88 BIOLOGY OF THE SEAS OF THE U.S.S.R. 



The brown colour of the surface layer of the sandy silt and silt is a well- 

 known and interesting phenomenon characteristic of the floor of various 

 oceans and seas, and widespread in the Barents Sea. 



The northern part of the Barents Sea and the trough south to Novaya 

 Zemlya have brown mud bottoms. The floor of the whole White Sea depres- 

 sion, most of the Kara Sea and that of the Arctic basin are covered with 



Fig. 28. Distribution of soils in the floor of the Barents Sea (Vinogradova). 

 1 Sand ; 2 Silty sand ; 3 Sandy silt ; 4 Mud ; 5 Clay-silt ; 6 Clay ; 7 Limit of 



brown soils. 



brown mud. A brown tint of the often very thin surface layer of the sea-bed is 

 commonly found on different kinds of bottom, even on sand. This brown 

 colour is due to the presence of ferric and manganic hydroxides ; and its pre- 

 sence leads to the suggestion that the bottom layer contains sufficient oxygen 

 for their oxidation. However, brown mud beds are undoubtedly situated 

 mainly either in depressions or within the regions of unfavourable aeration and 

 of considerable accumulation of free carbon dioxide in the bottom layer ; 

 brown mud is not formed under conditions specially favourable for aeration. 

 M. Klenova (1938, 1940) suggests that the brown colour of this soil may dis- 

 appear as a result of a plentiful benthos population which would create a 



