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

Type Article
Date 2006-02
Language English
Author(s) Gay Aurelien1, Lopez M2, Ondreas Helene3, Charlou Jean-Luc3, Sermondadaz G4, Cochonat Pierre3
Affiliation(s) 1 : NOCS, Southampton SO14 3ZH, Hants, England.
2 : Univ Montpellier 2, Lab Dynam Lithosphere, F-34095 Montpellier 05, France.
3 : IFREMER, Dept Geosci Marines, F-29280 Plouzane, France.
4 : TOTAL, F-64018 Pau, France.
Source Marine Geology (0025-3227) (Elsevier), 2006-02 , Vol. 226 , N. 1-2 , P. 81-95
DOI 10.1016/j.margeo.2005.09.011
WOS© Times Cited 82
Keyword(s) BIOZAIRE, ZAIANGO, Palaeochannel, Hydrates, Chemosynthesis, Seafloor facies, Fluid migration, Pockmark
Abstract 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 in wide Giant Pockmark is a composite feature due to the coalescence of multiple 100 in 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 mu mol/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.
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