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Diagenetic Carbonates from Seafloor Crusts and Subseafloor Concretions from the Nile Deep Sea Fan (East Mediterranean) as Indicators of Hydrocarbon-rich Fluid Seepage

Catherine Pierre, Swanne Gontharet, Marie-Madeleine Blanc-Valleron, Germain Bayon, and Jean Mascle

During the NAUTINIL (September-October 2003), MIMES (July 2004), BIONIL (October 2006) and MEDECO (November 2007) cruises, coring and submersible dives have been realized in the Nile deep sea fan (NDSF) area where active fluid venting sites were identified by the presence of living benthic organisms and by methane plumes in the bottom waters above the seeping structures. At all sites, hard carbonate crusts cover irregularly the sea floor. The sediments from the venting areas are organic-rich, contain sometimes small concretions and have a strong H2S smell indicative of active sulfate reduction. The mineralogy of carbonate crusts is dominated by aragonite and Mg-calcite; the mineralogy of concretions is more complex, with mixtures of Mg-calcite, dolomite and ankerite. The oxygen and carbon isotopic compositions of the carbonate from crusts and concretions exhibit large variations (-2.1< δ18O ‰ PDB <+9.5; -42.6< δ¹³C ‰ PDB <+22.4). The wide range of δ18O values reflects variable sources of fluids. Most of the authigenic carbonates from the Nile deep sea Fan were precipitated in isotopic equilibrium with the Mediterranean bottom water. The carbonate crusts and concretions from the brine seeps of the North western NDSF are enriched in 18O indicating that a source of 18O-rich fluids originated from depth. A few crusts and concretions from the eastern NDSF exhibit relatively low δ18O values, which are due to precipitation at warm temperatures. The very low δ¹³C values of the diagenetic carbonates indicate that methane and possibly other heavier hydrocarbons was the major source of carbon that was oxidized as bicarbonate mostly through bacterial sulfate reduction-anaerobic methane oxidation within the anoxic sediment. The very positive δ¹³C values of the diagenetic carbonates from carbonate concretions are related to the production of ¹³C-rich CO2 during methanogenesis within the subseafloor sediments.

AAPG Search and Discovery Article #90161©2013 AAPG European Regional Conference, Barcelona, Spain, 8-10 April 2013