Geological controls on focused fluid flow associated with seafloor seeps in the Lower Congo Basin

López, Michel; Berndt, Christian; Séranne, Michel

doi:10.1016/j.margeo.2007.06.003

Cited by 247 publications

(188 citation statements)

References 92 publications

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“…Indeed, other studies on the northern Congo Fan have shown that most of the pockmarks are today relatively dormant (Gay et al 2007;Sahling et al 2008;Nöthen and Kasten 2011). Moreover, the present findings are consistent with the general observation that fluid migration within pockmarks is highly variable both spatially and temporally, and imply that individual venting events in the study area are not triggered by changes in single external factors.…”

Section: Barite Precipitation Timesupporting

confidence: 92%

“…It is therefore argued that the SMT at the Worm Hole site has experienced a net downward migration of more than 8 m since the most intense past fluid migration and hydrate-forming event, which shifted the SMT close to the (former) sediment surface. It is indeed possible that the SMT did not migrate downward continuously but rather fluctuated over the examined sediment interval, considering the episodic nature of seepage at pockmark sites and the repeated pulses of upward methane flux that are likely to have occurred (e.g., Hovland and Judd 1988;Gay et al 2007). …”

Section: Discussionmentioning

confidence: 99%

“…Seismic and geological data collected during several research cruises to the Congo Fan have been presented by Uenzelmann-Neben et al (1997), Uenzelmann-Neben (1998), Ondréas et al (2005), Gay et al (2007), and Sahling et al (2008). During ODP Leg 175, three holes were drilled north of the Congo Canyon (Wefer et al 1998).…”

Section: Geological Settingmentioning

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: Barite Precipitation Timesupporting

confidence: 92%

Section: Discussionmentioning

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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“…Any seep may be composed of a single gas stream or several ones very close to each other such that they clearly define its perimeter. Submarine seeps, at cold seeps, occur worldwide along the continental margins and are usually related to geological structures with either positive reliefs such as submarine pingoes [1][2][3][4][5][6][7], carbonate concretions and pavements [8][9][10][11], and mud volcanoes [9,[12][13][14][15][16][17][18] or negative reliefs like pockmarks [13,[19][20][21][22][23]. They also occur in seafloor-reaching fault areas at tectonically active regions without being associated with a specific relief [24][25][26][27][28].…”

Section: Introductionmentioning

confidence: 99%

Gas Seepage along the Edge of the Aquitaine Shelf (France): Origin and Local Fluxes

et al. 2017

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During the scientific expedition GAZCOGNE2 at the Bay of Biscay nine gas seeps were sampled for the first time and their flux was measured using an in situ pressure-preservation sampler (PEGAZ, © IFREMER). Overall, three sites were investigated to determine the nature and the origin of the gases bubbling at the seafloor and forming acoustic plumes into the water column, as this was the question raised from the first geologic study of the area. This has guided our study and accordingly corresponds to the main purpose of the present article. Thus, the molecular and isotopic ( D and 13 C) analyses revealed that the gas seeps were primarily composed of methane. Both methane and ethane are of microbial origin, and the former has been generated by microbial reduction of carbon dioxide. Heavier hydrocarbons accounted for less than 0.06% mol of the total amount. Despite the microbial origin of methane, the samples exhibit subtle differences with respect to the 13 C CH 4 values, which varied between −72.7 and −66.1‰. It has been suggested that such a discrepancy was predominantly governed by the occurrence of anaerobic methane oxidation. The PEGAZ sampler also enabled us to estimate the local gas fluxes from the sampled streams. The resulting values are extremely heterogeneous between seeps, ranging from 35 to 368 mLn⋅min −1 . Assuming a steady discharge, the mean calculated methane emission for the nine seeps is of 38 kmol⋅yr −1 . Considering the extent of the seep area, this very local estimate suggests that the Aquitaine Shelf is a very appropriate place to study methane discharge and its fate on continental shelves.

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“…For this purpose we use the sediment section covering the past 1.2 Ma from Ocean Drilling Program (ODP) Site 1075, located on the lower Congo fan. This setting is well-documented for methane seepage (Gay et al, 2007) as well as for having a significant but variable terrigenous contribution that has been closely linked to orbitallydriven variations in monsoonal forcing and tropical African climate , modulated by sea surface temperature variations (Schefuß et al, 2003(Schefuß et al, , 2005. These features make the Congo Fan well suited to test a novel biomarker proxy for AMO in the sedimentary record, and discuss how possible variations in such biomarker concentrations are linked with Quaternary climate cycles and possible marine and terrestrial sources.…”

Section: Introductionmentioning

confidence: 78%

Synthesising bushmeat research effort in West and Central Africa: A new regional database

Taylor

Scharlemann

Rowcliffe

et al. 2015

Biological Conservation

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Methane (CH 4 ) is a strong greenhouse gas known to have perturbed global climate in the past, especially when released in large quantities over short time periods from continental or marine sources. It is therefore crucial to understand and, if possible, quantify the individual and combined response of these variable methane sources to natural climate variability. However, past changes in the stability of greenhouse gas reservoirs remain uncertain and poorly constrained by geological evidence. Here, we present a record from the Congo fan of a highly specific bacteriohopanepolyol (BHP) biomarker for aerobic methane oxidation (AMO), 35-aminobacteriohopane-30,31,32,33,34-pentol (aminopentol), that identifies discrete periods of increased AMO as far back as 1.2 Ma. Fluctuations in the concentration of aminopentol, and other 35-aminoBHPs, follow a pattern that correlates with late Quaternary glacial-interglacial climate cycles, with highest concentrations during warm periods. We discuss possible sources of aminopentol, and the methane consumed by the precursor methanotrophs, within the context of the Congo River setting, including supply of methane oxidation markers from terrestrial watersheds and/or marine sources (gas hydrate and/or deep subsurface gas reservoir). Compound-specific carbon isotope values of À30& to À40& for BHPs in ODP 1075 and strong similarities between the BHP signature of the core and surface sediments from the Congo estuary and floodplain wetlands from the interior of the Congo River Basin, support a methanotrophic and likely terrigenous origin of the 35-aminoBHPs found in the fan sediments. This new evidence supports a causal connection between marine sediment BHP records of tropical deep sea fans and wetland settings in the feeding river catchments, and thus tropical continental hydrology. Further research is needed to better constrain the different sources and pathways of methane emission. However, this study identifies the large potential of aminoBHPs, in particular aminopentol, to trace and, once better calibrated and understood, quantify past methane sources and fluxes from terrestrial and potentially also marine sources.

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Geological controls on focused fluid flow associated with seafloor seeps in the Lower Congo Basin

Cited by 247 publications

References 92 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

Gas Seepage along the Edge of the Aquitaine Shelf (France): Origin and Local Fluxes

Synthesising bushmeat research effort in West and Central Africa: A new regional database

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