Multi-disciplinary investigation of fluid seepage on an unstable margin: The case of the Central Nile deep-sea fan.

Description

We report on a multidisciplinary study of cold seeps explored in the Central Nile deep-sea fan of the Egyptian margin. Our approach combines in situ seafloor observation, geophysics, sedimentological data, measurement of bottom-water methane anomalies, pore-water and sediment geochemistry, and 230Th/U dating of authigenic carbonates. Two areas were investigated, which correspond to different sedimentary provinces. The lower slope, at ∼ 2100 m water depth, indicates deformation of sediments by gravitational processes, exhibiting slope-parallel elongated ridges and seafloor depressions. In contrast, the middle slope, at ∼ 1650 m water depth, exhibits a series of debris-flow deposits not remobilized by post-depositional gravity processes. Significant differences exist between fluid-escape structures from the two studied areas. At the lower slope, methane anomalies were detected in bottom-waters above the depressions, whereas the adjacent ridges show a frequent coverage of fractured carbonate pavements associated with chemosynthetic vent communities. Carbonate U/Th age dates (∼ 8 kyr BP), pore-water sulphate and solid phase sediment data suggest that seepage activity at those carbonate ridges has decreased over the recent past. In contrast, large (∼ 1 km²) carbonate-paved areas were discovered in the middle slope, with U/Th isotope evidence for ongoing carbonate precipitation during the Late Holocene (since ∼ 5 kyr BP at least). Our results suggest that fluid venting is closely related to sediment deformation in the Central Nile margin. It is proposed that slope instability leads to focused fluid flow in the lower slope and exposure of 'fossil' carbonate ridges, whereas pervasive diffuse flow prevails at the unfailed middle slope

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