Sea of Azov - Geology and Bathymetry

Geology and Bathymetry

The International Hydrographic Organization defines the limit of the Sea of Azov in the Kertch Strait as "The limit of the Black Sea", which is itself defined as "A line joining Cape Takil and Cape Panaghia (45°02'N)".

The sea is considered an internal sea of Russia and Ukraine, and its use is governed by an agreement between these countries ratified in 2003. The sea is 360 kilometres (220 mi) long and 180 kilometres (110 mi) wide and has an area of 39,000 square kilometres (15,000 sq mi); it is the smallest sea within the countries of the former Soviet Union. The main rivers flowing into it are the Don and Kuban; they ensure that the waters of the sea have comparatively low salinity and are almost fresh in places, and also bring in huge volumes of silt and sand. Accumulation of sand and shells results in a smooth and low coastline, as well as in numerous spits and sandbanks.

The Sea of Azov is the shallowest sea in the world with an average depth of 7 metres (23 ft) and maximum depth of 14 metres (46 ft); in the bays, where silt has built up, the average depth is about 1 metre (3 ft). The sea bottom is also relatively flat with the depth gradually increasing from the coast to the centre. The Sea of Azov is an internal sea with the passage to the Atlantic Ocean going through the Black, Marmara, Aegean and Mediterranean seas. It is connected to the Black Sea by the Strait of Kerch, which at its narrowest has a width of 4 kilometres (2.5 mi) and a maximum depth of 15 metres (49 ft). The narrowness of the Kerch Strait limits the water exchange with the Black Sea. As a result, the salinity of the Sea of Azov is low; in the open sea it is 10–12 psu, about one third of the salinity of the oceans; it is even lower (2–7 psu) in the Taganrog Bay at the northeast end of the Sea. The long-term variations of salinity are within a few psu and are mostly caused by the changes in air humidity and precipitation.

Although more than 20 rivers flow into the sea, mostly from the north; two of them, the Don and Kuban rivers, account for more than 90% of water income. The contribution of the Don is about twice that of the Kuban. The Kuban delta is located at the southeast, on the east side of the Kerch Strait. It is over 100 km long and covers a vast flooded area with numerous channels. Because of the spread, the delta has low contrast in satellite images, and is hardly visible in the map. The Don flows from the north into the large Taganrog Bay. The depth there varies between 2 and 9 metres, while the maximum depth is observed in the middle of the sea.

Typical values of the annual inflow and outflow of water to the sea, averaged over the period from 1923 to 1985, are as follows: river inflow 38.6 km3, precipitation 15.5 km3, evaporation 34.6 km3, inflow from the Black Sea 36–38 km3, outflow 53–55 km3. Thus, about 17 km3 of fresh water is outflowing from the Azov Sea to the Black Sea. The depth of Azov Sea is decreasing, mostly due to the river-induced deposits. Whereas the past hydrological expeditions recorded depths up to of 16 metres, more recent ones could not find places deeper than 13.5–14 metres. This might explain the variation in the maximum depths among different sources. The water level fluctuates by some 20 cm over the year due to the snow melts in spring.

The Taman Peninsula has about 25 mud volcanoes, most of which are active. Their eruptions are usually quiet, spilling out mud, and such gases as methane, carbon dioxide and hydrogen sulfide, but are sometimes violent and resemble regular volcanic eruptions. Some of those volcanoes are under water, near the shores of the peninsula. A major eruption on 6 September 1799, near stanitsa Golubitskaya, lasted about 2 hours and formed a mud island 100 metres in diameter and 2 metres in height; the island was then washed away by the sea. Similar eruptions occurred in 1862, 1906, 1924, 1950 and 1952.

The current vertical profile of the Sea of Azov exhibits oxygenated surface waters and anoxic bottom waters, with the anoxic waters forming in a layer 0.5 to 4 meters (1.6–13 ft) in thickness. The occurrence of the anoxic layer is attributed to seasonal eutrophication events associated with increased sedimentary input from the Don and Kuban Rivers. This sedimentary input stimulates biotic activity in the surface layers, in which organisms photosynthesise under aerobic conditions. Once the organisms expire, the dead organic matter sinks to the bottom of the sea where bacteria and microorganisms, using all available oxygen, consume the organic matter, leading to anoxic conditions. Studies have shown that in the Sea of Azov, the exact vertical structure is dependent on wind strength and sea surface temperature, but typically a 'stagnation zone' lies between the oxic and anoxic layers.

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