COARSE-GRAINED SEDIMENTS ARE POTENTIAL MICROBIAL FACTORIES FOR METHANE GENERATION IN MARINE SEDIMENTS

We present a new model of methanogenesis in marine sediments where methanogens are largely housed, and microbial methane is generated in coarse-grained sediments during burial. During burial, dissolved organic carbon is generated from organic matter in muds and diffused into sandy/silty beds to provide substrates for methanogens that generate methane. This model is based on observations that marine muds are much more compressible than sands/silts. As a result, the size of pores within marine muds (nm scale) decrease to smaller than the size of methanogens (1 μm) from tens of meters below the seafloor. This pore size limitation makes marine muds less habitable than marine sands/silts from tens of meters below the seafloor. With our model, we can simulate the presence of high-concentration microbial methane accumulations in either methane hydrate or natural gas reservoirs deep below the seafloor. Our model is fundamentally different from traditional models where microbial methane is assumed to be largely generated in muds. Knowledge of the depth and lithology of microbial methane generation is critical to understand the methane budget, methane distribution and carbon cycle under the seafloor.