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



Arctic and a migration of numerous representatives of the boreal fauna into 

 the Barents Sea have been observed during recent decades. It may be assumed 

 that similar climatic ameliorations occurred in former times, when warm 

 periods alternated with the cold ones. In the post-glacial epoch the highest 

 rise of temperature occurred during the Littorina stage, that is 5,000 years ago. 

 This considerable climatic amelioration (a rise in temperature of a few degrees) 

 left a definite trace in the eastern part of the Barents Sea in the form of resi- 

 dual warm-water forms, which penetrated into the Barents Sea and the White 

 Sea. Part of them are still living in the White Sea ; in the eastern part of the 

 Barents Sea they have died out. However, the shells of such molluscs as 

 Cardium edule, C. echinatum, Mactra elliptica, Nucella lapillus, Neptunea 

 despecta, Buccinum undatum and others, which no longer inhabit these parts, 

 are still found in a sub-fossil state there. 



By the end of the Tertiary period the bottom of the Barents Sea was 400 to 

 500 m higher than it is now, and the whole Sea was dry land. The Atlantic 

 waters penetrated into the Barents Sea at the beginning of the Ice Age. During 

 that period the bottom of the Barents Sea underwent a number of sinkings and 

 risings; numerous coastal terraces, some now below sea-level (70, 100, 180, 

 220 m), others considerably above it (up to 400 m), bear witness to these 

 changes. The Barents Sea at times grew shallow and dried up in considerable 

 areas, at times it became much deeper than it is at present. During the period 

 of the greatest glaciation (Riss stage) the Barents Sea was about 200 m 

 shallower than it is now and was blocked with ice. At that time the submarine 

 ridges between Scotland, Ireland and Greenland were near the surface and the 

 Atlantic waters could scarcely penetrate into the Arctic basin ; this must have 

 affected its climate considerably, causing a sharp drop of temperature. In the 

 opinion of some scientists this alone was sufficient to bring about a glacial 

 period. The lowering of Fenno-Scandia, which occurred at the height of the 

 Ice Age and which opened the Arctic basin to the Atlantic waters, caused the 

 melting of the ice. The boreal transition probably conditioned the mass pene- 

 tration of warm-water fauna into the Arctic basin and the Barents Sea ; how- 

 ever, it was soon exterminated by the arrival of a new glaciation phase (Wurm 

 stage). The coldest phase of the post-glacial period, which lasted for 20,000 

 years for the water bodies surrounding Fenno-Scandia, was the Yoldian 

 stage with Yoldia (Portlandia) arctica as its predominant form ; this latter 

 is still found in the coldest sections of the Barents Sea and elsewhere in 

 the Arctic. Several breeds of molluscs originating from Yoldia arctica 

 (C. Mosevitch, 1928) inhabit river estuaries and have maintained their relict 

 character, although they do not seem to prefer a cold-water environment. 



The alternation of colder and warmer phases in the course of the post- 

 glacial epoch resulted in one of the most characteristic features of the fauna 

 of the Barents and White Seas. This is a combination of cold- and warm- 

 water relicts which is frequently encountered even within small habitat areas. 

 The White Sea fauna displays this most clearly. 



The post-glacial cUmatic changes of the Arctic basin are due not only to the 

 fluctuations of its sea-level and its temperature. As has been mentioned above, 

 the changes of salinity must have been just as pronounced. 



