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An introduction to Mediterranean deep-sea biology* - page 8 / 32





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Plio-Quaternary pelagic and hemipelagic sediments drape the entire margin, recording the wide effects of the post-Messinian sea level rise (Rehault et al., 1985). This explanation of the Messinian salinity crisis led to one of greatest controversies (Cita, 1991) in the geological community. The signifi- cance of Messinian events can be judged from the estimate that as much as 6% of the ocean’s salt was transformed into giant deposits left on the Mediter- ranean sea floor by evaporation (MacKenzie, 1999). Krijgsman et al. (1999) show that the onset of the Messinian salinity crisis was synchronous over the entire Mediterranean basin, and date it at 5.96 mil- lion years ago. Isolation from the Atlantic Ocean was established between 5.59 and 5.33 million years ago, causing a large fall in the Mediterranean water level followed by erosion (5.59-5.50 million years ago) and deposition (5.50-5.33 million years ago) of non-marine sediments in a large ‘Lago Mare’ (Lake Sea) basin.

The water column of the present Mediterranean is characterised by oxygenated conditions above the sea-floor, as is indicated by the yellowish- brown bioturbated surface sediments in most regions (Stanley, 1985). However, the alternating sequences of light and dark sediments in Plio-Qua- ternary cores indicate that this condition was dif- ferent in the past. The dark, organic rich muds are known as sapropels, which are common in eastern Mediterranean sections, if the percentage of organ- ic carbon is greater than 2%, and as sapropelic sed- iments or organic-rich layers, which are found in some western Mediterranean sites, if the organic carbon content is between 0.5 and 2.0%. These petrologic changes could to record successive oxic-anoxic events. Two main models have been put forward to explain the origin of sapropels (Martínez-Ruíz et al., 2003): (1) the ‘stagnation model’, involving external physical processes (temperature, evaporation, circulation) that caused intense vertical density gradients, resulting in sta- ble stratification, reduced ventilation of deep water and anoxia, thus enhancing organic matter preser- vation; and (2) the ‘productivity model’, involving increased export production and rapid supply of organic matter to the sediment, which caused great remineralisation, leading to increased organic mat- ter burial and low oxygen in deep waters. A com- bination of the two models, bottom water anoxia and increased productivity, has also been suggest- ed. Local geographical settings and circulation pat- terns may, however, have induced various respons-

14 F. SARDÀ et al.

es to regional climate changes in different Mediter- ranean basins over time. This is supported by sig- nificant differences in the sedimentary records of the eastern and western Mediterranean sapropel event (Martínez-Ruíz et al., 2003).



The topographic characteristics of the Mediter- ranean basin force one to make a separation between the western and eastern Mediterranean basins, which are divided by the relatively shallow Strait of Sicily. The larger seas contained in the western Mediterranean Sea are the Alborán Sea, the Alger- ian, the Tyrrhenian , the Liguro-Provençal and the Catalan or Balearic (Fig. 1).

The 200 m depth isobath is commonly used in the western Mediterranean to separate the continen- tal shelf from the continental slope. The width of the continental shelf is quite variable, sometimes being particularly wide, as occurs in the Gulf of Lions (Fig. 1). In these wide and relatively shallow regions, sea breezes, strong northern winds and storms can greatly influence the flow, in contrast to waters beneath the 200 m isobath. The continental slope separates the continental shelf from the deep sea; it is a relatively narrow zone in which the change from 200 m down to 2000 m depth occurs within a few tens of kilometers.

The main water masses that can be found in the western Mediterranean basin are the Atlantic Water (AW), the Levantine Intermediate Water (LIW), the Tyrrhenian Deep Water (TDW), and the Western Mediterranean Deep Water (WMDW). Surface waters and intermediate waters have their origin in waters coming from the Atlantic and from the east- ern Mediterranean respectively. Thus, the surface dynamics in the Mediterranean Sea relies on the exchange of water masses through the Strait of Gibraltar: low dense Atlantic waters intrude as sur- face waters in the Mediterranean, while denser Mediterranean waters travel beneath in an opposite direction into the Atlantic ocean. Furthermore, an exchange of intermediate waters takes place in the Strait of Sicily. Due to topographic constraints, deep water masses have only a local origin: the formation of TDW is due to mixing, caused by the intrusion of LIW into the Tyrrhenian Sea by cascading from 200

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