Occurrences and Characteristics of Brine Accumulations on Mud Volcanoes from the Eastern Mediterranean

Caprais, J. C., P. Briand, P. Pignet, J. P. Foucher, J. Mascle, M. Sibuet, IFREMER, Villefranche/mer, France

Several mud volcanoes on the Mediterranean Ridge and on the Nile continental margin were visited by the submersible Nautile during the NAUTINIL expedition of the French research vessel L’Atalante in September-October 2003. One of the sites, in the western Nile continental slope, at a depth of 3000 meters, and known as Menes caldera, discharges brines at a temperature of around 50°C. These brines form on the seafloor pools and shal­low lakes reaching several dozens of meters in size on ‘Cheops’ and ‘Chefren’ cones both located inside Menes caldera which has a diameter up to 8 km. The thickness of the brine accumulation filling up the crater on top of ‘Chefren’ exceeds 200 m. Brines were sampled with pressure-tight titanium syringes and bottles. Measured salinities of the brines are up to 311 °/°°. Methane contents reach up 2.9 mmol/l and hydrogen sulfide 7.2 mmol/l A second site, also emitting brines and visited by the Nautile, is ‘Napoli’ mud volcano in the central domain of the Mediterranean Ridge. Shallow brine ponds, several meters across, have accu­mulated at various locations on the flat summit of the feature. Measured salinities are up to 268 °/°° . Methane contents are in the range 0.56-0.72 mmol/l and hydrogen sulfide 1.3-2.1 mmol/l. Biological activities in the brine environment are dominated by microbial mats and filaments, particularly on ‘Chefren’ and ‘Cheops’. Dense distribution of opportunistic Polychaete worms characterize ‘Chefren’ and ‘Cheops’. Large Vestimentifera tube worms are also observed, in association with carbonate crust linked to seepage on top of ‘Napoli’.