Abstract
The Sorokin Trough (NE Black Sea) and the Gulf of Cadiz (NE Atlantic) are both mud volcano (MV) provinces characterized by the presence of gas hydrates, methane-related carbonates, and chemosynthetic biota but possess differences in geological history, tectonics, composition of sedimentary cover, and water column conditions. This thesis compares the
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geological and biogeochemical processes taking place within these MV provinces. It attempted to elucidate the deep-fluid dynamics in both areas, to identify possible sources for ascending hydrocarbon gases, and to assess the source-strata for the erupted sediments. The environmental impact of expelled methane-rich fluids on the development of microbial communities involved in the anaerobic oxidation of methane (AOM), a process inducing carbonate precipitation, was also investigated. A significant contribution of non-microbial, mature hydrocarbons in the migrated fluid was apparent from the composition and the stable carbon isotope signatures of the gases. Comparison of the hydrocarbon gas and biomarker data revealed that the formation of the MV fluid starts with the discharge of over-pressurized hydrocarbons and is followed by dynamic entrainment of the surround sediments on its way up. The biomarker composition of the mud breccia matrix in the Sorokin Trough indicated different depths of de-fluidization related to the position of fracture zones or diapiric folds in the subsurface. The biomarker composition of various MVs from the Gulf of Cadiz designated similar source strata for the erupted material. The study of living microbial mats and related carbonates from the Sorokin Trough and an ancient methane-related carbonate crust from the Gulf of Cadiz revealed a substantial contribution of microbes involved in AOM to the microbial community and the dominant role of AOM in processes of carbonate formation. The lipid biomarker and 16S rRNA gene sequence data showed that AOM signals appear to be strongest within carbonates and are considerably weaker in the surrounding sediments, indicating that local oversaturation of pore waters with bicarbonate, (formed by AOM) induced carbonate precipitation. The data also revealed changes in the archaeal community structure during carbonate crust formation in the Gulf of Cadiz, which are interpreted as resulting from changes in the conditions (e.g. methane flux) during the crusts' accretion. Despite the presence of an anoxic water column in the Sorokin Trough and oxygenated waters in the Gulf of Cadiz, the lipid biomarker and 16S rDNA sequence data demonstrated AOM to be the dominant biogeochemical process for methane oxidation in both areas. The distribution of anaerobic methanotrophs is directly linked to the intensity of methane flux and to fluid chemistry. However, in comparison with the Sorokin Trough, the AOM signal in the Gulf of Cadiz is muted. This probably indicates the absence of active fluid transport in the Gulf although the ubiquitous presence of outcropping methane-related carbonates signifies episodes of fluid discharge in the past. Reduced AOM in the Gulf of Cadiz is likely the result of “fluid-flux-starvation” environments established after a period of MV eruptions followed by a consumption of allochthonous methane and other organic and inorganic constituents. In contrast, fluid inflow in the Sorokin Trough is still going on, resulting in development of chemosynthetic microbial communities and in carbonate precipitation.
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