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Physical Geography of Northern Eurasia

Climatic Change and the Development of Landscapes

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General Physiography of Northern Eurasia in the Cenozoic

When analysing the evolution of landscapes and climates of such a vast region, physiographic features which gradually developed during the meso-Cenozoic interval (termed 'geomorphic stage' by Gerasimov et al., 1974) require consideration.

At the beginning of the Cenozoic (see Table 1.1 for geochronology), about 66 per cent of Northern Eurasia, including the European territory, Western Siberia, the Turanian lowland, and the Central Siberian plateau, was a platform on consolidated basement and its planated surface was covered with weathering crust. Subsequently, the platform experienced epeirogenic movements resulting in transgressions of epicontinental seas. Other areas were montane regions of different ages, including young mountains, such as the Caucasus, the mountains of Central Asia and Kamchatka, and rejuvenated (epiplatform) mountains.

The Paleogene (65-24.6 million years [Ma] BP) featured predominantly weak tectonic activity across most of the continent (as indicated by the widespread epicontinental seas and planation surfaces on adjacent land) and a warm climate (Sinitsyn, 1980a, b). However, sea basins diminished during the late Eocene and Oligocene as a result of general (although intermittent) uplift of the continent and probably a global decline in ocean level caused by the onset of glaciation. Consequently, by the end of the Oligocene most of the East European plain had emerged as a land mass.

Throughout the Oligocene, a vast inland lake-sea basin in Western Siberia became detached from the withdrawing Paratethys. The Turanian lowland and plains of Kazakhstan, submerged in the middle Oligocene, began to rise at the end of the period. The most intensive uplift occurred in southern Central Asia, where large volumes of clastic materials accumulated at the base of the growing mountains. The Mesozoic fold belts of the north-eastern regions of Northern Eurasia experienced planation, while volcanic activity continued in the south and in the east.

Throughout the Paleogene, the climate of the plains was uniform and hyperzonality (a phenomenon referring to the expansion of a particular vegetation zone) was characteristic of the region. Forests dominated most of the continent. However, at the end of the period, climatic cooling brought about gradual changes in forest composition. Tropical and subtropical species declined and the development of latitudinal zonality began. Most of the first-order morphological units developed during the following Neogene stage (24.6-1.6 Ma BP).

In the Miocene, the uplift of the East European plain continued and topography became inreasingly dissected. All major uplands and lowlands (the Srednerusskaya (Central Russian) and Privolzhskaya uplands, the Donetsk Ridge and the Oka-Don lowland) were formed. Under the warm, wet conditions, surface runoff was abundant and intensive erosion developed. In southern regions, a sequence of transgressions and regressions occurred although the general trend was that of regression. However, a single marine basin existed in the place of the modern Black and Caspian Seas even during the regressions when the sea retreated to the southernmost part of the plain. During the transgressions, the Caspian Sea spread northwards and eastwards, sometimes merging with the Aral Sea.

In Western Siberia, flat topography and a humid climate favoured the accumulation of fine sediments in valleys and large lakes since the Oligocene. The area of sedimentation was shrinking gradually and retreated south-eastwards in the late Miocene. The Kazakh shield, as well as the eastern and south-eastern regions of the West Siberian platform, experienced an uplift during the Neogene, particularly in the mountainous regions of Southern Siberia, and were subject to differentiated erosion. In the extreme north-east of Northern Eurasia, the Miocene was characterized by weak tectonic activity and planation except for the Koryak upland and Kamchatka which experienced folding and active volcanism (Rezanov and Naimark, 1982; Olyunin, 1982).

The cooling trend persisted throughout the Miocene, although climatic warming occurred at least twice which resulted in the formation of a complicated zonal structure and the development of new biomes. Most importantly, steppes formed in southern Western Siberia and Kazakhstan in response to increasing aridity. Their development proceeded most intensively at the time of the Messinian crisis in the Mediterranean.

The Pliocene was marked by active orogenesis in the southern and eastern regions of Northern Eurasia. Locally, in Transcaucasia and Kamchatka, it was accompanied by intensive volcanism. The topography of the southern periphery of the East European plain became more dissected as a result of the rapid uplift of the mountains (particularly in the middle Pliocene) and tectonic subsidence in the southern Caspian basin. These processes also contributed to fluvial dissection in the Caucasus (Dumitrashko, 1974). Similar processes occurred in the south of the Turanian platform as indicated by the intensive deposition of coarse material throughout the Pliocene. On plains, the drainage network acquired a pattern similar to that of today. Thus, by the end of the Pliocene, the principal physiographic units of Northern Eurasia, which are recognized today, had formed.

In parallel with the large-scale restructuring of topography, climate changed gradually with warmer and colder phases alternating against the background of progressive cooling. One of the most pronounced warm phases was the Pliocene climatic optimum (approximately 4.2-4.0 Ma BP). In Central Asia and on the plains of Southern Siberia, aridity increased in response to the mountain-building, which isolated these regions from the influence of the Indian Ocean. Of great importance was a period of strong cooling which occurred between 2.7 and 2.1 Ma BP. Since that time, negative winter temperatures became typical over most of the plains. The first ice sheet may have spread across Scandinavia and valley glaciers may have developed in the mountains of the north-east region of Northern Eurasia at that time. Sediments dated to that interval reveal distinct evidence of permafrost occurrence (the Kutuyakh suite of north-eastern Siberia). A zonal structure similar to that of the present developed.

The Quaternary is the latest and the shortest period (1.6 Ma, according to the current chronostratigraphic schemes). It is subdivided into two units: Pleistocene (1.6-0.01 Ma BP) and Holocene (the latest 10.3 thousand [Ka]). In Russian-language paleogeographic literature the Quaternary is often subdivided into three units of different duration: Eopleistocene (1.6-0.7 Ma BP), Pleistocene (0.7 Ma BP - 10.3 Ka BP), and Holocene (the latest 10.3 Ka). During this period, as in the previous stages, landscape evolution was controlled by climatic change. What distinguishes the Quaternary from the previous epochs is that climatic cooling intensified. The temperature of each subsequent cold phase was lower than that of the preceding one and temperatures during the warm phases also declined progressively. Consequently, glaciations and the formation of permafrost, which originally developed in the Neogene, peaked during the Quaternary. In contrast to the previous stages, when climatic change resulted in the shift of zonal boundaries, cold and arid phases of the Quaternary featured hyperzonality (open landscapes with the significant participation of xerophytes in plant communities became widespread). A characteristic product of cold and arid environments with scarce vegetation was loess. In the Quaternary sequences, loess horizons alternate with fossil soils (Figure 2.1). The presence of the latter signifies episodes of warmer climate and richer vegetation. The loess-soil series may be correlated with tills farther north and marine terraces in the south, thus providing a reliable chronostratigraphy for the Quaternary landscapes.

Fig. 2.1 Principal paleogeographic components of the Quaternary on the East European plain. Compiled by A. Velichko

These natural processes and phenomena of the past strongly influenced various aspects of the environment. The vast ice sheets altered topography, which was also affected by glacioisostasy. Drainage systems were restructured due to ice damming and melt water. Vast areas of the continental shelves were exposed due to the glacioeustatic fluctuations of sea level (Alekseev, 1992). Some marginal seas temporarily lost their connection with the world ocean while inner water bodies experienced fluctuations in water level and area (Chepalyga, 1984).

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