Controls of anaerobic oxidation of methane in ocean margin sediments

Abstract

Organic rich sediments of continental margins contain large amounts of methane which is migrating upwards by molecular diffusion until it reaches the bottom of the sulfate zone. In this sulfate methane transition zone (SMTZ) anaerobic oxidation of methane (AOM) oxidizes 96 % of marine methane coupled to sulfate reduction (SRR) and thus prevents methane from contributing to the green house effect. The thesis investigates the effectiveness and controls of AOM and SRR, the microbial community that mediates the process and the kinetic and thermodynamic constraints. Measurements of microbial turnover rates demonstrate that AOM and SRR are not distributed equally over the SMTZ, but always occur at the bottom of this zone. The reason for this observation could be determined to be the kinetic constraint on the organisms, because methane concentrations are kinetically more limiting than sulfate. Even though the process has very low energy yields thermodynamic limitation does not exhibit significant inhibition and was demonstrated to supply sufficient energy for the organisms to meet their minimum energy requirement. At some sites methane oxidation seems to be rather ineffective and methane is tailing to the surface. This phenomenon seems to be related to ineffective methane oxidation in former limnic sediments like the Black Sea and the Baltic Sea and explains the occurrence of methane seeps at sites, where the marine deposits on top of limnic sediments are eroded.