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Daniel Praeg, Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS) (Italy)
Jean Mascle, Géosciences Azur (France)
Riccardo Geletti, Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS) (Italy)
Vikram Unnithan, Jacobs University Bremen (Germany)
Nigel Wardell, Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS) (Italy)
François Harmegnies, IFREMER Centre de Brest (France)
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We present evidence for the presence of gas hydrates on the central Nile fan, based on the first identification of a BSR in the Mediterranean Sea. This discovery forms part of ongoing investigations at OGS in Trieste into the potential occurrence of gas hydrates throughout the Mediterranean Sea, using theoretical modeling of hydrate stability to target observations of previously acquired seismic reflection data. Data available from the Nile deep-sea fan include multichannel seismic profiles acquired by OGS in 1973 (using explosive sources and a 2.4 km long streamer), reprocessed for this study; profiles acquired by Geosciences Azur from 1998-2002 (using airgun sources and streamers 0.3 to 4 km long); and measurements obtained using a geothermal corer during the recent MEDECO2 campaign of the Pourquoi pas?. The BSR is observed on the central Nile fan, as a reflection of inverse polarity that 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. The BSR is consistent with free gas at the base of a hydrate occurrence zone up to 250 m thick, comparable to the thickness beneath seabed of the modeled methane hydrate stability zone (MHSZ). The upper limit of the theoretical MHSZ extends upslope beyond the BSR, to an upper limit at depths of c. 1200 m.
The Nile fan is a slope with a known history of mass failures, and contains numerous seabed cold seeps that are venting gases in part derived from underlying hydrocarbon reservoirs. Modeling of hydrate stability for conditions representative of the last glacial stage in the Mediterranean Sea, when sea levels were lower and bottom waters cooler (by up to 4°C), shows that the MHSZ may have been up to 50% thicker and its upper limit up to 300 m shallower. The glacial to interglacial transition thus corresponded to a reduction in hydrate stability, with implications for slope stability along the upper limit of the MHSZ as it migrated downslope (between water depths of c. 900-1200 m), as well as for the activity of the many cold seeps that lie within the gas hydrate system on the central Nile fan. The recurrent release of free gas from hydrates in response to glacial-interglacial changes in bottom water temperatures in the Mediterranean Sea may have found expression in the sedimentary records both of slope failure and of cold seep functioning in the eastern Mediterranean Sea.
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