A steady state model for marine hydrate formation: Constraints on methane supply from pore water sulfate profiles

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−1, 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−1 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.