Janine Felden

• Methane is an important greenhouse gas and in the oceans cold seeps such as mud volcanoes or pockmarks are important methane emission sites. By using the chemical energy of hydrocarbons rising up with mud, gas or fluids from the deep subsurface, diverse seep communities with high biomasses develop at these structures. Therefore, cold seeps are fascinating hot-spot ecosystems as they link the deep geosphere with the biosphere at the seafloor surface. During this PhD study, methane efflux and consumption as well as related processes such as sulfate reduction and oxygen consumption were investigated at four different deep-sea by focusing on in situ quantification. The results showed that cold seeps are spatially heterogeneous ecosystems controlled by variations of fluid flow intensity influencing benthic biogeochemical processes. The highest fluid flow velocities are found at the central outflow of mud volcanoes in combination with high methane emission but low consumption rates. Outside of these main emission sites, chemosynthetic organisms such as mat-forming thiotrophic bacteria or siboglinid tubeworms are abundant. Here, medium to low fluid flow velocities with high methane oxidation rates were measured. Within one seep ecosystem there are spatial variations in methane emission and consumption, and the benthic biological methane filter of the different seep habitats removes a significant fraction of the total methane flow (up to 90%). For the methane budgets of geostructures and ocean basins, diffusive methane effluxes were previously not considered. However, based on the data of this PhD study, diffusive methane discharge significantly contributes to the total methane emission. Considering the diffusive methane release of the investigated deep-sea mud volcanoes, only mud volcanoes would release up to 15 ×10 12 g methane per year to the water column, which is a significant fraction of the total annual methane flux from the ocean to the atmosphere.