Submarine methane seepage in the Paleo Dnepr Area and Sorokin Trough and its influence on the Black Sea methane budget
Methane is a greenhouse gas that exerts a significant influence on the radiation budget and thereby the earth’s climate. Geological sources of methane, especially submarine, are particularly relevant in this regard because they emit significant quantities of methane into the atmosphere. However, it is difficult to estimate the magnitude of the global geological source because the extent of these methane-releasing areas and the processes, which influence the methane cycle, are not yet well understood. The Black Sea is a region characterized by numerous submarine methane sources. Methane emissions in the form of methane-containing fluids or gas bubbles have been documented here at all water depths, from the shelf down to the abyss. This marginal sea features a unique hydrographic structure resulting from the inflow of freshwater from rivers and the salty waters of the Mediterranean Sea over the Bosphorus. These inputs have very different densities, and this leads to a pronounced stratification of the water column that limits vertical mixing. High biological production in the surface waters leads to an intense consumption of oxygen in the upper water column, which can not be balanced by downward ventilation of oxygenated surface waters. The pronounced redox-gradient has in turn an important influence on the biochemical cycles in the water column and leads to anoxic conditions at depths below approximately 100 m. These particular conditions make the Black Sea the largest anoxic marine basin on earth. Chapters 2 and 3 of this work (as well as the articles in the appendix) explore the questions of what influence methane emissions have at different water depths on the methane cycle of the water column and what significance these submarine emissions have as sources of atmospheric methane. To this end, the zones of methane sources along the northwest shelf and continental slope (Paleo Dnepr Area) as well as in the northeastern deep sea region (Sorokin Trough) of the Black Sea were explored throughout 2003 and 2004. The observed depths span from between 60 and 2100 m and encompass the entire biogeochemical spectrum of the water column. The methane concentrations of the water column and the overlaying atmosphere were carefully recorded, as were the carbon isotope signatures of this methane. The physical description of the water column, the microorganisms involved in the methane cycle and their methane consumption rates were additionally documented. The area affected by the emission of methane from submarine sources was ascertained through extensive sampling campaigns. The dataset was used to model the methane flux at the sea/air interface and to evaluate the influence of methane sources, present in different water depth on the marine methane emission. In addition, the exploration of this area throughout two consecutive years provides insight into the seasonal variability of the source strength of these submarine emissions. Chapter 4 serves to combine the results of this investigation with those present in the literature for the purpose of constructing a methane budget that quantifies the significant sources and sinks of methane in the Black Sea. A transport-reaction model investigates the release of methane at discrete water depths and identifies those of particular interest. Various model scenarios were used to determine to what extent sizable methane emission events, for example from mud volcano eruptions or submarine landslides, affect the methane budget of the Black Sea and the associated methane emissions at the ocean surface. The following work cannot completely address the complex methane cycle of the Black Sea and its associated processes. This work serves rather to provide new insights, discuss relevant areas and point out still unanswered questions. The increase in research on methane in the Black Sea during recent years shows that its methane cycle is of great scientific interest.