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Holocene vegetation and climate variability in North Pre-Baikal region, East Siberia, Russia
Quaternary International 237 (2011) 74e82
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Holocene vegetation and climate variability in North Pre-Baikal region, East Siberia, Russia Elena V. Bezrukova a, b, *, Alexey V. Belov c, Lyubov A. Orlova d a
Institute of Archaeology and Ethnography, Siberian Branch of Russian Academy of Sciences, Novosibirsk 630090, Russia A.P. Vinogradov Institute of Geochemistry, Siberian Branch of Russian Academy of Sciences, Irkutsk 664033, Russia c V.B. Sochava Institute of Geography, Siberian Branch of Russian Academy of Sciences, Irkutsk 664033, Russia d Institute of Geology and Mineralogy, Siberian Branch of Russian Academy of Sciences, Novosibirsk 630090, Russia b
a r t i c l e i n f o
a b s t r a c t
Article history: Available online 18 January 2011
The paper presents the first reconstructions of vegetation and climate of the North Pre-Baikal region covering the last 9000 years based on detailed pollen records from peatlands. Recorded major changes in pollen spectra and vegetation are demonstrated to correlate with large-scale climatic events, such as the regional climatic optimum ca. 9000e6800 BP, termination of the optimum ca. 7000e6500 BP, and Neoglacial cooling culminated ca. 3000e2000 BP. The most warm and wet climate existed about 9000e6800 BP, the interval being referred to as the regional Holocene optimum. At that time, vegetation was dominated by dark coniferous forests of Abies sibirica, Picea obovata with Pinus sibirica. Since 6800 BP, the dark coniferous forests gave way to light coniferous ones (Pinus sylvestris and Larix) in response to gradual decrease of precipitation and summer temperatures to today’s values, agreeing well with earlier obtained data. The new records suggest the cold boreal plant communities are highly sensitive to climatic changes. Detailed dating of the sequences would facilitate inter- and intra-regional correlations of the obtained records and reconstructed events. Ó 2011 Elsevier Ltd and INQUA. All rights reserved.
1. Introduction The expected warming, as predicted by global climatic models, would be most pronounced in high latitudes of the Northern Hemisphere (ACIA, 2004). This accounts for the close attention focused on reconstructions of paleo-environments in various regions, and particularly in Siberia, opening the way to more objective forecast of environmental changes in future. The North Pre-Baikal region has particular interest for paleo-ecological and paleo-climatic studies. Due to its position in the central part of the continent, within the zone of cold boreal forests, where Atlantic air masses come into contact with Arctic ones, the region features unique environments. The climate with cold winter and moderately warm summer, as well as the presence of permafrost, favored formation of wetlands, and pollen records preserved in their deposits provide information on the Holocene environments. Climate changes in the Holocene were not so dramatic and did not induce complete restructuring of environments comparable to those in Late Pleistocene. However, the climate changes were
* Corresponding author. Institute of Archaeology and Ethnography, Siberian Branch of Russian Academy of Sciences, Novosibirsk 630090, Russia. E-mail address: [email protected] (E.V. Bezrukova). 1040-6182/$ e see front matter Ó 2011 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2011.01.012
sufficient for local and even regional plant communities to be noticeably changed. No paleo-environmental studies of the Holocene have been yet performed in the North Pre-Baikal region. The present paper gives the first reconstructions of vegetation and climate of this area during the last 9000 years, based on detailed pollen records obtained from peatlands. The objective of the investigation was to get new information on dynamics of vegetation and wetland ecosystems in the high mountains of the Baikalsky Ridge and Khanda depression (northeast of Lena-Angara Plateau). A comparison between the newly obtained records and other regional data would advance understanding of the evolution of climate and vegetation in this still insufficiently studied continental region. 2. Geography, climate and modern vegetation of the area under study 2.1. Northern part of the Baikalsky Ridge (North Baikalian basin) The high Baikalsky Ridge, with some peaks rising above 2000 m, forms the northwestern slope of the North Baikalian basin and extends for about 300 km from north to south. The coasts of the North Baikalian basin have a temperate continental climate, partly
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due to heating influence of the water mass. However, continuous permafrost suggests rather severe natural conditions. Atmospheric circulation in inner regions of Eurasia is mostly controlled by the Siberian anticyclone. In winter, it prevents zonal air currents from entering the regions, which results in cold and dry, mostly still weather. Occasional intrusions of arctic air mass are responsible for variability of the Siberian High impact in time and space (Lydolph, 1977). In summer the land surface is intensively heated and a zone of low atmospheric pressure develops over the greater part of Siberia, so that western oceanic air masses can easily penetrate there. Accordingly, maximum precipitation falls in summer. Vegetation of the Baikalsky Ridge displays a complicated pattern of vertical zonality. Its western slope facing wet air masses coming from west is covered mostly with dark coniferous forests of Pinus sibirica with noticeable Abies sibirica and Picea obovata. Betula sect. Albae is common in the lower parts of the slope, while the upper slopes are occupied with subalpine open woodlands of A. sibirica and Betula sect. Albae. Still higher is mountain shrub tundra with Pinus pumila and Betula sect. Nanae, and stone heathlands. On the leeside eastern slope are coniferous forests of P. sibirica, Larix, and Pinus sylvestris in its lower part, locally with steppe plant communities (Galazy, 1993). The studied peat deposits occur on the flat part of the mountain pass (Fig. 1, watershed of the Delbichinda and Okunaika rivers). The peatland is a part of mesotrophic bog system in the northern Baikalsky Ridge. Local vegetation near the drilling site of Okunaika peatland (55.52 N, 108.47 E, 1450 m asl.) has open forests of Betula sect. Albae and Larix, with some A. sibirica. The shrub layer consists of Duschekia (=Alnaster fruticosa) fruticosa, Pinus pumila, Betula nana L. subsp. exilis, and Salix. The bog itself is covered with several species of Carex, Sphagnum, and Eriophorum brachyantherum. 2.2. Khanda depression The Khanda depression is located in the northwest portion of the Pre-Baikalian foredeep at its contact with the Lena-Angara
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Plateau (Fig. 1). Geomorphologically, it is a depression about 50 km long and 15 km wide, with slopes rising as high as 200e250 m above its bottom. The bottom of the basin is composed of Quaternary gravelly sands and silt, as well as peat. The main water stream is the Khanda River. Climate is strongly continental, with long severe winters and short warm summers. Typical are temperature inversions in winter, when the air within the depression may be cooled to 45 to 50 S. Such low temperatures account for the development of permafrost, with associated processes, such as thermokarst, solifluction and frost heaving. The vegetation on the depression bottom and on differently oriented slopes shows conspicuous distinctions. The western side of the basin is dominated by forests of P. obovata and P. sibirica with Larix, while on its eastern side forests of P. sibirca, P. sylvestris, Larix with Betula sect. Nanae and Ledum palustre dominate. Within the depression are forests of P. sylvestris and Larix on the Khanda R. terraces. Primary forests are often destroyed by fires over considerable areas, overgrown with Larix and Betula sect. Albae. At the bottom of the depression, swamp vegetation prevails, and wetlands cover more than 80% of its total area. Riverine plant communities consist of Cyperaceae and Salix. Closer to the basin, river channels are fringed with forests of Larix and P. obovata with P. sibirica. In the lowest part of the depression there are peatlands covered with shrub and moss communities with occasional Larix in combination with Cyperaceae-Eriophorum-Sphagnum typical of low marshes. The general appearance of vegetation on the depression bottom has much in common with northern taiga communities with some tundra elements. Local vegetation near the drilling sites of Khanda and Khanda-1 peatlands (55.44 N, 107. 00 E, 713e714 m asl.) is mostly open woodlands of P. sylvestris, Larix gmelinii with rare trees of Betula sect. Albae. Betula nana sub. sp. exilis, Ledum palustre, Carex globularis, Rubus chamaemorus, Oxycoccus microcarpus, and Sphagnum are present in the shrub and herb layer. Carex diandra, Eriophorum gracile and Menyanthus trifoliate are found around small open water bodies. 3. Materials and methods
Fig. 1. Locations of the region and the pollen records discussed in the recent study.
A core 312 cm long was recovered from Okunaika peatbog in 2005, using a Russian hand-operated corer. The upper 292 cm of the core consists primarily of sedge and sphagnum peat. The lowermost 20 cm were clay with fine gravel. Every sixth centimeter was taken as a sample for pollen analysis (53 samples). In 2006, the same equipment was used in boring the peat massif at two points within the Khanda depression (Khanda and Khanda-1 sites, 7 km apart). In the Khanda core, the lower part of the sequence (232e70 cm) consists of wet, very dark, almost black, moderately decomposed peat, overlying light gray frozen fluvial sands of the Khanda R. From 70 cm to the surface, the peat becomes drier and less decomposed, with a brownish hue. Permafrost begins at a depth of 52 cm. Samples for pollen analysis were taken from every 5th cm (42 samples). The total length of core obtained from Khanda-1 peat deposit (7 km upstream of the Khanda R.) is 112 cm. The penetrated deposits are mostly sphagnum peat decomposed to a variable degree. The top of permafrost is at 62 cm depth from the peat surface. Every 2nd cm was sampled for pollen analysis (55 samples). The samples for pollen analysis were treated in the Laboratory of the Institute of the Earth’s Crust, Siberian Branch of the Russian Academy of Sciences, using standard techniques for pollen separation and concentration (Faegri and Iversen, 1989). Volume of samples (dry matter) varied from 1 to 2 cm3. Relative abundance (percentage) of the main taxa was calculated taking the pollen sum as 100%, with Cyperaceae pollen and Pteridophyta and Sphagnum
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spores excluded from the main sum. Percentages of Cyperaceae, Pteridophyta and Sphagnum in a particular sample were calculated with reference to total amount of pollen and spores counted in that sample. Data on the sub-recent pollen spectra composition described in all the altitudinal zones in the Baikal drainage basin (Belova, 1985; Bezrukova, 1999) have been used to interpret changes of vegetation in the past. Taxonomical identification of the plant remains (mostly to a level of genus or family) in the botanical composition of peat was carried out using atlases of fossil plants (Dombrovskaya et al., 1959; Kats et al., 1977) with a help of binocular microscope MBS-10 (magnification 70y). Proportion of plant remains differing in taxonomic status was calculated as a percentage of total quantity of plant remains according to standard procedures (Yurkovskaya et al., 1989; Elina et al., 2000). Fifteen radiocarbon dates were obtained in the Laboratory of Geology and Paleoclimatology, Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences (Novosibirsk); the dated material was total organic matter of the peat. The dates are calibrated using CalPal program (Danzeglocke et al., 2008). The results are given in Table 1. All the dates are given in the text as calibrated radiocarbon ages (cal. BP). 4. Results 4.1. Radiocarbon chronology Radiocarbon-dated sequence from the Okunaika section appeared to cover more than 9000 years. The age of deposits penetrated in sections Khanda and Khanda-1 is somewhat older than 6000 and 3000 BP, respectively (Table 1). 4.2. Pollen stratigraphy On the basis of variations in pollen assemblages and detailed 14C dating, five LPAZ (Local Pollen Assemblage Zones) in the Okunaika section (Okn1eOkn5, Fig. 2), three LPAZ in the Khanda section (Khd1eKhd3, Fig. 3) and a single LPAZ in the Khanda-1 section (Fig. 4) were visually identified. Depths of occurrence and ages of the upper and lower boundaries of zones are shown in Table 2. Description of pollen zones is summarized in Fig. 6. Average time resolution of pollen records is ca. 180, 150 and 60 years for Okunaika, Khanda and Khanda-1 sections, respectively.
Table 1 Radiocarbon determinations. Lab. no
Depth, cm Age (BP)
Okunaika bog SOANe6394 SOANe6395 SOANe6396 SOANe6397 SOANe6398 SOANe6399
49e53 100e106 148e152 202e208 250e256 302e308
3115 3485 4450 5670 6175 8050
140 120 110 110 185 215
3122e3478 3617e3921 4935e5260 6359e6605 6834e7259 8670e9243
(3300) (3769) (5098) (6482) (7047) (8957)
peat peat peat peat peat peat
Khanda SOANe6445 SOANe6446 SOANe6447 SOANe6448 SOANe6449 SOANe6450
11e13 68e70 121e123 143e145 188e190 226e228
130 1870 2800 3450 4080 5230
40 120 120 80 90 130
39e241 1660e1948 2816e3113 3619e3824 4473e4768 5843e6170
(140) (1804) (2965) (3722) (4621) (6007)
peat peat peat peat peat peat
188e416 (302) 2018e2247 (2133) 2987e3161 (3074)
peat peat peat
Khandae1 SOANe6451 8e10 SOANe6452 58e60 SOANe6453 110e112
260 45 2115 65 2915 55
Calendar age (cal. BP) Material dated
4.3. Peat botanical composition Analysis of botanical composition has been performed for 64 samples from the Okunaika section at 4 cm intervals. The lowermost peat layer (312-292 cm, w9500-8500 BP) could not be analyzed because of the high proportion of the mineral component. The results obtained for the remainder identified five Local Botanical Assemblage Zones (LBAZ), Okn1boteOkn5bot. Fig. 5 shows the chronological sequence of LPAZ in comparison with that of LBAZ in this section. 5. Reconstruction of palaeoenvironmental conditions Before discussing change of vegetation and climate in the region under study, it should be noted that in all probability the area did not experience human impact during the early-middle, or even in the late Holocene. Neither remains of settlements nor traces of any economic activities has been found near the peat massifs. Therefore, it may be safely suggested that the pollen stratigraphy thus obtained would primarily reflect changes in vegetation induced by variations of global and regional climate. The Okunaika pollen record (Okn) provides evidence of vegetation dynamics and climatic changes during the last 9000 years (or a little more). According to pollen data, at the initial stages of the peatland formation (Okn5, ca. 9000e7300 cal. BP) the area was covered with open larch and spruce forests of Larix, P. obovata, Betula sect. Albae, in good agreement with earlier data on other areas in the vicinity of Lake Baikal (Bezrukova et al., 2005; Demske et al., 2005; Tarasov et al., 2007; Bezrukova et al., 2008, 2009). The presence of arboreal pollen (no more than 50%) and abundance of Polypodiaceae and Lycopodiaceae spores also indicate the presence of isolated forested areas. At that time the climate was still cold and permafrost remained widespread. Nevertheless, relatively high spring and summer temperatures were favorable for the appearance of P. sibirica and expansion of A. sibirica ca. 8500 cal. BP. It is not inconceivable that the sediments dated to that interval were deposited in the immediate vicinity of an intermittent stream, accounting for the enrichment of organic deposits with mineral substance. The accumulation of proper peat began about 8500 cal. BP. An interval marked by reduction of tree species and expansion of shrubs (Salix, Alnaster fruticosa, Betula sect. Nanae) occurred 8500e7500 BP. Botanical composition of the peat provided evidence of Alnaster growth near the site, while A. sibirica seems to be absent there, as no macroremains of the species have been found. Pollen assemblages of Okn-5 zone coincided with a short-term cooling peaking about 8.2 ka BP (Prasad et al., 2009). There is no distinct response of vegetation to the “8.2 ka event” recorded in the studied spectra. It seems noteworthy, however, that arboreal pollen (A. sibirica in particular) is reduced in abundance in peat layers between the two lower dates (7047e8957 BP), while abundance of Betula nana-type and mesophytic herbs pollen noticeably increased. It is possible that such was the regional vegetation response to the cooling dated at ca. 8.2 ka BP. The interval 7500e6800 BP (Okn4) was marked by rapid (in 200e300 years, according to temporal resolution of the record) expansion of A. sibirica. Its presence near the peatbog is shown by the high abundance of its pollen in the assemblages, as well as by the botanical composition of the peat. Siberian pine (P. sibirica) could appear in the local plant communities for a short time, ca. 7000-6000 BP, as indicated by the presence of its wood remains in the peat. It was at that time interval that a peat layer was formed with A. sibirica and Poaceae dominant in its botanical composition. Such substantial changes in vegetation could result from dramatic changes of climate. A. sibirica requires mild winters and cool summers, thick snow cover and abundant precipitation, as well as
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Fig. 2. Percentage pollen diagram of selected pollen taxa from Okunayka site (Okn).
well drained fertile soils; the indicated interval may be considered optimal from the viewpoint of combination of all the above-stated environmental characteristics. The time of A. sibirica maximum in the Okunaika site vicinities agrees well with its maxima in other regions of the Baikal drainage basin (Kataoka et al., 2003; Demske et al., 2005; Tarasov et al., 2007; Bezrukova et al., 2008, 2009). Dark coniferous forests with A. sibirica were widespread in Siberia at that time under conditions of abundant precipitation (Tarasov et al.,
2007). High values of annual precipitation were typical of most temperate latitudes in the Northern Hemisphere between ca. 10000 and 6000 cal. BP, including the eastern Swiss Alps (Joerin et al., 2008), the modern steppe zone of Mongolia (Tarasov et al., 2000), and the Central Altai mountains (Blyacharchuk et al., 2004, 2007). In northern Kazakhstan, an increase in moisture supply is dated to 8500e7200 cal. BP, and in the steppe zone of SW Siberia to 7800e6600 cal. BP. (Davydova et al., 1995).
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Fig. 3. Percentage pollen diagram of selected pollen taxa from Khanda site (Khd).
A. sibirica was reduced in importance as a component of forests in the region after 6800 BP (Okn3b ca. 6800e4500 cal. BP), in common with other regions of Siberia (Blyacharchuk et al., 2004, 2007; Tarasov et al., 2007; Bezrukova et al., 2008, 2009). As that took place, P. obovata, P. sibirica and P. sylvestris gained in importance. An analysis of the peat botanical composition confirms the presence of Abies and Pinus near the site at the beginning of the interval. The Pinus remains have been identified to a genus level; they supposedly belong to P. sibirica, as at present this species is a forest-forming species in middle mountain taiga on the western
slopes of the Baikalsky Ridge, together with A. sibirica and P. obovata. A low content of P. sylvestris pollen (12% at most) suggests it was wind-borne. Judging from the Okunaika record, the expansion of pine forests began about 6800 BP, consistent with earlier dating of this event in Siberia (Kataoka et al., 2003; Blyacharchuk et al., 2004, 2007; Demske et al., 2005; Tarasov et al., 2007; Bezrukova et al., 2008, 2009; Müller et al., 2009). As follows from data on changes in available moisture and warmth supply, expansion of P. sylvestris went along with decreasing precipitation and increasing climate
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Fig. 4. Percentage pollen diagram of selected pollen taxa from Khanda-1 site (Khd-1).
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Table 2 Position of the pollen zones in the sections. Okunaika
Khanda-1
Khanda-2
Zone/Depth
Cal. BP
Zone/Depth
Cal. BP
Zone/Depth
Cal. BP
0e46 cm 46e122 cm 122e225 cm 260e225 cm 260e312 cm
w0e2700 w2700e4500 w4500e6800 w6800e7500 w7500e9000
0e68 cm 68e185 cm 185e232 cm
w0e2700 w2700e4500 w4500e6000
0e37 cm 37e72 cm 72e112 cm
w0e1300 w1300e2500 w2500e3000
continentality (Demske et al., 2005). Quantitative reconstructions of climate based on pollen record from the Baikal sediments show the mean annual precipitation ca. 7000e6500 BP could be 80e100 mm less than during the previous interval ca. 9000e7000 BP, when forests of A. sibirica were dominant in the region (Tarasov et al., 2007). In addition to pollen data, botanical composition of the samples points to increasing presence of Larix, Betula pendula, P. obovata, Salix sp., Ledum palustre, Hamaedaphne calyculata, and Spiraea sp. in the middle part and at the end of this interval. Carex vesicata was also found in abundance. According to the Khanda record (peat section located almost 100 km west of the Okunaika site and at elevation 750 m lower), the time interval ca. 6000-4500 cal. BP (Khd-1) in the Khanda depression is also marked by reduction of dark coniferous forests of A. sibirica, P. obovata, P. sibirica and gradual expansion of light coniferous forests of P. sylvestris. At present, P. sylvestris occupies limited habitats, although it makes up the bulk of pollen assemblages
(50e70%). It is reasonably safe to suggest, therefore, that ca. 6000e5000 BP P. sylvestris could occur as an insignificant admixture in the dark coniferous forests. Its growing percentage in pollen spectra may be indicative of expansion of P. sylvestris at a regional scale. The noticeable participation of herbs Artemisia, and Poaceae in the pollen spectra, along with Betula sect. Nanae and dwarf shrub Ericaceae, suggests open forests. Such vegetation is characteristic of northern taiga with forest-tundra elements under conditions of wet and moderately cold climate. Pollen assemblages suggest P. sibirica and P. sylvestris to be dominant near the crest of Baikalsky Ridge in its northern part about ca. 4500e2700 BP (Okn2). In the local vegetation near the Okunaika site, Larix, Pinus pumila and Cyperaceae presumably prevailed. The peat botanical composition confirms the presence of Betula pendula and Larix in local vegetation and adds Ledum palustre, Alnaster fruticosa, Oxycoccus microcarpa, Carex diandra, Carex dioica to the list of local species. The presence of Pinus pumila and
Fig. 5. Simplified percentage diagram of peat botanical composition of selected taxa from Okunayka site.
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Fig. 6. Comparative description of local pollen assemblage zones.
P. sibirica near the site, however, lacks support from the botanical composition analysis: in all probability, those species occurred on the lower slopes. The local vegetation of the Khanda Depression ca. 4500e2700 BP (Khd-2 and Khd-1c) was dominated by Larix, P. obovata, Betula sect. Nanae, Pinus pumila and Cyperaceae, with P. sylvestris and P. sibirica presumably growing on slopes of the depression. The presence of Pinus pumila on lower elevations is usually related with temperature inversions resulting in air temperature, dropping to 45 to 50 S in winter, and development of permafrost. It is not inconceivable, therefore, that somewhat enlarged ranges of Pinus pimula and Betula sect. Nanae ca. 4500e2700 BP noticeable in both records (Okh and Khd) indicate an increase in continentality and lower temperatures. It is also likely that, due to some local factors, the effect of temperature drop on permafrost was more pronounced at Khanda-1, as demonstrated by the higher proportion of Pinus pumila pollen in Khanda-1 record. On the whole, the suggestion that climate became less favorable is in good agreement with data on total deterioration of climate in the Lake Baikal basin at that time (Tarasov et al., 2007; Bezrukova et al., 2008). Later, from ca. 2700 BP to the present day, the proportion of dark coniferous trees in the local vegetation at Okunaika site decreased still further. The peat botanical composition indicates that Larix, Betula sect. Albae, and Betula sect. Nanae were present locally. Such changes could take place under conditions of steadily decreasing heat supply and somewhat growing effective moisture due to less intensive evaporation. Such a relationship between heat and moisture supply might be responsible for the expansion of shrubs and the formation of eutrophic bog with continuous cover of Cyperaceae. The presence of Iris laevigata and Eriophorum vaginatum remains in the upper peat layers also indicates the existence of wetlands at that time. Climatic change towards cooling and decrease of atmospheric precipitation between 3000 and 2000 BP has been recorded in many regions of the Northern Hemisphere,
including the mountains of Siberia (Solomina, 1999). The pine pollen in the Okunaika section was seemingly brought by wind from the lower altitudinal zone. The dynamics of pine expansion within the entire region is reflected in the Okunaika section as much as it is in other pollen records (Kataoka et al., 2003; Demske et al., 2005; Tarasov et al., 2007; Bezrukova et al., 2008). In the Khanda depression, the last 2700e2500 years are distinct for greater participation of dark coniferous trees and wide occurrence of Larix, in the Khanda site local vegetation in particular (Fig. 3). A specific feature of Khanda-1 site appears to be a rather long period (w1000 years) when Pinus pumila communities existed locally; their degradation is dated approximately at 1500 BP. Local environmental factors are likely responsible for reduction of Ericaceae ca. 2600 BP. At the end of the processes appeared communities of Menyanthes and Potamogeton, nearshore plants typical of stagnant or slowly flowing water. Appearance of those communities coincides with the Cyperaceae maximum. Later, about 2600e2200 BP, a sphagnum bog developed (unlike the sedge bog near the Khanda site). It is likely that the frequent changes in local plant communities reflected the general instability of wetland systems as a response to climate deterioration in the Northern hemisphere ca. 3000e2000 BP (Bond et al., 1997; Swindles et al., 2007). 6. Conclusions The paper presents the first high-resolution well dated pollen records obtained in the alpine zone of Baikalsky Ridge and Khanda depression, regions of the most severe climate in the Baikal region. The pollen records provided the basis for reconstructing vegetation and climate of the region spanning the last w9000 years. The reconstructions demonstrated considerable changes in local and regional vegetation during that time. The most significant changes in pollen assemblages and vegetation may be correlated with
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known large-scale climatic event, such as the regional optimum of w9000e6800 BP, the optimum termination about 7000e6500 BP, and Neoglacial cooling 3000e2000 BP. The warmest and wettest conditions are documented ca. 9000e6800 BP and tentatively correlated with the Holocene optimum clearly defined in the region. That interval was marked by dominating dark coniferous forests of A. sibirica and P. obovata with P. sibirica. The dark coniferous forests gave way to light coniferous ones (of P. sylvestris and Larix) after 6800 BP in response to gradual decrease of precipitation and summer temperatures to their present-day values, in good agreement with earlier results. The wetland ecosystem development at the bottom of Khanda depression, as well as the vegetation dynamics during the middle-late Holocene, were primarily controlled by the global climate cooling. True peat accumulation started ca. 6000 BP, at the time of the Holocene optimum termination all over East Siberia. The response of regional and especially local vegetation to changes in global and regional environments during the last 2700 years was markedly diversified, presumably due to differences in local climatic, topographic or hydrological factors. The Medieval Warm Period, as well as cooling of the Little Ice Age, are not distinct in the newly obtained pollen records. This may be attributed to short duration of both events and to location of the studied territory in the boreal forests. Not all the studied regions responded identically or synchronously to the global environmental changes. Thus, regional manifestations of the Holocene climate fluctuations might be highly diversified, and therefore, it is particularly important to obtain data from a geographically wider assortment of environments. The records definitely point to high sensitivity of cold boreal plant communities to climate changes. Adequate dating of sediments is instrumental in intra- and inter-regional correlations of records obtained and events reconstructed. Acknowledgements The authors are grateful to O.N. Shestakova for the treatment of samples in preparation for pollen analysis; to P.P. Letunova and A.A. Abzayeva for their help in performing the analysis, and to E.V. Sheifer for determination of the peat botanical composition. The studies were performed with financial support from the Russian Foundation for Basic Research, Project No. 09-05-00123-a, and Programs IX.81.1 and VII.65.1 of Siberian Branch of Russian Academy of Sciences. References ACIA, 2004. Impacts of a Warming Arctic: Arctic Climate Impact Assessment. Cambridge University Press, Cambridge. Belova, V.A., 1985. Vegetation and Climate of Late Cenozoic in the South of Eastern Siberia. Nauka Press, Novosibirsk, 176 pp. (in Russian). Bezrukova, E.V., Abzaeva, A.A., Letunova, P.P., Kulagina, N.V., Vershinin, K.E., Belov, A.V., Orlova, L.A., Danko, L.V., Krapivina, S.M., 2005. Post-glacial history of Siberian spruce (Picea obovata) in the Lake Baikal area and the significance of this species as a paleoenvironmental indicator. Quaternary International 136, 47e57.
Bezrukova, E.V., Krivonogov, S.K., Takahara, H., Letunova, P.P., Shichi, K., Abzaeva, A.A., Kulagina, N.V., Zabelina, Yu. S, 2008. Lake Kotokel as a stratotype for the late glacial and Holocene in Southeastern Siberia. Doklady Academii Nauk Geografiya 420 (2), 1e6. Bezrukova, E.V., 1999. Palaeogeography of Pribaikalie in the Late Glacial and Holocene. Nauka, Novosibirsk, p. 128 (in Russian). Bezrukova, E.V., Abzaeva, A.A., Letunova, P.P., Kulagina, N.V., Orlova, L.A., 2009. Evidence of environmental instability of the Lake Baikal area after the last glaciation (based on pollen records from peatlands). Archaeology Ethnology & Anthropology of Eurasia 37 (3), 17e25. Blyacharchuk, T.A., Wright, H.E., Borodavko, P.S., van der Knaap, W.O., Ammann, B., 2004. Late Glacial and Holocene vegetation changes on the Ulagan highmountain plateau, Altai Mountains, southern Siberia. Palaeogeography Palaeoclimatology Palaeoecology 209, 259e279. Blyacharchuk, T.A., Wright, H.E., Borodavko, P.S., van der Knaap, W.O., Ammann, B., 2007. Late glacial and Holocene vegetational history of the Altai mountains (southwestern Tuva Republic, Siberia). Palaeogeography Palaeoclimatology Palaeoecology 245, 518e534. Bond, G., Showers, W., Cheseby, M., Lotti, R., Almasi, P., deMenocal, P., Priore, P., Cullen, H., Hajdas, I., Bonani, G., 1997. A pervasive millennial-scale cycle in North Atlantic Holocene and Glacial climates. Science 278, 1257e1266. Danzeglocke, U., Jöris, O., Weninger, B., 2008. CalPal-2007 Online. Available at: http://www.calpal-online.de. Davydova, N.N., Martinson, G.G., Sevastyanov, D.V., 1995. History of Lakes in Northern Asia. Nauka, St. Petersburg (in Russian). Demske, D., Heumann, G., Granoszewski, W., Nita, M., Mamakowa, K., Tarasov, P.E., Oberhansli, H., 2005. Late glacial and Holocene vegetation and regional climate variability evidenced in high-resolution pollen records from Lake Baikal. Global and Planetary Change 46, 255e279. Dombrovskaya, A.V., Koreneva, M.M., Tyuremnov, S.N., 1959. Atlas of Fossil Plants Found in Peat. Gosenergoizdat, Moscow, Leningrad (in Russian). Elina, G.A., Lukashov, A.D., Yurkovskaya, T.K., 2000. The Late Glacial and Holocene of Eastern Fennoscandia (Paleovegetation and Paleogeography). Karelian Science Center, Petrozavodsk (in Russian). Faegri, K., Iversen, J., 1989. Textbook of Pollen Analysis, fourth ed.. John Wiley and Sons, Copenhagen. Galazy, G.I., 1993. Atlas of Lake Baikal. GUGK, Moscow (in Russian). Joerin, U.E., Nicolussi, K., Fischer, A., Stocker, T.F., Schluchter, C., 2008. Holocene optimum events inferred from subglacial sediments at Tschierva Glacier, Eastern Swiss Alps. Quaternary Science Reviews 27, 337e350. 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. Springer Verlag, Tokyo, pp. 207e218. Kats, N. Ya, Kats, S.V., Skobeeva, N.I., 1977. Atlas of Fossil Plants in Peat. Nedra, Moscow (in Russian). Lydolph, P.E., 1977. Climates of the Soviet Union (World Survey of Climatology 7). Elsevier Scientific Publishing, Amsterdam, p. 441. Müller, S., Tarasov, P.E., Andreev, A.A., Diekmann, B., 2009. Late glacial to Holocene environments in the present-day coldest region of the northern hemisphere inferred from a pollen record of Lake Billyakh, Verkhoyansk Mts, NE Siberia. Climate of the Past 5, 73e84. Prasad, S., Witt, A., Kienel, U., Dulski, P., Bauer, E., Yancheva, G., 2009. The 8.2 ka event: evidence for seasonal differences and the rate of climate change in Western Europe. Global and Planetary Change 67, 218e226. Solomina, O.N., 1999. Mountain Glaciation in Northern Eurasia in the Holocene. Nauchnyi Mir, Moscow (in Russian). 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 17, 177e182. 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 Palaoecology 252, 440e457. Tarasov, P.E., Dorofeyuk, N.I., Meteltseva, E.A., 2000. Holocene vegetation and climate changes in Hoton-Nur basin, northwest Mongolia. Boreas 29, 117e126. Yurkovskaya, T.K., Elina, G.A., Klimanov, V.A., 1989. Vegetation and paleogeography of forest and peatland ecosystems in the right side of the Pinega River (Arkhangelsk Region). Botanical Journal 12, 1711e1723 (in Russian).
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Quaternary International journal homepage: www.elsevier.com/locate/quaint
Holocene vegetation and climate variability in North Pre-Baikal region, East Siberia, Russia Elena V. Bezrukova a, b, *, Alexey V. Belov c, Lyubov A. Orlova d a
Institute of Archaeology and Ethnography, Siberian Branch of Russian Academy of Sciences, Novosibirsk 630090, Russia A.P. Vinogradov Institute of Geochemistry, Siberian Branch of Russian Academy of Sciences, Irkutsk 664033, Russia c V.B. Sochava Institute of Geography, Siberian Branch of Russian Academy of Sciences, Irkutsk 664033, Russia d Institute of Geology and Mineralogy, Siberian Branch of Russian Academy of Sciences, Novosibirsk 630090, Russia b
a r t i c l e i n f o
a b s t r a c t
Article history: Available online 18 January 2011
The paper presents the first reconstructions of vegetation and climate of the North Pre-Baikal region covering the last 9000 years based on detailed pollen records from peatlands. Recorded major changes in pollen spectra and vegetation are demonstrated to correlate with large-scale climatic events, such as the regional climatic optimum ca. 9000e6800 BP, termination of the optimum ca. 7000e6500 BP, and Neoglacial cooling culminated ca. 3000e2000 BP. The most warm and wet climate existed about 9000e6800 BP, the interval being referred to as the regional Holocene optimum. At that time, vegetation was dominated by dark coniferous forests of Abies sibirica, Picea obovata with Pinus sibirica. Since 6800 BP, the dark coniferous forests gave way to light coniferous ones (Pinus sylvestris and Larix) in response to gradual decrease of precipitation and summer temperatures to today’s values, agreeing well with earlier obtained data. The new records suggest the cold boreal plant communities are highly sensitive to climatic changes. Detailed dating of the sequences would facilitate inter- and intra-regional correlations of the obtained records and reconstructed events. Ó 2011 Elsevier Ltd and INQUA. All rights reserved.
1. Introduction The expected warming, as predicted by global climatic models, would be most pronounced in high latitudes of the Northern Hemisphere (ACIA, 2004). This accounts for the close attention focused on reconstructions of paleo-environments in various regions, and particularly in Siberia, opening the way to more objective forecast of environmental changes in future. The North Pre-Baikal region has particular interest for paleo-ecological and paleo-climatic studies. Due to its position in the central part of the continent, within the zone of cold boreal forests, where Atlantic air masses come into contact with Arctic ones, the region features unique environments. The climate with cold winter and moderately warm summer, as well as the presence of permafrost, favored formation of wetlands, and pollen records preserved in their deposits provide information on the Holocene environments. Climate changes in the Holocene were not so dramatic and did not induce complete restructuring of environments comparable to those in Late Pleistocene. However, the climate changes were
* Corresponding author. Institute of Archaeology and Ethnography, Siberian Branch of Russian Academy of Sciences, Novosibirsk 630090, Russia. E-mail address: [email protected] (E.V. Bezrukova). 1040-6182/$ e see front matter Ó 2011 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2011.01.012
sufficient for local and even regional plant communities to be noticeably changed. No paleo-environmental studies of the Holocene have been yet performed in the North Pre-Baikal region. The present paper gives the first reconstructions of vegetation and climate of this area during the last 9000 years, based on detailed pollen records obtained from peatlands. The objective of the investigation was to get new information on dynamics of vegetation and wetland ecosystems in the high mountains of the Baikalsky Ridge and Khanda depression (northeast of Lena-Angara Plateau). A comparison between the newly obtained records and other regional data would advance understanding of the evolution of climate and vegetation in this still insufficiently studied continental region. 2. Geography, climate and modern vegetation of the area under study 2.1. Northern part of the Baikalsky Ridge (North Baikalian basin) The high Baikalsky Ridge, with some peaks rising above 2000 m, forms the northwestern slope of the North Baikalian basin and extends for about 300 km from north to south. The coasts of the North Baikalian basin have a temperate continental climate, partly
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due to heating influence of the water mass. However, continuous permafrost suggests rather severe natural conditions. Atmospheric circulation in inner regions of Eurasia is mostly controlled by the Siberian anticyclone. In winter, it prevents zonal air currents from entering the regions, which results in cold and dry, mostly still weather. Occasional intrusions of arctic air mass are responsible for variability of the Siberian High impact in time and space (Lydolph, 1977). In summer the land surface is intensively heated and a zone of low atmospheric pressure develops over the greater part of Siberia, so that western oceanic air masses can easily penetrate there. Accordingly, maximum precipitation falls in summer. Vegetation of the Baikalsky Ridge displays a complicated pattern of vertical zonality. Its western slope facing wet air masses coming from west is covered mostly with dark coniferous forests of Pinus sibirica with noticeable Abies sibirica and Picea obovata. Betula sect. Albae is common in the lower parts of the slope, while the upper slopes are occupied with subalpine open woodlands of A. sibirica and Betula sect. Albae. Still higher is mountain shrub tundra with Pinus pumila and Betula sect. Nanae, and stone heathlands. On the leeside eastern slope are coniferous forests of P. sibirica, Larix, and Pinus sylvestris in its lower part, locally with steppe plant communities (Galazy, 1993). The studied peat deposits occur on the flat part of the mountain pass (Fig. 1, watershed of the Delbichinda and Okunaika rivers). The peatland is a part of mesotrophic bog system in the northern Baikalsky Ridge. Local vegetation near the drilling site of Okunaika peatland (55.52 N, 108.47 E, 1450 m asl.) has open forests of Betula sect. Albae and Larix, with some A. sibirica. The shrub layer consists of Duschekia (=Alnaster fruticosa) fruticosa, Pinus pumila, Betula nana L. subsp. exilis, and Salix. The bog itself is covered with several species of Carex, Sphagnum, and Eriophorum brachyantherum. 2.2. Khanda depression The Khanda depression is located in the northwest portion of the Pre-Baikalian foredeep at its contact with the Lena-Angara
75
Plateau (Fig. 1). Geomorphologically, it is a depression about 50 km long and 15 km wide, with slopes rising as high as 200e250 m above its bottom. The bottom of the basin is composed of Quaternary gravelly sands and silt, as well as peat. The main water stream is the Khanda River. Climate is strongly continental, with long severe winters and short warm summers. Typical are temperature inversions in winter, when the air within the depression may be cooled to 45 to 50 S. Such low temperatures account for the development of permafrost, with associated processes, such as thermokarst, solifluction and frost heaving. The vegetation on the depression bottom and on differently oriented slopes shows conspicuous distinctions. The western side of the basin is dominated by forests of P. obovata and P. sibirica with Larix, while on its eastern side forests of P. sibirca, P. sylvestris, Larix with Betula sect. Nanae and Ledum palustre dominate. Within the depression are forests of P. sylvestris and Larix on the Khanda R. terraces. Primary forests are often destroyed by fires over considerable areas, overgrown with Larix and Betula sect. Albae. At the bottom of the depression, swamp vegetation prevails, and wetlands cover more than 80% of its total area. Riverine plant communities consist of Cyperaceae and Salix. Closer to the basin, river channels are fringed with forests of Larix and P. obovata with P. sibirica. In the lowest part of the depression there are peatlands covered with shrub and moss communities with occasional Larix in combination with Cyperaceae-Eriophorum-Sphagnum typical of low marshes. The general appearance of vegetation on the depression bottom has much in common with northern taiga communities with some tundra elements. Local vegetation near the drilling sites of Khanda and Khanda-1 peatlands (55.44 N, 107. 00 E, 713e714 m asl.) is mostly open woodlands of P. sylvestris, Larix gmelinii with rare trees of Betula sect. Albae. Betula nana sub. sp. exilis, Ledum palustre, Carex globularis, Rubus chamaemorus, Oxycoccus microcarpus, and Sphagnum are present in the shrub and herb layer. Carex diandra, Eriophorum gracile and Menyanthus trifoliate are found around small open water bodies. 3. Materials and methods
Fig. 1. Locations of the region and the pollen records discussed in the recent study.
A core 312 cm long was recovered from Okunaika peatbog in 2005, using a Russian hand-operated corer. The upper 292 cm of the core consists primarily of sedge and sphagnum peat. The lowermost 20 cm were clay with fine gravel. Every sixth centimeter was taken as a sample for pollen analysis (53 samples). In 2006, the same equipment was used in boring the peat massif at two points within the Khanda depression (Khanda and Khanda-1 sites, 7 km apart). In the Khanda core, the lower part of the sequence (232e70 cm) consists of wet, very dark, almost black, moderately decomposed peat, overlying light gray frozen fluvial sands of the Khanda R. From 70 cm to the surface, the peat becomes drier and less decomposed, with a brownish hue. Permafrost begins at a depth of 52 cm. Samples for pollen analysis were taken from every 5th cm (42 samples). The total length of core obtained from Khanda-1 peat deposit (7 km upstream of the Khanda R.) is 112 cm. The penetrated deposits are mostly sphagnum peat decomposed to a variable degree. The top of permafrost is at 62 cm depth from the peat surface. Every 2nd cm was sampled for pollen analysis (55 samples). The samples for pollen analysis were treated in the Laboratory of the Institute of the Earth’s Crust, Siberian Branch of the Russian Academy of Sciences, using standard techniques for pollen separation and concentration (Faegri and Iversen, 1989). Volume of samples (dry matter) varied from 1 to 2 cm3. Relative abundance (percentage) of the main taxa was calculated taking the pollen sum as 100%, with Cyperaceae pollen and Pteridophyta and Sphagnum
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spores excluded from the main sum. Percentages of Cyperaceae, Pteridophyta and Sphagnum in a particular sample were calculated with reference to total amount of pollen and spores counted in that sample. Data on the sub-recent pollen spectra composition described in all the altitudinal zones in the Baikal drainage basin (Belova, 1985; Bezrukova, 1999) have been used to interpret changes of vegetation in the past. Taxonomical identification of the plant remains (mostly to a level of genus or family) in the botanical composition of peat was carried out using atlases of fossil plants (Dombrovskaya et al., 1959; Kats et al., 1977) with a help of binocular microscope MBS-10 (magnification 70y). Proportion of plant remains differing in taxonomic status was calculated as a percentage of total quantity of plant remains according to standard procedures (Yurkovskaya et al., 1989; Elina et al., 2000). Fifteen radiocarbon dates were obtained in the Laboratory of Geology and Paleoclimatology, Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences (Novosibirsk); the dated material was total organic matter of the peat. The dates are calibrated using CalPal program (Danzeglocke et al., 2008). The results are given in Table 1. All the dates are given in the text as calibrated radiocarbon ages (cal. BP). 4. Results 4.1. Radiocarbon chronology Radiocarbon-dated sequence from the Okunaika section appeared to cover more than 9000 years. The age of deposits penetrated in sections Khanda and Khanda-1 is somewhat older than 6000 and 3000 BP, respectively (Table 1). 4.2. Pollen stratigraphy On the basis of variations in pollen assemblages and detailed 14C dating, five LPAZ (Local Pollen Assemblage Zones) in the Okunaika section (Okn1eOkn5, Fig. 2), three LPAZ in the Khanda section (Khd1eKhd3, Fig. 3) and a single LPAZ in the Khanda-1 section (Fig. 4) were visually identified. Depths of occurrence and ages of the upper and lower boundaries of zones are shown in Table 2. Description of pollen zones is summarized in Fig. 6. Average time resolution of pollen records is ca. 180, 150 and 60 years for Okunaika, Khanda and Khanda-1 sections, respectively.
Table 1 Radiocarbon determinations. Lab. no
Depth, cm Age (BP)
Okunaika bog SOANe6394 SOANe6395 SOANe6396 SOANe6397 SOANe6398 SOANe6399
49e53 100e106 148e152 202e208 250e256 302e308
3115 3485 4450 5670 6175 8050
140 120 110 110 185 215
3122e3478 3617e3921 4935e5260 6359e6605 6834e7259 8670e9243
(3300) (3769) (5098) (6482) (7047) (8957)
peat peat peat peat peat peat
Khanda SOANe6445 SOANe6446 SOANe6447 SOANe6448 SOANe6449 SOANe6450
11e13 68e70 121e123 143e145 188e190 226e228
130 1870 2800 3450 4080 5230
40 120 120 80 90 130
39e241 1660e1948 2816e3113 3619e3824 4473e4768 5843e6170
(140) (1804) (2965) (3722) (4621) (6007)
peat peat peat peat peat peat
188e416 (302) 2018e2247 (2133) 2987e3161 (3074)
peat peat peat
Khandae1 SOANe6451 8e10 SOANe6452 58e60 SOANe6453 110e112
260 45 2115 65 2915 55
Calendar age (cal. BP) Material dated
4.3. Peat botanical composition Analysis of botanical composition has been performed for 64 samples from the Okunaika section at 4 cm intervals. The lowermost peat layer (312-292 cm, w9500-8500 BP) could not be analyzed because of the high proportion of the mineral component. The results obtained for the remainder identified five Local Botanical Assemblage Zones (LBAZ), Okn1boteOkn5bot. Fig. 5 shows the chronological sequence of LPAZ in comparison with that of LBAZ in this section. 5. Reconstruction of palaeoenvironmental conditions Before discussing change of vegetation and climate in the region under study, it should be noted that in all probability the area did not experience human impact during the early-middle, or even in the late Holocene. Neither remains of settlements nor traces of any economic activities has been found near the peat massifs. Therefore, it may be safely suggested that the pollen stratigraphy thus obtained would primarily reflect changes in vegetation induced by variations of global and regional climate. The Okunaika pollen record (Okn) provides evidence of vegetation dynamics and climatic changes during the last 9000 years (or a little more). According to pollen data, at the initial stages of the peatland formation (Okn5, ca. 9000e7300 cal. BP) the area was covered with open larch and spruce forests of Larix, P. obovata, Betula sect. Albae, in good agreement with earlier data on other areas in the vicinity of Lake Baikal (Bezrukova et al., 2005; Demske et al., 2005; Tarasov et al., 2007; Bezrukova et al., 2008, 2009). The presence of arboreal pollen (no more than 50%) and abundance of Polypodiaceae and Lycopodiaceae spores also indicate the presence of isolated forested areas. At that time the climate was still cold and permafrost remained widespread. Nevertheless, relatively high spring and summer temperatures were favorable for the appearance of P. sibirica and expansion of A. sibirica ca. 8500 cal. BP. It is not inconceivable that the sediments dated to that interval were deposited in the immediate vicinity of an intermittent stream, accounting for the enrichment of organic deposits with mineral substance. The accumulation of proper peat began about 8500 cal. BP. An interval marked by reduction of tree species and expansion of shrubs (Salix, Alnaster fruticosa, Betula sect. Nanae) occurred 8500e7500 BP. Botanical composition of the peat provided evidence of Alnaster growth near the site, while A. sibirica seems to be absent there, as no macroremains of the species have been found. Pollen assemblages of Okn-5 zone coincided with a short-term cooling peaking about 8.2 ka BP (Prasad et al., 2009). There is no distinct response of vegetation to the “8.2 ka event” recorded in the studied spectra. It seems noteworthy, however, that arboreal pollen (A. sibirica in particular) is reduced in abundance in peat layers between the two lower dates (7047e8957 BP), while abundance of Betula nana-type and mesophytic herbs pollen noticeably increased. It is possible that such was the regional vegetation response to the cooling dated at ca. 8.2 ka BP. The interval 7500e6800 BP (Okn4) was marked by rapid (in 200e300 years, according to temporal resolution of the record) expansion of A. sibirica. Its presence near the peatbog is shown by the high abundance of its pollen in the assemblages, as well as by the botanical composition of the peat. Siberian pine (P. sibirica) could appear in the local plant communities for a short time, ca. 7000-6000 BP, as indicated by the presence of its wood remains in the peat. It was at that time interval that a peat layer was formed with A. sibirica and Poaceae dominant in its botanical composition. Such substantial changes in vegetation could result from dramatic changes of climate. A. sibirica requires mild winters and cool summers, thick snow cover and abundant precipitation, as well as
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Fig. 2. Percentage pollen diagram of selected pollen taxa from Okunayka site (Okn).
well drained fertile soils; the indicated interval may be considered optimal from the viewpoint of combination of all the above-stated environmental characteristics. The time of A. sibirica maximum in the Okunaika site vicinities agrees well with its maxima in other regions of the Baikal drainage basin (Kataoka et al., 2003; Demske et al., 2005; Tarasov et al., 2007; Bezrukova et al., 2008, 2009). Dark coniferous forests with A. sibirica were widespread in Siberia at that time under conditions of abundant precipitation (Tarasov et al.,
2007). High values of annual precipitation were typical of most temperate latitudes in the Northern Hemisphere between ca. 10000 and 6000 cal. BP, including the eastern Swiss Alps (Joerin et al., 2008), the modern steppe zone of Mongolia (Tarasov et al., 2000), and the Central Altai mountains (Blyacharchuk et al., 2004, 2007). In northern Kazakhstan, an increase in moisture supply is dated to 8500e7200 cal. BP, and in the steppe zone of SW Siberia to 7800e6600 cal. BP. (Davydova et al., 1995).
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Fig. 3. Percentage pollen diagram of selected pollen taxa from Khanda site (Khd).
A. sibirica was reduced in importance as a component of forests in the region after 6800 BP (Okn3b ca. 6800e4500 cal. BP), in common with other regions of Siberia (Blyacharchuk et al., 2004, 2007; Tarasov et al., 2007; Bezrukova et al., 2008, 2009). As that took place, P. obovata, P. sibirica and P. sylvestris gained in importance. An analysis of the peat botanical composition confirms the presence of Abies and Pinus near the site at the beginning of the interval. The Pinus remains have been identified to a genus level; they supposedly belong to P. sibirica, as at present this species is a forest-forming species in middle mountain taiga on the western
slopes of the Baikalsky Ridge, together with A. sibirica and P. obovata. A low content of P. sylvestris pollen (12% at most) suggests it was wind-borne. Judging from the Okunaika record, the expansion of pine forests began about 6800 BP, consistent with earlier dating of this event in Siberia (Kataoka et al., 2003; Blyacharchuk et al., 2004, 2007; Demske et al., 2005; Tarasov et al., 2007; Bezrukova et al., 2008, 2009; Müller et al., 2009). As follows from data on changes in available moisture and warmth supply, expansion of P. sylvestris went along with decreasing precipitation and increasing climate
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Fig. 4. Percentage pollen diagram of selected pollen taxa from Khanda-1 site (Khd-1).
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Table 2 Position of the pollen zones in the sections. Okunaika
Khanda-1
Khanda-2
Zone/Depth
Cal. BP
Zone/Depth
Cal. BP
Zone/Depth
Cal. BP
0e46 cm 46e122 cm 122e225 cm 260e225 cm 260e312 cm
w0e2700 w2700e4500 w4500e6800 w6800e7500 w7500e9000
0e68 cm 68e185 cm 185e232 cm
w0e2700 w2700e4500 w4500e6000
0e37 cm 37e72 cm 72e112 cm
w0e1300 w1300e2500 w2500e3000
continentality (Demske et al., 2005). Quantitative reconstructions of climate based on pollen record from the Baikal sediments show the mean annual precipitation ca. 7000e6500 BP could be 80e100 mm less than during the previous interval ca. 9000e7000 BP, when forests of A. sibirica were dominant in the region (Tarasov et al., 2007). In addition to pollen data, botanical composition of the samples points to increasing presence of Larix, Betula pendula, P. obovata, Salix sp., Ledum palustre, Hamaedaphne calyculata, and Spiraea sp. in the middle part and at the end of this interval. Carex vesicata was also found in abundance. According to the Khanda record (peat section located almost 100 km west of the Okunaika site and at elevation 750 m lower), the time interval ca. 6000-4500 cal. BP (Khd-1) in the Khanda depression is also marked by reduction of dark coniferous forests of A. sibirica, P. obovata, P. sibirica and gradual expansion of light coniferous forests of P. sylvestris. At present, P. sylvestris occupies limited habitats, although it makes up the bulk of pollen assemblages
(50e70%). It is reasonably safe to suggest, therefore, that ca. 6000e5000 BP P. sylvestris could occur as an insignificant admixture in the dark coniferous forests. Its growing percentage in pollen spectra may be indicative of expansion of P. sylvestris at a regional scale. The noticeable participation of herbs Artemisia, and Poaceae in the pollen spectra, along with Betula sect. Nanae and dwarf shrub Ericaceae, suggests open forests. Such vegetation is characteristic of northern taiga with forest-tundra elements under conditions of wet and moderately cold climate. Pollen assemblages suggest P. sibirica and P. sylvestris to be dominant near the crest of Baikalsky Ridge in its northern part about ca. 4500e2700 BP (Okn2). In the local vegetation near the Okunaika site, Larix, Pinus pumila and Cyperaceae presumably prevailed. The peat botanical composition confirms the presence of Betula pendula and Larix in local vegetation and adds Ledum palustre, Alnaster fruticosa, Oxycoccus microcarpa, Carex diandra, Carex dioica to the list of local species. The presence of Pinus pumila and
Fig. 5. Simplified percentage diagram of peat botanical composition of selected taxa from Okunayka site.
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Fig. 6. Comparative description of local pollen assemblage zones.
P. sibirica near the site, however, lacks support from the botanical composition analysis: in all probability, those species occurred on the lower slopes. The local vegetation of the Khanda Depression ca. 4500e2700 BP (Khd-2 and Khd-1c) was dominated by Larix, P. obovata, Betula sect. Nanae, Pinus pumila and Cyperaceae, with P. sylvestris and P. sibirica presumably growing on slopes of the depression. The presence of Pinus pumila on lower elevations is usually related with temperature inversions resulting in air temperature, dropping to 45 to 50 S in winter, and development of permafrost. It is not inconceivable, therefore, that somewhat enlarged ranges of Pinus pimula and Betula sect. Nanae ca. 4500e2700 BP noticeable in both records (Okh and Khd) indicate an increase in continentality and lower temperatures. It is also likely that, due to some local factors, the effect of temperature drop on permafrost was more pronounced at Khanda-1, as demonstrated by the higher proportion of Pinus pumila pollen in Khanda-1 record. On the whole, the suggestion that climate became less favorable is in good agreement with data on total deterioration of climate in the Lake Baikal basin at that time (Tarasov et al., 2007; Bezrukova et al., 2008). Later, from ca. 2700 BP to the present day, the proportion of dark coniferous trees in the local vegetation at Okunaika site decreased still further. The peat botanical composition indicates that Larix, Betula sect. Albae, and Betula sect. Nanae were present locally. Such changes could take place under conditions of steadily decreasing heat supply and somewhat growing effective moisture due to less intensive evaporation. Such a relationship between heat and moisture supply might be responsible for the expansion of shrubs and the formation of eutrophic bog with continuous cover of Cyperaceae. The presence of Iris laevigata and Eriophorum vaginatum remains in the upper peat layers also indicates the existence of wetlands at that time. Climatic change towards cooling and decrease of atmospheric precipitation between 3000 and 2000 BP has been recorded in many regions of the Northern Hemisphere,
including the mountains of Siberia (Solomina, 1999). The pine pollen in the Okunaika section was seemingly brought by wind from the lower altitudinal zone. The dynamics of pine expansion within the entire region is reflected in the Okunaika section as much as it is in other pollen records (Kataoka et al., 2003; Demske et al., 2005; Tarasov et al., 2007; Bezrukova et al., 2008). In the Khanda depression, the last 2700e2500 years are distinct for greater participation of dark coniferous trees and wide occurrence of Larix, in the Khanda site local vegetation in particular (Fig. 3). A specific feature of Khanda-1 site appears to be a rather long period (w1000 years) when Pinus pumila communities existed locally; their degradation is dated approximately at 1500 BP. Local environmental factors are likely responsible for reduction of Ericaceae ca. 2600 BP. At the end of the processes appeared communities of Menyanthes and Potamogeton, nearshore plants typical of stagnant or slowly flowing water. Appearance of those communities coincides with the Cyperaceae maximum. Later, about 2600e2200 BP, a sphagnum bog developed (unlike the sedge bog near the Khanda site). It is likely that the frequent changes in local plant communities reflected the general instability of wetland systems as a response to climate deterioration in the Northern hemisphere ca. 3000e2000 BP (Bond et al., 1997; Swindles et al., 2007). 6. Conclusions The paper presents the first high-resolution well dated pollen records obtained in the alpine zone of Baikalsky Ridge and Khanda depression, regions of the most severe climate in the Baikal region. The pollen records provided the basis for reconstructing vegetation and climate of the region spanning the last w9000 years. The reconstructions demonstrated considerable changes in local and regional vegetation during that time. The most significant changes in pollen assemblages and vegetation may be correlated with
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known large-scale climatic event, such as the regional optimum of w9000e6800 BP, the optimum termination about 7000e6500 BP, and Neoglacial cooling 3000e2000 BP. The warmest and wettest conditions are documented ca. 9000e6800 BP and tentatively correlated with the Holocene optimum clearly defined in the region. That interval was marked by dominating dark coniferous forests of A. sibirica and P. obovata with P. sibirica. The dark coniferous forests gave way to light coniferous ones (of P. sylvestris and Larix) after 6800 BP in response to gradual decrease of precipitation and summer temperatures to their present-day values, in good agreement with earlier results. The wetland ecosystem development at the bottom of Khanda depression, as well as the vegetation dynamics during the middle-late Holocene, were primarily controlled by the global climate cooling. True peat accumulation started ca. 6000 BP, at the time of the Holocene optimum termination all over East Siberia. The response of regional and especially local vegetation to changes in global and regional environments during the last 2700 years was markedly diversified, presumably due to differences in local climatic, topographic or hydrological factors. The Medieval Warm Period, as well as cooling of the Little Ice Age, are not distinct in the newly obtained pollen records. This may be attributed to short duration of both events and to location of the studied territory in the boreal forests. Not all the studied regions responded identically or synchronously to the global environmental changes. Thus, regional manifestations of the Holocene climate fluctuations might be highly diversified, and therefore, it is particularly important to obtain data from a geographically wider assortment of environments. The records definitely point to high sensitivity of cold boreal plant communities to climate changes. Adequate dating of sediments is instrumental in intra- and inter-regional correlations of records obtained and events reconstructed. Acknowledgements The authors are grateful to O.N. Shestakova for the treatment of samples in preparation for pollen analysis; to P.P. Letunova and A.A. Abzayeva for their help in performing the analysis, and to E.V. Sheifer for determination of the peat botanical composition. The studies were performed with financial support from the Russian Foundation for Basic Research, Project No. 09-05-00123-a, and Programs IX.81.1 and VII.65.1 of Siberian Branch of Russian Academy of Sciences. References ACIA, 2004. Impacts of a Warming Arctic: Arctic Climate Impact Assessment. Cambridge University Press, Cambridge. Belova, V.A., 1985. Vegetation and Climate of Late Cenozoic in the South of Eastern Siberia. Nauka Press, Novosibirsk, 176 pp. (in Russian). Bezrukova, E.V., Abzaeva, A.A., Letunova, P.P., Kulagina, N.V., Vershinin, K.E., Belov, A.V., Orlova, L.A., Danko, L.V., Krapivina, S.M., 2005. Post-glacial history of Siberian spruce (Picea obovata) in the Lake Baikal area and the significance of this species as a paleoenvironmental indicator. Quaternary International 136, 47e57.
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