Sochi Club

 Physical features

 Physical features II

 Economic aspects

 Black Sea benefits as economies slip into the red

 Study and exploration

 Evidence found of Noah's ark flood victims

 Underwater Archaeology of the Black Sea

 Photo Gallery (Black Sea)

The Black Sea contains only a few small islands, the largest being Zmiyini (Fidonisi) east of the Danube delta and Berezan at the mouth of the Dniester River estuary. The submarine relief may be visualized as a series of concentric and occasionally asymmetrical rings. Beyond the shoreline a shallow shelf zone occupies about a quarter of the entire area. It is broadest in the west and at the head of Kerch Strait but elsewhere forms a rim about 6 to 7 miles wide, and the depth of the edge is usually less than 330 to 360 feet. The shelf gives way at its edge to a slope, which is broken by submarine valleys and is steep in its upper parts. Between the port cities of Sinop and Samsun (Turkey), the coastline is paralleled by a rugged range of underwater mountains extending for nearly 100 miles. The hollow forming the basin's core covers about a third of the total area and is a completely featureless flat plain, with depths increasing evenly toward the centre to a little more than 7,200 feet, with the axis of maximum depth displaced toward the Turkish coast.

Geology

Underlying rocks reflect a regional diversity of both type and age. The ancient Precambrian rocks of the southern tip of the structural block known as the Russian (or East European) Platform, dating from at least 570 million years ago, appear in the northwest. The associated Skifsky Platform has a deep cover of sedimentary rocks that were laid later. The deepwater depression, generally considered to be a geosyncline (or vast downwarp), has unique significance in the structure of the Earth's crust. The centre of the depression consists of sedimentary and basaltic crustal layers, respectively, with a granite layer thrust between them at the periphery. Seafloor deposits generally change from coarse pebbles and gravel at the periphery to fine silts at the centre of the basin.

The geologic history of the Black Sea is not fully known, but it clearly seems to be a residual basin of the ancient Tethys Sea, dating from about 250 to between 60 and 40 million years ago. The present form of the sea probably emerged at the end of the Paleocene Epoch (about 57.8 million years ago), when structural upheavals in Anatolia split off the Caspian basin from the Mediterranean. The newly created Black Sea basin became gradually isolated from the ocean, and its salinity was reduced; at that time the Crimea and the Caucasus probably were islands.

Early in the Miocene Epoch (about 23.7 million years ago), the Black Sea flowed into a chain of sea lakes but gradually became separated from the Caspian region. As mountains—the Pontic, Caucasus, Crimean, and Carpathians—rose around it, outwashed sediments filled the basin. Further earth movements and changes in sea level associated with Pleistocene glaciers then occurred and led to intermittent connections with the Mediterranean. During the last of the great glaciations, the freshwater Lake Novoevskinsky was formed; and 6,000 to 8,000 years ago the present connection with the Mediterranean Sea was made. Strong earthquakes—such as the Crimean earthquake of 1927—remain associated with the area.

Physical features

Climate

The climate of the landlocked Black Sea can be characterized generally as continental(i.e., subject to pronounced seasonal temperature variations), although climatic conditions in some parts of the basin are controlled to a great extent by the shoreline relief. A steppe climate, with cold winters and hot, dry summers, is found in the northwestern part of the basin exposed to the influence of air masses from the north. The southeastern portion of the sea, sheltered by high mountains, experiences a humid subtropical climate, with abundant precipitation, warm winters, and humid summers. In winter, spurs of the Siberian anticyclone (a clear, dry, high-pressure air mass) create a strong current of cold air, and the northwestern Black Sea cools down considerably, with regular ice formation. The winter invasion of polar continental air (which prevails for an average of 185 days annually) is accompanied by strong northeasterly winds, a rapid temperature drop, and frequent precipitation, with the air becoming warm and moist after passing over the milder eastern portions of the sea. Tropical air from the Mediterranean regions (87 days affected on average) is always warm and moist. Occasionally, winds from the Atlantic via eastern Europe bring rain and sharp squalls.

The average January air temperature over the central portion of the sea is about 46° F (8° C) and decreases to between 36° and 37° F (2° and 3° C) to the west. Spring air temperature everywhere approaches 61° F (16° C), rising to about 75° F (24° C) in the summer. Absolute minimums approach -22° F (-30° C) during the winter cold spells in the northwest, while in the Crimea the maximum may reach 99° F (37° C) in summer. Winds are strongest everywhere in the winter, with the cruel northeasterlies reaching hurricane force in the coastal region of Novorossiysk, just to the east of the Kerch Strait, and gale force on the sea itself.

Hydrology

The temperature of the Black Sea's upper layer has a marked yearly periodicity. In winter, water temperature ranges from 31° F (-0.5° C) in the northwest to about 48° to 50° F (9° to 10° C) in the southeast. The winter cooling forms an upper mixed layer extending to depths of about 160 to 330 feet, with temperatures at the lower boundary of about 44° to 46° F (6.5° to 8° C). In summer the surface layer is warmed to between 73° and 79° F (23° and 26° C). At depths of about 160 to 240 feet, a cold layer remains at 45° F (7° C), and lower depths do not change from their winter levels.

The salinity of the surface waters in the open sea averages between 17 and 18 parts per thousand, which is approximately half that of the oceans. A marked increase in salinity, up to 21 parts per thousand, occurs at depths of 160 to 500 feet, below which the salinity increase is much more gradual. The sea's deepest parts (below 1,300 feet) are distinguished by highly stable temperatures between 47° and 48° F (8.5° and 9° C) and salinities of 28 to 30 parts per thousand. Salinity increases to 38 parts per thousand at the Bosporus, where waters from the Sea of Marmara intrude. The chemical composition of Black Sea water is almost the same as that of the oceans.

A most important feature of the Black Sea is that oxygen is dissolved (and a rich sea life is made possible) only in the upper water levels. Below a depth of about 230 to 330 feet at the centre and 330 to 500 feet near the edge, there is no oxygen; in those reaches the sea is contaminated by hydrogen sulfide, which results in a saturated, gloomy, “dead” zone frequented only by adapted bacteria.

Currents in the Black Sea are wind-generated, with the main current running counterclockwise, its branches forming gyres (eddies) and sometimes large closed rotations. The current is relatively slow on the surface in the open sea, but near shore it reaches 16 to 20 inches (40 to 60 centimetres) per second; its speed is a mere inch or so per second in the depths. Flows in the Bosporus are complex, with surface Black Sea water going out and deep, saltier water coming in from the Sea of Marmara. Surface winds are an important complicating factor, especially in the shallow sill, or threshold, between the two basins. This situation also holds for flows to and from the Sea of Azov through the Kerch Strait. Water exchange through the Bosporus is relatively slow, and a complete recycling of Black Sea waters takes about 2,500 years.

The overall water balance of the sea results from a combination of the factors of precipitation, inflow from the continental mass and the Sea of Azov, surface evaporation, and exit through the Bosporus. The annual water level, therefore, varies slightly according to factors influencing any one or more of these components. Tides are virtually nonexistent, their range being exceeded by the foot or so variation induced by seiches (the changes in water level resulting from rapid movements of atmospheric pressure).

Vertical intermixing of water, except at or near the wind-whipped surface, is limited because of the compact, and hence stratified, nature of the sea. It has been estimated that hundreds of years are required to bring water in a cycle from depth to surface, although there is some limited bottom turbulence caused by the warmth of the Earth's crust and by chemical reactions in the seabed.

Marine life

Flora and fauna in the Black Sea derive from the Mediterranean, the result of a series of invasions from that area, and Caspian elements dominate in freshwater estuaries and river mouths. It has a rich biological productivity in some zones.

All the main groups of microorganisms, which in total biomass are one and a half times larger than that of the groups of phytoplankton and zooplankton combined, are found in the sea. Most of them occur in a thin surface layer, with a few anaerobic bacteria in the hydrogen sulfide zone (which otherwise is lifeless). The tiny phytoplankton number some 750 species; compared to numbers in the Mediterranean, however, the zooplankton are poorer, with but 80 species, including jellyfish. In coastal areas are found eggs and larvae of invertebrates and fish. The diffusion of sea-bottom (benthic) plants and animals is four to five times poorer than in the Mediterranean, again because of the effects of the hydrogen sulfide layer. In the shallow northwest section, there was a notable extensive field of the water plant known as phillophora, but since the 1960s this field has been drastically reduced by deterioration of the water quality.

There are about 180 species of fish, a fifth of them of commercial importance. The most important are khamsa, sprat, horse mackerel, and others, including the small Black Sea shark. Some seasonal migration of fish occurs, notably through the Bosporus.