ROV study of a giant pockmark on the Gabon continental margin

Ondréas, Hélène; Olu, Karine; Fouquet, Yves; Charlou, Jean-Luc; Dennielou, Bernard; Donval, Jean-Pierre; Fifis, A.; Nadalig, Thierry; Cochonat, Pierre; Cauquil, E.; Bourillet, Jean‐François; Moigne, Morgan Le; Sibuet, Myriam

doi:10.1007/s00367-005-0213-6

Cited by 106 publications

(101 citation statements)

References 46 publications

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“…The first seep community described in the South East Atlantic was discovered in 1998 on a giant pockmark along the Equatorial African margin at 3150 m water depth (Ondréas et al, 2005). Submarine pockmarks are crater-like depressions that are typically wide (10-800 m in diameter), shallow (up to 45 m deep) features, occurring in most seas, oceans and in many diverse geological settings (Hovland et al, 2002).…”

Section: Introductionmentioning

confidence: 99%

Deep-sea habitat heterogeneity influence on meiofaunal communities in the Gulf of Guinea

Gaever

Galéron

Sibuet

et al. 2009

Deep Sea Research Part II: Topical Studies in Oceanography

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To estimate the degree of spatial heterogeneity of benthic deep-sea communities, we carried out a multiple-scale (from m's to 200 km) investigation in the Congo-Angola margins (Equatorial West African margin, 3150-4800 m) in which we examined the metazoan meiofauna at a variety of habitats along the Congo Channel system and in the associated cold seep. We investigate the structure, density, vertical distribution patterns in the sediment and biomass of meiofaunal communities in the Gulf of Guinea and how they are controlled by hydrologic and biogeochemical processes. The meiofaunal communities in the Gulf of Guinea were shaped by heterogeneous conditions on the margin, and reflect the multiple-scale spatial variability that corresponds with different identified habitats. The two control sites, located at >100 km away from the canyon, were inhabited by very dense and the most diverse meiobenthic communities. Similar meiobenthic communities inhabited the transition zone between the canyon and the cold seep. Sites located along the Congo Channel were obviously affected by the local high-velocity bottom currents and unstable sedimentary conditions in this active submarine system. Extremely low meiobenthic densities and very low proportions in the most surficial sediment layers provided evidence for recently highly disturbed sediments at these sites. The ROV Victor 6000 provided images of the cold seep, showing a patchy distribution of several types of patchy distributed megafaunal communities dominated by three key symbiotic taxa (Mytilidae, Vesicomyidae and Siboglinidae). These cold seep sediments were colonised by a unique meiobenthic community, characterised by a high small-scale (m's) patchiness, low species richness and the prominent dominance of two large-sized nematode species: Sabatieria mortenseni, which is a cosmopolitan nematode known from littoral habitats, and an undescribed Desmodora species. The high individual body weight of Sabatieria mortenseni and its dominance at the cold seep site resulted in a significantly higher nematode biomass at the seep compared to the surrounding sites. In addition, the vertical A c c e p t e d m a n u s c r i p t 2 nematode profiles, with maximum proportions in subsurficial layers, points to a chemosynthesis-based meiobenthic community in this cold seep, in contrast to the phytodetritus-based communities at the control sites and at the transition zone.

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Section: Introductionmentioning

confidence: 99%

Deep-sea habitat heterogeneity influence on meiofaunal communities in the Gulf of Guinea

Gaever

Galéron

Sibuet

et al. 2009

Deep Sea Research Part II: Topical Studies in Oceanography

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“…Hovland & Judd (1988) found that North Sea pockmarks had spectacularly higher biological productivity compared with surrounding sediment. The large complex 'Regab' pockmark on the Gabon continental margin seems to contain a novel and important fauna, chemosynthetic communities and carbonate concretions (Ondréas et al 2005, Gay et al 2006, Olu-Le Roy et al 2007). Three pockmarks farther north of the 'Regab' pockmark also seem to contain a high abundance of typical seep fauna (vesicomyid clams and vestimentiferan worms) (Sahling et al 2008).…”

Section: Introductionmentioning

confidence: 99%

Benthic ecology of pockmarks in the Inner Oslofjord, Norway

Webb¹,

Barnes²,

Gray³

2009

Mar. Ecol. Prog. Ser.

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Despite pockmarks being one of the most widespread small-scale topographic features of the seabed, almost nothing is known of their influence on fauna. Here we present the first ecological study of fjordic pockmarks, focusing on the contrast between macrofauna inside and outside of these craters. We report an analysis of macrofauna from 27 pockmarks of the Inner Oslofjord, Norway. Five replicate grab samples were collected from each of 3 pockmarks at 3 sites plus a further control (non-pockmark) sample at each site. A single grab sample was collected from an additional 3 pockmarks at 6 sites and 6 control locations. We compared macrofaunal assemblages inside and outside of pockmarks and found important but subtle differences to those on non-pockmarked substrata. The fauna of pockmarks were typical of a disturbed fjord environment with a dominance of small opportunistic taxa, such as pioneer bivalves and polychaetes. The position of sites in the fjord gradient drove the most obvious faunal differences, but contrary to expectations, we could find no influence of pockmarks on the composition of the fauna at any taxonomic level. This makes them very unusual amongst marine topographic features, which usually have considerable influence on the nature of benthic communities. However, we found that pockmarks do significantly alter the abundances of key species and, as such, we suggest that the presence of pockmarks in the Oslofjord has a considerable cumulative influence on densities and populations of benthic organisms. KEY WORDS: Benthic community · Pockmarks · Oslofjord · Environmental factorsResale or republication not permitted without written consent of the publisher

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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%

“…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%

“…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%

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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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ROV study of a giant pockmark on the Gabon continental margin

Cited by 106 publications

References 46 publications

Deep-sea habitat heterogeneity influence on meiofaunal communities in the Gulf of Guinea

Deep-sea habitat heterogeneity influence on meiofaunal communities in the Gulf of Guinea

Benthic ecology of pockmarks in the Inner Oslofjord, Norway

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

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