Even in the most severe climatic conditions the spatial distribution of permafrost is not universally continuous. Within the permafrost zone the frozen ground can be absent within many sections of river valleys and watersheds, on south-facing slopes, under lakes, at sites of concentrated discharge of groundwater or of its recharge by seepage, volcanic craters, calderas, under some of the present glaciers and above the interior-Earth thermal anomalies associated with oxidation reactions, etc.
Within the region of distribution of continuous permafrost, unfrozen ground occupying small areas and existing continuously for more than a year is termed a talik. When the area of unfrozen ground appears to be comparable with or larger than the area of frozen ground the unfrozen ground is termed a talik zone or massif of unfrozen ground. According to the relation of taliks and talik zones to the surrounding frozen ground, they can be divided into open (penetrating through the whole frozen stratum) and closed (sometimes they are termed 'pseudotaliks'), penetrating into the frozen stratum to some depth and underlain by permafrost. Often thawed and unfrozen layers, lenses, channels and bodies of other form, bordered at the top, bottom and sides by permafrost are found in geological sections of the permafrost zone. Such formations are termed inter- and intrapermafrost taliks in the literature.
Strictly speaking, in addition to taliks proper (frozen earlier, but thawed now), the so-called original taliks, represented by ground not frozen before and existing in the unfrozen state, can be found in nature. For example, it is known that the ground under many rivers of Siberia and the Far East as well as under large lakes of tectonic, glacial and volcanic origin has never been frozen. In conditions of continuous permafrost the taliks are being formed at the present time also. Many of them are formed as a direct effect of human activity within the permafrost zone in regions under development.
Thus, taliks within the permafrost zone can be of various origin. Some of them should be considered as existing continuously in the course of perennial freezing (for many hundred thousand years), the others - as occurring naturally at the present time (after the thermal minimum of the Late Pleistocene) or as a direct effect of human activity.
There exists a great number of talik classifications reflecting in one way or another the nature of occurrence of thawed and unfrozen ground within the permafrost zone. Such kind of elaborations were carried out by N.I. Tolstik-hin, I.Ya. Baranov, N.A. Vel'mina, N.N. Romanovskiy, N.A. Nekrasov, S.M. Fotiyev, Ye.S. Sukhodol'skiy and others. As the basis of these classifications, various particular features of general or specific character are brought together under headings such as topographic position, (watershed, valley, and slope taliks), conditions and degree of water saturation (waterbearing, impermeable, or dry), shape and size (rounded, slightly and strongly extended, thawed fissures, craters, etc.), lifetime (stable, unstable, seasonal, perennial), hydrologic peculiarities of the ground (water-absorbing, water-removing, water-conducting, water-containing), the occurrence of waterbearing strata in relation to the permafrost (suprapermafrost, intrapermaf-rost and subpermafrost), mechanisms of heat transfer (conductive, convec-tive and their combinations), heat sources (solar, ground, surface and atmospheric water energy, the geothermal flux, exothermic reactions, etc.) and others.
The classification by N.N. Romanovskiy according to which taliks are divided into types with respect to the causes of their occurrence is most commonly used. The main causes are: radiation-heat balance on the Earth's surface including heat flow from the interior by both conduction and convection; warming effect of water courses, water reservoirs and glaciers; exothermal oxidation reactions; volcanic activity etc. (Table 13.1). On the basis of these main causes the taliks are subdivided into seven types: radiation-thermal, hydrogenic, hydrogeogenic, glaciogenic, chemogenic, volcanogenic and technogenic (Fig. 13.1). This subdivision of taliks points out the details of their origin. Thus, the radiation-thermal type is subdivided into three subtypes: radiation, thermal and pluvial-radiation.
Taliks of the radiation subtype are formed on account of solar energy arriving at the Earth's surface. Positive ground temperatures are maintained within the areas composed of water-impermeable rocks mainly by way of conductive heat transfer without the effect of infiltrating atmospheric precipitation. Such taliks are most widely distributed near the southern limit of the permafrost regions with a great number of sunny days and small amounts of snow (Middle Asia, Southern Siberia, southern part of the Far East).
Table 13.1. Classification of taliks (after N.N. Romanovskiy using those of I.A. Nekrasov and S.M. Fotiyev)
Source of heat Type Subtype
II Hydrogenic (water-thermal)
III Hydrogeogenic (underground-thermal)
Radiation, thermal, pluvial-radiation
Shelf, below an estuary, below a lake, below a river bed, near a river bed (flood-plain)
Subaerial, below a lake
Thermal taliks are formed due to the insulating effect of snow (increasing with depth and lower density) causing positive temperatures at the base of the seasonally freezing-thawing layer. This layer is usually represented by slightly water-permeable or water-saturated ground (within swamps) as in taliks of the radiation subtype. This kind of talik is typical of the regions with maritime and moderate continental climate with wind redistribution of
Fig. 13.1. (opposite) Position and structure of various types of taliks and their involvement in water exchange: 1-7 - geological structure; (1 - crystalline bedrock; 2 - terrigenous-sedimentary rocks; 3 - extrusives; 4 - intrusions;
5 - loose materials of various origins; 6 - tectonic dislocations; 7 - rocks with increased joints); 8 - pluvial-radiation infiltration open taliks;
9-17 - hydrogenous (underwater) taliks; (9 - stagnant, closed below a lake;
10 - stagnant, open below a lake; 11 - infiltration, closed below a lake;
12 - infiltration, open below a lake; 13 - pressure-seepage, open below a lake;
14 - infiltration, open below a river bed; 15 - pressure-seepage, open below a river bed; 16 - ground-seepage, closed below a river bed; 17 - ground-seepage, open below a river bed); 18-20 - hydrogeogenous taliks (18 - subaerial pressure-seepage, open; 19 - subaerial pressure-seepage, closed;
20 - pressure-seepage, open below a lake); 21-22 - glaciogenic taliks
(21 - infiltration open; 22 - pressure-seepage, open); 23-24 - volcanogenic taliks
(23 - infiltration open; 24 - pressure-seepage, open); 25-26 - boundaries
(25 - of permafrost; 26 - of areas of deep and ultra-deep freezing without groundwater); 27 - direction of groundwater flow;
28 - groundwater icings; 29 - lakes; 30 - glaciers.
snow (European North of Russia, Western Siberia, mountain regions of Southern Siberia and the Russian Far East).
Taliks of the pluvial-radiation subtype are formed by the heat effect of infiltrating rain-water. Such taliks are typical of low altitude (up to the absolute altitude of 900-1200 m), plane watersheds and gentle slopes composed of Quaternary formations and bedrock having good infiltration properties. They are formed in the southern parts of the permafrost region with discontinuous development of permafrost from the surface, where the amount of summer precipitation exceeds 300-400 mm. The pluvial-radiation taliks occupy the greatest areas (to 50-80% of the area). It is through such taliks that groundwater is replenished from atmospheric precipitation.
Romanovskiy divides the taliks of hydrogenic type (underwater taliks), formed under the warming effect of surface waters, into five subtypes: shelf taliks (under the effect of sea water), taliks below an estuary (under the warming effect of river and sea waters), below a lake (owing to the water containment effect), below a river bed (under the influence of the water flow) and flood-plain taliks (under the temporary, periodic effect of flooding waters).
It should be noted that taliks below a river bed and flood-plain taliks are not formed solely by the warming effect of surface waters. Powerful underground streams in the deposits of river beds have a very significant effect. In plan the free cross-sectional area of such taliks often extends beyond the flood-plain and into terrace deposits. Taliks below a lake (see Fig. 13.1) are clearly important in various ways for water exchange in the permafrost. They exist below lakes of various origins and can be both open and closed. Some amount of lake water feeds deep subpermafrost groundwater flow through open taliks of watershed lakes of glacial and tectonic origin. Some portion of this groundwater flow in turn discharges subaquatically through open taliks in lakes or trough valleys situated at lower levels.
It should be stressed that the presence of an open talik beneath most lakes in former river beds and lakes of thermokarst origin means the probability of groundwater recharge or discharge. However if water-impermeable clayey ground predominates in the section of the talik, or slightly permeable water-bearing ground alternates with water-impermeable ground, such taliks are practically free of water or are characterized by a stagnant or slowed-down regime of water exchange.
Glaciogenic taliks are formed by glacier meltwater, the streams of which are concentrated near the glacier base through fissures. Such taliks are found within high mountain systems of Pamir and Tien-Shan as well as below a number of glaciers of Verkhoyan'ye, Chukotka, and Koryakiya which have the most severe climatic and permafrost conditions.
Hydrogenic, chemogenic and volcanogenic types of taliks are classified based on the fact that they exist due to thermal anomalies resulting from processes in the Earth's interior. The origin of these hydrogeothermal anomalies is associated with convective heat transfer by subpermafrost waters under pressure. The conductive heat transfer causes anomalies of geothermal origin (exothermal, heat flux from the Earth's interior, hot gases outlets, volcanic heating).
Taliks of the technogenic type are formed in the course of human activity and develop as taliks of various types in natural conditions. They are formed below artificial water reservoirs, below straight stretches of river beds, under spoil heaps and fills of coal- and sulphide-containing rocks, below areas with the soil-turf layer artificially removed, with disturbance of the radiation-thermal regime existing at the ground surface, etc. The time of formation of technogenous taliks and their areas are different for the regions of continuous and discontinuous permafrost. Thus, in the south of the permafrost zone a small, often rather short-term, change in heat exchange conditions on the ground surface is sufficient for taliks to be formed; to the north long-term changes are necessary because the low-temperature permafrost has a more considerable thermal lag.
The character of taliks, the tendency for their development and the rate of the process depends in many respects on the relations with ground waters and on the type of water exchange. It is the water (if it is considered as an accumulator and carrier of heat from the Earth's interior, especially within the sections below lakes, where it moves vertically), which has most effect on the change in geotemperature pattern. Various types of taliks are subdivided into the following-classes: free of water (water-impermeable or 'sushentsy'), stagnant (water-containing), infiltration (water-absorbing), ground-seepage (water-conducting) and pressure-seepage (water-releasing). Groundwater recharge, movement in the zone of supra- and subpermafrost groundwater flow and pressure discharge occur through the taliks of the various classes.
Open taliks below a river bed can play various roles seasonally. Thus, in summer, when the abundant recharge of groundwater occurs, they exist as pressure-seepage taliks, while as the spring comes they change into infiltration ones within the upper reaches of valleys, as winter depletion of groundwater has taken place.
The nature of groundwater movement, as well as the rate of water exchange and the degree of participation of various taliks in it, depend on the cryogenic change of hydrogeological structures and vary with the different regions of permafrost development. Paradoxically as it may seem, the water exchange and groundwater movement are most active within the region of continuous permafrost. Concentration of the underground flow along the most washed-out rejuvenated tectonic dislocations takes place here. Within the regions of discontinuous permafrost development this groundwater flow becomes less concentrated. The cryogenic part of the geological section and the degree of washout of water-containing rocks in taliks is reduced significantly in this situation. Such a tendency is retained as a whole when moving from the areas of discontinuous cryogenic water-confining strata to massif-island, island and sporadic ones and is rather typical.
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