Methane Fluxes And Associated Biogeochemical Processes In Cold Seep Ecosystems

During this PhD study, methane efflux and consumption as well as related processes such as sulfate reduction were investigated at four different deep-sea cold seeps. The main focus was on in situ quantification of methane emission and oxygen consumption. The results showed that cold seeps are spatially heterogeneous ecosystems, which are 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 methane consumption rates. Outside of these main emission sites, chemosynthetic organisms such as matforming thiotrophic bacteria or siboglinid tubeworms are observed. Here, medium to low fluid flow velocities with high methane oxidation and high sulfate reduction rates were measured. Within one seep ecosystem there are spatial variations in methane emission and consumption, but 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 obtained data during this PhD study, diffusive methane discharge contributes significantly 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 x 1012 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.