Internal structure and the thermal and hydrological regime of a typical lithalsa: significance for permafrost growth and decay

摘要

This study presents new knowledge about the ice segregation and frost-heave processes taking place in a typical lithalsa. A tomodensitometric scanner was used to produce high-resolution computer images of ice lenses, soil layers, faults, sedimentary structures, and gas bubbles. Scan-image analysis allowed the interpretation of the ice lenses and soil cryostructures resulting from permafrost aggradation. It also provided an accurate estimation of volumetric contents of ice and gas present in the permafrost. Isotopic analyses on the various phases of the permafrost (i.e., ice, gas, and soil) provided supplementary information. O-18, deuterium, and tritium analyses were undertaken on ground ice and on surface water. Monitoring of the thermal regime of the lithalsa provided clues relative to gradients that drive groundwater movements and ice-lens growth. Compilation and interpretation of the data in a three-dimensional and temporal context suggest that the lithalsa under study grew under climate conditions slightly colder than those of the 20th century. However, post-aggradational water penetrated into the permafrost of the lithalsa. Ground temperatures increased since 2000, most likely because of underground warming owing to groundwater flow around the permafrost body. As a result, the mound has started to settle down, and an incipient thermokarst pond became conspicuous in 2003.