1 Introduction

Due to the uplift of the Qinghai-Tibet Plateau (QTP) since the Late Tertiary and Quaternary, an elevational cryosphere has been firmly established. In the Early Pleistocene, glaciers and permafrost were developed only on some very high summits on the QTP. At the beginning of the Middle Pleistocene, the Kunlun Mountains-Huanghe Movement (ca. 1.1-0.6 Ma B.P.) pushed the plateau to an average elevation of 3, 500 m, and the ensued climate cooling resulted in the Antepenultimate Glaciation (ca. 0.8-0.6 Ma B.P.), the maximum ever in the plateau history, with extensive and massive occurrence of glaciers and permafrost. During the Interglacial between the Antepenultimate and Penultimate Glaciations, climate was warm and moist, with large extents of forested lands as evidenced by the weathered laterite crust on the southern QTP. Shi (1998) thus inferred that glaciers and permafrost could have completely vanished during this period. Due to the combined effects of long-term denudation, transportation, deposition, tectonic deformations, surface water erosion and ground water percolation for hundreds of thousands to even millions of years, most paleo-periglacial micro-topography and paleo-permafrost strata remains prior to the mid-Quaternary had been greatly altered, making it almost impossible to rebuild the early evolution of permafrost and paleoenvironment on the QTP (e.g., Zhou, 1965; Wang et al., 1989; Pan and Chen, 1997; Jin et al., 2007; Zhao et al., 2014). Recently, with the support of the China 973 Program and the Chinese Academy of Sciences Strategic Pilot Science and Technology Program, and under the initiative of the International Permafrost Association Working Group on the Last Permafrost Maximum (LPM) (Vandenberghe et al., 2014), the chronologic data of permafrost remains, together with cross-reference of other proxy records, such as those from glaciation and paleo-geographic environments, were clarified and analyzed to roughly rebuild the evolutionary processes of permafrost and periglacial environment on the QTP during the Late Quaternary, i.e., since the Penultimate Glaciation (ca. 150 ka B.P.). In the meantime, the field observations and model predicted changes of permafrost on the QTP, and possible resultant consequences in hydrology, ecology, engineering and socio-economic consequences were also reviewed.

2 Indicators of paleo-permafrost and proxy reliability

Permafrost usually contains ice in soil and rocks. Generally, the age of ice formation is regarded as the age of permafrost formation. However, ice cannot be preserved after thawing of soil strata, which makes the dating for ice or permafrost even harder. In most cases, the formation age of permafrost soils is deduced by the ice formation periods, in combination with relationships such as bearing and ambient strata, reconstructed distribution of glaciers, glacial geology and geomorphology, periglacial phenomena, lithology and pollen data. Assuming similar regularities and spatial distributive features of permafrost at modern and past times, all these available and validated data are comprehensively integrated to estimate the formation, development and evolution of permafrost on the QTP during the Quaternary.

All the evidence or indicators for the occurrence of permafrost on the QTP can be divided into direct and indirect categories as follows. Direct indicators include items such as inactive ice wedges, ice wedge pseudomorphs, deeply buried permafrost and thick ground ice (layers), the past permafrost table, and pingo scars. Their processes relate to ground freezing, and occur in permafrost areas. Cryoturbations (involutions), sand/gravel and soil wedge casts, polygonal frost cracks, inactive rock glaciers, block fields, stone rings and strips are all indirect indicators. Their processes operate in the near-surface layer subject to seasonal thaw (the active layer), the near-surface permafrost located above the depth of zero-annual amplitude, and the zone of seasonal freezing and thawing in non-permafrost regions. Further indirect indicators include pollen (such as spruce, fir and other periglacial flora) in the strata, humic soil layers and paleosols, periglacial loess deposits, till and glacial landforms; periglacial fauna (such as mammoths, woolly rhinoceros and cold-climate ostracoda), fossils; reconstruction of paleo-temperature by mineralogical analysis and borehole measurements, to just name a few. All the indicators are unique to periglacial environments, and their spatial distributions are consistent with permafrost regions at the corresponding period. Remarkably, most of the indicators could have multiple interpretations, not necessarily just for indicating the occurrence of permafrost. In order to draw more reliable conclusions, integrated cross-examination with other indicators and paleoclimate proxies deems necessary.

The formation and expansion of permafrost generally lag behind changes of climate and glaciers. On the ground surface, periglacial remains by less intensive, the information for earlier occurrence of permafrost is often easily damaged or eliminated by more intensive, succeeding development or thawing of permafrost, making it difficult to reconstruct the detailed permafrost evolution. Fortunately, various permafrost remains have been reasonably preserved on the relatively arid QTP since the Last Glacial Maximum (LGM). The closer to present time, the more permafrost remains and evidences. With the rapid developing technology in soil dating during the last few decades, dating methods for permafrost soils have been greatly improved, and a variety of geographical materials and rich experimental and monitoring data provide more useful information. However, it is critical to determine the environment for the occurrence and development of past permafrost. Therefore, permafrost reconstruction relies heavily on studies in Quaternary glaciology, periglacial environment, paleoclimate, paleogeography and pollen analysis, clay minerals, stable oxygen isotopes and geological dating, in order to fathom the most plausible interpretations for environmental and climatic proxies.

3 Criteria for permafrost boundaries and statistics of paleo-permafrost

On the basis of traditional theories and analysis of the three-dimensional zonation of permafrost on the QTP (Cheng and Wang, 1982), models for the formation, distribution and evolution of permafrost were established. First, the maximal or minimal extent of plateau permafrost during the several major geological times, such as Penultimate Glaciation, the early stage of the Last Glaciation, LGM and the HMP, should be delineated. Then, regional contours or maps of plateau permafrost can be controlled. In the end, changes of permafrost at different periods can be inferred from these maps.

Among the past permafrost remains, wedge-like structures, such as ice wedge pseudomorphs, and sand/gravel and soil wedge casts in particular, are the most important evidence to more reliably estimate the boundaries of permafrost and environment changes in spite of recent challenges of their proxy values (Murton and Kolstrup, 2003). Therefore, it is crucial to correctly identify or clarify the casts and pseudomorphs. Then, statistical analysis of their lowest occurring elevations can be applied for estimating the distribution and the lower limits of permafrost (LLP) on the QTP. In the end, paleogeographic reconstruction according to climatic conditions for forming or sustaining these wedge structures and cross-examination with other paleodata become possible.

The forming age is of great importance in studying the remains of past permafrost. The authors refer to extensive statistics on glacial ages on the QTP during the Quaternary by Shi et al. (2006), and collects other relevant dating data for past permafrost, in order to have a division of permafrost evolution and spatial distribution on the plateau (Table 1 and Figure 1). In this paper, according to the ages of glacial geomorphology and permafrost remains, time gaps of forming ages, spatial distributive patterns of these evidences, in combination with climatic changes and paleogeographic successions during the glacial and interglacial periods, permafrost evolution and environmental changes on the QTP are outlined for the last 150, 000 years.

4 Evolution of permafrost and periglacial environment on the QTP

Up to the present, although there are no direct identified evidence for paleo-permafrost prior to the Penultimate Glaciation on the QTP, permafrost still could possibly have developed before the late Quaternary. According to the distribution of moraines, there were extensive glaciations during the Kunlun Glaciation (ca. 560-780 ka B.P.) in the early Middle Pleistocene, with glacial areal extents of at least 5.0×10⁵ km² (Shi et al., 2006). As a result of the Kunlun-Huanghe Movement (ca. 1.1-0.6 Ma B.P.), the plateau surface was uplifted to 3, 500 m a.s.l., resulting in a periglacial environment. At that time, permafrost should have developed on some highlands in the periphery of glaciers and ice sheets. In the following inter-glacial warm period (ca. 550-500 ka B.P.), there were conifer-broadleaf mixed forests on the Zoîgé Plateau in the northeast and forests on both sides of the Qinghai-Tibet Highway (QTH) in the interior. The extensive identification of buried residual crimson paleosols and rich plant remains on the southern QTP suggests that the plateau then was about 800 m lower than present, mean annual air temperatures (MAAT) about 5 ℃ higher, and annual precipitation considerably greater. Therefore, it is highly possible that most glaciers and permafrost on the QTP had vanished under a warming climate in this interglacial period.

The Middle Lianggan Glaciation occurred on the QTP at 480-420 ka B.P. when moraines (dated 462.9 ka B.P.) were formed in the Bailang River Valley in the Qilian Mountains and moraines (dated 477-460 ka B.P.) were deposited at the Gaowangfeng upland in the headwaters of Urumqi River in the Tianshan Mountains, Xinjiang, China. This glaciation largely corresponded to one of the important glaciations (MIS12). Plateau permafrost expanded, but then thawed again in the following interglacial period (Tong and Li, 1983).

The initial time of the Penultimate Glaciation remains unknown, but its later period is from ca. 150 to 130 ka B.P., and is marked by the considerably deformed higher lateral moraines and fragmented terminal moraines. Large-scale glaciers were well developed, as represented by the 100-km-long Guxiang Glacier in the Eastern Nyainqêntanglha Mountains, with its moraine age at 133.6 ka B.P. (MIS6). Similar dated glacial landforms were also identified at the northern piedmonts of the Gongger Mountains on the eastern Pamir Plateau, in the upstream valleys of the YulongKashgar River in the Western Kunlun Mountains, on the eastern slopes of the Western Nyainqêntanglha Mountains, and in the Tanggula, Nianbaoyuze, Gongga and Qilian Mountains. It is obvious that the scale of the Penultimate Glaciation is larger than that of the LGM (Shi et al., 2006). Numerous examples of past permafrost have been discovered for the late stage (since ca. 150 ka B.P.) of the Penultimate Glacial period on the QTP, and in the northeastern part in particular, where rich variety of relict periglacial landforms have been identified for various periods (Figure 1). Based on the dating of these past permafrost evidence (Table 1), in combination with various periglacial remains, the evolution of plateau permafrost can be divided into the following four expansion-degradation cycles: 150-130, 80-50, and 30-14 ka B.P. in the Late Pleistocene, and since 10.8 ka B.P. in the Holocene. The four cycles are discussed as follows.

4.1 Intensive development and expansion of permafrost during the late stage of the Penultimate Glaciation (ca. 150-130 ka B.P.)

The climate is harshly cold during the late stage of the Penultimate Glaciation. Many ice-wedge pseudomorphs have been found in the highlands and the fluvio-lacustrine terraces 3, 300~3, 400 m a.s.l. on the Northeastern QTP (e.g., Pan and Chen, 1997; Cheng et al., 2006; Harris and Jin, 2012). They were usually developed in groups in gravel layers or weathered bedrocks, accompanied by intense cryoturbations (Figure 2). Generally, ice-wedge pseudomorphs are 0.5~2.0 m in top width, 0.5~2.5 m in height, with a narrow and deep wedge structure and a sharp wedge-end (bottom). Only a few have branch-off wedges. The intense cryoturbation of the bearing soils surrounding the wedges reflects very intense extrusion. Wedges are filled with fine sand, occasionally with fallen gravel of 2~3 cm in diameter. The wedge walls are irregular, onto which there are brownish red paleosols attached. Based on these features, when the wedge-ice melted, fine sand filled in, forming the ice-wedge pseudomorphs.

These ice-wedge pseudomorphs can be represented by those at Da'heba (3, 350~3, 400 m a.s.l.), Xinghai County, southeastern Qinghai (Figure 2). They were developed in sand and gravel layers on fluvio-alluvial terraces of the Middle Pleistocene, overlaid by brownish red paleosols. The paleosols were again overlaid by the Malan loess nearly 1 m in thickness. The TL-dating at the middle part of the Malan Loessis 30.1±2.6 ka B.P., and ¹⁴C-dating of the black organic matter in the Holocene soil overlying the Malan loess is 4, 460±60 a B.P. This ice-wedge pseudomorph was filled with fine sand without distinguishable bedding and with a TL-dating of 135.7±10.5 ka B.P., reflecting that ice wedges developed in the late stage of the Penultimate Glaciation. According to temperatures of ice wedge development in gravel, MAATs then were 12~14 ℃ lower than present, and the LLP then was about 1, 600~1, 800 m lower than present (4, 350 m [on shadowy slopes and in paludal areas] approximately 4, 450 m [on sunny slopes and drylands]). It can be deduced that the LLP on the northeastern QTP should be about 2, 000~2, 200 m at that time.

Many ice-wedge pseudomorphs were also developed in loess at more than 20 locations near Yangsige 15 km north of Jungar Banner, Erdos (Dongsheng), Inner Mongolia Autonomous Region (39°50'N, 118°18'E, 1, 231 m a.s.l.). They are wide in the upper part, narrow in the lower part, and with only one sharp end (bottom). The upper part is pan-shaped, and the lower part wedge-shaped. The upper part was formed in the active layer, but the lower part in permafrost. The TL-dating of the bottom fillings is 132±13 ka B.P. (Zhou et al., 2008), i.e., the late stage of the Penultimate Glaciation. It thus can be inferred that the MAAT then was 14 ℃ lower than present in the Erdos (Dongsheng) region. The southern limit of latitudinal permafrost should be located between 33°N and 36°N in North China to the west of the Tai'hang Mountains. Latitudinal and elevational permafrost in China joined near Lanzhou, Gansu (Zhao et al., 2013). It was considerably cold during that period, and the environment was the harshest on the QTP. In the RM borehole in the Zoîgé Basin, the total content of organic carbon of soil cores at the MIS6 stage is the lowest, and pollen characteristic of alpine steppes (Wang et al., 1995).

The last interglacial occurred on the QTP during 130-80 ka B.P. (i.e., the MIS 5e stage). However, its time span is 125-75 ka B.P. as revealed from the Guliya ice-core in the Western Kunlun Mountains. According to analysis of this ice-core, the MAAT was 5 ℃ higher at the MIS 5e stage than present, and the climate was temperate and humid. This resulted in lush vegetation and lower carbonates content in the lacustrine sediment core from the adjacent Tianshui'hai Lake. In the RH borehole in the Zoîgé Basin, pollen records indicate an alpine evergreen dark coniferous forest on the eastern QTP, with an MAAT 2 ℃ warmer than present (Shi et al., 2006). However, on the basis of analysis of pollen assemblages in loess in Linxia, Gansu Province, the MAAT at the MIS 5e stage was 3 ℃ higher than present (Pan and Chen, 1997). All these data suggest that, there are many secondary climatic fluctuations under the major interglacial climate on the plateau. By inferring air temperatures in the Western Kunlun Mountains, there was only a remote possibility of permafrost existence on the plateau during this interglacial period. This may well explain the paucity of past permafrost and periglacial remains during this period.

4.2 Intensive development and expansion of permafrost during the early stage of the Last Glaciation (ca. 80-50 ka B.P.)

The remains of the early stage of the Last Glaciation were hard to distinguish because of overlapping by the late stage of the Last Glaciation. At Zhari'ali'he of the Yangbajing region in the Western Nyainqêntanglha Mountains, there are two sets (upper and lower) of moraines, with the age at 72.1±6.1 ka B.P. of the calcareous cements in the gravelly till of the upper moraines. In the Muzharte Valley of the Chinese Tianshan Mountains, the ESR-dating of terminal moraines is 64.2-71.7 ka B.P. (Zhao et al., 2010). Records of the Guliya ice-core in the Western Kunlun Mountains show that air temperatures sharply declined by as much as 12 ℃, climate was cold and dry at that time, but not as cold and dry as during the LGM (Shi et al., 2006). In addition, wind action prevailed during that period, with loess accumulation of 30~40 m in thickness. For example, thick loess covered the northern slopes of the Kunlun Mountains, such as on the bank of the Golmud Reservoir and the fourth terraces of the Golmud River at Nachitai, Qinghai. The ¹⁴C-dating of calcium nodules in the loess layer on the bank of the Golmud Reservoir indicates that periglacial loess is older than 50 ka, i.e., a product of the early stage of the Last Glaciation. Aeolian sands were also deposited in this period. The fixed sand dunes on the Chumar'he High Plain between the Kunlun and Tanggula mountains, near Wudaoliang in particular, were probably deposited in the same period (Wang, 1989).

In the early stage of the Last Glaciation, permafrost developed and expanded again on the plateau. The LLP in the north was close to the Golmud Reservoir; it declined to 3, 000~3, 200 m a.s.l. at Mt. Maomaoin, Tianzhu County, Gansu Province, with its peak of 4, 070 m a.s.l., an extension of Mt. Lenglong in the eastern Qilian Mountains. At present, the LLP is at about 3, 500~3, 600 m a.s.l.. In a intermountain basin, Songshantan Basin (2, 540 m a.s.l.), under the Malan loess, a buried peat pingo/palsa scar, 5 m in height and 25~30 m in horizontal diameter, was discovered. Based on ¹⁴C-dating, the age of the lower peat layer is more than 50 ka, but the upper peat 31, 100±150 years (Xu et al., 1984). This indicates that the pingo/palsa was formed in the early stage of the Last Glaciation, then it was overlaid by the Malan loess. The LLP at that time could be 2, 200~2, 300 m a.s.l., i.e., 1, 300~1, 400 m lower than present. The areal extent of plateau permafrost during this period could be slightly less than that in the late stage of the Penultimate Glaciation.

When the early stage of the Last Glaciation was over, climate became relatively warmer and wetter. Ice-core records show that the MAAT then was 4 ℃ higher than present. According to the analysis of water δD and δ¹⁸O in the fluid inclusion in halite in Charhan Salt Lake in the Qaidam Basin, the MAAT then was 2 ℃ higher than present (Zhang et al., 1994). There are several layers of paleosols in the sand dunes east of Wudaoliang, of which ¹⁴C-dating for the paleosols is 38-30 ka B.P. (Wang, 1989). In the periphery of Qinghai Lake, the vegetation was dominated by mixed conifer-broadleaf forests composed of pine, spruce, cedar, aspen, and walnut, reflecting a forest-steppe landscape under a cool and humid climate. Annual precipitation then could be up to 400 mm on the western QTP. Lakes were widely developed in this period (Li et al., 1991). For example, in the lacustrine sediment core of Tianshui'hai Lake in the Western Kunlun Mountains, carbonate content is considerably lower at the depth of 5.5~6.0 m (ca. 35-33 ka B.P.), and the combination of Doterocyprissciata (Müller)-Leucocythere mirabilis (Kaufmann) assemblage occurred at the same depth, indicating a warm freshwater environment (Li et al., 1998). Therefore, permafrost is quite difficult to form and preserve on the plateau during this period.

4.3 Expansion of permafrost during the LGM (30-14 ka B.P.)

The period of 30-14 ka B.P. was the late stage of the Last Glaciation, but most proxy records prove that the LGM during 24-18 ka B.P. was the coldest on the QTP, with a MAAT dip of 6~9 ℃ and a precipitation of 30%~70% of present. Records from the Guliya ice-core shows that the MAAT of 23 ka B.P. was the lowest, about 9~10 ℃ lower than present (Shi et al., 2006). A little lag after the LGM, or the Last Permafrost Maximum (LPM), saw the most favorable climatic conditions for permafrost development and expansion, and permafrost and periglacial remains of the LGM/LPM are well preserved and with many recognizable periglacial manifestations (Figure 3). A large number of wedge-shaped structures of the LPM have been found (Table 1), especially those widely distributed and identified sand/soil wedges and casts (Zhang et al., 1983; Xu et al., 1990; Wang and Bian, 1993; Pan and Chen, 1997; Cheng et al., 2006; Jin et al., 2007; Chang et al., 2011). They are usually exposed at the scarps of fluvial-lacustrine terraces, and overlaid by 1-to 2-m-thick deposits. In some wedge profiles, slumps structures are visible with fallen boulders and gravel in the wedges. This phenomenon is evidently related to the melting of ground ice.

Several representative wedges from the QTP are briefly described as follows.

1) Ice wedge casts groups at Qiejitan in Gonghe Basin

At Qiejitan (36°17'N, 101°09'E, 3, 100 m a.s.l.), Gonghe Basin, southern Qinghai, groups of ice-wedge pseudomorphs have been discovered in fluvio-lacustrine gravel layers. They are 50~80 cm in top width, 50~120 cm in height, with distorted bearing soils around these pseudomorphs. The ¹⁴C-age of filling in the lower part of a pseudomorph is 20, 430±430 a B.P., but the ¹⁴C-age of poorly developed paleosol covering the pseudomorph is 19, 430±350 a B.P. (Figure 4). This can be inferred that these casts formed ca. 20 ka B.P.

2) Sand-wedge pseudomorphs at Zhabuda in the source areas of Yellow River (SAYR), Southeastern Qinghai Province

A group of sand-wedge pseudomorphs have been identified in sandy clay of the Late Pleistocene fluvio-lacustrine strata at Zhabuda (36°20'N, 100°02'E, 2, 900 m a.s.l.) in the source areas of Yellow River (SAYR), Qinghai. They are 10~35 cm in top width and 30~50 cm in height, with slightly distorted bearing soils around the pseudomorphs. However, the wedge walls are uniform without evidence of slumping (Figure 5). The fillings are clean fine sand. The ¹⁴C-age of the bearing soil is 33, 880±3, 450 a B.P., but that of slightly dark paleosols overlying the pseudomorph is 14, 220±180 a B.P. Therefore, these sand-wedge pseudomorphs were probably formed during the LGM, but they stopped developing at about 14 ka B.P. because of climate change.

3) Sand-wedge pseudomorphs at Leng'hu, Qaidam Basin, Northern Qinghai

Primary sand-wedge pseudomorphs were found on the highest former shore (38°40'N, 93°15'E, about 3, 000 m a.s.l.) of Kunteyi Salt Lake near Leng'hu, Qaidam Basin, Northern Qinghai. They are 30~50 cm in top width and 160 cm in height. Downwards, the bearing soils are coarse sandy gravel, fine sandy gravel with horizontal bedding, and thin-layered loam (Figure 6). Surrounding coarse sand and gravel are up-churned. The fillings of sand-wedge pseudomorphs are clean fine sand, without slumped materials from surrounding soils. The TL-dating of the in-filled fine sand in a pseudomorph is 18, 510±2, 220 a B.P., but the ¹⁴C-age of the horizontal fine sandy gravel layer beneath the sand wedge is 31, 700±800 a B.P. Therefore, these sand-wedge pseudomorphs were formed about 20, 000 years ago.

4) Primary soil wedge at Nachitai along the QTH, Qinghai Province

South of Nachitai (35°40'N, 94°20'E, 3, 550~3, 600 m a.s.l.) along the QTH at the northern piedmont of the Kunlun Mountains, primary soil wedges were found in bluish gray loam on the second terrace of Kunlun River. They are 30~40 cm in top width and 120~130 cm in height. They are overlaid by a layer of loess-like sandy loam. There are slight cryoturbations of the bearing soil horizons at both sides of the wedges. The fillings are similar to the bearing soils, with vertical bedding (Figure 7). The ¹⁴C-age of the overlying layer, surrounding soil and filling material are 7, 207±387, 14, 041±399, and 15, 377±292 a B.P., respectively. According to these analyses, the soil wedges began to develop about 15, 000 years ago, then gradually expanded (Wang and Bian, 1993).

Additionally, many wedge-shaped structures and cryoturbations of this period have also been found in the Western Qinling Mountains, Hexi Corridor, source areas of Yellow River, Fenghuo Mountains, Western Kunlun Mountains and Ordos Basin (Table 1). According to the distribution of permafrost and periglacial remains, it can be inferred that plateau permafrost expanded to the marginal regions of the adjacent inland basins, such as Xinghai, Gonghe, Qinghai Lake, Qaidam, Ordos and Tarim basins. The LLP then was about 1, 200~1, 400 m lower than today. The LLP was at 2, 200~2, 300 m a.s.l. on northern flanks of the eastern Qilian Mountains, 2, 300~2, 400 m a.s.l. in the Gonghe Basin, 2, 600~2, 800 m a.s.l. in the margin of the Qaidam Basin, 2, 800~3, 000 m a.s.l. in the southern margin of the Tarim Basin, 2, 700~2, 800 m a.s.l. on the eastern Zoîgé Plateau, and 3, 700~3, 800 m a.s.l. in the upper reaches of the Yalu Zangpo River Valley. Permafrost extents were about 2.2×10⁶ km²on the QTP, roughly about that of the early stage of the Last Glaciation The latitudinal permafrost in North China and elevational permafrost joined together at that time, except in some areas lower than 2, 000 m a.s.l. along the Hexi Corridor and in southern and eastern Xinjiang where seasonal frost or taliks prevailed in deserts and gobi.

The plateau summer monsoon was weakened during the LGM. As a result, precipitation was considerably reduced, and many lakes shrank, with low lake levels and enhanced salinization. For example, in Qinghai Lake, deep-lake sediments were replaced by sandy gravel during 23, 640-14, 830 a B.P., reflecting a drying climate and shrinking lakes. On the southern slopes of the Western Kunlun Mountains, there are numerous lakes at 4, 800~5, 080 m a.s.l., such as Kushui'hai, Tianshui'hai and Aksai Chin. During the late stage of the middle Last Glaciation (50-30 ka B.P.), these lakes were unified into a large lake, i.e., the Pan-Aksai Chin Lake. Starting from the LGM, the lake level of the Pan-Aksai Chin Lake first lowered by about 40 m, and then disintegrated. Due to lowering lake levels, large quantities of lacustrine sediments were exposed providing abundant material sources for wind action and sand dune formation.

Under a very dry and cold glacial climate, plateau vegetation was on an extensive decline. Forests retreated to the eastern and southern margins, and desertified steppes and/or alpine tundra dominated the plateau surfaces. For example, in the Ren Co Lake area (4, 450 m a.s.l.) in Baxiu County, southeastern Tibet Autonomous Region, desertified steppe was dominated by Chenopodiaceae and Artemisia (Tang and Wang, 1988). Based on pollen analysis of the RH borehole, steppes dominated by Artemisia, Cyperaceae and Rosaceae was the main landscape during 20-18 ka B.P. on the Zoîgé Plateau (Liu et al., 1995). According to a soil profile at Wasong (3, 469 m a.s.l.) in Hongyuan County, western Sichuan Province, the upper limit (timberline) of forest was 1, 200 m lower than today; the surface was dominated by desert steppes with only appreciable pollen (Wang et al., 1995). The subtropical coniferous and broad-leaved mixed forests were only preserved in the lowlands, such as Mêdog and Zayü, on the southern slopes of the Himalaya Mountains.

The formation of an ice wedge requires cold and wet climate, or at least high relative humidity. Therefore, it was not conducive for ice wedge development in most regions during this period. That may explain why there are many discoveries of sand-and soil-wedge casts and primary sand/soil wedges but only a handful of ice wedge pseudomorphs have been identified on the interior and western QTP.

The cold and dry LGM climate dominated until 14 ka B.P. when many sand wedges stopped developing (Table 1). Afterwards, air temperature rose with fluctuations. Records of the Guliya ice-core show that climate warmed by 3 ℃ during 15.5-13.5 ka B.P., with a slight cooling at 13.0 ka B.P., and the warming peaked at ca. 12.0 ka B.P. when MAATs were 5 ℃ higher than the LGM. Permafrost then greatly shrank in areal extent, and the LLP also rose by 600~700 m in comparison with the LGM. For example, the LLP on the northern slopes of the Kunlun Mountains retreated to the southern vicinity of Nachitai (3, 600 m a.s.l.), as proved by cryoturbations on the second terrace of Kunlun River (Wang and Bian, 1993). By the end of the Late Pleistocene, the modern plateau permafrost body had taken its shape.

4.4 Permafrost evolution since the Holocene (since 10.5 ka B.P.)

Since the Holocene, the QTP has been uplifted to above 4, 000 m a.s.l.. However, under a fluctuating climate, permafrost has been in degradation in general in comparison with that in the LGM. Besides, six distinct periods of evident development, expansion and retreat can be distinguished.

1) Development of permafrost during the early Holocene with abrupt climate changes (from 10.8 to 8.5-7.0 ka B.P.)

The climate during the early Holocene was considerably unstable, with rapid fluctuations. Records of the Dunde ice-core show that δ¹⁸O content (-12.75‰) at 8.7 ka B.P. is the lowest in the Holocene, reflecting a rapid cooling in climate; that (-9.6‰) at 8.5-8.4 ka B.P. is the highest, indicating a warming episode (Yao and Shi, 1992). The climate in the early Holocene is cold-dry, and gradually transitioned to a wet-cool pattern. Inferred from the spatial distributive features of permafrost and periglacial remains, the northern LLP on the QTP was at about 3, 400~3, 500 m a.s.l. (north of Nachitai), and the southern LLP at 4, 200~4, 300 m a.s.l. (between Yangbajing and Damxung), i.e., 600~700 m lower than that of today. Permafrost was thermally stable, continuous, with an areal extent of 40%~50% larger than today.

According to borehole pollen records from 20 lakes, Tang (2000) summarized the distributive features of vegetation on the QTP in the early Holocene. On the southeastern QTP, the mesophytes, such as deciduous broad-leafed and coniferous forests, dominated during 10, 000-9, 100 a B.P. In the Qinghai Lake area, it was a subalpine steppe dominated by Artemisia and Chenopodiaceaeunder a cool-dry climate during 11, 000-8, 000 a B.P. On the western QTP, it was an Artemisia-based steppe under a cold-wet climate during 10, 000~7, 700 a B.P. Since then, the plateau climate was gradually warming and becoming wetter. Wetlands started to develop in some parts of plateau basins and valleys, accumulating peat and thick humus deposits. Thick peat, for instance, began to form at 8, 175±200 a B.P. at Qi'nongga in Yangbajing and at 9, 970±135 a B.P. in WumaQü in Damxung, Tibet Autonomous Region (Li, 1982). The borehole CK80-3 at Qingshuihe Riverside along the QTH reveals a black silty clay layer at a depth of 2.5~3.0 m, with ¹⁴C-age of 8, 800±305 a B.P. at a depth of 2.7~3.0 m. In the lower part of this borehole, strata change from yellow silty clay and medium to fine sands containing limestone fragments and carbonate nodules to black silty clay. This transition in soil strata can be attributed to climate warming and increased moisture, and resultant enrichment in soil organic matter. In the second terrace of ZuomaoxikongQü River on the northern foot of the Fenhuo Mountains, the age of humus-contained silty sand in the upper part of a sand wedge is 9, 218±189 a B.P., but it is 9, 160±170 a B.P. for sand wedges at the HMSS 82 on the southern foot of the Fenghuo Mountains.

This is a clear indication of abating cryogenic processes, and frost cracks stopped expanding. If frost cracks were fully filled with sand and gravel and sand wedges stopped developing, generally this would indicate a warming climate. Warmer and wetter climate is conducive to plant growth, which makes sand dunes, formed in the late stage of the Late Pleistocene, fixed or semi-fixed. One of the obvious evidence is the multi-layer of not-yet-decomposed plant roots and stems (¹⁴C-dating of 9, 716±270 a B.P.) buried in sand dune ridges 2 km southeast of Wudaoliang along the QTH in the interior of the QTP.

2) Extensive permafrost degradation during the Holocene Megathermal Period (HMP, from 8, 500-7, 000 a B.P. to 4, 000-3, 000 a B.P.)

The Middle Holocene Optimum, also called the Hypsithermal or Megathermal, was very conducive for organic growth during the Holocene. Most of thick peat and humus on the QTP was formed during this period (Table 1, Figure 8), reflecting a warm-humid climate. There are numerous instances for peat and humus formation in the Megathermal, such as the 4.4-m-thick humus in borehole No. 8 at Xidatan along the QTH (7, 530±300 a B.P.), ash-like peaty sand on the first terrace at Nachitai (4, 910±100 a B.P.), thick humus layers at the HMSS 109 south of Tanggula (5, 058±443 a B.P.) and at the HMSS 120 (4, 363-4, 576 a B.P.), thick humus at Qi'nongga in Yangbajing (3, 050±120 a B.P.) and at WumaQü in Damxung, Tibet Autonomous Region (3, 575±80 a B.P.).

On the eastern and northern QTP, examples include: 2-m-thick humus on the eastern slope of the Riyueshan Mountains (4, 920±80 a B.P. in the middle part), 5-m-thick humic soil layer on the southern slope of the Heka South Mountains with a ¹⁴C-dating of 4, 625±117 a B.P. at a depth of 2 m, thick humus layers at the terminus of inactive gelifluction lobe on the Wenbo South Mountains in Shiqu County, Sichuan Province (4, 395±215 a B.P.), and 4.15-m-thick humus on the moraine terrace in the Nianbaoyuze Mountains (5, 422±94 a B.P. in the lower part). Ash remains from human fire use have been found in many places from Nachitai to Xidatan on both banks of the Kunlun River, which shows that climate and environment in this area is quite inhabitable for humans. In the stratigraphic section of a peat farm at Hongyuan on the ZoîgéPeat Plateau (with a maximum peat thickness of 10.5 m), 5.2-m-thick peat was formed during 9, 350-370 a B.P., of which 3.3 m was continuously accumulated during 6, 350-3, 250 a B.P. (Sun, 1988). All these products of the HMP are widely distributed and thick, indirectly implying an extensive permafrost degradation.

In the late HMP, permafrost was replaced by seasonal-frost in most areas to the north of the Kunlun Mountains and south of the Tanggula Mountains (except very high mountains). On the Chumar'he High Plain between Kunlun and Tanggula Mountains, permafrost persistently thawed downwards during the Megathermal warming. The thawed depth is about 14~16 m, resulting in vertical detachment of permafrost from the seasonal thawing and the formation of thick-layered ground-ice at similar depths of probably more stable former permafrost table (Xing and Ou, 1983). When permafrost thaws and ground-ice melts at shallow depths, thermokarst lakes and depressions are formed, ice wedges are later filled and turn into ice wedge pseudomorphs. At that time, permafrost on the Chumar'he High Plain became sporadic in distribution, or deeply-buried, but it was still continuous in those relatively high areas, such as in the Kunlun, Fenghuo and Tanggula mountains.

Permafrost degradation was more intense on the eastern QTP than along and west of the QTH. Permafrost was completely converted to seasonally frozen ground in regions lower than 4, 200 m a.s.l. along the Qing (hai)-Kang (West Sichuan) Highway (QKH) from Xi'ning to Yushu, Qinghai, for instance. Permafrost thawed downwards to depths of 15~25 m at elevations of 4, 200~4, 400 m a.s.l. on both sides of the QKH from Huashixia and Qingshuihe. Finally, downward and lateral thawing of permafrost resulted in deeply-buried permafrost at depths of 10~20 m in some areas (Jin et al., 2006), and/or converted to seasonal frozen ground. However, in relatively high areas of the Bayan Har and Anyêmaqên Mountains, patches of permafrost persisted.

Based on these permafrost and periglacial remains, the HMP LLP was about 300~400 m higher than today, and MAATs 2~3 ℃ higher. High temperature lasted for a long period in the Megathermal. As a result of severe degradation, permafrost extent was only 40%~50% of that today.

3) Permafrost expansion in the Neoglaciation (4, 000~3, 000 a B.P. to 1, 000 a B.P.)

The Neoglaciation, the cold period of the late Holocene, was featured by significantly fluctuating cold climate and extensive advances of mountain glaciers on the QTP. The Neoglacial period in the interior is evidently marked by three to four rows of terminal and lateral moraines between terminal moraines of today and the LGM in the Kunlun and Tanggula mountains. Meanwhile, there are numerous Neoglacial features, such as the 20-m-thick permafrost near borehole No. 8 at Xidatan, a string of large pingo scars along the east-to-west-trending fault in eastern Xidatan (4, 250~4, 300 m a.s.l.), intensive cryoturbations on the first terrace of the Kunlun River south of Nachitai (ca. 3, 800 m a.s.l.), and the freeze-up of thick humus at the HMSSs 100 and 120 on southern slopes of the Tanggula Mountains.

On the eastern QTP, pingo groups were formed 40 km east of Shiqu County, Sichuan Province (2, 925±175 a B.P.) and 65 km north of the Maqin (Dawu)-Changma'he Highway (3, 925±185 a B.P.). On the northeastern QTP, numerous large polygonal ground, gelifluction and stone circles were formed in the Riyueshan Mountains (3, 450 m a.s.l.), Heka South Mountains (3, 600 m a.s.l.), la Mountains (3, 750 m a.s.l.), and on the northern slopes of the Bayan Har Mountains (4, 000~4, 100 m a.s.l.).

These widely discovered relict remains of past permafrost and periglacial manifestations suggest a considerably cold Neoglacial climate on the QTP. In comparison with the permafrost distribution at present, it can be referred that the Neoglacial LLP was 300 m lower than today, and air temperature 2 ℃ lower. On the basis of extensively degraded permafrost in the HMP, permafrost re-developed and expanded, reaching the largest extent (about 20%~30% larger than today) at the end of the Neoglaciation.

The LLP south of Nachitai along the QTH was at 3, 700~3, 800 m a.s.l., but it was at 4, 400~4, 500 m a.s.l. in the Damxung Valley in the south. On the Chumar'he High Plain (above 4, 500 m a.s.l.) in the interior, permafrost refroze downwards, forming a 30-m-thick permafrost-layer (Ding and Guo, 1982). The newly formed permafrost layer re-attached with the HMP permafrost table. Therefore, there is no deeply-buried or double layer permafrost discovered in this area.

In contrast, deeply buried permafrost and vertical talik have been extensively encountered between Huashixia and Qingshuihe along the QKH on the northeastern QTP. At the eastern plateau margin, the HMP thawed depth of permafrost was 15~25 m, but the refrozen depths of permafrost in the Neoglaciation were less, resulting in vertically detached, deeply-buried, or double-decked permafrost. Moreover, due to the secondary-level post-Neoglacial climate warming, the refrozen permafrost could be re-thawed to certain depths, resulting in alternating vertical distributive patterns of permafrost and taliks (Jin et al., 2006).

4) Permafrost degradation in the warm period of the late Holocene (from 1, 000 a B.P. to 500 a B.P.)

After the Neoglacial period, the plateau experienced several small-scale climatic fluctuations. Among them, the Medieval Warm Period, corresponding to the Sui and Tang Dynasties in Chinese history, lasted for hundreds of years. The MWP periglacial landforms and permafrost relics were well preserved on the plateau. Pingo were formed in the cold period of the Late Holocene at Xidatan, at 40 km east of Shiqu county, Sichuan Province and at K65 of the Maqên-Changma'he Highway in southeastern Qinghai were completely thawed, with the age of humus in the central of pingo scars and thermokarst depressions at about 720-625 a B.P.. In addition, in the vicinity of the HMSS 121 along the QTH, thick humic soils were formed on the permafrost islands at 780±131 a B.P.. These phenomena indirectly reflect the degradation and thawing of permafrost.

Meanwhile, plateau permafrost degraded regionally, with the depth of downward thaw as much as 10 m in some lowlands. For example, based on the borehole records of CK1 at Dinarantan, northeast of Huashixia and borehole ZK8 at Changma'he, the upper permafrost layer at depths of 9.7~12.3 m and 11.6~15.2 m, respectively, was the relict permafrost from the MWP (Jin et al., 2006). On the Chumar'he High Plain, there are two layers of past permafrost table according to the records of borehole CK224. The upper past permafrost table at a depth of 8.35 m was the product of the MWP, but the lower at 16 m was probably of the HMP. Thick-layered ground-ice beneath the two past permafrost tables could also be associated with the two warm periods (Xing and Ou, 1983). It is therefore estimated that the LLP was 200~300 m higher in the MWP than today, and MAATs 1.5~2.0 ℃ higher, and permafrost extent 20%~30% less.

5) Appreciable permafrost expansion in the Little Ice Age (LIA) (500 to 100 a B.P.)

The Little Ice Age (LIA) began 650-500 years ago on the plateau, reaching the coldest climate in the 17^th and 18^th centuries. The LIA climate was progressively drying and cooling during this period. In the meantime, most lakes on the plateau sharply shrank or dried up, resulting in the sharp reduction of lake water volume, and the more enrichment of salinity, with vanishing medium and small lakes and increasing number of saline or salt lakes. For instance, beneath the 50-to 60-cm-thick salt layer, silts were formed at 1, 094±344 a B.P. in the salt lake behind the HMSS 69 along the QTH, and at 1, 080±260 a B.P. in the Hajiang Salt Lake (formed later) to the east of the Ng ring Lake in the sources areas of the Yellow River. Fixed or semi-fixed sand dunes were covered by aeolian sand, and wind erosion became active again. During this period, plateau permafrost expanded again with increased thickness and areal extent, and some new permafrost islands were formed at the margin of the plateau. Along the QTH, thawed soils were refrozen downwards, and attached with the upper layer of the MWP permafrost table. For example, near the HMSS 121 along the QTH, the humus layer formed at 780 a B.P. refroze epigenetically and formed a permafrost island in the LIA. However, in some areas of the eastern plateau, refrozen permafrost in the LIA was too thin to be attached with the underlain deeply-buried former permafrost table, such as the permafrost at the depth of 1.5~8.0 m on the northern Ng ring lakeshore and at the depth 5.3~8.2 m in the water well at the QKH Qingshuihe (Jin et al., 2006).

In the Richa Village of Dari County, Qinghai, the lowest limit of block fields and streams near the highway was 4, 230 m a.s.l. during the LIA, where the age of humus under blocks was 425±85 a B.P.. Since the block fields and streams were formed later, they must be the product of the LIA climate. At present, the LLP is over 4, 300 m a.s.l., and 150~200 m higher than in the LIA. It can be inferred that MAAT was about 1.0~1.5 ℃ lower in the LIA than today, and permafrost extent was 10%~15% larger.

6) Extensive permafrost degradation during the 20^th century

During the 20^thcentury, and during the last few decades in particular, the climate was warming persistently. A great number of studies indicate that mean global air temperature has risen by about 0.3~0.6 ℃ since 1880, particularly since 1980s. Moreover, air temperature on the QTP has risen by 0.6~0.8 ℃, which undoubtedly has accelerated glacier melting and contraction. At present, the glacier area has been reduced by 21.2% and its ice reserves by 19.7%, respectively, in comparison with that in the LIA (Shi et al., 2011).

Increasing air temperature has led to a regional degradation of plateau permafrost (Wang, 1993; Wang et al., 2000; Jin et al., 2011). In comparison with the 1970s, mean annual ground temperatures have risen by 0.2~0.4 ℃, the LLP by 40~80 m; maximum thaw depth by 25~60 cm and maximum frost depth by 5~20 cm; permafrost extent by at least 1.0×10⁵~2.4×10⁵km², and about 60% of the shrinkage occurred during 1996-2006 (Jin et al., 2011). Isolated patches of permafrost in the peripheries of the QTP have degraded remarkably. For example, the extent of permafrost has shrunk by about 12% at Xidatan in the vicinity of the northern LLP along the QTH and by 20% at Liangdao'he near the southern LLP (Jin et al., 2006).

Due to increasing air temperature since the late 1980s, warm ( > -1 ℃) permafrost has started to degrade, and the degradation has gradually expanded to the zones of transitory (-1 to -2 ℃) and cold ( < -2 ℃) permafrost. Permafrost on the southern and southeastern plateau degrades markedly. It is projected that the degradation of permafrost is likely to accelerate, and substantial changes in the distributive features and thermal regimes of permafrost should be anticipated. However, regarding the relationships between degrading permafrost and the degradation of rangelands, it is still too early to draw reliable conclusions due to inadequate scientific criteria and evidence.

Because of permafrost degradation, seasonally frozen layer and the underlain permafrost are becoming detached vertically, with the talik about tens of centimeters to 2~3 m in thickness. In recent decades, permafrost degradation has been accelerated. On the northwestern plateau under the strong influences of the westerlies, climate is becoming warmer and wetter (Shi et al., 2003), precipitation in some permafrost regions may increase. Thus, surface vegetation and soil moisture may increase, which could lead to a slow degradation of permafrost.

7) Projected permafrost degradation during the 21^st century

It's predicted that MAATs on the QTP may rise by 2.2~2.6 ℃ during the next 50 years (Qin, 2002). Thus, the degradation of plateau cryosphere will accelerate, resulting in reduced river discharge, shrinkage of lakes and wetlands, and degraded and desertified lands, forming a vicious circle.

Therefore, a coordinated and sustainable development between cold regions environment and economy needs prudent management and practical measures on the basis of in-depth understanding of the constantly changing plateau climate and environment, in order to effectively alleviate frost hazards.

Due to the uplift of the QTP since the Quaternary, climate cooled with several distinct fluctuations between glacial and interglacial periods. During the Middle Pleistocene, permafrost began to develop on some high mountaintops. Afterwards, the development and decay of permafrost occurred alternatively in response to climatic fluctuations. Unfortunately, no direct evidences for permafrost occurrence prior to the Penultimate Glaciation have been found, and geocryologist's have to resort to deduction from glacial geology and other proxy reconstruction.

At present, all the identified relicts of permafrost or periglacial landforms were formed after the Penultimate Glaciation (about 150 ka B.P.). Based on these relicts and records of glaciers and paleo-geographic changes, the formation and evolution of permafrost and environmental changes on the QTP have been reconstructed for the past 150 ka, in which at least four extensive cycles of intensive development, expansion, degradation and decay of permafrost occurred. Moreover, in each cycle, there could be multiple secondary climatic fluctuations and permafrost changes.

Since the LGM, various permafrost remains have been relatively better preserved on the QTP. The closer to present, the more permafrost remains and discoveries.

In the past 150 ka, the pattern of climatic and environmental changes on the plateau has shown that there are differences in the timing of permafrost evolution sequence and the development of permafrost in different plateau regions, resulting in various permafrost types with their own features and environment. This is due to the alternating action of cold climate of west wind type in the glaciation and warm and wet climate of monsoon type in the interglacial, and the interpenetrations of various climatic types in different regions of the plateau.

In recent years, with the help from rapidly developing dating and modeling technology, and dating methods, more accurate dating with less dating material of various sources and rebuilding the large scale spatial and temporal changes of permafrost have become possible. It may be very helpful for rebuilding the detailed evolutionary processes of permafrost, glaciers, deserts, and lakes on the QTP and in the bordering regions. In prospect, more systematic, in-depth and coordinated studies on plateau permafrost evolution and changes, the reconstruction of Quaternary permafrost history will be more increasingly detailed and extending more retrogressively.

Acknowledgments:

Studies in this paper was supported by the Subproject No. XDA05120302 (Permafrost Extent in China during the Last Glaciation Maximum and Megathermal), Strategic Pilot Science and Technology Program of the Chinese Academy of Sciences (Identification of Carbon Budgets for Adaptation to Changing Climate and the Associated Issues) (Grant No. XDA05000000), Open Fund of State Key Laboratory of Frozen Soil Engineering (Grant No. SKLFSE201505), and under the auspices of the International Permafrost Association (IPA) Working Group on "Last Permafrost Maximum and Minimum (LPMM) on the Eurasian Continent."