Permafrost zoning on the basis of temperature field stability

Heat flows below and above the phase boundary (temperature close to 0 °C) are connected to each other by the Stefan relation (Duchkov et al., 2000):

where q is the heat flow determined in thawed rocks underlying a permafrost layer; qp is the heat flow in frozen rocks, Q is the melting heat for ice, W is the ice content in rocks, and V is the speed of phase boundary movement (speed of freezing/thawing).

The heat flow ratio (n = qp/q = 1+ Q-W-V/q) is used as a criterion of temperature field stationarity (Duchkov et al., 1995; Balobaev, 1991; Duchkov and Balobaev, 2001). In the stationary case V =0 and the coefficient n = 1. For temperature increases at surface (climate warming and permafrost decay) we get V <0 and n <1. For decreasing Ts (climate cooling and permafrost growth ) we get V > 0 and n > 1. Thus the coefficient n may serve as a tool for estimating the stability of modern temperature fields in permafrost and predicting its evolution in the near future. It is obvious that the method is applicable only if a phase transition occurs at the bottom of the permafrost (e.g. for the Meso-Cainozoic depressions). For its realization one needs heat flow values above and below the phase boundary.

This method was used for zoning permafrost in depressions of western and eastern Siberia (Fig. 2). The most detailed picture is given for western Siberia (Fig. 2A) where the permafrost, according to parameter n, consists of two sharply differing parts - one northern and the other southern. The boundary between them passes approximately along the latitude of the Polar Circle. Climate warming will mostly affect a rather narrow strip of the continuous permafrost located to the north of the Polar Circle, where n is small (n < 0.5). To the far north the permafrost forms a monolithic body whose temperature steadily remains negative. Here n is 0.5-0.8, showing some evidence of permafrost decay which corresponds to raising of its lower boundary. The similar ratio between qp and q is also established in the limits of the Viluyi depression (Fig. 2B), though here there is not enough experimental data. Zoning of the other eastern Siberia depressions was made using isolated thermal logs and thus is quantitative in nature. The permafrost of eastern Siberia as a whole is more cooled and resistant to climate warming. Today the entire Siberian region is characterized by a decaying type of permafrost (everywhere n < 1), and thus global climate warming may speed up the process of permafrost decay. This rate could be estimated given data on possible scenarios for climate warming in the 21st century.

Figure 2. Schematic zoning of the permafrost temperature field for western (A) and eastern (B) Siberia according to its stability level (Duchkov and Balobaev, 2001). A: 1 - n =0-0.1; 2 -n = 0.1-0.2; 3 - n =0.2-0.5; 4 - n = 0.5-0.8; 5 - n = 0.8-1; 6 - the southern permafrost boundary . B: 4 - site with permafrost thickness of about 1.5 km; 5 - site with talik.

Figure 2. Schematic zoning of the permafrost temperature field for western (A) and eastern (B) Siberia according to its stability level (Duchkov and Balobaev, 2001). A: 1 - n =0-0.1; 2 -n = 0.1-0.2; 3 - n =0.2-0.5; 4 - n = 0.5-0.8; 5 - n = 0.8-1; 6 - the southern permafrost boundary . B: 4 - site with permafrost thickness of about 1.5 km; 5 - site with talik.

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