Climate changes during the Upper Pleistocene and Holocene transition period

During the transition from the Pleistocene to the Holocene, the climate in eastern Asia, especially in China, was different from one region to another, according to the climatic belt to which each region belonged. While the most northwestern region belonged to the westerlies belt, the rest of China was influenced by the monsoon regime. Thus, while the first region was cold and humid during the last glacial period and became warmer and drier as the glaciers melted, most other regions, especially the inner ones, were dry and cold during the Ice Age and warm and moist as deglaciation proceeded (Li Jijun, 1990). This is manifested in the changes in the character of the different deposits. For example, Chaiwopu lake in the Xinjiang region, which is dominated by the westerlies, was 25-28 m higher than its present level depositing gray lacustrine deposits, during the period c. 15 ka to c. 12kaBP(JingtaiandKaqin, 1989). By comparison, the Qinghai and Qaidan lakes in the monsoon zone nearly or entirely dried up during the last glacial maximum (Kezau and Bowler, 1985; Ponyxi et al., 1988). However, when maximum interglacial conditions prevailed, monsoon influences could have also extended to northeastern China, which were usually under the influence of the westerlies (Zhong-wei Yan et al., 1990).

Wang Sumin and Li Jianren (1991) discussed the temporal and spatial distribution of lake sediments in China during the Late Cenozoic and the climatic environments revealed by lacustrine sediments since the last glacial period. They found an impressive variation in the evolutionary features of the lakes in different areas since the last glaciation. The Qinghai and Daihai lakes, lying on the northwestern margin of the east Asia monsoon area, had low lake levels during the last glaciation and the sediments were coarse. The palynological assemblage from Daihai Lake indicates that there was an Artemisia steppe, scattered with Ephedra shrub thickets and a few conifer or Abies-Picea-Pinus trees. The level of the lakes rose from 10 ka to 4.5 ka BP, building a terrace 40-45 m above present level.

3.1.2.a THE LOESS PLATEAU

On the whole, the climate changes during the Pleistocene as well as during the Pleistocene-Holocene transition period are clearly exhibited by the changes from loess to paleo-soil in sediment layers. In the loess plateau of China, influenced by the monsoon, the deposition of eolian loess characterized the dry and cold glacial periods. By comparison, paleo-soils formed during the warm and wet interglacial periods. In the most northerly region, influenced by the westerlies, brown and black soils were formed during the glacial periods, while during the interglacial periods, mainly eo-lian loess was deposited.

This general pattern varied with the specific local character of the different regions. For example, in the Yulin area, which is on the margin of the desert in the northern part of the monsoonal loess plateau, eolian sand, instead of loess, was deposited during the cold dry glacial period (Guarong, 1988).

An Zhisheng et al. (1991a) used the loess-paleo-soil sequence to identify variations in the east Asia monsoon for two time intervals: the last 130 ka years and the last 18 ka years. Here, only

Paloe Hyrdological Maps
Fig. 3.1. Map of eastern Asia.

the latter, shorter, time span will be discussed. The accelerated accumulation of thick loess layers from 18ka to 14kaBP is evidence of the domination of the PCAM or winter monsoon and the decline in the influence of the TOAM, or summer monsoon. The small amount of pollen, dominated by that of herbs resistant to arid conditions, indicates a substantial decrease in rainfall. Around 12kaBP, a paleo-soil developed in the western loess plateau, there was a significant increase in the pollen of broad leaves trees like spruce and fir of the lakes and the levels rose. All these indicate the strengthening of the summer monsoon. During the transition interval from Pleistocene to Holocene (c. 11 ka BP), there was another accelerated accumulation of dust, increase of herb pollen, the disappearance of spruce and fir, and a rapid lowering of levels of the lakes. These data indicate a return to the dominance of the PCAM, which may have lasted for only a few hundred years. Later, the climate changed quickly. From 9ka to 5 ka BP, a paleo-soil developed on the loess plateau, containing increased pollen of broad-leaved deciduous trees.

An Zhisheng et al. (1991b) also summarized the paleo-environmental changes in China during the last 18 ka as evidenced by dust accumulation, vegetation evolution, mountain glacier advance and retreat, and changes in sea level. They used certain magnetic properties of the sequence of loess, related to the precipitation and vegetation coverage ratio, as proxy-data for climate (Kukla and An Zhisheng, 1989; Versoub et al., 1993). From the composite curve, they concluded that the last full glacial age began to decline at about 14.5 ka BP, and there was a cool and humid ripple at about 12 ka BP. The climate then changed rapidly to a cold and dry regime around 11 ka BP and further transformed rapidly to a warm Holocene climate optimum, spanning from 9 ka to 5 ka BP. Afterwards, and continuing to the present, the climate reverted to a cool and dry regime, with occasional ripples of neo-glaciation. The same conclusions were reached by Zhou Weijian and An Zhisheng (1991), who reported on the correlation of 14C chronozones and other indications of the history of climatic changes during the Upper Pleistocene and Holocene on the loess plateau in China. They also considered paleo-soil layers with high "magnetic susceptibility" (MS; i.e., soils with a high iron content) to be indicative of low eolian accumulation and intensive soil-forming processes in which biological activity was involved (pedogenetic processes). Such processes occur most rapidly in a warm and humid climate. By comparison, low MS would indicate rapid dust accumulation and weak pedogenetic processes typical of dry-cold or dry-cool climate. In addition, they made use of the 'magnetic susceptibility age conversion equation' developed by Kukla and An Zhisheng (1989). From 13ka to 12kaBP, evidence for a mild climate was found in the form of a carbonate nodule horizon. From 11 ka to 10 ka BP there was a cold and dry climate, while a thick black paleo-soil, indicating a warm humid climate, was formed between 10 ka and 5kaBP. Since 5kaBP, the loess plateau has been dominated by the deposition of recent loess, generally reflecting a dry and cold climate. Some intercalations of weakly pedogenetic paleo-soils indicate periods of milder climate.

Hovan et al. (1989) established a connection between the Chinese loess sequence and the 518O chronostratigraphical and paleo-climatic sequence from a core in the northwest Pacific. Since the formation of the mid-latitude loess occurred during the glacial periods of the Pliocene-Pleistocene Epochs (Pye, 1987), a correlation was observed between the deposition of loess layers in China and greater accumulation of eolian material in the deep sea. From the correlation diagram, presented by Hovan et al. (1989), the period of melting of the ice at the end of the last glacial period, characterized by the oceanic water reducing its 518O contents, was also distinguished by a reduction in the eolian flux.

Huang et al. (2000) have investigated the loess profile in the southern part of the loess plateau. They found that crop cultivation began in this region at c. 7 ka BP, yet disturbance of the profile by cultivation has not masked the evidence for eolian dust deposition, which was characteristic of arid periods. During periods of humid climate, fertile brown soil developed. The following is the loess-soil sequence:

before 11kaBP: semi-desert conditions;

11 ka to 8.5 kaBP: post-glacial amelioration of climate;

8.5 ka to 6 kaBP: humid climate and formation of brown soil;

6 ka to 5 ka BP: arid climate deposition of loess;

5 ka to 3.1 kaBP: humid climate and formation of brown soil;

3.1kato 2.2kaBP: arid climate deposition ofloess;

2.2kaBP to present: humid climate and formation of brown soil.

The rather general character of the section limits the possibility of correlating it with other sections in the region or with conditions in the Levant. In general, it can be said that the dry period spread over time sections of mostly cold global periods, while the humid periods are those that were mostly warm.

3.1.2.b THE TIBETAN PLATEAU AND WESTERN CHINA Climatic changes in western China have been derived frompalyno-logical analysis of sediments, from the mountainous Lake Barkol in eastern Xingjiang (Han Shu-ti and Yuan Yu-Jiang, 1990). These sediments were continuous from the upper Pleistocene to recent times and would be used to examine the effect of the uplifting of the Tibetan plateau on the climates of western China and eastern China. This uplift, which reached its present elevation of 4000 m during the upper Pleistocene, reduced the influence of the east-western monsoon and resulted in changing the cold-dry/warm-wet to a cold-wet/warm-dry regime. The former, which was consistent and which still characterizes most of present-day China, prevailed in this region only during the Lower and Middle Pleistocene. The uplift also caused the aridization of this region, especially during the interglacial periods. During glacial periods, effective precipitation increased, resulting in lake level rise. During such cycles, low temperatures and an extended icebound season inhibited evaporation, and the greater available moisture caused the forest belt to expand. During the uppermost Pleistocene, most of the period from 21 ka to 12 ka BP was cold and wet, except for three cool and wet periods at 20 ka, 16 ka and 13 kaBP. During these periods, a retreat of the glaciers occurred. At 11 kaBP there was a warm dry spell, but at 10 kaBP the climate became warm and dry.

Li Shuan-Ke (1990) investigated the fluctuations of the levels of closed lakes and the variations of paleo-climates in the north Tibetan plateau, China. He concluded that these lakes went through four fluctuations from 18 ka BP to the present. In the first stage, from 18 ka to 12 ka BP, the level of the lakes was rising, while the mountain glaciers advanced. This was possibly related to the pattern of atmospheric circulation: the belt of westerlies shifted over the region and precipitation increased. In the second stage, from 12 ka to 8 ka BP, the lake levels dropped. During the third stage, from 8 ka to 6 ka BP, a rise in lake levels occurred, while in the fourth stage, from 6 ka BP to the present, there has been a drop of the level of the lakes.

3.1.2.c EASTERN CHINA

Xue Chunting et al. (1991), summarizing the history of Holocene coastal sedimentation in China, reported a transgression with a sea depth of 60 m all along the eastern part of the country from 11 kaBP, reaching its climax at 6kaBP. Since then the coasts have prograded seaward at various rates.

Yang Huai-Jan and Xie Zhiren (1984) reported on sea-level changes in east China over the past 20 ka, relating them to climatic changes. During the Holocene, they correlated minimum sea levels with maximum cold climatic phases observed in China. These were from 8.7 ka to 7.7kaBP (peak at 8.2ka), 6.7ka to 5.7 kaBP (peak at 5.8ka), 3.1 ka to 2.7kaBP (peak at 3ka) and 0.4ka to 0.1 ka BP (peak at 0.2 ka). During the cold periods, sea levels fell between 2 and 4 m. During the warm periods, a rise was observed. Three major rises could be discerned, from 10ka to 8.3 kaBP, from 8ka to 7kaBP and from 6ka to 5.5kaBP. These authors have also investigated the sea-level and climatic changes during the last 2000 years.

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