The Black Sea an unusual geography

The Black Sea is the largest inland located sea which has still regular communication with the world oceans.

This communication takes place through the narrow Bosphorous strait which leads to the Mediterranean Sea which in turn connects it with the Atlantic Ocean.

It has a more or less oval shape. Its longitudinal axis is about 1,150km long, and its latitudinal axis is 611km. The sea surface has an area of 423,000 sq km. The average depth is 1,300m, and the greatest depth is 2,245m. The volume of the Black Sea is 547,000 cu km.

Geological history. The Black Sea is considered to be a residual basin of the Central European Tethys Sea, which existed from the end of the Paleozoic era to the Middle Tertiary period. As a result of alpine orogenic movements in the Triassic period, the Tethys Sea became divided into a series of basins. One of them was Pontus, stretching from Moldavia in the west to the Aral Sea in the east. At the end of the Tertiary period this basin separated from the Caspian Sea and shrank to the dimensions of the present Black Sea.

During the Ice Age of the Pleistocene epoch, the level of the Black Sea rose, and the sea was connected to the Mediterranean and Caspian seas several times. But in the postglacial period the Black Sea contracted, became a freshwater sea, and its water level fell below that of the ocean. The Strait of Bosporus was formed 6,000-8,000 years ago. The salt water of the Mediterranean Sea entered the Black Sea and filled its basin. It destroyed the freshwater fauna of the Black Sea, which came to be replaced by the saltwater fauna of the Mediterranean Sea. The sea level rose, and the salt water flooded the lower parts of the river valleys, creating limans.

The continental shelf is the outer, coastal ring, less than 200m in depth. It slopes gently away from the coast and is covered with terrigenous deposits from mountains. It covers about one-quarter of the sea's area and is widest in the northwest and at the Kerch Strait.

The continental slope is the ring separating the continental shelf and the basin's core. The incline of the slope usually varies between 5° and 6°. At a depth of about 2,000m the incline of the slope suddenly decreases, and the slope merges with the basin bottom. The surface of the slope is usually very uneven and is covered with a layer of sticky mud, black on top and light gray underneath. The color is due to ferrous sulfide, which is deposited in the form of very fine grains or thin needles.

There are scarcely any islands in Dnipro-Boh Estuary, and Zmiinyi Island, near the delta of the Danube River, are tiny in size.

Climate. The climate of the Black Sea is predominantly continental. Only the southern coast of the Crimea and the northwestern coast of Caucasia are protected by mountains from the cold north winds and are visited by the mild winds of the Black Sea. Hence, they have a mild Mediterranean climate. The Atlantic Ocean has a great influence on the climate of the Black Sea. In winter northeast winds usually prevail over the sea, lowering the temperature and causing frequent storms. On the northeastern coast, near Novorossiisk, cold and violent mountain winds develop. The temperature of the northern part of the Black Sea averages about -3°C in January, but it occasionally falls as low as -30°C. The coastal waters freeze up for a month or more, while the shallow bays, river mouths, and limans freeze up for two to three months. During this season the southern coast of the Crimea and the sheltered eastern coast enjoy a temperature of 6-8°C, well above the freezing point. The average July temperature in the north is 22-23°C and, in the south, 24°C. The northwestern coast has the lowest annual precipitation (300 mm), and the Caucasian coast has the highest (up to 1,500 mm).

Sea water. 

The average volume of water in the Black Sea is 547,000 cu km. The volume changes depending on the following factors: precipitation, which is usually 230,000 cu km; the inflow from the continent - 310,000 cu km; the flow from the Sea of Azov - 25,000 cu km; evaporation loss - 357,000 cu km; and outflow through the Strait of Bosporus - 208,000 cu km.

Because of the Black Sea's relatively young geological age and the huge inflow of fresh water from the rive

rs and semifresh water from the Sea of Azov, the salinity of the Black Sea is almost 50 percent lower than the salinity of the ocean. It is lowest in the northwest (13 parts per thousand) and increases to 17-18 parts per thousand elsewhere. The salinity varies also: it is lowest at the surface and increases with depth to about 23 parts per thousand at 730 m, beyond which it remains almost constant.

By temperature the waters of the Black Sea are divided into two unequal layers: the upper layer, which extends to a depth of about 500 m, and the lower layer, from 500m to the bottom. The lower layer maintains an almost constant temperature of 9°C. In winter the temperature of the upper layer increases with depth, and in summer it decreases with depth. The vertical temperature changes are uneven. At the surface the temperature is lowest in February: from -0.5°C in the north, where the coastal waters freeze 10–20km offshore, to 5–8°C in the south. The waters reach their highest temperature in August—from 20°C in the north to 25°C or more in the south. In the Sea of Azov the waters reach 26–27°C, and even 30°C in the limans.

The water density depends on salinity and temperature. It is lowest at the mouths of rivers and the Kerch Strait—1.010–1.014 g/cc on the average—and increases towards the open sea and downward—it is 1.017 g/cc on the average at a depth of 150 m and almost constant below that depth.

Oxygen is the most important gas in the seawater for it supports organic life. In the upper layer it reaches 4–7 cc per liter. The quantity of oxygen decreases rapidly with depth: at 200 m only 0.5 cc is found in a liter of water. This is not sufficient to sustain life, and hence only a few bacteria (Microspira aestuarii) live at this depth. Another obstacle to life below 200 m is the high concentration of hydrogen sulfide, which increases to 6–7 cc per liter at a depth of 2,000 m.

The transparency of the water in the Black Sea is 16–22 m of depth on the average. It is lowest near the shore (2–3 m) and highest at the middle of the sea (20–27 m).

Water movements. The surface of the Black Sea is usually quiet, but waves arise in windy weather, particularly in wintertime. During storms the waves can reach a height of 15 m or more and present a threat to small vessels. The tide is almost imperceptible, for it reaches scarcely 10 cm in height.

There are two types of sea currents in the Black Sea: the surface currents, caused by the cyclonic pattern of the winds, and the double currents in the Bosporus Strait and Kerch Strait, caused by the exchange of waters with adjacent seas. The surface currents form two closed circles. The width of the western circle, opposite the Danube Delta, reaches 100 km and decreases towards the south. The velocity of the current is about 0.5 km per hour. The width of the eastern circle varies between 50 and 100 km, and the velocity is 1 km per hour.

The double current of the Strait of Bosporus consists of the exchange of waters between the Black Sea and the Sea of Marmara. The less saline and lighter waters of the Black Sea flow as a surface current into the Sea of Marmara at a velocity of 1–2 m per second. In exchange the more saline and denser waters of the Sea of Marmara flow at a depth of 50–120 m into the Black Sea at a rate of 4–6 m per second. In the Kerch Strait the surface current flows from the Sea of Azov to the Black Sea at 1–2 m per second, while the lower current flows in the opposite direction at a depth of about 5 m.

Besides horizontal currents there are also vertical currents, which are limited to the surface waters down to 200 m.

Flora and fauna. The most peculiar feature of the Black Sea is the absence of marine life at depths beyond 150–200 m, except for a few anaerobic bacteria. Living organisms are concentrated in the shallow waters of the continental shelf and river mouths along the northwestern coast.

The plant world consists of phytoplankton (350 unicellular species occur), bottom macrophytes (280 species), several grass species, and sea herbs. The Phillophora rubens herb, which covers over 15,000 sq km of the sea bottom and is harvested in quantities of over five million tonnes, has commercial value. The Zostera sea grass, which is consumed by fish, grows on the silt and sand bottoms of quiet bays.

The number of animal species in the Black Sea is small in comparison to the number in the Mediterranean Sea: 350 species of simple creatures, 650 species of crustaceans, over 200 species of molluscs, about 180 species of fish, and 4 species of mammals (the monk seal and 3 species of dolphin). The mammals and fish are of commercial value. The fishing industry brings in rich catches of sturgeon, mackerel, sardine, herring, anchovy, gray mullet, and other fish.

Reference:

http://www.encyclopediaofukraine.com/display.asp?linkpath=pages%5CB%5CL%5CBlackSea.htm