Constraints of Microbial Production from Physical Models of Hydrate Formation

We develop a numerical model to describe the physical processes that govern the volume and distribution of gas hydrate in marine sediments. We consider the environment of a deep continental margin where sedimentation adds organic material to the region of hydrate stability. Conversion of the organic material to methane by bacteria promotes hydrate formation and depletes the supply of organic carbon. We derive mass balance equations for the volume of hydrate and gas bubbles in the sediments and account for the changing concentration of dissolved methane and salts in the pore fluid. A biological component is incorporated in the model to allow the relationship between the biological communities and physical processes that govern hydrate formation to be studied. The effects of sediment compaction and the associated fluid flow are explicitly modeled. Allowances for deeper sources of fluid are also described, though we focus on the case of an idealized passive margin where carbon is input solely through sedimentation. The numerical calculations indicate that the key parameters in this model are the rate of sedimentation, the quantity and quality of the organic material, and a rate constant that characterizes the vigor of biological productivity. Model predictions for conditions that are representative of the Blake Ridge are compared with observations from Ocean Drilling Program Leg 164. We obtain a very good match to the observed chlorinity profile, including the region below the stability zone, without invoking any extraneous sources of freshening. We also predict that hydrate is unlikely to occupy more than 7% of the pore volume, in good agreement with observed estimates. Model predictions confirm that in situ microbial production provides a sufficient methane source for hydrate formation at these locales. Methane production is confined to the uppermost sediments where a high rate of carbon conversion (nominally 0.012 nmol mL^-1 per day) quickly degrades the available stock of metabolizable carbon.