Deep Sulfate Reduction Completely Mediated by Anaerobic Methane Oxidation in Sediments of the Upwelling Area off Namibia

Niewöhner, Christine; Hensen, Christian; Kasten, Sabine; Zabel, Matthias; Schulz, Horst D

doi:10.1016/s0016-7037(98)00055-6

Cited by 297 publications

(202 citation statements)

References 25 publications

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“…This geochemical phenomenon explains the close association of hydrotroilite with the increase of alkalinity in the interstitial water (Fig. 5) (Niewöhner et al 1998, Hinrichs et al 1999.…”

Section: Discussionmentioning

confidence: 72%

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The influence of methane fluxes on the sulfate/methane interface in sediments from the Rio Grande Cone Gas Hydrate Province, southern Brazil

et al. 2017

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Much research has been published regarding the relation between major gas hydrate accumulations and the global carbon cycle. In this context, the determination of the sulfate/methane interface (SMI) depth is of primary importance in order to understand the dynamics of methane flux in the shallow section. This paper identifies the depth of the SMI in sediments based on sulfate and methane concentration profiles in cores recovered in the Rio Grande Cone Gas Hydrate Province, Pelotas Basin, southern Brazil. The shape of methane and sulfate concentration profiles in the sediments can be linked to the local methane flux rate as follows: (i) near linear, high upward-diffusing methane flux coupled with high sulfate diffusion from seawater; (ii) irregular, variable methane flux rates; and (iii) kink-type profile, which is indicative of variable rather than strictly high upward methane flux. The areas in which a high methane flux was identified are spatially associated with gas chimneys in sediments within pockmarks, whereas profiles with low methane flux are present in adjacent areas. These chimneys appear as acoustic blankings in seismic records and can therefore be mapped in subsurface. The wavy-like seismic reflection following the SMI coincides with the occurrence of authigenic carbonate nodules and concretions. In addition, high methane fluxes and the occurrence of concretions and nodules carbonates were correlated by stratigraphic position of the concretions bearing intervals and sulfate profiles. KEYWORDS: gas hydrate; methane flux; Rio Grande Cone; Brazil. RESUMO: Muitas pesquisas têm sido publicadas relacionando elevadas acumulações de hidrato de gás e o ciclo global do carbono. Nesse contexto, a determinação da profundidade da interface sulfato/metano (SMI) é importante para entender a dinâmica do fluxo de metano em locais rasos. Este artigo científico identifica a profundidade da SMI em sedimentos com base em perfis de concentração de sulfato e metano em testemunhos recuperados na Província do Hidrato de Gás do

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“…This geochemical phenomenon explains the close association of hydrotroilite with the increase of alkalinity in the interstitial water (Fig. 5) (Niewöhner et al 1998, Hinrichs et al 1999.…”

Section: Discussionmentioning

confidence: 72%

“…The occurrence of AOM primarily at the SMI is indicated by a linear sulfate profile in areas with large amounts of methane diffusing upwards (Borowski et al 1996, Niewöhner et al 1998, Hinrichset al 1999. AOM can be represented by the reaction ahead (Eq.…”

Section: Introductionmentioning

confidence: 99%

The influence of methane fluxes on the sulfate/methane interface in sediments from the Rio Grande Cone Gas Hydrate Province, southern Brazil

et al. 2017

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“…For CH 4 analyses, syringe samples of wet sediment (cf. above) were filled into 50-ml septum vials containing 20 ml of a NaCl (1.2 M)/HgCl 2 (0.3 M) solution (Niewöhner et al 1998). These were stored cooled (4°C) upside down until analyzed in the home laboratory where, in each case, 20 μl headspace gas was injected into a VARIAN 3400 gas chromatograph equipped with a flame ionization detector.…”

Section: Sampling Sitesmentioning

confidence: 99%

“…It commonly occurs in a distinct sediment horizon, the sulfate/methane transition (SMT), where both sulfate and methane are consumed to depletion (e.g., Reeburgh 1976;Iversen and Jørgensen 1985;Niewöhner et al 1998). The depth of the SMT is controlled primarily by the upward flux of methane (Borowski et al 1999).…”

Section: Introductionmentioning

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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“…[7] In this study, we apply reaction-transport diagenetic modeling [Berner, 1980;Boudreau, 1997] to determine the combination of OSR and AOM that matches the basic geochemical observations from a site drilled in the northern Cascadia margin (Integrated Ocean Drilling Program (IODP) Site U1325 ). The modeling compares concentration gradients of sulfate and methane to estimate the relative importance of OSR and AOM, as done in previous studies [Niewöhner et al, 1998;Borowski et al, 2000;Moore et al, 2004;Treude et al, 2005;Snyder et al, 2007]. We also compute the concentration and d 13 C of DIC to determine how they are affected by the balance of OSR and AOM.…”

Section: Introductionmentioning

confidence: 99%

Modeling sulfate reduction in methane hydrate-bearing continental margin sediments: Does a sulfate-methane transition require anaerobic oxidation of methane?

Malinverno

Pohlman

2011

Geochem. Geophys. Geosyst.

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[1] The sulfate-methane transition (SMT), a biogeochemical zone where sulfate and methane are metabolized, is commonly observed at shallow depths (1-30 mbsf) in methane-bearing marine sediments. Two processes consume sulfate at and above the SMT, anaerobic oxidation of methane (AOM) and organoclastic sulfate reduction (OSR). Differentiating the relative contribution of each process is critical to estimate methane flux into the SMT, which, in turn, is necessary to predict deeper occurrences of gas hydrates in continental margin sediments. To evaluate the relative importance of these two sulfate reduction pathways, we developed a diagenetic model to compute the pore water concentrations of sulfate, methane, and dissolved inorganic carbon (DIC). By separately tracking DIC containing 12 C and 13 C, the model also computes d 13 C-DIC values. The model reproduces common observations from methane-rich sediments: a welldefined SMT with no methane above and no sulfate below and a d 13 C-DIC minimum at the SMT. The model also highlights the role of upward diffusing 13 C-enriched DIC in contributing to the carbon isotope mass balance of DIC. A combination of OSR and AOM, each consuming similar amounts of sulfate, matches observations from Site U1325 (Integrated Ocean Drilling Program Expedition 311, northern Cascadia margin). Without AOM, methane diffuses above the SMT, which contradicts existing field data. The modeling results are generalized with a dimensional analysis to the range of SMT depths and sedimentation rates typical of continental margins. The modeling shows that AOM must be active to establish an SMT wherein methane is quantitatively consumed and the d 13 C-DIC minimum occurs. The presence of an SMT generally requires active AOM.

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Deep Sulfate Reduction Completely Mediated by Anaerobic Methane Oxidation in Sediments of the Upwelling Area off Namibia

Cited by 297 publications

References 25 publications

The influence of methane fluxes on the sulfate/methane interface in sediments from the Rio Grande Cone Gas Hydrate Province, southern Brazil

The influence of methane fluxes on the sulfate/methane interface in sediments from the Rio Grande Cone Gas Hydrate Province, southern Brazil

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

Modeling sulfate reduction in methane hydrate-bearing continental margin sediments: Does a sulfate-methane transition require anaerobic oxidation of methane?

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