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Thermal, hydrological and geochemical dynamics of the active layer at a continuous permafrost site, Taymyr Peninsula, Siberia

Thermal, hydrological and geochemical dynamics of the active layer at a continuous permafrost site, Taymyr Peninsula, Siberia

Berichte zur Polarforschung 0(242): 1-104

About 65% of Russia is underlain by permafrost. In these regions, the upper ground, the active layer, seasonally freezes and thaws. The overall goal of this thesis was to improve our understanding on the coupled thermal, hydrological and geochemical regime of this layer in a continuous permafrost setting from field experiments. These experiments were conducted in the Levinson-Lessing Lake catchment on the Taymyr Peninsula, northern Siberia (74degree 32'N; 98degree 35'E) from July to September 1994 and May to October 1995. The objectives were to quantify the seasonal fluxes of water and heat in the active layer from spring thaw to fall freeze-back and to identify the sources of active layer water using the stable isotope ratios delta18O and deltaD. Time domain reflectometry (TDR) is evaluated as a field technique for measuring volumetric water content theta and bulk electrical conductivity sigmab in arctic soils. Calibration measurements of theta and sigmab were carried out on three different slopes that were characteristic for this catchment. Comparison of theta calculated from TDR using two different approaches and gravimetrically determined water contents show a close correlation. For the calculation of the soil water electrical conductivity sigmaw from TDR determined sigmab two theoretical models based on the pore geometry were tested using independent sigmaw of soil solutions obtained with suction cups. Unacceptable high deviations of predicted sigmaw resulted in the development of a new model. Best results for sigmaw are obtained using this regression model, with highest precision when probe specific calibration is carried out. Therefore TDR can be applied to obtain quantitative estimates of theta and sigmaw in the active layer for a range of arctic field soils in this permafrost setting. However, the application of TDR determined sigmab in different permafrost soils to infer sigmaw requires additional calibration. Based on these results, TDR is recommended as an in situ technique for studying the temporal dynamics of soil water and solutes in the active layer. Using TDR, liquid water was found to be present in frozen soil at temperatures down to -12degree C and its volumetric fraction increased with temperature until ground melting started. The ground thermal regime during spring thaw and fall freeze-back is dominated by the heat of fusion which stabilizes soil temperatures at 0degree C for extended periods ('zero curtain effect'). The thermal regime of the saturated active layer during the summer may be understood from assuming density driven convection as the mechanism of heat transfer. Convective transfer also appears to be dominating during fall, when large amounts of latent heat are released by freeze-back. Seasonal fluxes of heat and water are calculated using a simple zero-dimensional model of water and energy balance. Although this model neglects processes such as sensible heat fluxes and lateral water flow, it reveals that the dominant heat sink during spring and summer is evaporation. During fall freeze-back, the dominant heat-producing process is phase change.

Accession: 009641353

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