THE BLACK SEA 399 



Sea reflects the chart of water circulation : in the off-shore sand zone, shell 

 gravel and shallow- water muds are preponderant ; in the area of the currents 

 crossing the Sea in the direction of the Crimea and of the northern part of the 

 Caucasian coast the bottom becomes more coarse-grained; the halistatic 

 areas have the softest bottom. The amount of the fine fraction increases with 

 depth (up to 96-5 per cent) ; so does the amount of organic matter, and the 



Fig. 191. Distribution of contemporary deposits of Black Sea (Archangelsky and 

 Strahov, 1938, with the addition of Phyllophora beds). 1 Sand ; 2 Shell gravel ; 3 Mus- 

 sel ground ; 4 Phaseolin mud ; 5 Grey deep-sea clay ; 6 Grey clay with calcareous mud 

 interlays ; 7 Transitory mud ; 8 Same with grey mud interlays ; 9 Same with grey 

 mud and sand interlays; 10 Same with several grey mud interlays; 11 Calcareous 

 mud ; 12 Calcareous mud with grey clay interlays ; 13 Site free of contemporary 

 deposits; 14 Phyllophora beds. 



increase of its carbonate content with depth is, perhaps, the most character- 

 istic feature of the Black Sea. The mean values of these changes are given in 

 Table 159. 



The remarkable fact that the content of organic carbon in the present-day 

 Black Sea sediments of the hydrogen sulphide zone is practically the same, 

 down to the greatest depths, as that in 'normal' water basins was recorded 

 by N. Strahov (1941). This can be explained by the energetic decomposition 

 processes of organic residues and the return of the decomposition products 

 into the water column. A considerable amount of calcium carbonate in the 

 shallow- water muds is due to the presence of shell gravel. The calcium carbon- 

 ate of the deep-water oozes of the Black Sea is also of organic origin, but both 

 in its structure (a fine powder) and in the mode of its formation it differs from 

 that of the oxidation zone. It is mainly the product of the vital activities of the 

 desulphating and denitrifying bacteria which take part in the reduction of 

 sulphates (with the formation of hydrogen sulphide) and nitrates. 



