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An introduction to Mediterranean deep-sea biology* - page 5 / 32





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scan sonographs and sediment cores are the methodologies used to study the seafloor and sub- seafloor features resulting from a large debris flow event that affected the continental slope and base- of-slope. Canals et al. (2000) and Lastras et al. (2002) show the structure of a debris flow named BIG’95 on the Ebro continental slope that disturbs more than 2200 km2, including a 26 km3 deposit of remobilised sediment. In the source area, the head- wall scar located on the lower continental slope at ca. 1000 m water depth is 20 km long and up to 200 m high, and several other shallower secondary scars have been identified. Material released from these secondary scars partially buried the main headwall. Additional landslides, which occurred soon after the initial instability event, added more material to the debris flow deposit. The process of formation of the different scars and the releasing of material led to a rejuvenation of the slope relief, truncating canyons, burying the Valencia Channel and destroying gullies present in the rest of the conti- nental slope and base of slope. Following the for- mation of the main headwall, large blocks of sedi- ment were detached. These large blocks moved downslope while essentially keeping their internal coherence. Meanwhile, looser material from the slope moved in a different way and faster than the blocks, first partially burying the scars from which the blocks were detached, and then flowing through the depressions between the blocks. The decrease in the slope gradient and the Balearic margin counter- slope slowed down the debris flow, stopping the blocks, after a run-out in excess of 20 km. Looser material flowed farther on, reaching the distal depo- sitional area, slightly climbing over the lowermost Balearic slope, and turning NE before burying part of the Valencia Channel after more than 100 km of run-out from the source area.

Dating sediment cores gives a minimum age of 11000 calendar years BP for the BIG’95 debris flow. A set of triggering mechanisms, including seismici- ty and oversteepening of the slope due to the exis- tence of a volcanic structure underneath the main headwall, is invoked.

The main morphology of the continental slope is the submarine canyons. The submarine canyons are submarine valleys that are mostly incised in the con- tinental slope and form part of the drainage system of continental margins. The upper course and head of some canyons can be cut, however, on the conti- nental shelf and even reach the coastline (e.g. the Blanes and Palamós canyons in the northwestern

Mediterranean). The cross-section tends to be V- shaped along the upper course and U-shaped in the lower course, thus reflecting the prevalence of ero- sion and accumulation processes respectively. The course of submarine canyons is from straight to sin- uous or even meandering, as is perfectly exempli- fied by the Rhone Canyon and channel off the Gulf of Lions, the Blanes Canyon off Spain, and others. An intriguing inner course incised into a major course is a rather common feature, as was recently observed in the northwestern Mediterranean thanks to full coverage swath bathymetric mapping (Berné et al., 1999; Canals et al., 2000).

Submarine canyons are widespread in many con- tinental margins, but their density and development vary greatly. Complex canyon networks (e.g. the Gulf of Lions) are sometimes close to margin seg- ments with only linear canyons (e.g. the Catalonia margin) or no canyons at all (e.g. the North Balearic margin).

The origin of submarine canyons is probably not unique, since different canyons might have different origins, either submarine or subaerial, or both (Canals et al., 2002). The notion of submarine canyons as stable features must be discarded, since they are born, evolve and eventually die by sediment starvation or sediment filling. Controlling factors such as weakness zones, fault planes and river mouths is known to play a major role in the location of some submarine canyons.

Transport of sediment downcanyon is enhanced during the low sea-level periods but little is known about the frequency of turbiditic flows in different canyon settings. Most canyons are relatively inactive at the present high sea-level time, but recent studies show that the sediment transport by nepheloid layers at the head and upper reaches of active canyons is related either to flood events in nearby rivers or to storm events increasing orbital velocities and enhanc- ing density-driven currents (Puig et al., 2000).

During the 1990s, both European (EUROMARGE- NB, EUROMARGE-AS, CINCS, MATER…) and national projects conducted downward flux studies at selected sites along several Mediterranean conti- nental margins. The main objectives addressed in these projects were to identify the mechanisms that control the particle fluxes on continental margins with distinct geological characteristics, inside and outside canyons, and to estimate flux budgets and mass balances. The experiments conducted over the entire Mediterranean basin, from the Alboran Sea to the Aegean Sea, made it possible to define the major


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