 1998;Nöthen and Kasten 2011).…”

Section: Sampling Sitesmentioning

confidence: 99%

“…Barite has already been successfully applied as a proxy parameter to constrain changes in the upward flux of methane from the gas hydrate system on Blake Ridge (Dickens 2001b;Snyder et al 2007a), and from Cretaceous black shales in deposits of the Demerara Rise (Arndt et al 2009). Recently, Nöthen and Kasten (2011) used pore-water and solid-phase Sr/Ca and Mg/Ca ratios to infer the mineralogy of authigenic carbonates in the sediments of the so-called Kouilou pockmarks of the northern Congo Fan (Sahling et al 2008). They were able to identify active and fossil horizons of carbonate precipitation, and to reconstruct relative changes in seep activity.…”

Section: Introductionmentioning

confidence: 99%

“…The present study expands on that of Nöthen and Kasten (2011), using the depth distribution of solid-phase Ba with the objective to track the net downward movement of the SMT, thereby giving a time perspective to changes in upward methane flux and in gas hydrate dynamics. For this purpose, gravity cores were collected from the so-called Hydrate Hole and Worm Hole pockmarks of the Kouilou pockmark field.…”

Section: Introductionmentioning

confidence: 99%

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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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Section: Geological Settingmentioning

confidence: 99%

Section: Geological Settingmentioning

confidence: 99%

“…Sediment porosity was calculated from measurements of electrical conductivity performed at 2-cm intervals (cf. Niewöhner et al 1998;Nöthen and Kasten 2011).…”

Section: Sampling Sitesmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

et al. 2012

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Subsurface methane sources and migration pathways within a gas hydrate mound system, Gulf of Mexico

Wilson

Macelloni

Simonetti

et al. 2014

Geochem Geophys Geosyst

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[1] Geochemical profiles were coupled with seismic information to examine subsurface hydrocarbon source, migration, and fate at a Gulf of Mexico carbonate-gas hydrate mound (Woolsey Mound). Three seafloor features were investigated in detail: (1) major faults resulting from a rising salt body, (2) an acoustic backscatter anomaly, and (3) a pockmark associated with a major fault. We analyzed sulfate, chloride, dissolved inorganic carbon, and hydrocarbon concentrations, and carbon isotopes in pore water extracted from 20 m piston cores to characterize gas source and calculate methane flux. Dissolved biogenic methane dominated the off-fault sites, while the contribution of thermogenic methane increased near a major fault where thermogenic gas hydrates were recovered. Within the pockmark, methane concentrations were low and isotopes indicated a biogenic source. Since pockmarks are typically formed from expulsive fluid flow, this suggests that either the pockmark is the legacy of a conduit that has become plugged or that the expulsed fluid is confined within the fault walls. At the acoustic anomaly,

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Pore water geochemistry at two seismogenic areas in the Sea of Marmara

Ruffine

Germain

Polonia

et al. 2015

Geochem Geophys Geosyst

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Within the Sea of Marmara, the highly active North Anatolian Fault (NAF) is responsible for major earthquakes (Mw 7), and acts as a pathway for fluid migration from deep sources to the seafloor. This work reports on pore water geochemistry from three sediment cores collected in the Gulfs of Izmit and Gemlik, along the Northern and the Middle strands of the NAF, respectively. The resulting data set shows that anaerobic oxidation of methane (AOM) is the major process responsible for sulfate depletion in the shallow sediment. In the Gulf of Gemlik, depth concentration profiles of both sulfate and alkalinity exhibit a kink-type profile. The Sulfate Methane Transition Zone (SMTZ) is located at moderate depth in the area. In the Gulf of Izmit, the low concentrations observed near the seawater-sediment interface for sulfate, calcium, strontium, and magnesium result from rapid geochemical processes, AOM, and carbonate precipitation, occurring in the uppermost part of the sedimentary column and sustained by free methane accumulation. Barite dissolution and carbonate recrystallization have also been identified at deeper depth at the easternmost basin of the Gulf of Izmit. This is supported by the profile of the strontium isotope ratios ( 87 Sr/ 86 Sr) as a function of depth which exhibits negative anomalies compared to the modern seawater value. The strontium isotopic signature also shows that these carbonates had precipitated during the reconnection of the Sea of Marmara with the Mediterranean Sea. Finally, a first attempt to interpret the sulfate profiles observed in the light of the seismic activity at both sites is presented. We propose the hypothesis that seismic activity in the areas is responsible for the transient sulfate profile, and that the very shallow SMTZ depths observed in the Gulf of Izmit is likely due to episodic release of significant amount of methane.

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Reconstructing changes in seep activity by means of pore water and solid phase Sr/Ca and Mg/Ca ratios in pockmark sediments of the Northern Congo Fan

Cited by 136 publications

References 80 publications

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

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

Subsurface methane sources and migration pathways within a gas hydrate mound system, Gulf of Mexico

Pore water geochemistry at two seismogenic areas in the Sea of Marmara

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