CAUSES OF THERMAL SUFFOSION ATTENUATION ON THE BESTYAKH TERRACE OF THE LENA RIVER, CENTRAL YAKUTIA

This paper presents the results of thermal suffosion study in the Ulakhan-Taryn Creek valley, Central Yakutia, where intrapermafrost groundwater is discharged. Interpretation of satellite images and analysis of field data from landscape surveys, electrical resistivity tomography studies and ground temperature observations were used to determine the factors contributing to the attenuation of thermal suffosion in sandy deposits of the IV (Bestyakh) terrace of the Lena River. Successional stages of revegetation in the accumulation zones of sands washed off by subsurface flow were examined. Vegetation succession was found to modify geocryological conditions in the groundwater discharge area, sometimes leading to its complete freezing. Conditions favorable both for permafrost aggradation and degradation may develop, depending on the landscape type. Ground temperature data indicate that the active layer thickness in sands at a thermal suffosion site covered by sparse pine forest is 3.5 m, while the mean annual permafrost temperature at the depth of zero annual amplitude is –0.2°С. In 2014, a subaerial talik began to form at this site, the bottom of which lowered from 3.5 m down to 6.0 m. At a larch site with tussocks, the active layer thickness is within 0.5–0.8 m and the mean annual permafrost temperature is about –2.0°С. An ERT survey provided information on the current permafrost and groundwater conditions, where groundwater discharge has ceased. A scenario is proposed describing the changes in the discharge area caused by permafrost disturbance above an aquifer. Formation of an underground cavity and collapse of the overlying permafrost layer resulting in characteristic depressions and sinkholes at the surface promote ground freezing in the discharge zone. Mechanical blocking the flow paths reduces the flow velocity promoting the restoration of permafrost and cutting off the area of soil removal by thermal suffosion. Further on, this attenuates the thermal suffosion. Its active phase can shift spatially due to thawing the pore ice in the permafrost in contact with the confined aquifer, widening soil voids and forming new flow pathways. Thus, attenuation of thermal suffosion on the Bestyakh Terrace of the Lena River results from the combination of internal (impeded flow) and external (vegetation succession) factors.