T

per mean= per * mean, mean3

per mean= per * mean, mean3

Fig. 12.6. Variation of the permafrost thickness depending on the mean ground temperature for many-year periods of temperature fluctuations on the surface (after V.A. Kudryavtsev): 1-3 - envelopes of the many-year temperature fluctuations at fper = 0; 2 and 4°C, respectively.

mean r J

respectively.

However the mean temperature f^/an (for the period of fluctuation) at the permafrost surface has an effect not only on the permafrost thickness and its temperature regime but in accordance with the sign (minus or plus) and the

Fig. 12.7. Diagram of periodic variation of permafrost upper and lower boundaries under periodic temperature changes and various values of mean (fmean), maximum (fmax) and minimum (fmin) temperatures on the surface (after V.A. Kudryavtsev).

value of t^an, also on the formation of different types of permafrost. Actually, according to Kudryavtsev, three typical cases of permafrost formation are possible depending on the relationship between the mean (t^,"an), the maximum (i£farx) and minimum (t^J temperatures on the permafrost surface (Fig. 12.7): 1) Can < 0°C and Ape[ < CJan; 2) CJan < 0°C and Aper > CJani 3) Cean > 0 C, Aper > i£,ean.

In the first case, when Cean < permafrost existing during the whole period of fluctuation is formed, with the depth of its base changing in the range — B2 (see Fig. 12.7a). Such a type is widespread in severe climatic conditions of the northern geocryological zone, characterized by thick and hence rather stable permafrost. In the second case (see Fig. 12.7b) when Cean >0°C > tpeI, permafrost is formed with the episodic appearance (during part of the period T) of a layer of multiyear thawing in the range ax — a2 and with periodically changing depth of the permafrost base in the range Bj — B2. This permafrost is typical of zones having mean temperatures from 0 to — 2°C. In the third case, when ijj,®rn < 0°C < iper, permafrost appears periodically (during part of the period Tper) within the shaded layer (see Fig. 12.7c). This case is typical of the southern zone of permafrost having mean annual temperatures close to 0°C.

When perennial freezing occurs over a long time, the role of the geother-mal flux, and consequently, of the geothermal gradient in the unfrozen ground underlying the frozen, is substantially greater.

The lower boundary conditions are to a large extent responsible for the regime of movement of the permafrost base on account of the relationship between the heat fluxes on each side of the dividing line B2, that is, on account of the heat flux Qth arriving at this line from the underlying unfrozen ground and of the heat flux Q[r being removed from this line through the frozen ground (Fig. 12.8). The temperature on the dividing line 'unfrozen-frozen ground' proper appears to be fixed and equal to 0°C. When Q[r > gth, cooling and freezing of the underlying layers proceeds. The boundary of perennial freezing (Bi — B2) moves downward in this case. When Q(l < Qth, warming and thawing of the overlying frozen ground takes place and the lower boundary of the permafrost B2 moves upward. When Qu = Qth, temperature conditions at the base of the permafrost will be steady, and the boundary will be fixed. At the same time it is easy to show that if Qfr = X(r grad ffr = Xth grad fth = Qth, grad ffr = {XtJX fr) grad fth, i.e. temperature gradients in the permafrost near its base should be less compared with grad fth in the underlying thawed ground, because most often the values Xu > XA.

Thus, the permafrost thickness appears to be essentially dependent on the value of heat flux moving from below upward, i.e. on the geothermal gradient in the underlying thawed ground, gtii. The larger is the heat flux moving from below upward, and consequently, the geothermal gradient, the less the permafrost thickness. Estimation of this effect (at ^per = 6T,7Tera„ = 0°CU = 2.89kJ (mhr'C)"1, <2ph = 52375kJ m"3, rper = 100000 years) show that with the increase of geothermal gradient from 0 to 0.03 °C m"1, the permafrost thickness decreases by a factor of approximately two thirds to one half (Fig. 12.8). This rule is followed rather clearly in the part of the permafrost zone where heat flux differentiation is caused primarily by the different age of geological structures. For example, within the Siberian Platform the smallest heat flux values (from 13 to 25 mW m~2) at which the permafrost zone thickness is 800-900 m on the average noted within the most ancient structures of the basement (Anabar anticline). Increase of the heat flux value (up to 40-60 mW m~2) and decrease of the permafrost zone thickness (to 800-600 m) are noted within Mesozoic structures such as the Vilyuy (syncline) and the Predverkhoyansk and Yenisey-Khatangan depressions. A similar dependence of the permafrost thickness on heat fluxes is noted by V.V. Baulin for various structures of the Western Siberian Plate, where q values (in the range between 0.10-0.13 and 0.250.30 mW m ~ 2) are in accordance with the permafrost thicknesses of 350-400

Fig. 12.8. Variations of the thickness of permafrost depending on the value of geothermal gradient at Apcr = 6°C and rj|"an = 0°C (after V.A. Kudryavtsev): - geothermal gradient = 0; 0.01; 0.02 and 0.03°C m" \ respectively.

and 135-225 m, respectively.

The highest intensity of the heat fluxes (from 40 to 100 kW m ~ 2) are noted in Russian North-East (regions of active orogenesis) according to V.T. Balobayev's data. It is precisely this situation which is associated with the wide variations in permafrost thickness here (from 150-500 m), with low temperatures (from —6 to — 9°C). Still higher values of heat fluxes are a possibility within the regions of contemporary volcanism (for instance on Kamchatka) where the permafrost can be completely absent as a result.

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