HR: 0800h
AN: C41A-0055    [Abstracts]
TI: Permafrost degradation in the source area of the Yellow River, Northeastern Tibet
AU: * Ikeda, A
EM: aikeda@geoenv.tsukuba.ac.jp
AF: Graduate School of Life and Environmental Sciences, Univ. Tsukuba, Tsukuba, Ibaraki, 305-8572, Japan
AU: Matsuoka, N
AF: Graduate School of Life and Environmental Sciences, Univ. Tsukuba, Tsukuba, Ibaraki, 305-8572, Japan
AU: Sueyoshi, T
AF: Institute of Low Temperature Science, Hokkaido Univ., Kita-ku, Sapporo, 060-0819, Japan
AU: Ishii, T
AF: Geological Survey of Japan, AIST, Tsukuba, Ibaraki, 305-8567, Japan
AU: Gao, C
AF: China Institute for Geo-Environmental Monitoring, Haidian District, Beijing, 100081, China
AU: Ding, J
AF: Department of Land and Resources, Qhinghai Province, Xining, Qhinghai, 810001, China
AB: Frozen ground was investigated in 2003-2006 to evaluate the present-day distribution and ongoing degradation of permafrost in the source area of the Yellow River, located at the northeastern margin of the Tibetan Plateau. Distribution of permafrost was examined by seismic, electrical and/or thermal soundings at 18 sites between 3250 m and 4800 m ASL. Temporal variations in ground thermal and hydrological regimes were also investigated for two years at Madoi observatory (4273 m ASL), by automatic and manual observations of air and ground (0-8 m deep) temperatures, precipitation, snow depth, near-surface soil moisture and groundwater level. High P-wave velocities (>2 km/s) and relatively high DC resistivities (650-1100 ohmm) below a thin uppermost layer showed that permafrost 10-30 m in thickness occurred above 4300 m ASL. In contrast, low P-wave velocities (<1 km/s) throughout the sediments indicated that permafrost was absent below 4000 m ASL. On widespread alluvial plains between 4200 m and 4300 m ASL, permafrost was lacking or significantly degraded. Negative values of the mean annual surface temperature (MAST) also indicated widespread permafrost only above 4300 m ASL under the present climatic condition. The seasonal frost penetration reached a maximum depth of 2.6 m at the observatory. Intermittent and very shallow snow cover favored frost penetration. The ground between 4 m and 8 m in depth was kept at slightly positive temperatures (0-4°C) throughout two years, although the presence of permafrost at this site was suggested by a few reports in the 1980s. Assuming that the inter-annual variation in MAST follows that in the mean annual air temperature, permafrost is estimated to have significantly thawed on the alluvial plains at 4200-4300 m ASL during the last half-century. The resulting degradation of the permafrost is assumed to have extended 3000 km² on the alluvial plains in the source area.
DE: 0702 Permafrost (0475)
DE: 0772 Distribution
DE: 1807 Climate impacts
DE: 1823 Frozen ground
SC: Cryosphere [C]
MN: 2007 Fall Meeting