The evolution of natural conditions in the Late Cenozoic caused by global cooling of the Earth's climate led to the formation of the permafrost in the north, north-east and within high-elevation mountain systems on the south of the former USSR which continues to the present day. The history of the cryogenic development of the north-eastern part of Eurasia in the Late Cenozoic recorded in stratigraphy has not been studied uniformly and adequately as a whole. The initial period of the study of geocryological history is associated with investigations by A.I. Popov in the Taymyr Peninsula, P.A. Shumskiy, B.N. Dostovalov in Central Yakutiya, and V.N.Saks, V.V. Baulin, N.S. Danilova, G.I. Lazukov in Western Siberia. Ye.M. Katasonov, N.N. Romanovskiy, Yu.A. Lavrushin, B.I. Vtyurin, T.P. Kuznetsova, T.N. Kaplina, A.V. Sher, A.A. Arkhangelov, S.V. Tomirdiaro and other researchers have made a great contribution in the study of the geocryological history of the Russian North-East, as have and A.A. Velichko, S.E. Sukhodolskiy, I.D. Danilov, N.G. Oberman and others in the study of European Russia.
Reliability and comprehensibility of the palaeogeocryological reconstructions depend on completeness of the geocryological sections and on the study of the palaeogeographical and palaeoclimatological surroundings correlated with these sections. Sections revealing Cenozoic rock units, formed during a large time interval and suitable for visual investigation and sampling, are the key to reconstructing the history of the regional geocryological development.
In this case a method for studying the permafrost 'footprints' buried in layers of the Neogene and Quaternary deposits is central when studying the history. Originally formed ground veins (ground wedges, pseudomorphs of ice veins, parallel-layered ground wedges), ground involutions (cryoturba-tions), ice bodies of different shapes and sizes, various relationships between the lithological bedding and the layers with high iron content, peat inclu sions and ice streaks, shear and displacement of the layers observed in the exposures and in drill cores at the present time, constitute these 'footprints'. Such 'footprints' are of great variety and when occurring in situ give much information on the types of sedimentation and freezing.
This method of perception of the history of the permafrost is based on the principle of actualism, allowing the drawing of parallels between the present natural conditions and the cryogenic phenomena existing now and the related buried ancient forms. The analysis of such present-day forms gives us some idea of the conditions existing during the time of their formation. The essential features for the analysis are the occurrence of ground layers and ice streaks in the profile, their succession and the character of transformations of one layer into another.
Methods of Quaternary geology play an important role and thus the study of fossil animal bones and vegetation remains and inclusions (fragments of roots, branches and trunks, inclusions of lignites, coals, etc.) is also important in confirming the stratigraphy and origins of the frozen layers.
Palynologie data obtained as a result of spore and pollen analysis of samples from the sections under study allow one to estimate the climatic conditions for vegetation during the accumulation of the various horizons. Analysis of diatoms allows assessment of the moisture conditions and origin (marine or continental) of sediments.
Determination of the cryogenic age and of the type of freezing is possible only when studying the syncryogenic strata, the cryogenic and geological ages being the same. In this case the cryogenic features should testify to the absence of thawing during the warm periods, after freezing, i.e. these features should be the primary features. Otherwise these 'foot prints' will show the effects of repeated freezing and, consequently, the epigenetic cryogenic origin of the stratum. In the past two or three decades the 14C dating of organic inclusions as well as palaeotemperature reconstructions based on the isotope ratio 180/160 have played a great role in reconstructing the geological history in the Late Cenozoic.
The history of geocryological development (because of incomplete study in different regions of the permafrost zone) is not unambiguous and very often the same materials are treated differently by researchers. Weaknesses in the absolute dating and palaeotemperature methods, as well as ambiguity in the interpretation of palynospectra and of the other kinds of inclusions in deposits showing evidence of syngenesis or of subsequent thawing and new freezing, may be a factor. During different periods of study this situation caused underestimation as well as overestimation of the cryogenic age of the same layers and of the whole section, making it difficult to correlate palaeogeocryological materials in different regions. It must be added that because of the great rate of disintegration of ice-saturated materials in the exposures that are 'key' for the syncryogenic sections it is not always possible to repeat and to improve their description and sampling. For the epicryogenic strata the main evidence for the long duration of the permafrost and of the main stages of its development is the cryogenic structure across the section and the thickness of the whole permafrost layer, i.e. the depth of the 0°C isotherm. We can see the manifestation of cooling and warming periods in the epicryogenic bedrock in the formation of the cryogenic disintegration zones near the permafrost base and by the expansion of primary fissures in the ground as a result of water repeatedly freezing in them. The groundwater pressure deficit appearing as a result of incomplete filling by water of fissures in the thawing ground indicates thawing taking place at the base of the permafrost. We can recognize the refreezing of thawed frozen ground by the incomplete filling of fissures by ice. The distribution of ice content and the thickness of ice streaks proceeding downwards give much information about the conditions of freezing in loose epicryogenic materials. The combination of the features as a whole allows firm reconstruction of the history of the freezing of the stratum.
Nonuniformity of the evidence of permafrost over the territory and the ambiguity of the interpretation of the conditions of accumulation and of freezing of the cryogenic strata, as well as a weak understanding of the current regional geocryological environment, have led different scientists to separate and describe different stages and numbers of stages of geocryological development, giving each of them their own age range and assessment of the cryogenic role in the formation of the present features of the permafrost zone. The present description has been made in accordance with the four stages of geological and geocryological development of the Eurasian continent in the Cenozoic, distinguished by T.N. Kaplina, as well as with the four stages (with a detailed breakdown of the Holocene) distinguished by V.V. Baulin, N.S. Danilova, A.A. Velichko, etc.
At the beginning of the Cenozoic the Antarctic continent occupied its south-pole position and because of reduced solar radiation began to cool from the surface. The circumpolar oceanic currents sweeping the Antarctic continent were closed off with time and no longer picked up the heat from middle and equatorial latitudes, with a consequent sharp cooling of the atmosphere, lowering of the snow line and glacier formation on the elevated areas of the continent. The subsequent growing and joining of glaciers in the Middle and Late Miocene produced the great ice sheet of Antarctica and associated with this the lowering of world ocean levels by 55 m and more at the boundary between the Miocene and Pliocene. An increase in icebergs began to contribute to the world ocean cooling and caused the formation of the permafrost zone of the Earth's Northern Hemisphere in the Pliocene.
It should be remembered that along with the great land mass of the former USSR from west to east and with the transfer of Atlantic air masses dominating in the Cenozoic the cold periods were longer in the eastern part of the country and shorter in the western part. This spatial pattern was compensated by the latitudinal zonation of the solar radiation complicated by the altitudinal zonation in mountain regions. Therefore the longest cold periods existed in the north-east, the shortest ones in the south-west of the country. This tendency has persisted and caused the great variations in the duration or time intervals of the natural conditions during the stages of cryogenic development described (Fig. 14.5).
The first stage of geocryological development refers to the Pliocene-Early Pleistocene (N3 - Q,) and is the longest one (from 2.4-1.9 to 0.9-0.73 million years ago). The general cooling of the Earth's Northern Hemisphere climate is linked to the Early Pliocene (Fig. 14.6) when the polar basin glaciation, which has existed more or less uninterruptedly for the past 3 million years, began to develop because of the change in the warm Atlantic current and of the global cooling of the oceans. Thus, according to Ch. Emiliany's data, the bottom water temperature in subpolar regions of the Earth was the following on the background of a general cooling trend: in Middle Oligocene (30-35 million years ago) it was + 10.4°C; in Early and Middle Miocene (10-20 million years ago) it was + 7°C; in the Late Pliocene (3 million years ago) it was +2.2°C. At the boundary between Miocene and Pliocene the ocean level fell by a few hundred metres on the north-east of the Asian continent, and as a result of the Polar basin regression the whole Arctic shelf became a continent from which perennial freezing of the ground began, expanding southward. The great regression had a global character and was associated with the Antarctic glaciation as well as with the tectonic uplift of continents and the tectonic subsidence of the ocean bottoms.
The data on the oxygen <5180 isotope ratio in ice columns from Antarctica and Greenland reported by A.S. Monin and Yu.A. Shishkov (Fig. 14.7) allow us to follow the stages of the Earth's climatic cooling in the Cenozoic and the stages of the accumulation of thick ice sheets as well as the associated ground freezing. It has been determined with the help of K-Ar dating that the Greenland ice sheet, with the ice thickness as much as 3 km and more during the glacial maxima, was formed about 3 million years ago in the Pliocene.
The initial reasons for glaciations were not only the global climatic
within the present permafrost limits, possibly further south within the shelf of northern seas, on the coastal lowlands and on mountain summits of the former USSR North-East and south the highest summits of mountains; within plains: seasonal ground freezing
Permafrost distribution within area_
within the present
displacement of the southern Limit northward up to latitude 50° North within the Russian plain and to the south of the former USSR boundary in Siberia
Distribution of glaciers and ice sheets within the present permafrost limits, possibly further south on the northern lowlands and in depressions, in northern and central parts of Middle Siberia; in the mountain regions north-east and south in the former USSR, and in Zaba^Xal within the shelf of northern seas, on the coastal lowlands and on mountain summits of the former USSR North-East and south the highest summits of mountains; within plains: seasonal ground freezing a u u m XI C "I
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