Since the Eocene (55 million years ago) the small Juan de Fuca oceanic plate has been subducting beneath the northwestern margin of the North American plate along the Cascadia subduction zone. The current rate of subduction is 45 mm/yr. West of the subduction zone on the Juan de Fuca plate is the Cascadia basin where the sea floor is covered by pre-Pleistocene hemipelagic sediments overlain by Pleistocene turbidites. As the Juan de Fuca plate moves eastward, the hemipelagic silts and clays and sandy turbidites on the subducting plate and trench are scraped off forming an accretionary wedge.

Ongoing areas of study in the Hydrate Ridge region

The wedge is 2.5 to 3 kilometers thick and is characterized by thrust faulting and margin-parallel folds. Anticlinal ridges, with extensional fractures and breached folds, have developed on the edge of the accretionary wedge. Landward the accretionary wedge thickens. Vertical thickening and shortening, and associated thrust faulting caused by compressive forces has resulted in high-porosity sediments being emplaced deep within the accretionary sediments. As the sediments are consolidated, dewatering and fluid expulsion occurs. The deep faults and fractures that extend through the accretionary wedge act as conduits for the escaping fluids to reach the seafloor.

Two sites within the Cascadia Margin have been studied extensively: 1) the offshore region west of Vancouver Island and 2) "Hydrate Ridge", 100 kilometers west of the Oregon coast. Gas hydrates off the Cascadia Margin occur in a 30-kilometer wide zone parallel to and within the margin of the accretionary wedge. Both regions show the development of two, elongate anticlinal ridges along the margin of the accretionary wedge. These topographic highs rise up to 200 meters above the seafloor. Hydrate Ridge is located on the second frontal ridge. Water depths above the accretionary prism are 1400-1500 meters.

Locations of methane hydrate off the Cascadia Margin

Geochemical analyses of the methane in the gas hydrates show the methane to be mainly biogenic. However, the clastic sediments deposited offshore may not contain sufficient organic carbon for in-situ generation of gas to form methane hydrates. One model for hydrate concentration involves the transport of methane generated at depth by pore fluids that are being expelled duri ng consolidation of sediments within the accretionary wedge. The methane is removed from the upward-migrating pore fluids as they pass through the hydrate stability zone. Although the gas composition is primarily methane, hydrogen sulfide is present in significant amounts. Ethane, propane, and carbon dioxide are present in small amounts.

Methane hydrates beneath the sea floor along the Cascadia Margin and Hydrate Ridge have been mapped using seismic reflectors that record the contrast of higher velocity hydrate zones with lower velocity zones of gas-filled pore space below the hydrate. These seismic reflectors, known as bottom-simulating reflectors (BSR), are interpreted to represent the base of the hydrate stability field. Depth of the BSR ranges from approximately 64 to 225 meters below the sea floor. The Ocean Drilling Program encountered gas hydrates at depth between 4 and 17 and at 64 meters. Exploration of the sea floor by submersibles and remotely operated video instruments has detected gas hydrates directly outcropping on the sea floor. In places along Hydrate Ridge, hydrates outcrop in such abundance that hundreds of square meters of sea floor appear to be paved with hydrates. In shallow sediments the hydrates occur in joints and layers centimeters to decimeters thick.

In addition to the subsurface BSRs and direct observation of hydrates on the seafloor, both regions display features that indicate the widespread presence of gas hydrates. Migrating BSRs, clam and tube worm communities, and limestone chimneys are the result of the melting of hydrates at depth and the upward movement of methane charged waters that reach the sea floor.

Schematic representation showing the movement of methane and fluids through an accretionary wedge. Courtesy of Natural Resource Canada and Dr. Roy Hyndman.

As the sediment wedge is being consolidated, plumes of gas and fluids travel along faults that cut through sediments and hydrate. The plumes, called cold vents, are nevertheless warm enough to destabilize the hydrate structure, releasing fresh water and methane. As the fluids reach the hydrate stability zone, new hydrates form at shallower depths causing the bottom boundary of the hydrate zone and the BSR to migrate upward.

The plumes of methane and associated fluids, upon reaching the sea floor, can be quite large extending to hundreds of meters high and several kilometers wide. Along with methane, hydrogen sulfide and ammonia are released which are then oxidized to carbon dioxide, sulfate, and nitrate - the nutritional ingredients for chemical-eating bacteria. The bacteria in turn serve as a food source for higher lifeforms. Surrounding the areas where methane plumes vent onto the seafloor there exist communities of clams and tubeworms on an otherwise sparsely inhabited sea floor.

Figure model of tectonic fluid expulsion. Courtesy of Natural Resource Canada and Dr. Roy Hyndman.

Methane is also oxidized as the plume reaches the sea floor. The methane oxidizes to form bicarbonate, which combines with calcium in the seawater producing calcium carbonate, commonly known as limestone. The limestone precipitates around the mouth of the vents building chimneys and vent linings on the seafloor.

Although much of the methane is destroyed by oxidation or consumed by vent organisms, significant amounts are still being released into the ocean waters. Large releases of methane from hydrates beneath the ocean may become a significant source of methane in the atmosphere. Scientists are now beginning to evaluate the role and impact that methane may have on current and future climate changes.