Combined microbiological and (isotope-) geochemical investigations of marine sediments are an usefull approach to study complex methane oxidation processes.
Anaerobe methane oxidation was found to occur in sediments of North Sea coastal tidal flat area off Dangast (near Wilemshaven in northern Germany). The dark-grey to black coloured (organic-rich) sediments are usually fine-grained (up to 80 % clay and silt). The sediments exhibit natural methane concentrations in the range of 9158 vpm (ppm by volume) within the core section below 70 cm depth and of 2 vpm measured within the surface sediments (top 15 cm).
In the laboratory, radioactive labelled methane was injected into the sediments (down-core; cm-intervals) in order to study microbial methane oxidation. Three major oxidation zones were detected. Within the uppermost 10 cm of the sediments where the dissolved sulfate concentrations are still in the range of normal sea water (app. 2 mg/kg) an aerobic oxidation of methane was indicated by the formation of radioactive carbon dioxide. However, two distinct horizones with radioactive CO2 formation were also found at sediment depths of 36 to 42 cm and 50 to 58 cm. There the concentration of dissolved SO42- was reduced to 1,5 mg/kg indicating anaerobic sediment conditions. Stable carbon-(and hydrogen-) isotope analyses of CH4 and coexisting CO2 suggest bacterial CH4 formation to occur in sediments below 70 cm (d13C-CH4 values -72 to -77 ; dD= -130 to -163). The sediment horizones where anaerobic CH4 oxidation was observed, however, showed an enrichment of the heavy isotopes in the residual CH4 (d13C= -42 to -44 %o; dD= -138 to -116) probably due to a kinetic fractionation of stable isotopes in the course of microbial (CH4 oxidation) processes. As bacterial methane oxidation is known to occur at the base of anaerobic sulfate reduction it is assumed that additional horizones with distinct methane oxidation may occur at greater sediment depths.