Geographical Review of Japan Vol. 80, No. 5, 259-271, 2007

Permafrost Sounding (2003-2005) in the Source Area of the , 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 (-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 -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 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, , 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 , 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, ). The small precipitation is reflected 11 sites in early July 2005 with the SYSCAL in shallow winter snow cover (Matsuoka et al. R1 PLUS resistivity meter (Iris Instruments, 2005). These conditions favor deep seasonal France). The setting of the electrodes followed freezing, whereas the seasonally frozen layer the Schlumberger array. The length of a sound- 2.6 m deep at Madoi was completely thawed in ing profile varied from 200 m to 400 m at each June 2005 (Matsuoka et al. 2005). site. Modeled resistivity curves fitting with meas- ured values were calculated with WinSev6 soft- Methods ware (W_GeoSoft, Switzerland). First, the resis- tivities and boundary depths of subsurface lay- Subsurface structure ers were manually determined by fitting standard The presence of permafrost was examined by and auxiliary curves. Second, the initial values refraction seismic sounding at eight sites (Sites were automatically adjusted to measured resis- 2, 4, 5, 7, 9, 10, 13, 14) and one-dimensional tivities by least squares method (Koefoed 1979). (vertical) direct current (DC) resistivity sound- Where seismic data were available, boundary ing at eleven sites (Sites 2, 4, 5, 6, 7, 8, 9, 10, depths of resistivity layers were fixed to the 11, 12, 13) (Table 1, Figure 1). All sites but Sites nearby boundary depths of P-wave stratigra- 12, 13 and 14 are located on flood plains gentler phies. than 2° and wider than 2 km(see Figure 2A-B), In addition, the near-surface stratigraphy of which are underlain by fluvial sediments proba- the floodplains around Madoi was directly ob- bly much thicker than 10 m (Suzuki 1998: served with the cores recovered from a 10-m 362-363). Site 12 is located on a narrow flood- deep borehole at Site 10 (drilled on August 11, plain (<500 m wide) and Sites 13 and 14 are on 2004) and a 6-m deep borehole at Site 9 (August terraces (see Figure 2C). The terrace scarp 12, 2004). Site 10 lies at 4270 m ASL and Site 9 shows that the sediment is at least 10 m thick at at 4240 m ASL, which represent those of the Site 13 and more than 50 m thick at Site 14. Site widespread alluvial plains (4200-4300 m ASL) in 7 is located at a former shoreline of a recently the source area. degrading lake. Ground temperature In the seismic sounding, a seismic pulse pro- duced by a sledgehammer was sensed with re- Year-round ground surface temperatures were ceivers spaced at 2.5-5 m intervals along a 30-50 recorded at hourly intervals with a miniature m long survey line and transmitted to the Mc- data logger TR-51A (T & D Corporation, Japan), SEIS-3 seismograph (Oyo Corporation, Japan). which has a resolution of O.1ｰC and accuracy of The homogeneous topography of the sounding 0.5℃.Alogger was placed beneath a platy stone sites ensures the horizontally stratified structure (about 2 cm thick) at eight sites (Sites 1, 3, 4, within the target depth (<15 m deep). Thus, a 7, 11, 13, 14, 15) between 4790m and 3260m pair of travel time curves (i.e. forward and re- ASL in mid August 2003 (Figure 1, Table 1). All verse arrival times measured on one survey line) sites are located on flat terrain lacking shading was only measured at Sites 4 and 10, where the topography and vegetation. Data for the first and ground surface is slightly undulated or artificially second years were collected in mid August 2004 raked over. The P-wave velocities of two or three and in mid June 2005, respectively. layers and the depth of layer boundaries were Mean annual ground surface temperature determined using the intercept time method (MAST) is a useful indicator of permafrost

-263- 68 IKEDA A., SUEYOSHI T., MATSUOKA N., ISHII T., and UCHIDA Y.

(Ikeda and Matsuoka 2002; Ikeda 2006). Under ther saturated nor frozen cores. The whole sed- a stable climate, the upward geothermal flow iments consist mainly of gravels supported with leads to the lowest mean annual ground tem- sandy matrix, although the uppermost 0.15 m is perature (MALT) at the surface. Thus, a nega- embedded with silt. Silty layers (c. 0.5 m thick) tive MAST is required for the presence of per- are encountered at 2.1 m and 4.8m depths. mafrost at a few meters depth. In reality, how- P-wave velocity ever, the climatic warming has gradually re- versed the near-surface MAGI profile (e.g. Lu- The travel time curves of P-wave velocities are nardini 1996). Thus, degrading permafrost is classified into three types (Figure 3). The first likely present even under a slightly positive type shows a sharp break in a velocity profile MAST (e.g. Ikeda and Matsuoka 2002). MASTS within 10 m distance from the receiver (Sites 2, were computed from surface temperature data 4, 5, 7, 10). The second type shows two, more for 365 days. gentle breaks in a velocity profile indicating three layers (Sites 9, 13). The third type shows Results and Interpretation a nearly constant velocity (Site 14). The first layer of all curves has a velocity of 0.35- Borehole stratigraphy 0.49 km s-1 (Table 1). The first and second types At Site 10, the uppermost 2.4 m of sediments show a subsurface layer with a high velocity of consists of sandy silt lacking stones, which prob- 1.5-3.4 km s-1. The upper surface of each high ably represents loess. Below the topsoil, the ma- velocity layer of the first type lies at 2-4 m deep, terial is gravelly to the bottom of the borehole whereas that of the second type lies below 8 m (10 m deep) but mostly supported with sandy or deep. A velocity of 0.71-0.99 km s-1 was calcu- silty matrix, which is probably of a fluvial origin. lated for the second layer of Sites 2, 9, 13 and A clayey layer with pebbles is embedded be- 14. tween 5.4 m and 7 m depth. No core was ob- P-wave velocities lower than 1 km s-1 indicate tained in a frozen state. The groundwater level unconsolidated and unsaturated materials (Mil- lay at 2.7 m depth during the drilling. som 2003). The subsurface high velocities cor- At Site 9, a 6-m deep drilling recovered nei- respond to the values for various types of per-

Figure 3. Travel time curves of P-wave velocities. Calculated velocities and the depths of layer bases (D1 and D2) are indicated.

-264- Permafrost Sounding (2003-2005) in the Source Area of the Y ellow River 69

mafrost (1.5-4.7 km s-1: Hunter 1973) . Within ing trend shifts to slightly increasing or constant this velocity range, relatively high velocities at trend at the end . The increasing trend at Sites 7 Sites 2, 4, 5 and 13 (2 .3-3.4 km s-1) are also and 12 declines and becomes constant . Resistiv- equivalent to velocities of sandstone and shale ity continues to increase at Site 13 . bedrock, and relatively low velocities observed In general, electrical resistivity in ground in- at Sites 7, 9 and 10 (1.5-1.8 km s-1) to velocities creases when ground temperature falls below of unfrozen-saturated sediments (Milsom 2003) . 0℃ because of a decrease in liquid water con- In the former case, if the upper surface of the tent (e.g. Hoekstra and McNeill 1973) . Thus, as- high velocity layer is too shallow to be bedrock , suming homogeneous water content in the the high velocity layer is regarded as a frozen ground, the observed increase in resistivity sediment. Thus, the thin low-velocity top layer would indicate the presence of a frozen layer . (2-4 m thick) at Sites 2, 4 and 5 indicates the However, groundwater level and material com- presence of frozen ground at least above 4300 m position often lead to vertically variable water ASL, because bedrock lying at such shallow content, so that resistivity stratigraphies should depth is quite unusual. In contrast, locations be carefully interpreted by using other informa- below 4000m ASL (Sites 13 and 14) lack a near- tion (e.g. Ikeda 2006). surface high velocity layer (Table 1) . Site 13 All of the uppermost resistivity layers showing (3960 m ASL) indeed has a high velocity third alow resistivity of 16-270Ωm are thin:ner than layer (3.0 km s-1), although the top of this layer two meters(Table 1). This layer corresponds to lies at 13 m deep. The third layer probably rep- aloess and/or organic soil layer. The second re- resents bedrock, as exposed on the cliff fringing sistivity layers (140-960Ωm) mainly consist of the river terrace. gravels, as indicated by the excavated pits and Some seismic data fail to present unequivocal terrace outcrops. Thus, the downward increase interpretation. The intermediate velocity (1.5- in resistivity at a shallow depth results from the 1.7 km s-1) observed at 4200-4300m ASL indi- difference in grain size distribution between the cates either a frozen or an unfrozen-saturated fine-rich uppermost layer and the gravel-rich un- sediment. The uppermost 4 m thick unfrozen derlying layer. layer at Sites 2 and 5 possibly indicates degrad- At Sites 2, 4 and 5, the most resistant ing permafrost, because seasonal frost depth (650-1100Ωm) third layer corresponds to a rarely exceeds 3 m in the study area (Wang et al. frozen sediment indicated by the seismic sound- 2000; Matsuoka et al. 2005), whereas the accu- ing (Table 1). The thickness of the frozen ground racy of the measurement is ±1m(lkeda 2006). is estimated to be 27 m at Site 2, 10 m at Site 4 The top of the frozen layer at Site 4(≦3m deep) and 26 m at Site 5, assuming that the grain size indicates the thawing front in mid-August, so distribution is almost constant in the sediments. that whether the frozen layer is perennial or sea- In contrast, the resistivity at Sites 7, 9 and 10 sonal is also uncertain. decreases toward the third layer(30-140Ωm). The borehole cores at Sites 9 and 10 show that This layer corresponds to a P-wave velocity layer the low velocities (0.35-0.75 km s-1) correspond of 1.5-1.8 km s-1, which indicates the groundwa- to unsaturated and unconsolidated (i.e. un- ter level. At these three sites, permafrost is prob- frozen) sediments (Table 1). In addition, the sec- ably absent below the groundwater level because ond layer (1.7 km s-1) lying below about 3 m at the low resistivity layer(s) is thicker than several Site 10 does not correspond to a frozen layer but tens meters. The bedrock indicated by the seis- represents an unfrozen-saturated layer. mic profile at Site 13 has a higher resistivity

(500Ωm) than the overlying unfrozen sedi- DC resistivity meets. The apparent DC resistivity increases from the The resistivity curve at Site 6 has a similar ground surface downward at all sites (Figure 4). profile to those of Sites 2 and 4, although the ap- Further downward decrease in resistivity results parent resistivities decrease from a relatively in a convex-up curve at eight sites (Sites 2, 4, 5, shallow depth (Figure 4). The large downward 6, 8, 9, 10, 11). At Sites 9 and 10, the decreas- increase in resistivity at Sites 8 and 12 results

-265- 70 IKEDA A., SUEYOSHI T ., MATSUOKA N., ISHII T ., and UCHIDA Y.

Figure 4. Apparent electrical DC resistivity curves and calculated resistivities. Note that the calculated re- sistivities of the first layer thinner than 1 m are not displayed. from large difference in water content between Ground temperature the uppermost fine-rich layer and subsurface gravelly layer. Despite the presence of a high re- Figure 5 displays annual variation in daily sistivity layer (790-960Ωm), the presence of mean surface temperatures at four sites located permafrost is unclear at Sites 6, 8 and 12, be- at intervals of about 500m in elevation. A com- cause dry, coarse sediments have such a resis- parison between the four sites shows that eleva- tivity even in the unfrozen state. The large in- tion significantly affects summer temperatures, crease in resistivity near the ground surface may but it only slightly contributes to winter tem- mask a slight increase in resistivity between an peratures. Elevation is also responsible for time unfrozen layer and a frozen layer. The resistivity lags in the start of seasonal freezing and that of stratigraphy at Site 11 shows a high resistivity seasonal thawing. A rise in 1500 m from Site 15 layer(840 Ωm)at 16-44 m depth, which indi- to Site 1 causes one month earlier freezing and Gates possible presence of relict permafrost. one and half months later thawing. MASTS in 2003-04 at eight sites varied from

-266- Permafrost Sounding (2003-2005) in the Source Area of the Yellow River 71

Figure 5. Annual variation in mean daily surface temperature at four sites located at an interval of about 500m in elevation.

temperatures in winter (cf. French 1996: 63), which resulted in the similar daily mean tem- peratures in winter except at the lowermost Site 15 (Figure 5). Inversion of night temperatures between 4200m and 4800m ASL and between 3800 m and 4000 m ASL was observed from mid- October to January (Figure 7). This is probably because radiative cooling is more prominent in the broad basin (Sites 11, 14) than hilly loca- tions (Sites 1, 3, 4, 13). In addition, cloudy/foggy conditions in winter daytime may contribute to lower temperatures at lower locations near the foot of hills (Sites 4 and 7) than higher locations near the crests of hills (Sites 1 and 3) on the plateau. In particular, the smallest daily variation Figure 6. Altitudinal variation in the mean an- in temperature at Site 7 may be maintained by nual surface temperature (MAST). exceptionally cloudy/foggy conditions near a lake The regression line is expressed by (Figure 7B). MAST=-4.5H+20.2, where H is ele- Snow cover insulating ground surface was vation in km. mostly lacking in the study area, which was in- dicated by large daily variations in ground sur- -0 .8℃ to 5.7℃ with increasing elevation.(Table face temperatures and low precipitation. In fact, 1, Figure 6). The overall relation is given by precipitation from November to March was 13 MAST= -4.5H+20.2, where H is elevation in km. mm in 2003-04 and 40 mm in 2004-05 at the However, MASTS were similar between Sites 1, 3 Madoi meteorological station (after WeatherOn- and 4 and between Sites 13 and 14 despite the line Asia Limited). Continuous snow cover last- significant difference in elevation (200-400 m). ing two days or more occurred only three times This was probably due to strong inversion of in the winter 2004-05, with the longest period of

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Figure 7. Typical diurnal variations in ground surface temperature in December 2003 at 7 sites, showing al- titudinal inversion in temperature. (A) Temperatures from 4220 m to 4790 m ASL. (B) Tempera- tures from 3780 m to 4220 m ASL. Site 7 (4220 m ASL) is located near a lake.

20 days in February (Matsuoka et al. 2005). Dur- Permafrost Distribution and ing these three periods, snow depth never ex- Degradation ceeded 10 cm. Thus, snow cover rarely con- tributes to the ground surface temperatures in A combination of the seismic, electrical and the study area. thermal soundings suggests that relatively stable In 2004-05, batteries of six out of the eight permafrost occurs widely over 4300 m ASL, that data loggers were leaked in wet soils probably permafrost is mostly absent below 4200 m ASL, after heavy rainfalls in July 2005, which resulted and that the widespread alluvial plains between in data omission at the six sites. The rest of 4200 m and 4300 m ASL lack permafrost or have loggers showed MASTS of -0.9℃ at Site 4 and degrading permafrost below a supra-permafrost 2.7℃ at Site 13, which were within±0.2℃ of val- talik. In the study area, Wang et al. (2000) also ues in the previous years. These results were described that the lower boundary of permafrost consistent with the only slight decrease in MAAT lay at 4270 m ASL at the end of the 1990s, al- of 0.2℃ from the previous year at Madoi(after though they presented no specific data. Zhang et WeatherOnline Asia Limited). Assuming high al. (2004) reported that four out of eight bore- probability of the presence of permafrost where holes between 4200 and 4300m ASL showed MAST is negative under the present warming frozen sediments 2.5-8.5 m thick, and discussed trend, permafrost is considered to occur at least that permafrost was degrading'. A supra-per- above 4300 m ASL. This boundary elevation re- mafrost talik lay above 5-8 m deep at the two inforces the interpretation of the geophysical sites. The frozen sediments at the other two soundings that the frozen ground above 4300 m sites, however, probably included seasonally ASL is perennial. frozen part because the overlying unfrozen layer

-268- Permafrost Sounding (2003-2005) in the Source Area of the Yellow River 73

was too thin (c. 1 m thick) to be an active layer . In contrast, at least until the 1980s , 70-80% of Conclusions the plateau surface, i.e. the area except for lakes , streams and nearby swamps , was classified into Seismic, electrical and thermal soundings con- permafrost terrains (Wang 1987; Wang et al. ducted at 15 sites in the source area of the Yel- 1991)2. Thus, the source area of the Yellow River low River, northeastern Tibetan Plateau, lead to currently faces a rapid loss of the permafrost the following conclusions. area, since the elevations mostly belong to a High P-wave velocities (>2 km s-1) and rela- transitional condition between permafrost and tively high DC resistivities(650-1100 Ωm) seasonal frost environments. Several pits exca- below the thin uppermost layer (2-4 m thick) at vated on alluvial plains near Madoi showed that three sites show that permafrost occurs above permafrost was generally 15-20 m thick and the 4300 m ASL. In contrast, low P-wave velocities permafrost table lay at about 5 m depth in the (<1 km s-1) throughout the sediments at two early 1980s (Wang 1987). This indicates that per- sites indicate that permafrost is absent below mafrost at some places on the plateau began de- 4000 m ASL. At three sites located on the rep- grading before the 1980s, because seasonal frost resentative elevation of the plateau (4200-4300 depth rarely exceeds 3 m in the study area m ASL), subsurface low resistivities (30-140 (Wang et al. 2000; Matsuoka et al. 2005). The ab- Ωm)indicate the absence of permafrost below sence of permafrost at Sites 9 and 10 shows that the groundwater level detected by intermediate the reported permafrost has considerably disap- P-wave velocities (1.5-1.7 km s-1). In contrast, peared around Madoi after the 1980s. the presence of permafrost is possibly indicated MAAT of-1.8℃ in 2003-04 at Madoi(after by high DC resistivities at the other four sites be- WeatherOnline Asia Limited), which represents tween 4200 m and 4300 m ASL, although this in- the average condition in 2001-05(-2.0℃), was terpretation should be tested by other methods 2.2℃ higher than the long-term average of such as seismic sounding. MAATs of -4.0℃ (1953-1992, after Liu et al. Negative MASTS also indicate that permafrost 2002).Assuming that the difference between is present above 4300 m ASL under the present MAAT and MAST at a site is constant, average climatic conditions. In contrast, MASTS are

MASTS in 1953-1992 would be about 2℃ lower higher than 2℃ on the valley-fill terraces below than the MASTS in 2003-04. Thus, the average 4000 m ASL, which indicates the absence of per-

MASTS in 1953-1992 were estimated to be about mafrost. Degrading permafrost may remain be- -1℃ at 4220 m ASL (see Table l for the pres - tween 4200 m and 4300 m ASL, where MASTS are ent MAST at Sites 7 and 11). MASTS on the plain slightly positive(c.1℃). area between 4200 m and 4300 m ASL are con- Assuming that MAST has consistently in- sidered to have turned positive within the past creased with MAAT during the last half-century, several decades, which induces permafrost the permafrost at 4200-4300 m ASL is estimated degradation. to have significantly disappeared in the same pe- High DC resistivities observed at the four sites riod. As a result, permafrost has mostly thawed between 4200 m and 4300 m ASL may indicate along the mainstream of the Yellow River on the the presence of permafrost, although the inter- plateau, although a geocryological map edited in pretation should be tested by other methods the early 1990s included the whole area in a per- such as seismic sounding. In addition, dry per- mafrost region. mafrost, containing neither free water nor ice, is ruled out from this study, because its occurrence Acknowledgements is impossible to be detected by indirect geo- physical methods. Although dry permafrost We acknowledge Drs. C. Gao, Z. Han and Mr. J. Ding rarely exists, further study mainly based on mon- for logistical help and field assistance. The study was itoring of borehole temperatures will demon- supported by a national program `Sustainable Coexis- strate the degradation of permafrost in the study tence of Human Nature and the Earth' founded by the area more precisely. Ministry of Education, Culture, Sports, Science and

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Technology. sevler. (Received 13 April 2006) Lachenbruch, A. H., and Marshall, B. V. 1986. Chang- (Accepted 25 December 2006) ing climate: Geothermal evidence from permafrost in the Alaskan Arctic. Science 234: 689-696. Notes Liu, S., Lu, A., Ding, Y., Yao, T., Ding, L., Li, G., and Hooke, R. L. 2002. Glacier fluctuations and the in- 1. The dates of the investigation were not specified ferred climate changes in the A'Nyemagen Moun- in Zhang et al. (2004), whereas the discussion fo- tains in the source area of the Yellow River . Jour- cused on the distribution of permafrost in the late nal of Glaciology and Geocryology 24: 701-707 . 1990s. (CE) 2. Wang (1987) had estimated that the lower limit Lunardini, V. J. 1996. Climatic warming and the degra- of permafrost around Madoi lay at 4050 m ASL. dation of warm permafrost. Permafrost and However, Wang et al. (2000) revised the bound- Periglacial Processes 7: 311-320. ary in the 1970s at 4220 m ASL. Although the sur- Matsuoka, N., Ikeda, A., Sueyoshi, T., and Ishii, T. 2004. face of the plateau mostly lies above the revised Permafrost sounding (2003-2004) in the source elevation, the early study may overestimate the area of the Yellow River, China. Annual Report of distribution of permafrost. the Institute of Geoscience, the University of Tsukuba 30: 33-38. References Matsuoka, N., Ikeda, A., Sueyoshi, T., and Ishii, T. 2005. Monitoring frozen ground (2004-2005) at Madoi in French, H. M. 1996. The periglacial environment, the source area of the Yellow River, China. 2nd ed. Essex: Longman. Tsukuba Geoenvironmental Sciences 1: 39-44. Harris, C., Vonder Miihll, D., Isaksen, K., Haeberli, W., Milsom, J. 2003. Field geophysics, 3rd ed. Chichester: Sollid, J. L., King, L., Holmlund, P., Dramis, F., John Wiley & Sons. Guglielmin, M., and Palacios, D. 2003. Warming Osterkamp, T. E., and Romanovsky, V. E. 1999. permafrost in European mountains. Global and Evidence for warming and thawing of discontinu- Planetary Change 39: 215-225. ous permafrost in Alaska. Permafrost and Hoekstra, P., and McNeill, D. 1973. Electromagnetic Periglacial Processes 10: 17-37. probing of permafrost. In Proceedings of the Sec- Owen, L. A., Finkel, R. C., Haizhou, M., Spencer, J. Q., ond International Conference on Permafrost, Derbyshire, E., Barnard, P. L., and Caffee, M. W. 517-526. Washington: National Academy of Sci- 2003. Timing and style of Late Quaternary glacia- ences. tion in northeastern Tibet. Geological Society of Hunter, J. A. M. 1973. The application of shallow seis- America Bulletin 115: 1356-1364. mic methods to mapping of frozen surficial mate- Palmer, D. 1986. Refraction seismics. London: Geo- rials. In Proceedings of the Second Interna- physical Press. tional Conference on Permafrost, 527-535. Peng, X., Wu, Q., and Tian, M. 2003. The effect of Washington: National Academy of Sciences. groundwater table lowering on ecological envi- Ikeda, A. 2006. Combination of conventional geophys- ronment in the headwaters of the Yellow River. ical methods for sounding the composition of rock Journal of Glaciology and Geocryology 25: glaciers in the Swiss Alps. Permafrost and 667-671. (CE) Periglacial Processes 17: 35-48. Sharkhuu, N. 1998. Trends of permafrost development Ikeda, A., and Matsuoka, N. 2002. Degradation of talus- in the Selenge River Basin, Mongolia. In Proceed- derived rock glaciers in the Upper Engadin, Swiss ings of the Seventh International Conference Alps. Permafrost and Periglacial Processes 13: on Permafrost. ed. A. G. Lewkowicz and M. 145-161. Allard, 979-985. Sainte-Foy: Centre d'etudes Ikeda, A., Matsuoka, N., and Sueyoshi, T. 2004. Per- Nordiques. mafrost survey in the source region of the Yellow Suzuki, T. 1998. Kensetsu gijutsusha no tameno River: A preliminary report. Seppyo 66: 235-239. chikeizu dokuzu nyumon. Vol. 2. Teichi (In- (J) troduction to map reading for civil engineers. Jin, H., Li, S., Cheng, G., Wang, S., and Li, X. 2000. Per- Vol. 2. Plains). Tokyo: Kokon-shoin. (J) mafrost and climatic change in China. Global and Wang, G., Qian, J., Cheng, G., and Lai, Y. 2001. Eco- Planetary Change 26: 387-404. environmental degradation and causal analysis in Koefoed, O. 1979. Geosounding principles, 1. Resis- the source region of the Yellow River. Environ- tivity sounding measurements. Amsterdam: El- mental Geology 40: 884-890.

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