Terrestrial evidence for a spatial structure of tropical–polar interconnections during the Younger Dryas episode

Earth and Planetary Science Letters 191 (2001) 231^239 www.elsevier.com/locate/epsl

Terrestrial evidence for a spatial structure of tropical^polar interconnections during the Younger Dryas episode Weijian Zhou a; *, M. John Head a;e , Zhisheng An a , Patrick De Deckker b , Zhengyu Liu c , Xiaodong Liu a , Xuefeng Lu a , Douglas Donahue d , A.J. Timothy Jull d , J. Warren Beck d a

State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, P.O. Box 17, Xi'an 710054, PR China b Department of Geology, Faculty of Science, Australian National University, Canberra, ACT 0200, Australia c Department of Atmospheric and Ocean Sciences, University of Wisconsin-Madison, Madison, WI 53706, USA d NSF Arizona AMS Facility, University of Arizona, Tucson, AZ 85721, USA e School of Geosciences, Wollongong University, North¢elds Avenue, Wollongong, NSW 2522, Australia Received 6 June 2000; accepted 26 June 2001

Abstract The Younger Dryas chronozone, recognised in northern high-latitude areas as a cold event between 11 000 and 10 000 C yr BP (12 900^11 600 cal. yr BP), seems to manifest itself globally in different ways. Here, we examine well-dated stratigraphic sequences together with high-resolution proxy data plots from sites across our study area, the arid^semiarid transition zone in northern China. This climatically sensitive area of China records a cold, dry Younger Dryas climate which was punctuated by a brief period of summer monsoon precipitation. We have since found that similar climatic sequences have been reported from the Sahel and the equatorial region of Africa. Based on evidence from these sites, together with other published data, we postulate that precipitation during the Younger Dryas chronozone was indicative of a low-latitude driving force superimposed on the high-latitude cold background. This rain belt rearrangement was most probably caused by an interaction between cold air advection and summer moisture transport across the tropical Pacific Ocean. Examination of high-resolution proxies suggests short-term climate fluctuations indicative of a global teleconnection involving moist air transportation patterns from the tropics to higher latitudes, varying with ENSO and other tropical factors. ß 2001 Elsevier Science B.V. All rights reserved. 14

Keywords: Younger Dryas; paleoclimatology; global change; paleo-oceanography; general circulation models; Southern Oscillation; El Nino; North Atlantic oscillation; desert^loess transition zone

1. Introduction

* Corresponding author. Tel: +86-29-8320778; Fax: +86-29-8320456. E-mail address: [email protected] (W. Zhou).

Studies of recent climate £uctuations associated with El Nin¬o Southern Oscillation (ENSO) events in low latitudes have indicated a global connection, especially shown by precipitation increases or decreases in both high- and low-latitude areas

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[1^4]. Has this type of short-term global climate variability persisted over time? One of the most obvious connections between high- and low-latitude climate variability is the mid-latitude area of China, especially along the Chinese component of the Afro-Asian arid^semi-arid transition zone [5], depicted in its entirety and described in Fig. 3. This area covers the northern fringe of the Chinese Loess Plateau (the desert^loess boundary), which has been sensitive to periodic changes in dominance of the East Asian cold/dry winter and warm/humid summer monsoons, leading to the deposition of loess and palaeosol sequences respectively [6]. At the present time, the collision of cold continental air with warm, moist oceanic air produces summer monsoon rains, and about 80% of the annual precipitation in this region falls during June^September. This climate pattern has been extensively studied, and has been well documented for the last 130 000 years [7,8]. The loess/ palaeosol sequences correlate well with cold Heinrich events and N18 O records from deep sea and Greenland ice cores [9,10]. These studies have demonstrated a teleconnection between the Chinese Loess Plateau and high-latitude areas via westerlies associated with the Siberian high-pressure system, illustrated by computer simulations postulating strong storm tracks during the Last Glacial Maximum [11]. The data presented here suggest that moisture-controlling mechanisms related to atmosphere^ocean coupling between high and low latitudes [12], causing rain belt rearrangements [13], have dominant trends over long time scales, playing a signi¢cant part in regional climate variations worldwide. 2. Younger Dryas climates The Younger Dryas chronozone represents the period from 10 000 to approximately 11 000 14 C yr BP (12 900 to 11 600 cal. yr BP). Calibrated ages have been calculated using the revised Calib 4.1 14 C age calibration program [14]. In this period, the mean Younger Dryas cold signal controlled by high latitudes in the northern hemisphere has been modi¢ed by periods of regional increases in moisture controlled by low-latitude areas, as illus-

trated in Fig. 1. In low-latitude areas, precipitation:evaporation ratio (P:E) values calculated from reconstructed lake levels for Lake Victoria, East Africa (0³05PN, 32³48PE), have been interpreted as showing an initial aridity until about 10 900 14 C yr BP (12 800 cal. yr BP). This was interrupted by a moderately moist period which lasted until about 10 500 14 C yr BP (12 400 cal. yr BP), when more arid conditions returned [15]. Pollen data from core 17940 (20³07PN, 117³23PE) in the South China Sea (Fig. 1) show a domination of Picea, Abies and Tsuga pollen, indicating a cool, wet climate for southern China throughout the Younger Dryas chronozone [16]. The pollen concentration (montane conifer species) rose to a peak at about 10 600 14 C yr BP (12 500 cal. yr BP), then sharply dropped towards the Younger Dryas/Holocene boundary. These data do not have the ¢ne resolution to indicate the short-term £uctuations present within the other data sets, but the data do represent a wet phase being superimposed on a cool climate. Sea surface temperatures (SST) reconstructed from Sr/Ca ratios of coral samples collected from Esperitu Santo, Vanuatu, in the southwestern Paci¢c (15.5³S, 167³E) indicate a sharp rise from the commencement of the Younger Dryas chronozone to V1³C less than today. The proxy temperature values then fall sharply leading to the end of the Younger Dryas and the commencement of the Holocene period [17]. This con¢rms other evidence from the central Paci¢c (0³57PN, 138³57PW) [18], and the northern Paci¢c (21³35PN, 158³19PW) [19], estimating SSTs from alkenone concentrations and N18 O from planktonic foraminifera, that SSTs were about 1³C lower than they are today. In Fig. 1, the Vanuatu record is shown in combination with the N18 O record from the Greenland GISP2 ice core [10,20], as an indicator of the di¡erences between the lowlatitude and northern hemisphere high-latitude records. The GISP2 N18 O values shown here re£ect a sharp change to colder conditions at the beginning of the Younger Dryas chronozone, then, with a series of £uctuations, slightly warmer conditions, reverting to colder conditions towards the end of the Younger Dryas chronozone. Further high-latitude evidence is indicated by

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Fig. 1. Proxy data re£ecting sea surface temperatures (SST) and moisture levels from low-latitude through to high-latitude areas during the Younger Dryas chronozone. Lake level £uctuations found in a core from Lake Victoria, in equatorial east Africa, are reproduced from Stager et al. [15]. A plot of montane conifer pollen from a core collected in the South China Sea is reproduced from Sun and Li [16]. Proxy vT (³C) values obtained from corals collected on Esperitu Santo, Vanuatu, reproduced from Gagan et al. [17], are directly compared with N18 O values from the GISP2 Greenland ice core [19]. The SST data from the southwest Paci¢c indicate a signi¢cant rise during the mid Younger Dryas, while the data from the GISP2 ice core and from the North Sea, reproduced from Rochon et al. [21], re£ect cooler temperatures in polar latitudes. The North Sea evidence from both summer and winter SST plots shows strong seasonality.

reconstructed winter and summer SST values from the North Sea [21]. The winter SST values (between 31 and 0³C) were almost constant, but the summer SST values £uctuated signi¢cantly, as would be expected with the level of summer insolation at that time [22]. The North Sea record also shows an abrupt drop in summer SST occurring around 10 300 14 C yr BP (V11 900 cal. yr BP), just above a layer of Vedde ash [21]. In high latitudes, £uctuating summer SST may have been an important factor a¡ecting the East Asian monsoon variability [23]. 3. The Chinese Loess Plateau A logical place to look for the e¡ects of high/ low-latitude climate interactions is the northern Chinese Loess Plateau^desert boundary. This

forms portion of the Afro-Asian arid^semi-arid transitional zone extending from West Africa through the southern Sahara^Sahel, the Arabian Peninsula and northwestern India to northern China [5] (illustrated as part of Fig. 3). In Fig. 2 (see table 1 in the Background Dataset1 ), a comparison of the Younger Dryas stratigraphy of three sites along the desert^loess boundary in China, and Ari Koukouri in Africa, is contrasted with that of a swamp at Dignan, in southern China. Dongxiang (35³33PN, 103³35PE) is located to the southwest of the desert^loess boundary, and is a typical loess/palaeosol pro¢le. The Younger Dryas sequence consists of loess, followed by a weakly pedogenic palaeosol, dated from 1

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10 610 þ 160 to 10 190 þ 150 14 C yr BP (12 730 to 11 850 cal. yr BP), then loess to 9880 þ 150 14 C yr BP (11 380 cal. yr BP). The Shengmu (38³48PN, 110³27PE) pro¢le is along the desert^loess boundary, and the Younger Dryas sequence consists of coarse aeolian sand overlain by an immature sandy palaeosol, dated from 10 590 þ 100 to 10 270 þ 80 14 C yr BP (12 790 to 12 130 cal. yr BP). This is followed by a further layer of coarse aeolian sand. Also along the desert^loess boundary, the Jingbian pro¢le (37³39PN, 108³37PE) has a Younger Dryas sequence consisting of lacustrine silts, overlain by greyish black silty peat, dated from 10 610 þ 80 to 10 170 þ 220 14 C yr BP (12 730 to 11 850 cal. yr BP). The sequence is

completed by a layer of coarse aeolian sand. At the Jingbian site, samples were taken at 5 cm intervals for the measurement of pollen concentration and N13 C (Fig. 2) [23^25]. Both tree and herb pollen counts show a signi¢cant increase within the peat layer, signifying an increase in precipitation levels during this period. Also, as would be expected, the N13 C values become more negative during this period, signifying an increase in bacterial activity in conjunction with the peat formation [26]. A similar sequence of climate £uctuations during the Younger Dryas has been found in sites within the Sahel region of Africa, along the Afro-Asian arid^semi-arid transitional zone. The

Fig. 2. Sediment deposition patterns along the Afro-Asian arid^semi-arid transition zone as compared with those from a site (DingNan, 24³15PN, 115³2PE) in southern China during the Younger Dryas (depicted within the dotted lines). The ¢rst site (Ari Koukouri, 14³20PN, 13³10PE) is used as a representative of the transition zone in Sahelian central Africa, and is reproduced from Gasse and Fontes [27]. The sites from North China (Dongxiang, 35³33PN, 103³35PE; Shengmu, 38³48PN, 110³27PE; and Jingbian, 37³39PN, 108³37PE) are from our study area along the desert^loess boundary in the northern Chinese Loess Plateau. These sites are representative of a loess/palaeosol sequence, a coarse aeolian sand/palaeosol sequence, and an aeolian sand/silt/peat sequence. 14 C AMS determinations were carried out on pollen concentrates from Dongxiang and Shengmu. Charcoal and wood cellulose were dated for the Jingbian pro¢le. Proxy data as well as pollen plots from Jingbian are also presented to further de¢ne variations within the Younger Dryas chronozone. The Dignan site in southern China is a swamp (14 C dating on plant residues) that shows very little £uctuation in sediment type, but slight variations in the proxies indicate a comparative increase in summer monsoon activity.

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Ari Koukouri (14³20P;N, 13³10PE) dry stratigraphy [27] was punctuated by Mg-rich calcite containing ostracods, mollusc shells and diatoms, indicating a wetter period, dated from 10 620 þ 230 to 10 340 þ 120 14 C yr BP (12 630 to 12 150 cal. yr BP). These results agree with those from the Chinese Loess Plateau to within 1 standard deviation. In contrast, the stratigraphic pro¢le from Dignan (lacustrine mud), in south China (24³15PN, 115³2PE; measurements at 1 cm intervals) shows a slight increase in the organic carbon content, coinciding with a decrease in what is a low magnetic susceptibility (lower debris concentrations). This suggests slight summer monsoon strengthening compared to the periods before and after the Younger Dryas. In summary, the Younger Dryas sequences from the three pro¢les in northern China re£ect an initial cold, dry phase (loess, aeolian sand, silt), V11 000 to V10 500 14 C yr BP (V12 900 to V12 420 cal. yr BP), followed by a more humid phase (palaeosol, sandy palaeosol, peat) from V10 500 to 10 200 14 C yr BP (V12 420 to V11 960 cal. yr BP), then an extremely cold, dry phase (aeolian sand, loess), from V10 200 to V10 000 14 C yr BP (V11 960 to V11 500 cal. yr BP) [28]. 4. A comparison of modern global precipitation patterns with those of the Younger Dryas chronozone Recent meteorological data and models have shown a direct relationship between short-term variations in the Asian monsoon system, the Eurasian snow cover, and ENSO [4,29]. It has been suggested that when the Eurasian winter snow cover is relatively heavy, the subsequent Asian summer monsoon systems are weaker than normal, and ENSO is in the `El Nin¬o' mode [30], though at this stage, no positive correlation with the Eurasian snow cover has been made. Detailed studies of the space^time association between the Southern Oscillation and Indian summer monsoon rainfall for the 104 year period from 1871 to 1974 were carried out with the ¢nding that weak Indian summer monsoons have mostly co-

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incided with `El Nin¬o' events. In recent years, this trend has changed, in that strong El Nin¬o events have coincided with Indian summer monsoon rains [31]. It has been suggested that between 1981 and 1997, increased premonsoon surface temperatures over Eurasia have exceeded the warming events in the Indian Ocean, producing a stronger land^sea thermal gradient that has induced summer monsoon rain, apparently overriding the in£uence of El Nin¬o. Data from the Arabian Sea show that during the Younger Dryas chronozone, the southwest (Indian) summer monsoons were particularly weak [32], with their intensity increasing greatly after the Younger Dryas period, at 11 450 þ 150 cal. yr BP. Studies carried out in the Thar Desert, in India [33], indicate a continuous period of high aridity. This observed weaker Indian monsoon in the Younger Dryas is likely to have been a¡ected by the tropical SST, as occurs presently during El Nin¬o events. Fig. 3 reproduces compilations of meteorological data from recent strong `El Nin¬o' events [1^ 3], indicating global rainfall anomalies. Areas having higher than normal (purple) or lower than normal (yellow) precipitation are indicated. This situation is approximately reversed during strong `La Nin¬a' events. We have also collected reported global variations in precipitation patterns during the Younger Dryas from the literature [7,15,19,27,32^43]. These have been matched with the present-day pattern. Blue circles and white circles indicate wet and dry conditions respectively during the Younger Dryas, while blue/white circles indicate £uctuating wet/dry phases. As can be seen in Fig. 3, correlation is good, considering the number of data points available. The Younger Dryas chronozone in northwestern Europe has been reported as being initially cool and moist from about 11 000 to about 10 500 14 C yr BP, with Scandinavia cold and dry [43], re£ected in the North Sea SST data [21]. This suggests that an ocean^atmosphere-coupled response similar to the NAO in positive mode, which, at present, is associated with the generation of strong surface westerly winds across the North Atlantic onto Europe [44], may have been

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Fig. 3. This indicates areas found to have increased (purple) or decreased (yellow) precipitation levels, compiled from data collected during a series of recent strong El Nin¬o events [1,2]. The pattern is reversed when ENSO switches to the opposite mode. The present Afro-Asian arid^semi-arid transition zone is shown by a solid red line, the transition zone boundary during the Last Glacial Maximum is indicated by a dashed blue line, and the boundary during the Holocene Optimum is shown by a dotted green line. Blue and white circles indicate wet and dry conditions respectively during the Younger Dryas, while blue/white circles indicate £uctuating wet/dry phases.

dominant at that time. The strength of the NAO would have a¡ected the intensity of the Siberian high-pressure system [45], thus aiding the strengthening of the Asian winter monsoon system. From 10 500 to about 10 200 14 C yr BP, northern Europe was cold and dry, with intensive dune formation indicated in the Netherlands, [39]. This suggests a switching of the NAO system to negative values, simultaneous with weakening of the westerlies associated with the Siberian high system [45], which provides conditions more favourable for strengthening of the East Asian summer monsoon, generating precipitation. This may have been related to a dominance of ENSO for this period, and there is some evidence of this, but not really enough to justify this assumption. From 10 200 to about 10 000 14 C yr BP, northern Europe reverted to being cool and moist [36,39,43], indicating a further dominance of the NAO in positive mode, thus strengthening the Asian winter monsoon, and inducing cold, dry conditions along the desert^loess boundary in China.

In northern Australia, a well de¢ned arid aeolian dust peak from the Gulf of Carpentaria dates between 11 375 and 10 430 14 C yr BP. This represents intensi¢cation of southerly winds over northern Australia during winter, which may be related to ENSO [40], transporting moisture across the equator, possibly adding strength to the Asian summer monsoons [8]. Ocean^atmosphere interaction mechanisms such as ENSO and the NAO produce short-term inter-annual or annual £uctuations. However, the frequent dominance of one phase over another can produce longer-term climate trends. This is supported by a compilation of the global trend of SST collected from 1945 to 1993 from COADS data [46] (Fig. 4). The trend shows warm tropical Paci¢c SSTs and cold North Atlantic SSTs. Although global ocean^atmosphere interaction mechanisms may have been di¡erent from those in operation during the Younger Dryas chronozone, the similarities indicated in Fig. 3 are striking. This suggests that a cold Younger Dryas phase in the North Atlantic can co-exist with an

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Fig. 4. The linear trend of SST from 1945 to 1993 analysed from COADS data [46]. This illustrates the immense complexity of the global climate system, and shows the e¡ect of ocean^atmosphere interactions on the global picture. This ¢gure complements the previous ¢gure as the relationship between high and low SSTs forms a similar pattern to that indicated in Fig. 3.

ENSO Paci¢c condition at interdecadal or longer time scales. 5. Conclusions The Younger Dryas was in£uenced by changed orbital forcing caused by insolation, post-glacial warming, the presence of residual ice sheets on the northern continents, and North Atlantic ocean interactions [47,48] under the transition from the last glaciation to post-glaciation, which was induced by orbital forcing. We propose an additional series of short-term £uctuating ocean^atmosphere interactions (such as ENSO) where moisture from low latitudes can be transported via summer monsoon winds to mid^high latitudes, superimposing a moist signal onto a highlatitude cold background. Present-day observations (the last 100 years) have suggested that the ENSO^Asian monsoon system is modulated by interdecadal-scale NAO polarity, presumably through teleconnectivity in the Atlantic^Eurasian sector and related land^atmosphere interactions [44,45,49,50].

Although the Younger Dryas climate in the northern hemisphere shows a general cold trend, its spatial structure re£ects complex atmosphere^ ocean coupling processes, linking tropical winds with high-latitude cold air advection. If there is a strong similarity between the pattern of wet/dry conditions during the Younger Dryas and the recent ENSO precipitation picture, precipitation variability during the Younger Dryas may be related to ENSO. In summary, our evidence shows that the cold/ dry Younger Dryas chronozone in the mid-latitude sensitive area of the Chinese Loess Plateau has been punctuated by a short cool/wet period. This indicates that the shorter-time scale climate £uctuations of high/low-latitude interactions show up very clearly in these sensitive regions. The fact that these climate change punctuations can be observed elsewhere tends to suggest that their existence in many other sites may become evident with future high-resolution research. Recent evidence from Ecuador [51] argues against ENSO events during the Younger Dryas. We suggest that ENSO could have existed in a more weakened mode than the ENSO of today. However,

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since this point of view is controversial, we believe that more data need to be produced before we can be certain one way or the other. Since short-term climate patterns such as that shown in Fig. 4 are being debated as a possible ¢ngerprint of global warming, we believe that further studies of past climate patterns with respect to low/high-latitude interactions that occur at decade to century time scales can provide useful information. Acknowledgements This project was funded by the National Science Foundation of China 49725308, 40023003, NKBRSFG199043400, and Chinese Academy of Sciences. We thank Prof. S.C. Porter for support and encouragement. We also thank Professor John Kutzbach and an anonymous referee for very constructive suggestions.[EB] References [1] R. Allan, J. Lindesay, D. Parker, El Nin¬o, Southern Oscillation and Climate Variability, CSIRO, Melbourne, 1997, pp. 61^67. [2] K.E. Trenberth, General characteristics of El Nin¬o-Southern Oscillation, in: M.H. Glantz, R.W. Katz, N. Nicholls (Eds.), Teleconnections Linking Worldwide Climate Anomalies, Cambridge University Press, Cambridge, 1991, pp. 13^42. [3] K.-M. Lau, P.J. Shen, Teleconnections in global rainfall anomalies: seasonal to inter-decadal time scales, in: M.H. Glantz, R.W. Katz, N. Nicholls (Eds.), Teleconnections Linking Worldwide Climate Anomalies, Cambridge University Press, Cambridge, 1991, pp. 227^256. [4] P.J. Webster, V.O. Magan¬a, T.N. Palmer, J. Shukla, R.A. Tomas, M. Yanai, T. Yasunari, Monsoons: Processes, predictability, and the prospects for prediction, J. Geophys. Res. 103 (1998) 14,451^14,510. [5] Z.W. Yan, N. Petit-Maire, The last 140 Ka in the AfroAsian arid/semi-arid transitional zone, Palaeogeog. Palaeoclim. Palaeoecol. 110 (1994) 217^233. [6] Z.S. An, T.S. Liu, Y.C. Lu, S.C. Porter, G. Kukla, X.H. Wu, Y.M. Hua, The long-term paleomonsoon variation recorded by the loess-paleosol sequences in Central China, Quat. Int. 7/8 (1991) 91^95. [7] W.J. Zhou, D.J. Donahue, S.C. Porter, T.A. Jull, X.Q. Li, M. Stuiver, Z.S. An, E. Matsumoto, G.R. Dong, Variability of monsoon climate in East Asia at the end of the last glaciation, Quat. Res. 46 (1996) 219^229.

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