Seafloor geological studies above active gas chimneys off Egypt (Central Nile Deep Sea Fan)

Abstract

Four mud volcanoes of several kilometres diameter named Amon, Osiris, Isis, and North Alex and located above gas chimneys on the Central Nile Deep Sea Fan, were investigated for the first time with the submersible Nautile. One of the objectives was to characterize the seafloor morphology and the seepage activity across the mud volcanoes. The seepage activity was dominated by emissions of methane and heavier hydrocarbons associated with a major thermal contribution. The most active parts of the mud volcanoes were highly gas-saturated (methane concentrations in the water and in the sediments, respectively, of several hundreds of nmol/L and several mmol/L of wet sediment) and associated with significantly high thermal gradients (at 10 m below the seafloor, the recorded temperatures reached more than 40 °C). Patches of highly reduced blackish sediments, mats of sulphide-oxidizing bacteria, and precipitates of authigenic carbonate were detected, indicative of anaerobic methane consumption. The chemosynthetic fauna was, however, not very abundant, inhibited most likely by the high and vigorous fluxes, and was associated mainly with carbonate-crustcovered seafloor encountered on the southwestern flank of Amon. Mud expulsions are not very common at present and were found limited to the most active emission centres of two mud volcanoes, where slow extrusion of mud occurs. Each of the mud volcanoes is fed principally by a main narrow channel located below the most elevated areas, most commonly in the centres of the structures. The distribution, shape, and seafloor morphology of the mud volcanoes and associated seeps over the Central Nile Deep Sea Fan are clearly tectonically controlled.

Seep communities from two mud volcanoes in the deep eastern Mediterranean Sea: faunal composition, spatial patterns and environmental control

“…However, the first evidence of benthic communities based on chemosynthetic processes was found on the Napoli MV (Corselli & Basso 1996). It is now known that several of the MR sites (Salas & Woodside 2002, Olu-Le Roy et al 2004, Werne et al 2004) as well as those in the NDSF are colonised by various chemosynthesis-based faunal assemblages , Dupré et al 2007, Ritt et al 2011.…”

Sedimentary sources of the mud-breccia and mud volcanic activity in the Western Alboran Basin

“…Mud volcanoes active over the last 3 Ma were recently discovered along the off-shore Calabrian Arc, at the tip of the Apenninic-Maghrebide accretionary system (Praeg et al, 2009). Occurrence of mud volcanism and diapirism in the Eastern Mediterranean is also recorded in an extensional setting: the Nile Deep Sea Fan (e.g., Loncke et al, 2004;Dupré et al, 2007) Mud volcanoes are well known west of the Gibraltar Strait, in the Gulf of Cadiz, along the Atlantic counterpart of the Alboran Basin in the Gibraltar Arc System (GAS; e.g., Pinheiro et al, 2003;Fernández-Puga et al, 2007) where the related phenomena such as cold seepage, pockmarks, hydrocarbon venting, methanederived authigenic carbonates and gas hydrates have been investigated since 1996 (Baraza and Ercilla, 1996;Baraza et al, 1999;Ivanov et al, , 2001Kenyon et al, 2000;Somoza et al, 2000Somoza et al, , 2003Gardner, 2001;Mazurenko et al, 2002;Pinheiro et al, 2003Pinheiro et al, , 2006Magalhães et al, 2012). Their origin is associated with recent compressive tectonics on shale and salt deposits of the olistostrome/accretionary complex units, emplaced as an allochthonous unit in the Late Miocene as a result of Africa-Eurasia convergence (Maldonado et al, 1999;Somoza et al, 2003).…”

Biogeochemistry and Community Composition of Iron- and Sulfur-Precipitating Microbial Mats at the Chefren Mud Volcano (Nile Deep Sea Fan, Eastern Mediterranean)

“…Methane oxidation rates were measured using 14 CH 4 gas, based on previously described methods (24,81). Subcores were injected with 10 l of 14 CH 4 (2.5 kBq [total radioactivity] dissolved in double-distilled H 2 O) and incubated for 24 h in the dark at the in situ temperature, 14°C. Following incubation the cores were sectioned and fixed with 2.5% NaOH.…”