A kinetic model for the methane hydrate precipitated from venting gas at cold seep sites at Hydrate Ridge, Cascadia margin, Oregon

Abstract

We develop a kinetic model for hydrate crystallization from methane gas venting through shallow sediments at Hydrate Ridge on the Cascadia margin of Oregon that predicts how pore water chlorinity, temperature, and crystallized hydrate evolve after the onset of steady venting. Predictions are compared to observations at Ocean Drilling Program Site 1249. In the preferred model, calculated gas hydrate saturation and chloride concentrations reach those observed at depths less than 20 m below seafloor (bsf) under the southern summit of Hydrate Ridge in ∼650 years, and the vertical water flux must be less than 50 kg/m2/yr. Hydrate accumulates more slowly between 20 m bsf and the base of the hydrate stability zone where there is no free gas, accumulating to observed levels of a few volume percent of hydrate in 10 5 to 106 years, depending on the water flux that is assumed through this zone. This dichotomy means that the presently observed gas venting must have been diverted to this area ∼650 years ago, or be episodic and infrequent. If the gas venting for the last 650 years has been as observed today, the latent heat of hydrate precipitation in the upper 20 m of sediments would have caused the temperature to increase ∼0.8°C at ∼20 m bsf and ∼0.2°C at ∼100 m bsf. This would have caused a ∼5 m rise in the elevation of the base of hydrate stability zone, and decreased the rate of hydrate crystallization from 1.5 kg CH4/m2/yr 650 years ago to 0.7 kg CH4/m2/yr today. Key Points Methane hydrate crystallization rateNumerical modeSouthern summit of Hydrate Ridge © 2013. American Geophysical Union. All Rights Reserved.