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An introduction to Mediterranean deep-sea biology* - page 7 / 32

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divided into three sub-basins, the Messina Plain the northern basin, and is delineated by the 3600 m iso- bath that reaches a depth of 4100 m, the Sirte Plain located to the west of the Mediterranean Ridge (3900 m) and the Herodotus Trough, which is a nar- row faulted depression (3000-3500 m depth) at the base of slope of Libya.

The Hellenic Trough is located between the Mediterranean Ridge and the western continental margin of Greece. This abyssal plain is defined by the isobath of –3000 m (Carter et al., 1972). The floor of this plain shows a complex structure due to a combination of crustal plate kinematics with halokinesis and gravity processes. The floor is defined by a series of discontinuous mounts and trenches. Trenches are irregularly shaped depres- sions in echelon, forming small sediment ponds at different levels. These depressions, such as the North Matapan-Vavilov Trench which reaches a depth of 5121 m (Vanney and Gennessaux, 1985), are the deepest places in the Mediterranean.

The Aegean Sea is characterised by a basin deep- and-shallow platform pattern (Vanney and Gennes- saux, 1985). In this context, the Cretan Trough, the largest and deepest of the Aegean basins, is an elon- gated depression deepening from west (1300 m) to east (2500 m) parallel to the Cretan Island Arc (Crete, Kasos, Karpathos and Rhodes islands).

The Levantine basin has several abyssal plains bordering the Mediterranean Ridge. The Herodotus Abyssal Plain is located between the Mediterranean Ridge and the African continental margin, while the Rhodes Basin, the Antalya Basin and Cyprus Abyssal Plain are bordered to the north by the Turk- ish continental margin and to the south by the Mediterranean Ridge. The Herodotus Abyssal Plain, the largest basin plain in the eastern Mediterranean, is a NE-SW elongated depression bounded by the isobaths of 2800 or 3000 m with a maximum depth of 3156 m (Carter et al., 1972; Rothwell et al., 2000), lying seaward of the northwestern part of the Nile Cone (Rothwell et al., 2000).

Deep-sea plains commonly form the ultimate store of redeposited muds caused by instability on adjacent margins. Sediments filling the Mediter- ranean abyssal plains have been dominated by the deposition of turbidites. Two main controls have been identified, affecting sediment remobilisation and redeposition in the Mediterranean Sea (Roth- well et al., 2000): (1) climate change, principally linked to Quaternary glaciations, causing large-scale eustatic regressions and transgressions; and (2) the

tectonic framework resulting from the long-term northward movement of the African plate into Europe. However, after Stanley (1985), the domi- nant control of Plio-Quaternary sediment thickness, sediment types, basin plain depth and margin con- figuration is geologically recent tectonic activity rather than sedimentary processes. An exception is the Mediterranean margin, which is affected mainly by riverine supplies (Nile, Rhone, Po, and Ebro rivers). Rothwell et al. (1998 and 2000) reported that some large-volume thick mud beds, called “megaturbidites”, were transported by turbidity cur- rents to the deep sea basins during glacial times. Thick structureless turbidite muds have long been reported from cores of Mediterranean troughs and basins (see references in Stanley, 1985). However, thick, laterally extensive ‘megabeds’ that are clearly very large-volume pounded mud turbidites have been described from the larger abyssal plains of the Mediterranean, on the western Mediterranean and Calabrian Ridges (Kastens and Cita, 1981), on the Ionian Abyssal Plain (Hieke, 1984), on the Herodotus Abyssal Plain (Cita et al., 1984 and Roth- well et al., 2000), and on the Balearic Abyssal Plain (Rotwell et al., 1998 and 2000).

Below the pelagic and hemipelagic (megatur- bidites) Plio-Quaternary sediments (more than 1000 m thickness), extremely thick evaporitic sediments (from 2000 to 3000 m thick) were deposited. In 1970, Leg 13 of the Deep Sea Drilling Programme (DSDP) discovered that evaporitic deposits of Messinian age (5-6 million years ago) are present in the sub-bottom of all the major basins of the Mediterranean (the Balearic, Tyrrhenian, Ionian and Levantine basins) (Ryan et al., 1973). In 1975, Leg 42 showed that the evaporític deposits were not restricted to the shallow marginal basins but were widespread in all of the deeper Mediterranean basins, as was imaged by basinwide seismic profiles and sampled by drilling. According to Hsu et al. (1973), during the Messinian, the Mediterranean Sea became isolated from the Atlantic Ocean, which caused some areas to dry out and large salty lakes recharged by rivers flowing through deep canyons replaced the previous marine basins (Hsü et al., 1973; Hsü et al., 1973b; Cita, 1991; MacKenzie, 1999). This was the so-called Messinian salinity cri- sis. During this low sea level event, the emerged Mediterranean margins were subjected to intense erosion. The erosional surfaces that formed during the Messinian are quite varied, and a complex net- work of canyons incised the continental margin.

MEDITERRANEAN DEEP-SEA BIOLOGY 13

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