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Stable isotope records of plant cover change and monsoon variation in the past 2200 years: evidence from laminated stalagmites in Beijing, China JU ZHI HOU, MING TAN, HAI CHENG AND TUNG SHENG LIU

Hou, J. Z., Tan, M., Cheng, H. & Liu, T. S. 2003 (June): Stable isotope records of plant cover change and monsoon variation in the past 2200 years: evidence from laminated stalagmites in Beijing, China. Boreas, Vol. 32, pp. 304–313. Oslo. ISSN 0300–9483. Two stalagmites collected from the Shihua cave in the southwestern suburb of Beijing were dated by annual layer counting. The results are consistent with thermal ionization mass spectrometry 230Th dating. Stable carbon isotope variation of stalagmites is dominated by plant cover change, which largely reflects climate change and monsoon variation. Oxygen isotopes are mainly affected by precipitation, which is related to summer and winter monsoon intensity. The combination of carbon and oxygen isotopes can therefore be a proxy of plant cover change and monsoon variation. Our stable isotope results show that lower carbon isotope values of the stalagmites between 200 BC and AD 1000 probably imply dense plant cover and an episode dominated by humid summer monsoon. From 1000 to AD 1450, the dominant monsoon alternated between the winter monsoon and the summer monsoon. Since AD 1450, a significant jump in carbon isotope ratios and increasing oxygen isotope ratios has been demonstrated, indicating less plant cover and the probable dominance of dry winter monsoon. The results are consistent with historical documents of the region. Ju Zhi Hou (e-mail: [email protected]), Ming Tan (e-mail: [email protected]), Tung Sheng Liu (e-mail: [email protected]), Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China; Tung Sheng Liu, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an, Shaanxi Province 710054, China; Hai Cheng, Department of Geology and Geophysics, University of Minnesota, Minneapolis MN 55455, USA; received 8th March 2002, accepted 16th July 2002.

The east Asian monsoon regime is a subsystem of the Asian monsoon circulation, which is formed as a result of thermal differences between the Eurasian continent and the Pacific Ocean, and is enhanced by the thermal and dynamic effect of the Tibetan Plateau (An 2000). In winter, cold air from high latitudes can extend to subtropical South China, even crossing the Equator; while in summer, warm and humid air originating from the low latitude oceans migrates northward into China’s interior as far as the China–Mongolia border (Chen et al. 1991). The east Asian monsoon is characterized by prominent seasonal alternation between winter and summer. Much research has been carried out on various records of east Asian monsoon variation over the past 130 ka and 20 ka (e.g. Porter et al. 1992; Shi et al. 1992; Ding et al. 1992; Jarvis 1993; Liu & Ding 1993; Rutter & Ding 1993; Ding et al. 1994; Zhou et al. 1994; Porter & An 1995; Thompson et al. 1997; Yamano et al. 1998; Wang et al. 1999; An 2000; Phadtare 2000). Stalagmites have been used as a high-resolution indicator of climate change ever since growth layers were verified to be annual in the 1960s (Broecker et al. 1960). Stable isotope ratios, lamina thickness, greyness, luminescence and trace elements, etc., have been extracted from stalagmites in the study of palaeoclimate (e.g. Baker et al. 1993; Demarchelier et al. 2000; Genty

1993; Genty & Quinif 1996; Lauritzen 1995; Neff et al. 2001; Qin et al. 1998; Qin et al. 1999; Roberts et al. 1999). In recent years, some effective tracers of monsoon have been extracted from stalagmite, such as stable isotopes (e.g. Li et al. 1997; Burns et al. 1998; Ku & Li 1998; Yadava & Rames 1999; Li et al. 2000; Neff et al. 2000; Neff et al. 2001), stable isotopes combined with layer thickness (e.g. Burns et al. 1999; Fleitmann et al. 2000), concentrations of magnesium and strontium (e.g. Ku & Li, 1998) and petrology (e.g. Denniston et al. 2000). In this research, we extract plant cover change and climate records from stable carbon and oxygen isotope analysis from two stalagmites collected from Beijing Shihua Cave.

Site and sample description The Shihua Cave (39°50'N, 115°40'E, entrance at 150 m a.s.l.) is developed in Ordovician limestone and is situated in Baihua hill, Fangshan County, about 50 km southwest of downtown Beijing (Fig. 1). Lots of stalagmites and stalactites were found in all six levels of the cave. Stalagmite laminae and their palaeoclimatic significance in the study of global change have been reported previously (Liu et al. 1997; Tan et al. 1997) DOI 10.1080/03009480310002000. # 2003 Taylor & Francis

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Stable isotope records of climate change, China

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Fig. 1. Sketch map of Beijing and site of the Shihua Cave.

and the laminae have been demonstrated to be annual (Tan et al. 1997; Qin et al. 1998). Stalagmite LS9602 and stalagmite TS9701 were collected in 1996 and 1997, respectively. Both stalagmites were still growing at the time of sampling and no coupling stalactite was deposited. Stalagmite LS9602 is located in the first and uppermost level of the Shihua Cave. It is column-like with a height of 16 cm. The bottom diameter is 10 cm and the top diameter 7 cm. Stalagmite TS9701 is located in the third level of the cave. It is column shaped and is 24 cm in height. The bottom diameter is 12 cm and the top diameter 10 cm (Fig. 2). The stalagmites were bisected along the growth axis. One half was mounted on a glass slide to produce a thin section slice, while the remaining half was kept for other analyses. Their very homogeneous and almost featureless petrographical structures suggest that growth had been continuous. Observed in transmitted light and fluorescent light, the slices of both stalagmites

show the typical layer characteristics of stalagmites in northern China, i.e. in transmitted light the layers consist of a thin opaque interface and a thick transparent calcite band; in ultra-violet light the stalagmite is bright fluorescent, while the dark layer observed under transmitted light turns to a bright layer under fluorescent light. Non-coincidence was not found between the transmitted layer and the fluorescent layer (Genty et al. 1997; Tan et al. 1999). That is to say, there are bioptical microcycles in these samples, which means both of the stalagmites were in a stable growth environment from their beginning.

Methods TIMS dating Thermal ionization mass spectrometry

230

Th dating is

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Fig. 2. Longitudinal section of stalagmite TS9701 and LS9602. (A) TS9701, (B) LS9602, (C) transmitted layers (left) and fluorescent layers (right) of TS9701, (D) transmitted layers (left) and fluorescent layers (right) of LS9602. The data on the section are TIMS 230Th corrected age and annual layer counting results.

now the best method used for dating stalagmites up to 550 ka (Cheng et al. 1998; Li et al. 1989). Both stalagmites were dated at the Department of Geology and Geophysics, University of Minnesota, USA. Four samples of Stalagmite LS9602 and five of Stalagmite TS9701 obtained for dating were taken with a dental drill along the growth axis. Sample size is 0.6 g, while the sampling groove is about 2 mm wide, 6 mm deep and 20 mm long, paralleling the layer interface (Fig. 2). All samples were prepared in an ultra-clean chemical laboratory. The powder samples were dissolved in 14N nitric acid and 229Th-233U spikes were added. U and Th were then precipitated with Fe(OH)3; U was separated from Th with anion exchange resin; and U and Th were dissolved with HNO3 and covered on Re belt (Th is covered with graphite too). Finally, U and Th were tested with a Finnegan-MAT 262-RPQ mass spectrometer.

The criterion of defining annual layers was decided before the layers were counted, i.e. only layers with the following characteristics were counted: layers where the interface appeared to be carved, and there is a light line adjacent to the interface that moves back and forth slightly when the microscope slide is adjusted. This criterion has been discussed previously (Hou et al. 2002). There is a likelihood of minor error in counting the layers and in reconstructing the time series because the results of annual layer counting depend on the optical conditions during observation. Each stalagmite was counted by at least two counters in accordance with the criterion. The first counter marked the stalagmite slide every 20 layers, while the results were calibrated by others. The error generated by different optical conditions could be eliminated by this calibration. Stable isotope

Layer counting The dating methods commonly used, such as AMS 14C dating, U-series alpha counting and even thermal ionization mass spectrometry 230Th dating, could not provide an age corresponding exactly to calendar ages in the last 2000 years, so we had to resort to the ‘natural clock’, namely tree rings, stalagmite layers, etc.

Isotope equilibrium test. – Before discussing the stable isotope composition of the two stalagmites, we have to consider whether deposition occurred under isotope equilibrium conditions, which is essential for stalagmite stable isotope climate analysis. The criteria for the recognition of equilibrium conditions have been well documented previously (e.g. Hendy 1971; Schwarcz


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Table 1. TIMS dating and annual layer counting results of TS9701 and LS9602.*

Sample

No. of annual layers from top

TS9701-1 TS9701-2 TS9701-3 TS9701-4 TS9701-5 LS9602-1 LS9602-2 LS9602-3 LS9602-4

51–69 410–430 874–902 1840–1850 2290–2380 54–58 301–308 795–809 989–999

230 238

232

234U** measured

170.4 0.3 180.1 0.1 211.2 0.4 154.1 0.1 124 0.2 70.4 0.4 60.4 0.1 48.4 0.1 84.6 0.2

602.5 15.1 917.8 27.6 286.2 10.5 370.2 12.1 192.3 8.1 125 8 172.9 9.1 222.7 7.6 507 8

1411 6.4 1439 2 1374.2 6.7 1339.7 2.6 1490.4 6.5 1283 21 1274.9 6.8 1225.3 6.6 1243 11

U (ppb)

Th (ppt)

Th age (year) uncorrected

230

Th/238U (activity)

Th age (year) corrected

234U*** initial

0.00208 0.00031 0.01148 0.00111 0.02090 0.00098 0.03989 0.00076 0.05313 0.000105 0.00180 0.00046 0.00687 0.00068 0.01696 0.00071 0.02218 0.00069

94 14 515 50 965 45 1875 36 2350 47 86 22 330 33 835 35 1084 34

52 26 454 58 948 46 1845 39 2332 48 63 25 294 37 775 46 1006 52

1411.3 6.4 1441 2 1377.9 6.7 1346.9 2.6 1500.3 6.6 1283 21 1276.1 6.8 1228.2 6.6 1246 11

230

*230 = 9.1577 10 6y 1,234 = 2.8263 10 6y 1,238 = 1.55125 10 10y 1s. **234U = ([234U/238U]activity-1) 1000. ***234U initial was calculated based on 230Th age (T), i.e. 234Uinitial = 234Umeasured e234 T. Corrected 230Th ages assume the initial 230 Th/232Th atomic ratio of 4.4 2.2 10 6. Those are the values for a material at secular equilibrium, with the crustal 232Th/238U values of 3.8. The errors are arbitrarily assumed to be 50%.

1986). Briefly, the conditions are (i) that 18O remains constant along a single growth layer, while 13C varies irregularly, and (ii) that there is no correlation between 18O and 13C along a growth layer. Four sets of subsamples of stalagmite LS9602 were taken along growth layers at 14, 40, 97 and 125 mm from the top, with 5 samples in one set. For TS9701, five samples were analysed from each of four sets at 33, 54, 123 and 172 mm. Powder samples were taken with a dental drill. The sample size is about 5–8 mg. Samples were converted to CO2 by reacting with pure phosphoric acid and kept for at least 2 h at 50°C and then measured by MAT-252. All values are reported in per mille (%) for both oxygen and carbon relative to Pee Dee Belemnite.

Stable isotope time series. – Powder samples were collected along the growth axis from the top of both stalagmites, leaving about 20 annual layers in the interval between the two neighbouring samples. Each sample was dated by annual layer counting. Stable isotopes were measured as indicated above.

Results TIMS dating The dating results of the two stalagmites with 2 are listed in Table 1. In general, the precision of TIMS 230 Th dating of stalagmite correlates positively with the

Fig. 3. Comparison of layer counting results and TIMS 230Th dating. (A) Stalagmite TS9701, (B) stalagmite LS9602. TIMS age and the layer counting results are coincident in both stalagmites, which indicate that annual layer counting is valid.

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Fig. 4. Hendy test: Plots of 18O against 13C and 18O against distance from growth axis along the growth layers. The numbers in legends indicate the distance from top of stalagmites. Equilibrium deposition is indicated by lack of significant trend in isotopic ratios towards heavier values with distance from the growth axis and a lack of statistically significant correlation between 18O and 13C.

content of initial 238U, while being negatively correlated to the content of initial 232Th in the stalagmite.

Layer counting Layer counting results were plotted against TIMS 230Th age in Fig. 3, also listed in Table 1 (column 2). The layer counting results are consistent with the high-resolution TIMS U series disequilibria dating technique, suggesting that layer counting is an effective tool for dating stalagmites and also that the growth of both stalagmites was continuous. As both stalagmites were still growing when sampled, the first layer from the top can be assumed to be the year of sampling. As such, the first layer from the top of stalagmite TS9701 was formed in 1997, and the first layer of LS9602 in 1996. The second layer of each stalagmite was formed in the year prior to sampling, and so on, so that we can progressively

define the age, layer by layer, back to 2400 a BP for TS9701, and back to 1000 a BP for LS9602. Stable isotope Isotope equilibrium tests. – The test for Hendy criteria is provided in Fig. 4. For stalagmite TS9701, no clear correlation between carbon isotope and oxygen isotope ratios is shown in the examination, and there is no clear increase of stable oxygen isotopes in a single layer, except sample 33, which indicates that the isotopic equilibrium might not be reached everywhere in the stalagmite. For LS9602, although the tendency of 18O against the distance from the growth axis of 40 mm is slightly insignificant, those of other sets of subsamples were clear enough. In each case, the correlation between 13C and 18O was statistically insignificant. It is thus likely that both stalagmites grew under equilibrium conditions.

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Fig. 5. Comparison of stalagmite TS9701 and LS9602 isotopic records and historical records. The calendar age comes from annual layer counting. Oxygen isotope records of both stalagmites are shown in the upper part, carbon isotope records in the lower part; the solid line indicates isotope records of TS9701, while the broken line indicates LS9602. The climate index comes from historical records (2 = heavy rain, 1 = rain, 0 = normal, 1 = drought, 2 = heavy drought). Few records before AD 1000 were preserved, the rain was noted year by year since AD 1270. I, II, III are the stages mentioned in text.

Stable isotope time series. – The stable isotope time series are plotted against the annual layer counting results of each isotope sample in Fig. 5. In the past 2200 years, the entire profile of 13C in TS9701 ranges from 11.2% to 4.7% (PDB), with the highest value occurring at 280 a BP and the lowest at 1270 a BP, while oxygen isotope ratios vary slightly. Taking carbon and oxygen isotope in combination, the entire profile was mainly divided into three stages in the past 2200 years. Carbon isotope ratios of LS9602 range from 1.7% to 7.8%, which was divided into two stages combined with oxygen isotope. Two stages of LS9602 correspond to the latter two stages of TS9701 (Fig. 5). The first stage ended at about AD 1000, the second at AD 1450, with the last one continuing to the present. Stage I, from 200 BC to AD 1000, can be divided into two phases: (a) from 200 BC to about AD 820, and (b) from AD 820 to 1000. During the first phase (200 BC to

AD 820) the 13C decreases to its minimum 11.2% gradually and is accompanied by several evident undulations. The 18O increases continuously to its maximum 6.7% at the end of this phase, although oxygen isotope ratios varied slightly. In the second phase, from AD 820 to 1000, the carbon isotope ratios vary adversely compared to the former phase, as do oxygen isotope ratios. In this stage, nearly all the 13C values are lower than the overall mean values of 13C. Stage II, from 1000 to 1450, lasts about 450 years. Variation of carbon isotope ratios in TS9701 and LS9602 generally increases. 13C of TS9701 decreases from 8.4% to 10.4% in about 120 years, and then increases to 8.0% in the last 300 years. On the whole, carbon isotope ratios of stalagmite LS9602 increase with several anomalies. 18O of both stalagmites varies irregularly. Stage III occurs from AD 1450 to the top of both

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stalagmites. At the beginning of this stage, carbon isotope ratios increase. About 200 years later, carbon isotope ratios of stalagmite TS9701 increase abruptly from 9.0% to 4.7%. Although there is no such evident jump in LS9602 samples, carbon increases by 3%. Following this jump is an abrupt decrease of carbon isotopes, which lasts for about 50 years. 13C of stalagmite TS9701 decreases from 4.7% to 6.5% and 13C of LS9602 decreases slightly too. Variations in the trends of oxygen isotopes are similar to those of carbon isotopes for both stalagmites, though the amount is less than that of carbon isotopes. After this abrupt change of 13C during this stage, carbon and oxygen isotope vary slightly. Although there are similarities in 13C between stalagmite TS9701 and LS9602, there is a minor discordance in the two records. The carbon isotope record of TS9701 is much higher than that of LS9602 after AD 1500.

Discussion Calcite of stalagmites mainly originates from the seepage of rainwater. Dissolution of CO2 produced by plant respiration and decomposition of organic matter in the soil zone drives dissolution of carbonate bedrock during infiltration. In the cave, the seepage water becomes supersaturated and CaCO3 is precipitated to form stalagmites. The isotope composition of stalagmites therefore originates from seepage water. The dominant controls on carbon isotope variation differ from those on oxygen isotope. The carbon isotope composition of a stalagmite is best considered with respect to the carbon isotope composition of the soil atmosphere. Since variations of carbon isotopes in stalagmites depend on many complex processes, there is little consensus on their interpretation. However, we can identify at least five different processes that will affect 13C of stalagmites: (a) rainfall, controlling the percentage of plant cover, which is the main source of CO2 in the soil; (b) photosynthetic pathways (C3/C4 pathways), i.e. vegetation type, affecting the isotopic composition of CO2 dissolved in the water; (c) temperature, affecting biological activity and isotopic equilibrium; (d) bedrock proportion, varying the isotopic composition of seepage water when passing through the rock; and (e) drip rate, influencing the deposition equilibrium between water and stalagmites (Lauritzen 1999; Genty & Massault 1999; Genty et al. 1999, 2001). The variation in the 13C signal is unlikely to be related to temperature changes directly, because the mean annual temperature varied less than 2°C in this region during the Holocene (Shi et al. 1993). Such a change in temperature could account for only 0.6% variation in calcite 13C (Wang 2001), while the range of carbon isotopic variation amounts to 6% in this

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study. Because the Hendy test has shown that both stalagmites grew under isotopic equilibrium, and assuming that carbon isotope equilibrium is maintained over the past 2200 years, the dripping rate effect could be eliminated. The bedrock effect seems to be ascribed to plant cover change, which dominates soil CO2 concentration. The soil CO2 dominates limestone dissolution intensity, which can vary the proportion of carbon from the bedrock. Then the dominant controls on carbon isotope composition of stalagmites could be the photosynthetic pathway and precipitation. Generally, plants are rich in the light carbon isotope in photosynthesis. Plants utilizing the C4 (Hatch-Slack) photosynthetic pathway respire and decompose into carbon dioxide with higher 13C values than C3 plants (Calvin cycle). Both vegetation types are frequently found in close association, but the natural succession of C3 and C4 plant is a long process. If there is an abrupt variation in carbon isotope ratio in the record, it could not result from natural vegetation change, but from human activity. The carbon isotope incorporated in plants is roughly negatively correlated with the soil water (Farquhar et al. 1982). Soil water comes mainly from rainwater, which significantly controls plant cover change. Therefore the 13C values of speleothem calcite reflect plant cover change and climatic conditions (Dorale 1992; Denniston 2000). The oxygen isotope ratios of calcite in speleothem are derived almost exclusively from infiltrating meteoric water. Under typical monsoon climate, mean annual precipitation for 30 years (1951–1980) in Beijing was 632 mm, about 70% of which fell in July, August and September, the warmest period in this region. The rainfall originates mainly from the northwest Pacific Ocean and is carried to the continent by Southeast Asian summer monsoon; cold flows originate from the high latitude region of the northern hemisphere. Even in midsummer, high latitude cold flows frequently penetrate into southern China, because no high mountain barriers exist in eastern Asia. The high latitude cold flow meets the southeasterly warm flow, forming the Mei-Yu front near 30°N, which results in persistent rainfall called Mei-Yu. If the high latitude cold flow is strong enough to block off the Pacific High, the Mei-Yu front will stay across 30°N and the episode of Mei Yu in the Yangtze River will be much longer. With continuous precipitation from air masses, the remaining vapour is progressively depleted in the heavy isotope, because the heavy isotope tends to condense into the liquid phase more easily than the light isotope. During the summer, the Pacific High moves northward, shifting the rain belt from southern to northern China. 18O of the rainwater is very low in northern China, as is that of stalagmites provided that the temperature of the CaCO3 precipitation is stable. If the mid-latitude cold flow is too weak or the Pacific High is too strong to form the Mei-Yu front, the episode of Mei Yu may be short or absent near the Yangtze River. When the Pacific High enters northern

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China quickly, the rainy season of Beijing begins earlier than usual, and the rainwater is enriched in the heavy oxygen isotope, which will be reflected in stalagmites as higher 18O. The amount of precipitation controls the variation of oxygen isotopes in stalagmites in different ways. In drought years, however, when evaporation exceeds precipitation, the heavy oxygen isotope tends to condense in the soil water and infiltrates into the bedrock, and is finally recorded in the stalagmites. In general, the former case results from abundant precipitation (controlled by summer monsoon), and the latter from less precipitation (controlled by winter monsoon). Carbon isotope ratios of stage I and stage II are lower than the overall mean value of 13C in the entire profile and much lighter than that of stage III, according to Farquhar et al. (1982), which may indicate that there is more soil water in the former two stages than in stage III. In stage I, from 200 BC to AD 820, the slowly decreasing 13C of the stalagmite probably indicates increasing plant cover, from progressively greater amounts of precipitation during this episode, while in the latter phase 13C becomes heavier, suggesting decreased plant cover and variable precipitation compared to the former phase. These assumptions are supported by slight variations of oxygen isotopes. At first, the slowly increasing oxygen isotope ratios suggest that the summer monsoon was strengthening year-byyear. The following decrease of 18O may suggest adverse variations of the summer monsoon. Taking the interpretations of both the carbon and oxygen isotope records together, we can infer that stronger summer monsoon dominated the climate during this period. The jump in 13C in both stalagmites at the beginning of stage III probably suggests an abrupt change in plant cover, which may have resulted from serious drought conditions (Fig. 5). Exacerbating the change was the synchronous deforestation ordered under the command of the Emperor during the Ming Dynasty in the middle of the 15th century; the wood was used to construct the Imperial Palace (Hou 1985). After the drought and deforestation, the content of CO2 produced by plant respiration decreased and soil erosion was aggravated. The amount of CO2 dissolved in soil water therefore dropped, since plants enriched in the light carbon isotope during photosynthesis, the CO2 in seepage water would have been relatively rich in the heavy carbon isotope. The Little Ice Age roughly corresponds to the beginning of this stage. In the following 100 years, carbon isotope ratios decrease abruptly, possibly resulting from the recovery of plant cover. Since 1800, slight variations in carbon isotopes may reflect natural changes in plant cover. In this stage, less precipitation, revealed by historical records, and less plant cover, revealed by heavy carbon and oxygen isotopes, suggest that the climate was probably dominated by the cold and dry winter monsoon.


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In stage II, carbon isotope ratios of both stalagmites generally increase, with some anomalies, and no clear trend in oxygen isotope variation is shown. No certain results were extracted from the isotope records. It is assumed that this is a transitional period from dominant summer to winter monsoons, because it is between stage I (summer monsoon dominates) and stage III (winter monsoon dominates). The above results are supported by the distribution of drought and wetness records revealed by historical documents. It is said that from AD 1 to 1000, the borderline between drought and wetness is roughly along 113°E (which means that the summer monsoon penetrated into northern China and resulted in abundant precipitation). From AD 1000 to 1500, the borderline alternated between a north–south division and an east– west division. From AD 1500 to 1900, the borderline of drought and wet is oriented east–west and is situated more southward than in stage II, while between AD 1501 and 1700, it is 2° farther southward than stage I, and from AD 1701 to 1900, it is 1° farther southward than stage I (Zheng et al. 2000).

Conclusion The high-resolution records of stable isotopes in stalagmite TS9701 and LS9602, collected from Shihua Cave, Beijing, show that vegetation and climate underwent the following changes. From 200 BC to AD 1000, climatic conditions were wet and plant cover was dense due to the dominance of the summer monsoon. Between AD 1000 and 1450, climate alternated between periods of wet and dry. Since AD 1450, the climate has been dry and the plant cover has been less than ever, which could be due to the winter monsoon and to human activity. It is still uncertain why carbon isotope ratios of stalagmite LS9602 are lighter than those of stalagmite TS9701. The difference probably results from different depths in the cave. The present stable isotope record is fairly rough; further year-by-year or layer-by-layer isotopic analysis will provide some particulars of monsoon variation and human effect on plant cover change in the future.

Acknowledgements. – We gratefully acknowledge Zhaoyan Gu, Jingtai Han, Jimin Sun and Luo Wang at the Institute of Geology and Geophysics, Chinese Academy of Sciences for their instructive discussion while the manuscript was being prepared; Tieying Li for his assistance in sampling; and R. L. Edwards for help with dating the samples. Special thanks to Dominique Genty and S. E. Lauritzen for their excellent comments on drafts of this manuscript. Critical reviews by Jason Cosford at the University of Regina, Canada are sincerely appreciated. This study is supported by CMST project G1999043402, CAS project KZCX2-SW-118, KZCX3-SW-120 and NSFC project 40072098.

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