GENERAL CHARACTERISTICS OF FAR EASTERN SEAS 687 



conclusion that these Seas were formerly dry land, either partly or even wholly ; 

 and that even during the Quaternary Period, when the single common river 

 systems did exist, the level of the Ocean underwent a considerable change (up 

 to 500 m). This worker suggests that during the Quaternary Period the Far 

 Eastern Seas underwent alteration of the phases of regression and trans- 

 gression no less than three times. In addition to such fluctuations of the sea- 

 level G. Lindberg also maintains that the formation of the Seas of Japan and 

 of Okhotsk and the Bering Sea trench was due to downwarping. He casts 

 doubts upon the permanent or even more or less prolonged existence of the 

 trenches and of the Pacific Ocean itself and of ' the existence in comparatively 

 recent times of a continental link joining the Islands of Melanesia, Micronesia 

 and Polynesia to the Hawaiian Islands and likewise to southeast Asia' (1948). 



According to the latest opinion of Soviet geologists (P. Kropotkin, 1956, 

 I. Andreeva and G. Udintzev, 1958) the trench in the Sea of Japan is very 

 ancient (lower Palaeozoic). In its structure it closely resembles other trenches 

 on the western edge of the Pacific, and the bed of the Ocean ; it should there- 

 fore be regarded as a relict of this bed. 



Bottom deposits of 1-5 km thick were found in the southern part of the Sea 

 of Japan by seismo-acoustic methods. Associated with this, many geologists 

 assume a raising of the edges of the Sea of Japan at the end of the Pliocene 

 Period, until the Sea was completely separated from the Pacific (P. Kropotkin, 

 1954, 1956). 



The history of the existence of links between the Bering Sea and the Arctic 

 Ocean is equally obscure. The solutions of all these problems are most 

 important for the understanding of the history of the fauna and in particular 

 of such phenomena as amphi-boreal distribution. 



The analysis of long cores from the sea-bed and the examination of their 

 content of the remains of diatomaceous Radiolaria, Foraminifera, spores 

 and plant pollen are exceptionally valuable for the understanding of the 

 palaeo-geographical past of the Far Eastern Seas and of the palaeo-climatic 

 changes. 



T. Sechkina (1959) has analysed a 17 m long core obtained from the Vityaz 

 in 1957 from a depth of 3,504 m in the northern part of the trench in the Sea 

 of Japan, approximately on the latitude of the Strait of Sangara. The quanti- 

 tative and qualitative compositions of the diatoms were found to alter con- 

 siderably with the length of the core. Sechkina divided the core according to 

 its diatom content into five horizons (0 to 140 cm, 140 to 280 cm, 280 to 

 590 cm, 590 to 1,033 cm and 1,033 to 1,706 cm). The uppermost horizon 

 resembles the contemporary one in the composition of its diatoms ; the second 

 one differs from it greatly, reflecting a considerable decrease of temperature. 

 In contrast, the diatoms of the third horizon bear witness to a considerable 

 rise of temperature and there is in it a pronounced admixture of tropical 

 diatoms, while the Arctic ones are absent. The upper four metres of the fourth 

 horizon are, as it were, 'dumb', containing no diatoms. There is a thin (23 cm) 

 layer of cold-water Arctic flora of diatoms under it (the 'dumb' column cor- 

 responds to the beginning of a great fall in temperature). The 'dumb' layer 

 probably corresponds to the period of the greatest fall in temperature, to a 



