Evidence of methane venting and geochemistry of brines on mud volcanoes of the eastern Mediterranean Sea

As a part of the Dutch-French MEDINAUT diving expedition in 1998, cold seeps and mud volcanoes were studied and sampled in two distinctive tectonic settings in the eastern Mediterranean Sea. The first setting was the Olimpi Mud Volcano field (OMV area), including Napoli, Milano, Maidstone and Moscow...

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Published in:Deep-sea research. Part I, Oceanographic research papers Oceanographic research papers, 2003-08, Vol.50 (8), p.941-958
Main Authors: CHARLOU, J. L, DONVAL, J. P, ZITTER, T, ROY, N, JEAN-BAPTISTE, P, FOUCHER, J. P, WOODSIDE, J
Format: Article
Language:English
Subjects:
Continental interfaces, environment
Earth sciences
Earth, ocean, space
Exact sciences and technology
External geophysics
Geochemistry
Isotope geochemistry
Isotope geochemistry. Geochronology
Methane
Mineralogy
Ocean, Atmosphere
Oceans
Physical and chemical properties of sea water
Physics of the oceans
Sciences of the Universe
Silicates
Volcanoes
Water geochemistry
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Summary:As a part of the Dutch-French MEDINAUT diving expedition in 1998, cold seeps and mud volcanoes were studied and sampled in two distinctive tectonic settings in the eastern Mediterranean Sea. The first setting was the Olimpi Mud Volcano field (OMV area), including Napoli, Milano, Maidstone and Moscow mud volcanoes, south of Crete on the Mediterranean ridge. The second setting was the Anaximander Mountains (AM area), southwestern Turkey, where Amsterdam, Kazan and Kula mud volcanoes were explored. Large methane concentrations (45-892nmol/kg) were measured in the water column not only above mud volcanoes but also in seeps and vents along related fault systems, indicating intense degassing related to fluid circulation in sediments. The tracer results show that there is considerable variability in terms of gas seepage and matter flux between these mud volcanoes. Brine accumulations found as shallow pools on Napoli or associated with deep faults (Nadir Lake) outside mud volcanoes exhibit variable chlorinity, mineral and gas composition. The brines are significantly enriched in d18O relative to ambient seawater and are consistent with evaporated seawater. In the Nadir Brine Lake, the level of methane is as high as 5.93mu mol/kg, lower than the methane saturation level of 120mu mol/kg theoretically found at the salinity (120), pressure (200bar), and temperature (13.6 degrees C) conditions of Nadir lake. In contrast, the shallow brine pools on Napoli mud volcano (also OMV area) have methane levels of only 4.45mu mol/kg. In all brines, helium data show a clear radiogenic isotopic ratio (R=0.06 x Ra), in excellent agreement with recently published data for the Urania basin. Methane to ethane ratios (>1000) and d13C(CH4) values (-65.6% PDB) indicate that the CH4 is microbially produced. Unlike mid-ocean ridges, where abiogenic methane and helium have a common origin in the brines, the large variation in the CH4/He ratio indicates that CH4 and helium sources are unrelated, a fact that adds further support to the biogenic origin of methane. These results show that mud volcanoes in the eastern Mediterranean Sea are important sites of extensive biogenic methane fluxes, which are probably related to widespread occurrences of gas hydrates. [PUBLICATION ABSTRACT]
ISSN:0967-0637
1879-0119
DOI:10.1016/S0967-0637(03)00093-1