Fig. 1 – Geomorphologic zonation of the Black Sea (after Panin & E. and G. Ion, 1997)

Legend; 1, continental shelf; 2, continental slope; 3, basin apron: 3 a - deep sea fan complexes; 3 b - lower apron; 4, deep sea (abyssal) plain; 5, paleo-channels on the continental shelf filled up with Holocene and recent fine grained sediments; 6, main submarine valleys - canyons; 7, paleo-cliffs near the shelf break; 8, fracture zones expressed in the bottom morphology.

One of the most prominent physiographic features is the very large shallow (less than 200 m deep) continental shelf within the north-western Black Sea (about 25 % of the total area of the sea).

The Black Sea is located within a complex of high folded mountain chains of the Alpine system, which is represented by the Balkanides-Pontides belt to the south and by Caucasus , Crimea and the North Dobrogea Mountains to the north, north-east and the north-west respectively. Only in the north-west, there are low-standing plateaux and the Danube delta lowland.

Geologists consider the Black Sea a back-arc marginal extensional basin, which originated from the northward subduction of the Neo-Tethys along the southern margin of the Eurasian plate under a Cretaceous-Early Tertiary volcanic arc, as a result of the northward movement of the Arabic plate (fig.2).

Fig.2 – Tectonic sketch of the Black Sea Region (after Dinu et al., 2003; Panin et al., 1994) Legend: 1, Orogene overthrust front; 2, Gravitational faults of the rift; 3, Major strike-slip faults; 4, Major faults; 5, Limits of depressions and/or ridges; 6, Zone without granitic crust; 7, Thinned crust. Explanation of abbreviations : I. Platform regions: East European, Scytian, Moesian: II. Orogenic regions: North Dobrogea Orogene, Greater Caucasus, South Crimea Orogene – SCO, Balkanides, Western and Eastern Pontides ; III. Depressions and ridges: PDD – Pre-Dobrogean Depression; NKLD – North Kilia Depression; KD – Karkinit Depression; HD – Histria Depression; SD – Sorokin Depression; KTD – Kerci-Taman Depression; NKD – Nijne-Kamchiisk Depression; BD – Burgas Depression; ATD – Adjaro-Trialet Depression; TB – Tuapse Basin; SSR – Suvorov-Snake Island Ridge; KR – Krymskyi Ridge; AR – Azov Ridge; GR – Bubkin Ridge; IV. WBS – Western Black Sea ; V. EBS – Eastern Black Sea ;

Large-scale sea level changes and consequently drastic reshaping of land morphology, large accumulation of sediments in the deep part of the sea and modifications of environmental settings occurred all along the Black Sea geologic history. The Quaternary was especially characterised by very spectacular changes, which have been driven by the global glaciations and de-glaciations.

During these changes the Black Sea level behaviour was influenced by the restricted connection with the Mediterranean Sea through the Bosphorous – Dardanelles straits. When the general sea level lowered below the Bosphorous sill, the further variations of the Black Sea level followed specific regional conditions, without being necessarily coupled to the ocean level changes. One of the main consequences of the lowstands was the interruption of the Mediterranean water into the Black Sea , which became an almost freshwater giant lake.

The main glacial periods of the Quaternary in Europe corresponded to the regressive phases of the Black Sea , with lowstands of the water level down to –120 m. As mentioned above, the regressions represent phases of isolation of the Black Sea from the Mediterranean Sea and the World Ocean . Only the connection with the Caspian Sea could sometimes continue through Manytch valley. Correspondingly, during regressions, under fresh water conditions, the particularities of fauna assemblages had a pronounced Caspian character. On the contrary, during the inter-glacials, the water level rose to levels close to the present level; the Black Sea was reconnected to the Mediterranean sea , and the environmental conditions as well as the fauna characteristics underwent marine Mediterranean influences.

or example, during the Karangatian phase (since 125 ka BP to ~ 65 ka BP) of the Black Sea , which corresponds to the warm interglacial period (fig.3), the water level exceeded the present-day level by 8 to 12 m . The saline Mediterranean water penetrated through the Bosphorous, and the Black Sea became saline, covering the lowlands in the coastal zone.

Fig.3 – Paleo-geographic reconstruction of the Black Sea during the Karangatian phase (Riss-Wurmian or Mikulinian interglacial) (After Tchepalyga, 2002)

The last Upper Wurmian glaciation (Late Valdai, Ostashkovian) corresponds to the very low-stand phase of the Black Sea, when the shoreline moved far away from the present-day position, especially in the north-western part of the Black Sea, and large areas of the continental shelf were exposed (fig.4).

Fig. 4 – Palaeo-geographic reconstruction of the Black Sea during the Neoeuxinian phase ( Upper Wurmian ) (After Tchepalyga, 2002)

At the beginning of the Holocene, some 9-7.5 ka BP, when the Mediterranean and the Black Seas have reached the same level (close to the present day one), the two-way water exchange was established, and the process of transformation of the Black Sea in an anoxic brackish sea started.

In the late 1990s, a new hypothesis was formulated by Ryan, Pitmann et al. (1997). The authors consider that, when the de-glaciation started during a short episode, the level of the Black Sea was high enough, and the fresh Pontic water flowed towards the Aegean Sea . At about 12 k.yr BP, the retreat of the ice-sheet front determined the reorienting, for a limited period of time, of the melt-water supply towards the North Sea . The Black Sea, without the income of the ice-melting water during the Younger Drias cooling (~11 ka BP) until 9 ka BP, under more arid and windy climate, experienced a new lowering of the level (down to -156 m). At the same time, the Mediterranean Sea continued to rise, reaching by 7.5 K.yr BP the high of the Bosphorous sill, and generating a massive input of salt water into the Black Sea basin. The flux was several hundred times greater than the world's largest waterfall, and it caused a rise of the level of the Black Sea , some 30 to 60 cm per day topping up the basin in few years time.

This new hypothesis is still under debate: numerous data from the straits of Bosphorous and Dardanelles , Marmara and Aegean seas and the Danube Delta do not support the new Ryan's hypothesis. These data indicates that the “classical” scenario of Black Sea water outflow is rather credible. There are also some hydraulic incompatibilities for accepting a catastrophic flooding event in the Black Sea , as well as a different time scale for reaching the present day salinity of the Black Sea waters.

The length of the Black Sea shoreline is about 4,100 Km. The Black Sea coastal zone can be divided in 17 main zones characterised by different geology and morphology, as well as specific littoral water circulation, sediment drift systems and sedimentary budget. In a more general approach the Black Sea coast zone could be subdivided into three main morphodynamic categories, with very specific characteristics and behaviour: (1) Low, accumulative coasts mostly related to the main rivers mouth zones; (2) Erosive coasts within low standing plateaux and plains, with active cliffs in loess and loess-like deposits sometimes with very narrow beaches in front of the cliffs; (3) Mountainous coasts, with cliffs, certain number of marine terraces, land slides, sometimes with sandy or gravely beaches.

The low, accumulative coasts (the first category described above) are the most influenced by the global changes, specifically by the sea level changes and by the changes in the river sediment inputs. The decreasing of sediment supply and changes in littoral sediment drift due to anthropic activities (river damming, hydrotechnical regularisation, littoral structures etc.), especially when the sandy beaches are low, added to the rising of the sea level and the increasing of littoral sea energy could determine in certain conditions a very active and almost continuous recession of the beach line up to 20-30 m/y, as it happens in some sections within the Danube Delta littoral (fig.5). This process is causing land losses, environmental changes and economic degradation of the coastal zone. If the region represents the coastal zone of an important delta, which plays essential role in the normal structuring, and functioning of ecosystems, any changes of delta/sea interaction zone environments could be fatal and irreparable.