Regional vegetation patterns at lake Son Kul reveal Holocene climatic variability in central Tien Shan (Kyrgyzstan, Central Asia)

Quaternary Science Reviews 89 (2014) 169e185

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Regional vegetation patterns at lake Son Kul reveal Holocene climatic variability in central Tien Shan (Kyrgyzstan, Central Asia) Marie Mathis a, *, Philippe Sorrel a, *, Stefan Klotz b, Xiangtong Huang c, d, Hedi Oberhänsli d, e a

Laboratoire de Géologie de Lyon (UMR 5276), Université Claude BernardeLyon 1, Villeurbanne, France Fachbereich Geowissenschaften, Universität Tübingen, Rümelinstrasse 19-23, 72070 Tübingen, Germany c State Key Laboratory of Marine Geology, Tongji University, 200092 Shanghai, People’s Republic of China d Helmholtz-Centre Potsdam, German Geoscience Research Centre (GFZ), Section 5.2, Telegraphenberg, D-14473 Potsdam, Germany e Museum für Naturkunde, Leibnitz-Institute Berlin (Mineralogy), Invalidenstrasse 43, 10115 Berlin, Germany b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 8 January 2013 Received in revised form 27 January 2014 Accepted 31 January 2014 Available online 18 March 2014

A multiproxy study was conducted on Holocene sediments from the alpine lake Son Kul (3010 m a.s.l, 41
480 33N/75
070 38E) in central Tien Shan (Kyrgyzstan). The combination of high-resolution pollen, palynofacies and magnetic susceptibility data allowed reconstruction of changes in sedimentary and vegetation dynamics regionally at Son Kul between 8350 and ca 2000 cal. BP. Using pollen data to quantify climatic parameters, a quantitative reconstruction of climatic conditions was performed using the “Modern Analogue Vegetation types” (MAV) method and a ranged index of seasonality. The most temperate (e.g. moister) climate conditions occurred between 8350 and 5000e4500 cal. BP when alpine meadow vegetation was enriched in plants requiring moister conditions and trees developed regionally. Conversely, more continental and arid conditions prevailed after 4500 cal. BP with the decline of arboreal vegetation (especially Juniperus) and the extension of an alpine steppe-meadow along with a regional decrease in Poaceae. This climate transition was associated with a change in seasonality as the continentality greatly intensified after 5000e4500 cal. BP. Our results are consistent with other records from the Tien Shan range and the Chinese Province Xinjiang showing that relatively wet conditions prevailed regionally before 5000 cal. BP, whereas reduced moisture conditions were established after that time. From a more global perspective, we highlight that regional rainfall in central Tien Shan and western Central Asia is likely to be predominantly controlled by the Eastern Mediterranean cyclonic system and North Atlantic climate, as based on the close correspondence between climatic archives from western Central Asia, the Levant, the Eastern Mediterranean and Caspian Sea regions. However, the effect of monsoonal dynamics on the regional climatic system in central Tien Shan still remains dubious, since recent modelling studies have shown that no dynamic link exists between humidity in Central Asia and the Indian Summer Monsoon. This study pinpoints the need to explore the effect of remote Eurasian atmospheric circulation patterns on past climate variability in Central Asia. Ó 2014 Elsevier Ltd. All rights reserved.

Keywords: Vegetation dynamics Climatic change Pollen analyses Palynofacies Holocene Western Central Asia (Tien Shan)

1. Introduction As one of the largest mountain systems in the world, the Tien Shan exerts a great influence on climatic processes and the hydrologic cycle in Central Asia. Remote from oceanic influences, this mountain range is a key site for high-resolution palaeoclimatic studies since different climate systems interact and control regional climate variability. Today, climate in the Tien Shan mountains is

* Corresponding authors. Tel.: þ33(0) 472 445 869. E-mail addresses: [email protected], (M. Mathis), [email protected] (P. Sorrel). http://dx.doi.org/10.1016/j.quascirev.2014.01.023 0277-3791/Ó 2014 Elsevier Ltd. All rights reserved.

[email protected]

controlled by two distinct climatic systems: the mid-latitude westerlies and the high pressures from the Siberian High (SHP) (Aizen et al., 1996). The interaction of these two climatic systems has strong implications for regional climate variability on annual to decadal timescales (Aizen et al., 1996). Surface runoff in Central Asia mostly relies on snow and glacier ice melting in the mountains and is, therefore, ultimately related to climate change, in particular the extent of available moisture (Hagg et al., 2007). In recent years, more concern has been given to water resources in Central Asia under the context of global warming. Recent investigations have shown that the glaciers of Tien Shan are experiencing a so rapid retreat that this region will face a threat of water shortage if the temperature continues to rise (Aizen et al., 2006). However, the

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mechanisms and linkages that control regional climate conditions and associated hydrological balance on longer timescales are still poorly understood and remain one of the most controversial issues in central Asia. In this regard, an out-of-phase linkage was suggested between the Asian monsoon and the westerlies (e.g. Chen et al., 2008, 2010a) during the Holocene, although this interplay is still a matter of debate and seems more complex than a competitive relationship (Wanner et al., 2008; Dallmeyer et al., 2013). Today the moisture availability in Central Asia is relatively limited and controlled by the westerlies, but a generally wetter early to mid-Holocene climate was reported from many lakes in western and arid Central Asia including northwestern China and Mongolia (Chen et al., 2008; Yang and Scuderi, 2010; Yang et al., 2011). However, despite a clear distinction between seasonal rainfall patterns in the regions influenced by the westerlies and those dominated by the Asian summer monsoon, we still lack highresolution data that would help to constrain the distribution and origin of moisture during the early to late Holocene in central Asia and especially at high-altitude settings. In order to better understand which mechanisms control hydrological and climatic variability in central Tien Shan, it is crucial to examine long-term environmental changes as preserved in welldated palaeoecological records. Lake records provide continuous, high-resolution, terrestrial archives of environmental change. Therefore they play a key role in the reconstruction of past climates and environments and allow comparison with other continuous archives such as marine sediments, speleothems and ice cores. In recent years, a series of lake records from the Tien Shan region and western arid central Asia were investigated (e.g., Ricketts et al., 2001; Wünnemann et al., 2006; Mischke and Wünnemann, 2006; Beer et al., 2007a,b; Chen et al., 2008: Huang et al., 2009; Mischke et al., 2010; Li et al., 2011; Gómez-Paccard et al., 2012). However, available sedimentary archives covering the Holocene period with a high time resolution are still scarce in central Tien Shan, and very few of these studies are based on an integrated pollen record. In light of the above, the objective of this study is to reconstruct climatic change at lake Son Kul and the history of past moisture variability based on high-resolution palynological analyses (pollens and palynofacies). We aim to document robust and quantitative estimates of Holocene climate in central Tien Shan with special attention to the reconstruction of humidity changes and seasonality amplitude at the regional scale. We also discuss the sensitivity of western Central Asia to the climatic fluctuations recognised in monsoon-influenced areas and the North Atlantic and Mediterranean regions.

about 200 m below the modern ELA. This indicates that glaciers never fed Son Kul in the past 10,000 years. Lateral moraines found in some north-sloping valleys of the Boor-Albas Range must thus originate from the Pleistocene (Savoskul, 1997; Abramowski et al., 2006). The basin is semi-enclosed; it has a single outlet (3e5 m3/ s) in the southwestern part, which ultimately feeds the Syr Darya, one of the two rivers supplying the Aral Sea. Stretching 29 km in length and 18 km in width, Son Kul has an area of w275 km2 and a catchment area of w1450 km2. It is the second largest water body in Kyrgyzstan after Lake Issyk-Kul, and also one of the largest highmountain lakes in the world. The lake receives inflow from several small mostly episodic rivers fed by rainfall, groundwater and melting snow from neighbouring mountains (Aizen et al., 1995). The water volume is w2.82 km3 with a maximum water depth of 15.1 m. 2.2. Climatic setting Today the major sources of humidity in central Tien Shan are depressions from the North Atlantic steered eastwards by the westerlies along preferred major storm tracks (Aizen et al., 1995, 2001). The location of Son Kul in the central Tien Shan, surrounded by high mountain ranges, complicates the transport of moisture and results in reduced precipitation, especially under the strong influence of the Siberian anticyclone in winter. However, precipitation increases with altitude in central Tien Shan (Aizen et al., 1996). A late winter/spring precipitation maximum occurs in MarcheMay and July (45e55% of the annual precipitation) (Aizen et al., 1996) (Fig. 1b). Winter precipitation accounts for only 8e10% of the annual precipitation at the regional scale. In summertime the condensation level rises, leading to increased precipitation at high elevations (Aizen et al., 1995). Both the development of convection and a strengthening of unstable atmospheric stratification result in a summer precipitation maximum in June and July due to the entrance of westerly cold and moist air masses (Aizen et al., 1995). In the Tien Shan, elevation is the main factor influencing air temperature distribution at the macro-scale. Strong continental climate with high annual amplitudes of monthly air temperature (up to 38
C) is typical for some regions of the central Tien Shan. The mean annual temperature at 3000 m a.s.l. in the Son Kul basin is 1.5
C (New_LocClim_1.1, 2005; ftp://ext-ftp.fao.org/SD/ Reserved/Agromet/New_LocClim/), and mean annual precipitation is 470 mm yr1 (Aizen et al., 1997). The lake is frozen for 6–9 months of the year. 2.3. A complex duality in local/regional vegetation

2. Regional environmental setting 2.1. Geographical setting Lake Son Kul is a high-altitude alpine lake in the western part of the Tien Shan range (Fig. 1a), central Kyrgyzstan (41
46.1960 N, 75
08.2040 E, 3010 m a.s.l.). It fills the central depression of a plateau located between two generally eastewest stretching mountain ranges of metamorphic, magmatic and strongly folded sedimentary rocks, with the Son KuleToo Range in the north (altitude 3200e 3500 m a.s.l.) and the Boor-Albas Range in the south (3600e3800 m altitude). Palaeozoic metamorphic granodioritic-granitic gneisses form the northern range, and Carboniferous and Devonian metamorphic rocks constitute the southern range (Vereshchagin et al., 1975). The recent equilibrium line altitudes (ELA) determined for glaciers on north-facing slopes in southern Kyrgyzstan occur at ca 4100 m a.s.l., and thus the mountain ranges surrounding the lake are not currently glaciated. Within the Holocene, maximal depression of the ELA occurred during the Little Ice Age when it was

Lake Son Kul is situated on a high plateau surrounded by mountain meadows. Due to the local topography (characterised by sharp altitudinal changes over short distances), the vegetation changes rapidly within a few kilometres, with the development of an alpine meadow in the Son Kul watershed at ca 3000 m a.s.l., whereas steppe-desert landscapes predominate in open lowlands at lower elevations (Fig. 1a, d, e). The Son Kul watershed is located in a treeless basin with extensive grass cover. The vegetation is not affected by water shortages and the herbaceous plants are characteristic of those from lush mountain meadows (Fig. 1e). The vegetation around the lake is mostly composed of temperate grasslands evolving from graminoid to forb tundra, as based on the Chen et al. (2010b) nomenclature. Woodlands (Piceae schrenkiana or Betulaceae), which are absent in the lake basin today, are confined to the riparian forest and shady slopes outside the basin. The environment is still preserved today but pastoralism has developed during the past hundreds years (Khazanov, 1994; Golden, 2011), with possibly

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Fig. 1. (a) Location map of lake Son Kul and the study area with a remote sensing image showing the study site in central Tien Shan using a NDVI (Normalized Difference Vegetation Index) transformation (NIR-R/NIR þ R). The Son Kul watershed is outlined in red, and the Naryn River in blue. The shading depicts vegetation density (Image Landsat worked with ENVI 4.3). Points 1 and 2 correspond to the photos (d) and (e), and illustrate the local vegetation from these sites. ‘Aral’ and ‘Caspian’ refer to the Aral Sea and the Caspian Sea, respectively. Note the core locations (black circles) for the Caspian Sea (cores TM and GS05/CP14), which refer to the sites discussed in Sections 5.1 and 5.2. (b) Annual precipitation diagram at 3000e3500 m a.s.l. (Son Kul; full) and at 1000e1500 m a.s.l. (Lake Issyk Kul; dashed). (c) The Son Kul area with the location of core SK07 (red cross), and the sampling sites for surface pollen samples SK1, SK4 and SK5 (yellow points). (d) The typical vegetation association from the dry lowlands, characterised by a sparse vegetation. (e) the landscape around Son Kul showing the alpine meadow (Photos: Marie Mathis, 2012). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

10,000 heads of cattle around the lake in summertime (100 heads per family and possibly 100 families in the Son Kul basin during summertime) as based on personal observations. Conversely, the lowland vegetation (located at elevations below 2000 m a.s.l.) is mainly composed of sagebrush steppe (Artemisia) and Chenopodiaceae (Fig. 1d). This vegetation is typical of steppe and semi-arid environments, temperate xerophytic shrublands or desert biomes (Chen et al., 2010b). At a more regional scale, Son Kul is located between two regions with different vegetation characteristics: the Tibetan Plateau to the east and southeast, and the lowlands to the west with the Aral Sea basin. The different phytogeographical assemblages reflect, therefore, the diversity of the landscapes, climate and soils (Epple, 2001). Hence, the pollen assemblages from Son Kul sediments may contain pollens from different sources, representing the local and regional vegetation from both high- and low-elevation settings. According to the recent studies of Cour et al. (1999) and Yang et al. (2013), the predominant wind directions today are from the west with the Westerly Jet Stream (up to north China and Inner Mongolia) (Yang et al., 2013) and possibly including the influences from southern monsoon currents at the seasonal scale (Cour et al., 1999).

3. Material and methods 3.1. Site, sediments, core processing During a field campaign in the summer of 2008, piston cores SK07 (1.52 m) and SK04 (1.34 m) were retrieved from a platform equiped with a Usinger piston corer (http://www.uwitec.at) at lake Son Kul (41
46.1960 N, 75
08.2040 E; water depth 12.5 m) (Fig. 1). Core SK04 (41
46.5660 N; 75
08.1820 E) was retrieved close to core SK07, at ca 1.15 km from the coast. Core SK04 has a more proximal habitus than core SK07. Although we used core SK04 for crosscorrelation (and establishment of the age model as explained in Section 3.2), the present study was conducted on core SK07 solely. Due to the coring procedure, the topmost 0.6 m is missing from core SK07. In addition, three modern surface pollen samples (e.g., SK1, SK4, SK5) were collected from mosses in the summer of 2012 in the Son Kul basin (see Fig. 1 for location). Low field magnetic susceptibility (c or MS, expressed in SI ¼ System International) was measured directly after core opening on the surface of split core halves with a Bartington MS2E sensor (GFZ Potsdam) at a resolution of 1e2 mm. Low field

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Fig. 2. Simplified lithology and age model for Son Kul core SK07, as based on 11 AMS 14C dates. The chronology is based on the combination of cores SK07 and SK04. See also Table 1 and text for details.

magnetic susceptibility is a measure of the extent to which sediments are magnetised when subjected to a low magnetic field. Changes in low field MS is, therefore, regarded as a proxy for tracing variations in detrital influx (of aeolian of terrestrial origin) to the lake (Fig. 3), although biogenically magnetic particles (eg. magnetite, greigite) formed in-situ within the sediment must also be considered. Indeed, the precipitation of biomagnetite in sediments has been documented in several lake systems (Peck et al., 1996; Demory et al., 2005). 3.2. Age model Fourteen AMS (Accelerator Mass Spectrometry) 14C dates were obtained on cores SK07 and SK04 (Table 1). The correlation between cores SK07 and SK04 was performed by matching laminations, using photographs and physical properties (MS) for the upper 90 cm of core SK07 (Supplementary Fig. A). This correlation

provides an age of 1952  37 cal. BP at the topmost part of core SK07, 2864  51 cal. BP at 30 cm, 4108  165 cal. BP at 67 cm, and 4959  81 cal. BP at 86 cm in core SK07 (Supplementary Fig. A). It should be noted that ca 60 cm of sediments are missing at the top of core SK07, due to the fact that no short corer could be used in the field. Hence the past ca 2000 years are missing from core SK07. For the upper part of core SK04, AMS radiocarbon ages were determined using bulk organic matter. For each sample, AMS 14C dating was performed using between 0.2 and 1.0 mg of pure extracted carbon. To estimate the reservoir effect in Son Kul sediments, two 14 C dates were generated at a similar stratigraphic level (i.e., at about the same depth) on different samples from core SK07: one 14 C date was generated from bulk sediment at 113 cm (6257  23 cal. BP) whereas another date was made on a leaf at 113 cm (6225  26 cal. BP) (Table 1). Both dates are remarkably similar (within error) indicating that the reservoir effect in Son Kul is negligible at least during the mid- to late-Holocene (i.e., the past

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Fig. 3. Magnetic susceptibility and palynofacies data. The chronology is based on the combination of cores SK07 and SK04 (see also Table 1). Ages are given in cal. yr BP. For lithology, see Fig. 2.

ca 6500 years). Radiocarbon ages were converted to calibrated ages (cal. BP) using the program CALIB.6.1.0 and the IntCal09 calibration curve (Reimer et al., 2009). According to our age model for core SK07, the average sedimentation rate is ca 0.24 mm yr1 although there is some variability downcore with values ranging from 0.1 to 0.6 mm yr1 (Table 1). 3.3. Pollen and palynofacies analyses Sediment samples were collected about every 5 cm from core SK07, which provides a time resolution of ca 175 years. A total of 36 samples, each consisting of about ten grams of bulk sediment, were treated for pollen analysis using the method of Faegri and Iversen (1989). Sediment samples were treated with cold HCl (35%) and cold HF (70%) to remove carbonates and silicates. Denser particles were separated from the organic residue using ZnCl2 (density ¼ 2.0). Residues were filtered through a 150-mm nylon sieve to eliminate coarser particles including organic macroremains. Palynomorphs were further concentrated using a 10-mm nylon sieve. Palynological residues were homogenised and mounted onto microscopic slides using glycerol. A transmitting light microscope using 400 and 1000 magnifications was used for pollen identification. Pollen and spore identification was performed using pollen atlases (Reille, 1999). In combination with pollen analyses, a detailed analysis of the sedimentary organic matter (or palynofacies) was conducted on core SK07 using transmitted light microscopy (Tyson, 1995). Palynofacies analysis involves the identification of palynomorphs, plant

debris and amorphous components, their relative abundance and preservation state (Combaz, 1964; 1980). The palynofacies analysis aims to decipher the different organic matter components and the depositional conditions in the lake, as well as to identify changes in redox conditions in core SK07. It reflects, therefore, both the physical and chemical lake properties and the environmental conditions in the direct vicinity of the lake and in the catchment area. The palynomorphs were split into six main groups of constituents: (I) pollen (bisaccate and non-saccate pollens) and spores; (II) Pediastrum boryanum; (III) Botryococcus braunii; (IV) other algae; (V) phytoclasts: all opaque (e.g. inertinite) to translucent (e.g. cutinite) land-plant fragments (thus excluding algal macrophytes) and (VI) Amorphous Organic Matter (or AOM) (Tyson, 1995; Traverse, 2008) including all particulate organic components that appear structureless at the scale of light microscopy (Tyson, 1995). Each sample analysed for pollen counting was then also screened for variations of the palynofacies in core SK07. 3.4. Climate reconstruction and derivation of quantitative climate data For the reconstruction of climatic signals, the Modern Analogue Vegetation type method (or MAV) described by Klotz and Pross (1999) and Klotz et al. (2003) was used, which numerically deduces palaeoclimate values from fossil pollen floras. The basic principle of the MAV relies on the best-fit selection of modern pollen surface samples (with known climate parameters) representing analogues of fossil pollen floras (for

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Table 1  Radiocarbon Laboratory (Poland). Radiocarbon ages were converted to calibrated (cal.) ages Radiocarbon dates for core SK07 (Son Kul). AMS 14C ages were measured at Poznan using the IntCal09 calibration curve (program CALIB Rev. 6.0.1.). They indicate values with 2 standard deviation errors (95% of confidence level). Lab. ID

Composite depth (cm)

Sampling interval

Dated material

AMS

14

a

0.5 30 67 86 106 109.2 110 113 113 121 132 133 143 151

Son Son Son Son Son Son Son Son Son Son Son Son Son Son

TOC TOC TOC TOC TOC TOC TOC Leaf TOC TOC TOC TOC TOC TOC

2000 2770 3750 4730 5270 5240 5250 5460 5410 6200 6330 6790 6880 7570

             

Poz-26363 Poz-26364 Poz-26366 a Poz-26367 a Poz-26371 Poz-26368 Poz-26375 Poz-37469 a Poz-37470 a Poz-26372 a Poz-26373 a Poz-26370 a Poz-37471 a Poz-26374 a a

a

Kul Kul Kul Kul Kul Kul Kul Kul Kul Kul Kul Kul Kul Kul

4A 4A 4A 4A 7A 4A 7A 7A 7A 7A 7A 4A 7A 7A

0e75 0e1 cm 0e75 30e31 cm 75e150 0e1 cm 75e150 20e21 cm 120e180 0e1 cm 75e150 40e41 cm 120e180 5 cm 120e180 cm 7.2 cme8.6 cm 120e180 cm 7.2 cme8.6 cm 120e180 15.5e16.5 cm 120e180 26.5e27.5 cm 75e150 65e66 cm 120e180 cm 37.5e38.5 cm 120e180 45.5e46.3 cm

C age (yr BP) 30 40 35 40 40 40 40 40 40 50 50 40 50 50

Calendar year (yr BP) 1950 2865 4110 4960 5980 6100 6030 6225 6260 7095 7270 7625 7750 8370

             

35 50 65 80 45 60 60 25 25 70 60 30 50 50

Sedimentation rate (mm/year) 0.33 0.33 0.29 0.23 0.20 e 0.25 0.25 0.25 0.10 0.06 0.06 0.23 0.13

Denotes the date used to establish the age-depth model in Fig. 4.

which climate parameters are to be estimated), taking into account the composition and relative abundances of the participating taxa. Whereas the consideration of relative abundances allows selecting modern samples in terms of a general equivalency, the additional consideration of vegetation types allows selection of only equivalent combinations of dominant and associated plants. These vegetation types represent closer representatives of regional palaeoclimate and palaeoecologic conditions than only proportions and thus lead to a better choice of modern analogues, also reducing the problem of taphonomically altered fossil pollen floras. In practice, all pollen floras in the modern data set of 2653 surface samples in Europe and adjacent Asia (Peyron et al., 1998; Davis et al., 2013) and all modern and fossil pollen floras of the Son Kul record have been identified in terms of vegetation types. These vegetation types are represented by field observed major modern dominant and associated plant combinations in Europe and adjacent Asia, reflecting the major ecological environments throughout the sampling area. The bestfit calculation procedure uses squared log-transformed Euclidean distance measurements, calculating the equivalence (or in a strong sense the dissimilarity) between the fossil samples and the modern surface samples (on the basis of 43 taxa considered). For the purpose of having additional information about the reliability of our reconstruction for the fossil pollen samples, three modern surface samples were collected in the summer of 2012 in the Son Kul basin (see Fig. 1). We further calculated one-sample t-tests in order to verify whether the reconstructed quantitative climatic parameters (mean annual temperature or MAT; mean temperature for the coldest month or MTC; mean temperature for the warmest month or MTW; mean annual precipitation or MAP) were similar to actual climatic values. Nevertheless, as a matter of the scarcity of suitable modern analogues for fossil pollen assemblages especially older than 4000 cal. BP, the reconstructed climate development occasionally fluctuates, but visibly yields a general trend yet (see Chapter 4.4). This trend, however, is best quantitatively represented when applying an index of seasonality (or ranged index), which basically depicts the variation in seasonality amplitude at the regional scale based on the quantitative climatic data. This index allows continuous adjustment of the magnitude of the descriptors. The method of ranging, proposed by Sneath and Sokal (1973), reduces values of a variable to the interval [0,1] by first subtracting the minimum observed for each variable and then dividing by the range:

y0i ¼ ðyi  yminÞ=ðymax  yminÞ; where y ¼ (mean temperature for the warmest month, or MTW)(mean temperature for the coldest month, or MTC).

4. Results and interpretation 4.1. Lithology, palynofacies and magnetic susceptibility Sediments from core SK07 (Fig. 2) consist of dark-brownish organic muds interbedded with aragonitic-rich layers in the lower part of core SK07 and greyish clayey to silty calcareous muds in the top half of the core. The sediments, which are finely laminated, contain material of various origins (terrigenous, biogenic and chemogenic) and size (from clays to fine silts with mollusc shell fragments). Fig. 3 shows evidence for a good correspondence between the magnetic susceptibility (MS) record and variations in plant cuticle (¼phytoclast) abundances. The occurrence of phytoclasts in Son Kul sediments most likely stems from detrital influxes of vascular plant debris from the near shore (or from the direct vicinity of the lake) linked to weathering in the catchment area. Therefore the correlation between MS and the phytoclasts is regarded as a proxy for tracking changes in terrestrial (fluvial) input into the lake. Based on the major changes in palynofacies and MS, we describe the main changes in environmental conditions in core SK07, which mostly coincide with the different lithological facies. Lithozone A (152e87 cm; ca 8350e5000 cal. BP is partly disturbed, probably due to cryoturbation or regional tectonic activity (Fig. 2). Between 152 and 122 cm (lithozone A1), the sediments are mostly greyish calcareous mud layers interbedded with dark greyish calcareous mud layers, whereas a more homogeneous mud interval (with few laminations) is found between 137 and 122 cm. In this interval the palynofacies is dominated by AOM, however with a prominent increase in “undifferentiated algae” at 131 cm. MS decreases although values are amongst the highest downcore, with a high peak (five-fold higher than in the rest of the core) between 149 and 146 cm (8100e7850 cal. BP). It is indicative of a prominent increase of detrital inputs in the lake, suggesting an abrupt change in environmental conditions at that time, coeval to the 8200 cal. BP event widely documented in Europe and South Asia (e.g. Alley and Agustsdottir, 2005; Rohling and Pälike, 2005) though it has never been reported from Central Asia yet. However, taken into account the limit of our age model, it appears that this

M. Mathis et al. / Quaternary Science Reviews 89 (2014) 169e185 Fig. 4. Detailed pollen diagram for Son Kul core SK07, and modern surface pollen samples (e.g., SK1, SK4, SK5) collected from mosses in the summer of 2012 in the Son Kul basin (see Fig. 1 for location). Light-shadings correspond to 5 exaggeration percentages. Pollen zones P1eP3 were established based on a cluster analysis complemented by a K-means clustering using the Past software. The chronology is based on the combination of cores SK07 and SK04.

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climatic signature may have significantly affected the hydrological cycle in central Tien Shan judging from enhanced weathering and soil erosion in the lake catchment of Son Kul. Lithozone A2 [122e87 cm] reveals a pattern of extremely finely laminated calcareous muds interbedded with white aragonite-rich (further confirmed by XRD and SEM analyses; not shown) and dark organic-rich layers rich in ostracods. Recurrent paper-sheet-like layers, representing 1e3 cm-thick filamentous terrestrial mats made of plant debris are most common in this unit. This zone also sees the conspicuous dominance of AOM in the palynofacies (>60%). B. braunii is present in very low abundances (or even absent between 115 and 103 cm). MS is generally low in this zone. Lithozone B (87e63 cm; ca 5000e4000 cal. BP) is characterised by the dominance of greyish to dark-greyish calcareous mud layers. Ostracods are relatively enriched in the greyish layers. Mollusc shells are absent in this unit. Sediments are progressively lighter and less laminated, with a dominance of minerogenic successions as compared to lithozone A2, in which organic and aragonitic successions predominate. A mud interval with rare laminations (87e73 cm) evolves towards a more homogeneous clayey interval (73e63 cm). Plant debris are very frequent between 87 cm and 73 cm. Accordingly, this zone is characterised by the highest percentages of phytoclasts. This increase matches fairly well with higher MS values, suggesting that the removal of soils and subsequent detrital inputs are enhanced in this zone. Lithozone C (63e5 cm; 4000e1950 cal. BP) is mostly composed of greyish and yellowish clayey to silty calcareous muds forming 0.5e5 cm thick beds (Lithozone C2), while darker greyish calcareous mud layers characterize the interval 63e37 cm (Lithozone C1). Ostracods and diatoms are evenly distributed, while molluscs are found preferentially in the greyish beds in Lithozone C2. This zone is characterised by the occurrence of the chlorophycean algae P. boryanum. Its presence, within an increasing trend towards the top of core SK07, suggests an important change in lake water properties, probably linked to eutrophication in the surface waters (Jankovská and Komárek, 2000). A coeval presence of P. Boryanum was reported in Lake Wulungu (Liu et al., 2008) and linked to the possible first influences of human activity in the region. This occurrence also matches a prominent decrease in pollen concentration at Son Kul (see Fig. 4), which suggests a substantial environmental change at ca 4000 cal. BP. This hypothesis should, however, be validated by other proxy data. Abundances in B. braunii increase (attaining 75% at 37 cm) and then decrease towards the top of the core (30%). Contents of AOM increase, although the upper part of this zone (37 cm; ca 3100 cal. BP) shows a marked and abrupt decrease in AOM. This may indicate an interval with a wellmixed water column that was oxygenated down to the lake bottom since AOM is not preserved under oxic conditions (Tyson, 1995). We observe above a return to high abundances in AOM (averaging 50%), suggesting that the lake was again stratified between ca 3100 and 1950 cal. BP. Abundances in plant cuticles and MS data are very low in the uppermost 37 cm, suggesting that soil weathering (and thus detrital input into the lake) was minimal at that time. Sediments from this interval are very rich in juvenile specimens of the bivalve species Pisidium sp, which occur for the first time in this zone (and especially in Lithozone C2). 4.2. Pollen analyses Pollens are very well preserved in core SK07 and abundant in all samples. 47 taxa were identified in the samples and 300e880 pollens have been counted for each sample. Pollen concentration was estimated using Cour’s method (Cour, 1974). Pollen concentration varies from 5000 to ca 58,000 grains g1 (Fig. 4). Three ecostratigraphic pollen zones (P1eP3) have been distinguished

based on the major changes in pollen assemblages (Fig. 4). The results are shown in a synthetic pollen diagram, in which pollen taxa are depicted in 21 different groups based on their ecological preferences, in order to decipher the most prominent changes in vegetation dynamics. These ecostratigraphical pollen zones were further validated by applying a cluster analysis complemented by a K-means clustering, using the “Past” software. Prior to the cluster analysis, the pollen data were previously converted using an angular transformation to normalize the distribution, and processed with a BrayeCurtis similarity index (Faith et al., 1987). Pollen zone P1 (152e115 cm; 8350e6500 cal. BP): This zone is characterised by a decrease in Artemisia abundances from 50% to 33% between 152 and 115 cm. Conversely, Chenopodiaceae abundances reach a maximum at 130 cm with more than 20% of total pollen values. Percentages of trees are the highest in this zone, with Juniperus attaining 9% and Picea 4%. Abundances in Ephedra (2%) and Juniperus (9%) are also higher in this zone. Pollen concentration is, on average, the highest in this zone. Pollen zone P2 (115e75 cm; 6500e4500 cal. BP): This zone shows a conspicuous increase in abundances of Artemisia (from 115 to 99 cm; 50%). Thalictrum attains a maximum at the base of this zone, averaging 2.8%. Chenopodiaceae and Poaceae remain present at high values, at ca 16%. Juniperus decreases dramatically from the previous zone from 5.2 to 1.7%. Whereas Ephedra also decreases, Caryophyllaceae have a marked contribution in this zone, with percentages fluctuating around 2.5e3%. A peak in abundance in Asteraceae occurs at 95 cm (4.3%). Pollen zone P3 (75e5 cm; 4500e1950 cal. BP): This zone is characterised by a decrease in pollen concentrations (from 2000e 3000 to 500 pollen gr1) along with tree abundances, and the large supremacy of Artemisia (w60e70%), although lower values occur at 37 cm. Abundances in Chenopodiaceae are generally constant, but increase gradually until 37 cm. Poaceae decline significantly from P2 to P3 (7% on average from P1/P2 to P3), and continue decreasing in P3. Juniperus sp. is still present, but with decreasing values at the top of P3. Ephedra reaches a minimum in P3 with, on average, 0.18% in the upper part of P3 (from 40 to 0 cm). It is worth noting that a major change is recorded at 75 cm (ca 4500 cal. BP) as based on significantly lower contributions of all herbaceous taxa, except Artemisia. 4.3. Vegetation and environmental patterns derived from the pollen data Herbs, predominant in all samples (Fig. 4), are characterised by an overwhelming presence of Artemisia that accounts for 33e70%, and on average 55% of the total pollen sum, while pollens of Chenopodiaceae (mean: 15.6%) and Poaceae (mean: 12.3%) are also common. The sum of further non-arboreal taxa is usually between 4% and 14%. Consistently occurring taxa include non-Artemisia Asteraceae, Rosaceae, Ranunculaceae, Brassicaceae, Caryophyllaceae, Apiaceae, Rumex and Thalictrum. The arboreal pollen content (ranging between 1.5% and 12%) consists mainly of Juniperus, Betula, Pinus and Abies; other tree taxa never reach 1%. Pollen data therefore suggest that open vegetation types with steppe elements (shrubs, herbs) were always predominant in the Son Kul region from ca 8350 to 1950 cal BP. This implies that xeric conditions prevailed in the region (not restricted to the Son Kul basin only, but more at the regional scale in central Tien Shan and especially from the surrounding areas including the low-lands and the adjacent valleys), interrupted by periods of enhanced moisture. Juniperus is moderately under-represented in the pollen rain and values <5% are reported in areas where Juniperus is abundant in the regional vegetation (Beer et al., 2007a). Therefore abundances of Juniperus higher than 5% in core SK07 suggest favourable conditions

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Table 2 Abundances (%) of selected pollen taxa for the three modern surface samples (e.g., SK1, SK4, SK5) taken in July 2012 in the Son Kul basin (see Fig. 1c). Reconstructed quantitative climate parameters: mean annual temperature (MAT in
C), mean temperature of the coldest month (MTC in
C), mean temperature of the warmest month (MTW in
C) and mean annual precipitation (MAP in mm yr1). We also calculated one-sample t-tests in order to verify whether the reconstructed climatic parameters are similar to actual climatic values (actual climatic data are generated by New_LocClim_1.1 for mean annual temperature and precipitation). Samples

Artemisia

Chenopodiaceae

Ephedra

Cyperaceae

Poaceae

Asteraceae

Other herbs

Juniperus sp.

Deciduous trees

Conifers

SK1 SK4 SK5

10.7 11.2 24.2

10.7 3.7 11.2

1.0 0.0 0.3

5.8 41.0 2.3

6.8 9.0 12.4

16.5 10.6 8.9

39.8 13.8 23.1

1.9 0.5 2.9

2.9 4.3 10.1

3.9 4.3 4.6

Samples

MAT (
C)

MTC (
C)

MTW (
C)

MAP (mm)

T-test MAT

T-test MTC

T-test MTW

T-test MAP

SK1 SK4 SK5

0.7 3.2 5.9

12.8 9.9 8.7

14.3 15.9 19.3

693.2 547.0 636.7

0.78 0.1 0.2

0.37 0.005 0.06

0.75 0.05 0.38

0.065 0.002 0.12

for the development of Juniper trees during the early- to middleHolocene, implying a higher tree line and probably higher temperatures. Higher moisture levels are also inferred for that time from increased contents of Betula and Poaceae, suggesting that optimal climate conditions most probably occurred between 8350 and 4500 cal. BP in central Tien Shan. However, we can not exclude the possibility that pollen data in core SK07 may contain inputs not only from local and regional plants, but also from vegetation outside the region (Pamir, Kunlun and Karakoram Mountains), as shown in an atmospheric pollen study (Cour et al., 1999). Longdistance pollen transport, however, accounts for less than 0.5% in many cases. Moreover, if the southern monsoon influence is thought to be of some relevance in the Kunlun Mountains, such an influence is much less plausible in the Tien Shan and the Pamir due to the distances under consideration, even in the Karakoram (Cour et al., 1999). This is in line with grain size analysis results from Son Kul, which reveal that the influence of far-distance aeolian inputs is considerably low (if any) at Son Kul (H. von Suchodoletz; personal communication). Contributions of river pollen also should not be dismissed, although we currently have no pertinent results in this regard. However, the catchment area of Son Kul is small (w1450 km2), and the contribution of riverine pollen input should be regarded as a local, rather than regional, signal. No significant

shift in pollen concentration due to drastic changes in sedimentation rates was observed here (see also Table 1 and Fig. 4). Studies of atmospheric pollen composition indicate that both Artemisia and Chenopodiaceae are high pollen producers (Van Campo et al., 1996; Cour et al., 1999). At present in Central Asia, Artemisia and Chenopodiaceae are characteristic elements of steppe, semi-desert and desert environments (Tarasov et al., 1998a,b). Artemisia also has the greatest abundances in modern pollen spectra from montane and alpine steppe vegetation, whereas Chenopodiaceae values are highest in sites corresponding to temperate and alpine deserts (Herzschuh, 2007). From field investigations in July 2012 (Table 2), we report that Artemisia is sparse and restricted to specific places (e.g. freshly open paths and gullies, rock falls) within the Son Kul basin. The vegetation is not steppic around the lake, but rather corresponds to an alpine meadow. In addition, Chenopodiaceae appear to be absent in the watershed today. During the same field expedition, 3 samples of mosses (e.g., SK1, SK4, SK5) were collected downslope, in the meadow, for the study of modern pollen rain (see also Fig. 1c). These results show that the surrounding vegetation is over-represented in the surface samples (Table 2), even if a significant presence of Artemisia and Chenopodiaceae is also found in the pollen data (between 15 and 35% of the pollen sum with on average 8.9% of Chenopodiaceae and 15.4% of Artemisia; Fig. 4).

Fig. 5. Reconstructed climatic development based on the quantitative pollen data from Son Kul core SK07: (a) the arboreal pollen sum (or AP sum, %); (b) the aridity pollen index [ratio of (Artemisia plus Chenopodiaceae) to Poaceae] to distinguish dry steppe (values >5) from moist-meadow steppe and forest vegetation (values <5) (Fowell et al., 2003); (c) the ranged index of seasonality from MAV reconstruction, which basically depicts the variation in seasonality amplitude at the regional scale (adapted from Sneath and Sokal (1973)).

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These results are in accordance with modern pollen data from surface samples in other central Kyrgyzstan sites, especially at Kichikol located 200 km southwards from Son Kul (Artemisia: 55%; Beer et al., 2007a), and in Holocene sediments from Issyk Kul (Artemisia: 60%; Giralt et al., 2004; pp. 272e273) and Kichikol (Artemisia: 15e60%; Beer et al., 2007b). It was also reported that pollen assemblages in surface dust flux samples from northwest Tibet (including the Kunlun Mountains) were dominated by Artemisia and Chenopodiaceae, with peak contributions in autumn (Artemisia) and summer (Chenopodiaceae). Also, as shown by theoretical models of pollen transport, the proportion of extra regional pollen deposited in lake sediments increases with lake size (Sugita, 1994; Jackson and Lyford, 1999). Accordingly, pollen spectra from large lakes (>0.25 km2) usually have a higher contribution of long-distance transported pollen than those of small lakes from the same region. Local provenances are thus diluted by the more regional influx of airborne pollens, which mostly originate from the adjacent lower valleys (e.g. lowlands) where Artemisia and Chenopodiaceae flourish. Therefore, we emphasize that the palaeoecological signal contained in the pollen data must be interpreted at the regional scale since it is largely representative of regional vegetation. From 8350 to 6500 cal. BP abundances in Artemisia decreased (from ca 50% to 30%), compared to the uppermost sediments (P2 and P1; 50e70%). This observation matches with the coeval, significant decrease in tree taxa percentages (Juniperus, Betula, Salix) and Poaceae; these results imply that regional climatic conditions were most probably moister during the early- to mid-Holocene in central Tien Shan. However, the onset of drier conditions is evidenced at ca 6300 cal. BP judging from a steady, continuous increase in Artemisia and high content of Chenopodiaceae, along with persistently low tree abundances and a conspicuous decrease in regional pollen concentrations. One may therefore suggest that this decrease in pollen production is associated with an increase in the regional, short-distance transport component, as it has been reported elsewhere (Tarasov et al., 1998b; Cour et al., 1999; Herzschuh, 2006). This observation is consistent with other pollen records from alpine lakes in the Tien Shan. A similar increasing trend in Artemisia was indeed reported in Lake Kichikol (south Kyrgystan) between 6300 and 4000 cal. BP, together with a coveal decrease in abundances of Poaceae (Beer et al., 2007b). A comparable pattern is also obvious for other tree and plant components, as for Betula, Ephedra and Chenopodiaceae, suggesting that similar mechanisms controlled the vegetation dynamics in north and south Kyrgystan during the middle to late Holocene. The traditional interpretation of pollen records is usually based on the presence and abundance of selected indicator taxa (e.g. Birks and Birks, 1980). Semi-quantitative climatic and vegetation information in Central Asia is often obtained through the calculation of pollen ratios (e.g., Herzschuh, 2007). Fowell et al. (2003) developed an aridity pollen index [ratio of (Artemisia plus Chenopodiaceae) to Poaceae] to distinguish dry steppe (values >5) from moist-meadow steppe and forest vegetation (values <5). They assumed that Artemisia and Chenopodiaceae characterise dry environments, whereas Poaceae are more abundant in comparatively moist settings (such as meadows). For similar consideration, the arboreal pollen sum (or AP) is widely used for palaeoclimatic interpretation to document changes in the landscape openness of forest steppe and montane/ sub-alpine environments. It has a significant positive correlation with MAP (r2 ¼ 0.44) in modern surface samples (whereas the link with summer temperature is more complex; Herzschuh, 2007), which supports applicability of the AP sum as a semi-quantitative indicator for regional precipitation changes (Fig. 5). The aridity pollen index and AP sum applied to the core SK07 pollen data reveal moisture changes over a large gradient at the regional scale (Fig. 5). The most significant change in regional

humidity likely occurred abruptly during 5000e4500 cal. BP, as a transition between a moist-meadow steppe stemming from high values of AP (up to 6%) with low values of the aridity pollen index (<5) to a drier environment (steppe desert vegetation) with high values of the aridity index (>5; up to 12 at ca 2000 cal. BP) and persistently decreasing AP content (<2%). It is noteworthy that this shift corresponds to a drastic change in lithology (end of the laminated interval; Lithozone A), which most likely documents the onset of lower lake levels along with a well-mixed and oxygenated (unstratified) lake system as can be observed today. Our results are in good concordance with other records documenting a humid climate during the early to mid-Holocene across western Central Asia and the Xinjiang region (which is the nearest Chinese province to the east of Son Kul), as discussed below. 4.4. Climate reconstruction The reconstructed climate parameters for modern surface samples (collected in 2012 in the direct vicinity of Son Kul; see Table 2) are in accordance with present-day climatic conditions in Central Asia as inferred from an Inverse Distance Weighted Averaging (or IDWA) applied using the software New_locClim 1.1. Accordingly, mean annual temperature (MAT) and mean annual precipitation (MAP) reconstructed from the surface pollen samples range between ca 1
and 6
C (present-day value is 1.5
C) and 550e700 mm year1, respectively (present-day value is 550 mm year1 as determined from the New_locClim software). For the purpose of testing statistical similarity between our modern surface samples reconstructions and present-day climate conditions, we calculated one-sample t-tests (using IBMSPSS 19). Data basis is, that for every surface sample, the reconstruction with the MAV method yields 10 best-fit surface samples (with their climatic properties) from its underlying database. Based on a 95%-confidence interval, the results show that the reconstructed and present-day values are fairly similar (except for the sample SK4 where MTC and MAP show significant differences from the present-day values). Hence this observation supports the robustness of our data, and validates the range of values obtained from our ranged index of seasonality (Fig. 5; see also Section 3.4). Between 8350 and 5000 cal. BP, seasonality was weak as the annual thermal amplitude was low and the smoothed index never exceeds 0.5. In contrast, the seasonality greatly intensified after 5000 cal. BP, with values persistently fluctuating above 0.8. Similar results were obtained by applying the Gorczinsky index (a continentality index in which the annual difference in temperature is divided by the sin of the latitude; Gorczinsky, 1920), with a clear pattern of increased continentality between 8350 and 5000 cal. BP (not shown). A marked seasonality is not surprising in a continental context, but a diminution of the continentality before 5000 cal. BP is indicative of a less continental climate (most probably associated with a change in seasonality). As emphasised earlier, the reconstructed climatic conditions in this study should be regarded at the regional, rather than at a local, scale in central Tien Shan. Also, the reconstruction of climatic parameters is somewhat hampered by the scarcity of existing modern regional pollen datasets (due to the lack of dust flux and/or pollen-trap samples regionally). 5. Discussion 5.1. Holocene environmental and climatic change in the Tien Shan and arid Central Asia 5.1.1. Regional pattern in Kyrgyzstan (Central Asia) Based on the combination of pollen, palynofacies and MS data, we distinguished variations in temperature and humidity in the Son Kul watershed and its adjacent environments.

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Fig. 6. Comparison between our pollen-based vegetation record from Son Kul and other regional paleoenvironmental records in Central Asia: (a) This study, Lake Son Kul; (b) Lake Issyk Kul (Ricketts et al., 2001); (c) Kichikol lake (Beer et al., 2007b); (d) Karakol Lake (Mischke et al., 2010); (e) Yili valley (Li et al., 2011); (f) Aibi lake (Wu, 1995; Wu et al., 1996 in; Feng et al., 2006); (g) Manas Lake (Rhodes et al., 1996); (h) Wulungu lake (Yang and Wang, 1994 in; Feng et al., 2006); (i) Bosten Lake (Wünnemann et al., 2006); (j) Balikun Lake (Tao et al., 2010; An et al., 2012); (k) Hoton Nur Lake (Rudaya et al., 2009); (l) Grusha Lake (Blyakharchuk et al., 2007); (m) the Guliya ice core (Thompson et al., 1997); (n) Lake Sumxi lake (Gasse et al., 1991); (o) Bangong lake (Gasse et al., 1996); (p) Selin Co lake (Gu et al., 1993 in Feng et al. (2006)).

Environmental reconstruction based on the pollen record implies that the distribution of biotic communities differed between 8350e 5000 and 5000e1950 cal. BP. The vegetation evolved from an alpine meadow dominated by herb assemblages (e.g. Poaceae) to an alpine steppe-meadow characterised by a decrease in Poaceae and the rarefication of trees in the catchment area. Pollen data indicate that Son Kul experienced relatively wet conditions, with high effective moisture during the early- to middle Holocene, whereas the onset of a regional aridification (along with an increasing continentality) is recorded during the middle to late Holocene, with two major shifts at 6500 cal. BP and 5000e4500 cal. BP. In order to clearly identify whether the paleoenvironmental record obtained in Son Kul represents a local, or regional signal, we compared our results with other palaeoclimate archives from western and arid Central Asia. Other proxy records from Central Asia are in line with our reconstruction, demonstrating a regionally consistent pattern of pronounced environmental and climatic change at ca 6000e5000 cal. BP. Lake Issyk Kul (Fig. 6), which is the nearest lake in the northern Tien Shan, evolved from an open, freshwater, well-mixed lake between 8300 and 6900 cal. BP to a closed, more saline, less well-mixed lake between 6900 and 4900 cal. BP (Ricketts et al., 2001). Based on the stable isotopic and trace element compositions of ostracods in lake sediments, the authors proposed that the reduction of humidity levels at ca 6900 cal. BP may have been linked with the intensification of the Siberian High Pressure, thereby blocking the penetration of westerly air masses into Central Asia (Fig. 6). Conversely, the wettest period was attained between 5100 and 4000 cal. BP in lake Kichikol, south Kyrgyzstan (Beer et al., 2007b).

5.1.2. Central arid Asia including northwestern China and Mongolia Farther to the east at Yili valley, Li et al. (2011) have reported a humid period during 6500e5200 cal. BP and a transition from a temperate steppe to desert-steppe vegetation at that time, based on pollen data (Fig. 6). This is further consistent with paleohydrological conditions in the Xingjiang region, in the northwestern most part of arid and semi-arid China. Most records show that this area was wet after 8000e7000 cal. BP, whereas the humidity decreased following 6000e5000 cal. BP. At Wulungu Lake, high lake levels were attained between 7000 and 5000 cal. BP under a warm and wet climate (Feng et al., 2006), whereas the period 5000e3000 cal. BP was characterised by dry conditions. The pollen record from the mountainous Lake Aibi indicates that a warm and wet climate dominated the middle Holocene (8000e3500 cal. BP), but a mild and dry climate was inferred for the late Holocene. A similar trend was observed at Lake Manas, with a waning of effective moisture around 6000 cal. BP. The reconstruction of vegetation history from Manas Lake sediments revealed that a steppe environment prevailed during the period 9000e4000 cal. BP, but later deteriorated to a steppe-desert landscape at ca 4000 cal. BP (Rhodes et al., 1996). This is also consistent with the Lake Bosten (Eastern Tien Shan) palaeoenvironmental record, which documents a period of increased precipitation between 8300 and 4300 cal. BP, and a trend towards aridity since about 4300 cal. BP (Wünnemann et al., 2006) (Fig. 6), though a relatively dry climate was reported based on the A/C ratio from Huang et al. (2009) between 8000 and 6000 cal. BP. More to the east, Lake Balikun experienced humid conditions from 7900 to 4300 cal. BP, with the development of a steppe vegetation and

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several patch-birch woodlands, before more arid conditions were established shortly after 4300 cal. BP (Tao et al., 2010; An et al., 2012) (Fig. 6). Hence all sites in Xinjiang have recorded more severe and drier climates after 5000 cal. BP. Likewise, a shift from boreal woodlands to a steppic vegetation was concurrently documented at ca 4900 cal. BP from Lake Hoton Nur sediments (northwestern Mongolia), with a drastic decrease (>200 mm) in annual precipitation based on pollen signals (Rudaya et al., 2009). Further north, a significant decrease of the trees was reported at Grusha Lake at ca 5500 cal. BP (Blyakharchuk et al., 2007), though only one 14C date is available in that record. This is also in good agreement with records from northern Eurasia documenting that the optimal interval for boreal forest extension was reached at ca 7000 cal. BP, whereas a northwards retreat was recorded after 4000 cal. BP in arctic Siberia (Kremenetski et al., 1998; MacDonald et al., 2000). 5.1.3. Tibetan Plateau Many lacustrine and ice-core records from the northwestern Tibetan Plateau further support the hypothesis of a warm and wet climate from 10,000 to 5000e4000 cal. BP, followed by a fluctuating but dry stage between 5000 and 4000 and 3000 cal. BP. Studies at Bangong Lake showed that high lake levels prevailed during the period 8300e3300 cal. BP, before declining since 3300 cal. BP (Gasse et al., 1996). At nearby Sumxi Lake, reconstructed vegetation changes implied that Artemisia-dominated semi-arid steppe predominated in the lake area until 6500 cal. BP, while a climate deterioration commenced after that time (Gasse et al., 1991). The Guliya ice core also documents a decrease in regional humidity around 6000 cal. BP (Thompson et al., 1997). Additionally, a paleoenvironmental record from the Selin Lake suggests that optimal climatic conditions occurred between 8400 and 5500 cal. BP (or during 7500e6000 cal. BP based on pollen data) before the onset of a cooler and drier climate at 5500 cal. BP and even more drastic conditions from 4000 cal. BP onwards (Feng et al., 2006 and references therein). Hence all proxy data in central and arid Central Asia, including northwestern China and Mongolia further to the east (Yang and Scuderi, 2010; Yang et al., 2011, 2013), confirm that wet conditions prevailed regionally before 4500 cal. BP, whereas a reduced moisture supply is inferred thereafter (Fig. 6). Interestingly, this period also matches the time at which both Aral Sea and Caspian Sea lake levels were recorded to fall in western Central Asia (Rychagov, 1997; Boomer et al., 2000) after 5000 cal. BP, as also evidenced in cores TM and GS05/CP14 (Leroy et al., 2013a,b; Leroy et al., 2007; see also Fig. 1). Climate conditions were coevally optimal at Lake Van (eastern Anatolia, Turkey) until 4000 cal. BP, with a maximal extension of steppe-forests at about 6200 cal. BP, whereas the onset of aridity was established at 4000 cal. BP (Wick et al., 2003). 5.2. Holocene climate changes in western Central Asia: towards a connection with the eastern Mediterranean region? The pattern of climatic change across the Tien Shan and western Central Asia for the time interval 8350e2000 cal. BP is regionally consistent, and points to the onset of drier conditions around 5000e4500 cal. BP. However, the history of past moisture availability in western Central Asia is still poorly understood in

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terms of the mechanisms controlling the observed temporal and spatial changes. Climatologically speaking, there are two different parts of Asia with an out-of-phase moisture (precipitation) variability that are responding to global change, i.e., monsoondominated and westerly-dominated arid Central Asia during the Holocene (Chen et al., 2008, 2010a). We evaluate here the influences of these two circulation systems and their underlying mechanistic controls on the regional climatic pattern in central Tien Shan. It has been suggested that climate variability in Central Asia is coupled to variability of the tropical monsoon climate at long distances from the ocean (Schiemann et al., 2007), with moisture transport to the Tibetan Plateau controlled by the Indian Summer Monsoon (ISM) and large-scale circulation patterns (Chen et al., 2012). A significant relationship has been proposed between summer runoff of the Amu Darya in the Pamir Mountains and the intensity of the Indian Summer Monsoon or ISM (Schiemann et al., 2007). However no such relationship exists for the Syr Darya, which originates in the Son Kul area within central Tien Shan. It has been shown that the modern physical mechanism linking Amu Darya runoff and ISM intensity does not stem from spillover precipitation due to the direct penetration of air masses into the Central Asian mountains, but rather to the response of tropospheric temperatures to changes in monsoon intensity (Schiemann et al., 2007), and to Rossby-wave interactions on seasonal precipitation (Barlow et al., 2005). A northern advance of the ISM into the continental interior of Central Asia has also been suggested by proxy and modelling studies at times during the Holocene, and especially for the Mid Holocene Optimum. Based on modelling studies, Braconnot et al. (2007) and other studies (Wanner et al., 2008) proposed a northward shift of the ITCZ by 3e10
in latitude depending on the region, and suggested an increase in mean precipitation on the northern edge of the ITCZ (i.e., approximately at latitudes of lake Son Kul). Yet no climate model has identified a role for the ISM in Central Asian precipitation variability; today this region lies outside the influence of the ISM (Dallmeyer et al., 2013). Instead it was demonstrated that a strong ISM over India coincides with enhanced temperatures in the upper troposphere, with a warm ridge over Central Asia, that would in turn increase stability with respect to moist convection and suppress summer precipitation in Central Asia (Schiemann et al., 2007). On the other hand, strong summer insolation on the Tibetan Plateau results in strong uplift of air masses that enhance subsidence over the lowland areas adjacent to the northern Tibetan Plateau, which increases aridity in the western part of arid Central Asia (Broccoli and Manabe, 1992). This mechanism, however, was recently disputed based on modelling studies, and it is believed that subsidence has small effects on effective moisture in central Asia (Jin et al., 2012). In this regard, stronger monsoon activity over the Tibetan Plateau during the early and mid-Holocene would lead to enhanced aridity and induce a dry climate in Central Asia further to the north (Herzschuh, 2006), as further suggested by modeling experiments using a numerical general circulation model (Bush, 2002; Masson et al., 2009; Dallmeyer et al., 2013). Furthermore, a recent study based on a series of sensitivity experiments using a coupled climate model revealed that no dynamic link exists between humidity in Central Asia and the ISM (Jin et al., 2012).

Fig. 7. Comparison between the Son Kul record in central Tien Shan (western Central Asia) and other palaeohydrological records from the Eastern Mediterranean region and the Near East: (a) Holocene lake-levels changes in the Caspian Sea (Rychagov, 1997). The black and grey bars refer to the Gousan highstand (8400e3900 cal. BP) and the Neocaspian relative highstand (2100e0 cal. BP), respectively (Leroy et al., 2013b); (b) Israeli coastal plain paleosol data (Gvirtzman and Wieder, 2001); (c) a stacked lake isotope record based on d18O values from six lakes in the East Mediterranean (Roberts et al., 2011); (d) pollen-based proxy data from Son Kul (core SK07): the arboreal pollen sum (or AP) and the Fowell’s aridity pollen index indicating wetter/drier conditions in central Tien Shan; (e) the d18O record from Jeita Cave, Israel (Verheyden et al., 2008); (f) the d18O record from Soreq Cave, Israel (Bar-Matthews et al., 1999); (g) the aridity index from the northern Gulf of Aqaba (Arz et al., 2003); (h) the d18O record Gruber from the southeast Aegean Sea, marking enhanced freshwater flux to the eastern Mediterranean Sea (Rohling et al., 2002); (i) the d18O record of Gruber from the North Red Sea (Arz et al., 2003).

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Alternatively, North Atlantic climate variability is believed to have an impact on the hydrological cycle in Central and East Asia (Vandenberghe et al., 2006; Bothe et al., 2011 and references therein). These teleconnections may appear as waves along the westerly wave guide (Fujinami and Yasunari, 2009), and imply linkages between moisture changes in Central Asia and North Atlantic sea-surface temperatures. Modern circulation data show that episodes of extremes and severe dryness are dominated by various upstream standing wave patterns from the North Atlantic to Central Asia (Bothe et al., 2011). At present, precipitation in western Central Asia depends predominantly on the amount of water vapour transported by the mid-latitude westerlies from the North Atlantic Ocean and from inland seas and lakes along the westerly storm paths (Böhner, 2006). Depressions develop over the eastern Mediterranean Sea and subsequently move along a northeast trajectory during late spring and summer bringing moist air to Central Asia (Lioubimtseva et al., 2005; Syed et al., 2006), and as far as the central and eastern Mongolian Plateau (Sato et al., 2007). The strength and trajectory of westerly circulation is therefore likely to be a major control on moisture in Central Asia. Thus we may expect elevated precipitation in Son Kul and western Central Asia when moisture-transporting cyclones are stronger in the eastern Mediterranean region and, if so, we should find a similar humidity pattern in areas influenced by eastward moving storms between 8350 and ca 2000 cal. BP. A warm and wet early Holocene was evidenced by several proxy data in the Levant region (e.g. Frumkin et al., 1994; Rossignol-Strick, 1999; Gvirtzman and Wieder, 2001; Rohling et al., 2002; Arz et al., 2003; Bar-Matthews et al., 2003; Affek et al., 2008; Kotthoff et al., 2008; Develle et al., 2010), which match well with the highest humidity conditions at Son Kul (Fig. 7). This is further consistent with a recent palynological study from the Caspian Sea (cores TM and GS05/CP14; see also Fig. 1) which shows that the highest water levels were attained during the first part of the Holocene (Leroy et al., 2013a,b). Furthermore, a previous study based on data compilation showed that the early Holocene may have been the wettest phase of the last 15,000 years across much of the Levant and the Eastern Mediterranean (Robinson et al., 2006), with a possible >20% increase in rainfall in southern Levant as based on calculations using cave calcite isotopes (Bar-Matthews et al., 2003). Enhanced winter precipitation for the Eastern Mediterranean during the early to mid-Holocene was also inferred from climate reconstructions (Dormoy et al., 2009; Peyron et al., 2011) and climate models (Brayshaw et al., 2011) whereas summer precipitation simultaneously reached a minimum (Dormoy et al., 2009), or was absent (Brayshaw et al., 2011). Such a pattern has been suggested to be consistent with a southward shift in the North Atlantic storm track and an intensification over the Mediterranean (Brayshaw et al., 2010). This period also witnessed higher Lisan Lake and Dead Sea levels (Frumkin et al., 1994; Migowski et al., 2006), and enhanced precipitation as inferred from snail isotope records in Israel (Goodfriend, 1999) and speleothems in Lebanon (Verheyden et al., 2008). A concurrent drastic waning of humidity around 5000e 4500 cal. BP is recorded in central Asia and in marine, lake and speleothem records from the south and Eastern Mediterranean (Fig. 7). The onset of drier conditions at that time corresponds to a sudden increase in d18O in the Jeita and Soreq cave stalagmites that is coeval with higher d18OG.ruber in sediment cores from the Aegean and Red seas, together with a drastic decline of lake levels in the Levant at the regional scale (Fig. 7). Moreover, this trend of aridification is tied to the most drastic Holocene water level decrease in the Caspian and Aral seas, which occurred at about 5000 cal. BP. It has been proposed that this pronounced climatic change occuring at ca 4500 cal. BP may have resulted from a significant

reorganization in seasonality, and a non-linear climate system response to the gradual summer insolation decrease during the mid- to late Holocene transition (Peyron et al., 2011; Magny et al., 2012, 2013). Some studies have implicated the role of the North Atlantic Oscillation (NAO) on the control of humidity patterns in Central Asia during the Holocene (Huang et al., 2009; An et al., 2012). Moreover, a statistical analysis examining the relationship between NAO anomalies and precipitation variations from 57 to 88 hydroclimatic stations in mid-latitudes Central Asia showed that negative NAO anomalies correspond to increased precipitation (Aizen et al., 2001). This is not consistent with the conclusions of Syed et al. (2006) based on data reanalyses, who reported that increased precipitation over Central Asia is favoured during positive NAO conditions when the westerly drift encounters low pressure over Afghanistan and Central Asia, and moisture is replenished from the Caspian Sea. Similarly, the relationship between Middle Eastern rainfall in the Eastern Mediterranean and the phase of the NAO is non-linear (Black, 2012; Roberts et al., 2012), along with a known lack of stationarity in NAO teleconnections, although increased precipitation occurs more frequently when the NAO is in a positive phase (Black, 2012). Therefore, the reconciliation of Holocene climate anomaly patterns at millennial scales in western Central Asia and the Eastern Mediterranean region relying on underlying NAO dynamics at annual to- decadal timescales remains too speculative. Rather, various other atmospheric teleconnection patterns are involved in Central Asia; among them, the two Eurasian wave train features of the Scandinavian and East Atlantic/ western Russia patterns represent the most distinct influences on regional precipitation, as has also been suggested for the Mediterranean area (Magny et al., 2013). Such teleconnections rely on arching atmospheric bridges from the Atlantic over Northern Europe to Central and Eastern Asia. In this regard, Kutiel et al. (2002) define an atmospheric index between the North Sea and Caspian Sea (North Sea-Caspian Sea index or NCPI) that has been shown to have a significant impact on Eastern Mediterranean climate. A change from positive (drier) to negative (wetter) values of the spring NCPI was documented between 1960 and the late 1980s, a time interval that corresponds to a prominent precipitation increase in central Tien Shan, the Pamir and the plains of middle Asia (Aizen et al., 1997). Although no Holocene reconstruction exists for the NCPI, one should investigate whether periods of elevated moisture in central Asia are associated with periods of persistent negative phases of the NCPI. An improved understanding of the role of such linkages between the North Atlantic Ocean and western Central Asia should thus remain of high priority for future investigations. 6. Conclusions 1. A quantitative reconstruction of vegetation and environmental change is documented based on high-resolution pollen and sedimentary data from the Holocene record of lake Son Kul (central Tien Shan). Our reconstruction suggests a trend toward drier conditions during the middle to late Holocene, with two main shifts in climate conditions recorded at 6500 cal. BP and 5000e4500 cal. BP. The local vegetation evolved from an alpine meadow before 6500 cal. BP to a more open and drier landscape after that time. The seasonality was weak between 8350 and ca 5000e4500 cal. BP, as the thermic amplitude between the warmest and the coldest months was low. In contrast, the continentality greatly intensified between 4500 and 1950 cal. BP, with colder winters and warmer summers; 2. A prominent (and abrupt) increase in minerogenic inputs was recorded during the interval 8300e7800 cal. BP in Son Kul. We suggest that this drastic environmental change in central Tien

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Shan may coincide with the cooling event at 8200 cal. BP evidenced in Greenland ice cores (Alley et al., 1997; Rohling and Pälike, 2005). In view of the very few records available to date in Central Asia, we emphasize that more detailed information is needed on the timing and duration of this event for a better understanding of the processes linking North Atlantic and central Asian climates; 3. The occurrence of P. boryanum at ca 4000 cal. BP may indicate the first influences of human activity in central Tien Shan, at high altitude settings; 4. The reconstructed climatic conditions at Son Kul should be regarded at the regional, rather than local, scale in central Tien Shan. Also, the reconstruction of climatic parameters is somewhat hampered by the scarcity of existing modern regional pollen datasets (due to the lack of dust flux and/or pollen-trap samples regionally); 5. A crucial issue in future research is investigation of the link between Central Asian rainfall patterns and ISM dynamics on long timescales during the Holocene. In light of this study based on pollen analyses, we conclude by hypothesising that climate variability in central Tien Shan and western Central Asia is likely coupled to variability of the Eastern Mediterranean storm track and North Atlantic climate. This coupling was probably nonlinear during the Holocene, and may have only occurred when the Atlantic overturning circulation became robustly established after the mid-Holocene transition. Acknowledgements This work received financial support from the ANR PaleoSyr/ PaleoLib Project. Marie Mathis was financially supported by the French Ministry of Research. Xiangtong Huang acknowledges the financial support provided by Helmholtz e CSC e Fellowships, the Natural Science Foundation of China (grant 40902046, 40830107, 91128208). We are also very grateful to Matthew Makou (Laboratoire de Géologie de Lyon, France) for language editing, and to Gilles Escarguel (Laboratoire de Géologie de Lyon, France) for constructive comments. We also thank Dr. Suzanne Leroy, as well as two anonymous reviewers, for reviewing the manuscript; they provided very constructive comments that greatly contributed to manuscript improvements. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.quascirev.2014.01.023. References Abramowski, U., Bergau, A., Seebach, D., Zech, R., Glaser, B., Sosin, P., Kubik, P.W., Zech, W., 2006. Pleistocene glaciations of Central Asia: results from 10Be surface exposure ages of erratic boulders from the Pamir (Tajikistan) and the Alaye Turkestan range (Kyrgyzstan). Quat. Sci. Rev. 25, 1080e1096. Affek, H.P., Bar-Matthews, M., Ayalon, A., Matthews, A., Eiler, J.M., 2008. Glacial/ interglacial temperature variations in Soreq cave speleothems as recorded by ‘‘clumped isotope’’ thermometry. Geochim. Cosmochim. Acta 72 (22), 5351e 5360. Aizen, V.B., Aizen, E.M., Melack, J.M., 1995. Climate, snow cover, glaciers, and runoff in the Tien Shan, Central Asia. Water Resour. Bull. 31 (6), 1113e1129. Aizen, V.B., Aizen, E.M., Melack, J.M., 1996. Precipitation, melt and runoff in the northern Tien Shan. J. Hydrol. 186, 229e251. Aizen, V.B., Aizen, E.M., Melack, J.M., Dozier, J., 1997. Climatic and hydrologic changes in the Tien Shan, Central Asia. J. Clim. 10, 1393e1404. Aizen, E.M., Aizen, V.B., Melack, J.M., Nakamura, T., Ohta, T., 2001. Precipitation and atmospheric circulation patterns at mid-latitudes of Asia. Int. J. Climatol. 21, 535e556. Aizen, V.B., Kuzmichenok, V.A., Surazakov, A.B., Aizen, E.M., 2006. Glacier changes in central and northern Tien Shan during the last 140 years based on surface and remote sensing data. Ann. Glaciol. 43, 202e213.

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Alley, R.B., Mayewski, P.A., Sowers, T., Stuiver, M., Taylor, K.C., Clark, P.U., 1997. Holocene climatic instability: a prominent, widespread event 8200 yr. ago. Geology 25 (6), 483e486. Alley, R.B., Agustsdottir, A.M., 2005. The 8k event: cause and consequences of a major Holocene abrupt climate change. Quat. Sci. Rev. 24, 1123e1149. An, C.-B., Lu, Y., Zhao, J., Tao, S., Dong, W., Li, H., Jin, M., Wang, Z., 2012. A highresolution record of Holocene environmental and climatic changes from Lake Balikun (Xinjiang, China): implications for Central Asia. Holocene 22, 43e52. Arz, H.W., Lamy, F., Pätzold, J., Müller, P.J., Prins, M., 2003. Mediterranean moisture source for an Early-Holocene humid period in the Northern Red Sea. Science 300, 118e121. Bar-Matthews, M., Ayalon, A., Kaufman, A., Wasserburg, G., 1999. The Eastern Mediterranean paleoclimate as a reflection of regional events: Soreq Cave, Israel. Earth Planet. Sci. Lett. 166, 85e95. Bar-Matthews, M., Ayalon, A., Gilmour, M., Matthews, A., Hawkesworth, C.J., 2003. Seaeland oxygen isotopic relationships from planktonic foraminifera and speleothems in the Eastern Mediterranean region and their implication for paleorainfall during interglacial intervals. Geochim. Cosmochim. Acta 67, 3181e3199. Barlow, M., Wheeler, M., Lyon, B., Cullen, H., 2005. Modulation of daily precipitation over Southwest Asia by the MaddeneJulian oscillation. Mon. Weather Rev. 133, 3579e3594. Beer, R., Tinner, W., Carraro, G., Grisa, E., 2007a. Pollen representation in surface samples of the Juniperus, Picea and Juglans forest belts of Kyrgyzstan, Central Asia. Holocene 17, 599e611. Beer, R., Heiri, O., Tinner, W., 2007b. Vegetation history, fire history and lake development recorded for 6300 years by pollen, charcoal, loss on ignition and chironomids at a small lake in southern Kyrgyzstan (Alay Range, Central Asia). Holocene 17 (7), 977e985. Birks, H.J.B., Birks, H.H., 1980. Quaternary Palaeoecology. Edward Arnold, London. Black, E., 2012. The influence of the North Atlantic oscillation and European circulation regimes on the daily to interannual variability of winter precipitation in Israel. Int. J. Climatol. 32 (11), 1654e1664. Blyakharchuk, T.A., Wright, H.E., Borodavko, P.S., van der Knaap, W.O., Ammann, B., 2007. Late Glacial and Holocene vegetational history of the Altai Mountains (southwestern Tuva Republic, Siberia). Palaeogeogr. Palaeoclimatol. Palaeoecol. 245, 518e534. Böhner, J., 2006. General climatic controls and topoclimatic variations in Central and High Asia. Boreas 35, 279e295. Boomer, I., Aladin, N., Plotnikov, I., Whatley, R., 2000. The palaeolimnology of the Aral Sea: a review. Quat. Sci. Rev. 19, 1259e1278. Bothe, O., Fraedrich, K., Zhu, X., 2011. Large-scale circulations and Tibetan Plateau summer drought and wetness in a high-resolution climate model. Int. J. Climatol. 31, 832e846. Braconnot, P., Otto-Bliesner, B., Harrison, S., Joussaume, S., Peterchmitt, J.Y., AbeOuchi, A., Crucifix, M., Driesschaert, E., Fichefet, T., Hewitt, C.D., others, 2007. Results of PMIP2 coupled simulations of the Mid-Holocene and Last Glacial MaximumePart 2: feedbacks with emphasis on the location of the ITCZ and mid-and high latitudes heat budget. Clim. Past 3, 279e296. Brayshaw, D.J., Hoskins, B.J., Black, E.C.L., 2010. Some physical drivers of change in the winter storm tracks over the Atlantic and Mediterranean during the Holocene. Philos. Trans. Royal Soc. Lond. 368, 5185e5223. Brayshaw, D.J., Rambeau, C.M.C., Smith, S.J., 2011. Changes in Mediterranean climate during the Holocene: insights from global and regional climate modelling. Holocene 21, 15e31. Broccoli, A.J., Manabe, S., 1992. The effects of orography on midlatitude northern hemisphere dry climates. J. Clim. 5, 1181e1201. Bush, A.B., 2002. A comparison of simulated monsoon circulations and snow accumulation in Asia during the mid-Holocene and at the Last Glacial Maximum. Glob. Planet. Change 32, 331e347. Chen, F.H., Yu, Z., Yang, M., Ito, E., Wang, S., Madsen, D.B., Huang, X., Zhao, Y., Sato, T., Birks, J.B., others, 2008. Holocene moisture evolution in arid Central Asia and its outof-phase relationship with Asian monsoon history. Quat. Sci. Rev. 27, 351e364. Chen, F.-H., Chen, J.-H., Holmes, J., Boomer, I., Austin, P., Gates, J., Wang, N.-L., Brooks, S.J., Zhang, J.-W., 2010a. Moisture changes over the last millennium in arid central Asia: a review, synthesis and comparison with monsoon region. Quat. Sci. Rev. 29, 1055e1068. Chen, Y., Ni, J., Herzschuh, U., 2010b. Quantifying modern biomes based on surface pollen data in China. Glob. Planet. Change 74, 114e131. Chen, B., Xu, X.D., Yang, S., Zhang, W., 2012. On the origin and destination of atmospheric moisture and air mass over the Tibetan Plateau. Theor. Appl. Climatol. 110 (3), 423e435. Combaz, A., 1964. Les palynofacies. Rev. Micropaleontol. 7, 205e218 (in French). Combaz, A., 1980. Les kérogènes vus au microscope. In: Durand, B. (Ed.), Insoluble Organic Matter from Sedimentary Rocks. Technip, Paris, pp. 55e111. Cour, P., 1974. Nouvelles techniques de détection des flux et des retombées polliniques: étude de la sédimentation des pollens et des spores à la surface du sol. Pollen Spores 16, 103e141 (in French). Cour, P., Zheng, Z., Duzer, D., Calleja, M., Yao, Z., 1999. Vegetational and climatic significance of modern pollen rain in northwestern Tibet. Rev. Palaeobot. Palynol. 104 (3e4), 183e204. Dallmeyer, A., Claussen, M., Wang, Y., Herzschuh, U., 2013. Spatial variability of Holocene changes in the annual precipitation pattern: a model-data synthesis for the Asian monsoon region. Clim. Dyn. 40, 2919e2936. Davis, B.A.S., et al., 2013. The European Modern Pollen Database (EMPD) project. Veg. Hist. Archaeobot. 22 (6), 521e530.

184

M. Mathis et al. / Quaternary Science Reviews 89 (2014) 169e185

Demory, F., Novaczyk, N.R., Witt, A., Oberhänsli, H., 2005. High-resolution magnetostratigraphy of late quaternary sediments from Lake Baikal, Siberia: timing of intracontinental paleoclimatic responses. Glob. Planet. Change 46, 167e186. Develle, A.-L., Herreros, J., Vidal, L., Sursock, A., Gasse, F., 2010. Controlling factors on a paleo-lake oxygen isotope record (Yammouneh, Lebanon) since the Last Glacial Maximum. Quat. Sci. Rev. 29, 865e886. Dormoy, I., Peyron, O., Combourieu Nebout, N., Goring, S., Kotthoff, U., Magny, M., Pross, J., 2009. Terrestrial climate variability and seasonality changes in the Mediterranean region between 15 000 and 4000 years BP deduced from marine pollen records. Clim. Past 5, 615e632. Epple, C., 2001. A vegetation study in the walnut and fruit-tree forests of Southern Kyrgyzstan. Phytocoenologia 31, 571e604. Faegri, K., Iversen, J., 1989. Textbook of Pollen Analysis. John Wiley and Sons, Chichester, UK, p. 328. Faith, Daniel P., Minchin, Peter R., Belbin, Lee, 1987. Compositional dissimilarity as a robust measure of ecological distance. Vegetation 69 (1e3), 57e68. Feng, Z.D., An, C.B., Wang, H.B., 2006. Holocene climatic and environmental changes in the arid and semi-arid areas of China: a review. Holocene 16, 119e130. Fowell, S.J.B., Hansen, C.S., Peck, J.A., Khosbayar, P., Ganbold, E., 2003. Mid to late Holocene climate evolution of the Lake Telmen Basin, North Central Mongolia, based on palynological data. Quat. Res. 59, 353e363. Frumkin, A., Carmi, I., Zak, I., Magaritz, M., 1994. Middle Holocene environmental change determined from the salt Caves of Mount Sodom, Israel. In: BarYosef, O., Kra, R.S. (Eds.), Late Quaternary Chronology and Paleoclimates of the Eastern Mediterranean, Radiocarbon, pp. 315e332. Fujinami, H., Yasunari, T., 2009. The effects of midlatitude waves over and around the Tibetan Plateau on submonthly variability of the East Asian summer monsoon. Mon. Weather Rev. 137, 2286e2304. Gasse, F., Arnold, M., Fontes, J.C., Gibert, E., Huc, A., Li, B., Li, Y., Liu, Q., Mélières, F., Van Campo, E., Wang, F., Zhan, Q., 1991. A 13,000 year climate record from western Tibet. Nature 353, 742e745. Gasse, F., Fontes, J.C., Van Campo, E., Wei, K., 1996. Holocene environmental changes in Bangong Co basin (Western Tibet). Part 4: discussion and conclusions. Palaeogeogr. Palaeoclimatol. Palaeoecol. 120, 79e92. Giralt, S., Julià, R., Klerkx, J., Riera, S., Leroy, S., Buchaca, T., Catalan, J., De Batist, M., Beck, C., Bobrov, V., Gavshin, V., Kalugin, I., Sukhorukov, F., Brennwald, M., Kipfer, R., Peeters, F., Lombardi, S., Matychenkov, V., Romanovsky, V., Podsetchine, V., Voltattorni, N., 2004. 1,000 year environmental history of Lake Issyk-Kul. In: Nihoul, J., Zavialov, P., Micklin, P. (Eds.), Dying and Dead Seas, Climatic Versus Anthropic Causes, NATO ASI Series, vol. 36. Kluwer Academic publishers, ISBN 1-4020-1902-5, pp. 253e285. Golden, P.B., 2011. Central Asia in World History. Oxford University Press. Gómez-Paccard, M., Larrasoaña, J.C., Giralt, S., Roberts, A.P., 2012. First paleomagnetic results of midto late Holocene sediments from Lake Issyk-Kul (Kyrgyzstan): Implications for paleosecular variation in central Asia. Geochem. Geophys. Geosyst. 13, 1e18. Goodfriend, G.A., 1999. Terrestrial stable isotope records of Late Quaternary paleoclimates in the eastern Mediterranean region. Quat. Sci. Rev. 18, 501e514. Gorczinsky, W., 1920. Sur le calcul du continentalisme et son application dans la climatologie. Geogr. Ann. 2, 324e331. Gvirtzman, G., Wieder, M., 2001. Climate of the last 53,000 years in the Eastern Mediterranean, based on soil-sequence stratigraphy in the coastal plain of Israel. Quat. Sci. Rev. 20, 1827e1849. Gu, Z.Y., Liu, J.Q., Yuan, B.Y., 1993. Plateau monsoon variations in the Tibetan Plateau during the past 12 kyr BP. Chin. Sci. Bull. 38, 61e64 (in Chinese). Hagg, W., Braun, L.N., Kuhn, M., Nesgaard, T.I., 2007. Modeling of hydrological response to climate change in glacierized Central Asian catchments. J. Hydrol. 332, 40e53. Herzschuh, U., 2006. Palaeo-moisture evolution in monsoonal Central Asia during the last 50,000 years. Quat. Sci. Rev. 25, 163e178. Herzschuh, U., 2007. Reliability of pollen ratios for environmental reconstructions on the Tibetan Plateau. J. Biogeogr. 34, 1265e1273. Huang, X.Z., Chen, F.H., Fan, Y.X., Yang, M.L., 2009. Dry late-glacial and early Holocene climate in arid Central Asia indicated by lithological and palynological evidence from Bosten Lake, China. Quat. Int. 194, 19e27. Jackson, S.T., Lyford, M.E., 1999. Pollen dispersal models in Quaternary plant ecology: assumptions, parameters, and prescriptions. Bot. Rev. 65, 39e75. Jankovská, V., Komárek, J., 2000. Indicative value of Pediastrum and other Coccal green algae in Palaeoecology. Folia Geobot. 35 (1), 59e82. Jin, L., Chen, F.-H., Morrill, C., Otto-Bliesner, B.L., Rosenbloom, N., 2012. Causes of early Holocene desertification in arid Central Asia. Clim. Dyn. 38, 1577e1591. Khazanov, A.M., 1994. Nomads and the Outside World, second ed. University of Wisconsin Press, Madison. Klotz, S., Pross, J., 1999. Pollen-based reconstructions in the European Pleistocene: the modified indicator species approach as an tool for quantitative analysis. Acta Palaeobot., 481e486. Suppl. 2. Klotz, S., Guiot, J., Mosbrugger, V., 2003. Continental European Eemian and early Würmian climate evolution: comparing signals using different quantitative reconstruction approaches based on pollen. Glob. Planet. Change 36, 277e294. Kotthoff, U., Pross, J., Müller, U.C., Peyron, O., Schmiedl, G., Schulz, H., Bordon, A., 2008. Climate dynamics in the borderlands of the Aegean Sea during formation of sapropel S1 deduced from a marine pollen record. Quat. Sci. Rev. 27, 832e 845.

Kremenetski, C.V., Sulerzhitsky, L.D., Hantemirov, R., 1998. Holocene history of the northern range limits of some trees and shrubs in Russia. Arct. Alp. Res. 30 (4), 317e333. Kutiel, H., Maheras, P., Türkes, M., Paz, S., 2002. North SeaeCaspian Pattern (NCP)d An upper level atmospheric teleconnection affect- ing the eastern MediterraneandImplications on the regional climate. Theor. Appl. Climatol. 72, 173e 192. Leroy, S.A.G., Marret, F., Gibert, E., Chalié, F., Reyss, J.-L., Arpe, K., 2007. River inflow and salinity changes in the Caspian Sea during the last 5500 years. Quat. Sci. Rev. 26, 3359e3383. Leroy, S.A.G., Kakroodi, A.A., Kroonenberg, S.B., Lahijani, H.A.K., Alimohammadian, H., Nigarov, A., 2013a. Holocene vegetation history and sea level changes in the SE corner of the Caspian Sea: relevance to SW Asia climate. Quat. Sci. Rev. 70, 28e47. Leroy, S.A.G., Tudryn, A., Chalié, F., López-Merino, L., Gasse, F., 2013b. From the Allerød to the mid-Holocene: palynological evidence from the south basin of the Caspian Sea. Quat. Sci. Rev. 78, 77e97. Li, X., Zhao, K., Dodson, J., Zhou, X., 2011. Moisture dynamics in Central Asia for the last 15 ka: new evidence from Yili Valley, Xinjiang, NW China. Quat. Sci. Rev. 30, 3457e3466. Lioubimtseva, E., Cole, R., Adams, J.M., Kapustin, G., 2005. Impacts of climate and land-cover changes in arid lands of Central Asia. J. Arid Environ. 62 (2), 285e 308. Liu, X.Q., Herzschuh, U., Shen, J., Jiang, Q.F., Xiao, X.Y., 2008. Holocene environmental and climatic changes inferred from Wulungu Lake in northern Xinjiang,China. Quat. Res. 70, 412e425. MacDonald, G.M., Velichko, A.A., Kremenetski, C.V., Borisova, O.K., Goleva, A.A., Andreev, A.A., Cwynar, L.C., Riding, R.T., Forman, S.L., Edwards, T.W.D., Aravena, R., Hammarlund, D., Szeicz, J.M., Gattaulin, V.N., 2000. Holocene Treeline history and climate change across Northern Eurasia. Quat. Res. 53, 302e311. Magny, M., Peyron, O., Sadori, L., Ortu, E., Zanchetta, G., Vannière, B., Tinner, W., 2012. Contrasting patterns of precipitation seasonality during the Holocene in the south- and north-central Mediterranean. J. Quat. Sci. 27, 290e296. Magny, M., Combourieu-Nebout, N., de Beaulieu, J.-L., Bout-Roumazeilles, V., Colombaroli, D., Desprat, S., Francke, A., Joannin, S., Ortu, E., Peyron, O., Revel, M., Sadori, L., Siani, G., Sicre, M.A., Samartin, S., Simonneau, A., Tinner, W., Vannière, B., Wagner, B., Zanchetta, G., Anselmetti, F., Brugiapaglia, E., Chapron, E., Debret, M., Desmet, M., Didier, J., Essallami, L., Galop, D., Gilli, A., Haas, J.-N., Kallel, N., Millet, L., Stock, A., Turon, J.-L., Wirth, S., 2013. North-south palaeohydrological contrasts in the central Mediterranean during the Holocene tentative synthesis and working hypotheses. Clim. Past 9, 2043e2071. Masson, J.A., Lu, H., Zhou, Y., Miao, X., Swinehart, J.B., Liu, Z., Goble, R.J., Yi, S., 2009. Dune mobility and aridity at the desert margin of northern China ata time of peak monsoon strength. Geology 37 (10), 947e950. Migowski, C., Stein, M., Prasad, S., Agnon, A., Negendank, J.F.W., 2006. Dead Sea levels, climate variability and human culture evolution in the Holocene Near East. Quat. Res. 66, 421e431. Mischke, S., Wünnemann, B., 2006. The Holocene salinity history of Bosten Lake (Xinjiang, China) inferred from ostracod species assemblages and shell chemistry: possible palaeoclimatic implications. Quat. Int. 154, 100e112. Mischke, S., Rajabov, I., Mustaeva, N., Zhang, C., Herzschuh, U., Boomer, I., Brown, E.T., Andersen, N., Myrbo, A., Ito, E., et al., 2010. Modern hydrology and late Holocene history of Lake Karakul, eastern Pamirs (Tajikistan): a reconnaissance study. Palaeogeogr. Palaeoclimatol. Palaeoecol. 289, 10e24. Peck, J.A., King, J.W., Colman, S.M., Kravchinsky, V.A., 1996. An 84-kyr paleomagnetic record from the sediments of Lake Baikal, Siberia. Journal of Geophysical Research 101 (B5), 11365e11385. Peyron, O., Guiot, J., Cheddadi, R., Tarasov, P.E., Reille, M., de Beaulieu, J.-L., Bottema, S., Andrieu, V., 1998. Climatic reconstruction in Europe from pollen data, 18,000 years before present. Quat. Res. 49, 183e196. Peyron, O., Goring, S., Dormoy, I., Kotthoff, U., Pross, J., de Bealieu, J.L., DrescherSchneider, R., Magny, M., 2011. Holocene seasonality changes in the Central Mediterranean region reconstructed from the pollen sequences of Lake Accesa (Italy) and Tenaghi Philippon (Greece). Holocene 21, 131e146. Reille, M., 1999. Pollen et Spores d’Europe et d’Afrique du Nord, second ed. Laboratoire de Botanique Historique et Palynologie, Marseille (France), ISBN 29507175-3-5, p. 536. Reimer, P.J., Baillie, M.G.L., Bard, E., Bayliss, A., Beck, J.W., Blackwell, P.G., BronkRamsey, C., Buck, C.E., Burr, G.S., Edwards, R.L., Friedrich, M., Grootes, P.M., Guilderson, T.P., Hajdas, I., Heaton, T.J., Hogg, A.G., Hughen, K.A., Kaiser, K.F., Kromer, B., McCormac, F.G., Manning, S.W., Reimer, R.W., Richards, D.A., Southon, J.R., Talamo, S., Turney, C.S.M., van der Plicht, J., Weyhenmeyer, C.E., 2009. IntCal09 and Marine09 radiocarbon age calibration curves, 0e50,000 Years cal BP. Radiocarbon 51, 1111e1115. Rhodes, T.E., Gasse, F., Lin, R., Fontes, J.C., Wei, K., Bertrand, P., Gibert, E., Mélières, F., Tucholka, P., Wang, Z., others, 1996. A Late PleistoceneeHolocene lacustrine record from Lake Manas, Zunggar (northern Xinjiang, western China). Palaeogeogr. Palaeoclimatol. Palaeoecol. 120, 105e121. Ricketts, R.D., Johnson, T.C., Brown, E.T., Rasmussen, K.A., Romanovsky, V.V., 2001. The Holocene paleolimnology of Lake Issyk-Kul, Kyrgyzstan: trace element and stable isotope composition of ostracodes. Palaeogeogr. Palaeoclimatol. Palaeoecol. 176, 207e227.

M. Mathis et al. / Quaternary Science Reviews 89 (2014) 169e185 lu, C., Fiorentino, G., Caracuta, V., 2011. CliRoberts, N., Eastwood, W.J., Kuzucuog matic, vegetation and cultural change in the eastern Mediterranean during the mid-Holocene environmental transition. Holocene 21 (1), 147e162. Roberts, N., Moreno, A., Valero-Garcés, B.L., Corella, J.P., Jones, M., Allcock, S., Woodbridge, J., Morellón, M., Luterbacher, J., Xoplaki, E., Türkes, M., 2012. Palaeolimnological evidence for an east-west climate see-saw in the Mediterranean since AD 900. Glob. Planet. Change 84e85, 23e34. Robinson, S.A., Black, S., Sellwood, B.W., Valdes, P.J., 2006. A review of palaeoclimates and palaeoenvironments in the Levant and Eastern Mediterranean from 25,000 to 5000 years BP: setting the environmental background for the evolution of human civilisation. Quat. Sci. Rev. 25, 1517e1541. Rohling, E.J., Mayewski, P.A., Abu-Zied, R.H., Casford, J.S.L., Hayes, A., 2002. Holocene atmosphere-ocean interactions: records from Greenland and the Aegean Sea. Clim. Dyn. 18, 587e593. Rohling, E.J., Pälike, H., 2005. Centennial-scale climate cooling with a sudden cold event around 8,200 years ago. Nature 434, 975e979. Rossignol-Strick, M., 1999. The Holocene climatic optimum and pollen records of sapropel 1 in the Eastern Mediterranean, 9000e6000 BP. Quat. Sci. Rev. 18, 515e530. Rudaya, N., Tarasov, P., Dorofeyuk, N., Solovieva, N., Kalugin, I., Andreev, A., Daryin, A., Diekmann, B., Riedel, F., Tserendash, N., others, 2009. Holocene environments and climate in the Mongolian Altai reconstructed from the HotonNur pollen and diatom records: a step towards better understanding climate dynamics in Central Asia. Quat. Sci. Rev. 28, 540e554. Rychagov, G.I., 1997. Holocene oscillations of the Caspian Sea, and forecasts based on Palaeographical reconstructions. Quat. Int. 41/42, 167e172. Sato, T., Tsujimura, M., Yamanaka, T., Iwasaki, H., Sugimoto, A., Sugita, M., Kimura, F., Davaa, G., Oyunbaatar, D., 2007. Water sources in semi-arid northeast Asia as revealed by field observations and isotope transport model. J. Geophys. Res. 112. http://dx.doi.org/10.1029/2006JD008321. Savoskul, O.S., 1997. Modern and Little Ice Age glaciers in "humid" and "arid" areas of the Tien Shan, Central Asia: two different patterns of fluctuation. Ann. Glaciol. 24, 142e147. Schiemann, R., Glazirina, M.G., Schär, C., 2007. On the relationship between the Indian summer monsoon and river flow in the Aral Sea basin. Geophys. Res. Lett. 34, p.L05706. Sneath, P.H.A., Sokal, R.R., 1973. Numerical Taxonomy: the Principles and Practice of Numerical Classification. W.H. Freeman, San Francisco. Sugita, S., 1994. Pollen representation of vegetation in Quaternary sediments: theory and methods in patchy vegetation. J. Ecol. 82, 881e897. Syed, F.S., Giorgi, F., Pal, J.S., King, M.P., 2006. Effect of remote forcings on the winter precipitation of central southwest Asia: Part 1. Observations. Theor. Appl. Climatol. 86, 147e160. Tao, S.C., An, C.B., Chen, F.H., Tang, L.Y., Wang, Z.L., Lv, Y.B., et al., 2010. Pollen inferred vegetation and environmental changes since 16.7 ka BP at Lake Balikun, Xinjiang. Chin. Sci. Bull. 55, 2249e2257. Tarasov, P.E., Cheddadi, R., Guiot, J., Bottema, S., Peyron, O., Belmonte, J., RuizSanchez, V., Saadi, F., Brewer, S., 1998a. A method to determine warm and cool steppe biomes from pollen data; application to the Mediterranean and Kazakhstan regions. J. Quat. Sci. 13, 335e344. Tarasov, P.E., Webb III, T., Andreev, A.A., Afanas’eva, N.B., Berezina, N.A., Bezusko, L.G., Blyakharchuk, T.A., Bolikhovskaya, N.S., Cheddadi, R.,

185

Chernavskaya, M.M., Chernova, G.M., Dorofeyuk, N.I., Dirksen, V.G., Elina, G.A., Filimonova, L.V., Glebov, F.Z., Guiot, J., Gunova, V.S., Harrison, S.P., Jolly, D., Khomutova, V.I., Kvavadze, E.V., Osipova, I.M., Panova, N.K., Prentice, I.C., Saarse, L., Sevastyanov, D.V., Volkova, V.S., Zernitskaya, V.P., 1998b. Present-day and mid-Holocene biomes reconstructed from pollen and plant macrofossil data from the former Soviet Union and Mongolia. J. Biogeogr. 25, 1029e1053. Thompson, L.G., Yao, T.D., Davis, M.E., Henderson, K.A., Thompson, E.M., Lin, P.N., Beer, J., Synal, H.A., Dai, J.C., Bolzan, J.F., 1997. Tropical climate instability: the Last Glacial cycle from a QinghaieTibetan ice core. Science 276, 1821e1825. Traverse, A., 2008. Paleopalynology, Second ed. Springer. Tyson, R.V., 1995. Sedimentary Organic Matter: Organic Facies and Palynofacies. Chapman & Hall, London; New York. Van Campo, E., Cour, P., Sixuan, H., 1996. Holocene environmental changes in Bangong Co basin (Western Tibet). Part 2: the pollen record. Palaeogeogr. Palaeoclimatol. Palaeoecol. 120, 49e63. Vandenberghe, J., Renssen, H., van Huissteden, K., Nugteren, G., Konert, M., Lu, H., Dodonov, A., Buylaert, J.P., 2006. Penetration of Atlantic westerly winds into Central and East Asia. Quat. Sci. Rev. 25, 2380e2389. Vereshchagin, V.N., Ronov, A.B., Tazikin, N.N., 1975. Paleogeography of the USSR. In: Explication Note to the Atlas of Lithologic-paleogeographic Maps of the USSR. Nedra Publication, Moscow, pp. 5e200 (in Russian). Verheyden, S., Nader, F.H., Cheng, H.J., Edwards, L.R., Swennen, R., 2008. Paleoclimate reconstruction in the Levant region from the geochemistry of a Holocene stalagmite from the Jeita cave, Lebanon. Quat. Res. 70, 368e381. Wanner, H., Beer, J., Bütikofer, J., Crowley, T.J., Cubasch, U., Flückiger, J., Goosse, H., Grosjean, M., Joos, F., Kaplan, J.O., Küttel, M., Müller, S.A., Prentice, I.C., Solomina, O., Stocker, T.F., Tarasov, P., Wagner, M., Widmann, M., 2008. Mid- to Late Holocene climate change: an overview. Quat. Sci. Rev. 27, 1791e1828. Wick, L., Lemcke, G., Sturm, M., 2003. Evidence of Lateglacial and Holocene climatic change and human impact in eastern Anatolia: high-resolution pollen, charcoal, isotopic and geochemical records from the laminated sediments of Lake Van, Turkey. Holocene 13, 665e675. Wu, J.L., 1995. Holocene depositional history and environment in the Aibi Lake area. Geogr. Sci. 5, 39e46 (in Chinese). Wu, J.L., Wang, S.M., Wang, H.D., 1996. Paleoclimatic and paleoenvironmental changes in the Aibi Lake in the Xinjiang region during the Holocene. J. Ocean Lake Stud. 27, 524e530. Wünnemann, B., Mischke, S., Chen, F., 2006. A Holocene sedimentary record from Bosten Lake, China. Palaeogeogr. Palaeoclimatol. Palaeoecol. 234, 223e238. Yang, X.D., Wang, S.M., 1994. Holocene environmental reconstruction in the Wulungu Lake area. Arid Geogr. 11, 67e72 (in Chinese). Yang, X., Scuderi, A.L., 2010. Hydrological and climatic changes in deserts of China since the Late Pleistocene. Quat. Res. 73, 1e9. Yang, X., Scuderi, L.A., Paillou, P., Liu, Z., Li, H., Ren, X., 2011. Quaternary environmental change in the drylands of China e a critical review. Quat. Sci. Rev. 30, 3219e3233. Yang, X., Wang, X., Liu, Z., Li, H., Ren, X., Zhang, D., Ma, Z., Rioual, P., Jin, X., Scuderi, L., 2013. Initiation and variation of the dune fields in semi-arid northern China e with a special reference to the Hunshandake Sandy Land, Inner Mongolia. Quat. Sci. Rev. 78, 369e380.