A multiproxy lake record from Inner Mongolia displays a late Holocene teleconnection between Central Asian and North Atlantic climates

Quaternary International 227 (2010) 170e182

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A multiproxy lake record from Inner Mongolia displays a late Holocene teleconnection between Central Asian and North Atlantic climates Huei-Fen Chen a, *, Sheng-Rong Song b, Teh-Quei Lee c, Ludvig Löwemark d, Zhenqing Chi e, Yong Wang e, Eason Hong a a

Institute of Applied Geosciences, National Taiwan Ocean University, 2 Pei-Ning Road, Keelung 20224, Taiwan, ROC Institute of Geosciences, National Taiwan University, Taipei 10617, Taiwan, ROC Institute of Earth Sciences, Academia Sinica, Taipei 11529, Taiwan, ROC d Department of Geology and Geochemistry, Stockholm University, Stockholm 10691, Sweden e Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China b c

a r t i c l e i n f o

a b s t r a c t

Article history: Available online 18 March 2010

In order to study how the Holocene Central Asian climate is coupled to the global climate system, a 4.24 m long lake core from western Inner Mongolia in China was studied using a multiproxy approach. Sedimentology and geochemical parameters such as gypsum and dolomite content, presence of lakeshore sand changing to aeolian sand, and changes in paleomagnetic properties bear witness to a trend toward a generally drier climate over the late Holocene. Aridification is linked to the southward retreat of the northern boundary of the Asian summer monsoon, leaving central Asia under the influence of the westerly belt. The weakening of the Asian summer monsoon in turn was caused by an orbitally driven decrease in summer insolation. The weakening summer insolation also likely increased the intensity of the Siberian High pressure system, further promoting aridification of central Asia. On a shorter time scale, the multiproxy record shows the climate to have been relatively dry during the Medieval Warm Period (AD 800e1100) with the ensuing humid environment at the end of this period gradually turning to become extremely dry (AD 1100e1550) at the Little Ice Age Maximum. Switches in the North Atlantic Oscillation caused these changes through a teleconnection in the form of westerlies. These westerlies provided most of central Asia’s moisture after the retreat of the Asian summer monsoon. The central Asian climate therefore corresponds closely with late Holocene European climate changes. Ó 2010 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction The Holocene Epoch is particularly interesting from a climate change perspective for two main reasons. First, abundant resolution and age control of paleoclimatic archives is considerably better for this period than for earlier times. Second, the boundary conditions of the system are rather well known and have changed in a predictable way during the Holocene, compared to earlier stages of the glacialeinterglacial cycle (Wanner et al., 2008). For example, the gradual decrease in summer insolation on the northern hemisphere has resulted in a general weakening of the summer monsoon and a shift toward more arid climates, especially in Asia and northern Africa (An et al., 2008; Kröpelin et al., 2008). The general gradual trend toward a more arid climate in the mid Holocene has been observed in many Asian paleoclimate records

* Corresponding author. Tel.: þ886 2 24622192x6519; fax: þ886 2 24625038. E-mail address: [email protected] (H.-F. Chen). 1040-6182/$ e see front matter Ó 2010 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2010.03.005

(An et al., 2000; He et al., 2004; Xiao et al., 2004; Feng et al., 2006; Chen et al., 2008; Cosford et al., 2008). Numerous accounts show the environment becoming abruptly drier between 3500 and 3000 BP over wide parts of China (Huang et al., 2000; He et al., 2004; Li et al., 2004; Xiao et al., 2004; Dykoski et al., 2005; Cosford et al., 2008), especially in Inner Mongolia (Zhang et al., 2000; Liu et al., 2001; Wang et al., 2001; Mischke et al., 2003; Shi and Song, 2003; Yang and Williams, 2003; Feng et al., 2006). During the late Holocene, variations in climate also occurred in the Medieval Warm Period and the Little Ice Age. These two climatic events have been well described for Europe and the North Atlantic region (Lamb, 1965; Jelgersma et al., 1995; Borja et al., 1999; Clarke et al., 2002; Bradley et al., 2003; Wilson et al., 2004; Clemmensen et al., 2007). In addition, the importance of late Holocene climatic changes and the role of human activity have also been well recognized. In arid northwestern China, pollen, organic carbon, and pigment contents in sediments are all very low (Qu et al., 2000), with only salts and soluble ions being able to indicate salinity variations in

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lake sediments (Zhang et al., 2004; Hartmann and Wünnemann, 2009). This study examined a core extracted from East Juyanhai Lake (Sogo Nur); this lake is now an individual lake on the northern side of Paleo-Juyanze (Fig. 1c) (Mischke et al., 2003; Herzschuh et al., 2004). Paleo-Juyanze is also named Juyanze Paleolake. A previous core, G36, was extracted here near the most easterly side } nnemann and Hartmann, 2002; Hartmann, of Paleo-Juyanze (Wu 2003; Hartmann and Wünnemann, 2009). The present core from

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East Juyanhai Lake exhibits less salinity than Juyanze Paleolake (G36). In addition, time scales, given by the cores for the two lakes, are different. Core G36 shows the time period for eastern Juyanze Paleolake from 11,000 BP to 1580 BP. The core, for East Juyanhai Lake, covers 4000 BP to circa 400 BP. Hartmann and Wünnemann (2009) used XRD to count quartz, feldspar, mineral salts, and soluble-ion contents of core G36 to estimate changes in lake volume of Juyanze Paleolake. However, they did not analyze the

Fig. 1. (a) The Juyanhai area (star) of northern China. When the Asian summer monsoon is enhanced in summer, the westerly jet moves northward. When the Siberian High is enhanced in winter, the jet moves southward (as per Gao (1962) and revised from Zhang et al. (1992)). (b) Alashan Plateau including the Haihe and Shiyang Rivers, both of which derive from the Qilian Mountains of northwestern China (modified from Herzschuh et al., 2004). The core is marked with a circle, and the cross is the record of Juyanze Paleolake from Mischke et al. (2003). (c) The satellite map released from the Remote Sensing Technique Applied Center of China Institute of Water Resources and Hydropower Research in August (2002). Core location (circle) is on a northern slope of East Juyanhai Lake, and the dashed line area is Paleo-Juyanze, a desiccated zone.

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contents of clay minerals to show the relative total mineral variations in the core. This paper takes a more advantageous approach by first presenting age data post-2000 BP to investigate lake variations during the Medieval Warm Period and Little Ice Age. The relative contents of all minerals and major elements were examined to compare chemical variations in lake sediments. Finally, magnetic S-ratios were used to indicate the degree of salinity in the lake. Rapid desertification in the region and its consequences for the people who live there is of great concern. A late Holocene multiproxy record of environmental changes from Western Inner Mongolia, which is near the boundary between two major climate systems (Fig. 1a); i.e., the northern margin of the Asian summer monsoon and the westerly belt (Wang and Feng, 1992), is presented. To differentiate the two, the Asian summer monsoon system is generally characterized by warm-wet summers and cooldry winters. On the other hand, the westerly belt produces warmdry summers with high evaporation and cool-wet winters. The westerly belt system brings less rain annually than the summer monsoon system. Consequently, the region is extremely sensitive to retreat of the summer monsoon, which would leave the westerly belt in control and increase desertification. Comparisons over a longer time scale indicate that the climate has shifted from cold and wet during glacial periods to warm and dry during interglacial intervals since the late Pleistocene in northwestern China (Li, 1990). It is interesting that when the Asian summer monsoon decreased during the mid Holocene in the Qilian Mountains (Fig. 1b), the Juyanhai region of north China began to dry out, and the influence of the westerlies increased. In the discussion of this paper, climate change in Central Asia relative to changes in Europe, especially the dynamic evolution of mid-latitude westerlies during the Medieval Warm Period and Little Ice Age, are examined. 2. Study area and regional geology The core used in this study was extracted in the Juyanhai area (Gaxun Nur Basin, Hartmann and Wünnemann, 2009) of western Inner Mongolia in north China (Fig. 1a), on the Alashan Plateau near the Gobi Altai and Badain Jaran Desert (Fig. 1b). It was drilled on a northern slope near the East Juyanhai Lake (42 20.130 N, 10115.350 , Fig. 1c), 904 m above sea level. The Gaxun Nur Basin includes the West Juyanhai Lake (Gaxun Nur), East Juyanhai Lake (Sogo Nur), and Paleo-Juyanze. They are individual lakes at the end of the Haihe River. The Paleo-Juyanze is now dry (Fig. 1c). The Haihe and Shiyang Rivers originate in the northwestern part of the Qilian Mountains (Fig. 1b). These two rivers both have terminal lakes at the fringes of deserts, but these lakes have become increasingly dry in recent decades. At one time, a single unified lake in the Gaxun Nur Basin covered both the West and East Juyanhai Lakes, and included the Paleo-Juyanze (dashed line in Fig. 1c), which once } nnemann and Hartmann, 2002; covered eastern core G36 (Wu Hartmann, 2003). Over time, the tectonic graben structure in the basin of the region has assisted in the lakes gradually drying up (Becken, 2005). The Paleo-Juyanze (42 000 N, 101340 E, Fig. 1b and c) withered around 2700 BP (Mischke et al., 2003; Herzschuh et al., 2004), while the records of eastern core G36 (41.75e40 N, 101.5e102 E) continue to 1580 BP (Hartmann and Wünnemann, 2009). Presently, the inland lake receives its water from the Qilian Mountains due to ice melt in spring. Sediments are carried by the Haihe River via the Hexi Corridor (Gansu Corridor) toward the wide Quaternary basin and end lakes. They include sediments from loess regions, flood-plain, lacustrine and other aeolian deposits. The upper Haihe River travels from the northern Qilian Mountains, which is composed of complex rocks, including: mud, carbonate, andesitic and basaltic rocks, as well as metamorphic rocks which

are characteristic of a paleo-oceanic crust. The paleo-oceanic crust contains abundant mafic elements, so the sediments supplied from the upper Haihe River are rich in Fe and Mg. In addition, a comparison of the southern and northern sources of the Haihe River shows that the northern source, from Gobi Altai, brings coarse quartz and feldspar over a short transporting distance, giving sediments rich in Si, Na, and K. According to recent meteorological records, the mean temperature is about 11.7  C in January and 26.1  C in July. Local annual precipitation is only about 38.2 mm/y, whilst the evaporation rate reaches about 3700 mm/y (Qu et al., 2000). Presently, the lakes are supplied mostly by water from the Haihe River, which comes from spring ice melt to give the long transport distance in the end lakes. So, as the water travels over the wide basin, high evaporation rate produces a negative lake budget. Hypersaline lakes exist at the end of the Haihe, and some of these desiccated paleolakes are composed of salt minerals. In brief, the climate is marked by high evaporation rates and exhibits a typical inland continental climate with relatively warm, dry summers and cold, wet winters. 3. Material and methods A 424-cm dry lake core was extracted from western Inner Mongolia in Northern China. Only a section of the core between 64 cm and 424 cm below ground was available for sampling as the upper portion was composed of coarse sand. The core section below 64 cm was sampled at 3-cm intervals (corresponding to a sampling interval of 30e40 years) and then analyzed for mineral composition, major element variations, and paleomagnetism. The paleomagnetic parameters were determined with a cryogenic magnetometer (2G Enterprise Co., model 755 SRM). Different magnetic minerals acquire their saturated isothermal remnant magnetizations (SIRM) under different strengths of an applied field. Therefore, the ratio (S-ratio) between the soft component (acquired by magnetite) over the total SIRM of the samples can be used to estimate the relative content of different magnetic minerals. The S-ratios in this study were calculated following Bloemendal et al. (1992). For example, magnetite acquires its SIRM below an applied field of about 300 mT, while that of goethite is about 1 T and that of hematite over 1.5 T. The S-ratio varies between 0 for pure hematite and 1 for pure magnetite. A high S-ratio indicates a predominance of anoxic sedimentary conditions and the preservation of magnetite (Yancheva et al., 2007). However, both oxidation and salinity affect S-ratio values, and both can be expected to change synchronously with lake variations. Katari and Tauxe (2000) utilized an experimental method to show that salinity is more influential when pH values are higher than 5. The hypersaline lakes in this inland region all have high pH of about 9e10 based on the Badain Jaran Desert records of Yang and Williams (2003). Consequently, two major factors will influence the amounts of magnetite and hematite in the sediments, one is oxic conditions and the other is salinity of lake water. When the lake shrinks, the lake level descends and sediments are subaerially oxidized causing the S-ratio to decline. On the other hand, the shrinking of the lake will cause salinity to increase and when the pH value becomes higher than 5, magnetite will decrease and the S-ratio ought to drop rapidly. The latter factor is more significant, because both the salinity and pH value show large fluctuations when the lake volume changes. Gypsum was quantified by detecting SO2 4 using Ion Chromatography (Metrohm, 753 Supperssor Model), which has a detection limit of 0.5 ppm for anions and 1 ppm for cations. Other minerals were semi-quantified by X-ray Powder Diffraction (XRD) using the methods of Biscaye (1965) and Chen (1973). Because different salt minerals have different solubility in water, they can be used as

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proxies to investigate fluctuations in the lake level. As a general rule, as the lake volume decreases, calcite (CaCO3) precipitates prior to the formation of gypsum (CaSO4.2H2O) (Inoue et al., 1998). If the alkalinity of the lake is high enough, aragonite and dolomite both appear after calcite precipitates. From mineral identification, quartz, feldspar, and clay minerals are of detrital origin (Fig. 4), while calcite, gypsum, dolomite, and aragonite are authigenic salt minerals (Fig. 4). Consequently, a clay-rich signal is indicative of a relatively high lake level, while high amounts of quartz and feldspar are the result of lakeshore sand and aeolian supply. Especially, feldspar increased following a rise in quartz in unit IV (Fig. 4), which implies aeolian dust arriving from the northern Gobi Altai during lake shrinkage. As for quartz, it is very stable and hardly decomposes to other minerals under physical or chemical weathering. Feldspar, however, is easily altered to clay minerals by chemical weathering through hydrolysis in wet conditions. Consequently, aeolian sand preserves more feldspar than lakeshore sand and lacustrine sediments. In another work, in marine core MD012404 from the central Okinawa Trough, that feldspar content increased during the late stage of the LGM, and decreased in the warm period in MIS 5 (Chen et al., submitted for publication). This appears to indicate aeolian sources from northern China responding to climatic changes. Major elements were determined by X-ray Fluorescence (XRF) on a RIGAKU RIX-2000 (Japan). About 2.0 g of dry sample was combusted at 950  C for 2 h. This process, loss on ignition (L.O.I.), lets the volatile components escape, and the residual sample is used for XRF analyses. For a typical analyses process and the precision of all elements refer to Yang et al. (1996) and Liu et al. (2006). The analytical errors of: SiO2, Al2O3, Fe2O3, TiO2, CaO, K2O, Na2O, MgO, and MnO etc. were less than: 0.9%, 1.7%, 1%, 1%, 1%, 1.2%, 1%, 1.9%, and 3.7%, respectively. Major elements can reflect bulk composition of total minerals, meaning they can help explain certain mineral origins. Based on the weathering rule, Al2O3 and TiO2 are considered residual elements in clay minerals and heavy minerals. In order to show variations in element ratios relative to residual elements, other elements are normalized to Al. This can help identify those components that are enriched or depleted in the sediment. In addition to the above, some samples were used to determine particle size. Firstly, samples were dissolved in 10% HCl to wash away carbonate content and salt minerals. The residual samples were then used to detect particle size in a laser particle analyzer (Malven Masterizer 2000), which is capable of detecting sizes from 0.02 to 2000 mm. 4. Age model The age model for the studied core is based on three 14C dates, which were measured at the Rafter Radiocarbon Laboratory in New Zealand for AMS 14C dating (Fig. 2). The three organic grass samples suitable for 14C dating were found at 144-cm, 161-cm, and 181-cm depths (Fig. 3), respectively. The grass samples gave ages of 638  40 14C BP (NZA 21094), 815  30 14C BP (NZA 21138), and 1187  30 14C BP (NZA 21139). Their calibrated ages are 580 (25) cal BP, 714 (30) cal BP, and 1115 (55) cal BP (before AD 1950) with 1s standard deviation, respectively (Fig. 2). The calibration ages were calculated following the method of Stuiver et al. (1998). In the lower part of the core, no suitable material for 14C dating was found. However, below 320e330 cm a distinct lithological sequence consisting of a thin silty sand layer, with a distinct waterescape structure up to 285 cm, underlain by a carbonate-rich muddy layer, was found (Fig. 3). The silty sand is obviously representative of a shallow lake and it bears a little aeolian sand, which is

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Fig. 2. Age model with three d14C data points of this study (C), and two possible ages of aeolian sand (B) in Eastern Juyan region (data from Mischke et al., 2003).

characterized by a spotty-hole surface texture (Fig. 7). For lack of better evidence, an age of around 3 ka is assigned to this aeolian event (Fig. 2) for the Juyanhai region, according to lithostratigraphic correlation. By examining the nearby profile of the Paleo-Juyanze and its eastern core G36, the expected age can be roughly determined. The Paleo-Juyanze dried up around 2700 BP and recorded a dry episode with aeolian sand during 3200e2900 cal BP (Mischke et al., 2003; Herzschuh et al., 2004). Site G36 in eastern Juyanze Paleolake has an age of 3355 cal BP at the top of the lower carbonate lake deposits, near the boundary of the upper aeolian sand (Hartmann and Wünnemann, 2009). This coeval aeolian sand is underlain by a layer of carbonate rich mud similar to the greenishgray mud of East Juyanhai Lake in Unit I of the core. In addition, many lake records in Shiyang River, which also originates from the Qilian Mountains (Fig. 1b), has carbonate rich mud layers from 5 ka to 3 ka (Chen et al., 2004). 5. Results 5.1. Lithology Four lithounits were recognized from the core (Fig. 3). The units have been numbered I to IV from the lowermost to uppermost units. Unit I (424e330 cm) is mainly composed of pale greenishgray calcareous mud (Fig. 4) with scattered aggregates of platy gypsums (Fig. 3) that had been deposited in the deeper part of a lake. This unit is characterized by brownish silty mud from 345 cm to 330 cm. Three and five gravels occur respectively at 338 cm and 334 cm. The larger grain size and oxidized condition indicate an energetic shallower lake setting. Unit II (330e285 cm) is composed of thin layers of sand and reddish muddy silt. Water-escape structures are not uncommon in this unit. No gypsum crystal or any salt minerals are observed. The lower part is of a thin oxidized brownish sandy layer at 330 cme320 cm, indicating lakeshore facies while the upper part, from 320 cm to 285 cm, characterizes a shallow lake facie. Unit III (285e118 cm) mainly consists of reddish silty mud with interbedded silty laminations. From 285 cm to 280 cm is a thin reddish mud layer. Above that the reddish silty mud accumulates up to a depth of 118 cm. From 280 cm to 220 cm, water-escape structures occur in the silty mud layers. Gypsum crystals are observed again at depths of 240e230 cm and 185e170 cm, respectively. Furthermore, three grass samples, discovered at depths of 144 cm, 161 cm and 181 cm, were sampled to do the 14C

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Fig. 3. Sediment sequence of the core from East Juyanhai Lake, which indicates the deep lakes transformed to a shallow lake and eventual dried out.

dating (Fig. 3). This unit is interpreted as deposited in a shallow lake environment. The uppermost unit (Unit IV, above 118 cm) is composed of yellowish-brown sand that coarsens upwards. From 118 cm to 100 cm, it is composed of laminated silt with little mud. Laminated fine sand occurs from 100 cm to 70 cm. From 70 cm to 30 cm, a very thick sand layer characterized by low-angle parallel laminations is apparent. At 29e30 cm (Fig. 3), a dry, hardened brownish-red mud occurs indicating dry oxidization. From 29 cm to the surface, the sandy unit is dominated by small-scale trough cross-bedding. The coarse sand in the uppermost part of Unit IV appears to have been

deposited from aeolian swept off the northern slope of East Juyanhai Lake from the Gobi Altai (Fig. 1c). An erosional unconformity is evident on an outcrop close to where the core was extracted. Such an observation implies that an ephemeral stream flowed across the core site. In addition to the core, a trench was dug nearby to about 1.8 m depth (Song et al., 2003). The vertical section of the trench can be correlated to the uppermost part of the core. Based on color and grain size, the depositional setting seems to have evolved from a considerably reducing environment into an oxidizing one, ending up, finally, as a drought environment. The

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Fig. 4. XRD semi-quantification of detrital and salt minerals, correlations of clay/feldspar (dotted line) with grain size (simple line), and S-ratios.

first dry event occurred in Unit II and is indicated by lakeshore sand with a little aeolian sand (SEM image) (Fig. 7). This aeolian-sand event is also evident in Paleo-Juyanze (Mischke et al., 2003; Herzschuh et al., 2004) and its eastern site (G36) (Hartmann and Wünnemann, 2009). In unit IV, the coarsening-upward sequence and the horizontal laminations replaced by low-angle parallel laminations (Fig. 3) indicate the occurrence of a second dry event. The latter dry event in Unit IV led the lake to wither gradually. 5.2. Geochemistry and mineral composition As per the analytic proxies in Section 3, lake conditions were reconstructed as follows: (1) Sources of detrital sediments, based on content variations of quartz, feldspar and clay minerals, and also comparative variations of major elements; (2) Modes of sediment transportation, based on grain size and lithological descriptions; (3) Variations in lake level and hydrological status, based on grain size, kaolinite/feldspar ratios, content of salt minerals, and S-ratios in lacustrine facies. Preliminary findings for divisions in sources indicate more quartz and feldspar being derived from the northern Gobi Altai; however, most clay minerals are derived through transport over long distance from the southern Haihe River. As for chemical composition of sediments, the northern source is rich in Si, Na and K, and the southern source in Fe, Mg and Ca. Grain size is only representative of transport distance and flow energy. Regarding lake shrinkage, it is evident this core comes from a lakeshore environment. With the enhancement of the winter monsoon, aeolian sand covered this area. This phenomenon also occurred in neighboring Paleo-Juyanze. Both lake shrinkage and aeolian transport contribute to larger grain sizes in sediments. Consequently, the lithological structure and grain surface texture needs to be examined by SEM (Fig. 7) to confirm means of transportation. When grain size is very fine and sediments are rich in clay fractions, sediment transport by suspending over a long

distance through a large lake. Finally, variations in lake level are estimated utilizing grain size, kaolinite/feldspar ratios, and the hydrological status of the lake by its salt contents and S-ratios, which indicate salinity. A high lake level is represented by a fine grain size and a high kaolinite/feldspar ratio. The clay-fraction proxy is more sensitive than gain size, because alteration due to hydrolysis of feldspar to kaolinite was predominant along the Haihe drainage area as the climate became wetter. Moreover, during lake shrinkage, water salinity rises, precipitating more mineral salts. These are given in the following order: carbonate (calcite, aragonite and dolomite), sulfate minerals (gypsum, mirabilite and thenardite), and chloride minerals (halite). At the same time, S-ratios decrease abruptly as salinity increases and salt-mineral composition changes. The records of eastern Juyanze Paleolake (G36), where thenardite and halite are prevalent in lake sediments (Hartmann and Wünnemann, 2009), had a higher salinity than this lake, in which only carbonate and gypsum precipitates (Fig. 4). This implies that East Juyanhai Lake dried out after Juyanze Paleolake, and therefore, can record evidence of climatic changes for latter periods. Given the above analysis, the core can be subdivided into four distinct facies that are generally supported by variations in mineral compositions (Fig. 4) and major elements (Fig. 5). The largest difference appears within Unit I, which is characterized by high clay/feldspar ratios, carbonate content (calcite and dolomite), fine grain size (Fig. 4), and strongly varying Ca/Al ratios (Fig. 5). In the latter stage of Unit I, aragonite, dolomite, and gypsum levels rise, and the S-ratio drops, apparently. Evidently, the lake was experiencing shrinkage. Unit II displays some increases in quartz, feldspar and grain size (Fig. 4), and Si/Al, Na/Al and K/Al ratios rise (Fig. 5) in the sand layers. This indicates a change in detrital source, i.e., contribution from the northern part of the lake, as well as aeolian sources. This first dry event corresponds to a synchronous aeolian event during 3300e2900 BP in Paleo-Juyanze and its eastern core

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Fig. 5. Variations in the ratios of normalized major elements to Al (by XRF) and the loss on ignition (L.O.I.) in the core.

(G36) (Mischke et al., 2003; Herzschuh et al., 2004; Hartmann and Wünnemann, 2009). From 3500 BP, the lake underwent shrinkage and kaolinite/feldspar ratios decreased (Fig. 6c). Unit III shows the southern Haihe River as being a stable source, with only high fluctuations in gypsum (Fig. 4). A high salinity event appeared at 172 cm, accompanied by a distinctive drop in S-ratio, and increases in dolomite, aragonite and gypsum. In addition, peaks in Mg/Al and Ca/Al indicate that the drier conditions evident in Unit III occurred

at about 960 cal BP. Unit IV is very distinct since it only contains quartz and feldspar sands, and is practically devoid of other minerals. Above 118 cm, quartz increased obviously and transformation to feldspar also unexpectedly rose (Fig. 4). This indicates that the sand source above 100 cm is different from that in the lower part of this unit (Unit IV, Fig. 3). However, the upper 64 cm was not analyzed because it was too coarse and the sedimentary structure could be easily investigated from the trench profile. The

Fig. 6. (a) d18O variations in the Dongge Cave in southern China (modified from Wang et al., 2005). (b) Northern Hemisphere mean temperatures quoted from Moberg et al. (2005). (c) The kaolinite/feldspar ratios and the drier index of gypsum contents in this study.

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sand components reflected sudden increases in Si/Al, Na/Al and K/ Al ratios, but decreases in Fe/Al and Mg/Al (Fig. 5), indicating a source derived from the northern region. 5.3. Paleoenvironmental interpretation Unit I bears witness to a significant climate change where the lake environment changed from deep to relatively shallow conditions. The high calcite and low gypsum content in the lower part of Unit I (424 cme350 cm) in the core indicate the presence of a larger, stable lake body during this period. This is also corroborated by higher total clay/feldspar (Fig. 4) and kaolinite/feldspar ratios (Fig. 6c), indicating that chemical weathering was quite pronounced in this period, and consequently that the climate must have been warmer and more humid. A wetter and warmer climate caused by a stronger summer monsoon at this stage is supported by higher d18O in the speleothem records from Dongge Cave in southern China (Fig. 6a) (Wang et al., 2005). The decreasing ratios of clay/feldspar and increasing grain size and amounts of dolomite and gypsum suggest a shrinking and progressively more saline lake as the climate gradually became drier. A generally drier climate after 4000 BP is also indicated by lake records from the Badain Jaran desert (Zhang et al., 2000; Yang and Williams, 2003), and many lakes in Inner Mongolia (Wünnemann et al., 2007). This implies that the intensity of the summer monsoon was quickly weakening during this period. Unit II is interpreted as being deposited in the following sequence: an abrupt transform from a short period of lakeshore facie to shallow lake facies with some aeolian sand (Fig. 7) input mixed with the fine sand layer from 330 cm to 320 cm. A similar sequence with aeolian sand has also been observed in the PaleoJuyanze and core G36. Therefore, this interval is taken to represent an ensuing drought episode. However, the poorly constrained age model in this part of the core does not allow a firm correlation with other climate events. Nevertheless, a correlation is proposed to the first cool-dry event of China’s West Chou Dynasty (Liu, 1992) during 3000e2850 BP. The return to muddy sediments after the sandy layer of Unit II and the decrease in grain size, Si/Al and Na/Al ratios indicate that the lake expanded again and that the aeolian influence weakened. However, the oxidative reddish mud and lesser clay/feldspar ratios

Fig. 7. SEM image of the sand surface in 285e313 cm of the core.

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indicate that the lake was shallower than in Unit I (Fig. 4). Although the area returned to lacustrine conditions after 2900 BP, with the prevailing climate being more humid than in the previous interval of Unit II, it was still drier than the climate displayed in Unit I. One particular minor dry event is well constrained (1195e875 cal BP) and corresponds with the general timing of the Medieval Warm Period. After this period, there is very good age control (Fig. 3). The distinctive drop in S-ratio at 172 cm, indicative of more saline conditions, together with the concomitant peaks in dolomite, aragonite and gypsum in Unit III occurred at about 960 cal BP. Chinese historical records have the warmest age reportedly taking place around AD 985, after which the climate then gradually became colder (Liu, 1992). So, higher temperatures coinciding with the Medieval Warm Period did not lead to a wetter environment: rather, they resulted in relative dry conditions. The Juyanhai area was an ancient oasis until the Tang Dynasty (AD 618e906) and was even a military base for a short time in the Yuan Dynasty (AD 1271e1368) (Li, 1998). However, this region was abandoned in the Sung Dynasty (AD 960e1279), and outside clans from China’s northern frontier invaded China at the same time. Therefore, the drier climate during the Medieval Period could have been responsible for inducing war in China. In the ensuing period, post AD 1100 to the Little Ice Age, the climate appears to have varied from humid to extremely dry as implied by low gypsum content at the end of Unit III switching to a sand layer in Unit IV (Fig. 4). Unit IV indicates that the East Juyanhai Lake became drier above a depth of 118 cm and in the core’s location was completely dry above 29 cm only recently (Fig. 3). It appears that as the lake shrank gradually, and some sand was deposited from the Gobi Altai (Fig. 1c) because feldspar content quickly rises above 100 cm (Fig. 4). By extrapolating the age of the sediment from the 14C date at 144 cm (580 cal BP), the area started to become more arid above 118 cm from around 400 years ago. This is concomitant with the Northern Hemisphere temperature minima experienced during the Little Ice Age Maximum (Fig. 6b). 6. Discussion 6.1. How the central Asian climate is coupled to the European climate According to evidence of modern climate modeling in Fig. 8, westerlies brought some moisture to the region in winter, and to freeze on the snowline of the Qilian Mountains. In summer, evaporation is higher than rainfall in this arid region. After spring, the ice thawed to feed the Haihe River and water flowed into the end lakes. Fig. 8 indicates that the main source of winter moisture was dependent upon the westerlies and was not from the Arctic Ocean. On the other hand, in summer this location is somewhat far from the modern Asian summer monsoon limit (Fig. 1a), so moisture is unlikely to be from this source. The central Asian climate is coupled to the monsoon system, but also to the North Atlantic climate through variations in strength and position of the westerlies. This connection becomes especially important in the late Holocene after the Asian summer monsoon retreated southward. According to Seager et al. (2007), the position and strength of the westerlies in Europe are influenced by the North Atlantic Oscillation (NAO). A positive NAO is caused by a stronger than usual subtropical highpressure center and a deeper than normal Icelandic low (Hurrell and Dickson, 2004). This results in a warming of the Sargasso Sea, a drying of the Mediterranean region, and a wetter northern Europe (Fig. 9a). The shift in the westerlies during positive NAO also has been shown to cause drier conditions in western North America, the Middle East, and Central Asia (Hoerling and Kumar, 2003). For

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Fig. 8. The modeling of moisture supplement in the Juyanhai area during winter (November, December, January, February) from AD 1948 to 2009 based on the NCEP (National Centers for Environmental Prediction) (Provided by Prof. Huang-Hsiung Hsu, Department of Atmospheric Sciences, National Taiwan University). The arrows of the figures represent moisture delivery capacity, and the negative blue color represents the convergence current and the positive red color represents the divergence current. The air pressures of: 100, 925, 850, 700 hPa represent the altitudes of near ground at 600, 1500, and 3000 m, respectively. Moisture is mainly supplied from the southwest red area in the Qilian Mountains below an altitude of 3000 m. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Fig. 9. The movement of westerlies controlled by the NAO influence and Siberian High during the Medieval Warm Period (a) and Little Ice Age (b): (a) the brown dots are dry records and the green ones are wet records during the Medieval Warm Period; (b) the red dots are the onsets of the sand invasion event in Europe and drying of Central Asia during the Little Ice Age. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

example, the existence of a wetter climate for the Medieval Warm Period in Scotland and England, at latitudes of 50e60 N (Lamb, 1965; Proctor et al., 2002), but drier conditions for Holland, Spain, Italy and Israel at latitudes between 30 and 50 N (Dragoni, 1998; Bar-Matthews et al., 1998; Tol and Langen, 2000; Benito et al., 2003) have been linked to a positive NAO during the Medieval Warm Period (Fig. 9a). Late Holocene changes in Central Asia are primarily controlled by shifts in the westerlies, and ultimately controlled by variations in North Atlantic climate as represented by the NAO. The northward shift of the westerlies caused by a positive NAO during the Medieval Warm Period resulted in wetter conditions in northeastern Europe and drier conditions in south-central Europe as well as in central Asia. In the results from Unit III, the period of time associated with the Medieval Warm Period (1195e875 BP) in the core was relatively dry. Although numerous records of climatic changes related to shifts in the westerlies are available from Western Europe and North America, there are conspicuously few records from central Asia during this period (Kang et al., 2002; Zhang et al., 2004). The record from the East Juyanhai Lake is important as it allows for an assessment of how changes in the westerlies affected Central Asia. 6.2. Evolution of the central Asian climate during the mid Holocene Environmental changes agree fairly well with the general trend of a weakening Asian monsoon during the mid Holocene found in

many of China’s historical records (An et al., 2000; He et al., 2004; Xiao et al., 2004; Feng et al., 2006; Chen et al., 2008; Cosford et al., 2008). The decrease in the summer monsoon was indicated in oxygen isotopes from Dongge Cave (Fig. 6a) (Dykoski et al., 2005; Wang et al., 2005) and Lianhua Cave (Cosford et al., 2008), and reflected in temperature changes in Chinese historical records (Zhu, 1979). In north China, pollen data and sand dune records also point to a dry episode during 3500e3000 BP, including Diaojiao Lake (Shi and Song, 2003), Daihai Lake (Li et al., 2004; Xiao et al., 2002), and the Loess Plateau (Huang et al., 2000). According to archeological and agricultural records on the history of China, the climate was wetter and warmer than today (Zhang et al., 1992; Liu, 1992) and flooding occurred regularly along the Yellow River in northern China prior to 3600 BP (Wang and Feng, 1992). In general, since the Holocene Climatic Optimum, monsoon intensity has gradually decreased and the boundary between the monsoon system and the westerly belt has slowly shifted from northwest to the southeast China (An et al., 2000). On the other hand, many deserts have paleosol layers dated between 3400 and 4400 BP in northern China, and sand dune growth was extensive after this period (Gao et al., 1992). This may have led to more kaolinite being hydrolyzed from feldspar in the source region (Fig. 6c), indicating a high chemical weathering environment before 3500 BP. The existence of a warmer and wetter climate can be established by correlation between the d18O of Dongge Cave and the kaolinite/feldspar ratios (Fig. 6a and c), which

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show a large lake existing before 3500 BP. A comparison with temperature data in China (Zhu, 1979) shows the climate of the mid Holocene being wet and hot. However, after 3500e2900 BP, a more arid climate arose leading to negative water budgets in Chinese desert lakes. This process is linked to global climate deterioration, retarding the influence of the summer monsoon in northwestern China (Wünnemann et al., 2007). This dry episode culminates around 3300e2900 BP with the appearance of aeolian deposits in the Gaxun Nur Basin. This drying trend began to reverse after 2000 BP, when the climate changed to display warm-dry and coldwet variations. This was also indicated in the dry index for gypsum content in the core (Fig. 6b and d). Such climate characteristics confirm the retreat of the summer monsoon from this area, and the prevalence of influence from the westerly belt. 6.3. Climate during the Medieval Warm Period and the Little Ice Age After the dry episode of the mid Holocene, the climate of the following two millennia appears to have been relatively stable with only a few minor dry spells manifested as silt laminations in the otherwise reddish mud of Unit III (Fig. 3). The stable salt minerals, S-ratio, and major element ratios (Mg/Al and Ca/Al) indicate that relatively stable climate conditions continued until the onset of the Medieval Warm Period (Figs. 4 and 5). This relatively dry medieval period has been mentioned in Section 5.3. Other research in East Juyanhai Lake also showed a relatively dry environment at AD 850e1090, and a more arid condition after AD 1400 (Zhang et al., 2004). Tree-ring research has also proved that the most arid periods were during AD 938e958 and AD 1051e1057, with a minor dry spell during the 14the15th centuries in upper Haihe River (Kang et al., 2002). Drought records are also available from tropical regions, including equatorial East Africa, North Africa, and in tropical Peru (Chu et al., 2002). Temperature evidence from tree-ring records in Central Asia indicates warmer conditions during the Medieval Warm Period from AD 800 to AD 1100 (Esper et al., 2002). This warm duration coincides with the relatively dry period. On the other hand, Mann et al. (2009) collected global data to identify the Medieval Warm Period from AD 950 to AD 1250, and the Little Ice Age from AD 1400 to AD 1700. Evidently, onset times for increasing temperatures during the medieval period are not synchronous in different places. The combination of the moisture record from East Juyanhai Lake with the mean Northern Hemisphere temperature records (Esper et al., 2002; Moberg et al., 2005) suggests that the climate displayed warm-dry and cold-wet trends after 2000 BP (Fig. 6b). This indicates that the westerlies controlled moisture supply here and the summer monsoon retreated fully. Central Asian and European climates during the Medieval Warm Period were controlled by the same primary driver, namely variations in the North Atlantic Oscillation governing strength and position of the westerlies. When the westerlies were enhanced by a positive NAO and moved northward making northwest Europe wetter and central and southern Europe drier, this same northward shift of the westerlies resulted in Central Asia being drier, too (Fig. 9a). According to the gypsum record of the core, after the Medieval Warm Period, the dry climate changed to humid conditions indicated by a decrease in gypsum, and then gradually dried again as indicated by increasing gypsum onset at circa AD 1400. After that, extreme drought conditions prevailed based on the sand layer of Unit IV, and no gypsum appeared after AD 1550 (Fig. 6). This change to a cooler climate is roughly coeval with the beginning of the 14th century in northern Europe (Grove, 2001). After that, the climate became unusually cold, with especially harsh winters in central Europe during the late 17th century (Wanner et al., 1995; Pfister, 1999). This climatic change has been linked to an increasingly

negative NAO index resulting in a cooler and stormier climate. It seems this cooling and increased storminess progressed gradually from northern to southern Europe (Clarke and Rendell, 2006). Records show that sand invasions, resulting from stronger winds, began occurring first in Northern Ireland at AD 1300 (Wilson et al., 2004) and then moved across Europe with stronger winds in: northwestern Scotland from AD 1400 (Dawson et al., 2004), East England from AD 1450 (Bateman and Godby, 2004), southwestern France from AD 1480 (Clarke et al., 2002), Denmark form AD 1560 (Clemmensen et al., 2007), and western Portugal from AD 1770 (Clarke and Rendell, 2006) (Fig. 9b). The record from Central Asia displays climate change occurring initially with a slightly drier environment trending toward extremely arid conditions during AD 1400 to AD 1550. After AD 1550, the lake gradually withered and aeolian sand swept to cover the Sogo Nur region. This region is at the latitude between France and Portugal (Fig. 9b). Although some Chinese historical records show more people utilizing water to irrigate in the upper Haihe River after about AD 1400 (Zhang et al., 2004), tree-ring research proves that river runoff from the mountain mouth of the upper Haihe drainage indeed dropped during the 14the15th centuries (Kang et al., 2002). Therefore, climate changes were driven by two mechanisms: (1) the spatial position of the westerly belt; and (2) changes in moisture supply and strength of the westerly belt. NAO in Europe and pressure variations between the Asian summer and winter monsoon in Asia directly influence the position of the westerly belt (Figs. 1a and 9). A higher Siberian High will cause the westerlies to move southward (Fig. 1a). When the westerlies swept Central Asia after the MWP, the climate became wetter in Central Asia and Central Europe. When the westerly belt moved outside of this region, the Siberian High directly cover the region, reducing moisture supply and whipping up winter winds to bring aeolian sand to the region. K/Al and feldspar signals can be used to identify the strong winter monsoon. High-resolution (8e10 samples/yr) records from the GISP2 ice core from Greenland show a substantial increase in potassium just after AD 1400 (Meeker and Mayewski, 2002). It is believed that the potassium signal was supplied from feldspar in aeolian sand, which also coincides with the results that show K/Al ratios increasing in Unit IV (Fig. 5). This was caused by influence of the Siberian High increasing after AD 1400. As to the strength of the westerlies, the great difference between the Medieval Warm Period and Little Ice Age is due to relative pressure variation between the Azores High and Icelandic Low, and between the Icelandic Low and the Siberian High (Fig. 9). During the Medieval Warm Period, the pressure gradient increased between the Azores High and Icelandic Low, and this resulted in a positive NAO and strong westerlies moving northward with somewhat drier conditions in Central Asia (Fig. 9a). By contrast, a negative NAO during the Little Ice Age caused the westerlies to shift southward and made Central Asia once again wet after the Medieval Warm Period. At the same time, the increasing pressure of the Siberian High caused the gradient between the Icelandic Low and Siberian High to increase. Thus the reversed pressure direction gently decreased the strength of westerlies in Central Asia, and the strong Siberian High lead to distinctly drier conditions as the westerlies moved southward beyond this area after AD 1550 during the Little Ice Age Maximum (Fig. 9b). 7. Conclusions The study of a multiproxy lake record from the East Juyanhai Lake in Inner Mongolia reveals how the Late Holocene climate in Central Asia has been controlled by the complex interplay between Northern Hemisphere insolation, the Asian summer monsoon, the Siberian High pressure system and North Atlantic climate.

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Observations from sedimentalogy, minerals and elemental compositions, as well as sedimentary magnetic properties reveal several important clues to how the climatic drivers work, and the affinities of the teleconnections between Central Asia and the North Atlantic. -

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The general decrease in insolation on the Northern Hemisphere during the Holocene led to a gradual change from a monsoon dominated wet and humid climate to a cool and dry climate as reflected by the drying out of East Juyanhai Lake. After about 3000 years ago, the climate of the study area appears to have been primarily dominated by variations in the westerlies. Increased silt, gypsum, and dolomite content combined with a dramatic decrease in S-ratio indicate that the Medieval Warm Period was characterized by reduced precipitation and drier conditions. This can be linked to northward westerlies caused by the generally positive North Atlantic Oscillation, which made north Europe wet and central Europe dry. After the Medieval Warm Period, when the North Atlantic Oscillation changes to negative and the westerlies gradually shifted southward, climate in Central Asia were initially humid as reflected by a decrease in e.g. gypsum, dolomite and silt content in the core sample. However, climate changed to extremely dry under the stronger influence of a decreasing Northern Hemisphere insolation that strengthened the Siberian High pressure system and enhanced sand invasion over Central Asia during the Little Ice Age Maximum. These results clearly show the strength of sedimentary multiproxy records when reconstructing climatic variations in continental settings where other records are sparse.

Acknowledgments We are grateful for the assistance of Mr. Li-Wei Kuo, and Mrs. Sao-Yi Huang, and Hsio-Chih Chen for their lab experiments. Particularly, we deeply appreciate Prof. Huang-Hsiung Hsu of National Taiwan Univerity (Atmopspheric Sciences) for providing the climate modeling of Fig. 8 in this paper. We also thank Professor Ping-Mei Liew and Louis Suh-Yui Teng of National Taiwan University (Geosciences) for their valuable comments on the manuscript. This research was supported by APEC II of Academia Sinica, R.O.C. and NSC 96-2116-M-019-001 from National Science Council in Taiwan. The coring budget was from the project number: 40572100 of the National Science Foundation of China. References An, C.B., Chen, F.H., Barton, L., 2008. Holocene environmental changes in Mongolia: a review. Global and Planetary Change 63 (4), 283e289. An, Z.S., Porter, S.C., Kutzbach, J.E., Wu, X.H., Wang, S.M., Liu, X.D., Li, X.Q., Zhou, W.J., 2000. Asynchronous Holocene optimum of the East Asian monsoon. Quaternary Science Reviews 19, 743e762. Bar-Matthews, M., Ayalon, A., Kaufman, A., 1998. Middle to late Holocene (6,500 Yr. period) paleoclimate in the eastern Mediterranean region from the stable isotopic composition of speleothems from Soreq Cave, Israel. In: Issar, A.S., Brown, N. (Eds.), Water, Environment and Society in Times of Climatic Change. Kluwer Academic Publishers, Dordrecht, pp. 203e214. Bateman, M.D., Godby, S.P., 2004. Late-Holocene inland dune activity in the UK: a case study from Breckland, East Anglia. The Holocene 14 (4), 579e588. Becken, M., 2005. Properties of magnetotelluric transfer functions-with a case study from the Gaxun-Nur basin, NW-China. PH. D Thesis, TU Berlin pp. 1e120. Benito, G., Diez-Herrero, A., Fernandez, V.M., 2003. Magnitude and frequency of flooding in the Tagus basin (Central Spain) over the last millennium. Climatic Change 58, 171e192. Biscaye, P.E., 1965. Mineralogy and sedimentation of recent deep-sea clay in the Atlantic Ocean and adjacent seas and oceans. Geological Society of America Bulletin 76, 803e883. Bloemendal, J., King, J.W., Hall, F.R., Doh, S.J., 1992. Rock magnetism of late Neogene and Pleistocene deep-sea sediments: relationship to sediment source,

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