Abstract. We develop a numerical model to predict 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. 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.
[1] We develop a steady state model to describe the formation of hydrate below the seafloor. The model includes the sulfate reducing zone (SRZ), which permits sulfate measurements to be used in conjunction with chloride data to better constrain the supply of methane at hydrate locations. The steady state model is applied at Ocean Drilling Program Site 997 to resolve the methane source at the Blake Ridge. Sulfate measurements rule out a shallow source of methane directly below the SRZ because the predicted sulfate concentration overestimates the measurements by a factor of 2 or more. Although a deep source of methane reproduces the main features of the sulfate and chloride data when the upward fluid velocity is 0.25 mm yr À1 , the deep source fails to reproduce the increased freshening observed between 200 and 450 m below seafloor. We find that an in situ methane source (located in the lower part of the hydrate stability zone) together with an incoming methane bearing fluid at 0.23 mm yr À1 gives the best fit to both the sulfate and chloride data. In addition, the predicted sulfate profile indicates that anaerobic methane oxidation is the primary pathway of sulfate depletion at the Blake Ridge.
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