HR: 0800h
AN: OS21A-1199    [Abstracts]
TI: Origin of the diagenetic carbonate crusts and concretions from the mud volcanoes of the Nile deep-sea fan
AU: Gontharet, S
EM: swanne.gontharet@lodyc.jussieu.fr
AF: LODYC-UPMC, 4 Place Jussieu, Paris, 75252 France
AU: * pierre, c
EM: cat@lodyc.jussieu.fr
AF: LODYC-UPMC, 4 Place Jussieu, Paris, 75252 France
AU: Blanc Valleron, M
EM: valleron@mnhn.fr
AF: MNHN, 43 rue Buffon, Paris, 75005 France
AU: Rouchy, J
EM: rouchy@mnhn.fr
AF: MNHN, 43 rue Buffon, Paris, 75005 France
AU: Fouquet, Y
EM: yves.fouquet@ifremer.fr
AF: IFREMER-G‚osciences Marines, BP 70, Plouzan‚, 29280 France
AU: Bayon, G
EM: germain.bayon@ifremer.fr
AF: IFREMER-G‚osciences Marines, BP 70, Plouzan‚, 29280 France
AB: During the NAUTINIL cruise (September -October 2003), 22 submersible dives have been realized in the Nile deep-sea fan area to investigate by a multidisciplinary approach, selected mud volcanoes which are very abundant and of various morphologies in the whole area (Loncke et al., 2004). The deepest site (3019 m) located in the western part of the deep-sea fan, corresponds to a large caldera (about 8 km of diameter) where brines are seeping along the flanks of the structure and are sometimes collected in pools and lakes. The other sites in the central and eastern parts of the Nile deep-sea fan correspond respectively to pock-marks located at 2120 m and to a mud volcano located at 1130 m where active fluid ventings were identified by the presence of living benthic organisms (mainly vestimentiferan worms; rarely bivalves). At these three sites, hard carbonate crusts cover irregularly the sea floor and are sometimes present as dispersed fragments within the topmost sediments. The sediments from the venting areas are organic-rich and have a strong H$_{2}$S smell which is indicative of active sulfate reduction. Petrographic observations and XRD analyses of the carbonate crusts indicate that aragonite, calcite, Mg-calcite are the dominant authigenic carbonate phases with a minor contribution of dolomite ; small concretions of ankerite occur occasionally in the sediments of the eastern delta. Millimeter sized barite concretions have also been discovered in the pock-marks sediments. The oxygen and carbon isotopic compositions of the bulk carbonate from crusts and concretions exibit large variations : -0.67 $<$ \delta$^{18}$O\permil PDB $<$ 4.66 -44.17$<$ \delta$^{13}$C \permil PDB $<$ 3.10 The distribution of the isotopic values is explained by the mixing of the authigenic carbonates with the sedimentary matrix which corresponds itself to a mixture of pelagic sediments and mud breccia issued from the mud volcano activity. The rather large range of \delta$^{18}$O values might reflect variable sources of diagenetic fluids. Typically, the very low \delta$^{13}$C values of the authigenic carbonates indicate that CH$_4$ was the major source of carbon which was oxidized as CO$_{2}$, either through bacterial sulfate reduction within the sediment, or via bacterial aerobic oxidation at the sea floor. Similar isotopic values were previously measured in the diagenetic carbonate crusts from the mud volcanoes of the Mediterranean Ridge area (Aloisi et al., 2000) as well as in other areas of cold seeps outside the Mediterranean sea (for instance Gulf of Mexico, Cascadia margin, Barbados prism). References: Aloisi G., Pierre C., Rouchy J.M., Foucher J.P., Woodside J. and the Medinaut Scientific Party, 2000. E.P.S.L., 184, 321-338. Loncke L., Gaullier V., Bellaiche G., and Mascle J., 2004. A.A.P.G. Bull
DE: 3675 Sedimentary petrology
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
DE: 1045 Low-temperature geochemistry
DE: 1050 Marine geochemistry (4835, 4850)
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting