Vegetation History And Indian Summer Monsoon Variation During The Last Glaciation And The Holocene Documented By Xingyun Lake,Yunnan

As one of the main Asian summer monsoon systems,the Indian Summer Monsoon(ISM,or southwest monsoon in China)not only controls the precipitation within southwest China but may also affect the whole of eastern Asia through large-scale water vapor transport from the Indian Ocean.Thus it is important for maintaining the biodiversity and ecological balance of the natural ecosystem,as well as for the socio-economic and cultural development of this densely populated region.Therefore,understanding the variability of the ISM on various time-scales is of practical importance,and is helpful for the prediction of possible future climatic changes in the region in the context of enhanced global warming.Significant progress in the investigation of climate change in the Indian monsoon regions has been made during the last few decades.However,the results of previous studies remain the subject of debate and the patterns of monsoonal climate changes are still poorly understood,mainly due to the uncertainties in chronologies and differences in the sensitivity of various proxies to changes in the ISM.Thus well-dated records with unambiguous ISM proxies(such as of precipitation)are necessary to reconstruct and understand the history of the ISM.Yunnan Plateau,located in southwestern China at the southeastern edge of the Tibetan Plateau,lies in the typical Indian monsoon region.The occurrence of a large number of tectonic fault lakes,together with its high biodiversity,makes the Yunnan Plateau an important region for palaeovegetational,palaeoenvironmental and palaeomonsoonal research.We selected a semi-closed shallow alpine lake(Xingyun Lake)in the central Yunnan Plateau to investigate vegetation changes and the history of the Indian summer monsoon.Firstly,we investigated the modern vegetation of the lake and its catchment,and a total of 25 top-soil samples and 10 surface lake sediment samples were collected and analyzed to characterize the modern pollen assemblages.In addition,two cores(XY08A and XY13B),with respective lengths of 974 cm and 34cm length,were extracted from a water depth of 9 m from the centre of the lake.A total of 15 samples of plant macrofossils and charcoal were picked from core XY08A for radiocarbon dating using accelerator mass spectrometry(AMS).An age-depth model was established by fitting spline functions to the age control points.In addition,in order to obtain a high resolution reconstruction of the last two millennia,15 organic samples and 2 humic acid samples,picked from the top 245 cm of the core,were also used for radiocarbon dating.Combined with ²¹⁰Pb and ¹³⁷Cs ages from core XY13B,a chronology for the top 245 cm of the core was established.From the sediment cores,a total of 204 samples were used for pollen analysis with an average of 780 pollen grains being counted for each samples.In addition,other proxies,such as grain size(a total of 485 samples),magnetic susceptibility(a total of 486 samples),total organic carbon(TOC)and total nitrogen(TN),as well as the ratio of total organic carbon and total nitrogen(C/N,a total of 124 samples)were also analyzed.Finally,based on the age models and analysis of modern pollen assemblages,the regional vegetation and climate histories during the Last Glaciation and the Holocene,as well as the last two millennia,were reconstructed mainly from the fossil pollen data.Subsequently,changes in mean annual precipitation(MAP)during the Holocene and the last two millennia were reconstructed using a pollen-climate calibration-function model based on modern pollen assemblages.Combined with our reconstruction and previous proxy records both from Xingyun Lake and other regions,the Indian summer monsoon variability during the Holocene and the last two millennia was reconstructed.The principal conclusions are as follows:1.Modern pollen analysis of Xingyun Lake and its catchmentAnalysis of modern pollen assemblages revealed that Pinus,evergreen and deciduous oaks,Alnus,Poaceae,Artemisia,Crucifereae and Chenopodiaceae are the major pollen types,--consistent with the modern vegetation composition.The pollen distribution appears to be significantly affected by pollen deposition and re-deposition processes which result in the pollen grains being evenly distributed in Xingyun Lake.The pollen assemblages from the surface lake sediments reflect the regional vegetation distribution,while these of topsoil samples only represent the vegetation composition in the vicinity of the sampling site.2.Chronology of sediment coresAccording to the 15 AMS ¹⁴C dates of plant remains from core XY08A,a hiatus occurs between 485-490 cm,which is also reflected by dramatic changes in various proxies.Comparison with previous published records from the same lake indicates that some depth intervals were not samples during the collection of the core.Consequently the interval used for constructing an age model for reconstructing climatic change was divided into two sections.The chronology of the lower section(490-974 cm)was established using seven AMS ¹⁴C dates of plant macrofossils,which indicates that the age range is between 36,400 and 13,400 cal yr BP.The chronology of the upper section is based on eight AMS¹⁴C dates from plant macrofossils and charcoal,and indicates that the age range is from 8500 cal yr BP to the present.3.Vegetation history and climatic changes during the Last Glaciation(36,400-13,400 cal yr BP)1)Pollen analysis was conducted on 62 pollen samples(with an average time resolution of 275 a for the interval section from 974-616 cm,and 460 a for the interval from 616-490 cm).The results reveal that Pinus,Quercus(E),Abies/Picea,Tsuga,Quercus,Poaceae and Cyperaceae are the major pollen types.Three stages of regional vegetation changes are documented.During 36,400-29,200 cal yr BP the region was covered by pine forest and evergreen broadleaved forest dominated by evergreen oaks,and on the surrounding high mountains hemlock forest and Abies/Picea forest was also developed.This stages can be divided into three sub-stages.From 36,400-33,900cal yr BP,the region was covered by both pine forest and evergreen broadleaved forest;in addition,mixed forest of Tsuga and evergreen broadleaved trees was also distributed on the surrounding mountains.From 33,900-31,500 cal yr BP,the mixed Tsuga and evergreen oaks forest expanded and the Abies/Picea began to develop.From 31,500-29,200 cal yr BP pine forest and evergreen broadleaved forest expanded,while hemlock and Abies/Picea decreased.During 29,200-17,600 cal yr BP,Abies/Picea forest expanded significantly on the surrounding mountains and reached its maximum extent.The distribution of evergreen broadleaved forest growing in the mountain valleys or on the arid barren hillsides increased.During 17,600-13,400 cal yr,Tsuga forest and Abies/Picea forest began to decline until they finally disappeared,while pine forest and evergreen broadleaved forest expanded and dominated the catchment.PCA results based on pollen taxa suggest that the PCA axis 1(PCA-1)mainly represents changes in summer temperature,while PCA axis 2(PCA-2)represents changes in effective moisture.2)The results of the analyses of pollen,sediment grain-size and other proxies(MS,TOC,TN and C/N ratios)indicate a drying climate from 36,400-13,400 cal yr BP,suggesting that the ISM gradually weakened.During 36,400-29,200 cal yr BP,the climate was the wettest and thus summer monsoon was the strongest.From29,200-17,600 cal yr BP,the variations of all the proxies suggest a drying climate and weakened monsoon.From 17,600-13,400 cal yr BP,the climate was the driest and the summer monsoon was weakest.3)The pollen percentages from Xingyun Lake reflect temperature variations during the Last Glaciation.During 36,400-29,200 cal yr BP,the climate was relatively warm but that it cooled gradually during the latter part of the period.From 29,200 to17,600 cal yr BP,all the proxies indicated a gradually cooling climate with coldest interval occurring from 28,000 to 17,600 cal yr BP,corresponding to the Last Glaciation Maximum(LGM),when the Abies/Picea forest distributed on the surrounding mountains reached a maximum extent.Based on the modern distribution limit of Abies/Picea and the given temperature gradient,the temperature of the LGM may have been at least 3?lower than at present.During 17,600-13,400 cal yr BP,the climate became warm.4)The variations of the precipitation/moisture during the Last Glaciation in the Xingyun Lake region was controlled by Northern Hemisphere summer insolation,modulated by the tropical SST.However,the variations of summer temperature in this region was controlled by spring insolation,and therefore there is no correlation between the precipitation/moisture and summer temperature.In other words,precipitation/moisture and temperature did not change synchronously:the climate was mild and wet during 36,400-29,200 cal yr BP,coldest but not the driest during the LGM,and was warmest and the driest during the early part of the Last Deglaciation.4.Vegetation history and climatic changes during the Holocene(since 8500 cal yr BP)1)Pollen analysis was conducted on 41 pollen samples(with an average time resolution of 293 a for the interval from 490-240 cm,and 89 a for that from 240-0 cm).The pollen assemblages are dominated by Pinus,Quercus(E),Quercus,Alnus,Liquidambar,Ulmus,Poaceae and Artemisia.The pollen records document four stages of vegetation history.During 8500-5500 cal yr BP the region was covered by mixed forest dominated by pine and evergreen oaks,with thermophilous and hygrophilous taxa such as Liquidambar and Fagus,together with Dacrydium,growing on the surrounding mountains.From 5500-3500 cal yr BP Pine forest,together with deciduous Quercus and Alnus,expanded,while evergreen oaks declined and the thermophilous and hygrophilous trees almost disappeared.During 3500-1000cal yr BP the regional vegetation became less diverse and more open,with the maximum extent of the distribution of Pinus and deciduous oaks.Since 1000 cal yr BP deforestation caused by human activity accelerated,and was characterized by the increase of secondary trees such as Alnus,by non-arboreal taxa such as Poaceae and by the significant decrease of other trees.2)During the early Holocene(8500-6000 cal yr BP)the pollen-based MAP was relatively high,with the highest precipitation(1480 mm)occurred during the interval from 7800-7500 cal yr BP.Subsequently the MAP commenced a clear decreasing trend,a decrease of about 21%at 2000 cal yr BP.During the last 2000years,MAP fluctuated frequently with remarkably high values from 1200 to 800 cal yr BP,corresponding to the Medieval Warm Period(MWP).The variations of KCM simulated MAP in the region of 20-30°N,90-105°E,together with various multi-proxy records(grain size,TOC,TN and C/N ratios),suggest that the ISM was strongest during the early Holocene(8500-5500 cal yr BP)and then weakened gradually,but was enhanced during the MWP.3)The reconstructed ISM trend inferred from pollen-based mean annual precipitation is compatible with the variations of oxygen isotope records from speleothems and other proxies records from the Indian monsoon domain.All of these records exhibit a plateau of strong summer monsoon intensity in the early Holocene,followed by a subsequent rapid decrease,although there are slight differences in the timing of the termination.The almost linear decrease of the ISM observed in these records is similar to changes of June insolation in the Northern Hemisphere,suggesting that the ISM evolution is mainly forced by summer insolation.However,our observation from Xingyun Lake of a strong ISM during the early Holocene disagrees with some other records from Yunnan Plateau.The climate conditions inferred from these records may not only pertain to a strong summer monsoon but may also be related to temperature changes.Thus,the separation of precipitation and temperature may further our understanding of the summer monsoon evolution.4)Our reconstruction of the ISM from Xingyun Lake is similar to that portrayed by all of the speleothem?¹⁸O records from both southeast China and the ISM domain but is significantly different from other records(such as pollen and loess-palaeosol records)of Holocene EASM evolution,which exhibit a mid-Holocene monsoon maximum.Our results support the hypothesis that the ISM and EASM evolved asynchronously during the Holocene.5.Vegetation history and climatic changes during the past two millenniaThe lake sediment records of the Holocene show that despite the trend of weakening ISM,an interval of relatively wet condition occurred from 1200 to 800 cal yr BP,corresponding to the MWP.The characteristics of the ISM during this period need to be further studied using high-resolution analyses.We have obtained more radiocarbon dates(15 organic samples and 2 humic samples),which are combined with the results of ²¹⁰Pb and ¹³⁷Cs analysis from core XY13B reconstruct an age model for the last 2300 years.1)We conducted pollen analyses of 122 pollen samples(with an average time resolution of 18 a)from the top 245 cm of the core.The result show that Pinus,Quercus(E),Quercus,Alnus,Poaceae and Artemisia are the major pollen types.Four stages of vegetation change can be revealed from the pollen records.During2300-1250 cal yr BP(350 BC-700 AD),the region was covered by mixed forest with pine and oaks,which grew sporadically on the surrounding high mountains.In addition,the present of cereal-type Poaceae pollen suggest that the region was affected by human activity.During 1250-550 cal yr BP(700-1400 AD),broadleaved forest expanding but that the pine forest shrunk.Intensified human activity is indicated by the increasing trend of cereal-type Poaceae and grasses.During 550-50cal yr BP(1400-1900 AD),pine forest increased slightly but that the broadleaved trees declined significantly,and that Abies/Picea forest occurred on the surrounding high mountains.This was accompanied by an increase in herbs and ferns in the understory,and the vegetation became more open.In addition,there was an acceleration of deforestation,caused by large-scale human activity.Since 50 cal yr BP(since 1900AD),the regional vegetation are dominated by the secondary forest with Pinus and Alnus.Human activity continued to intensify.2)On the basis of pollen,grain size and MS,the history of human activities during the last 2000 years was reconstructed.Before 500 AD,cereal-type pollen appeared sporadically,and at the same the Pediastrum present and exhibit an increase trend.The natural environment was only slightly influenced by human activity;however,early agriculture did appear in this region.During 500-1050 AD,cereals increased markedly while the tree pollen and pollen concentration decrease sharply.The sediment grain-size exhibited high-frequency fluctuation,and the MS increased abruptly to very high values.These suggest that both the intensity and scale of the agricultural increased rapidly,and that large-scale land reclamation was carried out by means of deforestation.The extremely high and continuously increasing Pediastrum representation suggests that the water quality deteriorated leading to the eutrophication of Xingyun Lake.Since 1050 AD,the catchment was influenced by a sustained high level of human activity.3)The pollen record of the last two millennia from Xingyun Lake still reflects climate changes,although the intensified human activity played an important role in determining the vegetation composition.The pollen-based MAP fluctuated frequently and commenced a clear decreasing trend,with remarkably high values from1100-1400 AD,corresponding to the MWP.Subsequently,the MAP decreased to its lowest value during 1500-1700 AD,corresponding to the Little Ice Age.The results of other proxy records(grain size,TOC,TN and C/N ratios),as well as the comparison with the stalagmite oxygen isotope records from ISM domain,suggest that the ISM intensity exhibits a general weakening trend over the past two millennia.The results also suggest a wet MWP and a relatively dry LIA.The pollen-based MAP exhibit high amplitude fluctuations during the last 1000 years,and in addition the impact of intensive human activity may at least partly also reflect climatic instability.