Location of Blake Ridge with respect to Atlantic coastline. Source: B. E. Tucholke, R. E. Houtz, and W. J. Ludwig, 1982, Sediment thickness and depth to basement in the western North Atlantic Ocean basin, AAPG Bulletin 66:1393.

Blake Ridge is a bathymetric high located on the Atlantic continental rise approximately 400 km east from Charleston, South Carolina. The sedimentary feature is situated in water depths between 2,000 to 4,800 meters and is approximately 500 km in length. Over the past 30 years, the Blake Ridge has become perhaps the best-studied occurrence of methane hydrate in the United States.

In 1970, the Deep Sea Drilling Project (DSDP) began drilling in the Blake Ridge as part of their effort to understand the evolution of the world's oceans and oceanic processes. Sediment cores recovered during drilling contained high concentrations of methane. The correspondence between the presence of methane and the observation of anomalously high seismic velocities led to the hypothesis that methane hydrates exist at Blake Ridge. Confirmation of hydrate presence came in 1980 when an actual hydrate sample was retrieved from beneath the ocean floor.

Core from Blake Ridge

The Blake Ridge formed through the deposition of sediments carried parallel with the shoreline by ocean bottom currents. During the Paleocene, the initiation of the Gulf Stream brought warm waters northward along the Atlantic coast. The opening of the Greenland Sea in the Oligocene allowed cool, dense water from the Arctic Ocean to flow south (Western Boundary Current) along the western Atlantic margin. Blake Ridge marks the location where these two major currents interact, lose speed, and drop a portion of their sediment load.

To date, no sediments older than Miocene have been recovered from Blake Ridge. Miocene sediments cored at Blake Ridge are siliceous gray-green muds. Middle Miocene through present sediments are commonly carbonaceous gray-green muds with varying amounts of silts and sands.

The occurrence of hydrate at Blake Ridge is connected to the high rates of sedimentation in the area. Estimates of sedimentation rates range from 160-350 meters per million years for the Miocene to 40 meters per million years for the Pleistocene. Such high rates of sedimentation allow for quick and deep burial of organic matter that is transported to the site. At depth, bacteria convert the organic matter to methane and release it into the sediments. In addition, deeper marine shales and carbonates may provide methane for hydrate formation.

At Blake Ridge, a large amount of methane is thought to be trapped within the hydrate layer and beneath the hydrate as free gas. Repeated episodes of deep hydrate burial, hydrate dissociation, methane release and recapture is recognized as a method to explain the relative concentration of free gas and hydrate at Blake Ridge.

Topographic map showing the locations of landslides and diapirs on top of the BSR in the Blake Ridge area. Source: W. P. Dillon and M. D. Max, 2000, The U.S. Atlantic Continental Margin; the Best-Known Gas Hydrate Locality, in M. D. Max (ed.), Natural Gas Hydrate in Oceanic and Permafrost Environments.

Scientists from the USGS have mapped the distribution and thickness of methane hydrate deposits at Blake Ridge using seismic lines and side-scanning sonar. Maps of depth to the BSR (Bottom Simulating Reflector) show thinning of the hydrate layer linked to disruptions of the sea floor. Because hydrates are sensitive to changes in temperature and pressure, external forces that affect these parameters, such as a drop in mean sea level or a mass movement of sediment, can initiate hydrate dissociation which may lead to pressure build-up and subsequent methane release and seafloor disruptions.

Cape Fear Slide and Cape Lookout Slide are two mass movement features near Blake Ridge (see Seismic Line figure below). Seismic lines show weaker and shallower BSRs beneath the slides compared to undisturbed adjacent areas. It is likely that decreased overburden pressure upon the sediments beneath the slides resulted in dissociation of hydrate. Although hydrate dissociation may not have triggered the slides, dissociation may have involved more sediment in the mass movement.

One of the most intriguing aspects of the Blake Ridge is the presence of a major depression on the crest that may be linked to methane dissociation and release. Although its development history is not fully understood, two theories currently exist to explain the structure. One stresses catastrophic expulsion of gas and sediment. During an unspecified glacial episode, sea-level drop resulted in reduced pressure and consequent hydrate dissociation. However, the gas was unable to effectively dissipate due to the low permeability of the Blake Ridge sediments. Pressure built up, the structure inflated, and eventually blew out. A second theory suggests the feature is composed of rapidly deposited sediment waves that allowed for more gradual methane expulsion through

Seismic Line Over Blake Ridge Depression Feature. Courtesy of W.S. Holbrook.

high-permeability pathways linking the zone of hydrate dissociation to the sea-bottom.