Ninth International Conference on Permafrost ... - IARC Research

Current State and Dynamics of Permafrost in the Siberian PlatformM.N. ZheleznyakMelnikov Permafrost Institute SB RAS, Yakutsk, RussiaV.T. BalobaevMelnikov Permafrost Institute SB RAS, Yakutsk, RussiaV.G. RusakovMelnikov Permafrost Institute SB RAS, Yakutsk, RussiaThe Siberian Platform is a major geological structurein Asia located in the north-central part of the continent.A characteristic feature of the Siberian Platform is thewidespread occurrence of permafrost, the thickness andareal distribution of which vary greatly depending onclimatic conditions, topography, and the complex interactionof external and internal factors.The geothermal data collected by the authors during thelast 20 years were analyzed to provide a characterizationof the geothermal field and permafrost conditions in theregion. Heat flow varies across the Siberian Platform from15 to 65 mW/m 2 , and is determined by the structural andtectonic setting of the region. Thermal properties of rocksdepend on the age, composition, and moisture content of thematerial, and vary widely from 1.2 to 7.3 W/m⋅K. Groundtemperatures range from -5.0°C (in the presence of thickpermafrost) to 16.0°C (where permafrost is absent) at depthsof 500 m and from 0°C to 38.0°C at 1500 m, depending onthe heat flow and rock type.Permafrost occurs in 65% of the Siberian Platform. Itssouthern boundary is at 63°N latitude over most of theregion, rising northwards to the latitude of Igarka in thewest. Patches of frozen ground are encountered south of thisboundary, where favorable local conditions exist.The thickness of permafrost in the Siberian Platformvaries from a few meters to as much as 1370 m in the AnabarShield. It is determined by the ground surface temperature,the thermal properties of subsurface materials, and thegeothermal heat flow. The latter two factors remain relativelyconstant over long periods of time, while the ground surfacetemperatures have changed repeatedly during the permafrosthistory. The present thermal state of the permafrost is mainlydetermined by the difference between the present-daytemperature and the temperature in the last cold period (theSartan) (Zheleznyak 2005).The thermal state of the permafrost is in steady state insome parts of the region and in unsteady state in others.The steady-state thermal regime is characterized by theconstant position of the permafrost base due to the equal heatflows in unfrozen and frozen ground at the phase boundary.Equilibrium permafrost occurs in the areas composedof Early Mesozoic and Paleozoic sedimentary rocks orcrystalline and metamorphic rocks. They occupy most ofthe Siberian Platform, the Verkhoyansk-Chukotka FoldedRegion, the Aldan-Stanovoy Massif, the Anabar Shield, andsome minor uplifted basement blocks. The rocks comprisingthese structures have low porosity, are poorly fractured, andcontain very little water, so their temperatures rise above0°С rapidly, with minimum heat involved. The high thermalconductivity of these rocks facilitates rapid smoothing of thethermal state and maintains the steady-state regime.Disequilibrium permafrost is characterized by thedifference of heat flows at the lower phase boundary. Sincethe present epoch is warmer than the previous one, the heatflow in unfrozen ground is greater than in frozen ground,because the internal heat is partially absorbed at the phaseboundary during the thawing of ice inclusions. As a result,the lower phase boundary of permafrost rises slowly, and thethickness decreases. In the Siberian Platform, disequilibriumpermafrost is confined to the areas of exposed UpperMesozoic (Jurassic and Cretaceous) and Cenozoic rocks.They occur in the pre-Yenisei zone, the Vilyuisk Basin,Table 1. Geothermal parameters of permafrost in the Vilyuisk Basin and the Verkhoyansk Trough in Recent Epoch and 200,000 years BP.Geothermal measurement site Recent Sartan ΔТ°С ΔН, m Rate of thaw,Н o, m Т o,°С Н s, m Т s°Сcm/yrBakhynai 650 -5.0 720 -12.4 7.4 -70 -1.7Balagachi 700 -5.0 760 -13.6 8.6 -60 -1.8Lindenskaya 400 -2.1 520 -11.6 9.5 -120 -1.9Srednevilyuisk 485 -1.8 568 -11.8 10.0 -83 -2.2Vilyuisk 600 -3.0 730 -13.3 10.3 -130 -1.5Ust-Vilyui 150 -2.8 360 -10.4 7.6 -210 -2.1Badaran 500 -2.0 610 -11.5 9.5 -110 -1.7Sobo Khaya 80 -1.7 350 -11.3 9.6 -270 -2.3Khailakh 600 -5.0 785 -12.5 7.5 -185 -1.4Namtsy 480 -3.2 580 -10.4 7.2 -100 -1.9Yakutsk 350 -2.5 578 -10.8 8.3 -228 -1.7363