nature Vol 4454 January 2007doi:10.1038/nature05431 LETTERS Influence of the intertropical convergence zone on the East Asian monsoon Gergana Yancheva',Norbert R.Nowaczyk',Jens Mingram',Peter Dulski,Georg Schettler',Jorg F.W.Negendank, Jiaqi Liu2,Daniel M.Sigman',Larry C.Peterson&Gerald H.Haug The Asian-Australian monsoon is an important component of the We conclude that this anticorrelation is best explained by migra- Earth's climate system that influences the societal and economic tions in the intertropical convergence zone.Similar migrations of activity of roughly half the world's population.The past strength the intertropical convergence zone have been observed in Central of the rain-bearing East Asian summer monsoon can be recon- America for the period AD 700 to 900(refs 4-6),suggesting global structed with archives such as cave deposits',but the winter climatic changes at that time.From the coincidence in timing,we monsoon has no such signature in the hydrological cycle and has suggest that these migrations in the tropical rain belt could have thus proved difficult to reconstruct.Here we present high-resolu- contributed to the declines of both the Tang dynasty in China and tion records of the magnetic properties and the titanium content the Classic Maya in Central America. of the sediments of Lake Huguang Maar in coastal southeast China Instrumental and historical records reaching back several centuries over the past 16,000 years,which we use as proxies for the strength show considerable interannual to decadal variability in monsoonal of the winter monsoon winds.We find evidence for stronger win- strength.Although El Nino warm events tend to weaken the summer ter monsoon winds before the Bolling-Allerod warming,during rainfall in both the Indian and East Asian monsoons',the instru- the Younger Dryas episode and during the middle and late Holo- mental data do not reveal a straightforward relationship among these cene,when cave stalagmites suggest weaker summer monsoons'-3. major climate elements".Palaeoclimate records can provide additional 0 2 10 11 12 13 14 15 16 LLLLLLLLLLLL ate and Middle Holocene Earty Holocene Allerod -0.92 0.94 0.96 00 0.98 1,000 1,000 00 40 20 10933 15 10- Anoxic Anoxic lake floor lake floor AMS 14C dates 0 0612 3.8 83 10.611.Z 13.0 14.8(cal.kyr BP) 0 4 5 8 10 11 12 13 14 15 16 Age(cal.kyr BP) Figure 1 Palaeoclimate time series of Lake Huguang Maar.Rock magnetic saturated at 1 after 7.8 kyr BP.Five AMSC dates of leaves(black)and four parameters(magnetic susceptibility and S-ratio),and Ti and TOC content of bulk sediment (blue)are shown with an uncertainty interval of 26.Inset, from the sediment sequence during the past 16 kyr.Distinct intervals of locations of Lake Huguang Maar,Hulu cave and the Chinese loess plateau. anoxic conditions at the lake bottom are shaded in yellow.The S-ratio is GeoForschungsZentrum(GFZ)Section 3.3,Telegrafenberg Potsdam D-14473,Germany.2Institute of Geology and Geophysics,Chinese Academy of Sciences,PO Box925Beijing 100029,China.3Department of Geosciences,Princeton University,Princeton,New Jersey 08544,USA.Rosenstiel School of Marine and Atmospheric Science,University of Miami Miami,Florida 33149,USA. 2007 Nature Publishing Group
LETTERS Influence of the intertropical convergence zone on the East Asian monsoon Gergana Yancheva1 , Norbert R. Nowaczyk1 , Jens Mingram1 , Peter Dulski1 , Georg Schettler1 , Jo¨rg F.W. Negendank1 , Jiaqi Liu2 , Daniel M. Sigman3 , Larry C. Peterson4 & Gerald H. Haug1 The Asian–Australian monsoon is an important component of the Earth’s climate system that influences the societal and economic activity of roughly half the world’s population. The past strength of the rain-bearing East Asian summer monsoon can be reconstructed with archives such as cave deposits1–3, but the winter monsoon has no such signature in the hydrological cycle and has thus proved difficult to reconstruct. Here we present high-resolution records of the magnetic properties and the titanium content of the sediments of Lake Huguang Maar in coastal southeast China over the past 16,000 years, which we use as proxies for the strength of the winter monsoon winds. We find evidence for stronger winter monsoon winds before the Bølling–Allerød warming, during the Younger Dryas episode and during the middle and late Holocene, when cave stalagmites suggest weaker summer monsoons1–3. We conclude that this anticorrelation is best explained by migrations in the intertropical convergence zone. Similar migrations of the intertropical convergence zone have been observed in Central America for the period AD 700 to 900 (refs 4–6), suggesting global climatic changes at that time. From the coincidence in timing, we suggest that these migrations in the tropical rain belt could have contributed to the declines of both the Tang dynasty in China and the Classic Maya in Central America. Instrumental and historical records reaching back several centuries show considerable interannual to decadal variability in monsoonal strength. Although El Nin˜o warm events tend to weaken the summer rainfall in both the Indian and East Asian monsoons7 , the instrumental data do not reveal a straightforward relationship among these major climate elements8 . Palaeoclimate records can provide additional 1 GeoForschungsZentrum (GFZ), Section 3.3, Telegrafenberg, Potsdam D-14473, Germany. 2 Institute of Geology and Geophysics, Chinese Academy of Sciences, PO Box 9825, Beijing 100029, China. 3 Department of Geosciences, Princeton University, Princeton, New Jersey 08544, USA. 4 Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida 33149, USA. Anoxic lake floor Anoxic lake floor AMS 14C dates (cal. kyr BP) Bølling– Allerød Late and Middle Holocene YD Early Holocene Age (cal. kyr BP) 0.6 1.2 2.3 3.8 8.3 10.6 11.7 13.0 14.8 Loess distribution in China 1000 km Lake Huguang Maar Hulu cave S-ratio Ti (counts s–1) Beijing Magnetic susceptibility (10–6 SI) TOC (wt%) 100 100 1,000 1,000 Figure 1 | Palaeoclimate time series of Lake Huguang Maar. Rock magnetic parameters (magnetic susceptibility and S-ratio), and Ti and TOC content from the sediment sequence during the past 16 kyr. Distinct intervals of anoxic conditions at the lake bottom are shaded in yellow. The S-ratio is saturated at 1 after 7.8 kyr BP. Five AMS 14C dates of leaves (black) and four of bulk sediment (blue) are shown with an uncertainty interval of 2s. Inset, locations of Lake Huguang Maar, Hulu cave and the Chinese loess plateau. Vol 445| 4 January 2007| doi:10.1038/nature05431 74 ©2007 NaturePublishingGroup
NATURE Vol 4454 January 2007 LETTERS insights into monsoon dynamics,as the past includes larger ampli- with a 9 point triangular window.Magnetic susceptibility was mea tudes of climate change that may reveal more robust linkages. sured in 2.5 mm steps directly on the surface of the split core halves. Previous palaeoclimate reconstructions generally agree that the Additionally,sediment slices of 4 mm thickness(for the time interval Asian summer monsoon was weaker during cold phases in the of Termination I and the early Holocene)were analysed for their rock Northern Hemisphere1,when the intertropical convergence magnetic properties (see Methods) zone (ITCZ)tends to move southward'1,as it does during El Two rock magnetic parameters,magnetic susceptibility and the Nino years1 S-ratio (Fig.1),measure the concentration of magnetic minerals Here we present a new palaeoclimatic record with nearly annual and the mean oxidation state of iron in those minerals,respectively. time-resolution from a sediment core recovered in Lake Huguang The S-ratio is a (nonlinear)estimate of the abundance of magnetite Maar,southeast China(219'N,11017'E),which extends back to compared to that of antiferromagnetic minerals,mainly haematite22 16.2 kyr ago(Fig.1).The sedimentation rates range from 41cm kyr In Lake Huguang Maar,a high S-ratio indicates the availability of before the Bolling-Allerod to 112 cmkyrduring the past 4,000 yr. bottom water oxygen,and it is interpreted to reflect wind-driven lake The age model is based on 5 AMS(accelerator mass spectrometry) mixing.Magnetic susceptibility is sensitive to both lake redox con- 4C dates of leaves and 4 of bulk sediment,with dating errors of less ditions and the aeolian input,both of which are affected by wind than +160yr within the 1o interval of the AMS C method. strength (see Methods). Adjustments using the well-dated records from Cariaco basin are The Ti content of the sediment is used to reconstruct the aeolian within the error of the originalC-based age model (Fig.1;see also input into the lake(Fig.1).The main lithogenic source to Huguang Supplementary Information).Lake Huguang Maar today lies 23m Maar sediments is dust transported by the winter monsoon winds above sea level and has a water depth of 20 m.The surface area of the from the arid areas in the north-for example,the loess plateau-and lake is 2.25 km,and it drains an extremely small catchment of possibly local sources.Changes in Ti are interpreted as a measure of 3.2 km2.Because of its small catchment and a lack of stream inputs, winter monsoon winds,with stronger winds more effectively trans- the lake receives a minimal quantity of material by runoff and thus porting dense Ti-and Fe-rich grains(including the magnetic miner- acts as a natural sediment trap for dust delivered to the site by the als)over the lake.Ti(rather than Fe)is used here as the dust input northerly winds of the winter monsoon.The Huguang Maar sedi- indicator because of its lack of redox sensitivity;however,the two are ments record the strength ofthe winter monsoon in two independent highly correlated(data not shown). ways:(1)the accumulation of wind-blown material,and (2)the During cold climates,for instance,before 14.8 kyr ago and during redox-sensitive characteristics and total organic carbon (TOC)con- the Younger Dryas(between 12.8 and 11.6 kyr ago),Ticontent,mag- tent of the sediment as a result of changes in wind stress and water- netic susceptibility and S-ratio are high(Figs 1,2)while TOC content column mixing. is low's.In contrast,during the Bolling-Allerod and early Holocene Our palaeoclimate time series are based on continuous measure- (before 7.8 kyr ago),Ti content and rock magnetic amplitudes drop, ments of sediment elemental composition and magnetic susceptibil- and TOC increases(Fig.1).Over the course of the Holocene,mag- ity,augmented by discrete measurements of additional magnetic netic susceptibility clearly increases while the Ti content shows a properties and TOC content(Fig.1).Micro X-ray fluorescence ele- weaker trend towards higher values;over this same time interval, ment scanning was performed at a resolution of 0.5 mm,smoothed the TOC content decreases (Fig.1). 9 95 10 10.5 12 12.5 13 13.5 14.5 15 Figure 2 Comparison of the monsoon sensitive sedimentary reborea Younger Dryas Bolling-Allerod records from Lake Huguang Maar with other climate records.These are from the Cariaco basin",in the Dongge ca southern Caribbean off Venezuela, and Hulu and Dongge caves.The Bolling-Allerod,Younger Dryas and Preboreal are highlighted. 0.92 9-0L 200 500 North (-s squnoo) 25 Cariaco basin (ref.17) Icz (Cariaco basin) 15 10- South 10.5 11 11.5 12 12.5 13.5 145 Age (cal.kyr BP) 75 82007 Nature Publishing Group
insights into monsoon dynamics, as the past includes larger amplitudes of climate change that may reveal more robust linkages. Previous palaeoclimate reconstructions generally agree that the Asian summer monsoon was weaker during cold phases in the Northern Hemisphere1–3,9–15, when the intertropical convergence zone (ITCZ) tends to move southward16–18, as it does during El Nin˜o years19–21. Here we present a new palaeoclimatic record with nearly annual time-resolution from a sediment core recovered in Lake Huguang Maar, southeast China (21u 99 N, 110u 179 E), which extends back to 16.2 kyr ago (Fig. 1). The sedimentation rates range from 41 cm kyr21 before the Bølling–Allerød to 112 cm kyr21 during the past 4,000 yr. The age model is based on 5 AMS (accelerator mass spectrometry) 14C dates of leaves and 4 of bulk sediment, with dating errors of less than 6160 yr within the 1s interval of the AMS 14C method. Adjustments using the well-dated records from Cariaco basin are within the error of the original 14C-based age model (Fig. 1; see also Supplementary Information). Lake Huguang Maar today lies 23 m above sea level and has a water depth of 20 m. The surface area of the lake is 2.25 km2 , and it drains an extremely small catchment of 3.2 km2 . Because of its small catchment and a lack of stream inputs, the lake receives a minimal quantity of material by runoff and thus acts as a natural sediment trap for dust delivered to the site by the northerly winds of the winter monsoon. The Huguang Maar sediments record the strength of the winter monsoon in two independent ways: (1) the accumulation of wind-blown material, and (2) the redox-sensitive characteristics and total organic carbon (TOC) content of the sediment as a result of changes in wind stress and watercolumn mixing. Our palaeoclimate time series are based on continuous measurements of sediment elemental composition and magnetic susceptibility, augmented by discrete measurements of additional magnetic properties and TOC content (Fig. 1). Micro X-ray fluorescence element scanning was performed at a resolution of 0.5 mm, smoothed with a 9 point triangular window. Magnetic susceptibility was measured in 2.5 mm steps directly on the surface of the split core halves. Additionally, sediment slices of 4 mm thickness (for the time interval of Termination I and the early Holocene) were analysed for their rock magnetic properties (see Methods). Two rock magnetic parameters, magnetic susceptibility and the S-ratio (Fig. 1), measure the concentration of magnetic minerals and the mean oxidation state of iron in those minerals, respectively. The S-ratio is a (nonlinear) estimate of the abundance of magnetite compared to that of antiferromagnetic minerals, mainly haematite22. In Lake Huguang Maar, a high S-ratio indicates the availability of bottom water oxygen, and it is interpreted to reflect wind-driven lake mixing. Magnetic susceptibility is sensitive to both lake redox conditions and the aeolian input, both of which are affected by wind strength (see Methods). The Ti content of the sediment is used to reconstruct the aeolian input into the lake (Fig. 1). The main lithogenic source to Huguang Maar sediments is dust transported by the winter monsoon winds from the arid areas in the north—for example, the loess plateau—and possibly local sources. Changes in Ti are interpreted as a measure of winter monsoon winds, with stronger winds more effectively transporting dense Ti- and Fe-rich grains (including the magnetic minerals) over the lake. Ti (rather than Fe) is used here as the dust input indicator because of its lack of redox sensitivity; however, the two are highly correlated (data not shown). During cold climates, for instance, before 14.8 kyr ago and during the Younger Dryas (between 12.8 and 11.6 kyr ago), Ti content, magnetic susceptibility and S-ratio are high (Figs 1, 2) while TOC content is low23. In contrast, during the Bølling–Allerød and early Holocene (before 7.8 kyr ago), Ti content and rock magnetic amplitudes drop, and TOC increases (Fig. 1). Over the course of the Holocene, magnetic susceptibility clearly increases while the Ti content shows a weaker trend towards higher values; over this same time interval, the TOC content decreases (Fig. 1). Age (cal. kyr BP) Magnetic susceptibility (10–6 SI) Ti (counts s–1) stronger stronger weaker weaker Bølling–Allerød Ti (counts s–1) –9 –8 –7 –6 –9 –8 –7 –6 (Cariaco basin) Preboreal Younger Dryas Dongge cave (ref.3) Hulu cave (ref.1) Cariaco basin (ref.17) Winter monsoon (Lake Huguang Maar) Summer monsoon (Hulu cave) Figure 2 | Comparison of the monsoon sensitive sedimentary records from Lake Huguang Maar with other climate records. These are from the Cariaco basin17, in the southern Caribbean off Venezuela, and Hulu and Dongge caves1–3. The Bølling-Allerød, Younger Dryas and Preboreal are highlighted. NATURE| Vol 445| 4 January 2007 LETTERS 75 ©2007 NaturePublishingGroup
LETTERS NATURE Vol 4454 January 2007 The sediment S-ratio and TOC content,which indicate changes in correlation between summer and winter monsoons recognized here bottom water oxygen,are functionally independent from Ti content, is also evident for the Indian monsoon system,with the Younger an indicator of dust input.Thus,their inter-consistency,reflecting Dryas representing a time of weaker summer and stronger winter high lake mixing whenever Ti-rich dust input is high,makes a com- monsoons'.The high-resolution records from East Asia indicate pelling case for interpreting these records as a robust measure of that this summer/winter monsoon anti-correlation also applies on winter monsoon strength.We deduce that the pre-Bolling-Allerod, shorter timescales (Fig.2). the Younger Dryas,and the later Holocene were all characterized by Migration of the annual mean position of the ITCZ provides a strong winter monsoons in East Asia.During those intervals,low single coherent explanation for the observed trends in both winter TOC content and higher magnetic susceptibility and S-ratios indicate and summer monsoons over the past 16 kyr,as well as for the strong enhanced wind mixing of the lake's waters,resulting in a well oxyge- anti-correlation between them.When the ITCZ is displaced north- nated lake floor,good preservation of magnetic minerals,and ward,the summer monsoon should strengthen,while the winter increased degradation of organic matter.In parallel,the higher Ti monsoon will weaken.A northward shift in the ITCZ would be content during these times suggests intensified winter winds in cent- expected during times of Northern Hemisphere warming2,such as ral China leading to an increased input of dense Ti-bearing dust.In the Bolling-Allerod and the early Holocene,times when,indeed,the contrast,during periods of warmer climate such as the Bolling- East Asian summer monsoon was strong'2 and the winter monsoon Allerod and the early Holocene,Ti concentrations are lower,while was weak.This mode of explanation for climate change has recently lower magnetic susceptibility and S-ratios are coupled with higher proven fruitful for interpreting palaeoclimate records from the trop- TOC content,implying reduced winter monsoon winds and stable ical Americass(Fig.2;see also Supplementary Information).The stratification of the lake. data reported here,in concert with existing data'- 3202127 suggest that The Lake Huguang Maar records of winter monsoon strength these ITCZ migrations extended across the Pacific show a remarkable relationship with the radiometrically dated The role of climate and environmental change in the success or 88O records from stalagmites at Dongge cave2 and Hulu cave', failure of societies is a matter of intense debate4322 It would be East Asia(Fig.2).Strong winter monsoon phases reconstructed from simplistic to imagine that all episodes of societal change are driven by Huguang Maar correlate with higher O at Hulu and Dongge caves, climatic events,especially in an advanced and complex society such which indicates lower summer precipitation rates and thus a weaker as dynastic China.Nevertheless,we note that,on the basis of our new summer monsoon'.Thus,our data,when compared with the spe- Huguang Maar data,major changes in Chinese dynasties occurred leothem'2 and South China Sea2 records,argue for an inverse cor when the winter monsoon was strong(Fig.3).The anti-correlation relation between the strengths of the winter and summer monsoons between winter and summer monsoon strength indicated by com- (see also Supplementary Information).Moreover,the similarity of parison of the Huguang Maar data with the cave records would the records implies that monsoon changes during the latest glacial, suggest that dynastic transitions tended to occur when the summer Bolling-Allerod,Younger Dryas and Holocene were synchronous monsoon was weak and rainfall was reduced.Dynastic changes in and common to large regions of coastal Southeast Asia.The inverse China often involved popular uprisings during phases of crop failure Figure 3The Lake Huguang Maar 910 AD810 palaeoclimate records during the past 4,500 yr in the context of major events in the cultural history of China.a,Major changes in Chinese dynasties occurred during dry phases,as indicated by the titanium and magnetic susceptibility records from Lake Huguang Maar and applying the observed anti-correlation between the winter and summer monsoons, na maa while the described 'golden ages' n centun tended to occur during wet phases. b,Comparison of titanium records 000 1.050 1,100 1,150 1,200 1.250 1,300 from Lake Huguang Maar and the Age (cal.yr Bp) Cariaco basin.The shared features of the two climate records as well as the similar timing of Chinese*and ang Fra of Mayan societal changes suggest a disunity role for coherent climate changes 1.000 (that is,ITCZ migration)across the Pacific in the events of widely 2.000 dispersed civilizations. 3.000 100 000 120 5.000 140 160 180 500 1.000 1,500 2.000 2.500 3.000 3.500 4.000 Age (cal.yr Bp) 16 82007 Nature Publishing Group
The sediment S-ratio and TOC content, which indicate changes in bottom water oxygen, are functionally independent from Ti content, an indicator of dust input. Thus, their inter-consistency, reflecting high lake mixing whenever Ti-rich dust input is high, makes a compelling case for interpreting these records as a robust measure of winter monsoon strength. We deduce that the pre-Bølling–Allerød, the Younger Dryas, and the later Holocene were all characterized by strong winter monsoons in East Asia. During those intervals, low TOC content and higher magnetic susceptibility and S-ratios indicate enhanced wind mixing of the lake’s waters, resulting in a well oxygenated lake floor, good preservation of magnetic minerals, and increased degradation of organic matter. In parallel, the higher Ti content during these times suggests intensified winter winds in central China leading to an increased input of dense Ti-bearing dust. In contrast, during periods of warmer climate such as the Bølling– Allerød and the early Holocene, Ti concentrations are lower, while lower magnetic susceptibility and S-ratios are coupled with higher TOC content, implying reduced winter monsoon winds and stable stratification of the lake. The Lake Huguang Maar records of winter monsoon strength show a remarkable relationship with the radiometrically dated d18O records from stalagmites at Dongge cave2,3 and Hulu cave1 , East Asia (Fig. 2). Strong winter monsoon phases reconstructed from Huguang Maar correlate with higher d18O at Hulu and Dongge caves, which indicates lower summer precipitation rates and thus a weaker summer monsoon1 . Thus, our data, when compared with the speleothem1,2 and South China Sea24 records, argue for an inverse correlation between the strengths of the winter and summer monsoons (see also Supplementary Information). Moreover, the similarity of the records implies that monsoon changes during the latest glacial, Bølling–Allerød, Younger Dryas and Holocene were synchronous and common to large regions of coastal Southeast Asia. The inverse correlation between summer and winter monsoons recognized here is also evident for the Indian monsoon system, with the Younger Dryas representing a time of weaker summer and stronger winter monsoons10. The high-resolution records from East Asia indicate that this summer/winter monsoon anti-correlation also applies on shorter timescales (Fig. 2). Migration of the annual mean position of the ITCZ provides a single coherent explanation for the observed trends in both winter and summer monsoons over the past 16 kyr, as well as for the strong anti-correlation between them. When the ITCZ is displaced northward, the summer monsoon should strengthen, while the winter monsoon will weaken. A northward shift in the ITCZ would be expected during times of Northern Hemisphere warming25, such as the Bølling–Allerød and the early Holocene, times when, indeed, the East Asian summer monsoon was strong1,26 and the winter monsoon was weak. This mode of explanation for climate change has recently proven fruitful for interpreting palaeoclimate records from the tropical Americas17,18 (Fig. 2; see also Supplementary Information). The data reported here, in concert with existing data1–3,20,21,27, suggest that these ITCZ migrations extended across the Pacific. The role of climate and environmental change in the success or failure of societies is a matter of intense debate4,5,28,29. It would be simplistic to imagine that all episodes of societal change are driven by climatic events, especially in an advanced and complex society such as dynastic China. Nevertheless, we note that, on the basis of our new Huguang Maar data, major changes in Chinese dynasties30 occurred when the winter monsoon was strong (Fig. 3). The anti-correlation between winter and summer monsoon strength indicated by comparison of the Huguang Maar data with the cave records would suggest that dynastic transitions tended to occur when the summer monsoon was weak and rainfall was reduced. Dynastic changes in China often involved popular uprisings during phases of crop failure Age (cal. yr BP) Age (cal. yr BP) 5,000 4,000 1,000 1,500 2,000 2,500 3,000 3,500 4,000 1,000 1,050 1,100 1,150 1,200 1,250 1,300 500 AD1644 AD1368 AD1279 AD960 AD1127 AD907 AD618 AD9-24 AD1271-1368 AD618 202BC 256BC 1752BC 1752BC 2183BC 0 3,000 2,000 1,000 0 Wet Wet Dry Dry Ti (counts s–1) Ti (counts s–1) Ti (counts s–1) Era of disunity W Shang Xia Zhou Ming Suang Tang Han E Zhou South NorthYuan Qin a b Eighth century: decline of AD907: collapse of Tang dynasty Peak of Tang dynasty Tang dynasty Major phases of collapse of ‘Classic Maya civilization’ Peak of ‘Classic Maya period’ AD~910 AD~860 AD~810 Magnetic susceptibility (10–6 SI) Lake Huguang Maar Cariaco basin Figure 3 | The Lake Huguang Maar palaeoclimate records during the past 4,500 yr in the context of major events in the cultural history of China. a, Major changes in Chinese dynasties30 occurred during dry phases, as indicated by the titanium and magnetic susceptibility records from Lake Huguang Maar and applying the observed anti-correlation between the winter and summer monsoons, while the described ‘golden ages’30 tended to occur during wet phases. b, Comparison of titanium records from Lake Huguang Maar and the Cariaco basin. The shared features of the two climate records as well as the similar timing of Chinese30 and Mayan4,5 societal changes suggest a role for coherent climate changes (that is, ITCZ migration) across the Pacific in the events of widely dispersed civilizations. LETTERS NATURE| Vol 445| 4 January 2007 76 ©2007 NaturePublishingGroup
NATURE Vol 445 4 January 2007 LETTERS and famine,consistent with a linkage to reduced rainfall.The Tang 4. Diamond,J.Collapse (Penguin,London,2005). dynasty has been described as a high point in Chinese civilization,a 5. Fagan,B.Floods,Famines and Emperors:El Nino and the Fate of Civilizations (Pimlico London,2000). golden age of literature and art.The power of the dynasty began to 6. Haug,G.H.et al Climate and the collapse of Maya civilization.Science 299 ebb in the eighth century,starting with a defeat by the Arab army in 1731-17352003). AD 751.Rebellions further weakened the Tang empire,and it fully Kumar,K.K.,Rajagopalan,B.Cane,M.A.On the weakening of the relationship collapsed in AD 907 (ref.30). between the Indian monsoon and ENSO.Science 284,2156-2159 (1999) It is intriguing that the rise and collapse of the Classic Maya's Wang,B.The Asian Monsoon (Springer,Berlin,2006). 9. 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Haug G.H.,Hughen,K.A.,Sigman,D.M.,Peterson,L.C.Rohl,U.Southward Bartington MS2E sensor directly on the surface of split core halves.Magnetic migration of the Intertropical Convergence Zone through the Holocene.Science susceptibility of the discrete samples was measured with a Kappabridge KLY-3S 293,1304-1308(2001). (AGICO). 18.Wang,X.et al.Wet periods in northeastern Brazil over the past 210 kyr linked to S-ratio and TOC.The S-ratio varies between 0 for pure haematite and 1 for pure distant climate anomalies.Nature 432,740-743(2004). magnetite2.A high S-ratio indicates a predominance of oxic sedimentary con- 19. Cane,M.A.The evolution of El Nino,past and future.Earth Planet.Sci.Lett.230, 227-240(2005). ditions and the preservation of magnetite.The strong correlation between high 20.Koutavas,A.Lynch-Stieglitz,J.Marchitto Jr,T.M.Sachs,J.P.EI Nino-like S-ratio and low TOC in our record has two alternative interpretations:(1)the pattern in ice age tropical Pacific sea surface temperature.Science 297,226-230 supply of oxygen to the sediments is high,or(2)the flux oforganic matter to the (2002). lake bottom is low.In the pervasively eutrophic Lake Huguang Maar,large 21.Ivanochko,T.5.et al Va iation in tropical convection as an amplifier of changes in productivity are unlikely and difficult to cause by climate change global climate change at the millennial scale.Eorth Plonet.Sci.Lett.235,302-314 Thus,the measured changes in S-ratio and TOC are most logically interpreted as (2005). the result of wind-driven lake mixing,with more mixing and thus higher S-ratio 22.Bloemendal,J.,King.J.W.,Hall,F.R.Doh,S.J.Rock magnetism of Late Neogene and lower TOC occurring during times of strong winter monsoon winds.This and Pleistocene deep-sea sediments:Relationship to sediment source,diagenetic interpretation is supported by sedimentary manganese and biogenic opal con- processes and sediment lithology.J.Geophys.Res.97,4361-4375(1992) centrations and accumulation rates(see Supplementary Information). 23.Mingram,J.et al.The Huguang maar lake-a high-resolution record of Magnetic susceptibility.The sharp increase in magnetic susceptibility at palaeoenvironmental and palaeoclimatic changes over the last 78,000 years from South China.Quat.Int.122,85-107 (2004). -7.8 kyr ago,which is simultaneous with a saturating increase in S-ratio and a 24.Wang,L.et ol.East Asian monsoon climate during the Late Pleistocene:high- decrease in TOC content,requires that increasing wind mixing caused a thresh- resolution sediment records from the South China Sea.Mar.Geol.156,245-284 old in lake oxygen content to be crossed,such that magnetite is subsequently 1999). preserved in the sediments (Supplementary Fig.4).However,the continued 25.Hastenrath,5.Greischar,L.Circulation mechanisms related to northeast Brazil gradual increase in magnetic susceptibility over the mid-to late Holocene has rainfall anomalies.J.Geophys.Res.98,5093-5102 (1993). two potential explanations.It may result from a continued increase in the annual 26.Wang,L.et al.Holocene variations in Asian monsoon moisture:a bidecadal mean oxygen content of the lake,due to a continued increase in wind mixing.In sediment record from South China Sea.Geophys.Res.Lett.26,2889-2892(1999) this case,the lack ofchange in S-ratio would be due to saturation of this index at a 27.Fleitmann,D.et al.Holocene forcing of the Indian Monsoon recorded in a stalagmite from Southern Oman.Science 300,1737-1739 (2003). value close to 1 (Supplementary Fig.4).Alternatively,the gradual increase in 28. deMenocal,P.B.Cultural responses to climate change during the late Holocene magnetic susceptibility may result from an increase in the aeolian delivery of Science292,667-673(2001). magnetic minerals because of stronger winter winds,as magnetic minerals such 29.Hodell,D.A.Brenner,M.,Curtis,J.H.Guilderson,T.Solar forcing of drought as magnetite are much (roughly two times)denser than most aluminosilicates frequency in the Maya lowlands.Science 292,1367-1370(2001). The gradual decrease in TOC content through the mid-to late Holocene may 30.Blunden,C.Elvin,M.Cultural Atlos of China (Checkmark Books,New York, indicate either an increasing wind-driven ventilation of the lake or progressively 1998). greater dilution by aeolian inputs.In either case-an increase in lake mixing and/ or an increase in aeolian input-the combined data require a Holocene strength- Supplementary Information is linked to the online version of the paper at www.nature.com/nature. ening of the winter monsoon. Acknowledgements We thank M.Cane,R.Seager,P.deMenocal and S.Clemens Received 27 January;accepted 6 November 2006. for discussions,comments and reviews.This work was supported by the Deutsche Forschungsgemeinschaft (DFG).D.M.S.and G.H.H.thank the Humboldt 1.Wang Y.J.et al.A high-resolution absolute-dated Late Pleistocene monsoon Foundation for support.D.M.S.was also supported by BP and Ford Motor Company record from Hulu Cave,China.Science 294,2345-2348 (2001). through the Princeton Carbon Mitigation Initiative. 2.Yuan,D.et ol.Timing,duration,and transitions of the Last Interglacial Asian Monsoon.Science 304,575-578(2004). Author Information Reprints and permissions information is available at 3.Dykoski,C.A.et al.A high-resolution,absolute-dated Holocene and deglacial www.nature.com/reprints.The authors declare no competing financial interests. Asian monsoon record from Dongge Cave,China.Earth Planet.Sci.Lett.233, Correspondence and requests for materials should be addressed to G.H.H. 71-86(2005). (haug@gfz-potsdam.de). 公 2007 Nature Publishing Group
and famine, consistent with a linkage to reduced rainfall. The Tang dynasty has been described as a high point in Chinese civilization30, a golden age of literature and art. The power of the dynasty began to ebb in the eighth century, starting with a defeat by the Arab army in AD 751. Rebellions further weakened the Tang empire, and it fully collapsed in AD 907 (ref. 30). It is intriguing that the rise and collapse of the Classic Maya4,5 coincided with the golden age and decline of the Tang dynasty in China30. Comparison of the Ti records from Lake Huguang Maar and the Cariaco basin reveals similarities, including both a general shift towards drier climate at about AD 750 and a series of three multi-year rainfall minima within that generally dry period (Fig. 3), the last of which coincides with the final stage of Maya collapse as well as the end of the Tang dynasty. Given these results, it seems possible that major circum-Pacific shifts in ITCZ position catalysed simultaneous events in civilizations on opposite sides of the Pacific Ocean. METHODS X-ray and magnetic measurements. Element scanning was carried out with a micro X-ray fluorescence spectrometer EAGLE BKA (Ro¨ntgenanalytik Meßtechnik GmbH). Isothermal remanent magnetizations (IRM), saturation as well as back field, were imprinted with a 2G Enterprises pulse magnetizer in magnetic fields of 2 T and 20.3 T, respectively. All IRMs were measured on a Molyneux spinner magnetometer (Minispin). The S-ratios were calculated after ref. 22. Continuous logging of magnetic susceptibility was performed with a Bartington MS2E sensor directly on the surface of split core halves. Magnetic susceptibility of the discrete samples was measured with a Kappabridge KLY-3S (AGICO). S-ratio and TOC. The S-ratio varies between 0 for pure haematite and 1 for pure magnetite22. A high S-ratio indicates a predominance of oxic sedimentary conditions and the preservation of magnetite22. The strong correlation between high S-ratio and low TOC in our record has two alternative interpretations: (1) the supply of oxygen to the sediments is high, or (2) the flux of organic matter to the lake bottom is low. In the pervasively eutrophic Lake Huguang Maar, large changes in productivity are unlikely and difficult to cause by climate change. Thus, the measured changes in S-ratio and TOC are most logically interpreted as the result of wind-driven lake mixing, with more mixing and thus higher S-ratio and lower TOC occurring during times of strong winter monsoon winds. This interpretation is supported by sedimentary manganese and biogenic opal concentrations and accumulation rates (see Supplementary Information). Magnetic susceptibility. The sharp increase in magnetic susceptibility at ,7.8 kyr ago, which is simultaneous with a saturating increase in S-ratio and a decrease in TOC content, requires that increasing wind mixing caused a threshold in lake oxygen content to be crossed, such that magnetite is subsequently preserved in the sediments (Supplementary Fig. 4). However, the continued gradual increase in magnetic susceptibility over the mid- to late Holocene has two potential explanations. It may result from a continued increase in the annual mean oxygen content of the lake, due to a continued increase in wind mixing. In this case, the lack of change in S-ratio would be due to saturation of this index at a value close to 1 (Supplementary Fig. 4). Alternatively, the gradual increase in magnetic susceptibility may result from an increase in the aeolian delivery of magnetic minerals because of stronger winter winds, as magnetic minerals such as magnetite are much (roughly two times) denser than most aluminosilicates. 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Supplementary Information is linked to the online version of the paper at www.nature.com/nature. Acknowledgements We thank M. Cane, R. Seager, P. deMenocal and S. Clemens for discussions, comments and reviews. This work was supported by the Deutsche Forschungsgemeinschaft (DFG). D.M.S. and G.H.H. thank the Humboldt Foundation for support. D.M.S. was also supported by BP and Ford Motor Company through the Princeton Carbon Mitigation Initiative. Author Information Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests. Correspondence and requests for materials should be addressed to G.H.H. (haug@gfz-potsdam.de). NATURE| Vol 445| 4 January 2007 LETTERS 77 ©2007 NaturePublishingGroup
