Mid-holocene climates and environments in China

Global and Planetary Change, 7 (1993) 219-233 Elsevier Science Publishers B V , A m s t e r d a m

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Mid-Holocene climates and environments in China Shi Yafeng a,c, Kong Z h a o z h e n g b, Wang Sumin c, Tang Lingyu d Wang Fubao e, Yao T a n d o n g a, Z h a o Xitao f, Z h a n g Peiyuan g and Shi Shaohua c " Lanzhou Institute of Glaciology and Geocryology, Chmese Academy of Sciences, Lanzhou, China b Institute of Botany, Chinese Academy of Sciences, Beoing, China Lake Sediment and Ent'tronment Lab_ Nanjlng Institute of Geography and Llmnology, Chinese Academy of Sciences, Nanjmg, China a Nanjlng Institute of Geology and Paleontology, Chinese Academy of Sciences, Nanjmg, Chma e Department of Geo and Ocean Sciences, Nanjmg Umt'erstty, Nanjing, Chma f Institute of Geology, Chinese Academy of Sciences, Betjing, China g Institute of Geography, Chinese Academy of Sciences, Betting, Chma (Recewed December 26, 1991, revised and accepted July 30, 1992)

ABSTRACT Shl Yafeng, Kong Zhaozheng, Wang Summ, Tang Lmgyu, Wang Fubao, Yao Tandong, Z h a o Xltao, Z h a n g Pelyuan and Shl Shaohua, 1993 Mld-Holocene climates and environments in China_ Global Planet Change, 7' 219-233 This paper focuses mainly on the chmatlc and environmental varmtlons from 8 5 to 3 ka B P with particular reference to one stable warmer and wetter mlllennmm (7 2 - 6 ka B P )_ Some large-scale centennial warming was accompanied by an increase in precipitation owing to the expansion of the monsoon circulation, e g rapid warming during the period from 8 5 to 8 3 ka B P and was reflected in the high level of some mland lakes and the sudden expansion of vegetahon m pollen diagrams The growth of the Neohthlc culture with agriculture and settlement m the present semi-and area of Northwest China was undoubtedly related to the dramatic warming and wetting before 8 ka B_P_ According to the proxy data of palynological studies, the dewation of annual mean temperature from today's about 7 - 6 ka B P. was roughly estimated at about I°C in South China, 2°C m the Changjlang (Yangtze) Valley, 3°C m North China and Northeast China. The strongest warming by 4-5°C was recorded m Qlnghai-XIzang (Tibet) Plateau The winter temperature rise was much greater than the annual average temperature In the eastern half of China, the vegetation zones are mainly parallel to the latitude During the chmax of the Holocene warm period, the northern limit of the tropical monsoonal rain forest shifted less than 1° of latitude northward, the subtropical broad-leaved evergreen forest about 1° of latitude, the north subtropical deciduous and evergreen broad-leaved mixed forest about 3 ° of latitude In the coastal area but only 1-1.5 ° of latitude in mid-West China and the warm temperate deciduous forest moved 4 ° of latitude northward, greatly expanding ~ts distribution Further north, the cool temperate boreal forest withdrew from its southern boundary by about 2 ° of latitude Global climatic warming in the mid-Holocene resulted in a sea-level rise During the period from 6 to 5 ka BP the sea-level was about 1-3 m higher than the present level. Large area of coastal lowland was submerged by sea water and frequency of occurrence of storm surges also increased during the high sea-level period_

Introduction

Global warming is an important problem universally interested all over the world People are anxious to know how the climates and environments will change in next decades or even in the next century. IPCC (Intergovernmental Panel On Climatic Change) (1990) estimated that the global temperature might be I°C higher m 2025 A.D.

than the present and 3°C higher in 2100 A.D The main purpose of this paper is to illustrate the basic features of the mld-Holocene climates and environments in China, in order to provide useful mslght into climate process, to improve the evaluation of the chmate scenario in the next century by chmate models. At the beginning of the 1970s, according to historical and phenological records Zhu Kezhen

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M I D - H O L O C E N E CLIMATES AND E N V I R O N M E N T S IN CHI NA

(1972) first pointed out that the average temperature in the lower H u a n g h e Rwer area from 5 to 3.1 ka B P. was 2°C higher than the present and the winter temperature was 3-5°C higher. In the last 20 years, a great number of Holocene deposits with accurate date have been found which are related to stratigraphy, palynology, archaeology, paleosols, paleolimnology, sea-level change and ice cores. An Zhisheng et al. (1990) have made a preliminary summary of the environmental evolution of China over last 20 Ka. We will deal some important data concerning the Holocene, and try to describe the climatic fluctuations, and the environmental features during that time.

Paleosols Multiple-layered paleosols have been found from the loesses and eolian sands in the semi-arid and semi-humid region especially. During the Holocene, more paleosol layers forming relevant peaks In their magnetic susceptibility curves, means that there was a more humid, dense vegetation and strong pedogenic processes But there is an obvious difference between the profiles of paleosols within a short distance Therefore it is necessary to carefully distinguish the result of local environmental variation from that of regional climatic changes. The paleosols from several sites in the middle Huanghe area are shown in Fig. 3.

Sources of data

Paleohmnology The locations of all the proxy data on mid-Holocene climates and environments are given in Fig 1.

Spore-pollen and other plant remams Data illustrating the Holocene climates come mainly from the study of spore-pollen in the Holocene strata described in more than 200 papers Most of the data may be used for explaining quahtatively the changes in cold-warm and drywet climates, and a few may be used for inferring semi-quantitatively the changes in temperature with transfer functions. Some plant nuts may be used for deducing the annual mean temperature and winter temperature in the hght of modern environments. Small-scale and rapid fluctuations of climate are not easily shown by spore-pollen assemblages, because of the time lag and uncertainty of spore-pollen data corresponding to climatic changes. Fewer 14C dates and the inaccurate datings may result in an incorrect age and the human actwltles during the Neolithic also had considerable effects on natural vegetation in some places. The temperature curves reconstructed according to spore-pollen assemblages in the locations in Fig. 2 are somewhat inconsistent with each other and the reasons for this phenomenon remain unknown

The hthology, fOSSIlS, chemical components of lacustrine sediments and the extstence of stephke lake terraces indicate lake-level fluctuations, expansion or shrinkage, freshening or sallnization. Inland lakes are especially sensitive indicators of climatic changes, and particularly effective moisture state. Thirteen lake records are used in the paper. The fluctuations in levels of some lakes in Holocene are shown in Fig. 4

Paleoglactal and ice core evtdence Mountain glaciers are another sensitive indicator of climatic changes Moraine tills dated by lichen or 14C, represent the climate becoming cold or humid, resulting in glaoal advance, while glacial regressions, during the warming period, left fewer morphostratigraphic traces. The analysis of an ice core drilled from the ice cap in mountains with low melting rate can be used to obtain the temperature fluctuation with a high resolution indicated by the variation of 6180 values. There is only one ice core in China from Dunde, Mount Qihan (Thompson et al., 1990; Yao Tandong, 1991). But the-6180 values of ice core In the Holocene cannot be simply translated Into temperature by means of relations between present 6180 value and temperature, because there is a rather large difference in 6180 values

SHI YAFENG ET AL

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slthermal do exist in China, the b e g i n n i n g varying from 9 to 7.5 ka B.P. and the e n d from 5 to 2.5 ka

b e t w e e n the m o i s t u r e of the c o n t i n e n t a l and oceanic air mass, m o r e o v e r the rainfall proportion from oceanic air mass in the m l d - H o l o c e n e may have b e e n much higher than that at p r e s e n t

B P Based o n D u n d e ice core records (Fig. 5), two strong h i g h - t e m p e r a t u r e events in the H o l o c e n e occurred respectively in 8.4-8.5 ka B P and 2 . 9 - 3 0 ka B.P. T h e r e were a b r u p t changes in climate before and after the two events. Before the 8 4 - 8 5 ka B P h i g h - t e m p e r a t u r e event, or

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rather m a b o u t 8 7 - 8 9 ka B.P., an evident cold event o c u r r e d A c c o r d i n g to the r e l a t i o n s h i p of 61~O value with t e m p e r a t u r e in west China, it lS

F a u n a , paleo sealevel, archaeology, p a l e o - p e r mafrost records a n d a n c i e n t legends w r i t t e n down m historical books, for example, the story about the G r e a t Floods, are also utilized in this paper.

inferred roughly that t e m p e r a t u r e increased sharply up to a b o u t 4 5°C d u r i n g 8 7 - 8 5 ka B.P A l t h o u g h D u n d e ice core records and palynological data from several localittes indicate that the t e m p e r a t u r e at 10 ka B P. was as high as that of today and the t e m p e r a t u r e at 9 ka B P was shghtly higher t h a n at p r e s e n t After 2 9 ka BP, there was a severe fluctuation, m d l c a t m g the t e m p e r a t u r e decreases a n d o t h e r e n v t r o n m e n t a l deteriorating

The chronology, phases and important events in the mid-Holocene

Chronology of the Holocene m China Different views on chronology of the m i d - H o locene w a r m phase, i.e. M e g a t h e r m a l or Hyp-

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Lake Bangong m west Tibet was 30-35 m higher than that of today. This shows that Indian monsoon rainfall sharply increased with warming. In the Qarhan saline lake, Qaldam Basin freshened clay was deposited. Qlnghm Lake, Daihal and Huangqlhal lakes in Inner Mongolia also had high water levels. These facts show that the summer monsoon rainfall areas expanded greatly northward By about 8 ka B.P., the warm and humid climate had caused a great change in vegetational dlStrlbuUon The warm temperate deciduous broad-leaved forest expanded 3 ° of latitude further north According to pollen analytical data from Lake Qinghai (Kong Zhaochen et al, 1990), Loess Plateau (Sun Jianzhong et al., 1991), Belsuhai in Inner Mongoha (Kong Zhaochert et al, 1991), East Hebel (Tong Guobang,

Chmattc fluctuatlons and dwtstons of the mtd-Holocene There were many climatic and environmental fluctuations from 8.5 ka B.P to 2.9 ka B P. Thus, it is necessary to divide the period into a number of phases as follows

8_5-7.2 ka B.P_ This phase is characterized by unstable temperature fluctuations The abrupt climatic change caused by sharply increasing warmth before 8 5 ka B.P. would have led to serious calamities and would have been harmful to many orgamsms. However, there was a time lag in adaptation of the vegetation, whale the precipitation changed quickly In 8.5-8.3 ka B.P., the water level of Age S u s c e p t i b i l i t y (Ka BP) 40 60 80

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1991), South Llaonlng (Gulyang Institute of Geochemistry, 1977), Luojie Mountain (Wang Qlan, 1990), etc., the temperature around 8 ka B P. was 2-4°C higher than that of today Many nuts of Juglans mandshuna were found from the Xinlun ruins of Chlfeng City (42°10'N, l19°E) in present temperate grasslands of Inner Mongolia, dated at 8135 + 270 yr B.P. Ceratopterls now growing in subtropical lakes and swamps of the Yangtze valley was found from the sediments in the suburbs of Tlanjin (39°N) dated at 8 ka B.P. Members of the Phizophoraceae now growing to the south of the Tropic of Cancer were discovered from sediments in Nlngbo (30°N, 120°E) dated at 8.5-7 ka B P. Such favourable conditions resulted in a rapid development of the Neolithic cullture e g agriculture appeared as the main mode of production and the people settled down in both Huanghe and Changliang valleys. According to records from the Dunde ice core, two low temperature events occruued at about 7.8 ka B.P. and 7.3 ka B.P. In response to the cooling events, in the Beijing region some dark coniferous species, which were restricted to the mountain areas earlier, spread

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M I D - H O L O C E N E CLIMATES AND E N V I R O N M E N T S IN CHI NA

onto plains at about 7.7 ka BP (Kong Zhaochen, 1982). These events coincided also with a sharp decrease in cultural remains during that time in the middle reach of the Huanghe. Denton (1973) regarded the period of 7.8 ka B.P. as the peak value time of the cold period in the northern hemisphere. On the Rothlisherger's (1986) diagram of glacier fluctuation, there was a common glacial advance not only in mountains of the northern hemisphere, but also in New Zealand and tropic mountains of South America in the southern hemisphere around 7.3 ka B.P.

7.2-6 ka B P. This was a stable, warm and wet phase, i.e. the climax of the Holocene optimum period Except for a few places, such as Qaldam, where the climate was drier than before and salt deposits appeared due to high temperature and strong evaporation, the other places had a warm and wet chmate. There was monsoon rainfall almost throughout China and plants unprecedentedly flourished For example, the coniferous and deciduous broad-leaved mixed forest grew around the Lake Qlnghal during that time. From fragmentary branches of Plcea purpurea (6245 +_ 180 yr B P.), It is inferred that the annual precipitation was about 600 mm and the temperature was about 3°C higher than that of today (Shi Yafeng, 1990) The deciduous broad-leaved forest of the warm temperate zone formed in the Shanjiang Plain and Changbai Mountains (Xia Yumel, 1988, Liu Jinling, 1989) in northeast China and the temperature was 3-4°C higher that at present. High lake levels occurred in all inland lakes in Inner Mongolia, Xinjlang, Qlnghai and Xizang. Lakes and swamps were well developed in North China Plain. At the same time, deserts greatly reduced Most of the mobile sand dunes from the Ordos desert to the western part of Northeast China Plain were once covered by the meadow steppe. In the Loess Plateau in the middle reach of the Huanghe and sand dune area in Inner Mongolia the paleosols were well developed. In the middle and lower reaches of the Changjiang where the deciduous broad-leaved and evergreen mLxed forest occurred, the temperature during 6.5-6.0 ka B.P. was 2.7°C higher than that of

225

today (Tang Linyu et al., 1990). In the south of the lower Changjlang Plain, Castanopsts and Cyclobalanopsis attained their greatest development since 10 ka B.P., also indicating a much warmer and wetter climate there. Favourable climatic conditions during this phase made the production, population and settlements increase rapidly. The neolithic Yangshao Culture in the Huanghe catchment and the Majlabing Culture in the lower Changjlang Plain were a period of great prosperity. The number of cultural ruins dated by 14C is greater than in the preceding and following phases More than 30 ruins of the microlithic culture, maybe referable to this phases in age (Llu Zechun, 1991), were discovered from northwest Xizang now unsuitable to human settlement. This indicates that warming in the Qinghai Xizang Plateau was stronger than at other places. By the end of the phase, the highest sea level appeared (Zhao Xttao, 1990) Additional and detailed discussion about migration of vegetation zones and environmental changes during the phase will be given later

3. 6-5 ka B P. This phase is characterized by strong climatic fluctuations and adverse environments. On one side, the warm and wet climate in the former phase still remained to some extent. For example, at about 5245 + 90 and 5140 + 120 yr B P. a large number of subtropical animals and plants were discovered respectwely from Talcheng County in Shandong Province and Baohua Hill in the eastern suburb of Nanjing and members of Ceratoptens and Proteaceae indicate that the temperature was perhaps 3.6°C higher than that of today. On the other hand, the results of studies of the Dunde ice core show that three cooling events took place at 5.9, 5.4 and 4.9 ka B.P The middle one IS especially obvious in north and east China (Yang Zigeng, 1979; Hong XueqIn, 1989). In the lower Changjiang Plain, palynological data suggest that the temperature in 6-5 ka B.P. was over I°C lower than that in 6.5-6 ka B.P. (Tang Lingyu et al., 1990). The Yangshao Culture in North China also declined and its sites rapidly reduced m number. All these probably indicate that the

226

human hvlng condittons during this phase were relatively poor The period 5.8-4.9 ka B.P. was called by Denton (1973) the second Neoglaclal period, with the coldest peak occurrmg about 5 3 ka B.P. Corresponding glacial advances took place lll all mountains throughout the world and there was also an advance occurring in the Urumql Vally of Ttanshan in 5380 _+ 150 yr B.P. The view that the pertod from 6.0 to 5.0 ka B.P is the climate optimum period IS obviously unsuitable for Chma. 4.5-3 ka B.P. Before 4 ka BP, a substable, warm and humid climate existed. There was a sharp increase in the number of archaeological sttes, such as the Long-

SH1 YAFENG ET AL

shan Culture in North China characterized by black pottery and the Llangzhun Culture discovered in the lower Changltang Plain respectively The sporepollen assemblage (4635-4245 yr B.P.) is dominated by pine forest in the Haiyuan ruin (36°15'N,105°40'E) located in the present semiarid steppe At the Taosi site (4500-3900 yr B.P.) in Xxangfen (36°N, 111°25'E), deciduous broadleaved and pine-dominated coniferous mixed forest existed. In the Changbei Mountains, the deciduous broad-leaved forest was still flourishing. These data show that the temperature was about 3°C higher than that of today Although the lakes at 45-50°N shrank already (Barzenkova, 1990) htgh water level still existed in the lakes in South Inner Mongolia, Qinghal and Xlzang. It seems

Fig 6 M o d e r n v e g e t a t i o n dlvlslon in C h i n a ( a f t e r W u Z h e n g y l et a l , 1980) 11 = T r o p i c a l m o n s o o n r a m f o r e s t 12 = E v e r g r e e n b r o a d - l e a v e d f o r e s t I j = D e c i d u o u s a n d e v e r g r e e n b r o a d - l e a v e d m i x e d forest. 14 = D e c i d u o u s a n d b r o a d - l e a v e d f o r e s t I 5 = C o m f e r o u s a n d d e c i d u o u s b r o a d - l e a v e d r e l i e d forest_ 16 = B o r e a l c o n d l f e r o u s forest_ //1 = S t e p p e lie = D e s e r t I / / = H i g h l a n d vegetation

M I D - H O L O C E N E CLIMATES AND E N V I R O N M E N T S IN CHINA

that the northern margin of monsoon rainfall moved southwards and the chmate was still warmer and wetter than that of today in the most parts of China. Around 4 ka B.P., many calamities happened and a wide and shallow cold trough appeared in the 31SO curve of the Dunde ice core. (Ftg. 5) In the northeastern part of Loess Plateau, the temperature and the precxpltat~on decreased suddenly and the northern boundary of the agricultural zone moved southwards by 1° of latitude. In east China, the legendary catastrophic flood lasting several dynasttes m~ght have led to the end of the Longshan and Lmnzhu Cultures. The story about I~ng Dayu's regulating rivers and water courses shows that by then people were organized to struggle against natural calamities and made great achievements. From the end of the calamitxes to 3 ka B.P., the chmate was still warm and humid. One of the ewdences is the fact that the Elephus maxlmus now living in the southern tropical zone stall existed in

227

Yangyuan (40°N, 114°10'E) of North China at that t~me. The palynological data from Guxiangtun in the Changbei Mountains (Lm Jlnhng, 1989), Qlnfeng (33°50'N, 120°E) of North Jiangsu (Tang Lmyu, 1990) and Luojle Mount in S~chuan (Wang Jlan, 1990) also support the idea that the vegetation did not decline until 3 ka B.P., which ~s basically m agreement with Prof. Zhu's view that the mld-Holocene warming period ended at about 3.1 ka B.P in China (Zhu Kezhen, 1972)

Reconstruction of climates and environments during the Holocene optimum Northward and westward migration of cegetatton zones Based on pollen and other paleobotanlcal data, the vegetation dlvlslon map of the optimum period was compded (Fig 7) and can be compared with modern vegetation division map (Fig. 6).

Fig_ 7 Vegetation dwlslon during Holocene optimum in China_ 11 = Tropical monsoon ram forest /2 = Evergreen broad-leaved forest I3 = Deciduous and evergreen broad-leaved mixed forest 14= Deciduous broad-leaved forest Is = Coniferous and deciduous broad-leaved mixed forest 16= Boreal comferous forest 111= Steppe III~ Highland forest and steppe 1~ = Desert 11I2 Highland steppe 1/~ = Forest-steppe I113 Highland desert

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Latttudmal mtgratton of vegetatton zone in east China Vegetation zones in East China display obvious but differential northward migration during this period. The southern limit of the Boreal coniferous forest of the cool temperate zone in northeast China, mainly composed of Larix, Pinus and Picea was at 50°N at the time, which was 2 ° of latitude further north than that at present. Coniferous and deciduous broad-leaved mixed forest of the temperate zone, characterized by Pmus, Quercus, Fraxmus, Betula, etc., shrank drastically and was only limited to the area of the Changbal and Xlaoxlngan Mountains in the eastern part of Northeast China. Deciduous broadleaved forest of the warm temperate zone extended its northern limit by about 5 ° of latitude The main surviving species are Quercus and PInus. The pollen assemblage in the northern part of the great plain in Northeast China were often mixed with a certain amount of Ptcea and Abtes In the Balyangdlan Lake (38°50'N, 116°E) of the North China Plain, a few subtropical elements, such as Ltqutdambar and Ceratoptens, often appeared. The deciduous and evergreen broadleaved mixed forest of the north subtropical zone

SHI YAFENG ET AL

was mainly composed of Fagaceae of the oak family, mixed with some elements of evergreen trees, bushes and ferns. Its northern limit reached the line around 35°N, 2 - 3 ° of latitude further north. The broad-leaved evergreen forests of middle and south subtropical zones were composed of Fagaceae associated with Anacardlaceae, Euphorbiaceae, Magnoliaceae, Louraceae and Proteaceae. Its northern limit reached the line 32°N, only 1° of latitude further north than now. The movement of the limit of tropical rain forest was even less than 1° of latitude

Longitudinal migration of vegetation zones The steplike relief from the East China plain to the Qinghai-Xizang Plateau (i.e. "the roof of the world") and the large distance from the Pacific and the Indian Ocean to most of inner Asia causes an evident gradient of heat and moisture. As the temperature and humidity increased during the mid-Holocene, the vegetation belts showed a westward expansion, displaying the contemporary vertical and horlzonal transition, that is, the expansion of the steppe and shrinkage of the desert In northwest China and the enlargement of the forest at the eastern edge of the Qinghal-

Fig 8 Isogram of the temperature deviation during the mld-Holocene from present m China

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MID-HOLOCENE CLIMATES AND ENVIRONMENTS IN CHINA

Xizang Plateau and the Hengduan Mountains. The western boundary of forest and forest-steppe (a transitional vegetation from forest to steepe) moved about 3 - 5 ° of longitude compared with that at present. Similarly, the true steppe also moved about 3 - 4 ° of longitude westward The vegetation of the Qinghai-Xizang Plateau was dominated by highland forest and steppe The area of the desert in northwest China and in the western part of the Qlnghai-Xizang Plateau were greatly diminished; it was limited to the Tarlm Basin, western part of Inner Mongolia, the Qaidam Basin and south of the Kunlun Ranges.

Reconstructton of temperature On the basis of above-mentioned vegetation divisions ~t is possible to reconstruct the temperature situation in terms of the relationship between vegetation zone and annual average temperature at present It is estimated that the temperature in the boreal forest area of the cool temperate zone was 3°C higher, at the northern border of the subtropical zone 2°C or more, from 30°N to the Zhujiang Delta only 0.5-1.5°C higher, i.e. about I°C. In the third terrace of Peikuco Lake (29°N, 85°30'E), 4900-5100 m a.s I in South Xizang, the pollen assemblage during 6 ka BP is composed of Pzcea dominated by coniferous trees and a few broad-leaved trees, such as Quercus, Betula, indicating that the average temperature then was 5°C or more higher than that at present. The pollen assemblage of Pinus, Betula, Abtes, Ptcea, etc .. in the lake sediment dated at 7 0 ka B P from Zhabuya Lake (31°30'N, 88°E) 4400 m a.s.l, on the northern slope of the Gandis Range, Xizang, also indicates that the annual average temperature was 4-5°C higher than that at present. The lsogram of temperature deviation during the mid-Holocene from present is roughly shown in Fig 8. It can be estimated from Fig. 8 that the average temperature In east China during the period was 2 5°C higher than at present and in west China 3-4°C, much higher than the average value of the northern hemisphere (1.5°C) in 6.5-7 ka B.P. estimated by Frenzel (1991), not to say that by Vehchko et ai. (1991) 0.6-0.7°C ( 6 - 5 5 ka B.P ). It may be concluded that temperature rise

in China during mid-Holocene period was the greatest of an equivalent latitude in the world especially in the QinghaI-Xizang Plateau The winter temperature difference between North and South China is much greater than that of summer in East China. The presence of Ceratoptens m the Baiyangdian area, south of Beijing, at 6 ka B.P. and in Tancheng County of Shandong (34°40'N, 118°20'E) at 5.2 ka B.P. and now living in subtropical lakes and swamps, indicate a 3.5°C rise of annual average temperature and 6°C rise of winter temperature (Kong Zhaochen et al., 1991). The discovery of nuts of Hehcla dated at 5140 + 120 yr B.P. m Baohua Hill, east of Nanjing, also shows a rise of 3 5°C m annual average temperature and of 6 3°C in January temperature_ There was a continuous and stable forest vegetation in the Qinghai Lake area at 8.5-3 ka B.P, indicating a rise of 2°C m the highest monthly average temperature and of 8°C in the lowest one (Kong Zhaochen et al, 1990) The three cases show that the winter temperature rise was 70-100% more than that of the annual mean temperature, i.e in most part of East China, a possible rise of 4-5°C in winter temperature contrasts to rise merely I°C in summer temperature The great rise in winter and the small one in summer greatly narrowed the annual range of temperature, which severely weakened the high pressure system in Siberia, the source of winter monsoon or cold tide, so that it could not affect the vast territory of China as present With such a favorable climate and environment, Elephus maxtmus and Rhmoceros sondatcus, which now live in the tropical and south subtropical zone, could exist In the northern area from 34°N to 41°N during the period.

Reconstruction of preclpttatton Based on the information about migration of vegetation zones, It is generally implied that the precipitation In the mld-Holocene was more plentiful than that at present. But the data on specific changes in precipitation amount are not adequate to compile the precipitation map at this time. A simple outline can be formed by the following data

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During the mid-Holocene in the mid-east of Inner Mongoha, the deciduous broad-leaved forest and temperate coniferous forest moved 3 ° in latitude northwards. It can be estimated that the annual average t e m p e r a t u r e increased by 3°C and precipitation by 100 mm, while forest moved 300-400 m up the mountains (Cul Halting et al., 1991). At 6.7 ka B P , the high lake level of Dalhal Lake (40°15'N, 112°45'E) indicated that, under the condition of temperature rise of 3°C, the precipitation was about 40% (150 mm) more than that at present, according to the water and energy balance calculation (Wang Sumln, 1991) At 6 - 7 ka B.P., the existence of Ceratopterts Brougu at Baiyangdlan Lake (38°51'N, 116°E) indicates a 450 m m increase in precipitation and this led to the formation of some large lakes in the North China Plain. At 6.3 ka B.P_, the presence of Picea purpurea indicated m the Qlnghal Lake (37°N, 100°E) an annual rainfall of 600-650 mm, 70-80%, higher than at present (Wang Sumin, 1991) and an annual precipitation of 540 mm by using the water and energy balance models for lake levels In the Ordos area of Inner Mongolia, accordIng to the paleosols and other data, it is estimated that annual rainfall was 250-650 mm m the humid period of the Holocene, while it is 200-400 m m at present. The most humid period probably occurred at 6 2 ka B.P (Shi Peljun, 1991) Many Yangshao Culture rums with the agricultural features m the present pasturage area proved a plentiful precipitation at that time The possible reason for the distinct increase of precipitation in the vast area of arid and semi-arid in Asia is the strengthening and expanding of summer monsoon The traces of flooding across the Taklamakan desert in the Tarlm Basin show that the increase of precipitation during the mxdHolocene was not enough to change the desert environment and the sand dunes were still active It should be noticed that the precipitation change is much more complex than the temperature change Even two adjacent positions have different precipitation situations in the same period This may be explained by the palesol data The formation of paleosol layer intercalated in

SHI YAFENG

ET AL

loess and eohan sands in the semi-arid area of Northwest China depends mainly on the improvement of moisture conditions. When the precipitation increased, the vegetation thrived and the pedogenic processes prevailed, correspondingly magnetic susceptibility values rose For example, in Baxle Loess profile (Fig 3) in the western part of the Loess Plateau, four layers of paleosols appeared from 8.2 to 5.5 ka B P. and the largest magnetic susceptibility value from 8 2 to 7 7 ka B.P. (An et al., 1991). But the another adjacent section has only two layers of paleosols from 9 to 5 5 ka B P (Chen et al., 1991; Zhou et al, 1991) The discordant paleosol occurrence stated above might result from local climatic and landform differences The presence of paleosol layers around 6 5 ka B P. in a vast area from the sand land in Northeast China (Jlu Sanwen, 1991) to the Gonghe Basin (36°N, 101°E) in northeastern margin of the Qlnghai-Xlzang Plateau (Gao Sanyu, 1991), revealed a prevailing humid period during the Holocene chmatlc optimum period But the absence of paleosol and the red scorched layer close to the Ebl Lake (45°N, 83°E) North Xlnjiang probably show that a relatively dry situation still remained The high sea level as an important aspect of paleo-envlronment mainly occurred at 6.5 ka B P and ended at 4-5 ka B P and only in a few sites lasted untd 2 8 ka B.P. The maximum height of sea level was 1-3 m higher than that at present During the high sea level period, the sediments in Qlnfeng profile show some concrete evidences of storm surges and perhaps indicate a frequent occurrence of tropical cyclones (Zhao et al., 1991) Conclusions and discussion

Based on the study since the 1970s, of palynology, paleobotany, paleozoology, archeology, paleopedology, paleohmnology, ice core and sea level fluctuations in the Holocene, it is inferred that there were a series of climatic fluctuations of cold and warm, dry and wet, abrupt change and gentle transitions from 8 5 ka to 2.9 ka B.P. and they can be divided into several phases, unstable cold and warm fluctuations from 8 5 to 7 2 ka B.P., a stable warm and humid phase from 7.2 to 6 ka B.P, i.e.

MID-HOLOCENE CLIMATESAND ENVIRONMENTS IN CHINA

the Holocene optimum; intensive oscillation from 6 to 5 ka B P. with strong cold events causing enwronmental deterioration; the first millennium in the period of 5 - 3 ka B P. being substable and gently oscillated warm and humid stage, and the second millennium an intensified oscdlatton stage. Through reconstructton of the mid-Holocene climates and environments Jt seems that temperature rtse was accompanied by an increase in precipitation in centenary time scales. Owing to the northward and westward expansion of the summer monsoon, the vegetation system moved northwards and westwards The southern limit of boreal forest in Northeast China migrated 2 ° of latitude northwards, the northern limit of temperate coniferous and deciduous broad-leaved mixed forest moved 5 ° of latitude northwards, north subtropical deciduous and evergreen broad-leaved forest moved 2 - 3 ° of latitude northwards, but the northward movement of middle and south subtropical evergreen broad-leaved forest was only 1° of latitude and the tropical evergreen rain forest even less than 1° of latitude. The Elephus maxtrnus, Rhinoceros sandatcus, Alhgator smensls which now live in tropical and subtropical areas, could be found in areas north of 35°N. Because of the improvement of warmth and moisture conditions, the forest and forest steppe zones moved 3-5 ° of longitude westwards. The realm of desert in Northwest China and the Qinghai-Xlzang Plateau shrank greatly. Based on the division of mid-Holocene vegetation, the deviation value of annual average temperature in South China is about I°C higher than that at present, the Changjmng area about 2°C; northeast, northwest, and north China about 3°C, the central and southern parts of the QinghaiXlzang Plateau even reaching 4-5°C. The temperature rise in China was the greatest of that latitude in the world According to some pollen data, the winter temperature rise was 70-100% more than annual average temperature, therefore China was one of the areas with the greatest winter temperature rise in the world. By contrast w~th winter temperature, the summer temperature rise was much less than annual average temperature change, so the annual range of temperature was much smaller than that at present,

231

Indicating a weakening of cold air currents from Attic region at that time. Although the precipitation change was much more complex than the temperature change, it is certain that m a vast area, including north China, northwest China, Xizang and south Mongolia, the precipitation m the mid-Holocene was from tens to hundreds of milhmetres more than that at present, resulting In high lake levels and freshening of inland lakes, development of large lakes and swamps in north and northeast China and appearance of multi-layered paleosols m the eolian sand and loess areas. In the modern desert and steppe areas approximately east of 105°E, most acttve sand dunes were then fixed, but from 105°E westward the sand dunes were still mobile. Another feature of the mId-Holocene environment is the occurrence of a sea level 1-3 m higher than today's from 65 to 5 ka B.P. and hence in east China a plain area of about 7 × 104 km 2 was submerged Contemporaneously, the area of glaciers and permafrost greatly reduced. The natural environment in the mid-Holocene was propitious to the development of productivity and expansion of settlements during the Neolithic civilization, Le. the well known Yangshao Culture, in China

Acknowledgements This research received support from the Chinese Natural Science Foundatton. In the preparation of this paper, Mr. Wu Tongjla, Shl Hulquan, Liu Ta, Wu Shouyun, Ms. Ma Yah rendered valuable help. To them the authors express their sincerest thanks Here we take this opportunity to thank Dr H Faure heartily for his constructive amendments to thts manuscript.

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