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

Gay, A.; Lopez, M.; Ondreas, H.; Charlou, J-L.; Sermondadaz, G.; Cochonat, P.. 2006 Seafloor facies related to upward methane flux within a Giant Pockmark of the Lower Congo Basin. Marine Geology, 226 (1-2). 81-95.

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The origin of the cold fluid venting from a Giant Pockmark within the Lower Congo Basin has been elucidated based upon results of precise mapping, submersible dives, gravity coring and isotopic analyses realized under a TOTAL-IFREMER cooperative project (ZAIANGO and BIOZAIRE projects). During four dives of the IFREMER ROV-Victor 6000, the bottom was filmed, hard and soft samples were lifted from the sediment, and water samples were collected with a CTD-rosette system. The detailed dip map shows that the 800 m wide Giant Pockmark is a composite feature due to the coalescence of multiple 100 m wide depressions that displays a broad range of biological, mineralogical and chemical features on the seafloor, leading to a seafloor anomaly recorded on the multibeam imagery. Methane-rich fluids migrating through the sedimentary column from a buried palaeochannel clearly react with the sulphate-rich circulating sea water to produce hydrogen sulphide and bicarbonate ions. This situation leads to a concentrical arrangement of the sedimentary facies, with methane-related features in the centre and sulphide-related features at the periphery. This organization is correlated with high levels of methane (up to 20 μmol/l) measured in the centre of the Giant Pockmark, responsible for the crystallization of gas hydrates at the bottom. In this model, the concentrical organization of mineralogical and biological features reflects a geochemical partitioning related to the peripheral progressive mixing of the methane flux.

Item Type: Publication - Article
Digital Object Identifier (DOI):
ISSN: 0025-3227
Additional Keywords: pockmark; fluid migration; seafloor facies; chemosynthesis; hydrates; palaeochannel; ZAIANGO; BIOZAIRE
Date made live: 04 Jan 2007 +0 (UTC)

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