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EVIDENCE OF ENVIRONMENTAL INSTABILITY OF THE LAKE BAIKAL AREA AFTER THE LAST GLACIATION (BASED ON POLLEN RECORDS FROM PEATLANDS)
ARCHAEOLOGY, ETHNOLOGY & ANTHROPOLOGY OF EURASIA Archaeology Ethnology & Anthropology of Eurasia 37/3 (2009) 17–25 E-mail: [email protected] PALEOENVIRONMENT. THE STONE AGE
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E.V. Bezrukova1, A.A. Abzaeva1, P.P. Letunova1, N.V. Kulagina2, and L.A. Orlova3 Laboratory of Archaeology and Paleoecology, Irkutsk State University, K. Marxa 1, Irkutsk, 664003, Russia E-mail: [email protected] 2 Institute of the Earth’s Crust, Siberian Branch, Russian Academy of Sciences, Lermontova 128, Irkutsk, 664033, Russia E-mail: [email protected] 3 Laboratory of Cenozoic Geology and Paleoclimatology, Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences, Akademika Koptyuga 3, Novosibirsk, 630090, Russia E-mail: [email protected] 1
EVIDENCE OF ENVIRONMENTAL INSTABILITY OF THE LAKE BAIKAL AREA AFTER THE LAST GLACIATION (BASED ON POLLEN RECORDS FROM PEATLANDS)*
Pollen analysis of two dated sedimentary cores from lacustrine-boggy sediments in various parts of the Lake Baikal area yielded the ¿rst complete record of deep changes in the lake catchment area during the Late Glacial and Early Holocene. The Early Middle Holocene record shows an optimum – a humid and mild climate with warm winters between ca 10,000 and 7000 BP. During the Late Holocene, the climate grew more and more continental, and dark coniferous forests were replaced by light coniferous ones. Comparison of variation ranges of paleogeographic events in the Late Pleistocene and Holocene recorded in our samples with previously known records for the Lake Baikal area and other regions of Eurasia indicated that major changes of vegetation and climate mostly correlate with the global ice retreat, solar radiation level, and the concentration of carbon dioxide in the atmosphere. Less signi¿cant short-term Àuctuations of vegetation and climate recorded in our archives can be regarded as regional ecosystem responses to solar activity changes of a quasi-millenary scale. Regional pollen records demonstrate a distinct relationship with the climate of the Northern Hemisphere as a whole. The amplitude of these changes is higher in the northeastern Lake Baikal area than in its southern part. Keywords: Pollen analysis, paleoclimate, paleoecology, Late Glacial, Holocene, Lake Baikal catchment area.
Introduction To evaluate the anthropogenic climatic changes and their relationship with natural environmental changes, one must first of all assess the directionality of the *This study was supported by the Russian Foundation for Basic Research (Project 09-05-00123-а) and the Baikal Archaeological Project.
evolution of the climate during the Late Glacial and in the Holocene (Rind, Overpeek, 1993). Before the natural dynamics of the recent past, i.e. of Termination 1 and of the Holocene, universally represented by sediments, has been adequately described and interpreted, it will be impossible to evaluate the magnitude of the anthropogenic impact on the environment and climate. Our knowledge of the recent geological past is limited. Currently, the most reliable oxygen-isotopic record
Copyright © 2009, Siberian Branch of Russian Academy of Sciences, Institute of Archaeology & Ethnography of the Siberian Branch of the Russian Academy of Sciences. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.aeae.2009.11.001 doi:10.1016/j.aeae.2009.11.002
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of temperature changes in the Late Glacial and in the Holocene, based on the ice core from Greenland, contains signals of sharp short-lived climatic uctuations during the Late Glacial and evidences a relatively stable climate in the Holocene (GRIP Members, 1995). However, data concerning the geochemical admixture in the same core attest to climatic instability and the irregularity of the Holocene characteristics at least in Greenland (Mayewski et al., 1997). Climatic fluctuations in the Holocene have been demonstrated in many regions (Enzel et al., 1999; Wurster, Patterson, 2001; Zhao et al., 2007), and various mechanisms explaining these uctuations have been suggested (Bond et al., 2001; Visbeek, 2002). It is impossible to reconstruct the temporal and spatial variation of the Late Glacial and Holocene climate and environment without information from various regions of the planet, especially those where the environment is particularly sensitive to climatic changes. Earlier studies have shown that geochemical and diatom records from the bottom sediments of Lake Baikal meet this requirement (Uchastniki..., 1998; Khursevich et al., 2001; BDP-99..., 2005; and others). However, earlier reconstructions of climate in these studies are based on averaged signals from deep-water cores. Meanwhile, the Lake Baikal basin spans nearly four degrees of latitude and, because its southern and northern parts display climatic differences at the present, such differences must have existed in the past as well. The relevant information can be obtained from peatland ecosystems, where a thick layer of organic sediments contains a continuous record of environmental changes with a high time resolution over the last
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15 thousand years (Kataoka et al., 2003; Bezrukova, Krivonogov, Abzaeva et al., 2005; Bezrukova, Belov, Abzaeva et al., 2006; Bezrukova, Belov, Letunova et al., 2008; Bezrukova, Krivonogov, Takahara et al., 2008). As the study of these records has demonstrated, climatic uctuations in the Holocene had a larger amplitude and higher frequency than previously believed, although the range was lower than during the Late Glacial. The objective of the present article is to develop a high-resolution reconstruction of Late Glacial and Holocene environmental changes in the Lake Baikal area using dated pollen records from marsh systems, which today are situated in areas with different biological and climatic parameters – those in the southern and the northeastern shores of the lake. Before the mid17th century, neither area was subjected to a signicant anthropogenic impact. Therefore the sedimentation records from these ecosystems will hopefully reect the natural environmental changes. Study areas Duguldzera. The core was taken at the eastern Baikal shore (Fig. 1), in the forest ecosystem of the middle mountain zone. Larch, Scots and Siberian pines dominate this ecosystem. Forests composed of larch, Siberian pine, and spruce occupy more elevated areas on mountain slopes and in valleys. Open Siberian pine and r forests of a primarily valley type grow higher. This region is characterized by an extreme continental climate. According to data of the nearest weather station at Davsha, mean January and July temperatures are –22 ºС and +14 ºС, respectively; the mean annual temperature equals –3.3 ºС. Mean annual precipitation ranges from 350 to 400 mm. Insular permafrost occurs in this region (Baikal..., 1993). Dulikha. The peat bog is located on the southern shore of Baikal (Fig. 1), where southern Siberian taiga composed of Siberian pine and r predominates. Larch trees are rare in marshlands. Birches form secondary forests, replacing dark coniferous forests in felling areas and re-sites. The climate is moderate continental (Ibid.). Mean July, January, and annual temperatures are +14.4, –17.7, and –0.7 oC, respectively. Mean annual precipitation is 600–650 mm. Thus, the temperatures in the two areas differ by 4–5 oC in January, by 2 oC in July, and by approx. 2.5 oC on average, whereas the difference between mean annual precipitation levels is nearly 250 mm.
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Materials and methods Fig. 1. Map showing the location of the core sites. 1 – Duguldzera; 2 – Dulikha; 3 – VER93-2 bottom sediments, station 24 GC.
The Duguldzera core is 400 cm long. The upper 330 cm are represented by peat of various composition;
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the lower 70 cm are formed by lacustrine gyttja with an admixture of clay mineral particles. Every fourth centimeter was subjected to pollen analysis; therefore the temporal resolution of the record is 150–200 years. The chronological model of the section is based on seven radiocarbon dates. The depth of Late Glacial and Holocene peat sediments in the Dulikha core is 500 cm (Bezrukova, Krivonogov, Abzaeva et al., 2005). Every fourth centimeter of the core was treated with pollen analysis. The average temporal resolution of the record is 100–150 years. The chronological model of this core is based on three dates (Table). Chronological limits of pollen zones were estimated by linear interpolation between the dates. To evaluate the possible mechanisms behind the vegetation changes and to correlate the temporal limits of these changes with those for the Northern Hemisphere, radiocarbon dates were calibrated using CalPal software (Danzeglocke, Jöris, Weninger, 2008). Age estimates in pollen diagrams are calibrated. The classication of pollen taxa used for the calculation of pollen indices of temperature and moisture corresponds to the taxa grouping applied in the biome reconstruction method (Prentice et al., 1996; Tarasov et al., 2000; Demske et al., 2005). Pollen diagrams are presented here in the most general form for several reasons: (1) the complete diagram of the Dulikha section has already been published (Bezrukova et al., 2005), although dates were given in conditional uncalibrated 14C values and no pollen indices of temperature or moisture were provided; (2) the complete diagram of the Duguldzera section was published by Abzaeva et al. (2008); (3) for the purpose
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of the present article, the diagrams themselves are less important than the indices of environmental changes based on these diagrams. Duguldzera is designated Dz and Dulikha is labeled Dl. Results and interpretation Four zones have been recognized in the Duguldzera pollen diagram (Fig. 2). They are described from bottom to top. The zones are characterized by the most signicant pollen taxa for paleoenvironmental reconstructions. Dz4: Artemisia – Betula alba-type – Picea; > 16,000 BP; depth 400–385 cm. Sediments are represented by mineralized gyttja. Spore-pollen spectra (hereafter, SPS) evidence the first maxima of pollen of spruce (Picea obovata) and birches of both sections (Betula sect. Albae and Betula sect. Nanae). The herbaceous group is dominated by Artemisia pollen. Dz3д: Artemisia – Salix – Betula alba-type; meadowsteppe motley grassland; ~16,000–14,700 BP; depth 385– 355 cm. SPS formed in lacustrine gyttja. Tall birch pollen prevails along with pollen of shrub birch, willow, and mesoxerophytic herbs. Dz3г: Betula alba-type – Cyperaceae – Salix; ~14,700–14,000 BP; depth 355–345 cm. Birch and willow pollen dominate SPS; sedge pollen occurs in large quantities. Dz3в: Duschekia – Picea – Larix – Betula alba-type – Equisetum; ~14,000–13,200 BP; depth 345–325 cm. SPS were accumulated in gyttja. Spruce pollen appears again; the amount of alder pollen increases. Dz3б: Larix – Betula alba-type – Duschekia; ~13,200– 12,800 BP; depth 325–315 cm. SPS are characterized by
Results of radiocarbon dating of sediments Interval in the core (cm from the surface)
Radiocarbon age, years
Laboratory code
240 ± 45
SO AN-5705
25–30
1485 ± 50
SO AN-5706
1391 ± 55
»
90
4515 ± 40
АА-37969*
5179 ± 92
Wood
94–96
4805 ± 65
SO AN-5707
5531 ± 66
Peat
193
8020 ± 45
SO AN-37970*
8893 ± 93
Seeds
323
11,295 ± 55
АА-37971*
13,194 ± 101
Gyttja
378
12,950 ± 90
АА-37972*
15,767 ± 422
»
8425 ± 32
Seeds
Calibrated age, years
Dated material
Duguldzera 0–2
275 ± 114
Peat
Dulikha 300
7620 ± 115
NUTA-5615*
399
9185 ± 55
AA-37974*
10,362 ± 79
Peat
475
11,110 ± 120
NUTA-6038*
13,010 ± 128
»
*Dated by the accelerator mass spectrometry technique at the Nagoya University Center for Chronological Research.
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Fig. 2. Pollen diagram of Duguldzera sediments. In Fig. 2 and 3, the thick line in the column “Pollen indices” shows changes of the temperature index, and the thin line shows those of the moisture index.
the second major spruce pollen maximum. Sediments represent a gyttja to peat transition. Dz3а: Picea – Duschekia – Betula alba-type; ~12,800– 11,300 BP; depth 315–265 cm. SPS were formed in peat sediments. They are characterized by the greatest amounts of birch and alder pollen. Dz2б: Larix – Betula alba-type – Picea – Polypodiophyta; ~11,300–10 000 BP; depth 265– 225 cm. SPS abound in larch and birch pollen; the rst fern spores and sedge pollen maxima are recorded. The amount of spruce pollen constantly varies. A maximum of pollen representing hygrophytic plants of Potamogeton and Typha can be observed in the beginning of the zone. Dz2а: Abies – Larix – Picea – Betula; ~10,000–6000 BP; depth 225–110 cm. Fir pollen continually presents, while the amount of spruce pollen decreases. Small quantities of Siberian pine pollen are constantly recorded. The content of sedge pollen and that of fern and horsetail spores is high. Dz1б: Pinus sylvestris – Pinus sibirica – Pinus pumila, ~6000–2500 BP; depth 110–55 cm. Pollen of all the mentioned pine species dominates SPS. Dz1a: Larix – Pinus sylvestris – Pinus sibirica – Pinus pumila – Betula nana-type; ~2500 – 0 BP; depth 55–0 cm. SPS are characterized by the second maximum of shrub
birch pollen and by a high amount of arboreal birch pollen and sphagnous moss spores. Four zones are recognizable in the Dulikha pollen diagram (Fig. 3). They are described from bottom to top. Dl4: Larix – Picea – Salix – Betula nana-type – Betula alba-type; > 13,200 BP; depth 500–480 cm. Shrub and herb pollen dominates SPS. Pollen of spruce, larch, and birch of both sections prevails in the group of arboreal plants. Dl3: Artemisia – Larix – Picea – Betula nana-type – Betula alba-type – Cyperaceae – Polypodiophyta; ~13,200–10 600 BP; depth 480–405 cm. Shrub and herb pollen is still predominant in PSS. Birch and larch pollen dominates the group of arboreal plants. The share of mesoxerophytic herbs, gramineous plants, sedge, and fern increases Dl2в: Abies – Picea – Betula alba-type; ~10,600– 10,000 BP; depth 405–370 cm. Arboreal pollen becomes more abundant. The pollen of Siberian pine, Scots pine, and Siberian dwarf pine appears in spectra. Dl2б: Pinus sibirica – Betula alba-type – Abies; ~10,000–9200 BP; depth 370–330 cm. Fir pollen dominates SPS; the amount of pollen of both Siberian and Scots pine increases. Dl2а: Abies – Pinus sibirica – Betula alba-type; ~9200–6200 BP; depth 330–215 cm. The share of r pollen
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Fig. 3. Pollen diagram of Dulikha sediments.
constantly decreases, while that of Siberian and Scots pine continues to rise. The amount of birch pollen is stable. Dl1: Pinus pumila – Betula – Pinus sylvestris – Pinus sibirica; ~6200–0 BP; depth 215–0 cm. Discussion: Reconstruction of the paleoenvironment Pollen records and results of radiocarbon dating of lacustrine and marsh sediments make it possible to reconstruct climate and vegetation evolution and to partially trace changes of the hydrological regime on the southern and northeastern Baikal shores after the last glacial maximum. Vegetation and climate of the last glacial period The Duguldzera pollen record encompasses a longer time period than the Dulikha record. The pollen analysis of the Duguldzera core provided a base for the first reconstruction of vegetation and climate changes of the entire Lake Baikal record for the transitional period from the last glaciation to the modern interglacial. The lowest section of the Duguldzera pollen record (up to 16,000 BP, Dz4) reflects the environmental
conditions under which mineralized gyttja formed in the peat bog near the vegetated lake. At that time, the northeastern shore was covered with forest and tundra vegetation composed of larch, birch, spruce, and tundra herbs and shrubs. Vegetation of this type currently occurs in the Pechora River delta characterized by a cold climate with an average annual temperature of –4 °С, an average July temperature of approx. 13 °С, and an average annual precipitation of approx. 400 mm (Valiranta, Kaakinen, Kuhry, 2003). The time period of this vegetation existence coincided with the beginning of deglaciation after ~17,000 BP (Bowen et al., 2002) and relative climatic warming. Temperature and moisture indices suggest that the climate was cold (especially in the winter) and humid. Judging by the vegetation composition, the humidity index diagnoses a high soil moisture level caused by melting permafrost and lower summer temperatures rather than high atmospheric precipitation. Almost a complete disappearance of spruce pollen and decrease in the amount of arboreal pollen in the spectra formed in the mineralized gyttja ~16,000– 14,700 years ago, on the one hand, and dominance of pollen of birch, willow, and mesoxerophytic herbs, on the other hand, testify to deterioration of conditions for forest vegetation growth (Dz3д). This could possibly be determined by a general climate worsening during one of the stadials of Termination 1, whose minimum occurred ~15,500 years ago (Wehrli, Tinner, 2007). Temperature and moisture indices suggest a lowering level of warmth
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and moisture accessible to plants. The warmth level rose slightly ~14,700–14,000 years ago (Dz3г) and coincided with the rst signicant warming of Termination 1, while the humidity level continued to drop. The vegetation was dominated by shrubby yernik and willow tundra with patches of open birch forests. The rst recorded significant maximum of sedge pollen suggests the beginning of local bog formation. Between ~14,000 and 13,200 BP (Dz3в), forest-tundra vegetation with larch and later on with spruce, birch, and alder spread in the lake environs. At the same time (before ~13,200 BP), the southern Baikal shore was dominated by open birch and spruce forests with larch. This represented a shift toward milder climatic conditions of the interstadial warming that corresponded to the Allerøed. A short-term culmination of warm and humid conditions that occurred ~13,200– 12,800 years ago (Dz3б) entailed expansion of spruce in the Duguldzera region. At that time, spruce forest-tundra prevailed on the southern shore. A new period of warmth level lowering occurred between 12,800 and 11,300 BP (Dz3а). At that time, the humidity was unstable. Spruce-birch forest-tundra and alder tundra prevailed on the northeastern shore. Conditions unfavorable for forest vegetation probably resulted from climate deterioration synchronous with the Younger Dryas (Dansgaard et al., 1993). Approximately at the same time period, on the southern shore, the climate became more continental; it favored the development of birch-larch forest tundra with spruce and of herb-shrubby tundra (Dl3). Pollen indices attest to the coldest and the most humid climate over the entire period examined. Vegetation character points to a high soil moisture level (melting permafrost) due to lower summer temperatures (lower evaporation rate). In the southern Baikal area, this period was longer: it lasted almost to 10,600 BP. Vegetation and climate of the Holocene humid optimum At the onset of the Holocene (~11,300–10,000 BP (Dz2б)), a lowland sedge marsh formed in the place of the vegetated lake in the northwestern Baikal area. Larch and spruce became more abundant in the surroundings of the Duguldzera marsh. On the southern shore, on the contrary, areas vegetated by larch diminished, while territories covered by spruce expanded rapidly. Such shifts in plant composition suggest that the climate became less extreme continental; total annual precipitation values and mean winter temperatures increased. These changes marked the beginning of expansion of humid r taiga and thus the onset of the Holocene humid optimum on the southern shore. The maximum of the r taiga development occurred ~10,000–9200 years ago (Dl2б). On the southern shore, the humid optimum terminated
gradually from ~9200 to ~6800 BP (Dl2а). On the northeastern shore, the humid maximum manifested itself in a different way. It started after 10,000 BP and reached its maximum ~7000–6000 BP (the highest amount of r pollen). However, despite such a late onset of the humid optimum on the northeastern shore, it terminated there between 7000 and 6000 BP, as on the southern shore. Quantitative characteristics of the humid optimum reconstructed previously on the basis of the pollen record from the Baikal bottom sediments (VER93-2, station 24 GC (Fig. 1)) have shown that ~9500–6500 years ago, mean annual precipitation exceeded modern values by 80–100 mm; mean winter temperatures were 2–4 °С higher than nowadays. Mean July temperatures, however, could be close to those of today (Tarasov et al., 2007). A combination of mild, snowy winters, absence of spring frosts, and cool and moist summer seasons favored the development of r forests. Pollen indices demonstrate a steady tendency toward decrease of humidity and increase of warmth ca 10,000–6000 BP in the northeast and 10,500–7000 BP in the southern part of the lake. When temperature and humidity levels became close to modern ones, r taiga ceased to be predominant. The Holocene optimum is a very important period as it may be used for modeling future climatic changes. The optimum is usually believed to coincide with the period of the postglacial thermal maximum (Winkler, Wang, 1993). In Northern Europe this period is characterized by a warm and generally moist climate. However, in China, as in the Baikal basin, the Holocene optimum is dened as a period of maximal precipitation values rather than as a thermal maximum (Xiaoqiang Li et al., 2004; Porter, Weijian, 2006). A less continental climate and the predomination of r forest in the Baikal area might have been a consequence of a greater thermal gradient between the ocean and the land, which resulted in a greater transport of moist air masses to the continent. Having attained the lake area, these masses caused a greater convective rainfall. Various paleoclimatic records contain evidence of the Holocene optimum characterized by wet and cool conditions almost all over the Northern Hemisphere (Herzschuh et al., 2005; Blyakharchuk et al., 2004; Mudie et al., 2007). New facts suggesting the highest atmospheric precipitation and temperate continental climate with cool summers and warm winters that existed ca 11,000–7000 years ago have been recently obtained for the western TransBaikal region, too (Bezrukova, Krivonogov, Takahara et al., 2008). Vegetation and climate of the post-optimum period of the Holocene During the ~7000–6000 BP interval in the south and after ~6000 BP in the northeast of Lake Baikal, drastic
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changes in the composition of forest vegetation occurred: Siberian and Scots pine replaced r and spruce. This shift occurred under the conditions of a signicant lowering of humidity and increasing heat supply. Eco-edaphic requirements of these new forest elements presuppose increasing continentality of the climate due to lowering atmospheric humidity, drop of winter temperatures, and rise of summer temperatures (Tarasov et al., 2007). The termination of the humid optimum in the Baikal basin coincided with the onset of neoglaciation on the Loess Plateau (Porter, Weijian, 2006) and with the documented ice-rafted debris peak in the North Atlantic ca 6000 BP (Bond et al., 2001). At about the same time (ca 5500 BP), the relatively cool and moist “green” period in Northern Africa, which contributed to the existence of numerous lakes in what is now the Sahara, came to an end (Renssen et al., 2006). The transition to a considerably more continental climate all over Eurasia meant that the underlying mechanisms were global. The Baikal landscapes responded to these global changes by radical restructuring: the dark coniferous forest of the Early and Middle Holocene was replaced by a light coniferous and more xerophytic vegetation of the Late Holocene. Beginning from ca 6000 BP, the level of moisture available to plants demonstrates a stable reduction with minor uctuations, whereas relative temperature level demonstrates short-term small uctuations around the modern mean (see pollen indices in Fig. 2 and 3). Consequently, temperature and moisture levels did not remain permanent, evidencing climatic instability in the Late Holocene. The frequency and range of these fluctuations requires additional study, although the available records make it clear that the climatic changes affected the landscapes of the Baikal ecosystem. The changes, however, were mostly expressed in the local landscapes. In the Late Holocene (ca 2400–2500 BP), cooling resulted in changes of the hydrological regime of the marshland on the southern shore of the lake. After 2400 BP, yernik associations began to expand. The process was even more marked on the northeastern shore of Baikal at about 2500 BP. A distinct pollen signal of climate deterioration followed by the distribution of yernik associations which has been obtained for Lake Kotokel basin as well (Bezrukova, Krivonogov, Takahara et al., 2008) suggests that an adequate response of the entire Baikal ecosystem to the decrease of solar radiation occurred after 2700 BP, when the climate deteriorated in both hemispheres (Swindles, Plunkett, Roe, 2007). Even shorter uctuations of the landscape and climate in the Lake Baikal area, coinciding with well-known events such as the Medieval Warm Period and the Little Ice Age, are registered by pollen records from the northern shore of the lake (Bezrukova, Belov, Abzaeva et al., 2006).
Conclusions Results of pollen and radiocarbon studies of lacustrine and boggy ecosystems on various shores of Lake Baikal, and the comparison of these results with dated records of respective changes in adjacent regions led to the elaboration of a detailed record of environmental evolution in the Baikal area since the end of the last glaciation (17,000–16,000 BP). Considerable changes in atmospheric circulation in the Northern Hemisphere in the beginning of the deglaciation contributed to warmer and dry summers 16,000–12,000 years ago in Siberia (Schirrmeister et al., 2002). This was the time when peat deposits began forming on the southern and northeastern shores of Baikal (at 13,000 and 11,500 BP, respectively). Generally, high-resolution pollen records presented in this article attest to profound changes of vegetation and climate of the lake catchment area during the Late Glacial and in the Early Holocene, and to a high climatic variability during the modern interglacial period. Pollen records demonstrate instability of landscapes and climate in the Late Glacial and Early Holocene, resulting in frequent changes of plant associations. The probable reason behind this interchange was the melting of global ice sheets and regional mountain glaciers, leading to the instability of the ocean–atmosphere–cryosphere system. Pollen records conrm the beginning of a long period of Holocene optimum with a moist and mild climate and warm winters at ca 11,000–10,000 BP, characterized by the predominance of r, spruce, and pine forests in various areas of the Lake Baikal catchment area under increased insolation in high latitudes of the Northern Hemisphere. The optimal period ended about 7000– 6000 BP, when insolation decreased and the world ocean attained its modern level. The optimum was followed by a period when the climate became progressively more continental, atmospheric precipitation decreased, the winter became colder, and summer became warmer. As a result, dark coniferous forests gave way to light coniferous ones. The principal factors behind the climatic changes of such magnitude were variability in insolation and concentration of carbon dioxide in the atmosphere. Lesser and short-term uctuations of vegetation and climate in the Holocene (ca 2500–2400, 1600–1200, and 500–400 BP) evidenced by our pollen records may be viewed as responses of the regional ecosystem to quasi-millennium changes of insolation (Meeker, Mayewski, 2002). Records from the Dulikha and Duguldzera cores demonstrate a marked correlation with climatic uctuations in the Northern Hemisphere as a whole. The range of these changes is higher on the northeastern shore of Lake Baikal than on its southern shore. In addition to climate, local factors such as geological and geomorphological structure, vegetation, changing level of ground waters, thickness and depth of
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permafrost, may have been important in the evolution of the Lake Baikal environment. To discern smaller secular variations on the southern shore, new records, having a higher resolution and a more detailed chronological control, are required. References Abzaeva A.A., Bezrukova E.V., Letunova P.P., Belov A.V. 2008 Detalnaya paleoklimaticheskaya rekonstruktsiya pozdnelednikoviya i golotsena severo-vostochnogo poberezhiya ozera Baikal po palinologicheskim dannym. Novosti paleontologii i stratigra¿i, vol. 49, No. 10/11: 375–379. Baikal. Atlas. 1993 Moscow: Federalnaya sluzhba geodezii i kartograi. BDP-99 Baikal Drilling Project Members. 2005 A new Quaternary record of regional tectonic, sedimentation and paleoclimate changes from drill core BDP-99 at Posolskaya Bank, Lake Baikal. Quaternary International, vol. 136: 33–48. Bezrukova E.V., Belov A.V., Abzaeva A.A., Letunova P.P., Orlova L.A., Kulagina N.V., Fisher E.E. 2006 Pervye vysokorazreshayuschie datirovannye zapisi izmeneniya rastitelnosti i klimata srednego – pozdnego golotsena severnogo poberezhiya oz. Baikal. Doklady RAN, vol. 411, No. 2: 254–258. Bezrukova E.V., Belov A.V., Letunova P.P., Abzaeva A.A., Kulagina N.V., Fisher E.E., Orlova L.A., Sheifer E.V., Voronin V.I. 2008 Biostratigrafiya torfyanykh otlozhenii i klimat severozapadnoi chasti gornogo obramleniya ozera Baikal v golotsene. Geologiya i geo¿zika, vol. 49, No. 6: 547–558. Bezrukova E.V., Krivonogov S.K., Abzaeva A.A., Letunova P.P., Orlova L.A., Takahara H., Mieshi N., Nakamura T., Krapivina S.M., Kavamuro K. 2005 Landshafty i klimat Pribaikaliya v pozdnelednikovie i golotsene po rezultatam kompleksnykh issledovanii torfyanikov. Geologiya i geo¿zika, vol. 46, No. 1: 21–33. Bezrukova E.V., Krivonogov S.K., Takahara H., Letunova P.P., Shichi K., Abzaeva A.A., Kulagina N.V., Zabelina Y.S. 2008 Ozero Kotokel – opornyi razrez pozdnelednikoviya i golotsena yuga Vostochnoi Sibiri. Doklady RAN, vol. 420, No. 2: 248–253. Blyakharchuk T.A., Wright H.E., Borodavko P.S., Knaap W.O. van der, Ammann B. 2004 Late Glacial and Holocene vegetational changes on the Ulagan high-mountain plateau, Altai Mountains, southern Siberia. Palaeogeography, Palaeoclimatology, Palaeoecology, vol. 209: 259–279. Bond G., Kromer B., Beer J., Muscheler R., Evans M.N., Showers W., Hoffmann S., Lotti-Bond R., Hajdas I., Bonani G. 2001 Persistent solar inuence on north Atlantic climate during the Holocene. Science, vol. 294: 2130–2136.
Bowen D.Q., Phillips F.M., McCabe A.M., Knutz P.C., Sykes G.A. 2002 New data for the last glacial maximum in Great Britain and Ireland. Quaternary Science Review, vol. 21: 89–101. Dansgaard W., Johnson S.J., Clausen H.B., Dahl-Jensen D., Gundestrup N.S., Hammer C.U., Hvidberg C.S., Steffensen J.P., Sveinbjörnsdottir A.E., Jouzel J., Bond G. 1993 Evidence for general instability of past climate from a 250-kyr ice core record. Nature, vol. 364: 218–220. Danzeglocke U., Jöris O., Weninger B. 2008 CalPal-2007. (Online resource). Access: http://www.calpalonline.de Demske D., Heumann G., Granoszewski W., Nita M., Mamakowa K., Tarasov P.E., Oberhansly H. 2005 Late Glacial and Holocene vegetation and regional climate variability evidenced in high-resolution pollen records from Lake Baikal. Global and Planetary Change, vol. 46: 255–279. Enzel Y., Ely L.L., Mishra S., Ramesh R., Amit R., Lazar B., Rajaguru S.N., Baker V.R., Sandler A. 1999 High-resolution Holocene environmental changes in the Thar Desert, northwestern India. Science, vol. 284: 125–128. GRIP Members. 1995 Climate instability during the last interglacial period recorded in the GRIP ice core. Nature, vol. 364: 203–207. Herzschuh U., Zhang C., Mischke S., Herzschuh R., Mohammadi F., Mingram B., Kurschner H., Riedel F. 2005 A late Quaternary lake record from the Qilian Mountains (NW China): Evolution of the primary production and the water depth reconstructed from macrofossil, pollen, biomarker, and isotope data. Global and Planetary Change, vol. 46: 361–379. Kataoka H., Takahara H., Krivonogov S., Bezrukova E., Orlova L., Krapivina S., Kawamuro K. 2003 Pollen Record from the Chivyrkui Bay Outcrop on the Eastern Shore of Lake Baikal since the Late Glacial. In Long Continental Records from Lake Baikal. Tokyo: Springer Verlag, pp. 207–218. Khursevich G.K., Karabanov E.B., Prokopenko A.A., Williams D.F., Kuzmin M.I., Fedenya S.A., Gvozdkov A.N., Kerber E.V. 2001 Insolation regime in Siberia as a major factor controlling diatom production in Lake Baikal during the past 800,000 years. Quaternary International, vol. 80/81: 47–58. Mayewski P.A., Meeker L.D., Twichler M.S., Whitlow S., Yang Q., Lyons W.B., Prentice M. 1997 Major features and forcing of high-latitude Northern Hemisphere atmospheric circulation using a 110000 year long glaciochemical series. Journal of Geophysical Research, vol. 102, No. 263: 45–66. Meeker L.D., Mayewski P.A. 2002 A 1400-year high-resolution record of atmospheric circulation over the North Atlantic and Asia. The Holocene, vol. 12: 257–266.
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Mudie P.J., Marret F., Aksu A.E., Hiscott R.N., Gillespie H. 2007 Palynological evidence for climatic change, anthropogenic activity and outflow of Black Sea water during the late Pleistocene and Holocene: Centennial- to decadal-scale records from the Black and Marmara Seas. Quaternary International, vol. 167/168: 73–90. Porter S.C., Weijian Z. 2006 Synchronism of Holocene East Asian monsoon variations and North-Atlantic drift-ice tracers. Quaternary Research, vol. 65: 443–449. Prentice I.C., Guiot J., Huntley B., Jolly D., Cheddadi R. 1996 Reconstructing biomes from palaecological data, a general method and its application to European pollen data at 0 and 6 ka. Climate Dynamics, vol. 12: 185–194. Renssen H., Brovkin V., Fichefet T., Goosse H. 2006 Simulation of the Holocene climate evolution in Northern Africa: The termination of the African Humid Period. Quaternary International, vol. 150: 95–102. Rind D., Overpeck J. 1993 Hypothesized causes of decade-to-century-scale climate variability: Climate model results. Quaternary Science Reviews, vol. 12: 357–374. Schirrmeister L., Siegert C., Kuznetsova T., Kuzmina S., Andreev A., Kienast F., Meyer H., Bobkov A. 2002 Paleoenvironmental and paleoclimatic records from permafrost deposits in the Arctic region of Northern Siberia. Quaternary International, vol. 89: 97–118. Swindles G.T., Plunkett G., Roe H.M. 2007 A delayed climatic response to solar forcing at 2800 cal. BP: Multiproxy evidence from three Irish peatlands. The Holocene, vol. 17, No. 2: 177–182. Tarasov P., Bezrukova E., Karabanov E., Nakagawa T., Wagner M., Kulagina N., Letunova P., Abzaeva A., Granoszewski W., Riedel F. 2007 Vegetation and climate dynamics during the Holocene and Eemian interglacials derived from Lake Baikal pollen records. Palaeogeography, Palaeoclimatology, Palaeoecology, vol. 252: 440–457.
Tarasov P.E., Volkova V.S., Webb T., Guiot J., Andreev A.A., Bezusko L.G., Bezusko T.V., Bykova G.V., Dorofeyuk N.I., Kvavadze E.V., Osipova I.M., Panova N.K., Sevastyanov D.V. 2000 Last glacial maximum biomes reconstructed from pollen and plant macrofossil data from northern Eurasia. Journal of Biogeography, vol. 27: 609–620. Uchastniki proekta Baikal-Burenie. Nepreryvnaya zapis izmenenii paleoklimata za poslednie 5 millionov let iz donnyh otlozhenii ozera Baikal. 1998 Geologiya i geo¿zika, vol. 39, No. 2: 135–154. Valiranta M., Kaakinen A., Kuhry P. 2003 Holocene climate and landscape evolution East of the Pechora Delta, East-European Russian Arctic. Quaternary Research, vol. 59: 335–344. Visbeck M. 2002 The ocean’s role in Atlantic climate variability. Science, vol. 297: 2223–2224. Wehrli M., Tinner W. 2007 Ammann B. 16,000 years of vegetation and settlement history from Egelsee (Menzingen, central Switzerland). The Holocene, vol. 17, No. 6: 747–761. Winkler M.G., Wang P.K. 1993 The late-Quaternary vegetation and climate of China. In Global Climates Since the Last Glacial Maximum. Minneapolis: Univ. of Minnesota Press, pp. 221–264. Wurster C.M., Patterson W.P. 2001 Late Holocene climate change for the eastern interior United States: Evidence from highresolution d18O value of marital otoliths. Palaeogeography, Palaeoclimatolog,y Palaeoecology, vol. 170: 81–100. Xiaoqiang Li, Zhou Jie, Shen Ji, Weng Chengyu, Zhao Hongli, Sun Qianli. 2004 Vegetation history and climatic variations during the last 14 ka BP inferred from a pollen record at Daihai Lake, northcentral China. Review of Palaeobotany and Palynology, vol. 132: 195–205. Zhao Y, Cheng Yu Z., Chen F., Ito E., Zhao C. 2007 Holocene vegetation and climate history at Hurleg Lake in the Qaidam Basin, northwest China. Review of Palaeobotany and Palynology, vol. 145: 275–288. Received September 26, 2008.
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E.V. Bezrukova1, A.A. Abzaeva1, P.P. Letunova1, N.V. Kulagina2, and L.A. Orlova3 Laboratory of Archaeology and Paleoecology, Irkutsk State University, K. Marxa 1, Irkutsk, 664003, Russia E-mail: [email protected] 2 Institute of the Earth’s Crust, Siberian Branch, Russian Academy of Sciences, Lermontova 128, Irkutsk, 664033, Russia E-mail: [email protected] 3 Laboratory of Cenozoic Geology and Paleoclimatology, Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences, Akademika Koptyuga 3, Novosibirsk, 630090, Russia E-mail: [email protected] 1
EVIDENCE OF ENVIRONMENTAL INSTABILITY OF THE LAKE BAIKAL AREA AFTER THE LAST GLACIATION (BASED ON POLLEN RECORDS FROM PEATLANDS)*
Pollen analysis of two dated sedimentary cores from lacustrine-boggy sediments in various parts of the Lake Baikal area yielded the ¿rst complete record of deep changes in the lake catchment area during the Late Glacial and Early Holocene. The Early Middle Holocene record shows an optimum – a humid and mild climate with warm winters between ca 10,000 and 7000 BP. During the Late Holocene, the climate grew more and more continental, and dark coniferous forests were replaced by light coniferous ones. Comparison of variation ranges of paleogeographic events in the Late Pleistocene and Holocene recorded in our samples with previously known records for the Lake Baikal area and other regions of Eurasia indicated that major changes of vegetation and climate mostly correlate with the global ice retreat, solar radiation level, and the concentration of carbon dioxide in the atmosphere. Less signi¿cant short-term Àuctuations of vegetation and climate recorded in our archives can be regarded as regional ecosystem responses to solar activity changes of a quasi-millenary scale. Regional pollen records demonstrate a distinct relationship with the climate of the Northern Hemisphere as a whole. The amplitude of these changes is higher in the northeastern Lake Baikal area than in its southern part. Keywords: Pollen analysis, paleoclimate, paleoecology, Late Glacial, Holocene, Lake Baikal catchment area.
Introduction To evaluate the anthropogenic climatic changes and their relationship with natural environmental changes, one must first of all assess the directionality of the *This study was supported by the Russian Foundation for Basic Research (Project 09-05-00123-а) and the Baikal Archaeological Project.
evolution of the climate during the Late Glacial and in the Holocene (Rind, Overpeek, 1993). Before the natural dynamics of the recent past, i.e. of Termination 1 and of the Holocene, universally represented by sediments, has been adequately described and interpreted, it will be impossible to evaluate the magnitude of the anthropogenic impact on the environment and climate. Our knowledge of the recent geological past is limited. Currently, the most reliable oxygen-isotopic record
Copyright © 2009, Siberian Branch of Russian Academy of Sciences, Institute of Archaeology & Ethnography of the Siberian Branch of the Russian Academy of Sciences. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.aeae.2009.11.001 doi:10.1016/j.aeae.2009.11.002
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of temperature changes in the Late Glacial and in the Holocene, based on the ice core from Greenland, contains signals of sharp short-lived climatic uctuations during the Late Glacial and evidences a relatively stable climate in the Holocene (GRIP Members, 1995). However, data concerning the geochemical admixture in the same core attest to climatic instability and the irregularity of the Holocene characteristics at least in Greenland (Mayewski et al., 1997). Climatic fluctuations in the Holocene have been demonstrated in many regions (Enzel et al., 1999; Wurster, Patterson, 2001; Zhao et al., 2007), and various mechanisms explaining these uctuations have been suggested (Bond et al., 2001; Visbeek, 2002). It is impossible to reconstruct the temporal and spatial variation of the Late Glacial and Holocene climate and environment without information from various regions of the planet, especially those where the environment is particularly sensitive to climatic changes. Earlier studies have shown that geochemical and diatom records from the bottom sediments of Lake Baikal meet this requirement (Uchastniki..., 1998; Khursevich et al., 2001; BDP-99..., 2005; and others). However, earlier reconstructions of climate in these studies are based on averaged signals from deep-water cores. Meanwhile, the Lake Baikal basin spans nearly four degrees of latitude and, because its southern and northern parts display climatic differences at the present, such differences must have existed in the past as well. The relevant information can be obtained from peatland ecosystems, where a thick layer of organic sediments contains a continuous record of environmental changes with a high time resolution over the last
1
3
15 thousand years (Kataoka et al., 2003; Bezrukova, Krivonogov, Abzaeva et al., 2005; Bezrukova, Belov, Abzaeva et al., 2006; Bezrukova, Belov, Letunova et al., 2008; Bezrukova, Krivonogov, Takahara et al., 2008). As the study of these records has demonstrated, climatic uctuations in the Holocene had a larger amplitude and higher frequency than previously believed, although the range was lower than during the Late Glacial. The objective of the present article is to develop a high-resolution reconstruction of Late Glacial and Holocene environmental changes in the Lake Baikal area using dated pollen records from marsh systems, which today are situated in areas with different biological and climatic parameters – those in the southern and the northeastern shores of the lake. Before the mid17th century, neither area was subjected to a signicant anthropogenic impact. Therefore the sedimentation records from these ecosystems will hopefully reect the natural environmental changes. Study areas Duguldzera. The core was taken at the eastern Baikal shore (Fig. 1), in the forest ecosystem of the middle mountain zone. Larch, Scots and Siberian pines dominate this ecosystem. Forests composed of larch, Siberian pine, and spruce occupy more elevated areas on mountain slopes and in valleys. Open Siberian pine and r forests of a primarily valley type grow higher. This region is characterized by an extreme continental climate. According to data of the nearest weather station at Davsha, mean January and July temperatures are –22 ºС and +14 ºС, respectively; the mean annual temperature equals –3.3 ºС. Mean annual precipitation ranges from 350 to 400 mm. Insular permafrost occurs in this region (Baikal..., 1993). Dulikha. The peat bog is located on the southern shore of Baikal (Fig. 1), where southern Siberian taiga composed of Siberian pine and r predominates. Larch trees are rare in marshlands. Birches form secondary forests, replacing dark coniferous forests in felling areas and re-sites. The climate is moderate continental (Ibid.). Mean July, January, and annual temperatures are +14.4, –17.7, and –0.7 oC, respectively. Mean annual precipitation is 600–650 mm. Thus, the temperatures in the two areas differ by 4–5 oC in January, by 2 oC in July, and by approx. 2.5 oC on average, whereas the difference between mean annual precipitation levels is nearly 250 mm.
2
Materials and methods Fig. 1. Map showing the location of the core sites. 1 – Duguldzera; 2 – Dulikha; 3 – VER93-2 bottom sediments, station 24 GC.
The Duguldzera core is 400 cm long. The upper 330 cm are represented by peat of various composition;
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the lower 70 cm are formed by lacustrine gyttja with an admixture of clay mineral particles. Every fourth centimeter was subjected to pollen analysis; therefore the temporal resolution of the record is 150–200 years. The chronological model of the section is based on seven radiocarbon dates. The depth of Late Glacial and Holocene peat sediments in the Dulikha core is 500 cm (Bezrukova, Krivonogov, Abzaeva et al., 2005). Every fourth centimeter of the core was treated with pollen analysis. The average temporal resolution of the record is 100–150 years. The chronological model of this core is based on three dates (Table). Chronological limits of pollen zones were estimated by linear interpolation between the dates. To evaluate the possible mechanisms behind the vegetation changes and to correlate the temporal limits of these changes with those for the Northern Hemisphere, radiocarbon dates were calibrated using CalPal software (Danzeglocke, Jöris, Weninger, 2008). Age estimates in pollen diagrams are calibrated. The classication of pollen taxa used for the calculation of pollen indices of temperature and moisture corresponds to the taxa grouping applied in the biome reconstruction method (Prentice et al., 1996; Tarasov et al., 2000; Demske et al., 2005). Pollen diagrams are presented here in the most general form for several reasons: (1) the complete diagram of the Dulikha section has already been published (Bezrukova et al., 2005), although dates were given in conditional uncalibrated 14C values and no pollen indices of temperature or moisture were provided; (2) the complete diagram of the Duguldzera section was published by Abzaeva et al. (2008); (3) for the purpose
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of the present article, the diagrams themselves are less important than the indices of environmental changes based on these diagrams. Duguldzera is designated Dz and Dulikha is labeled Dl. Results and interpretation Four zones have been recognized in the Duguldzera pollen diagram (Fig. 2). They are described from bottom to top. The zones are characterized by the most signicant pollen taxa for paleoenvironmental reconstructions. Dz4: Artemisia – Betula alba-type – Picea; > 16,000 BP; depth 400–385 cm. Sediments are represented by mineralized gyttja. Spore-pollen spectra (hereafter, SPS) evidence the first maxima of pollen of spruce (Picea obovata) and birches of both sections (Betula sect. Albae and Betula sect. Nanae). The herbaceous group is dominated by Artemisia pollen. Dz3д: Artemisia – Salix – Betula alba-type; meadowsteppe motley grassland; ~16,000–14,700 BP; depth 385– 355 cm. SPS formed in lacustrine gyttja. Tall birch pollen prevails along with pollen of shrub birch, willow, and mesoxerophytic herbs. Dz3г: Betula alba-type – Cyperaceae – Salix; ~14,700–14,000 BP; depth 355–345 cm. Birch and willow pollen dominate SPS; sedge pollen occurs in large quantities. Dz3в: Duschekia – Picea – Larix – Betula alba-type – Equisetum; ~14,000–13,200 BP; depth 345–325 cm. SPS were accumulated in gyttja. Spruce pollen appears again; the amount of alder pollen increases. Dz3б: Larix – Betula alba-type – Duschekia; ~13,200– 12,800 BP; depth 325–315 cm. SPS are characterized by
Results of radiocarbon dating of sediments Interval in the core (cm from the surface)
Radiocarbon age, years
Laboratory code
240 ± 45
SO AN-5705
25–30
1485 ± 50
SO AN-5706
1391 ± 55
»
90
4515 ± 40
АА-37969*
5179 ± 92
Wood
94–96
4805 ± 65
SO AN-5707
5531 ± 66
Peat
193
8020 ± 45
SO AN-37970*
8893 ± 93
Seeds
323
11,295 ± 55
АА-37971*
13,194 ± 101
Gyttja
378
12,950 ± 90
АА-37972*
15,767 ± 422
»
8425 ± 32
Seeds
Calibrated age, years
Dated material
Duguldzera 0–2
275 ± 114
Peat
Dulikha 300
7620 ± 115
NUTA-5615*
399
9185 ± 55
AA-37974*
10,362 ± 79
Peat
475
11,110 ± 120
NUTA-6038*
13,010 ± 128
»
*Dated by the accelerator mass spectrometry technique at the Nagoya University Center for Chronological Research.
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Fig. 2. Pollen diagram of Duguldzera sediments. In Fig. 2 and 3, the thick line in the column “Pollen indices” shows changes of the temperature index, and the thin line shows those of the moisture index.
the second major spruce pollen maximum. Sediments represent a gyttja to peat transition. Dz3а: Picea – Duschekia – Betula alba-type; ~12,800– 11,300 BP; depth 315–265 cm. SPS were formed in peat sediments. They are characterized by the greatest amounts of birch and alder pollen. Dz2б: Larix – Betula alba-type – Picea – Polypodiophyta; ~11,300–10 000 BP; depth 265– 225 cm. SPS abound in larch and birch pollen; the rst fern spores and sedge pollen maxima are recorded. The amount of spruce pollen constantly varies. A maximum of pollen representing hygrophytic plants of Potamogeton and Typha can be observed in the beginning of the zone. Dz2а: Abies – Larix – Picea – Betula; ~10,000–6000 BP; depth 225–110 cm. Fir pollen continually presents, while the amount of spruce pollen decreases. Small quantities of Siberian pine pollen are constantly recorded. The content of sedge pollen and that of fern and horsetail spores is high. Dz1б: Pinus sylvestris – Pinus sibirica – Pinus pumila, ~6000–2500 BP; depth 110–55 cm. Pollen of all the mentioned pine species dominates SPS. Dz1a: Larix – Pinus sylvestris – Pinus sibirica – Pinus pumila – Betula nana-type; ~2500 – 0 BP; depth 55–0 cm. SPS are characterized by the second maximum of shrub
birch pollen and by a high amount of arboreal birch pollen and sphagnous moss spores. Four zones are recognizable in the Dulikha pollen diagram (Fig. 3). They are described from bottom to top. Dl4: Larix – Picea – Salix – Betula nana-type – Betula alba-type; > 13,200 BP; depth 500–480 cm. Shrub and herb pollen dominates SPS. Pollen of spruce, larch, and birch of both sections prevails in the group of arboreal plants. Dl3: Artemisia – Larix – Picea – Betula nana-type – Betula alba-type – Cyperaceae – Polypodiophyta; ~13,200–10 600 BP; depth 480–405 cm. Shrub and herb pollen is still predominant in PSS. Birch and larch pollen dominates the group of arboreal plants. The share of mesoxerophytic herbs, gramineous plants, sedge, and fern increases Dl2в: Abies – Picea – Betula alba-type; ~10,600– 10,000 BP; depth 405–370 cm. Arboreal pollen becomes more abundant. The pollen of Siberian pine, Scots pine, and Siberian dwarf pine appears in spectra. Dl2б: Pinus sibirica – Betula alba-type – Abies; ~10,000–9200 BP; depth 370–330 cm. Fir pollen dominates SPS; the amount of pollen of both Siberian and Scots pine increases. Dl2а: Abies – Pinus sibirica – Betula alba-type; ~9200–6200 BP; depth 330–215 cm. The share of r pollen
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Fig. 3. Pollen diagram of Dulikha sediments.
constantly decreases, while that of Siberian and Scots pine continues to rise. The amount of birch pollen is stable. Dl1: Pinus pumila – Betula – Pinus sylvestris – Pinus sibirica; ~6200–0 BP; depth 215–0 cm. Discussion: Reconstruction of the paleoenvironment Pollen records and results of radiocarbon dating of lacustrine and marsh sediments make it possible to reconstruct climate and vegetation evolution and to partially trace changes of the hydrological regime on the southern and northeastern Baikal shores after the last glacial maximum. Vegetation and climate of the last glacial period The Duguldzera pollen record encompasses a longer time period than the Dulikha record. The pollen analysis of the Duguldzera core provided a base for the first reconstruction of vegetation and climate changes of the entire Lake Baikal record for the transitional period from the last glaciation to the modern interglacial. The lowest section of the Duguldzera pollen record (up to 16,000 BP, Dz4) reflects the environmental
conditions under which mineralized gyttja formed in the peat bog near the vegetated lake. At that time, the northeastern shore was covered with forest and tundra vegetation composed of larch, birch, spruce, and tundra herbs and shrubs. Vegetation of this type currently occurs in the Pechora River delta characterized by a cold climate with an average annual temperature of –4 °С, an average July temperature of approx. 13 °С, and an average annual precipitation of approx. 400 mm (Valiranta, Kaakinen, Kuhry, 2003). The time period of this vegetation existence coincided with the beginning of deglaciation after ~17,000 BP (Bowen et al., 2002) and relative climatic warming. Temperature and moisture indices suggest that the climate was cold (especially in the winter) and humid. Judging by the vegetation composition, the humidity index diagnoses a high soil moisture level caused by melting permafrost and lower summer temperatures rather than high atmospheric precipitation. Almost a complete disappearance of spruce pollen and decrease in the amount of arboreal pollen in the spectra formed in the mineralized gyttja ~16,000– 14,700 years ago, on the one hand, and dominance of pollen of birch, willow, and mesoxerophytic herbs, on the other hand, testify to deterioration of conditions for forest vegetation growth (Dz3д). This could possibly be determined by a general climate worsening during one of the stadials of Termination 1, whose minimum occurred ~15,500 years ago (Wehrli, Tinner, 2007). Temperature and moisture indices suggest a lowering level of warmth
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and moisture accessible to plants. The warmth level rose slightly ~14,700–14,000 years ago (Dz3г) and coincided with the rst signicant warming of Termination 1, while the humidity level continued to drop. The vegetation was dominated by shrubby yernik and willow tundra with patches of open birch forests. The rst recorded significant maximum of sedge pollen suggests the beginning of local bog formation. Between ~14,000 and 13,200 BP (Dz3в), forest-tundra vegetation with larch and later on with spruce, birch, and alder spread in the lake environs. At the same time (before ~13,200 BP), the southern Baikal shore was dominated by open birch and spruce forests with larch. This represented a shift toward milder climatic conditions of the interstadial warming that corresponded to the Allerøed. A short-term culmination of warm and humid conditions that occurred ~13,200– 12,800 years ago (Dz3б) entailed expansion of spruce in the Duguldzera region. At that time, spruce forest-tundra prevailed on the southern shore. A new period of warmth level lowering occurred between 12,800 and 11,300 BP (Dz3а). At that time, the humidity was unstable. Spruce-birch forest-tundra and alder tundra prevailed on the northeastern shore. Conditions unfavorable for forest vegetation probably resulted from climate deterioration synchronous with the Younger Dryas (Dansgaard et al., 1993). Approximately at the same time period, on the southern shore, the climate became more continental; it favored the development of birch-larch forest tundra with spruce and of herb-shrubby tundra (Dl3). Pollen indices attest to the coldest and the most humid climate over the entire period examined. Vegetation character points to a high soil moisture level (melting permafrost) due to lower summer temperatures (lower evaporation rate). In the southern Baikal area, this period was longer: it lasted almost to 10,600 BP. Vegetation and climate of the Holocene humid optimum At the onset of the Holocene (~11,300–10,000 BP (Dz2б)), a lowland sedge marsh formed in the place of the vegetated lake in the northwestern Baikal area. Larch and spruce became more abundant in the surroundings of the Duguldzera marsh. On the southern shore, on the contrary, areas vegetated by larch diminished, while territories covered by spruce expanded rapidly. Such shifts in plant composition suggest that the climate became less extreme continental; total annual precipitation values and mean winter temperatures increased. These changes marked the beginning of expansion of humid r taiga and thus the onset of the Holocene humid optimum on the southern shore. The maximum of the r taiga development occurred ~10,000–9200 years ago (Dl2б). On the southern shore, the humid optimum terminated
gradually from ~9200 to ~6800 BP (Dl2а). On the northeastern shore, the humid maximum manifested itself in a different way. It started after 10,000 BP and reached its maximum ~7000–6000 BP (the highest amount of r pollen). However, despite such a late onset of the humid optimum on the northeastern shore, it terminated there between 7000 and 6000 BP, as on the southern shore. Quantitative characteristics of the humid optimum reconstructed previously on the basis of the pollen record from the Baikal bottom sediments (VER93-2, station 24 GC (Fig. 1)) have shown that ~9500–6500 years ago, mean annual precipitation exceeded modern values by 80–100 mm; mean winter temperatures were 2–4 °С higher than nowadays. Mean July temperatures, however, could be close to those of today (Tarasov et al., 2007). A combination of mild, snowy winters, absence of spring frosts, and cool and moist summer seasons favored the development of r forests. Pollen indices demonstrate a steady tendency toward decrease of humidity and increase of warmth ca 10,000–6000 BP in the northeast and 10,500–7000 BP in the southern part of the lake. When temperature and humidity levels became close to modern ones, r taiga ceased to be predominant. The Holocene optimum is a very important period as it may be used for modeling future climatic changes. The optimum is usually believed to coincide with the period of the postglacial thermal maximum (Winkler, Wang, 1993). In Northern Europe this period is characterized by a warm and generally moist climate. However, in China, as in the Baikal basin, the Holocene optimum is dened as a period of maximal precipitation values rather than as a thermal maximum (Xiaoqiang Li et al., 2004; Porter, Weijian, 2006). A less continental climate and the predomination of r forest in the Baikal area might have been a consequence of a greater thermal gradient between the ocean and the land, which resulted in a greater transport of moist air masses to the continent. Having attained the lake area, these masses caused a greater convective rainfall. Various paleoclimatic records contain evidence of the Holocene optimum characterized by wet and cool conditions almost all over the Northern Hemisphere (Herzschuh et al., 2005; Blyakharchuk et al., 2004; Mudie et al., 2007). New facts suggesting the highest atmospheric precipitation and temperate continental climate with cool summers and warm winters that existed ca 11,000–7000 years ago have been recently obtained for the western TransBaikal region, too (Bezrukova, Krivonogov, Takahara et al., 2008). Vegetation and climate of the post-optimum period of the Holocene During the ~7000–6000 BP interval in the south and after ~6000 BP in the northeast of Lake Baikal, drastic
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changes in the composition of forest vegetation occurred: Siberian and Scots pine replaced r and spruce. This shift occurred under the conditions of a signicant lowering of humidity and increasing heat supply. Eco-edaphic requirements of these new forest elements presuppose increasing continentality of the climate due to lowering atmospheric humidity, drop of winter temperatures, and rise of summer temperatures (Tarasov et al., 2007). The termination of the humid optimum in the Baikal basin coincided with the onset of neoglaciation on the Loess Plateau (Porter, Weijian, 2006) and with the documented ice-rafted debris peak in the North Atlantic ca 6000 BP (Bond et al., 2001). At about the same time (ca 5500 BP), the relatively cool and moist “green” period in Northern Africa, which contributed to the existence of numerous lakes in what is now the Sahara, came to an end (Renssen et al., 2006). The transition to a considerably more continental climate all over Eurasia meant that the underlying mechanisms were global. The Baikal landscapes responded to these global changes by radical restructuring: the dark coniferous forest of the Early and Middle Holocene was replaced by a light coniferous and more xerophytic vegetation of the Late Holocene. Beginning from ca 6000 BP, the level of moisture available to plants demonstrates a stable reduction with minor uctuations, whereas relative temperature level demonstrates short-term small uctuations around the modern mean (see pollen indices in Fig. 2 and 3). Consequently, temperature and moisture levels did not remain permanent, evidencing climatic instability in the Late Holocene. The frequency and range of these fluctuations requires additional study, although the available records make it clear that the climatic changes affected the landscapes of the Baikal ecosystem. The changes, however, were mostly expressed in the local landscapes. In the Late Holocene (ca 2400–2500 BP), cooling resulted in changes of the hydrological regime of the marshland on the southern shore of the lake. After 2400 BP, yernik associations began to expand. The process was even more marked on the northeastern shore of Baikal at about 2500 BP. A distinct pollen signal of climate deterioration followed by the distribution of yernik associations which has been obtained for Lake Kotokel basin as well (Bezrukova, Krivonogov, Takahara et al., 2008) suggests that an adequate response of the entire Baikal ecosystem to the decrease of solar radiation occurred after 2700 BP, when the climate deteriorated in both hemispheres (Swindles, Plunkett, Roe, 2007). Even shorter uctuations of the landscape and climate in the Lake Baikal area, coinciding with well-known events such as the Medieval Warm Period and the Little Ice Age, are registered by pollen records from the northern shore of the lake (Bezrukova, Belov, Abzaeva et al., 2006).
Conclusions Results of pollen and radiocarbon studies of lacustrine and boggy ecosystems on various shores of Lake Baikal, and the comparison of these results with dated records of respective changes in adjacent regions led to the elaboration of a detailed record of environmental evolution in the Baikal area since the end of the last glaciation (17,000–16,000 BP). Considerable changes in atmospheric circulation in the Northern Hemisphere in the beginning of the deglaciation contributed to warmer and dry summers 16,000–12,000 years ago in Siberia (Schirrmeister et al., 2002). This was the time when peat deposits began forming on the southern and northeastern shores of Baikal (at 13,000 and 11,500 BP, respectively). Generally, high-resolution pollen records presented in this article attest to profound changes of vegetation and climate of the lake catchment area during the Late Glacial and in the Early Holocene, and to a high climatic variability during the modern interglacial period. Pollen records demonstrate instability of landscapes and climate in the Late Glacial and Early Holocene, resulting in frequent changes of plant associations. The probable reason behind this interchange was the melting of global ice sheets and regional mountain glaciers, leading to the instability of the ocean–atmosphere–cryosphere system. Pollen records conrm the beginning of a long period of Holocene optimum with a moist and mild climate and warm winters at ca 11,000–10,000 BP, characterized by the predominance of r, spruce, and pine forests in various areas of the Lake Baikal catchment area under increased insolation in high latitudes of the Northern Hemisphere. The optimal period ended about 7000– 6000 BP, when insolation decreased and the world ocean attained its modern level. The optimum was followed by a period when the climate became progressively more continental, atmospheric precipitation decreased, the winter became colder, and summer became warmer. As a result, dark coniferous forests gave way to light coniferous ones. The principal factors behind the climatic changes of such magnitude were variability in insolation and concentration of carbon dioxide in the atmosphere. Lesser and short-term uctuations of vegetation and climate in the Holocene (ca 2500–2400, 1600–1200, and 500–400 BP) evidenced by our pollen records may be viewed as responses of the regional ecosystem to quasi-millennium changes of insolation (Meeker, Mayewski, 2002). Records from the Dulikha and Duguldzera cores demonstrate a marked correlation with climatic uctuations in the Northern Hemisphere as a whole. The range of these changes is higher on the northeastern shore of Lake Baikal than on its southern shore. In addition to climate, local factors such as geological and geomorphological structure, vegetation, changing level of ground waters, thickness and depth of
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