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Abstracts of oral presentations 49 2 and as widespread, disseminated chunks in Unit 3. The presence of hydrate structure I was deduced from the LMWHC composition and confirmed by X-ray diffraction techniques. Comparative calculations including all pressurized cores were performed to assign nominal Unit-specific concentrations of dissolved methane. The results confirm that highest nominal methane concentrations occur in Unit 3 deposits. Respective in situ concentrations of dissolved methane in Unit 3 deposits ranged between about 770 and 2,400 mM. To the best of our knowledge, this is the first high-resolution investigation of in situ amounts of gas and gas hydrates in a high-intensity seepage area. The results obtained allow for an estimation of the total amount of gas hydrates and methane-bound carbon in shallow deposits of the entire Batumi seep area. References Klaucke I., Sahling H., Weinrebe W., Blinova V., Bürk D., Lursmanashvili N., Bohrmann G., (2006) Acoustic investigation of cold seeps offshore Georgia, eastern Black Sea. Marine Geology, 231, 51-67. Tishchenko P., Hensen C., Wallmann K., Wong C.S. (2005) Calculation of the stability and solubility of methane hydrate in seawater. Chemical Geology, 219, 37-52. Direct petrographic evidence of the past occurrence of gas hydrates in Oligo-Miocene sediments of the Tertiary Piedmont Basin (NW Italy). C. Petrea 1 , L. Martire 1 , M. Natalicchio 1 , P. F. Dela Pierre 1 , S. Cavagna 1 , P. Clari 1 1 Dipartimento di Scienze della Terra, Torino University, Italy The Tertiary Piedmont Basin (TPB) and in particular the Monferrato region are among the first areas in the world where CH4-derived authigenic carbonates have been described. Methane-derived rocks occur at different stratigraphic levels in a succession consisting mainly of clastic sediments and spanning in age the Oligocene and Miocene. They include both classical chemoherms, characterized by richness of chemosymbiotic mollusc remains, and other products of methane oxidation, devoid of seep fossils, that are interpreted to result from carbonate precipitation in the pores of buried sediments at some metres or tens of metres below the sea floor. All the investigated carbonates share very depleted δ 13 C values (down to -52 ‰ PDB) and some samples also show positive δ 18 O values (up to + 8 ‰ PDB). Such positive O values suggest that dissociation of clathrates could be the source of methane-rich fluid flow to the surface as observed on many present day continental margins. A detailed study of the TPB CH4-derived rocks, conducted with a multidisciplinary approach (petrography, cathodoluminescence, UV epifluorescence, SEM-EDS, isotope geochemistry, fluid inclusion analysis, Raman microspectrometry) allowed to identify unusual diagenetic features that point to the possible past occurrence of gas hydrates. Different kinds of features may be distinguished in several samples coming from different localities: (1) antigravitational structures: cm-sized cavities are filled with concentrically arranged laminated microcrystalline carbonates separated or even floating in limpid calcite spar. Microcrystalline carbonates mainly consist of dolomite and display spherulitic and peloidal fabric referable to microbial activity. (2) irregular distribution of sparry and fibrous calcite cements within cm-sized cavities corresponding to syneresis cracks or gas conduits. Cathodoluminescence evidences that cements do not make isopachous rims but pinch out on cavity walls giving rise to very asymmetric authigenic fillings. (3) complex, irregular, crusts of mm thickness occurring at fissure walls: they consist of very thin stromatolitic-like microcrystalline microbial laminae with a very large open framework now centripetally filled with dolomite spar. (4) geopetal fills of secondary pores: sharp-edged mm-sized cavities within muddy sand dykes are geopetally filled with mud and authigenic pyrite on the bottom and by sparry dolomite. The genesis of the first three puzzling fabrics can be explained with the former presence of a solid compound prone to a progressive volume reduction that gave rise to increasingly larger new open spaces. Gas hydrates, that may form at the sea floor or within buried sediments and short after dissociate depending on P and T changes, are the best candidates for explaining these features. The new open spaces could occur either at the boundary between gas hydrate and cavity walls or within the gas hydrate mass itself and were filled with authigenic carbonates whose precipitation was induced by microbial oxidation of methane. Geopetal structures (4), instead, were generated by dissociation of small clasts of gas hydrates: sediment particles contained within the gas hydrate clast fell on the bottom of the resultant pore that was plugged by dolomite cement concomitantly with pyrite authigenesis. The described features may be considered a new type of diagenetic structures, that can be defined “melt and seal”, and provide a direct documentation of the presence and dissociation of gas hydrates in ancient sedimentary successions. Given the small size of these structures, their recognition requires a detailed petrographic analyses but the effort is rewarding because it greatly helps in reconstructing hydrocarbon and fluid migration in fossil systems.
50 Abstracts of oral presentations Authigenic carbonate crusts from active cold seep sites in the Eastern Mediterranean: New results from MEDECO cruise (HERMES Project) C. Pierre 1 , G. Bayon 2 , M.-M. Blanc-Valleron 3 , J.-M. Rouchy 3 , J. Mascle 4 , S. Dupré 1 , I. Bouloubassi 1 , J. Sarrazin 2 , J.-P. Foucher 2 and the Medeco scientific team 1 UPMC, LOCEAN, Paris, France, 2 Ifremer, Brest, France, 3 Muséum National d’Histoire Naturelle, Paris, France, 4 Géosciences Azur, Villefranche sur Mer, France. Active cold seeps associated to mud volcanoes and pockmarks are widely distributed in the Eastern Mediterranean.Sea. In situ studies on these geological structures initiated ten years ago in the frame of the french-dutch MEDINAUT survey and of the ESF-MEDIFLUX program during which two oceanographic cruises (NAUTINIL and BIONIL) were realized. These operations have demonstrated the importance to use a submersible to obtain in situ observations, sampling and measurements at precise locations of fluid seepage. During fall 2007, MEDECO cruise, onboard the « R.V. Pourquoi Pas ? », has allowed to investigate, during two months, deep Mediterranean ecosystems using the ROV-Victor. A large part of the time was devoted to study several active cold seeps already known on the Mediterranean Ridge, Anaximander Mountains and the Nile deep sea fan. At cold seep sites, special attention was given to sample authigenic carbonate crusts covering the sea floor either as massive and indurated large pavements, or as thin and highly porous laminated crusts. These carbonate crusts represent hard substrates where abundant fauna are living either fixed or sheltered within cavities. The carbonate mineralogy indicates low-Mg calcite that originates from the pelagic sediment, whereas the authigenic carbonate cement is composed of Mg-calcite, aragonite and dolomite. No obvious difference was observed in carbonate composition of the crusts from the different sites, with the exception of the absence of dolomite in the crusts from Cheops mud volcano located in the western Nile deep sea fan. The oxygen and carbon isotopic compositions of the carbonate crusts vary within wide ranges : 2.47< δ 18 O ‰V- PDB
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