Fluid flow regimes and growth of a giant pockmark

Type Article
Date 2014-01
Language English
Author(s) Marcon Yann1, Ondreas Helene2, Sahling Heiko1, Bohrmann Gerhard1, Olu KarineORCID3
Affiliation(s) 1 : Univ Bremen, MARUM, D-28359 Bremen, Germany.
2 : IFREMER, Ctr Bretagne, REM GM, F-29280 Plouzane, France.
3 : IFREMER, Ctr Bretagne, REM EEP, F-29280 Plouzane, France.
Source Geology (0091-7613) (Geological Soc Amer, Inc), 2014-01 , Vol. 42 , N. 1 , P. 63-66
DOI 10.1130/G34801.1
WOS© Times Cited 38
Note This work was supported by IFREMER and by SENSEnet, a Marie Curie Initial Training Network (ITN) funded by the European Commission 7th Framework Programme, contract no. PITN-GA-2009-237868
Abstract Pockmarks are seafloor depressions commonly associated with fluid escape from the seabed and are believed to contribute noticeably to the transfer of methane into the ocean and ultimately into the atmosphere. They occur in many different areas and geological contexts, and vary greatly in size and shape. Nevertheless, the mechanisms of pockmark growth are still largely unclear. Still, seabed methane emissions contribute to the global carbon budget, and understanding such processes is critical to constrain future quantifications of seabed methane release at local and global scales. The giant Regab pockmark (9 degrees 42.6' E, 5 degrees 47.8' S), located at 3160 m water depth near the Congo deep-sea channel (offshore southwestern Africa), was investigated with state-of-the-art mapping devices mounted on IFREMER's (French Research Institute for Exploitation of the Sea) remotely operated vehicle (ROV) Victor 6000. ROV-borne micro-bathymetry and backscatter data of the entire structure, a high-resolution photo-mosaic covering 105,000 m(2) of the most active area, sidescan mapping of gas emissions, and maps of faunal distribution as well as of carbonate crust occurrence are combined to provide an unprecedented detailed view of a giant pockmark. All data sets suggest that the pockmark is composed of two very distinctive zones in terms of seepage intensity. We postulate that these zones are the surface expression of two fluid flow regimes in the subsurface: focused flow through a fractured medium and diffuse flow through a porous medium. We conclude that the growth of giant pockmarks is controlled by self-sealing processes and lateral spreading of rising fluids. In particular, partial redirection of fluids through fractures in the sediments can drive the pockmark growth in preferential directions.
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