Submarine canyons – Submarine canyons are defined as “steep-walled, sinuous valleys with V-shaped cross sections, axes sloping outward as continuously as river-cut land canyons and relief comparable to even the largest of land canyons” (Shepard, 1963).  “Large” canyons were mapped in this study based on the definition of Harris and Whiteway (2011), which requires canyons to extend over a depth range of at least 1,000 m and to be incised at least 100 m into the slope at some point along their thalweg.

Submarine canyons are diverse and complex in terms of their origins, hydrography, geologic settings and biodiversity.  The combination of steep rocky slopes, strong currents and enhanced access to food makes submarine canyons places of special ecological significance.  Canyons often are iconic, defining features of marine reserves in many locations because of their association with higher biomass and biodiversity.  For example, the La Jolla Canyon off southern California has been shown to have as much as 50 times the biomass than surrounding areas of shelf and slope (Vetter and Dayton, 1998).

Statistics of submarine canyons (from Harris et al., 2014).

OceanAll canyons Area km2All canyons Area%All canyons No.All canyons mean area km2All canyons mean length kmAll canyons mean incision depth (m)
Arctic Ocean359,6502.7840489058.92,132
Indian Ocean760,4201.081,59048044.42,399
Mediterranean & Black Sea163,0405.3981720026.61,601
North Atlantic 738,4301.651,54848042.02,433
North Pacific 816,5800.9982,08539038.82,360
South Atlantic 291,2900.70745364049.22,441
South Pacific 694,7900.7962,00935035.72,378
Southern Ocean569,4402.805711,00059.42,300
All Oceans4,393,6501.219,47746041.12,308

Submarine canyon statistics, continued.

Ocean% Slope that is canyon areaSelf-incising Area km2Self-incising No.Self-incising average size km2Shelf-incising mean length kmShelf-incising mean incision depth (m)Ratio of area of shelf-incising to blind
Arctic Ocean16.1162,020752,16099.61,6190.820
Indian Ocean11.2222,69029575456.01,4010.414
Mediterranean & Black Sea13.894,43030730733.11,0931.38
North Atlantic 10.4292,33029399763.81,5650.655
North Pacific 10.2367,71048975156.91,4240.819
South Atlantic 8.965,3207389466.01,3490.289
South Pacific 10.2214,96036858446.61,3460.448
Southern Ocean15.1194,4101761,10463.71,5750.518
All Oceans11.21,613,8602,07677754.81,3950.581

Submarine canyon statistics, continued.

OceanBlind canyon Area km2Blind canyon No.Blind canyon average size km2%Blind versus shelf incising by area%Blind versus shelf incising by no.Blind canyon mean length kmBlind mean incision depth (m)
Arctic Ocean197,63032960055.081.449.72,249
Indian Ocean537,7401,29541570.781.441.72,627
Mediterranean & Black Sea68,61051013442.162.422.71,907
North Atlantic 446,1001,25535560.481.136.82,636
North Pacific 449,2201,59628155.076.533.22,647
South Atlantic 225,83038059477.583.946.02,650
South Pacific 479,6401,64129269.081.733.22,609
Southern Ocean375,02039594965.969.257.52,623
All Oceans2,779,7907,40137563.378.137.32,563

Formation of submarine canyons

Submarine canyons are formed via erosion and mass wasting events, particularly on steep continental slopes but also on the flanks of volcanic islands.  Canyons serve as conduits for terrigenous (land-derived) sediment derived from the continents to the deep ocean basins (Shepard, 1963).  Many of the world’s largest submarine canyons commence on the continental shelf, and sometimes at the mouths of large rivers or glaciers, and are incised into the continental slope.  Sediment delivered to the coast by rivers or glaciers accumulates, forming unstable deposits perched at the head of the canyon.  The deposit is dislodged usually by a trigger mechanism, such as an earthquake or tsunami, although simple gravitational slope failure can also be a trigger (Kennett, 1982).  For example, Thunnell et al. (1999) have documented earthquake-generated turbidity flows on the continental slope off northern Venezuela.

The mobilised material forms a dense mixture of water and sediment that flows down-slope under the influence of gravity as a turbidity current.  Turbidity currents may reach speeds of 40 to 55 km/hr and they have been blamed for breaking submarine telecommunication cables off Newfoundland as a result of the 1929 earthquake, for example (Piper et al., 1999).  Over geologic time, submarine canyons are formed by the repeated erosion of the slope by turbidity currents flowing down the canyon axis.  Retrogressive slope failures may expand the canyon head, eroding landwards and expanding the size of the canyon.   Canyon lengths typically range from 50 to 300 km, are hundreds of meters deep, kilometres wide and have steep “V”-shaped cross sections in their upper parts.  Canyon axes are generally aligned normal to the coastline (although they may be deflected along-slope by structural features) and they may occur as single channels or involve complex tributary systems.  Canyons incised into steep continental slopes are generally more closely spaced than those incised into gentle slopes.

Fig. 4.10 Three-dimensional view of the west Tasmanian continental slope, incised by numerous submarine canyons.  Arrows indicate canyon heads that incise the continental shelf.

The rise and fall of global sea level over glacial-interglacial cycles plays an important role in controlling the timing of sediment delivery to the head of the canyon.  This is because under high sea level conditions (such as exist at the present time), the heads of most of the world’s large submarine canyons are located seawards from coastal river mouths and they do not presently receive a direct supply of sediment.  Rather, sediment supply to these canyons takes place during the ice ages, when sea level was up to 120 m below its present position and the river mouths and glacial outlets were located on the outer shelf, at or near the heads of the canyons.  Therefore, turbidity flows and canyon erosion are mainly ice-age phenomenon and are atypical of the modern, high sea level earth we inhabit today.  Nevertheless, there are modern examples of canyons that do extend a considerable distance landward and have their head’s close to (or within) coastal river mouths.  Examples are the Congo River Canyon in Africa, Scripps and Monterey Canyons in California (Shepard, 1963), the “No Ground” Channel incised into the Ganges-Brahmaputra Delta in Bangladesh (Kuehl et al., 1997) and the Sepic River in Papua New Guinea (Walsh and Nittrouer, 2003).  Studies of these systems provide insights into the processes governing canyon formation, turbidity flow generation and off-shelf sediment transport.

Enhanced ocean productivity over canyons

Oceanographically, canyons may affect local upwelling patterns and enhanced primary productivity which extends up the food chain to include birds and mammals (Hickey, 1995).  Consequently, commercially important pelagic and demersal fisheries as well as cetacean feeding grounds are commonly located at the heads of submarine canyons (Hooker et al., 1999).  The driving force behind the enhanced productivity is the upwelling and mixing of cold, nutrient-rich waters affected by canyon geomorphology interacting with ocean currents and internal waves.  For example, ocean mixing rates inside Monterey Canyon are as much as 1,000 times greater than rates measured in the open ocean (Carter and Gregg, 2002).  Key physical processes driving the mixing of canyon waters and upwelling include internal waves, coastally-trapped waves, the modification (eg. bathymetric steering) of shelf and upper slope currents and internal tides (Hickey, 1995; Carter and Gregg, 2002).

Shelf-incising canyons and down-slope transport of organic matter

Submarine canyons that extend across the continental shelf and approach the coast are known to intercept organic-matter-rich-sediments being transported along the inner shelf zone.  This process causes organic rich material to be supplied to the head of Scripps Canyon, for example, and transported down-slope, where it provides nourishment to feed a diverse and abundant macro fauna (Vetter and Dayton, 1998; 1999).  Gage et al (1995) reported finding seagrass at 3,400 m water depth at the base of Setubal Canyon off Portugal.  Canyons that do not have a significant landward extension would presumably not intercept littoral sediments and would not be expected to contain such a rich fauna. Applying the precautionary principle suggests, therefore, that shelf systems having landward extensions should be distinguished from those that do not.  It follows that the representation of the two canyon types should be recognised separately within the MPA selection process, and that lumping all canyon types into a single category misses important ecological differences (Williams et al., 2009).

Headless canyons and cold-seep communities

The above descriptions refer to submarine canyons that are associated with a significant terrestrial drainage system, but in fact most canyons have no such association and they are “headless”; they do not extend across the continental shelf, tend to have a regular spacing and are isolated from down-slope erosive flows. Orange et al. (1994) have attributed the origins of some of these canyons to fluid seepage-induced slope failure.  Once a slump scar is formed, there is an increase in the pressure gradient of internal fluids that drives further failures such that the original slump scar is enlarged.  Fluid escape along a section of continental slope prone to slump-failure will then theoretically produce a self-organised, regular pattern of alternating spurs and canyons (Orange et al., 1994).  The escape of methane- and sulphide-rich fluids from the sediments (cold seeps; see above) gives rise to specialised benthic communities that have been documented by deep submersible expeditions in headless canyons located on slopes of the western United States and Japan (Barry et al., 1996; 2002).  In this way, cold-seep benthic communities co-occur with regularly-spaced, headless submarine canyons.

Connectivity between canyons

Overall, submarine canyons are among the most energetic of oceanic environments, and canyon-associated currents affect the dispersal of eggs, larvae and juveniles.  The oceanographic setting of submarine canyons is complex and involves the interaction of a number of oceanographic processes, including currents having strong vertical components; examples include density currents, internal waves that propagate across the density interface of stratified water columns and upwelling currents.  The channelling of tidal flows along canyons can also induce locally strong currents.  In addition, canyon heads that extend landward and intercept sediment and organic matter in transit along the inner shelf establish a link from the inner shelf to the slope and abyssal biomes (Vetter and Dayton, 1998; 1999).  These processes affect the vertical transport of water and suspended material and collectively, they differentiate submarine canyons from other slope habitats.

In addition to these vertical motions, canyons are subject to the normal range of waves and currents that impinge upon the continental margin, and which cause the dispersal of eggs, larvae and juveniles along the margin, from one canyon to the next.  Surface ocean transport in a coast-parallel direction will be typical for canyons located on margins subject to strong boundary currents; counter-currents occurring at depth provide an opportunity for larvae dispersal in the opposite direction to the surface currents.  Hence dispersal takes place in all directions: along the margin by surface ocean currents and deeper counter-currents; and normal to the margin along the canyon axis by tides, density currents and upwelling currents.  One or all of these processes may affect the dispersal of larvae, depending on the life histories of different species.

Canyons may be the focal point of a species life cycle, with some life stages spent outside the canyon environment, but with reproduction occurring within canyons.  For example, within submarine canyons of the George’s Bank, Cooper et al. (1987) noted that “concentrations of lobsters (adolescents and adults) are substantially greater in submarine canyons than in areas nearby; lobsters seen inside the canyons are usually adolescents (<80 mm) while those nearby but outside the canyons are usually adults.”   Katz et al. (1994) modeled ocean currents, wind velocities and various swimming speeds for different lobster larval stages. Their results show that larvae released within submarine canyons were the most likely source of more mature larvae and juveniles found outside of the canyons.

Submarine canyon benthic biodiversity

Studies of the benthos of submarine canyons have increased in recent years.  Eight submarine canyons surveyed in Tasmania, Australia, by Williams et al. (2009) also displayed depth-related patterns in the occurrence of benthic fauna, in which the percentage occurrence of faunal coverage visible in underwater video achieved a peak at 200-300 m water depth, where averages of over 40% faunal coverage occurred.  Coverage was reduced to less than 10% below 400 m depth.  Ecological differences between canyons depends on their “depth, size, complexity, anthropogenic impact, and other factors” (Williams et al., 2009, p. 222).  Submarine canyons that intrude onto shelf and have rocky exposed rocky substrate along their rims are ecologically different from those that do not have these attributes.  Yoklavich et al. (2000) noted different fish assemblages were associated with rocky versus soft-sediment substrate in Monterey Canyon, and that rock outcrops of high relief are less accessible to fishing activities and provide a natural refuge for economically important fishes.  Gage et al (1995) found that soft-sediment fauna at 3,400 m depth at the base of Setubal Canyon off Portugal was 10 times more abundant but of lower diversity than fauna measured at a site on the adjacent Tagus Abyssal Plain.  The upper sections of shelf-incising canyons often support iconic, cold-water, coral communities such as occur, for example, in the Cap de Creus canyon in the Mediterranean (Lo Iocano et al., 2012) or canyons that incise the continental shelf of Antarctica (Post et al., 2012).

Image of corals in an Antarctic submarine canyon, from Post et al. (2012).


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