Expulsion of methane gas
 through sediment waves in a large methane hydrate province

W. Steven Holbrook
University of Wyoming

Wednesday, June 27, 2001
3:00 p.m.–Pacific Forum

Methane hydrate, a solid form of methane and water that is widespread in continental margin sediments, comprises a large proportion of Earth's organic carbon, but its role in global carbon cycles remains uncertain. Hydrate has been linked to several episodes of paleoceanographic and climate change through its signature of isotopically light carbon. Proposed mechanisms of methane release from hydrate reservoirs generally invoke large-scale sediment failure, sea-level changes, or thermal dissociation of hydrate via warming of bottom water. New seismic data from the Blake Ridge suggest a new mechanism by which methane gas was expelled directly from the free gas zone, without thermal or mechanical disruption of the overlying hydrate layer. Rapid, post-2.5 Ma deposition of large sediment waves created permeable pathways connecting free gas to the seafloor, allowing methane gas expulsion. The amount of missing methane, 0.6 Gt, is equivalent to approximately 15% of total present-day atmospheric methane. Similar features elsewhere on the Blake Ridge suggest that methane expulsion through large sediment waves on hydrate-bearing contourites may be common. Our results imply that significant amounts of methane gas can bypass the hydrate stability zone and be expelled directly into the ocean/atmosphere system.