HR: 1340h
AN: V13B-1347    [Abstracts]
TI: Seismic Evidence of a Gas Hydrate System on the Nile Deep-Sea Fan
AU: * Praeg, D
EM: dpraeg@ogs.trieste.it
AF: Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Borgo Grotta Gigante 42/c, Trieste, 34010, Italy
AU: Mascle, J
EM: mascle@geoazur.obs-vlfr.fr
AF: Géosciences Azur, Observatoire Océanologique, La Darse, B.P. 48, Villefranche-sur- Mer, 06235, France
AU: Geletti, R
EM: rgeletti@ogs.trieste.it
AF: Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Borgo Grotta Gigante 42/c, Trieste, 34010, Italy
AU: Unnithan, V
EM: v.unnithan@jacobs-university.de
AF: Jacobs University Bremen, P.O. Box 750 561, Bremen, DEU 28725,
AU: Wardell, N
EM: nwardell@ogs.trieste.it
AF: Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Borgo Grotta Gigante 42/c, Trieste, 34010, Italy
AB: The Nile Fan is a depocentre on a passive margin that hosts an active hydrocarbon province and a wide range of seabed cold seeps. A bottom simulating reflection (BSR) is identified, for the first time in the Mediterranean Sea, based on re-examination of two sets of academic multichannel seismic data: profiles acquired by OGS in 1973 (using explosive sources and a 2.4 km streamer), reprocessed for this study; and profiles acquired by Géosciences Azur from 1998-2002 (using airgun sources and 0.3 to 4 km streamers). On the central Nile Fan, a reflection of inverse polarity can be traced over a depth range of 2000-2500 m, deepening from c. 220-330 ms below seabed and in places cross-cutting (discontinuous) stratal reflections. Comparison with the modeled stability zone for methane hydrate in seawater (MHSZ) indicates the BSR to be consistent with free gas at the base of a hydrate occurrence zone up to 250 m thick. The MHSZ extends upslope beyond the observed BSR, to a limit at c. 1200 m depth. Modeling of hydrate stability for glacial-stage conditions in the Mediterranean Sea, when bottom waters were much cooler (at least - 4°C), shows that the MHSZ would have been at least 50% thicker and its upper limit at least 300 m shallower. Deglaciation thus corresponded to a dramatic reduction in hydrate stability, with consequences for slope stability along the upper limit of the HSZ as it migrated downslope (across water depths of c. 900-1200 m). The inferred deglacial release of gas and water also has implications for the functioning of cold seeps, which occur across the depth range of the BSR on the central Nile Fan and are linked to underlying hydrocarbon reservoirs. The episodic formation and dissociation of hydrates in response to large glacial-interglacial changes in bottom water temperatures may represent an important driving mechanism for shallow fluid flow, finding expression in the sedimentary records both of slope failure and of cold seep systems on the Nile Fan and elsewhere in the eastern Mediterranean Sea.
DE: 3004 Gas and hydrate systems
DE: 3025 Marine seismics (0935, 7294)
DE: 3045 Seafloor morphology, geology, and geophysics
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting