8 

 General Characteristics and Geological History 



I. GENERAL CHARACTERISTICS 



The Black, Azov, Caspian and Aral Seas, and to some extent the Mediter- 

 ranean and even the Red Sea, for all the differences in their physical geo- 

 graphy, have a number of important features in common. All these Seas 

 possess a salinity of their own, different from that of the ocean ; this was parti- 

 cularly so in the historical past, when at times it exceeded the normal salinity 

 of the ocean in areas with a negative balance of fresh-water inflow (through- 

 out the Mediterranean and Red Seas, in many gulfs, inlets and the Sivash of 

 the Black, Azov and Caspian Seas). At times it decreased below that of the 

 ocean (the Black, Azov, Caspian and Aral Seas). 



Equally characteristic of all these bodies of water, which are isolated from 

 the open ocean, is the temperature of their deep layers ; excluding the Azov 

 and Aral Seas, this temperature is high in comparison with the open ocean 

 —about 9° in the Black Sea, 5° to 6° in the Caspian, 13-5° to 13-7° in the 

 Mediterranean and 21-5° in the Red Sea. Their temperatures correspond, to 

 some extent, to the lower average temperature of their upper layers in winter. 



These common features of the system of seas from the Black Sea to the 

 Aral Sea are chiefly due to their common origin, which is linked with the geo- 

 logical past of the so-called South Russian geosyncline. This, it is assumed, 

 constitutes a remnant of the ancient Tethys geosyncline, which underwent a 

 complex process of the isolation of sea-basins during almost the whole Neo- 

 genic Period. 



A considerably lower salinity (10 to 22-5% ), as compared with the normal 

 marine salinity, and a significant difference between the surface and deep- 

 layer salinities, are also very characteristic of the South Russian bodies of 

 water. The marked saline stratification is accentuated by an abrupt tempera- 

 ture stratification which appears in the warm season of the year, when surface 

 water is at times warmed to 27° to 30°. In winter, on the other hand, the sur- 

 face layer of water becomes very much cooled, and a larger or smaller ice- 

 cover is formed. Saline, and sometimes temperature, stratification causes the 

 formation of hydrogen sulphide on the bottom, when, either at certain seasons 

 or throughout the year, deep waters in the more or less thick layers are con- 

 taminated. A. Archangelsky (1938) thinks that the contamination of the 

 Black Sea with hydrogen sulphide is not peculiar to its present phase, but is a 

 characteristic phenomenon common to all the bodies of water of the South 

 Russian geosyncline of the Neogene system. 



Lastly, the historical basis of the fauna of the Southern Seas of the u.s.s.r. 

 is a peculiar relict fauna which is itself, in the final analysis, a remnant of the 

 Tethys fauna (Sarmatian, Pontic and Caspian fauna) formed by a complicated 

 succession of lower and higher salinity phases. To this fauna are added in 

 greater or lesser numbers immigrants from fresh waters and far-travelled 



z 353 



