Chironomid record of Late Quaternary climatic and

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Quaternary Science Reviews 26 (2007) 705–731

Chironomid record of Late Quaternary climatic and environmental changes from two sites in Central Asia (Tuva Republic, Russia)—local, regional or global causes? Boris P. Ilyashuk, Elena A. Ilyashuk Institute of North Industrial Ecology Problems, Kola Science Centre, Russian Academy of Sciences, 14 Fersman Street, 184209 Apatity, Murmansk Region, Russian Federation Received 29 April 2006; received in revised form 23 September 2006; accepted 7 November 2006

Abstract Sediment cores from two mountain lakes (Lake Grusha at 2413 m a.s.l. and Ak-Khol at 2204 m a.s.l.) situated in the Tuva Republic (southern Siberia, Russia), just north of Mongolia, were studied for chironomid fossils in order to infer post-glacial climatic changes and to investigate responses of the lake ecosystems to these changes. The results show that chironomids are responding both to temperature and to changing lake depth, which is regarded as a sensitive proxy of regional effective moisture. The post-glacial history of this mountain region in Central Asia can be divided into seven successive climatic phases: the progressive warming during the last glacial–interglacial transition (ca 15.8–14.6 cal kyr BP), the warm and moist Bølling-Allerød-like interval (ca 14.6–13.1 cal kyr BP), the cool and dry Younger Dryas-like event (ca 13.1–12.1 cal kyr BP), warmer and wetter conditions during ca 12.1–8.5 cal kyr BP, a warm and dry phase ca 8.5–5.9 cal kyr BP, cold and wet conditions during ca 5.9–1.8 cal kyr BP, as well as cold and dry climate within the last 1800 years. The chironomid records reveal patterns of climatic variability during the Late-glacial and Holocene, which can be correlated with abrupt climatic events in the North Atlantic and the Asian monsoon-dominated regimes. Apparently, the water balance of the studied lakes is controlled by the interrelation between the dominant westerly system and the changing influence of the summer monsoon, as well as the influence of alpine glacier meltwater supply. It is possible that monsoon tracks could have reached the southwest Tuva, resulting in an increase in precipitation at ca 14.6–13.1 and ca 12.1–8.5 cal kyr BP, whereas cyclonic westerlies from the North Atlantic were likely responsible for considerable moisture transport accompanying the global Neoglacial cooling at ca 5.9–1.8 cal kyr BP. These events suggest the changes of the regional pattern of atmospheric circulation, which could be in turn induced by the global climatic shifts. Some discrepancies compared with other reconstructions from Central Asia may be associated with regional (spatial) differences between the changing predominant circulation mechanisms and with local differences in uplift and descent of air masses within the complicated mountain landscape. In this paper, we also discuss the possibilities and perspectives for using chironomids in reconstructions of past temperatures and climate-induced changes in water depth of lakes in Central Asia. r 2006 Elsevier Ltd. All rights reserved.

1. Introduction In recent years, climatic variability of the current interglacial in different areas of the Earth has recently been the focus of attention of many geoscientists (Wright et al., 1993; Roberts, 1998; Mackay et al., 2003; Battarbee et al., 2004; Pienitz et al., 2004). Understanding the climatic instability at millennial timescales can determine the Fax: +7 81555 74964.

E-mail address: [email protected] (B.P. Ilyashuk). 0277-3791/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2006.11.003

background and trend of natural climatic variations. In order to establish the geographic patterns and study the mechanisms of the climatic instability, temporal sequences of the post-glacial records must first be documented on a regional scale, especially for climate-sensitive regions (Jacoby et al., 2000). The Tuva Republic of the Russian Federation in southern Siberia, situated in the central part of the Asian continent north of Mongolia, is one of the most climatesensitive regions. Tuva is located in a broad belt of mountains and intermontane plains at elevations between

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B.P. Ilyashuk, E.A. Ilyashuk / Quaternary Science Reviews 26 (2007) 705–731

520 m and 4000 m a.s.l. (Efimtsev, 1957). It is encircled by the Sayan Mountains in the north and east, the Altai Mountains in the west, and the Tannu Ola Mountains in the south; mountains occupy about 80% of Tuva’s territory. Mountain regions are characterized by steep environmental gradients over rather short distances (Huber et al., 2005). Almost all landscape types of the temperate climatic zone occur in Tuva, within a latitude range of only 41 (between 49 and 531N). Its natural landscapes consist of luxuriant meadows, boundless steppe and dusty semideserts on the plains and hollows, coniferous taiga and deciduous forests on the mountain slopes, alpine meadow and tundra, and glaciers on the mountain crests (Kokorin et al., 2001). Accordingly, the elevational position of the ecotonal boundaries between different landscapes is climate related and may be shifted by climatic change (Lotter and Psenner, 2004). Alpine glaciers may also respond sensitively to climatic changes of regional and/or global significant. Tuva is frequently called a ‘country of blue lakes’, for lakes of various sizes are widespread on the landscape. Mountain lakes that are commonly characterized by small water volumes and prolonged ice cover are known to be particularly susceptible to climatic change, since regional and even hemispherical climate strongly affects synoptic weather patterns in mountain regions (Livingstone, 1997; Lotter and Psenner, 2004). Tuva is far from both the warm Atlantic and Pacific oceans and the cold Arctic Ocean, and it is located in the region of the highest degree of continentality on the Earth. The extremely continental climate is reflected in the annual temperature range of about 96–98 1C. There is also high daily temperature range. Everywhere along the area, the average annual temperature is below zero, from 3.3 to 6.1 1C (Shaktarzhik, 1993; Lebedev and Polulyakh, 2005). The present-day climate of Tuva is largely controlled by the Siberian-Mongolian high atmospheric pressure zone, along with westerly low-pressure systems linked to the North Atlantic (Efimtsev, 1957; Alpatev et al., 1976). The Arctic air masses, significantly transformed during their long passage over land, also penetrate southern Siberia (Alpatev et al., 1976). In wintertime, the Siberian anticyclone results in frosty and sunny weather. In summertime, dominating western and northwestern cyclones bring warm and moist temperate air masses. However, summer precipitation is limited, because the mountain ranges prevent the northwestern humid airflow from reaching Tuva (Efimtsev, 1957). Prolonged winters (November– April) are cold and windless with little snow, whereas short summers are moderately warm in the mountains, and hot and dry on the plains. One of current environmental problems is global warming, which will have a substantial effect on ecosystems at high latitudes and high altitudes. It is expected that climatic changes in the Altai-Sayan mountain country, including Tuva’s territory, will be almost as strong as in the

Arctic zone and much stronger than in the temperate latitudes of the Northern hemisphere (Kokorin et al., 2001). In this regard, a deep comprehension of the frequency and magnitude of past climatic changes, particularly in mountain regions, becomes very important. It is also decisive for the effective management of their natural resources and for adequate evaluation of human impacts superimposed on natural background and trends. One way to gain this understanding is by multidisciplinary researches of lake ecosystems in mountain regions, as mountain lakes directly register environmental change. The physical (e.g., ice cover, stratification of the water column) and chemical (e.g., pH, alkalinity, nutrients, oxygen) responses of a mountain lake to local weather conditions determine its biotic response (Lotter and Psenner, 2004). The lakes have a ‘memory’, for their sediments are excellent environmental archives and different fossils are proxies for specific climatic reconstructions. Among biotic proxies, non-biting midges or chironomids (Diptera: Chironomidae) are of special interest. These true flies have a short generation time and are able to respond rapidly to climatic change, highly synchronous with responses of other organisms, in the absence of latitudinal migrational lags (Ammann et al., 2000). Sometimes, in the presence of latitudinal migrational lags, chironomids respond somewhat more rapidly than some other fossil groups, e.g., terrestrial vegetation (Smol et al., 1991; Caseldine et al., 2003; Andreev et al., 2004, 2005). Chironomid analysis just recently become a powerful tool for reconstruction of past environmental conditions and is widely used in palaeoecology and palaeoclimatology (Walker, 2001; Porinchu and MacDonald, 2003; Ilyashuk and Ilyashuk, 2004). However, mono-proxy climatic reconstructions are an approach of the past, whereas multi-proxy and multi-site investigations of lake sediments are becoming increasingly popular, because they offer potentially independent lines of evidence, provide an integrated view of palaeoenvironmental conditions both in a lake and its catchment areas, and are able to extract a reliable regional climatic signal (Ammann and Oldfield, 2000; Battarbee, 2000; Lotter, 2003; Velle et al., 2005). In 2000, an American-Swiss-Russian team cored the sediments of two mountain lakes, Lake Grusha and Ak-Khol, located in the southwestern part of the Tuva Republic. The well-dated sediment sequences spanning the entire post-glacial period were the object of a multiproxy study, including analyses of aquatic and terrestrial biotic proxies. Reconstructions of climatic changes were made from diatom (Westover et al., 2006) and pollen (Blyakharchuk et al., 2006) records from these lakes. In this paper, we report the results of a chironomid-based palaeoenvironmental study. We hope that our climatic inferences will contribute to understanding the dynamics of climatic change on a regional scale and enhance our knowledge of Holocene climatic variability in Central Asia.

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2. Site description Lake Grusha and Ak-Khol are located near a valley of the Mogen-Buren River between the Mongun-Taiga Mountains (up to 3970 m a.s.l.) and the Chikhacheva Range (up to 4029 m a.s.l) of the Altai Mountains (Fig. 1). On the axis of the Chikhacheva Range is the administrative boundary between the Tuva Republic and the Altai Republic. Lake Grusha (the lake shape resembles that of the fruit of pear-tree, and ‘‘grusha’’ means ‘‘pear’’ in Russian) is a small, nutrient-poor, weakly acidic lake with relatively simple bathymetry (Table 1). The lake is situated at an elevation of 2413 m a.s.l. and fed by three streams draining the Chikhacheva Range. It has a single outlet to the Chedi-Tei River, a tributary of the upper Mogen-Buren River. Ak-Khol (‘‘ak-khol’’ means ‘‘white lake’’ in Tuvinian, as the lake water seems white owing to the surrounding snowcovered ranges that are mirrored on the water surface) is situated at an elevation of 2204 m a.s.l. about 15 km downstream of the Chedi-Tei River. In the north, the lake adjoins a channel of the Chedi-Tei River that forms an inlet

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and an outlet of the lake (Fig. 1). Thus the hydrological regime of the Chedi-Tei River strongly affects the water level and hydrological balance of Ak-Khol as in a system between river and floodplain lake. Ak-Khol is a small oligotrophic circumneutral lake with relatively simple bathymetry. The submerged non-rooted macrophyte Ceratophyllum sp. colonizes the littoral zone of the lake at a depth of up to 2 m. The underlying bedrock of the area is composed of sandstone, limestone, schist, and quartzite. Steppe plant communities dominate the terrestrial vegetation in the catchment of both lakes. Of special interest is the presence of numerous Scythian grave monuments around Ak-Khol (Skorobogatko, 2004). One of the closest meteorological stations, Kosh Agach (501010 N, 881440 E), is situated ca 68 km southwest of Ak-Khol at an elevation of 1758 m a.s.l. Monthly mean air temperature and precipitation records at the station have been collected during the last 10 years (http://www. weatherbase.com/weather/weather.php3?s=095263&refer =&units=metric). According to these records, the mean

Fig. 1. Maps of: (a) Eastern hemisphere; (b) Tuva Republic and adjacent areas; and (c) the southwestern part of the Tuva Republic showing the location of the sampled lakes.



Table 1 Geographic, morphometric and hydrochemical characteristics of the lakes Parameter

Lake Grusha

Ak-Khol

Latitude Longitude Elevation (m a.s.l.) Surface area (km2) Maximum depth (m) pH Ptot (mg l1) Ntot (mg l1) TOC (mg l1) Mn (mg l1) Fe (mg l1) Na (mg l1) K (mg l1)

501160 N 891270 E 2413 1.7 3.55 6.5 13.7 9.1 0.3 o0.2 0.10 o5.0 o1.0

501150 N 891370 E 2204 6.0 2.80 7.3 12.7 9.1 0.4 o0.2 0.12 190.0 1.4

January and July air temperatures in the area studied are –28 and 13 1C, respectively. Average annual precipitation is 170 mm, occurring largely in June–August (20, 40 and 20 mm, respectively). According to Alpatev et al. (1976) and Lebedev and Polulyakh (2005), in the Altai and Sayan mountains at an elevation of ca. 2000 m a.s.l. mean July air temperature is only about 8 1C. 3. Methods 3.1. Coring and loss-on-ignition analysis The sediment cores were retrieved from a rubber boat in the deepest central part of the lakes with a square-rod piston sampler (Wright, 1991) in August 2000. The core segments of 1-m length and 5 cm in diameter were transported in plastic tubes to the laboratory for cold storage (4 1C) and analyses. The uppermost unconsolidated sediments were sampled with a piston corer in a clear plastic tube, subsampled in the field, and stored in plastic bags. The core segments from each lake were correlated by sequence slotting (Birks and Gordon, 1985) of loss-onignition (LOI) profiles aided by visual inspection. LOI analyses were carried out on 1-cm3 samples of sediment at 2–10 cm intervals following the recommendations in Heiri et al. (2001). The depth scale of the Ak-Khol sediment profile was slightly modified in the older part of the sequence (namely it was shortened by 12 cm) as compared to Westover et al. (2006), following re-correlation of the two lowermost core segments (K.S. Westover and B. Ammann, personal communication, June 2005). Depths below the sediment surfaces were used for sediment description. 3.2. Radiocarbon dating and chronology Dispersed organic carbon in bulk sediment samples was used for radiocarbon dating by accelerator mass spectrometry (AMS) from 13 and 12 levels in Lake Grusha and Ak-Khol, respectively (Table 2). Additionally, three AMS

radiocarbon dates were obtained on macroscopic remains of terrestrial plants picked from the Lake Grusha sediments. Calendar-year ages expressed as 95.4% probability envelopes were obtained by calibration of the radiocarbon dates based on the INTCAL98 calibration data set (Stuiver et al., 1998), using the CALIB REV4.4.2 radiocarbon calibration software (Stuiver and Reimer, 1993). One date on the sample from 416 cm depth of the AkKhol sediment profile is inconsistent with stratigraphic order, and is obviously too old compared to the other dates in the profile (Table 2). This date was excluded from the age–depth chronology. The mean median probability dates were used for age–depth calculations. The age–depth models were constructed as a composition of a threetermed polynomial and a linear interpolation to account for an abrupt change in sedimentation rate at ca 413 cm in Ak-Khol and the so-called radiocarbon plateau in the interval of ca 180–135 cm depth at the Lake Grusha sediment profile (Fig. 2). The radiocarbon plateau recorded in the Lake Grusha sediment sequence will be discussed more in detail later. 3.3. Chironomid analysis Samples for chironomid analysis appear to span the interval from the last Glacial Termination (approximately from 15.8 cal kyr BP) to the present. One-centimetre slices of sediment were used as samples for the chironomid analysis. In Lake Grusha the samples were taken every 5 cm down to the 40-cm level, and every 10 cm between 40 and190 cm. Below 190 cm sampling occurred every 2–4 cm to the base of the core. In total, 45 samples were obtained from the Lake Grusha core. The Ak-Khol core was sampled every 5 cm between 0 and 35 cm and every 10 cm for the rest of the core, resulting in a total 48 samples. The sediments were mixed only with distiled water and were not treated with KOH or sieved prior to sorting, following Walker (2001). The samples were sorted in Bogorov counting tray at 25–35 magnification under a stereomicroscope. Chironomid remains were picked out and mounted on glass slides in glycerol for microscopic identification. At least 50 chironomid head capsules were counted and identified in each sample, except for the basal sample (246 cm; 38 head capsules) from the Lake Grusha core. Such a number of head capsules provides a representative count for quantitative analyses (Heiri and Lotter, 2001; Larocque, 2001; Quinlan and Smol, 2001a). Chironomid concentrations were calculated as headcapsule abundance per gram wet sediment. Taxonomic identification of the chironomid remains is primarily based on descriptions of genera provided in Wiederholm (1983). Identifications of the chironomid head capsules to a more precise taxonomic level were carried out using the descriptions of Hofmann (1971) for Chironomus, Contreras-Lichtenberg (1986) for Dicrotendipes, Makarchenko and Makarchenko (1999) for Sergentia, and Tang et al.


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Table 2 Radiocarbon dates of samples from the sediment cores recovered in Lake Grusha and Ak-Khol Depth (cm)

Sample no.a

Material analysed

Radiocarbon age71s (14C yr BP)

Calendar age (cal yr BP)

2s range (cal yr BP)

Lake Grusha 55 95 130 135 145 155 155 165 165 175 185 195 210 225 232 239

CURL5550 CURL5551 AA44429 CURL5552 AA44430 AA44431 AA44433 AA44432 AA44434 CURL5553 AA44435 CURL5554 CURL5555 CURL5556 CURL5557 CURL5558

Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment Macrofossil Bulk sediment Macrofossil Bulk sediment Macrofossil Bulk sediment Bulk sediment Bulk sediment Bulk sediment Bulk sediment

6060740 8120745 9655787 9710750 9834774 10008770 10003778 9691776 9617776 9770765 98607100 10270755 10710750 11330755 11820760 12270755

6900 9070 10 950 11 130 11 230 11 480 11 480 11 050 10 930 11 180 11 280 12 070 12 800 13 290 13 860 14 660

6790–7010 8990–9260 10 740–11 200 10 860–11 230 11 110–11 360 11 230–11 700 11 220–11 760 10 750–11 230 10 730–11 170 11 070–11 260 11 090–11 650 11 690–12 380 12 620–12 970 13 140–13 480 13 470–14 090 14 080–15 400

Ak-Khol 36 95 154 214 276 325 386 402 412 416 420 426

CURL5536 CURL5537 CURL5538 CURL5539 CURL5540 CURL5546 CURL5547 AA44428 CURL5548 AA44427 AA44426 CURL5549

Bulk Bulk Bulk Bulk Bulk Bulk Bulk Bulk Bulk Bulk Bulk Bulk

2150745 3580750 4660735 5320740 6210760 6950750 8100745 8491768 8460750 123207130 99807100 11450755

2140 3870 5400 6080 7110 7760 9050 9490 9480 14 600 11 470 13 380

2000–2310 3720–3990 5310–5470 5990–6200 6950–7250 7670–7860 8980–9130 9400–9560 9420–9540 14 080–15 400 11 180–11 770 13 160–13 510

a

sediment sediment sediment sediment sediment sediment sediment sediment sediment sediment sediment sediment

CURL, INSTAAR AMS Laboratory, Colorado, USA; AA, NSF Arizona AMS Laboratory, USA.

Fig. 2. Depth–age models for the sediment profiles from: (a) Lake Grusha and (b) Ak-Khol. The median probability calibrated dates are shown by closed circles and 2 the upper and lower bounding dates (72s) by vertical lines. The sample in the open circle was excluded from age–depth calculations.



(2004) for Propsilocerus. The morphotypes of Micropsectra, Paratanytarsus and Tanytatsus were identified from the description in Heiri et al. (2004). The morphotype Tanytarsus lugens-type was amalgamated with Corynocera oliveri, as these taxa could not be differentiated in fossil material (Walker and Cwynar, 2006). Most chironomid taxa could be identified to the generic level, but in some cases, larger taxonomic grouping was necessary (e.g., Cricotopus/Orthocladius, Stempellinella/Zavrelia). In order to summarise and to estimate major trends in the chironomid assemblages through time, a preliminary detrended correspondence analysis (DCA) showed that the first axis length is larger than 2 standard deviation units (3.327 SD units) in the chironomid stratigraphy for Lake Grusha and smaller than 2 standard deviation units (1.233 SD units) in the Ak-Khol stratigraphy. Therefore, for the Lake Grusha assemblages a unimodal response model was assumed and DCA was selected as the appropriate ordination method, whereas for the Ak-Khol assemblages a linear response model was used together with principal components analysis (PCA) based on Euclidean distances. All ordinations were accomplished with the program CANOCO for Windows version 4.0 (ter Braak and Sˇmilauer, 1998). A constant equal to the largest negative sample score on the PCA was added to the sample scores to remove all negative scores prior to stratigraphic plotting. In order to identify the major assemblage trajectories through time in Ak-Khol, sample scores were plotted as trajectories of 5-point running means. Stratigraphic diagrams were produced with the software packages TILIA 2.0 b.4 and TILIAGRAPH 2.0 b.5 (Grimm, 1991). The stratigraphy was zoned with the constrained optimal sum of square partitioning (Birks and Gordon, 1985), and the number of statistically significant zones was determined with the broken-stick approach (Bennett, 1996) using the software package Psimpoll 4.10

(Bennett, 2002). In all the numerical analyses, percentage species data were transformed to square roots in an attempt to stabilize variances among taxa. All ordinations were made after data transformation and with rare taxa down-weighted. 3.4. Sedimentation limit, spatial pattern of sedimentary environment, and benthic invertebrates The most important aspects of various theoretical and empirical models of sedimentation in lakes are: (i) the effective fetch of the open water surface over which wind can generate waves, and (ii) the so-called ‘sedimentation limit’ (Sly, 1978), or ‘wave base’ (Johnson, 1980), or ‘critical limit’ (Ha˚kanson, 1982a, b), i.e. the ‘critical’ water depth separating the lake volume into two zones (Fig. 3). Within the first zone (Zet) that lies above the critical water depth, wind/wave-induced processes of sediment erosion and/or transport dominate and wind-induced turbulence prevents deposition of fine-grained sediments. Within the other zone (Za) that lies below the critical water depth and is unaffected by wind-induced turbulence, processes of accumulation dominate and allow deposition of finegrained sediments. In shallower zone Zet coarse (sand and gravel) deposits dominate, whereas deposits within deeper zone Za are fine-grained and comparatively loose, with a high water and organic-matter content (Ha˚kanson and Jansson, 1983). The grain size of sediments generally decreases with increasing water depth (Sly et al., 1983), and in the context of the two zones the limit between fine and coarse sediments is about 6 mm, i.e. as ‘medium’ silt (Ha˚kanson, 1982b). Shuman (2003) showed that sediment organic matter content, as measured by LOI, varies very little (2–5%) across zone Za of shallow lakes, whereas LOI decreases 5–7 times at the sedimentation limit.

Fig. 3. Conceptual schema showing the position of ‘critical’ water depth, zone Zet and area Aet of sediment erosion and/or transport, zone Za and area Aa of sediment accumulation in a hypothetical shallow lake with relatively simple bathymetry at high (a; lake elevation ¼ X m a.s.l.) and low (b; lake elevation ¼ X1.5 m a.s.l.) lake stand (after Ha˚kanson, 1982a, b).


Benthic invertebrates, including chironomid assemblages, can be strongly influenced by the substrate available for colonization by the larvae (e.g., Pinder, 1986), and, thus, the differences between zones Zet and Za in the sediment dynamics, the sediment organic matter content and the grain size of sediments induce a strong difference between zones in the types of habitats available for chironomids. Heiri (2004) studied variability of fossil chironomid assemblages across shallow lakes and presented evidence that the sediment organic matter content and water depth are statistically significant in explaining the variance in the chironomid data. In broad-scale biogeographical studies presented by Francis (2004) and Nyman et al. (2005), the sediment organic matter content has been also shown to be one of main factors affecting chironomid distributions in shallow lakes. In our study of the shallow and unstratified lakes, the separation of indicator taxa of zones Zet and Za was used for the interpretation of changes in a ratio Aet:Aa between area of sediment erosion and/or transport (Aet) and area of sediment accumulation (Aa) (Fig. 3). This ratio was regarded as a function of the complex pattern of sedimentary environment in the lake, which integrates such main factors as the mean lake depth, the sediment organic matter content, and the grain size of sediments. The changes in the Aet:Aa habitat ratio through time were

711

used for the ensuing climatic (effective moisture) interpretation in terms of lake-level fluctuations and changes in the mean lake depth. Possible alternative explanations for the changes in the Aet:Aa ratio were also considered, following Dearing (1997). 4. Results and interpretation 4.1. Lithology The 247 cm sediment sequence from Lake Grusha was separated into two lithostratigraphic units (Fig. 4). The lowest ca 13 cm are highly minerogenic grey-beige silt. From a sediment depth of ca 234 cm to the core top the sediment consists of dark/light brown to olive-grey gyttja, with three layers of abundant fibrous macrofossil plant fragments (Fig. 4). The lowest layer (235–226 cm) includes fragments of aquatic mosses. The two upper layers (193–155 and 75–45 cm) include predominantly sedge (Carex sp.) fragments. The 443 cm sediment sequence from Ak-Khol was also separated into two lithostratigraphic units (Fig. 5). Below ca 400 cm sediment depth, the deposits consist of carbonate-rich light grey silt and sand, with a dark sandy layer between 400 and 402 cm. The upper 400 cm section consists of lighter to darker olive-grey gyttja.

Fig. 4. Stratigraphy of the sediment profile from Lake Grusha: lithology, selected chironomid taxa as percentages of the total number head capsules, chironomid head-capsule concentration (capsules g1 of wet sediment), the scores of the first two DCA axes (standard deviation units), organic content of sediments as percentage loss-on-ignition at 550 1C (LOI 550 1C), and statistically significant zones for chironomid assemblages. Environmental phases follow the synthesized outline in the text and Fig. 8.



Fig. 5. Stratigraphy of the sediment profile from Ak-Khol: lithology, selected chironomid taxa as percentages of the total number head capsules, chironomid head-capsule concentration (capsules g1 of wet sediment), the scores of the first two PCA axes (Euclidean distances), organic content of sediments as percentage loss-on-ignition at 550 1C (LOI 550 1C), and statistically significant zones for chironomid assemblages. Environmental phases follow the synthesized outline in the text and Fig. 8.

4.2. Notes: Chironomid taxa and their ecological preferences In total, 35 chironomid taxa were identified in the sediment sequence from Lake Grusha. Only 24 taxa have at least two occurrences, with a minimum relative abundance of 2% (Fig. 4). Rare taxa (abundance o2%) are Bryophaenocladius, Cladopelma, Constempellina brevicosta, Cryptochironomus, Endochironomus albipennis-type, Georthocladius luteicornis-type, Glyptotendipes, Hydrobaenus, Mesopsectrocladius, Microchironomus, and Parachironomus arcuatus-type. In the sediment sequence from Ak-Khol, 33 chironomid taxa and Acarina were identified. Only 20 chironomid taxa have abundances X2% in at least two samples (Fig. 5). Rare taxa (abundance o2%) are Camptocladius, Eukiefferiella/ Tvetenia, Nanocladius, Monopsectrocladius septentrionalistype, Pseudochironomus prasinatus, Cryptochironomus, Endochironomus, Pagastiella orophila, and Glyptotendipes. In the study lakes, among more common taxa are Paracladius, Monodiamesa, Protanypus, Micropsectra radialis-type, M. insignilobus-type and Stictochironomus, which are commonly associated with cold environments, and Dicrotendipes, Cladotanytarsus mancus-type, Ablabesmyia, Microtendipes pedellus-type, Chironomus plumosus-

type, Ch. anthracinus-type, Einfeldia, Tanytarsus mendaxtype (as Tanytarsus sp. B in Brooks and Birks (2001)), Polypedilum, Parakiefferiella bathophila-type, and Stempellinella/Zavrelia, which are commonly associated with warm environments, according to the different training sets (cf. Lotter et al., 1997; Walker et al., 1997, 2003; Olander et al., 1999; Brooks and Birks, 2001; Larocque et al., 2001; Francis, 2004; Heiri and Millet, 2005; Barley et al., 2006). Taxa with a broader range of thermal tolerances according to the mentioned above training sets, such as Procladius, Psectrocladius sordidellus-type, Sergentia coracina-type, and Corynoneura scutellata-type, are also common. By using the information about modern bathymetric preferences of chironomids provided in Sæther (1979), as well as a wide range of other literature (Brundin, 1949; Pankratova, 1970, 1983; Wiederholm, 1983; Makarchenko and Makarchenko, 1999; Heiri, 2004; Barley et al., 2006; Boggero et al., 2006), all recorded taxa can be separated into three groups as inhabitants of shallow zone Zet, inhabitants of deeper zone Za, and eurybathic taxa. Dicrotendipes, Cladotanytarsus mancus-type, Ablabesmyia, Microtendipes pedellus-type, Pagastiella orophila, Cladopelma, Einfeldia, Psectrocladius and Stempellinella/Zavrelia are frequently associated with minerogenic sediments or aquatic


macrophytes of shallow zone Zet. Sergentia coracina-type, Protanypus, Chironomus plumosus-type and Ch. anthracinustype frequently prefer habitats along fine-grained and organic-rich sediments within deeper zone Za. The remaining taxa occur across a wide bathymetric range. In the study lakes, Paratanytarsus penicillatus-type and Tanytarsus lugens-type are the most abundant taxa. In Fennoscandia, P. penicillatus-type (Brooks and Birks, 2001) and T. lugens-type (Olander et al., 1999; Larocque et al., 2001) show little sensitivity to temperature and apparently both taxa are characterized by a wide range of thermal tolerances. However, these taxa likely differ in the habitat preferences along bathymetric gradient. Paratanytarsus penicillatus according to Sæther (1979) is a typical member of shallow-water communities. T. lugens-type is characterized as deep-water inhabitant (e.g., Hofmann, 1988; Korhola et al., 2000), and, in shallow lakes, it is also more associated with deeper zone Za of lake basin (Heiri, 2004). 4.3. Chironomid stratigraphy 4.3.1. Lake Grusha Five statistically significant zones were determined in the chironomid stratigraphy from Lake Grusha (Fig. 4). The first zone (Gr-1; 246–239 cm) corresponds to the initial phase of the lake formation. It is characterized by the lowest concentration of chironomid head capsules. The zone is associated with a wide diversity of cold-temperature indicators, such as Micropsectra insignilobus-type, Paracladius, Stictochironomus, Monodiamesa and Protanypus, along with two warm-temperature indicator taxa Polypedilum and Parakiefferiella bathophila-type. The deepwater inhabitant Tanytarsus lugens-type is also common. Zone Gr-2 (239–225 cm) is also characterized by a high abundance of cold-temperature indicator taxa Micropsectra radialis-type and M. insignilobus-type. Taxa with a broader range of thermal tolerances, such as Psectrocladius sordidellus-type and Paratanytarsus penicillatus-type, become common, along with the warm-temperature indicators Microtendipes pedellus-type and Chironomus anthracinustype. Tanytarsus lugens-type is present throughout the zone. Zone Gr-3 (225–185 cm) is distinguished by faunistic heterogeneity. The deep-water inhabitant Tanytarsus lugens-type disappears. Only in the lower and upper parts of the zone, warm-adapted taxa Chironomus anthracinus-type and Dicrotendipes are common, whereas in the middle part of the zone cold-adapted taxa Micropsectra radialis-type and Paracladius are major components of the assemblages. Psectrocladius sordidellus-type and Paratanytarsus penicillatus-type remain abundant through the whole zone. In zone Gr-4 (185–43 cm) cold-temperature indicator taxa become minor components of the chironomid assemblages. Paratanytarsus penicillatus-type attains its highest abundance. Warm-adapted shallow-water inhabitants Cladotanytarsus mancus-type, Ablabesmyia, Einfeldia,

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Tanytarsus mendax-type (as Tanytarsus sp. B in Brooks and Birks (2001)) and Stempellinella/Zavrelia appear in this zone, and along with Dicrotendipes become common. Deep-water inhabitants are presented by warm-adapted Chironomus plumosus-type and Ch. anthracinus-type. Zone Gr-5 (43–0 cm) is associated with the reappearance and the highest values of Tanytarsus lugens-type and Sergentia coracina-type. Paracladius becomes again more common. All warm-temperature indicators, except for Microtendipes pedellus-type, disappear or show extremely low abundances. A low chironomid head-capsule concentration characterizes the zone. 4.3.2. Ak-Khol Single remains of Cricotopus/Orthocladius were found only in the two basal samples (438 and 433 cm) from AkKhol. Four statistically significant zones were determined in the chironomid stratigraphy of the upper sediment profile, where Paratanytarsus penicillatus-type with a broad range of thermal tolerance dominates throughout (Fig. 5). The first zone (Ak-1; 423–360 cm) is associated with a high abundance of the deep-water inhabitant T. lugenstype and the lowest concentration of chironomid head capsules. Zone Ak-2 (360–200 cm) is characterized by a shift from Tanytarsus lugens-type to the shallow-water inhabitants Cladotanytarsus mancus-type, Tanytarsus mendax-type, and Dicrotendipes, which prefer warm environments. Among other warm-adapted taxa of this zone, Chironomus plumosus-type shifts to Ch. anthracinus-type. The lower part of the zone (315–285 cm) is characterized by an abrupt short-term decrease in C. mancus-type and T. mendax-type and an increase in Paratanytarsus penicillatus-type. In zone Ak-3 (200–32 cm), Tanytarsus lugens-type again becomes the dominant taxon. The abundance of all warmadapted shallow-water inhabitants declines, with a shift from Chironomus anthracinus-type to Ch. plumosus-type. The highest chironomid head-capsule concentration is observed in this zone. The last zone Ak-4 (32–0 cm) is characterized by gradual decrease in Tanytarsus lugens-type and increase in Sergentia coracina-type and cold-temperature indicator Paracladius. The warm-adapted taxa Psectrocladius sordidellustype and Chironomus plumosus-type disappear completely, and another warm-adapted taxon Ch. anthracinus-type reaches a maximum. All other warm-temperature indicators show extremely low abundances. The chironomid head-capsule concentration decreases upwards within this zone. 4.4. Assemblage successions and patterns of change 4.4.1. Lake Grusha The results of the DCA ordination (Fig. 6) also reveal five distinct clusters from the Lake Grusha assemblages that correlate with five statistically significant zones determined by optimal partitioning. DCA produces two



Fig. 6. Scatter bi-plot of a detrended correspondence analysis (DCA) of the Lake Grusha chironomid stratigraphy showing the sample scores (a) and the species scores (b) of the most abundant taxa and some indicator taxa. Arrows indicate the directions of possible major environment variables (dashed line) and chironomid assemblage trajectory (unsmoothed) through time (solid line).

significant axes that together explain 43% of cumulative variance in the chironomid data. The assemblages dominated by cold-adapted taxa are located on the left-hand side of the ordination, whereas the assemblages dominated by warm-adapted taxa are located on the right-hand side of the ordination. Such distribution of the assemblages along DCA axis 1 suggests that this axis can be considered as a temperature axis. DCA axis 2 appears to be driven mainly by the deep-water inhabitant Tanytarsus lugens-type. The assemblages dominated by T. lugens-type are located in the lower part of the ordination, whereas the assemblages characterized by a low abundance or lack of this taxon are located in the upper part of the ordination. Such distribution of the assemblages along the second axis suggests that this axis can be considered as a depth axis. Taking into account the species scores, the overall assemblage trajectory in the Lake Grusha ordination suggests that changes in the late-glacial assemblage mainly follow the temperature gradient towards ca 13.2 cal kyr BP (Fig. 6). The trajectory of assemblages appears to be driven by both the depth and temperature gradients at ca

Fig. 7. Scatter bi-plot of a principal components analysis (PCA) of the Ak-Khol chironomid stratigraphy showing the sample scores (a) and the species scores (b) of the most abundant taxa and some indicator taxa. Open circles indicate samples from zone Ak-1 and Ak-3, squares zone Ak2, and diamonds zone Ak-4. Arrows indicate the directions of possible major environment variables (dashed line) and chironomid assemblage trajectory through time (solid line); the assemblage trajectory was based on five-sample running mean values of the sample scores.

13.2–11.9 and ca 6.5–5.3 cal kyr BP. Between ca 11.9 and ca 6.5 cal kyr BP and after ca 5.3 cal kyr BP, there is little variation along both gradients in the ordination. 4.4.2. Ak-Khol The results of PCA ordination, based on Euclidean distances (Fig. 7), agree with the pattern of optimal partitioning of the Ak-Khol assemblages into the statistically significant zones. PCA produces two significant axes that together explain 44% of the total variance in the chironomid data. The assemblages dominated by coldadapted taxa are located on the left-hand side of the ordination, whereas the assemblages dominated by warmadapted taxa are located on the right-hand side of the ordination. The pattern of distribution of the Ak-Khol assemblages along the second PCA axis is generally similar


to one of the Lake Grusha assemblages along the second axis in the DCA ordination. This suggests that the first axis can also be considered as a temperature axis and the second axis as a depth axis in the PCA ordination of the Ak-Khol assemblages just as in the DCA ordination of the Lake Grusha assemblages. Taking into account the species scores, the assemblage trajectory in the Ak-Khol ordination suggests that earlyHolocene assemblage changes mainly follow the depth gradient towards ca 8.8 cal kyr BP (Fig. 7). The trajectory appears to be also driven largely by the temperature gradient during the period ca 8.8–8.5 cal kyr BP. The trajectory appears to be driven mainly by both gradients between ca 8.5 and ca 5.9 cal kyr BP. There is little variation along both gradients at ca 5.9–1.8 cal kyr BP. The trajectory mainly follows the depth gradient after ca 1.8 cal kyr BP. 5. Discussion The age–depth model for Lake Grusha suggests that the lake basin became open for the lacustrine sedimentation after deglaciation at ca 15.8 cal kyr BP (247 cm sediment depth). The lacustrine sedimentation in the Ak-Khol basin started at ca 11.5 cal kyr BP (423 cm sediment depth), according to the age–depth model. The presence and

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dominance of lacustrine chironomid taxa in the basal sediment samples from Lake Grusha (246 cm) and AkKhol (423 cm) confirm the onset of lacustrine conditions at these periods. On the basis of chironomid and lithological stratigraphy of the sediment profiles from both lakes, seven successive phases (phases I–VII) reflecting the prevailing environmental conditions can be distinguished through the history of the lakes and climatic changes in the area (Fig. 8). 5.1. Assemblage successions and environment changes 5.1.1. Phase I (ca 15.8–14.6 cal kyr BP) All the evidence from the Lake Grusha core points to a cool climate resulting in very low bioproductivity in this initial lake. The accumulation of clayey silt dominated in the lake. This inorganic allochthonous material was likely transported to the lake basin by glacier-fed streams and slope-wash at a time when erosion rates were high. The chironomid assemblages dominated by Micropsectra insignilobus-type indicate oligotrophic conditions (Sæther, 1979) and cold well-oxygenated water (Brodersen and Anderson, 2002) in the lake. A gradual increase in M. insignilobus-type, a taxon commonly preferring deep-water habitats in lakes (Heiri, 2004), and the presence of other deep-water inhabitants, such as Tanytarsus lugens-type and

Fig. 8. Synthesis of climatic and environmental changes based on the chironomid and lithological stratigraphy of the sediment profiles from Lake Grusha and Ak-Khol.



Protanypus, suggest a gradual rise in lake level throughout the phase. 5.1.2. Phase II (ca 14.6–13.1 cal yr BP) Subsequent changes in the Lake Grusha chironomid assemblages suggest a response to continued warming with the disappearance of cold-adapted taxa, such as Monodiamesa and Protanypus, and the appearance of warmadapted Microtendipes pedellus-type and Chironomus anthracinus-type. The lake level seemed to remain rather high at the increasing temperatures and accelerating evaporation rate during the phase. This may suggest that increasing effective moisture was due to higher precipitation coinciding with the warming. About ca 14.1 cal yr BP, a short-term warmer oscillation can be inferred from a brief appearance of Chironomus plumosus-type and peaks of Ch. anthracinus-type and Psectrocladius sordidellus-type. The temperature oscillation likely induced an extensive distribution of aquatic mosses at the bottom of this rather cold, deep and clear-water lake. The large amount of moss fragments found in the sediments between 235 and 226 cm depth (Fig. 4) is consistent with the suggested event. Aquatic mosses often dominate among submerged macrophytes in oligotrophic lakes with high water transparency (Karttunen and Toivonen, 1995; Toivonen and Huttunen, 1995). In cold Arctic and Antarctic lakes, mosses are typically the only macrophytes present (Priddle, 1980; Imura et al., 1999; Sand-Jensen et al., 1999) and can extend to the profundal zone up to 19 m depth (Welch and Kalff, 1974). A prominent feature of the chironomid record that may be related to the expansion of aquatic mosses includes the peak of Ch. anthracinus-type, which is frequently a dominant taxon in invertebrate communities of submerged moss beds (Ilyashuk, 1994). Thus, a peak of LOI values during the middle part of the phase may be mainly related to increased productivity of benthic biotopes, whereas pelagic bioproductivity in the lake remained low. The later part of phase II, namely the minor episode marked in the Lake Grusha record at ca 13.3–13.1 cal yr BP, may be characterized as a short-term abrupt warming, the onset of which is indicated by the appearance or reappearance of warm-adapted Dicrotendipes, Tanytarsus mendax-type, and Chironomus plumosus-type. It is noteworthy that the second peak of LOI values occurs in the moss-barren sediments corresponding to the later part of phase II (Fig. 4). An increase in the planktonic algal biomass may be a response to the warming during this episode. In turn, an increase in lake-water turbidity as a response to increase in phytoplankton biomass may commonly result in a decrease in water transparency affecting submerged macrophytes (Scheffer et al., 1993; Jeppesen et al., 1998). Thus, we assume that the increase in water turbidity, as a response to the increase in phytoplankton production during the warming, induced a shift of Lake Grusha from a clear-water state dominated by submerged mosses to a turbid state dominated by

phytoplankton during this short-term episode. This shift coincides with the reappearance of benthic diatom algae (226 cm; Westover et al., 2006), which occurred first at 236 cm sediment depth and disappeared when aquatic mosses began to dominate the Lake Grusha. 5.1.3. Phase III (ca 13.1–12.1 cal yr BP) The changes in the Lake Grusha chironomid assemblages during the phase suggest the prevalence of a cold and dry climate. Warm-adapted taxa and deep-water inhabitant Tanytarsus lugens-type that peaked in the previous period were succeeded by Micropsectra radialistype and Paracladius, which are more cold-adapted taxa according to different training sets (e.g., Lotter et al., 1997; Walker et al., 1997; Brooks and Birks, 2001; Larocque et al., 2001). Furthermore, the maximum of Psectrocladius sordidellus-type may also indicate cold conditions, for the taxon is characterized by its highest abundances in cold mountain lakes, although it has a broad range of thermal tolerance (Heiri and Millet, 2005). Broadly similar peaks of P. sordidellus-type have been recorded in the chironomid assemblages corresponding to the cold Younger Dryas in the Swiss Alps (Brooks, 2000). If M. radialis-type tends to dominate shallow-water assemblages, in contrast with T. lugens-type (Heiri, 2004), it is possible that Lake Grusha had a low stand and was characterized by a relatively high Aet:Aa habitat ratio during the phase as result of decreased effective moisture under fairly arid conditions. The lake apparently had a decreased bioproductivity, accounting for the lower LOI values registered for this period. Pollen assemblages reflect an expansion of tundra plant communities during the phase (Blyakharchuk et al., 2006). Corresponding diatom assemblages also suggest cold climatic conditions, as indicated by the dominance (up to 50%) of Staurosirella pinnata, the highest abundance of which can be found today in tundra lakes of Arctic Siberia (Westover et al., 2006). 5.1.4. Phase IV (ca 12.1–8.5 cal yr BP) Shortly following the onset of phase IV, the single remains of Cricotopus/Orthocladius, suggesting the existence of running water and/or semi-aquatic biotopes in the Ak-Khol depression, were replaced with abundant lacustrine chironomid assemblages, indicating that the lacustrine sedimentation started in the depression after ca 11.5 cal yr BP. The beginning of the phase (ca 12.1–10.8 cal yr BP) is characterized by an abrupt reversion to warmer temperatures. This event is inferred from the complete disappearance of cold-adapted Micropsectra radialis-type in Lake Grusha, and the presence of warmadapted taxa such as Dicrotendipes and Chironomus anthracinus-type in both lakes. After ca 10.8 cal yr BP, the chironomid assemblages of both lakes are characterized by an appearance of new warm-adapted taxa and a gradual increase in the proportion of all warm-adapted taxa. These changes point to a gradual warming at a later period (ca 10.8–8.5 cal yr BP).


Amelioration of climate commonly is accompanied by intensive glacier floods triggered by the outburst of water reservoirs in, on, underneath, and at the margins of glaciers in mountain areas (Barry, 1990). Accelerated warming leads also to an activation of other glacier-related processes and widespread occurrence of ice avalanches and debris flows in glacierised mountain areas (Alean, 1985; Kaä¨b et al., 2005). As reported by Ivanovskiy (1967, 1981), the ancient avalanche chutes likely corresponding to the last glacial–interglacial transition are widespread on flanks of the Altai Mountains. The last phenomenal glacial superflood occurred west of our study area in the Altai Mountains, in the Chuya and Katun valleys, at the lateglacial period (Baker et al., 1993; Rudoy, 2002). Apparently, the glacierized catchment-area of Lake Grusha responded dramatically to the rapid warming at the beginning of phase IV. The area contains flat landforms that would accumulate deep ice and snow as well steep slopes with coarse and unstable parent material that could have resulted in frequent ice and debris avalanches at the warming. Moreover, glacier lake outburst floods could have occurred in the catchment-area of Lake Grusha during this period. Material transported by ice/debris avalanches and glacier lake outburst floods is a source of large amount of terrestrial plant remains. Thus, a high sedimentation rate and the large amount of terrestrial plant macrofossils along the sedimentary record dated between ca 12.0 and ca 10.8 cal yr BP (Fig. 4) may be related to the large terrestrial origin input(s), like the supposed dramatic glacier-related event(s) at the catchment-area of Lake Grusha during this period. Perhaps, only the easy fraction of suspended allochthonous material, as slope-wash tracks, could have reached the Lake Grusha basin. It is noteworthy that a pooling of water in the Ak-Khol depression and an enlargement of water body started at ca 11.5 cal yr BP, which could have also been induced by intensive glacier-related hydrological and geomorphological processes. Now, due to the variety of dramatic hydrological and geomorphological processes in mountain areas at the warming, it is difficult to reconstruct exactly the pattern of glacier-related events in the region. Moreover, an increased rainfall during this warm and humid period could have also induced an intensive slope-wash and transport of allochthonous material into the lake basins. The reappearance of the deep-water inhabitants Chironomus anthracinus-type and Ch. plumosus-type in Lake Grusha suggests a rise in water level at the beginning of the phase. Probably, a high sedimentation rate affected some of inhabitants of the deep-water zone Za but resulted in the thriving of shallow-water Paratanytarsus penicillatus-type that prefers habitats where severe processes of sediment transport occur. The high abundance of the deep-water inhabitant Tanytarsus lugens-type in Ak-Khol also point out a high lake stand. Thus, the relatively high water level in both lakes throughout much of the phase (Figs. 6 and 7) implies a humid climate during this time span. This is in agreement with the diatom record from Lake Grusha

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Fig. 9. Stratigraphic profile of the proportions of planktonic diatoms (Cyclotella ocellata+Cyclotella spp.; Westover et al., 2006) and chironomid-inferred environmental phases at Lake Grusha.

(Westover et al., 2006), which indicates the maximum relative abundance of planktonic diatoms (up to 4–6%) during ca 12.0–8.5 cal yr BP, suggesting the high lake stand at this phase (Fig. 9). However, the pollen data suggest that more humid conditions occurred only after 11 cal yr BP, when afforestation began (Blyakharchuk et al., 2006; Westover et al., 2006). To all appearance, the difference can be explained by some time-lag in the response of terrestrial vegetation to the rapid environmental change because factors such as migration and pedogenesis play an important role in a development of terrestrial plants, while diatoms and chironomids may react with a minor or even without any time-lag to a rapidly changed climate. Of special interest is the finding of the so-called radiocarbon plateau (i.e., a part of sequence where there are relatively constant radiocarbon ages (Ammann and Lotter, 1989)) in the Lake Grusha sediment sequence. The plateau corresponds to the earlier part of the phase between ca 11.25 and ca 10.85 cal yr BP (i.e., approximately 400 years long). A radiocarbon plateau dated to ca 10.0



14

C kyr BP (ca 11.5 cal yr BP) is well-known (Wohlfarth, 1996) from other sediment sequences (e.g., Ammann and Lotter, 1989; Bjo¨rck et al., 2002) and dendrochronological records (e.g., Becker et al., 1991; Spurk et al., 1998). The plateau time span of several hundred years coincides with the period of the abrupt changes in temperature and atmospheric 14C/12C ratio and may be interpreted as reflecting the last glacial–interglacial transition (Becker et al., 1991; Bjo¨rck et al., 1996). Thus, the radiocarbon plateau recorded in the Lake Grusha sediment sequence is in agreement with the abrupt chironomid-inferred climatic changes during the beginning of the phase (ca 12.1–10.8 cal yr BP). 5.1.5. Phase V (ca 8.5–5.9 cal yr BP) Phase V is distinguished in the Lake Grusha lithostratigraphy and the chironomid stratigraphy of both lakes. The lowest relative abundance of Tanytarsus lugens-type and dominance of the shallow-water taxa in Ak-Khol suggests a low lake stand due to decreased effective moisture induced by warm and dry climate. The Ak-Khol bioproductivity was rising during the time span as reflected by the LOI values. In the Lake Grusha chironomid stratigraphy, the increase in Chironomus plumosus-type is marked during this phase. It suggests low oxygen availability and periods of anoxia resulting from enhanced respiration by decomposing organic material in sediments. An increase in organic matter content in sediments, accounting for the highest LOI values, could strengthen the process. This warm and dry period very likely led to a low water level and a reduction of the lake area. A former littoral zone of the lake became suitable for an expansion of wetland biotopes and associated plants. The large amount of wellpreserved sedge (Carex sp.) macrofossils and the highest LOI values recorded in the sediments of this period is consistent with the suggested environmental events. The increase in Cricotopus/Orhtocladius, which is commonly associated with macrophytes, is likely related to the expansion of wetland biotopes. A decrease in relative abundance of planktonic diatoms in Lake Grusha after ca 8.5 cal yr BP also implies the lower lake level and thus drier climate (Fig. 9). To all appearance, the lakes differ in mechanism and pattern of response to climatic changes owing to differences in the hydrological regime. Lake Grusha likely had a topographically closed basin during this phase as well as throughout the previous lake history. Accordingly, Lake Grusha was characterized by a high sensitive to effective moisture changes. Chironomid inferred lake-level changes suggest that a magnitude of this lake-level fall (phase V) was significantly less than it was during phase III (ca 13.1–12.1 cal yr BP). Though Ak-Khol apparently had a topographically open basin throughout the lake history, its hydrological balance strongly depended on the hydrological regime of the Chedi-Tei River, as in a system between river and floodplain lake. At dry periods, the water level of

Ak-Khol apparently responded to a low-flow hydrological regime of the river. Discussing the pollen records from these lakes, Westover et al. (2006) and Blyakharchuk et al. (2006) suggest that the period ca 8.2–6.6 cal yr BP was the warmest and the most humid, based on the maximum occurrence of fir Abies sibirica. However, it is very likely that favourable temperature rather than humidity was a more important factor for the extent of this most thermophilous species (among Siberian trees) along the slopes of surrounding ranges after ca 8.2 cal yr BP. Perhaps the effective moisture, which was high during the period ca 12.0–8.5 cal yr BP as shown by chironomids and diatoms, decreased after 8.5 cal yr BP but remained sufficient to support the existence of dark-coniferous forests as separate islands in wetter places. Moreover, the current forest limit is ca 40 km to west of the study site, and it is possible that the catchments of Lake Grusha and Ak-Khol were never forested (Westover et al., 2006). To all appearance, there was long-distance transport of tree pollen from sites where local temperature and humidity conditions were more favourable. 5.1.6. Phase VI (ca 5.9–1.8 cal yr BP) The highest relative abundance of Tanytarsus lugens-type in both lakes within phase VI indicates high lake stands. The period was apparently coldest and wettest during the Holocene. The reappearance of Sergentia coracina-type in both lakes and Paracladius in Lake Grusha, taxa preferring cold environment, supports this assumption. The presence of taxa having a high oxy-regulator capacity (Brodersen et al., 2004) and tolerance to low oxygen concentrations (Little and Smol, 2001), such as Chironomus plumosus-type in Ak-Khol, and Procladius in Lake Grusha, suggests that lake-water temperature stratification and decreased dissolved oxygen concentration in the bottom water and sediments of both lakes existed. It implies that the lakes were characterized by a high stand resulting from increased effective moisture. As evident from LOI curves, the AkKhol bioproductivity, which increased steadily during the previous warm phase, slightly decreased and persisted at nearly a constant level during phase VI. In Lake Grusha, it decreased distinctly. The appearance of shallow-water taxon Microtendipes pedellus-type in both lakes within this colder time span and its absence from the previous warm periods is interesting, for the taxon is regarded as a warm temperature indicator (e.g., Lotter et al., 1997; Larocque et al., 2001). The lateglacial samples from Lake Grusha corresponding to wet and cool climate (phase II) have also the high relative abundance of M. pedellus-type (Fig. 4). Most likely, the relatively low temperatures could induce a weakening of lake bioproductivity as well as a retardation of the microbial activity and an associated decrease in the decomposition of organic material in the sediments. As a consequence, the accumulation of coarse organic matter could increase. In turn, M. pedellus-type prefers a coarse


substratum in the low-productivity shallow lakes (Brodersen and Lindegaard, 1997, 1999). Apparently, the trophic conditions and the grain size of sediments in the lakes became more favorable for M. pedellus-type at these cool phases than during the warm periods. Thus, sediment composition and trophic conditions rather than temperature is likely the more important factor influencing the distribution of the taxon in lakes. The dramatic changes in the Lake Grusha chironomid stratigraphy, related to the increase in relative abundance of the deep-water inhabitants, imply a pronounced decrease in the Aet:Aa habitat ratio, suggesting a rapid lake-level rise at the beginning of the phase. The lake-level rise resulted in a reduction or a disappearance of the wetland biotopes and the associated helophytes (Carex sp., etc.), that encircled the lake during the previous phase V. To all appearance, the lake remained topographically closed up to ca 5.9 cal yr BP and reached the outlet threshold under the rapid climatic shift, i.e. the closedbasin lake was converted to a perennial topographically open-basin lake after ca 5.9 cal yr BP (Figs. 6 and 8). A significant reduction in initial volume of the lake basin owing to a gradual accumulation of sediment through the previous lake history was one more factor that promoted the crossing of a hydrological threshold and the opening of outlet to the Chedi-Tei River at the beginning of this cold and wet phase. Undoubtedly, the chironomid-based inference of the switch between a closed and open basin of Lake Grusha is tentative and may be tested and improved by using an analysis of oxygen and carbon isotopes designed to assess hydrologic variability in lakes (Valero Garce´s et al., 1995; Li and Ku, 1997). The chironomid-inferred cooling is in agreement with diatom and pollen evidence of colder conditions. In the diatom stratigraphy of Lake Grusha the increase in abundance of Staurosirella pinnata preferring cooler temperatures occurs after ca 6.0 cal yr BP (Westover et al., 2006), coinciding with the deforestation indicated by the pollen data from both lakes (Blyakharchuk et al., 2006; Westover et al., 2006). According to the assumptions from the pollen record, the reduction of forests was related to cooling and decreasing moisture (Blyakharchuk et al., 2006; Westover et al., 2006). However, as shown by the chironomid record, the effective moisture was rather high during this period. Therefore, we believe that low temperature, but not dryness, became the main factor restricting the extent of forests in this area after ca 6.0 cal yr BP. Apparently, the pronounced cooling resulted in the retreat of forests even from lower elevation areas, and the long-distance transport of tree pollen to the high elevation study site became hampered. 5.1.7. Phase VII (ca 1.8–0 cal yr BP) The last relatively short phase is recognized better in the Ak-Khol chironomid stratigraphy, since the time resolution for each sediment sample from this lake is much higher than from Lake Grusha owing to a much higher

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sedimentation rate during the late Holocene. For this reason it is difficult to detect the recent environmental changes in the Lake Grusha stratigraphy. In Ak-Khol, the decrease in warm-adapted taxa, such as Cladopelma, Tanytarsus mendax-type, Cladotanytarsus mancus-type and Dicrotandipes, and the presence of cold indicators Paracladius and Sergentia coracina-type, albeit at levels below 5%, likely points to slightly decreased temperatures. The steady decrease in Tanytarsus lugenstype towards the present suggests that the lake may have shallowed from the previous phase probably owing to drier climatic conditions. The observed replacement Chironomus plumosus-type by Ch. antracinus-type may have been also indirectly related to the decrease in lake depth through changes in lake-water thermal stratification and improvement of the bottom oxygen conditions. Moreover, as indicated by the training sets of Lotter et al. (1997) and Larocque et al. (2001), Chironomus antracinus-type is more tolerant to cooler temperatures than Ch. plumosus-type. The Ak-Khol bioproductivity seemed to change insignificantly during the period, as registered by the LOI curve. The pollen data point out that the role of steppe elements with Artemisia, grasses, sedges as well as some herbaceous alpine taxa increased especially after ca 2.0 cal yr BP, also suggesting drier climatic conditions (Blyakharchuk et al., 2006). 5.2. General implications for the post-glacial climate development in the region The expression of regional climates in mountain areas can be modified owing to spatial variability of the magnitude of change in limiting environmental factors (Barry, 1990; Huber et al., 2005). This is a basis for disagreement on climatic reconstructions derived even from relatively closely spaced sites as local conditions may notably differ. Some discrepancies occur also in climatic inferences based on different proxy records because controlling factors other than climate can affect biological indicators differently, leading to different patterns of climatic changes (Larocque and Bigler, 2004). Inconsistencies can partly be caused by poor sample resolution, different dating strategies and poor radiocarbon dating control (Lowe, 2001). The chironomid data from Lake Grusha and Ak-Khol are the first late-Quaternary chironomid records with reasonably good resolution and dating control from vast mountain areas of Central Asia. No considerable inconsistencies were detected in the climatic and environmental inferences based on two chironomid records from the different lakes. On the contrary, the records from two mountain lakes located very close together supplement each other, allowing us to make inferences that are more reliable. Below, we compare our results with other available late-Quaternary evidence from the region and adjacent areas, and consider possible causes of the inferred climatic and environmental changes. In the discussion, the



focus is relative temperature and humidity changes that occurred throughout the post-glacial period. 5.2.1. Last glacial–interglacial transition before ca 14.6 cal kyr BP There is evidence that during the Late Pleistocene, the western part of Tuva was covered by the extensive KaraKhol glacier (ca 16,000 km2), the terminal moraines of which are situated at an elevation from 1300 to 2300 m a.s.l. (Efimtsev, 1958, 1961; Prudnikov, 2005). Following the cold and dry glacial period, the progressive warming caused glaciers to melt and triggered a lacustrine pulse in depressions, especially in those situated close to glacier margins. These lake basins apparently filled rapidly, reaching a high stand. According to our records, the thermal reversal triggered the lacustrine pulse in the Lake Grusha depression after ca 15.8 cal kyr BP. Initial warming accompanied by an increase in relative moisture occurred at approximately the same time in the Altai Mountains (Blyakharchuk et al., 2004) and in the Baikal region (Demske et al., 2005) just east of our study site. The climate development induced by orbital forcing presents a global pattern. About 18 cal kyr BP, changes in Earth’s orbital parameters initiated an increase in Northern hemisphere summer insolation, which could in turn induce changes in sea-ice coverage of the northern North Atlantic and the North Atlantic thermohaline circulation, gradual warming of Atlantic Southern Ocean and southward seaice retreat, and synchronous increase in atmospheric concentrations of greenhouse gases (e.g., Imbrie et al., 1992; Stocker, 2003; Bianchi and Gersonde, 2004). All this could account for the onset of the last glacial–interglacial transition. 5.2.2. Bølling-Allerød-like event (ca 14.6–13.1 cal kyr BP) As revealed by the chironomid records, a significant climatic amelioration related to warming and higher effective moisture was marked at the studied area ca 14.6–13.1 cal kyr BP. Generally, lake levels are regarded as a sensitive proxy of regional effective moisture, which is a function of precipitation, evaporation and temperature. Lake Grusha has shown rising lake levels during the first half of the phase, and a relatively constant stand during the second half. Apparently, the intensified precipitation took place during the whole time span and was able to overcompensate for the increased evaporation accompanying the rising temperatures. This climatic event likely correlates with the BøllingAllerød (B/A) interval, which is well established in Europe and dated in the GRIP Greenland ice core to 14.7–12.7 icecore kyr BP (Bjo¨rck et al., 1998). Southern hemisphere records, as a rule, show almost no evidence for the prominent B/A warming. This phenomenon was restricted to the North Atlantic realm (Yu and Eicher, 2001; Andres et al., 2003). It is assumed that a meltwater pulse derived from melting of the Antarctic Ice Sheet during the first step of deglaciation could have induced a drastic reduction in

the Antarctic deep-water formation, a consequent turn-on and an increase of the North Atlantic deep-water formation, thus causing the onset of the B/A warming of the Northern hemisphere and the cold reversal in Antarctica (Weaver et al., 2003). There is ample evidence for a widespread B/A-like event in the Baikal region, including southern Eastern Siberia (Chebykin et al., 2002; Demske et al., 2005) and adjacent areas of northern Mongolia (Fedotov et al., 2004). The climatic amelioration at ca 14.5–13.0 cal kyr BP, accompanied by the establishment of taiga forests in the southern (Altai and Sayan) mountains of Siberia, has been also indicated by palaeoclimatic data from loess deposits (Chlachula, 2003). It is noteworthy that many records from the monsoon climate area of Asia reveal the strong intensification of the Southwest Indian summer monsoon, which is one of the major climate systems in the world (Webster, 1987), at the beginning of the B/A period (e.g., Overpeck et al., 1996; Herzschuh et al., 2005; Qin et al., 2005; Sinha et al., 2005). Likely, at the start of the warming, the pattern of atmospheric circulation could have abruptly changed so that the summer monsoon displaced northward beyond the Tibetan Plateau, China, and monsoon tracks began to reach Tuva and other adjacent areas of southern Siberia and/or an increased moisture transport from the west occurred, resulting in an increase in precipitation. 5.2.3. Younger Dryas-like event (ca 13.1–12.1 cal kyr BP) The Lake Grusha records suggest that the prevalence of a dry and cool climate causing the lake levels to drop took place in southwestern Tuva ca 13.1–12.1 cal kyr BP. The climatic oscillation is clearly recorded over a large area elsewhere in Siberia, including northeastern and eastern Siberia (Anderson et al., 2002), central and southern Yakutia (Andreev et al., 1997), the Baikal region (Demske et al., 2005) and the Altai Mountains (Butvilovskiy, 1993). The dry and colder episode probably corresponds to the Younger Dryas (YD) event originally identified in northwestern Europe and later dated in the GRIP Greenland ice core to ca 12.7–11.6 ice-core kyr BP (Bjo¨rck et al., 1998) and in the GISP2 Greenland ice core to ca 12.9–11.6 icecore kyr BP (Stuiver et al., 1995). The time of the YD event recorded by us in the Lake Grusha chironomid data slightly differs from that recorded in the Greenland ice cores. The difference may be explained by the using mainly 14 C dates obtained using bulk sediment samples for the Lake Grusha chronostratigraphy, while bulk sediment 14C dates provide ages that are generally a few hundred years older than more true ages based on 14C dates obtained using terrestrial macrofossils (e.g., Wohlfarth, 1996). A possible cause of the YD cooling has been identified in the melting of residual ice sheets on the northern continents, intense meltwater influx into the North Atlantic Ocean and a suppression of the North Atlantic deep-water formation (Lehman and Keigwin, 1992). The YD signal was commonly believed to be transferred by ocean circulation. Gradually, evidence of the cooling event have


been accumulated from different parts of the world, including eastern North America, Asia, the Pacific Ocean, and East Africa (Yi and Saito, 2004). Recent reports (IvyOchs et al., 1999; Tschudi et al., 2003) demonstrate glacial advances in both the Northern and Southern hemisphere, which are synchronous with the YD cold event in northwestern Europe, and imply atmospheric transport of this climatic signal. In this way the growth and wastage of the huge Laurentide ice sheet in North America could play an indirect role in the climatic history of Siberia during the late-glacial periods (Wright, 2005). Atmospheric effects of the colder North Atlantic included reduced monsoon strength and a steeper temperature gradient causing stronger winds, which transmitted the signal farther into the Southern hemisphere. Now it is assumed that this abrupt climatic change between 13 and 11 cal kyr BP are of an interhemispheric nature and a global character (Zhou et al., 2001; Andres et al., 2003). There is evidence of cold and dry conditions in areas from the monsoonal Central Asia ca 13.0–11.7 cal kyr BP, caused by a strengthened winter monsoon and a weakened summer monsoon (Yi and Saito, 2004; Qin et al., 2005; Herzschuh, 2006). Hong et al. (2003) documented that the weakest Indian Ocean summer monsoon occurred in the Younger Dryas period. Thus, variation of the Atlantic Ocean thermohaline circulation may take the responsibility for droughts in the lower latitudes, including the Tuva area. 5.2.4. Late-glacial–Holocene transition and the early Holocene (ca 12.1–8.5 cal kyr BP) Our reconstruction suggests that the abrupt climatic changes towards wetter and warmer conditions in the southern mountains of Tuva began shortly after ca 12.1 cal kyr BP. In view of the older ages based on the bulk sediment 14C dates, we may suppose that this climatic event as well as the YD cooling (see above for details) occurred a few hundred years later. The warming may be attributed to enhanced summer insolation of the northern high latitude and forced by elevated atmospheric CO2 and methane concentration during the early Holocene (Smith et al., 2004). This warm and humid phase is in agreement with North Atlantic warming (Bauch et al., 2001) and coherent with an atmospheric reorganization of climatic forcing mechanisms over southwestern Tuva and adjacent areas. It is known that the North Atlantic Oscillation (NAO), which represents a large-scale fluctuation in the air pressure difference between the Azores High and the Iceland Low, influences air temperature and precipitation over large areas of the Northern hemisphere in winter (Hurrell et al., 2003). When the northern high latitudes get warmed, positive values of the NAO index correspond to strong meridional pressure gradient that result in strong, northward displaced westerlies transporting warm, moist air across mid-latitudes (Hoerling et al., 2001). Thus, the warm and moist early Holocene period in southwestern Tuva is likely related to a weakened Siberian anticyclone and predominance of

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moisture-supplying Atlantic cyclones. The wet early Holocene was likely also related to high effective soil moisture resulting from snow and ice melting. Likewise, high temperatures and favorable moisture conditions after ca 10.0 cal kyr BP are reported from the Baikal region (Fedotov et al., 2004; Demske et al., 2005), the central areas of the Altai Mountains (Blyakharchuk et al., 2004), as well as from Mongolia and western China (Yang et al., 2004), and northwestern China (Yu et al., 2006). The peak insolation (7% more than the present value) from ca 12 to ca 8 cal kyr BP (Kutzbach and Gallimore, 1988) has also apparently stimulated weakening of the glacial climate boundary conditions, amplified the sea–land thermodynamic contrast, and generated the pressure gradient needed for a strong Indian and East Asian summer monsoons able to advance far northward into the present arid and semi-arid regions (Overpeck et al., 1996; An et al., 2000; Feng et al., 2004). There is evidence that the Indian monsoon peaked ca 12 cal kyr BP, and the East Asian monsoon had maximum ca 9 cal kyr BP (An et al., 2000). The high effective moisture during the early Holocene was inferred from most records from the area dominated by the Indian Monsoon (Herzschuh, 2006). The early Holocene monsoon maximum coincides with the highest lake levels in northwestern China (Herzschuh et al., 2005) and western and central Mongolia (Grunert et al., 2000; Lehmkuhl and Lang, 2001). High lake levels during the Late-glacial–Holocene transition and the early Holocene occurred in the lakes located in the mid-latitude of mainland China and influenced by the westerly climatic system (Xue et al., 2003). Likely, the enhanced summer monsoon brought large amount of the precipitation to the area beyond the present monsoon limits. This area is much more sensitive to the changes in palaeo-summer monsoon. We suppose that enhanced monsoonal rains were also able to reach southwestern Tuva during the early Holocene, causing there a peak in precipitation. However, not all records from the monsoonal Central Asia indicate optimal moisture conditions at the time following the Late-glacial–Holocene transition (e.g., Feng et al., 2006; Herzschuh, 2006). Rather extreme differences exist even when investigated sites are located close to each other, especially on plateaus, where even minor differences in height can cause notable differences in microenvironments. Herzschuh (2006) suggests that asynchronous climate changes in the whole region may be attributed to spatial differences between dominating circulation mechanisms and/or to the local differences in the uplift and descent of air masses. The local climatic differences demonstrate the necessity of a dense network of palaeoclimatic sites in combination with fine-resolution dating, to advance our knowledge about general patterns of climatic changes within the complicated mountain landscape. 5.2.5. First half of the mid-Holocene (ca 8.5–5.9 cal kyr BP) The records from Tuva lakes suggest that the period between ca 8.5 and 5.9 cal kyr BP was generally dry and



warm. Apparently, the rate of evaporation exceeded the rate of precipitation, leading to a decrease in effective moisture. Semi-desert landscapes likely dominated the area at that time. A mid-Holocene dry interval has also been well documented by Chen et al. (2003a; ca 9–5 cal kyr BP) and Chen et al. (2001, 2003b; ca 7–5 cal kyr BP) in various records from the arid and semi-humid areas in northwestern China, and by Peck et al. (2002; ca 7.1–6.3 cal kyr BP) and Fowell et al. (2003; ca 7.5–4.5 cal kyr BP) from a semi-humid area of north-central Mongolia, and is suggested to be universal phenomena (Chen et al. 2001). Evidently, the mid-Holocene drought extending over this large area may account for the maximum dust flux in Pacific Ocean sediments corresponding to this period (Pye and Zhou, 1989). Desertification of the area likely resulted from low precipitation connected with a weakening summer monsoon, and is related to the gradual regional shift in the maximum precipitation belt from northwest to southeast over the past 10 kyr (An et al., 2000). Thus, dry conditions might have begun and ended later at the further southeastern locations. In central Tibet, for example, far to the south of the studied Tuva lakes, rapid decreases in monsoon precipitation and lake level took place only at 7.0–7.5 cal kyr BP (Morrill et al., 2006). A series of records from southwest Asia also provides the evidence of a first decrease in monsoon strength at ca 7.0 cal kyr BP, following the early Holocene maximum (Morrill et al., 2006 and references therein). This scenario appears to support the notion of asynchronism of climatic changes in monsoonal Asia. Wet mid-Holocene has been documented, for example, in the intramontane Yanchi Basin, south of the Tien Shan range (Wu¨nnemann et al., 2006; ca 8.1–5.4 cal kyr BP), the central Tibetan Plateau (Wu et al., 2006; ca 8.6–5.7 cal kyr BP), the eastern Tibetan Plateau (Liu et al., 2002, as cited by Feng et al., 2006; ca 8.0–6.0 cal kyr BP), and in the northwestern part of the Chinese Loess Plateau (Feng et al., 2004; ca 8.0–6.0 cal kyr BP). The shift in the maximum precipitation belt is interpreted as a response to changing seasonality linked to orbital forcing of the climate (An et al., 2000). However, An et al. (2006) believe that a prolonged dry mid-Holocene in central and northern China occurred only in deserts, and corresponding records possibly reflect a local rather than regional climatic signal. Overall, across the Asian monsoon region, there are considerable regional differences in monsoon history. Dry but cool mid-Holocene conditions were reconstructed for the Baikal region (Demske et al., 2005) and the central areas of the Altai Mountains (Blyakharchuk et al., 2004). Our data, on the contrary, suggest that this period was warm, likely even the warmest during the Holocene. This is in agreement with the reconstructions of Tarasov et al. (1999), which imply that in northern Mongolia (adjacent to our study site), both summer and winter temperatures at 6 uncalibrated 14C kyr (ca 7 cal kyr BP) were 2 1C higher than today. In western and eastern China, the glaciers slowly retreated at 10–8 cal

kyr BP, while at 8–3 cal kyr BP they retreated rapidly (Zheng and Shi, 1982, as cited by Zheng et al., 1998). The Holocene thermal optimum was registered in Western Siberia (Blyakharchuk, 2003) and the central Altai Mountains (Blyakharchuk et al., 2004) before ca 7.5 cal kyr BP, but later in the Baikal region, after ca 6.2 cal kyr BP (Demske et al., 2005) or ca 4.5–2.5 uncalibrated 14C kyr (Karabanov et al., 2000). These variations in temperature might be related to changes in the position and strength of a polar high-pressure system and in the pattern and intensity of atmospheric circulation. Chironomid data from Ak-Khol (Fig. 7) indicate that within this dry and warm phase there was a climatic oscillation (ca 8.2–7.2 cal kyr BP) suggesting cold and still drier conditions. The episode is likely correlative to the abrupt climate event about 8.2 cal kyr BP (the ‘‘8.2k’’ event), when a big outburst flood freshened the North Atlantic and widespread climate anomalies developed across much of the Northern hemisphere. Compilation of different records from Africa and Asia (Alley and A´gu´stsdo´ttir, 2005) provides the evidence of spread of strong drought in these regions, following North Atlantic cooling. Much of this evidence is suggestive of a dry interval longer than the shortlived anomaly (or the 8.2k event) recorded in Greenland ice cores (Alley and A´gu´stsdo´ttir, 2005). 5.2.6. Second half of the mid-Holocene (ca 5.9–1.8 cal kyr BP) The chironomid records from both lakes point to the beginning of cooling in southwestern Tuva after ca 5.9 cal kyr BP. This agrees with Holocene climatic development proposed by Prudnikova (2005) for the Tuva Republic, indicating a general retreat of spruce forests around 5.0 uncalibrated 14C kyr, which was associated with cooling and increasing in continentality of climate. This abrupt mid-Holocene cooling likely marks the onset of the Neoglacial period, which was global in extent and is marked in many records from various regions in both hemispheres. This was a time of alpine glacial readvances and contrasting patterns of hydrological changes. The climate deterioration centred at 5.8 cal kyr BP over monsoonal Asia has been reported by Gasse et al. (1991), Sirocko et al. (1993), Overpeck et al. (1996), Enzel et al. (1999) and Chen et al. (2003b). The transition from warm to cold climate possibly resulted from a combination of different factors including orbital forcing, changes in ocean circulation and variations in solar activity (Magny and Haas, 2004). Our data also suggest that the climate in southwestern Tuva was wet during the period ca 5.9–1.8 cal kyr BP. The similar onset of cooler and wetter conditions in the Altai and Sayan Mountains at ca 4.5 uncalibrated 14C kyr has been inferred by Solomina (1999) on the base of different proxy data. A study of Lake Telmen in north-central Mongolia also suggests increased effective moisture after ca 6.3 cal kyr BP, and still more humid conditions since ca 4.5 cal kyr BP (Peck et al., 2002; Fowell et al., 2003). In the


northern Mongolian Plateau the most humid conditions seem to have occurred from ca 4.5 to ca 1.7 uncalibrated 14 C kyr (Feng et al., 2005). The most humid phase dated to ca 5.4–3.2 cal kyr BP was also marked in the sedimentary record from the Alashan Plateau, northwestern China (Herzschuh et al., 2004). The occurrence of a cooler and wetter climate at ca 4.5 cal kyr BP is reported for the Tengger Deserts, northwestern China (Ma et al., 2004). Zhang et al. (2000) note that cold temperature oscillations in arid northwestern China after ca 5.7 cal kyr BP coincided with high mountain glacier advances on the Tibetan Plateau and in the Tien Shan Mountains. To the north of China and Mongolia, in most mountain areas of the former USSR, including the Altai Mountains, a general cooling, accompanied by enhanced moisture, and glacier advances began after ca 3–2 uncalibrated 14C kyr (Serebryanny and Solomina, 1996; Solomina, 1999; Pattyn et al., 2003). The increased effective moisture accompanying the cooling in northwestern Central Asia was likely a result of reinforced cyclonic westerlies from the North Atlantic, which pushed farther south and east in response to a stronger thermal gradient between high and low latitudes, and caused increased precipitation in the middle latitudes. Considerable moisture transport to southwestern Tuva, northern Mongolia, northwestern China and the Tien Shan region could be influenced by the west-wind system. Most probably, not only enhanced summer rain precipitation but also increased early winter snowfall took place during this period. Additionally, the humidity could have begun to increase due to decreased evaporation when summer temperatures dropped. The enhanced precipitation, combined with the decreased evaporation, would have resulted in increased effective moisture. It should be mentioned that this global mid-Holocene climatic reversal coincided with wetter conditions prevailed over intermediate latitudes (between ca 401 and 601 latitudes) in west-central Europe, and drier conditions in Central Asia and some other regions of both hemispheres (Magny and Haas, 2004). Evidently, the prevalence of a dry climate in monsoonal Asia in the late Holocene was associated with a weakening and southward retreat of the Northern hemisphere monsoon systems. In turn, the Asian monsoon intensity follows summer insolation (Wang et al., 2005), and the weakening of the monsoon is linked to cooler sea-surface temperature and weaker thermal contrast between landmass and adjacent ocean (Morrill et al., 2003). The asynchronous reactions of lakes affected by different sources of moisture transport are the result of temporal and spatial shifts in moisture availability along the monsoon and west wind pathways (An et al., 2000; He et al., 2004). Likely, it is a cause for a contrasting pattern of hydrological changes in southwestern Tuva and adjacent areas, and in monsoon areas. However, some lakes can exhibit a different or even opposite behaviour because of local factors controlling the hydrological regime. Archaeological research in Tuva revealed evidence of Neolithic settlements of 3rd/2nd millennium BC as well as

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initial/early Scythian culture dated to late 9th/early 8th century BC (van Geel et al., 2004) and classic Scythian culture of 5th/3d century BC (Barkova, 1976). There is a hypothesis that the climatic shift towards wetter conditions after ca 3 cal kyr BP led to a modification of the landscape from hostile semi-deserts to attractive steppe, resulting in quick expansion and migration of the Scythians, a folk of nomadic horsemen, into Tuva (van Geel et al., 2004). 5.2.7. Late Holocene (ca 1.8–0 cal kyr BP) Our reconstruction suggests that the last 1800 years were a time of even greater cold climate in southwestern Tuva. This cooling trend is correlative with pronounced glacier expansion after ca 1.5 cal kyr BP and during the Little Ice Age in the Pamir-Alai mountain system located southwest of Tuva (Narama, 2002) and in the Himalayas/Karakorum (Rothlisberger and Geyh, 1985, as cited by Lami et al., 1998) as well as in western and eastern China (Zheng et al., 1998). Moreover, in many places of the world the Little Ice Age advances were the most significant glaciations since the Younger Dryas. During the last millennium, very prominent glacier advances occurred also in the most other mountain regions of the former Soviet Union (Solomina, 2005). At the same time, the period after ca 1.8 cal kyr BP is remarkable for aridity in southwestern Tuva. Likewise, in north-central Mongolia, lower lake level and drier conditions occurred after ca 1.6 cal kyr BP (Fowell et al., 2003). Rather dramatic environmental changes during the last 3 cal kyr BP occurred in arid/semiarid China, where most of the lakes became smaller in lake area, lower in water level, and more salted (Xue et al., 2003). Long dry events around 1.5 and 0.8 cal kyr BP (Chen et al., 2001) and desiccation of a lake after ca 1.7 cal kyr BP (Herzschuh et al., 2004) were marked in northwestern China. In north-central China, a cool and dry climate occurred as early as ca 2.9 cal kyr BP and became constant after ca 1.4 cal kyr BP (Xiao et al., 2004). This may suggest that climate change towards aridity was not only a local feature reconstructed for the southwestern part of Tuva, but might be also a regional attribute. Wu et al. (2006) suppose that the drying-up trend after 3 cal kyr BP in the central Tibetan Plateau might be referable to the effects of regional monsoon and/or global climatic aridity. It is probable that the cold and dry climatic conditions in southwestern Tuva and adjacent areas during this period are conditioned by the weakened intensity of the cyclonic westerly from the North Atlantic, and in turn, the strengthened position of the Siberian high atmospheric-pressure zone, which is controlled by the changes of continental ice sheet and global ice volume, and produces cold and dry winds in wintertime. 5.3. Perspectives for chironomid-based palaeoenvironmental reconstructions in Central Asia Unfortunately, the lack of a modern chironomid training set from Central Asia is a major impediment to chironomid-inferred quantitative estimates of past climatic and



hydrological changes in our study. Knowledge of longterm variability of air temperature and effective moisture is particularly important for the region, where hydroclimatic fluctuations have a substantial impact on water availability for natural ecosystems and human society, and quantitative estimates of past changes would be more useful to the climate-modelling community. In the last decade, the most dramatic advances in chironomid palaeoecology have been in the area of development of transfer functions for lake-water or air temperature inferences from chironomid fossils. However, at present, all available chironomid—temperature inference models from Eurasia (Lotter et al., 1997; Olander et al., 1999; Brooks and Birks, 2001; Larocque et al., 2001; Heiri and Lotter, 2005) are restricted to the cold, humid and dry sub-humid aridity zones of Europe, making their extension to the arid, semi-arid and dry sub-humid area of Central Asia questionable. Therefore, the development of chironomid—temperature transfer functions from this region is a promising prospect for new quantitative palaeotemperature estimates from Central Asia. Another very promising line is the prospect of chironomid-based reconstructing the hydrological balance of lakes and ensuing interpretations of the records in terms of either a direct climatic influence on water level in topographically closed-basin lakes through changes in effective moisture or as a climate-driven switch between hydrology of topographically open- and closed-basin lakes. Four basic approaches may be distinguished within this direction of study: (1) The balance between precipitation and evaporation (effective moisture) affects lake level, which in turn alters the concentration of dissolved salts in lake water. Chironomid distribution is strongly correlated with salinity, and chironomid—salinity inference models based on chironomid assemblages in surface sediments of lakes were developed in western Canada (Heinrichs et al., 2001) and Africa (Eggermont et al., 2006). In the future, it would be valuable to develop a similar model in Central Asia. (2) In many surface sample surveys, lake depth has been shown to be one of the main factors affecting chironomid distribution (e.g., Walker et al., 1991, 2003; Francis, 2004; Gajewski et al., 2005). This strong relationship suggests that chironomid remains may be useful indicators of past lake-level changes (Hofmann, 1998). However, the developed by Korhola et al. (2000) and Barley et al. (2006) the transfer functions for quantitative chironomid-based inference of changes in past maximum lake depths in humid climate has insufficient sensitivity to amplitude of lake-level fluctuations less than 3–4 m (Walker, 2001). Using threedimensional modelling of lake morphometry at possible lake levels, Stone and Fritz (2004) showed that palaeohydrological reconstructions should incorporate an detailed consideration of lake morphometry and

how available habitat areas of aquatic organisms change with lake level. Stone and Fritz (2004, p. 1547) showed also that ratios of diatom habitat areas may be a better predictor of mean lake depth than maximum lake depth, and suggested that ‘‘such approach can be applied easily to other organisms that are influenced by lake level, such as ostracodes and chironomids’’. Thus, a calibration model for chironomid-based inference of past lake-level fluctuations should, perhaps, be based on mean lake depth, as on a simple function of lake morphometry, or on a habitat areas ratio (Aet:Aa), as on more complex function of lake morphometry. Such a model for quantitative inferences should, perhaps, also be based on surface sediment samples from lakes with relatively simple bathymetry where littoral habitat areas change linearly with lake depth (see Stone and Fritz (2004) for details). (3) In deep lakes with well-developed thermal stratification where distribution of profundal chironomid assemblages is strongly correlated with hypolimnetic oxygen content (e.g., Brundin, 1951), changes in water depth affect hypolimnetic volume and hypolimnetic oxygen content. In North America, Quinlan and Smol (2001b) have developed a transfer function for quantitative chironomid-based inference of average volumeweighted hypolimnetic oxygen (avgVWHO) concentrations, which can be used in order to trace anthropogenic or climate-driven changes in bottom-water anoxia and mixing regimes of lakes. However, interpretation of chironomid-inferred avgVWHO concentrations in terms of lake-level changes can benefit from coupled estimation of past air temperature and avgVWHO concentration in order to clearly decipher a shifting mixing regime from changes in water balance of the lake. Applying this approach would enable semiquantitative estimates of the direction and magnitude of past lake-level changes. In northern Fennoscandia, Ilyashuk et al. (2005), using such approach to the chironomid stratigraphy from a small closed-basin lake, have inferred low and high lake-stand periods consistent with regional quantitative lake-level reconstructions based on fossil cladocera assemblages (cf. Korhola et al., 2005). (4) As supposed by Birks (1998), calibration models for variables such as water depth may be based on many surface samples from different water depths within the same lake. 6. Conclusions In this study, we present the first chironomid-based reconstruction of past climatic changes throughout the Late-glacial and Holocene in the mountain area of Tuva situated in the central part of the Asian continent. The climatic reconstruction indicates that chironomids are responding not only to temperature, but also to changing lake depth, which reflects changing moisture regimes.


Chironomid records from two lakes reveal seven successive climatic phases (I–VII) occurred since ca 15.8 cal kyr BP. Three phases (I–III) after the last glacial flourishing period and before Holocene correspond to: the progressive warming during the last glacial–interglacial transition (phase I, ca 15.8–14.6 cal kyr BP), which triggered the lacustrine pulse in the Lake Grusha depression; the warm and moist Bølling-Allerød-like interval (phase II, ca 14.6–13.1 cal kyr BP); and the cool and dry Younger Dryas-like event (phase III, ca 13.1–12.1 cal kyr BP). The abrupt climatic changes towards wetter and warmer conditions, which triggered the lacustrine pulse in the Ak-Khol depression, began shortly after ca 12.1 cal kyr BP and took place during the early Holocene (phase IV, until ca 8.5 cal kyr BP). The next period (phase V, ca 8.5–5.9 cal kyr BP) was generally dry and warm, likely even the warmest during the Holocene. The climate abruptly reversed after ca 5.9 cal kyr BP towards cold conditions, which continued until modern times. Two main phases (VI and VII) are recognized within the cool late Holocene (or Neoglacial interval). The first (phase VI) is marked between ca 5.9 and ca 1.8 cal kyr BP and characterized by wet conditions, which apparently have resulted in the crossing of hydrological threshold and the opening of outlet in Lake Grusha at the beginning of the phase. The second (phase VII) took place within the last 1800 years and differed by aridity. In general the chironomid-based climatic inferences are in agreement with the diatom and pollen records from studied lakes, and with other post-glacial archives available for the region. However, some discrepancies in chironomidand pollen-based reconstructions take place apparently because the aquatic and terrestrial biological indicators can have different proximate controlling factors, and thus in some cases reveal different patterns of climatic changes. To all appearance, the main factors controlling the hydrological regime of the lakes studied are the temperature and interrelation between the dominant westerly system and the changing influence of summer monsoon as a source for potential moisture availability, as well as the influence of temperature-driven meltwater supply. Our climatic inferences imply that the pattern of atmospheric circulation could have abruptly changed during the warming at ca 14.6–13.1 and ca 12.1–8.5 cal kyr BP so that the northern limit of the Asian summer monsoon moved landward, and monsoon tracks began to reach southwestern Tuva, resulting in an increase in precipitation. Apparently, the rate of precipitation exceeded the rate of evaporation, resulting in increased effective moisture and a relatively high stand of lakes during these warm periods. During all the other periods through the post-glacial interval, the southwestern part of Tuva was situated beyond the Northern hemisphere monsoon belt. The increased effective moisture accompanying the Neoglacial cooling at ca 5.9–1.8 cal kyr BP was likely a result of reinforced cyclonic westerlies from the North Atlantic.

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Comparability in the change trajectory and the happening time of our inferred climatic changes and the major climatic events (the last glacial–interglacial and the Lateglacial–Holocene transitions, B/A, YD, 8.2k, and Neoglaciation) marked in many records from both hemispheres imply that the events throughout post-glacial climatic history of southwestern Tuva were generally non-local appearances. These appearances point to the changes of the regional pattern of atmospheric circulation, which could be in turn induced by the global climatic shifts. Our climatic reconstruction exhibits some dissimilarity compared with other reconstructions from Central Asia. These apparent differences may most likely be attributed to regional (spatial) differences between dominating circulation mechanisms and to local differences in uplift and descent of air masses within the complicated mountain landscape. As demonstrated by Bjo¨rck et al. (2002), considerable differences in palaeoclimatic conditions may have occurred even within the non-mountain landscape of Greenland. Our study illustrates the necessity to retrieve new highresolution proxy data from a dense network of palaeoclimatic sites to advance our knowledge about the controlling mechanisms as well as about the temporal and spatial patterns of the late-Quaternary climatic and environmental changes within the continental Asia. Further chironomidbased investigations are also needed to reach the potential of chironomid fossils with respect to quantitative palaeoclimatic reconstructions in this sensitive area. Acknowledgements This research was supported through grants to Herbert E. Wright from the US National Science Foundation and the National Geographic Society. The fieldwork was organized by Tatyana A. Blyakharchuk and Pavel S. Borodavko. Michael Sturm performed the hydrochemical analysis. Karlyn S. Westover generously shared the results of diatom analysis. Florencia Oberli analysed the sediments for LOI, and W.O. van der Knaap assisted with the age–depth models. We thank Michael Broderick and Herbert E. Wright, who helped to improve the language in this manuscript. Our most cordial thanks go to Brigitta Ammann for fruitful discussions and improvement of the manuscript. We thank Scott Elias and Svetlana Kuzmina for valuable comments on earlier draft of the paper. References Alean, J.C., 1985. Ice avalanches: some empirical information about their formation and reach. Journal of Glaciology 31, 324–333. Alley, R.B., A´gu´stsdo´ttir, A.M., 2005. The 8k event: cause and consequences of major Holocene abrupt climate changes. Quaternary Science Reviews 24, 1123–1149. Alpatev, A.M., Arkhangelskiy, A.M., Podoplelov, N.Ya., Stepanov, A.Ya., 1976. Fizicheskaia geografiia SSSR: aziatskaia chast’ (Physical Geography of USSR: Asian Part). Vysshaia Shkola Press, Moscow (in Russian).



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