Geographical Review of Japan Vol. 80, No. 5, 259-271, 2007 Permafrost Sounding (2003-2005) in the Source Area of the Yellow River, Northeastern Tibet IKEDA Atsushi*, SUEYOSHI Tetsuo**, MATSUOKA Norikazu*, ISHII Takemasa***, and UCHIDA Youhei*** * Geoenvironmental Sciences , Graduate School of Life and Environmental Sciences, University of Tsukuba, Tsukuba 305-8572, Japan ** Institute of Low Temperature Science , Hokkaido University, Sapporo 060-0819, Japan *** Geological Survey of Japan , National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8567, Japan Abstract: Present-day distribution and ongoing degradation of permafrost were evaluated by geophysical means in the source area of the Yellow River, located at the northeastern margin of the Tibetan Plateau. Seismic, electrical and/or thermal soundings were undertaken at 15 sites be- tween 3260 m and 4790 m ASL in 2003-2005. High P-wave velocities(>2km s-1)and relatively high DC resistivities (650-1100Ωm)below a thin uppermost layer show that permafrost 10-30 m in thickness occurs above 4300 m ASL. In contrast, low P-wave velocities(<1km s-1)through- out the uppermost ten to fifteen meters of sediments indicate that permafrost is absent below 4000m ASL. On widespread alluvial plains between 4200 m and 4300 m ASL, some sites show subsurface intermediate P-wave velocities(1.5-1.7km s-1)and low resistivities(30-140Ωm)in- dicating the presence of unfrozen-saturated sediments, while others show high DC resistivities possibly indicating the presence of permafrost. Negative values of the mean annual ground sur- face temperature(MAST)also indicate widespread permafrost only above 4300 m ASL under the present climatic condition. 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 allu- vial plains at 4200-4300 m ASL during the last half-century. Key words: permafrost, ground temperature, seismic velocity, DC resistivity, global warming, Yellow River, Tibet terrain combine to restrict glaciers to develop on Introduction only a few mountains above 5000 m ASL (Wang and Derbyshire 1987; Owen et al. 2003). These Recent global warming has raised the temper- cryospheric conditions indicate that the hydro- ature of permafrost in both high-latitude low- logical system in the source area depends largely lands and mid-latitude mountains (e.g. Lachen- on the thermal state of the seasonally or perma- bruch and Marshall 1986; Osterkamp and Ro- nently frozen ground. Recent studies have re- manovsky 1999; Harris et al. 2003). In particu- ported desertification (degradation of the grass- lar, warm permafrost has been thinning rapidly land and meadow vegetation) of the plateau, in recent decades in Mongolia and the Tibetan which possibly originates from deepening frost (Qinghai-Xizang) Plateau (e.g. Sharkhuu 1998; table and overgrazing (Wang et al. 2001; Zhang Jin et al. 2000). et al. 2004). A remarkable feature is the ground- The source area of the Yellow River (Huang water level lowering at a rate of about 0.1 m a-1. He), which is located in the northeastern margin This has been mainly attributed to degradation of the Tibetan Plateau, appears to be one region of permafrost (Peng et al. 2003), because there broadly underlain by such warm permafrost. The was no significant change in precipitation during area comprises a plateau with elevations of over the last half-century (Yang et al. 2004). 3500 m, whereas dry climate and a lack of higher Whereas a few recent reports have indicated -259- 64 IKEDA A., SUEYOSHI T., MATSUOKA N., ISHII T., and UCHIDA Y. Figure 1. The source area of the Yellow River in the southeastern part of Qinghai Province, showing sites for seismic, electrical and thermal soundings. The thick contour line corresponds to 4000 m ASL. Contour interval 400 meters. rapid degradation of permafrost in the source ogy, we have investigated the present geothermal area of the Yellow River (Zhu et al. 1995, 1996; conditions in the source area of the Yellow River Jin et al. 2000; Wang et al. 2000), a large part of (Ikeda et al. 2004; Matsuoka et al. 2004, 2005), the area was considered to have been underlain as a part of an interdisciplinary research project by permafrost at least until the 1980s (Wang to model the groundwater circulation and to pre- 1987; Wang et al. 1991; Zhou et al. 2000). The dict near-future water resources of the whole evidence for the degradation is, however, ex- Yellow River basin. This paper discusses critical tremely limited in contrast to well-documented conditions for permafrost distribution on the ground temperatures along the Golmud-Lhasa basis of field monitoring of ground surface tem- Highway, lying 400 km west of the main road in peratures and sounding of near surface seismic the source area (Jin et al. 2000; Wang et al. and electrical stratigraphies. 2000). The degradation in the source area seems to be assumed without any data in the review pa- Study Area pers by Jin et al. (2000) and Wang et al. (2000). Even more specific papers written in Chinese The fieldwork was undertaken along the R214 lack relevant information such as the number, el- road that connects Xining and Yushu in the evations and dates of boreholes (e.g. Zhu et al. southeastern part of Qinghai Province (Figure 1995, 1996). A notable exception is Zhang et al. 1). The elevation of the measurement sites varies (2004), who, however, conclude that most of from 3260m to 479D m ASL (Table 1), which their boreholes are thermally affected by the ad- crosses the boundary between the permafrost jacent river and lake. In general, the mapping of and seasonal frost areas (Wang 1987; Zhu et al. permafrost in the area is still insufficient. 1995). In the area, mountain ranges reaching In order to verify ongoing degradation of per- above 4000 m ASL extend along WNW-ESE fault mafrost and its impacts on groundwater hydrol- systems. The main study area, Madoi County, is -260- Permafrost Sounding (2003-2005) in the Source Area of the Yellow River 65 Table 1. Results of seismic refraction and vertical electrical soundings in the source area of the yellow River. P-wave velocity (V),depth of layer base (D), length of sounding profile (AB) and calculated resistivity (ƒÏ). The mean annual ground surface temperature (MAST)in 2003-04 is also displayed -261- 66 IKEDA A., SUEYOSHI T., MATSUOKA N., ISHII T., and UCHIDA Y. located on an uplifted peneplain composing the slopes (Figure 2D). Thus, groundwater hydrol- northeastern part of the Tibetan Plateau. In this ogy is susceptible to the presence of permafrost area, valley-fill alluvial plains are widespread be- only on the alluvial plains and terraces. The land tween 4200m and 4300m ASL, and hills rise up surface is dominated by grassland (alpine to 500m from the surrounding plains (Figure meadow) subjected to widespread grazing activ- 2A). Steep mountains are lacking even near the ity. The plains partly involve wetlands and lakes. divide of the Yellow River basin (Figure 2B). Sev- Bare ground is exposed only on some high hills eral measurements were also undertaken in a situated in dry and windy locations and the pe- mountainous district of Xinghai County, fringing riphery of recently degrading lakes. Geologically, the plateau. The difference in elevation between the plateau consists mostly of Paleozoic to Meso- the mountain ridges and valley floors generally zoic sedimentary rocks. Sandstone and shale un- ranges from 500 m to 1000 m. Wide river terraces derlie the sediments of the sounding sites. with thick deposits fill large valleys, and pedi- The plateau area lies in a transitional zone be- ments are well-developed on the foot of steep tween discontinuous and sporadic permafrost. slopes (Figure 2C). The long-term meteorological records at Madoi Contemporary weathering seems to be too (98°13'E,34°55'N.4273m ASL; Site 10 in Fig- slow to produce thick debris on the hill and ure 1)in 1953-1980 show a cold-dry climate mountain slopes, because a number of artificial with a mean annual air temperature (MART) of -4 outcrops along the road show that bedrock di- .1℃,an annual thermal amplitude ranging rectly underlies a thin (<1 m) loess layer on the from -16.8℃ in January do 7.5℃ in July and an Figure 2. Landscapes of the study area. (A) An alluvial plain between 4200 m and 4300 in ASL (Site 9 near Madoi). (B) A wide valley lying at 4600 in ASL near the divide of the Yellow River (Site 2). (C) Terrace surface in the mountainous region fringing the plateau (Site 14, 3800 m ASL). (D) An ar- tificial outcrop at the foot of a hill near Madoi, showing bedrock underlying a loess layer of 0.4m thick. - 262- Permafrost Sounding (2003-2005) in the Source Area of the Yellow River 67 annual precipitation of 304 mm (Zhou et al. (Palmer 1986). The reciprocal method (Palmer 2000). Decadal mean air temperatures increased 1986) was employed to obtain more accurate P- by 0.7℃ from the 1960s to 1990s(Yang et al. wave velocities of the second layer at Sites 4 and 2004).More recent records (2001-2005) show 10 by eliminating anomalies caused by irregular further rising MAAT to -2.0℃ with an annual ground and refracting surfaces. The soundings thermal amplitude ranging from -13.6℃ in Jan- were carried out in late August 2003 and middle nary to 9.1℃ in July and steady annual precipi- August 2004. cation of 304 mm (after WeatherOnline Asia Lim- The DC resistivity sounding was performed at ited, China). The small precipitation is reflected 11 sites in early July 2005 with the SYSCAL in shallow winter snow cover (Matsuoka et al.