230 



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



Fig. 97. Kara Sea sea-bed soils. 1 Clay and mud ; 2 Silt ; 3 Sand-mud ; 4 Silty 



sand ; 5 Sand ; 6 No data ; 7 Ferromanganate concretion ; 8 Rock ; 9 Gravel ; 



10 Limit of distribution of underlying clay (Gorshkova). 



amount, and sometimes complete absence, of bicarbonates in the brown 

 mud. 



Life is always scarce in the brown mud, but it is not clear why this is so. 

 It may be caused by a lack of free oxygen (taken up from the deep-water layer 

 by manganese compounds), by an accumulation of carbon dioxide, or, as 



