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METRO: Working Area
The Black Sea is a marginal sea with the shallow Bosporus Strait being the only connection to the open Mediterranean Sea (Figure 1). Constricted water exchange and high frights of organic matter by riverine input cause the water column in the Black Sea to be anoxic, i. e., oxygen-free, at depth below 120 m. Thus, the Black Sea is the world's largest anoxic basin. In addition, it is the largest surface reservoir of dissolved methane having a methane inventory of 6x10exp12 mol (Reeburgh et al., 1991). A high amount of this inventory originates from methane seeps that expel methane, which was formed by microbial or thermocatalytic processes within the sediment layers of approximately 12 km thickness. These seeps are particularly active if the dissolved methane concentration exceeds saturation in the pore water and thus forms free methane gas and gas hydrates. Owing to focused gas and fluid flow part of the dissolved and free methane escapes into the water column at these seep or cold vent sites. In the Black Sea gas hydrates are stable at depth below about 750 m. It was in the Black Sea that samples of gas hydrates were recovered for the first time in marine sediments (Yefremova and Zhizchenko, 1974). The most effective methane sink in the Black Sea water column is the microbially mediated anaerobic methane oxidation which consumes about 99 % of the methane input (Reeburgh et al., 1991).
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Overview METRO
Working Area
Autoclave Technology
Subprojects:
SP1: Structural Analyses
SP2: Methane Budget
SP3: Mapping
SP4: Geochemistry
Expeditions
back to Gas Hydrate
Figure 1: Bathymetric map of the Black Sea and adjacent countries. Potential research areas are indicated (1-4). The thin lines indicate the track from RV METEOR cruise M52/1 MARGASCH.
Earlier expeditions caught sight of numerous cold vent sites in the western Black Sea at depths above and below 750 m. These vents and recovered pieces of gas hydrate often occurred in association with so called "mud volcanoes". These features accumulate, e.g., in the western basin of the Black Sea and in the Sorokin Troguh to the southeast of the Crimean Peninsula at depth between 600-2000 m. Because of the manifold structural diversity the Sorokin Trough was the area of main interest during the expedition METEOR M52/1 which was part of the previous project OMEGA (Bohrmann & Schenck, 2002). The emphases of the cruise were the distribution, composition, and the structure of gas hydrate occurrences, as well as their relationship to fluid migration through sediments and gas venting. The Dvurechenskii and Odessa mud volcanoes proofed to be excellent targets for these investigations (Figure 2). Both MVs are located within the gas hydrate stability zone but are considerably different from each other concerning their structure, sediments, and their fluid composition (Bohrmann et al., 2003).
