Abstract

The widespread presence of bottom-simulating reflectors (BSRs) on continental margins has bolstered suggestions that gas hydrates and free gas constitute a large dynamic reservoir of CH4 carbon and a vast potential source of energy. However, only a few hydrate-bearing areas have been drilled, and of these, the amount of CH4 has only been directly quantified in 18 discrete samples from 3 holes on Blake Ridge, east of Georgia. Here we report and discuss 30 direct measurements of CH4 concentration in sediments above and below the BSR at Hydrate Ridge on a tectonically active margin offshore Oregon. High CH4 concentrations (71–3127 mM) support abundant gas hydrate (occupying an average of ∼11% of porosity) and free gas (occupying ∼4% of porosity in 1 sample) in a restricted area where hydrocarbon gases migrate from the deep accretionary complex to the seafloor. In a larger area lacking this hydrocarbon supply, lower CH4 concentrations (10–893 mM) indicate less gas hydrate (average ∼1% of porosity) and little or no free gas. Overall, the amount of CH4 at Hydrate Ridge is significantly less than that at Blake Ridge. These results challenge certain interpretations, including the global volume of hydrate-bound CH4, which though large, may be four to seven times less than widely cited estimates. Speculations on the distribution and role of gas hydrate and free gas need revision.

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