Some aspects of plateau permafrost, Qinghai-Xizang (Tibet) Plateau, China, and a comparison with the Mackenzie Delta region, Canada.

Abstract

Differences in permafrost conditions between the Qinghai-Xizang (Tibet) Plateau and the Mackenzie Delta region primarily relate to their Quaternary histories and their current climates. For example, the Tibetan Plateau has experienced uplift of at least 3,000 m during the last 2 million years. Under the present climatic conditions, the lower altitudinal limit of the plateau permafrost in the north is at 4,150 m a.s.l. This suggests that if the Plateau were 1,000 m lower than its present elevation, there would be no permafrost. During the Quaternary, the Tibetan Plateau remained unglaciated. This has meant that little water was available for the formation of massive ground ice, in contrast to the Mackenzie Delta region. Located at 68-69$\sp\circ$N, the Mackenzie Delta region experiences a combination of low air temperature in winter, a long solar night and a short thaw period in summer. The result is a relatively thin active layer. Located at 30-34$\sp\circ$N, the Tibetan Plateau experiences much higher solar insolation, and a diurnal temperature rhythm. The thickness of the active layer is much greater than in the Mackenzie Delta region and varies between 1.3 and 3 m or more. Permafrost on the Tibetan Plateau is much warmer and thinner than that in the Mackenzie Delta region. One consequence is that it is more sensitive to any changes in climate and surface conditions. Deep ground temperatures in the Pleistocene Mackenzie Delta indicate a recent warming trend, while a cooling trend in the Modern Delta likely relates to local factors such as channel shifting and emergence and/or sedimentation in the Mackenzie River. Water bodies are a cause of geothermal disturbances common to both regions. Numerical simulation of rapid coastal retreat in the Mackenzie Delta region indicates that subsea permafrost is at least 3$\sp\circ$C warmer than adjacent terrestrial permafrost. On the Tibetan Plateau, faulting also disturbs the geothermal regime. Measurements of in situ permafrost creep in the Fenghuo Shan area are one indicator of the warmer permafrost temperatures on the Tibetan Plateau. The average creep velocity ranges from 0.16 cm/year at 2.8 m depth to 0.54 cm/year at 1.6 m depth. These velocities are greater than those recently obtained from the High Arctic of Canada and are approximately of the order of magnitude as those obtained in the Mackenzie Valley. Inter-continental comparison of creep data suggests that climate controls the regional (large scale) magnitude of creep, and that ground ice is a local factor controlling creep rate in a particular area or site. A constitutive relationship (secondary creep power flow law) was applied to the field creep data; and creep parameters A and n were determined for each of the three different depths in the West Valley, Fenghuo Shan area, Tibetan Plateau.