In: GEOLOGICAL PROCESSES ON EUROPEAN CONTINENTAL MARGIN (TTR-10) Post-Cruise Conference. Jan 28-Feb 4, 2001. Moscow Russia.

Carbonate chimneys in the Gulf of Cadiz: Initial report of their petrography and geochemistry

Diaz-del-Rio V.(1), Somoza L.(2), Martinez-Frias J.(3), Hernandez-Molina F.J.(4), Lunar, R.(5) , Fernandez-Puga M.C.(2) , Maestro A.(2) , Terrinha P.(6), Llave E.(2), Garcia A.(7), Garcia A. C. (8) and J.T. Vazquez (4).

(1) Instituto Español de Oceanografia IEO, Malaga, Spain. Email:
(2) Marine Geology Dv., Geological Survey of Spain IGME, Madrid, Spain.
(3) Facultad de Ciencias del Mar UCA, Cadiz, Spain.
(4) Centro de Astrobiología CSIC-INTA, Madrid, Spain.
(5) Dpt. Mineralogy, Universidad  Complutense de Madrid UCM, Madrid, Spain.
(6) Instituto Geologico e Mineiro de Portugal IGM, Lisbon, Portugal.
(7) Facultad de Ciencias del Mar UVI, Vigo, Spain.
(8) Universidade do Algarve UALG, Faro, Portugal.

Dolomite chimneys associated with hydrocarbon-rich fluid venting was discovered September 2000, as part of the TASYO project (Marine and Science Technology Spanish Programme) in the Gulf of Cadiz. The unexpected discovery occurred during the cruise Anastasya/2000 aboard of research vessel Cornide de Saavedra dredging a 870m-deep and 120m-tall carbonate mound called as the "Iberico". A suite of more than 60 individual structures of chimneys were collected, which displays distinct pipe-like morphologies that varies from 1 to 0.40 m long. Fragments of chimneys from the Hesperides mud volcanoe within the Tasyo field and along the Morrocan margin were also sampled. At same time, in all of these sites, strongly sulfide mud breccia and dolomite slabs have also been collected.

Targets were previously detected in May 2000 by carrying out a detailed mapping and ultra high resolution seismic, with an extent new data of more than 1200 km obtained during the cruise TASYO/2000 aboard of research vessel Hesperides. The  Simrad EM12S-120 system, a multibeam echo sounder system, was used for mapping sea-floor. The swath mapping provided bathymetric map contoured at an interval of 1 m also a subsidiary map showing the strength of the sea-floor backscattering of the sonar signal. A Parasound echosounder TOPAS (Topographic Parametric Sound) , which is a sub-bottom profiles was also used.

The chimneys are dominated by Fe-riched dolomite (ankerite) forming aggregates with minor amounts of pyrite, iron oxide, Ta-enriched rutile, zircon and quartz. Abundant, well preserved remains of foraminifera (globigerinoids and milioids) composed of Mg-calcite are present within the matrix. Dolomite aggregrates are remarkably depleted in 13C (-35 to -56 PDB)  and are therefore interpreted being the result of methane oxidation by sulphate-reducing bacteria. This microbial activity is shown by abundance spheroids to euhedral pentagonal monocryst composed by up to 60 cells of sulphate-reducing bacteria. Aggregates of single bacteria  of about 1?m diameter produces rounded to -pentagonal shaped framboids up to 60 ?m characteristics of pyrite. Presently, mostly these bacterial-origin framboids are replaced by haematite. At same time, some of framboids have been found in the interior of the foraminifer chambers, which could suggest some symbiotic association between foraminifer and chemosyntectic bacteria.

Dolomite chimneys are interpreted as cemented conduits formed as result of methane-enriched  fluid expulsion through a submarine mound, probably formed as a mud volcanoe. The abundant pseudo-pyrite framboids are related with the zone of shallow microbial sulphate reduction, a process fundamental to the nourishment of the chemosynthetic cold seep communities. In the last years, it has been considered the importance of carbonate cementation related with methane fluxes, both on submarine modern environments and on rocks of the fossil record. This carbonate cementation, in forms of chimneys, slabs, and crusts has been reported in several tectonic settings such as the Gulf of Mexico, Oregon margin, Otago slope, Monterey basin and Kattegat (e.g. Jorgensen,1992; Orpin 1997; Stakes et al. 1999).  Large fluxes of methane seem responsible for dolomite cementation, instead high-Mg calcite and aragonite when fluxes are lower. Recently, its has been hypothesis that extensive anerobial microbial communities exist in sedimentary layers below high-temperatures vent fields. Chemosyntetic bacteria are the primary producer of  hydrothermal producers that are fuelled by geothermal energy. The sum of the syntrophy cooperation between methane-oxidisers (archaeobacterias domain) and sulphate-reducers micro-organism produces carbonates and sulphides at the sulphate-methane interface, which depth below sea floor is dependent on the methane flux rate (DeLong 2000)

This research has been supported by the "TASYO" project (CYTMAR 98-0209) in the frame of the Spanish-Portuguese agreement for scientific cooperation.


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