Terrestrial canyons like the Grand Canyon, USA are spectacular geomorphic features carved out over millions of years by river erosion. They can extend for many hundreds of kilometres, have a width of up to 29 Km, and be as deep as 6 Km. However, the submerged margins of continental landmasses are also host to huge canyons systems; the scale of which can rival canyons on land. These submarine canyons extend from the shallow shelf down the deep oceans, and are host to a wide diversity of life. Unlike canyons on land, the main process creating submarine canyons is not rivers, but processes known as turbidity currents.
Simplified view of the submarine real, showing the location of most submarine canyons relative to the continental landmass and slope.
Turbidity currents are dense mixtures of sediment and water that move under gravity down the continental slope and into ocean basins. They can form when underwater landslides occur and disintegrate, when oceanic storms whip up sediment from the sea floor, and where rivers discharge sediment into the ocean. Over time these turbidity currents erode the seafloor and carve out vast submarine channels and canyons. The turbidity currents that carve these canyons can transport as much sediment in one week as all the world’s rivers do in one year, and are some of the largest mass transport process operating on our planet.
Schematic showing the different types of mass transport that occur on the continental slope. Turbidity currents transport sediment to the more distal areas of the ocean.
The Nazaré canyon, off the coast of Portugal, is one of the most spectacular examples of a sinuous submarine canyon. It begins 50 m offshore the town of Nazaré, and extends down to a water depth of 5 Km. Its deepest point relative to the surrounding seafloor is 1 Km, and at its widest it spans 9 Km. In addition to being one of Europe’s largest submarine canyons, it is also home to a unique marine ecosystem consisting of mesopelagic and benthic organisms.
Bathymetric map of the Nazaré Canyon and the Iberian Abyssal Plain, off the Central Portuguese Margin.
In recent decades human impact on the oceans has become strikingly clear. The vast quantity of plastic and other discarded material in the oceans is soon estimated to outweigh the quantity of marine fish. Despite their extreme depth, submarine canyons have not escaped the problems associated with marine pollution. Aluminium and plastic refuse have been observed by ROV cameras at a depth of 4,500 m in Nazaré Canyon.
Images of marine litter in the deep sea (Image credit: MBARI).
Submarine canyons are unique environments, and they are host to unique species and ecosystems. However, like all other habitats they are susceptible to pollution and degradation. While we know much about submarine canyons, we know relatively little about the ecosystems that exist within them, as well as the effects that environmental degradation is having on their trophic structures. Considerably more effort should be afforded to understanding this secret world.