Review of Holocene glacial chronologies based on radiocarbon dating in Tibet and its surrounding mountains

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1 JOURNAL OF QUATERNARY SCIENCE (2008) 23(6-7) Copyright ß 2008 John Wiley & Sons, Ltd. Published online in Wiley InterScience ( Review of Holocene glacial chronologies based on radiocarbon dating in Tibet and its surrounding mountains CHAOLU YI, 1 * HUALIANG CHEN, 1 JIANQIANG YANG, 1 BIN LIU, 1 PING FU, 1 KEXIN LIU 2 and SHIJIE LI 3 1 Laboratory of Tibetan Environment Change and Land Surface Processes, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing, China 2 Institute of Heavy Ion Physics, Peking University, Beijing, China 3 Institute of Environment and Engineering in Cold and Arid Regions, Chinese Academy of Sciences, Lanzhou, China Yi, C., Chen, H., Yang, J., Liu, B., Fu, P., Liu, K. and Li, S Review of Holocene glacial chronologies based on radiocarbon dating in Tibet and its surrounding mountains. J. Quaternary Sci., Vol. 23 pp ISSN Received 23 September 2007; Revised 17 April 2008; Accepted 30 June 2008 ABSTRACT: A compilation of C ages on moraines constrains the Holocene glacial history of Tibet and the surrounding mountains. Several Holocene glacial stages are identified. The Little Ice Age had three substages at cal. ka BP. However, the glacial advances may have occurred a earlier in southern and eastern bordering mountains than in the northern bordering mountains. The earlier glacial advance might have been driven by humid conditions in bordering areas of Tibet. Most glacial advances in the Neoglaciation occurred at cal. ka BP. They can be identified in almost every mountain range. An early Holocene glacial advance occurred cal. ka BP, in both central Tibet and bordering mountains. The timings of Holocene glacial advances are synchronous with the cooling periods in the d 18 O record from ice cores and with the results of optically stimulated luminescence dating and cosmogenic exposure dating in Tibet. Copyright # 2008 John Wiley & Sons, Ltd. KEYWORDS: Holocene; radiocarbon dating; glacial stages; Tibet. Introduction Knowledge of Holocene glacial chronology is poorly constrained despite its usefulness in predicting future climate change. Glacial advances during the Holocene were not caused directly by orbital Milankovitch forcing. Rather, they were quite possibly related to the local precipitation changes during a general cooling period. The Tibetan Plateau and its surrounding mountains, with a surface land area of more than 2 million km 2, is the highest place in the world and plays an important role in determining regional climate change in the Holocene. Morrill et al. (2003) compiled and analysed 36 previously published palaeoclimate records and found that Asian monsoon precipitation increased dramatically at the start of the Holocene (11.5 cal. ka BP), which is synchronous with an abrupt warming in the North Atlantic; those authors also observed a weakened monsoon strength at about cal. ka BP. Herzschuh (2006) analysed the Holocene climate changes in Central Asia from 75 published dates retrieved from lakes, loess and peat sediments. He concluded that the climate was wet during the early Holocene in the area dominated by the South Asia monsoon, while the optimal conditions prevailed in the area controlled by the Southeast Asia monsoon * Correspondence to: C. Yi, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Shuangqing Road 18, Beijing , China. clyi@itpcas.ac.cn and Westerlies during the mid Holocene. Lehmkuhl (1997) summarised Late Pleistocene glaciations in Tibet and derived two glacial readvances at about 3 ka BP and during the Little Ice Age (LIA). Most recently, cosmogenic radionuclide exposure dating has concentrated mainly on the last glaciation (e.g. Lehmkuhl and Owen, 2005), although an early Holocene glacial readvance was identified by several workers (e.g. Sharma and Owen, 1996; Richards et al., 2000; Tsukamoto et al., 2002; Owen et al., 2003; Damm, 2006). During the last few decades, a number of studies have provided preliminary radiocarbon dates to help define the timing of Holocene glaciation in Tibet and surrounding mountains. These dates are widely dispersed in the literature. Most of the publications are written in Chinese, and it is difficult for researchers outside of China to access them. In this paper, we therefore compile 53 published radiocarbon dates to examine the timing of Holocene glaciation throughout Tibet and the bordering mountains. These results have important implications for modelling glaciation, climate dynamics and hydrology in the world s most populated region. Data Source and Methods We collected data on 53 radiocarbon dates on glacial moraines in Tibet and the surrounding mountains (Table 1). Figure 1

2 534 JOURNAL OF QUATERNARY SCIENCE Table 1 Radiocarbon dates for Holocene glaciations in Tibet and surrounding mountains Site Dating material and geomorphic position Age limiting 14 C a BP Cal. 14 C a BP Source Tianshan Qilianshan West Kunlun East Kunlun Tanggula Qiangtang Plateau Nyainqentanglha Karakoram Pamirs Himalaya Hengduan Mountains Two of the largest dead lichens in a terminal moraine, the upper Urumqi River Valley Humic matter in buried soil interlayer in lower part of an overlapped terminal moraine, Baishui River valley Humic matter in a lacustrine interlayer in a terminal moraine, Dunde ice cap Humic matter in a lacustrine interlayer in terminal moraines, Keliya Glacier and North Glacier Humic matter cm deep in two terminal moraines, Guoluo Glacier Humic matter at the bottom of soil covering terminal moraines, Gaerqu River Humic matter in an exposure of a terminal moraine, Gangzari Mt Charcoal and humic soil in moraines, Zepu Glacier Wood in a lateral moraine, Aza Glacier Wood in a moraine, Ruoguo Glacier Humic matter in soil 1 2 m below the turf on the terminal moraine, Luzi Glacier Wood in moraines in front of Mina Glacier Wood root in terminal moraines, Kungai Peak Wood in a terminal moraine in front of Zelongnong Glacier Wood in moraine in front of Khumbu Glacier Organic matter in terminal moraines, Lantang Valley Humic soil buried in a terminal moraine, Raphsthreng Glacier, upper reaches of Lingshi Chu (river) and Goyok Chu Wood in lateral and terminal moraines, Gonggashan Median Yi et al. (2004) Wu (1984a,b) Maximum Li and Jiao (1990) Maximum Earlier than Jiao and Zheng (2006) Zheng et al. (1995) Jiao and Shen (2006) Median Li and Li (1992) , Jiao and Iwata (1993); Iwata and Jiao (1993) Li et al. (1986) Zheng and Li (1986) Earlier than Zheng (2006) minimum Median Maximum Maximum Derbyshire et al. (1984); Li et al. (1986) Ono et al. (1997) Zhang (1988) Fushimi (1978) Shiraiwa and Watanabe (1991) Iwata et al. (2002) Li et al. (1986); Zheng and Ma (1994) (Continues)

3 HOLOCENE GLACIAL FLUCTUATIONS IN TIBET 535 Table 1 (Continued) Site Dating material and geomorphic position Age limiting 14 C a BP Cal. 14 C a BP Source Humic matter in moraines, Diancangshan Black clay at the bottom of lacustrine sediment between two moraines, Haizishan Yang et al. (2006) Maximum Zheng and Ma (1995) Figure 1 Satellite image from Google Earth showing study regions in Tibet and its surrounding mountains shows the study sites. All radiocarbon dates presented are calibrated at 68.2% significance using Calib5.1 software (Stuiver and Reimer, 1993; We use the weighted mean for a calibrated timing span. Radiocarbon dates from selected regions Tian Shan Range Dating work on the moraines has been focused in the middle part of the Tian Shan ( N, E, Fig. 1). The radiocarbon dating on two of the largest dead lichens in a terminal moraine 0.5 km away from Glacier 1 in the upper Urumqi River Valley yielded minimum ages of and cal. ka BP (Yi et al., 2004). These data are supported by the lichenometry of Chen (1988), who dated this moraine to AD Qilian Range In the Qilian Range, a 3 km long moraine is present in the upper stream in the Gangshika Valley, Baishui River, on the south slope of eastern Qilian. Wu (1984a,b) dated organic soil beds within a terminal moraine downstream and obtained median ages of and cal. ka BP. Li and Jiao (1990) compared radiocarbon dating on the terminal moraine formed in early Holocene glacial advances in West Kunlun and Qilianshan and reported that the lacustrine interlayer in one of the terminal moraines in front of a glacier on the north side of Dunde ice cap was dated to cal. ka BP. The lacustrine sediment is believed to have been deposited before glacial advance. This date represents the maximum age of this glacial advance. Kunlun Range In West Kunlun, several glaciers radiate from the Chongce ice cap. Humic matter in the folded lacustrine interlayer is present within several terminal moraines. The top of the lacustrine sediment in one of the moraines, about 3.5 km from Keliya Glacier, dates to cal. ka BP. Another in North Glacier Valley dates to cal. ka BP (Li and Jiao, 1990; Jiao and Zheng, 2006). Lacustrine sediment was believed to have been deposited in proglacial lakes. Glacial advance

4 536 JOURNAL OF QUATERNARY SCIENCE overlapped the lacustrine sediment. The dates may represent the maximum age of this glaciation. In East Kunlun, humic matter was found at a depth of cm in two terminal moraines at 4100 m above sea level (a.s.l.) and 3 km from the modern glacier terminus on the northern slope of Guoluo Peak, Bayanhar Mountains. The humic matter dates to minimum ages of and cal. ka BP (Zheng et al., 1995). Tanggula Range Soil covers three terminal moraines in upper Womeitongdong River Valley, a branch of the Gaerqu River, on the eastern slope of Geladangdong Mountain. Humic matter at the bottom of the soil layer, 60 cm deep on the downstream moraine, dates to a minimum age of cal. ka BP and the upstream moraine to a minimum age of cal. ka BP (Fig. 2) (Jiao and Shen, 2006), suggesting that two glacial advances occurred earlier than these two dating ages. Qiangtang Plateau Qiangtang Plateau is located in central Tibet and has an average height of m a.s.l. Three terminal moraines lie 1 km from the glacier snout in the Gangzari Mountains ( N, E). Humic matter in a river exposure in one of the moraines dated to a median age of cal. ka BP (Li and Li, 1992). Nyainqentanglha Range Radiocarbon dating in the Nyainqentanglha Range focused on Zepu Glacier, Arza Glacier and Ruoguo Glacier, on the southern slope of eastern Nyainqentanglha (Figs. 3 and 4(A) and 4(B)). Since the dated materials were buried wood and organic matter in a palaeosol at or near the top of moraines, dating results give minimum ages of moraines. Jiao and Iwata (1993) dated samples in six moraines in a 13 km long valley below the Zepu Glacier and obtained ages of , , cal. ka BP (charcoal), and , and cal. ka BP (buried humic soil). Iwata and Jiao (1993) also provided three additional ages of cal. ka BP (charcoal), cal. ka BP (charcoal) and cal. ka BP (humic soil) on the terminal moraines in front of Zepu Glacier. Zheng and Li (1986) dated the buried wood in a lateral moraine in the valley of Ruoguo Glacier and obtained ages of and cal. ka BP. Li et al. (1986) dated buried wood in a high lateral moraine of the Arza Glacier 100 m above the glacier to an age of cal. ka BP. In the western Nyainqentanglha, at an elevation of 5100 m a.s.l., organic soil, 1 2 m below the turf on the terminal moraine of the Luzi Glacier, was dated to a minimum age of cal. ka BP (Fig. 4(C); Zheng, 2006). Karakoram Range Buried wood is present near the top or in the upper layer of moraines on the western side of the Minapin Glacier. Two lateral moraines were dated to minimum ages of and cal. ka BP (Fig. 5; Derbyshire et al., 1984), and a terminal moraine in front of Mina Glacier was dated to a minimum age of cal. ka BP (Li et al., 1986). Pamirs Ono et al. (1997) dated the two samples of buried wood root to minimum ages of and cal. ka BP in Figure 2 Map of the glacial sequences and chronology in the Gaerqu, Tanggula Mountains. Modified after Li and Li (1992)

5 HOLOCENE GLACIAL FLUCTUATIONS IN TIBET 537 Figure 3 Map showing glacial sequences and chronology in Zepu Glacier Valley, East Nyainqentanglha (modified after Jiao and Iwata, 1993) the upper layer of two terminal moraines m away from the modern glacier at the source of the Ayizhisu River, a tributary of the Wuyitak River on the northern slope of Kungai Peak. Himalaya Range Fushimi (1978) studied a series of moraines in the Khumbu Valley on the southern slope of Qomolangma Peak (Mount Everest) and dated buried wood in moraines to minimum ages of and cal. ka BP. Shiraiwa and Watanabe (1991) dated humic matter in three sets of moraines in Lantang Valley, Nepal Himalaya, to a minimum age of cal. ka BP for a hummocky moraine and an age of cal. ka BP, a little later than minimum age, for a terminal moraine, a minimum age of cal. ka BP and median age of cal. ka BP for a terminal moraine (Fig. 6). Buried wood in two moraines in front of the Zelongnongba Glacier was dated to minimum ages of and cal. ka BP on the western slope of Namjagbarwa Peak, eastern Himalaya (Zhang, 1988). Iwata et al. (2002) dated the buried soil layer about cm deep in the middle ridge of the three terminal moraines in front of the Raphsthreng Glacier, upper reaches of Lingshi Chu (river) and Goyok Chu on the eastern side of the Himalaya and obtained a minimum age of to cal. ka BP.

6 538 JOURNAL OF QUATERNARY SCIENCE Figure 4 Map showing (A) the dating sites in front of the Guoruo Glacier, (B) cross-section showing Arza Glacier and (C) sketch showing Luzi Glacier, Nyainqentanglha (modified after Zheng, 2006) Hengduan Range The Hengduan Range extends from south to north in eastern Tibet. Li et al. (1986) and Zheng and Ma (1994) dated buried wood in a series of latero-terminal moraines in Hailuogou Glacier Valley on the eastern slopes of Gongga Mountain. The buried wood yielded minimum ages of , and cal. ka BP for the lateral moraines and , , and cal. ka BP for terminal moraines (Fig. 7). Buried wood near the bottom of a lateral moraine at an elevation of 3660 m and 9 km from the glacier snout in Swallow Valley on the northeastern Figure 5 Cross-section sketch showing the stratigraphy of the sampled sites in front of Minaping Glacier, Hunza Valley, Karakoram Mountains (after Derbyshire, 1984)

7 HOLOCENE GLACIAL FLUCTUATIONS IN TIBET 539 Figure 6 Sketch map showing the locations of the dated sites in Lantang Valley, Nepal Himalaya (modified after Shiraiwa and Watanabe, 1991) slope of the mountain was dated to maximum ages of and cal. ka BP. At Diancang Mountain, in the southern part of the Hengduan Range, humic matter at the top layer of a moraine at the outlet of a glacial cirque was dated to a minimum age of cal. ka BP (Yang et al., 2004). Yang et al. (2006) also dated humic matter in the till silt that was deposited on a glacial erosional platform on the eastern side of Sanyang Peak, and the moraine covering a protruding rock in front of a cirque on the western side of Longquan Peak. They obtained minimum dates of and cal. ka BP, respectively. Zheng and Ma (1995) observed four terminal moraines in the lower reach of the Yangyingcuo Valley in Haizishan Mountain, eastern Tibet. Fluvial lacustrine sediment 20 m thick was deposited between the two upstream moraines. Black clay at the bottom of the lacustrine sediment was dated to cal. ka BP. The authors suggested that a proglacial lake was formed at the same time as the moraine close to upstream and the age of the lacustrine deposit could represent the maximum age of the moraine. Glacial fluctuations in holocene Based on earlier glacial geomorphology work and sedimentology in Tibet and its surrounding mountains, Holocene glacial activity has traditionally been divided into two stages: the LIA with three sub-advances, and the Neoglacial stadial with two or three subglacial advances. However, the radiocarbon

8 540 JOURNAL OF QUATERNARY SCIENCE Figure 7 Sketch map showing the locations of the dated sites in front of Hailuogou Glacier, Hailuogou Valley, Gongga Mountain, eastern Tibet (after Zheng and Ma, 1994) dating seems to show that there were more glacial advances in some regions. Timing of glacial advances Two or three terminal moraines lie within hundreds of metres to a few kilometres from the terminus of modern glaciers in most glaciated areas in western China, representing two or three advances in the LIA. Since the oldest ages from this series of terminal moraines were , and cal. ka BP in front of the Zepu Glacier of eastern Nyainqentanghla (Iwata and Jiao, 1993; Jiao and Iwata, 1993), and a lateral moraine was dated to cal. ka BP in the Hailuogou Glacier of Gonggashan (Zheng and Ma, 1994), we set the boundary of the LIA at cal. ka BP (Fig. 8). Some moraines date to a minimum age of to cal. ka BP in Pamirs, eastern Nyainqentanglha and Gonggashan of the Hengduan Mountains. A number of radiocarbon dates show that the minimum age of the downstream moraine of the three terminal moraines was to cal. ka BP in Tianshan, Namjagbarwa Peak of eastern Himalaya, and the southern slopes of Mt Everest, representing an earlier glacial advance during the LIA. However, the downstream moraine of the three terminal moraines dates to a minimum age of to cal. ka BP in eastern Nyainqentanglha, Gonggashan of eastern Tibet, Karakoram and Xixiabangma Peak of middle Himalaya. Some even older moraines date to to cal. ka BP in Gongga Mountain of eastern Tibet and eastern Nyainqentanglha. The minimum ages show that glacial advances were about a earlier than in other regions. It seems that a glacial advance in the LIA occurred earlier in southern and eastern bordering mountains than in western and northern bordering mountains. Based on the large number of observations in the field, there are clearly three terminal moraines beyond the moraines formed in the LIA in eastern and southern Tibet, while there are only two terminal moraines in central and northwestern Tibet. This indicates that there may be one more glacial advance in southern and eastern Tibet than in central and northern Tibet. Radiocarbon ages show that most of these glacial advances occurred at to cal. ka BP. A few dates show that glacial advances occurred at to cal. ka BP in Tangglhashan, Qiangtang Plateau and Diancangshan and to cal. ka in Kunlun and Hengduan Range, 1 3 ka earlier than the rest of the study areas (Table 1 and Fig. 8). It is difficult to determine the driving force for this timing difference of glacial advances. Four ages show a glacial advance beginning at to cal. ka BP in the western Kunlun Mountains and Daocheng of eastern Tibet, and the western Qilian Mountains, representing an earliest glacial advance in the Holocene. Comparison with other dating results Beryllium-10 exposure dating has been widely used to determine the timing of glacial advance in Himalaya and eastern Tibet in recent years. Three samples from the lateral moraines in MachaKhola Valley, Gorkha Himal, Nepal, gave exposure ages of , and ka (Zech et al., 2003). The early Holocene dates on the Chhukung glacial stage in Khumbu Himal, southern slope of Mount Everest, yields an age of 9.2 ka (n ¼ 5), and the previously undated Thuklha glacial stage yields an age of 3.6 ka (n ¼ 3) (Finkel et al., 2003). Two glacial advances occurred at (n ¼ 3) and (n ¼ 4) in Kyrrola Pass. The dates concerning laterofrontal moraines of the Kulti Glacial Stage in Lahul Himalaya, northern India, show that a minor glacial advance occurred at ka (Owen et al., 2001). A glacial advance constructed 1 2 km long, well-defined moraines with strong varnished and weathered boulders in Hunza Valley, Karakoram Mountains, northern Pakistan. Their age was determined to be ka (n ¼ 5) (Owen et al., 2002). The dates of moraines in Nanga Parbat show that two Holocene glacial advances occurred at ka (n ¼ 4) and ka (n ¼ 4) (Phillips et al., 2000). 3 He exposure dating also shows a significant retreat around 10 ka in glacial stadials at 8.5, 7.5 and 6 5 ka at the upper valley of the Mailun Khola, Ganesh Himal in central Nepal (Gayer et al., 2006). Owen et al. (2005) dated the series of latero-terminal moraines in Hailuogou Valley,

9 HOLOCENE GLACIAL FLUCTUATIONS IN TIBET 541 Figure 8 A diagram showing the timing sequence of Holocene glacial stadials dated by 14 C in Tibet and its surrounding mountains Gongga Mountain. The old moraine was dated to be ka (n ¼ 5), with an average age of 8.22 ka. The young moraines yield different ages: ka (n ¼ 2), ka (n ¼ 4), with average ages of 0.77, 1.33 and ka (n ¼ 2). Using optically stimulated luminescence dating techniques, several researchers determined the ages of some Holocene glacial advances in Tibet. Sharma and Owen (1996) dated aeolian silt 2 m thick overlying a thick finely laminated lacustrine deposit on the highest lateral moraine, upper Rudugaira Valley, and obtained two dates for the Bhujbas Glacial Advance: 4.8 and 5.1 ka. Richards et al. (2000) dated the beds of fine sands and silts in the moraines in the southern slope of Mt Everest, Khumbu Himal, and obtained ages of 10 ka for the Chhukung Glacial Stage and 2 1 ka for the Lobuche Stage. Owen et al. (2002) identified a Holocene glacial advance at ka in Hindu Kush, Pakistan. Tsukamoto et al. (2002) dated young moraines in Kyrnchenjunga Himal and obtained dates of glacial advances of 5 6 and 10 8 ka. Using lichenometry, Chen (1988) dated the moraine on which two of largest dead lichens were dated to and cal. ka BP by radiocarbon dating (Table 1; Yi et al., 2004) and obtained a similar age of AD These dating results can be summarised into three Holocene glacial stadials in the early Holocene at ka, middle Holocene at ka and late Holocene at and ka (Fig. 9). Most of the stadials are within the framework of Holocene glacial stadials determined by radiocarbon dating (Fig. 8). The glacial advance at ka supplements the glacial advance by radiocarbon dating results. d 18 O records from the Dunde ice core showed (Yao et al., 1992) that the highest concentration of d 18 O occurred at ka during the Holocene, indicating an extreme cooling event, and a low concentration occurred at ka, Figure 9 Left: timing sequence of Holocene glacial stadials dated through CRN, OSL and lichenometry in Tibet and its surrounding mountains. Each dot represents the average age of a moraine with two or more dates. Right: d 18 O profile of Dunde ice core since 10 cal. ka

10 542 JOURNAL OF QUATERNARY SCIENCE representing a warm period, within which the lowest concentration occurred at ka, representing the megathermal maximum. However, the variation in d 18 O value showed that temperature fluctuated during this warm period. A cooler period occurred 6 5 ka. d 18 O value has declined since 3 ka, suggesting that climate was cooling and that the coldest period was at AD 1000 AD. The warmest period during the last 5 ka occurred around 800 a (Yao and Thompson, 1992). There have been three apparent cold periods since AD 1400 AD. They occurred at AD , , , corresponding to the three moraines in the LIA (Yao et al., 1991). The radiocarbon dates show that glacial advances corresponded to the cold periods recorded by d 18 O concentration in the ice core (Fig. 9, right). Similarly, no glacial advances were found during the two warmest periods of 800 a and ka, corresponding to the ice-core records. A more moist period of minor amplitude is recorded for Bangong Co, Sumxi-Longmu Co and Selin Co between 3.5 and 2.1 ka (Gasse et al., 1996). The record from Selin Co indicates a wetter period since 1.4 ka to present (Gasse et al., 1996). These relative humid conditions might be the explanation of the glacial advances occurring in these periods. Conclusions Based on radiocarbon dating, we summarised several Holocene glacial advances in Tibet and its surrounding mountains. The LIA had three substages. They occurred at cal. a. The glacial advances may have occurred a earlier, however, in eastern and southern bordering mountains of Tibet. They might be driven by humid conditions in bordering areas of Tibet. Most of the glacial advances in the Neoglacial occurred at cal. ka. They can be identified in almost every mountain. An early Holocene glacial advance occurred at cal. ka. They occurred in both central Tibet and bordering mountains. It appears that they were synchronous. 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