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An introduction to Mediterranean deep-sea biology* - page 6 / 32

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features of shelf-to-slope particle transfer in the Mediterranean basin. The results are summarised in several publications (Fabres et al., 2002; Lipiatou, 1997; Lipiatou et al., 1999; Monaco et al., 1990; Puig and Palanques, 1998a; Stavrakakis et al., 2000; Tselepides and Polychronaki, 2000; and Monaco et al., 2002). All the studied margins show similar pat- terns, with the flux of particles in the continental margin environment showing an offshore decrease and depth increase. The increase in fluxes with depth is caused by lateral inputs. This supplemen- tary arrival of particles represents a substantial con- tribution ranging from 50% in the Gulf of Lions to 80% in the Cretan Sea, though as an exception to this pattern Fabres et al. (2002) reported an opposite trend in the Malaga margin of the Alboran Sea. The changes in total mass fluxes over time fluctuate from a short scale of a few weeks (flux peaks) to a seasonal time scale (winter fluxes higher than sum- mer fluxes). Increases in total mass fluxes are fairly simultaneous and are affected by driving forces that are either external (e.g. river discharges and storms) or internal (e.g. circulation patterns). The composi- tion of settling material is relatively stable, with a predominant lithogenic component, even at the Baleares and Crete sites without a major fluvial sup- ply; biogenic constituents (organic matter, calcium carbonate and biogenic silica) vary according to the location and period of the year and their variability decreases downstream due to the increase in total mass flux.

Submarine canyons draining sediments from the continental shelf and upper slope help to build vari- ous types of depositional bodies at the base-of- slope, mainly deep-sea submarine fans. Deep-sea submarine fans are thick sedimentary bodies that develop seaward of a major sediment input. They have gradients similar to continental slopes, decreasing from the upper to the lower fan. They are supplied with sediment by one or more feeder chan- nels, usually connected to slope canyons or canyon- channel systems. The major deep-sea submarine fans are the Rhone and Ebro fans in the western Mediterranean, and the Nile fan in the eastern Mediterranean. Channels in submarine fans are flanked by levees, the positive reliefs formed by the overspill from channelised turbidity currents.

Turbidity currents are gravity-driven suspensions of mud and water. Beds deposited by turbidity cur- rents are called turbidites. Turbidity currents may be caused by flood river discharges, storms, breaking of internal waves and (often) slope failure. Tur-

12 F. SARDÀ et al.

bidites show a characteristic sequence of fining grain size and vertical disposition of sedimentary structures, which record the decrease in flow veloc- ity. Turbidity currents can transport large volumes of sediments from the continental margin to the deep sea in a single event.

Abyssal plains

Abyssal plains are defined by several isobaths in the western and eastern Mediterranean. In the west- ern basin the 2600 m (Stanley, 1977) and 2700 m (Rehault et al., 1985) depths have been used as upper limit, with a maximum depth of 2855 m to the east of Corsica. The abyssal plain is the largest phys- iographic feature in the western Mediterranean Basin, the vast area known as the Algerian-Balearic Basin, bounded by the 2600 m isobath (Acosta et al., 2002). Roughly, the basin has a triangular shape. Its vertices are the Oran Rise, the Ligurian Trough and the Sardinia Channel. The overall area is ca. 240000 km2. Previous works define this abyssal plain as in general ‘‘featureless’’ due to depositional processes (Rothwell et al., 1998), though other authors (Stanley et al., 1974 and Acosta et al., 2002) indicate low seafloor reliefs (up to 35 m height) ascribed to diapiric intrusions of the Miocene infra- salt layer and/or to basement irregularities.

In contrast with the Balearic Abyssal Plain, for the Tyrrhenian Bathyal Plain Selli (1985) choose the term bathyal due to the gentle concave shape, high sedi- mentation rates and the intermediate type of underly- ing crust, showing a flat sea-floor surface resulting from turbiditic deposition. The deepest part of this Central Tyrrhenian Basin exceeds depths of 3600 m (Vanney and Gennessaux, 1985). The Tyrrhenian bathyal plain is spotted by seamounts that rise from the bathyal plain. These correspond to volcanic bod- ies of tholeitic petrology associated with crustal faults oriented N-S, such as Magnaghi, Vavilov and Marsili seamounts, or to crescent-shape bathymetric ridges (hosrts) bounded by normal faults, such as Vercelli and Cassinis ridges (Wezel, 1985).

In the eastern basin, the Ionian Abyssal Basin, recording effects of compression, shows a sharp, small and broken topography for the basin plains. This abyssal plain shows a triangular form con- strained by the Calabrian arc from the northwest, by the Mediterranean ridge from the east and by the folded system of the Sirte rise and the megafault system of the Sicily-Malta escarpment (Finetti, 1985). Specifically, the Ionian Abyssal Plain is sub-

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