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

Abstract

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 230 Th/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 not, vert, ~ 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.

Geological fluid flow in sedimentary basins

“…1). This observation is similar to what was already described for more conventional pockmark structures on the Nile margin (Bayon et al, 2008;Dupré et al, 2010;Migeon et al, 2014;Praeg et al, 2014) but it is the first time that such a specific seabed planform morphology associated with such a fluidcirculation activity is reported.…”

High-resolution mapping of large gas emitting mud volcanoes on the Egyptian continental margin (Nile Deep Sea Fan) by AUV surveys

“…The Nile Deep Sea Fan hosts numerous active fluid escape structures (Bayon et al 2009;Huguen et al 2009;Loncke et al 2004;Mascle et al 2006) including several large gas emitting mud volcanoes (Dupré et al 2007). These latter are characterized by emissions of thermogenic methane and higher hydrocarbons (de Lange et al 2006;Mastalerz et al 2007).…”

Using sediment geochemistry to infer temporal variation of methane flux at a cold seep in the South China Sea

“…Modern seep activity can be detected through seabed observations, pore water analysis, methane anomaly detection, and many other methods (Borowski et al, 1996(Borowski et al, , 1999Borowski, 2004;Newman et al, 2008;Bayon et al, 2009bBayon et al, , 2011Mazumdar et al, 2012aMazumdar et al, , 2014Brothers et al, 2013Brothers et al, , 2014Lemaitre et al, 2014;Skarke et al, 2014), but temporal variations in methane flux in the past are not well constrained because it is difficult to select the appropriate indicators in quantifying and age determining the occurrence of seep activity. Consequently, little is known about the impact of methane seepage on the surrounding sediment environment in seep settings.…”