H 3 4j

glaciers on mountain summits of the Parmirs, Tien Shan, Sayany, etc.

G Lacierisation on Chukotka and on Arctic islands

Ice cover over Arctic basin

Greenland ice sheet

Antarctic ice sheet 14 to 7 10* yr ago

changes but also the particular combinations of regional natural conditions. The main among them are regional uplift as a result of tectonic movement and the increase in solid precipitation. Given the limited precipitation during the periods of cooling, very deep ground freezing took place.

In the northern half of the former USSR the freezing began first of all on high peaks of mountain ridges with the formation of glaciers and then within the high elevation mountains of the middle and southern latitudes. It has been established using palynologic data, that as early as in the Eocene within the mountains of north-eastern Asia the dark-coniferous taiga (which is now typical of the northern zone of unfrozen ground) was displaced by larch forests with shrubs and mosses; mountain tundras appeared on the mountain summits for the first time. In the early Pliocene the north of the continent was occupied by coniferous forests undemanding of the soil-climatic conditions, with swampy areas, and at that time the permafrost had already appeared on the plains of the North-East of Russia. In the places where the continental accumulation of fine-grained sediments prevailed, the traces of cryogenesis have survived to the present.

The most ancient indications of the syncryogenic ground freezing, observed in the form of ground wedges and pseudomorphs of ice wedges, have been found in the Russian North-East in the Kolyma river basin (the Begunov suite and the lower bench of Oler suite) and on the Krestovka river (Kutuyakh suite) the sediments of which have been assigned by A.V. Sher, T.N. Kaplina, A.A. Arkhangelov to the Pliocene (2.4-1.9 million years ago). We can observe from one to four layers of pseudomorphs in these suites of sediments - associated by Kaplina with cooling-warming cycles and the consequent permafrost aggradations and degradations. The sizes of the pseudomorphs of ice wedges suggest what we might call severe conditions during the formation of syncryogenic lacustrine-alluvial sediments, with the ice wedges on the north of Chukotka being assigned by Arkhangelov to the Eopleistocene.

In Early Pleistocene such traces of perennial freezing have been studied in Western Siberia at latitude 60° N in alluvial and lacustrine deposits of the Irtysh river valley and in Central Yakutiya. The upper terraces of the

Fig. 14.5. (opposite) Diagram of the development of permafrost in the former USSR, in Late Cenozoic: 1 - curve for the climatic fluctuations (coolings and warmings); 2 - accumulation of syncryogenic deposits with (a) large and (b) small ice wedges; 3 - formation of (a) pseudomorphs of ice wedges and of (b) hillocky terrain as a result of ice thawing out; 4 - (a) increase of seasonal thawing layer and (b) regional development of thermokarst processes; 5 - regional development of (a) peatlands and of (b) swamps; 6 - time of existence of glaciers and ice sheets.

Curve The Cenozoic Oxygen Isotope
Fig. 14.6. Diagram of mean annual temperature fluctuations in the Cenozoic in Central Europe (after A.S. Monin and Yu.A. Shishkov).
Curve The Cenozoic Oxygen Isotope
Fig. 14.7. Profile of oxygen 5180 isotope ratio in an ice core from Camp Century - Greenland ice sheet (after A.S. Monin and V.A. Shishkov).

Lena river and its tributaries consists of deposits with ice wedges of up to 10-20 m and more in thickness, which can be attributed to syngenetic freezing in the early Pleistocene, according to T.P. Kuznetsova. Studies in the Russian North-East of pseudomorphs of ice wedges, indicate their syncryogenesis in the period of accumulation of the Pliocene-Early Pleistocene deposits of the Oler and Khroma suites. In the Oler suite, for example, an increase of size of the pseudomorphs of ice wedges and a decrease of distance between them has been revealed in the vertical section, pointing to continuous lowering of the ground temperatures and to increase of the amplitude of temperature fluctuations on the ground surface during the period of ice wedge growth.

The cold continental climate over the territory of the Siberian Platform at that time contributed to epicryogenic ground freezing over great areas.

Cyclic climatic fluctuations with relatively insignificant warming periods throughout the Quaternary period did not interrupt this pattern, because during most of the Pleistocene the climate was colder than at present.

The present permafrost thicknesses exceed 500-700 m in the Russian North-East and in Zabaykal'ye and 1000-1500 m on the Siberian Platform, and these depths lead us to assume that in these regions the epicryogenic frozen strata underwent freezing from the Early Pleistocene and, locally, possibly beginning from the Late Pleistocene. As the degradation periods associated with climatic warming were not so significant and long as the periods of cooling (Fig. 14.5), the thawed ground refroze entirely during the succeeding aggradation period. When the tendency towards increase of duration of the cooling periods and towards decrease of the warming periods took place, the degradation process was damped out and the aggradation process was amplified with time, as well as northward (and with height in mountains). The intensity of these processes varied over the region depending on the specific features of the natural surroundings. Therefore in the north of Siberia there was the possibility of the ground remaining frozen from the Early Pleistocene but mainly in epicryogenic ice-rich soils and bedrock. The syncryogenic deposits always produced relief during their formation and thawed in the warming epochs. They were reformed in the thermokarst during epigenesis with further formation of pseudomorphs in ice wedges.

As a whole the data presented here point to severe geocryological conditions throughout a stage when the ground temperature was not more than — 3°C and the southern limit of permafrost was situated at latitude 60° N and just south of it including Taimyr and the northern and middle part of the Middle Siberian Upland (Fig. 14.8).

The second stage (from 0.9-0.73 to 0.15-0.11 million years ago) refers to the Middle Pleistocene (Qn), which became the key stage in the permafrost formation process. During this stage the aggradation-degradation cycles took place mainly on a background of negative ground temperatures. Therefore the development of the cryogenic strata in the north of Siberia in the Middle Pleistocene has been continued practically without interruption up to the present time. In T.N. Kaplina's opinion the syncryogenic deposits formed in the Middle Pleistocene on the northern plains have persisted locally, to the present.

The geological (and geocryological) age of the deposits of this stage was determined in the lower course of the Kolyma, by dating of fossil rodent bones by A.V. Sher, T.N. Kaplina and others, from layers in the section of the syncryogenic strata that have persisted until the present. On the north-

Fig. 14.8. Possible permafrost distribution limits in Cenozoic: 1 - in Neogene-Early Pleistocene; 2 - in Middle Pleistocene; 3 - during the Holocene climatic optimum; 4 - during the Holocene climatic optimum and in Late Holocene (at the depth of 70-200 m from the surface); 5 - at the present time.

Fig. 14.8. Possible permafrost distribution limits in Cenozoic: 1 - in Neogene-Early Pleistocene; 2 - in Middle Pleistocene; 3 - during the Holocene climatic optimum; 4 - during the Holocene climatic optimum and in Late Holocene (at the depth of 70-200 m from the surface); 5 - at the present time.

ern plains of Yakutiya in all the studied sections of such age, ice-rich strata with thick ice veins occurring at various depths have been found.

Analysis of the permafrost traces in sediments of the Middle Pleistocene in other regions show that the permafrost was developed in this period not only within the present limits but also significantly south of them (see Fig. 14.8). Within the European plain the pseudomorphs of ice wedges and ground veins are reported by A.A. Velichko, A.B. Bogutskiy and others to latitude 50° N. In Western Siberia and Northern Kazakstan such traces have been observed to latitudes 49-50° N, and in the south of Central Siberia and in Zabaykal'ye, to the boundary of the former USSR. In T.N. Kaplina's opinion such wide permafrost distribution in the Middle Pleistocene points to much more severe geocryological conditions at that time compared to the present. Only late in the period and for up to forty thousand years (Kazantsev interglacial period) was the climate close to the present one. On the general background of these severe geocryological conditions in the Middle Pleistocene, periods of warming (for example, Tobol and Messov-Shirtin time in Western Siberia) are recognized by palaeogeographers and geocryologists. In N.S. Danilova's opinion the ex pansion of northern taiga and forest-tundra northward and thermokarst development with accumulation of lacustrine and bog deposits, are to be associated with those periods.

The third stage of the permafrost development is the Late Pleistocene (QnI), from 150-110 to 11-10 thousand years ago. This stage is characterized by sharp and strong climatic cooling (see Fig. 14.5). Wide development of surface ice sheets and permafrost over the whole region of the former USSR is associated with this cooling. This period has been termed in literature the Glacial period. The upper Pleistocene deposits in the North and North-East of the country are relief-forming and have been studied in a great number of key sections, with dating of fossil vegetation and bones and by 14C. They have inclusions of thick ice wedges, 50-80 m in vertical extent and of width 3-6 m. In the Northern Hemisphere the Late Pleistocene is divided into four periods according to the degree of climatic severity.

The beginning of the Late Pleistocene, Kazantsev time (Q1,,,) with a duration of about 30 thousand years, is described by V.V. Baulin and others as the time of the Kazantsev shallow sea transgression covering the vast territories of Western Siberia (according to G. I. Lazukov up to latitude 65° N). The Salekhard marine plain in Yamal and Gydan remained un-flooded. Syncryogenic littoral-marine and alluvial deposits of Kazantcevsky age containing syngenetic ice wedges have been described by G.I. Dubikov. Pseudomorphs of ice wedges occurring in latitudes more than 3-4° further south than the regions of present-day ice wedge formation, are found in deposits from the beginning of the Late Pleistocene in Western and Central Siberia. This points to more severe climatic conditions than the present.

The Zyryan (60-50 thousand years ago according to N.V. Kind) and Sartan (27-15 thousand years ago) periods are the coldest in this stage, with the ice sheets being at a maximum and the perennial ground freezing taking place under a temperature of 5-10°C below that of the present. Therefore it is thought that the Zyryan-Kargin-Sartan period (Q2in — Q4m) lasting about 70 thousand years is the period of the maximum buildup of permafrost thickness and its maximum extension southward over most of the former USSR. Traces of the severely perennially frozen ground (pseudomorphs of ice wedges, hillocky terrain, cryoturbations, etc.) were followed by A.A. Velichko over the vast area of the Russian plain extending to 48-49° N. In Western Siberia such traces have been found at 52° N. On the south of Taz peninsula at the latitude of the Arctic Circle, the vertical dimension of the pseudomorphs in such deposits reaches 12 m with width up to 2 m, with the underlying sand deposits carrying some evidence of the previous syngenesis. Under present conditions only irregular frost fractur ing takes place in peat terrain at this latitude. North of the Arctic Circle the Upper Pleistocene deposits of the III - I alluvial terraces have syngenetic sand and ice wedges which have persisted from the time of their accumulation.

In the north of Central Siberia, within the northern plains and in the Meso-Cenozoic depressions of Eastern Siberia, the accumulation of syn-cryogenic ice-saturated horizons continued during that period, with wedge ice of great size (up to 80 m and more) - the 'ice' complex (see §9.2) which still exists in relics of the 'yedoma' suite which form relief. Typical of this time in the North-East of Russia, was a sharp decrease of forest area and its replacement with a particular kind of tundra-steppe vegetation having no analogue at the present time. In mountain regions the mountain tundra and deserts were widespread. Under the severe conditions of the Sartan period (27-15 thousand years ago) as a result of marine transgression, the freezing of the shelf and cryopeg formation took place. According to T.N. Kaplina's estimation the mean annual temperatures possibly reached — 25 °C in that time, i.e. the temperatures were 10°C lower than at present. This extensive region of the country was occupied at that time by continuous permafrost of great thickness, 100-200 m greater than at present, containing thick syngenetic ice wedges in the Middle-Late Pleistocene deposits and epigenetic ice wedges in the deposits formed earlier - with minimal development of thermokarst and taliks below water courses, as well as of frost weathering in bed rock and eolian processes on sandy areas of the drained shelf and tundra-steppes. The long persistence and remarkable severity of freezing conditions led to ice wedges of great thickness, having no analogues previously, which are observed up to the present in the Upper Pleistocene deposits within-the northern plains.

The occurrence of the 'second' buried permafrost layer separated from the surface by thawed ground (see Fig. 14.8) in Western Siberia and on the European North-East of Russia points to the great thickness of frozen ground in the Late Pleistocene. In Western Siberia the base of the relict layer of permafrost has been found at the present time at a depth of 300-400 m in latitude 59-60° N, i.e. to the south of the Ob river's latitudinal course and south of the present occurrence of permafrost islands. The existence of the Barents Sea and Kara ice covers and deep ground freezing in Central and Eastern Siberia where the absence of great glaciations, due to the deficit of atmospheric moisture and precipitation, caused the formation of a thick cryogenic stratum, are considered to be associated with that time. By the end of the Late Pleistocene in the central part of the Siberian Platform this thick cryogenic stratum had reached the maximum thickness (of 1000-

1600 m) on the Earth as far as platform conditions are concerned and it is represented by a thick negative temperature layer with salt waters and brines overlain by the permafrost layer with ice in fissures. Severe geo-cryological conditions extended over most of the former USSR territory at the end of Late Pleistocene and the southern limit of permafrost was situated at 47-49° N on the south-west excluding only the Black Sea coast and plains of Central Asia, and beyond the boundary of the former USSR to the southeast (see Fig. 14.8). After the Sartan minimum the global climatic warming is apparent, most intensively in middle latitudes of the European part of the former USSR and in Western Siberia.

The fourth stage of the permafrost development is the Holocene one (QIV). The climatic optimum Q2IV) taking place from 10-9 to 4.5-3 thousand years ago, is shown very well within the whole territory of the former USSR and a great amount of palaeogeographical and palaeocryolithological evidence shows this was the main event at this stage. On the Russian plain the ice sheets disappeared, in the Russian North-East, in Zabaykal'ye and in other permafrost regions, the areas of mountain-valley glaciers decreased, the soil-vegetation zones were displaced northward, etc. The thawed layer occurring above the layer of Pleistocene permafrost on the European northeast and in Western Siberia is one of the important relics of the permafrost degradation in the Holocene. Temperature curves with low gradient or without any gradient in cryogenic strata in Western Siberia and in other regions are evidence for the geocryological conditions becoming milder in the Holocene optimum.

The permafrost distribution by the end of the climatic optimum was such that (see Fig. 14.8) permafrost 200 m in thickness, lay roughly along the present southern limit of the permafrost. The estimations by V.V. Baulin and others show that thawing from the surface of bedrock at a temperature higher than — 3°C, could involve 100-250m and that some part of this (approximately 100 m) could become frozen again in the period of the Late Holocene cooling. By and large it can be considered that the ground temperature was 3-4°C higher during the climatic optimum than at the present.

In Eastern Siberia the period of the Holocene climatic optimum is likely to have been 9.5-8 thousand years ago. Regional development of ther-mokarst and formation of lacustrine-thermokarst plains and swamp deposits took place within the northern plains composed of ice-saturated ground in that period.

The Late Holocene (Q4IV) was marked universally by cooling and by the aggradational development of the permafrost zone as a whole (see Figs. 14.5

and 14.8) reflected in its present state. Thus the permafrost up to 50-100 m and more in thickness of Late Holocene cryogenic age was formed in thawed ground in the southern part of the permafrost zone; the layers thawing in the optimum were partly or completely frozen again; the mean annual permafrost temperature became 1-2 °C lower; the southern limit of permafrost was displaced to the south by a few degrees of latitude; the ice wedges of 1.5-2 m began to be formed and to grow within the peat lands of the European North of Russia and in Western Siberia. Within the present southern permafrost zone perennial and seasonal ground heaving is actively taking place with hillocky peatlands being formed. Within the northern permafrost zone new fairly thin syncryogenic deposits are being formed on river and lake flood-plains; frost fracturing is active on flat ground; icing formation is developing widely as a result of river and stream beds freezing up; 'golets' ice is being formed in rock streams and in coarse block deposits.

Thus the permafrost strata show various cryogenic ages (times of onset of the perennial freezing) across the section through the territory of the former USSR. Thus it was reported by N.S. Danilova that within the northern part of the permafrost zone the younger strata (as far as the cryogenic age is concerned) are situated near the permafrost base. These are the frozen rocks of the Late Pleistocene cryogenic age, because the maximum cold pulse is associated with that period of geological development of the Earth's permafrost zone in the Cenozoic. Under the alternating conditions of sedimentation and freezing (in the southern geocryological zone, in the regions of sea transgressions and ice sheets etc.) complex strata (as far as their cryogenic age is concerned) are being formed. In syncryogenic deposits the lower part of a section is always of more ancient cryogenic age than the upper one. However the cryogenic age of the permafrost base in the epicryogenic part can be the same as in the syncryogenic.

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