Lateglacial and Holocene vegetational and climatic changes in the southern taiga zone of West Siberia according to pollen records from Zhukovskoye peat mire

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Quaternary International 237 (2011) 65e73

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Lateglacial and Holocene vegetational and climatic changes in the southern taigazone of West Siberia according to pollen records from Zhukovskoye peat mire

O.K. Borisova a,*, E.Yu. Novenko a, E.M. Zelikson a, K.V. Kremenetski b

a Institute of Geography RAS, Staromonetny 29, 119017 Moscow, RussiabUniversity of California, Los Angeles, CA, USA

a r t i c l e i n f o

Article history:Available online 21 January 2011

* Corresponding author.E-mail addresses: [email protected] (O.K.

edu (K.V. Kremenetski).

1040-6182/$ e see front matter � 2011 Elsevier Ltd adoi:10.1016/j.quaint.2011.01.015

a b s t r a c t

Pollen analyses and radiocarbon dates from Zhukovskoye peat mire (56�200N, 84�500E), situated in thesouth-eastern part of the boreal forest (taiga) zone of West Siberia, suggest that climatic oscillations ofthe Lateglacial were well expressed in the region. These events are tentatively correlated with the Allerödwarming and the Younger Dryas cooling in Europe. In the Alleröd, complex vegetation combined larchcopses with birch and spruce in wetter places with dry steppe communities dominated by Artemisia andPoaceae. The climate was cool and continental, with moderately warm summers. Due to the onset ofcolder and drier climate in the Younger Dryas the wooded areas were reduced, while xerophile herba-ceous communities with periglacial steppe elements expanded. Following the warming in the EarlyHolocene, woody vegetation was established in the area. At the early stage the woods were dominated bybirch, larch and Scots pine. Later an increase in moisture caused formation of Picea taiga forest with Abiesand Pinus sibirica. In the relatively warm and humid climate the process of mire development spreadover a major part of West Siberia. Comparison with the published data on other sites in West Siberianplain shows that the warmest conditions existed in the region approximately 6e5 ka BP (non-calibrated14C age). The warming is indicated by spread of more heat-demanding forest communities in the taigazone and by a shift of the forest/tundra boundary to the north. Cooling in the late Holocene causeda decline of the relatively thermophile species in the forests and a retreat of the northern tree line to thesouth. Development of the raised peat bogs in the southern taiga sub-zone of West Siberia reflected inpollen profiles shows that this cooling continued in Subatlantic time.

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1. Introduction

Peat mires of West Siberia are the second largest peat accu-mulative ecosystem in theWorld, after the Amazon Lowland. Miresin West Siberia have a considerable impact both on the environ-ment and on the industrial development of the region. Lately, thepeat-accumulating ecosystems have attracted close attention ofresearchers due to their important role in the global Carbon cycle.Among other aspects of the research of mires, the studies of theirhistory against the background of climate and vegetation changesare important, pollen analysis being themost useful tool facilitatingthis research. The first pollen diagrams of the West Siberian peatsequences were published over 70 years ago (Bronzov, 1930). In hismonograph devoted to the Holocene history of forests in NorthernEurasia, Neustadt (1957) presented 20 pollen diagrams for the peatsections in the West Siberian Plain. Invention of the radiocarbon

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method of dating strongly stimulated studies of peat deposits(Neustadt, 1967a, 1967b; Kind, 1969). In the following decades, theresults of many palynological studies of peat sections in WestSiberiawere published (Arkhipov and Votakh,1980; Arkhipov et al.,1980; Levina, 1980).

Unfortunately, due to the easier access, peat mires were studiedmainly in their marginal parts, especially in the natural peatoutcrops in the riverbanks. As the result, the interior parts of themires with the deepest and oldest peat deposits remained scarcelyinvestigated. The majority of the pollen diagrams based on the peatsequences were supplied by only a few radiocarbon dates. Recon-structions of the vegetation and climatic changes in West Siberia,based onwell-dated pollen diagrams (e.g. Volkov et al.,1973; Glebovet al., 1974; Blyakharchuk and Sulerzhitsky, 1999), are still few andfar apart, especially for the early Holocene and the Lateglacial.Therefore, new detailed palynological studies of the complete peatsequences with serial radiocarbon dates are required at present.

The Zhukovskoye section situated in the south-eastern part ofthe taiga zone in West Siberia was chosen for a study of the vege-tation because of its proximity to the southern and eastern limits of

O.K. Borisova et al. / Quaternary International 237 (2011) 65e7366

several important forest tree species, and therefore a high sensi-tivity to the climatic changes. A detail palynological examination ofthis section supplied with a series of radiocarbon dates providesa long-term view of vegetation dynamics, peatland change, andclimate history at the southern taiga boundary.

2. Study area

2.1. Physiography

The study area is situated in the south-eastern part of the WestSiberian Plain, in the interfluve area between theOb’ and Tom’ rivers(Fig. 1). The landscape represents an undulating erosion-accumu-lative plain with altitudes above sea level from 116 m in the north-east to 158m in thewest. Narrowvalleys (1e2.5 kmwide) formedbyglacial meltwater runoff during the Middle Quaternary time dissectthe plain, following a general slope of the area from south-west tonorth-east. These valleys are largely inherited by present-day rivernet. The plain is partly covered with eolian deposits.

2.2. Climate

Climate of the region is temperate continental. Mean Januarytemperature as measured at meteorological station closest to thesite (Tomsk), is �17.5�, and average July temperature is 18.1 �C,resulting in an annual mean air temperature of�0.6 �C. The climateis relatively humid, mean annual precipitation being about 637mm

Fig. 1. Location of Zhukovskoye mire (1) and vegetation zones of West Siberia (based on Iltundra; 4 e northern taiga; 5 e middle taiga; 6 e southern taiga; 7 e forest steppe; 8 e stTrofimov et al., 1980). Holocene peat sections: 2 e Gorno-Slinkino (Volkov et al., 1973); 3 e

1999); 5 e Nizhnevartovsk (Neustadt and Selikson, 1971); 6 e Entarny (Arkhipov et al., 198

(including 215 mm in winter). The snow cover persists on averagefor 176 days, from Oct. 31 until Apr. 19. The region is situatedbeyond the present-day limits of the permafrost zone.

2.3. Vegetation

According to the accepted bio-climatic subdivision of the WestSiberian Plain (Il’ina et al., 1985), the study area belongs to thesouth-eastern part of the sub-taiga sub-zone of the boreal forestzone. Forests occupy 70e80% of the area, herb and moss pineforests being predominant (65% of the total forested area). Mixedbirch-aspen-dark coniferous forests account for 10e15% of the area.Pure spruce (Picea obovata) and Siberian pine (Pinus sibirica) forestsare mainly confined to the river valleys. Small patches of birch andaspen forests occur after forest fires.

According to the world classification of mires (Katz, 1971), thestudy area corresponds to the West Siberian province of southerntaiga, birch forests and meso-eutrophic peatlands with smallerproportion of pineesphagnum bogs. Flat or slightly convex richfens and forested swamps are typical for this zone, mesotrophicfens and oligotrophic bogs being less common.

2.4. Study site

Zhukovskoye peat mire (56�200N and 84�500E) is situated in thesouth of the Tomsk district, 25 kmwest of Tomsk. It occupies part ofthe floodplain of the Zhukovka River, a small tributary of the Tom’

’ina et al., 1985). Vegetation: 1 e arctic tundra; 2 e typical (shrub) tundra; 3 e foresteppe; 9 e mountain taiga; 10 e southern boundary of discontinuous permafrost (afterNovy Tevriz (Liss and Berezina, 1981); 4 e Bugristoye (Blyakharchuk and Sulerzhitsky,0); 7 e Pur-Taz (Peteet et al., 1998).

Table 1Radiocarbon dates from the Zhukovskoye peat section.

Depth (cm) Sample type Age, 14Cyr. BP Calibrated age, BP Lab No.

90e100 Peat 1590 � 50 1450 � 50 IGAN-1875140e150 Peat 2610 � 80 2748 � 80 IGAN-2040190e200 Peat 3060 � 80 3290 � 80 IGAN-1873240e250 Peat 3440 � 50 3961 � 50 IGAN-1876340e350 Peat 4330 � 50 4870 � 50 IGAN-1881390e400 Peat 5410 � 60 6235 � 60 IGAN-1879490e500 Peat 6430 � 90 7300 � 90 IGAN-1878

CaCO3 6440 � 110 7315 � 110 IGAN-1874590e600 Peat 7160 � 90 7990 � 90 IGAN-1883

CaCO3 7680 � 100 8430 � 100 IGAN-1880690e700 Peat 8580 � 120 9496 � 120 IGAN-1888

CaCO3 8530 � 310 9486 � 210 IGAN-1885730e740 CaCO3 8960 � 70 9967 � 70 IGAN-1924750e760 CaCO3 10600 � 480 12529 � 480 IGAN-1940790e800 CaCO3 11060 � 130 11060 � 130 IGAN-1882

O.K. Borisova et al. / Quaternary International 237 (2011) 65e73 67

River. Channel width of the river is 1.5e3 m during summer lowstage and up to 5e7 m during the spring flood. The floodplainelevation above the summer water level is 0.6e0.8 m. Watershedsbetween Zhukovka and other small tributaries of the Tom’ Riverhave a flat gently undulating topography with numerous lakes inshallow depressions.

Within the area, peat mires occupy about 30e40% of water-sheds, as well as flat bottoms of river valleys. Many are hydrologi-cally connected through the groundwater flow to one another andwith river valleys. The mires are also fed by snow-melt surfacerunoff in spring, and by rain water.

The poorly drained surface of the Zhukovka River floodplain iscovered by rich fen with sedge-brown moss communities pre-dominant in the ground cover. The fen is open or covered by pine(Pinus sylvestris) and birch (Betula alba) open forest with rathersparse shrub understorey (Betula nana, Betula humilis). Carex dia-ndra, Carex lasiocarpa, Carex chordorriza, Thelypteris palustris,Menyanthes trifoliata, Equisetum palustre, Galium palustre, andSaxifraga hirculus are typical species in the herbaceous layer.Among mosses, Homatocaulis vernicosus, Drepanocladus aduncus,Tomenthypnum nitens, and Aulacomnium palustre prevail.

Birch and pine forests are also predominant in the area, adjacentto the fen. Sandy slopes in the vicinity of the site are covered withpine forest. Dark coniferous forests of Siberian pine (P. sibirica),Siberian spruce (P. obovata) and fir (Abies sibirica) occupy placeswith more clayey wet soil. Larch (Larix sibirica) occurs in the forestsin minor quantities.

Long-term field studies of forest-peatland relationships in thisarea were conducted by the Scientific Field Station of the ForestInstitute of Russian Academy of Sciences (Krasnoyarsk) since 1960.The Zhukovskoye peat mire has been previously studied palyno-logically by Piavchenko et al. (1973) with the sampling interval of50 cm. The oldest of three radiocarbon dates showed an age of lakedeposits, underlying 8.5 m thickness of peat, as 9650 � 110 BP(Piavchenko, 1983).

3. Field and laboratory methods

A sediment core 880 cm long was collected from the deepestinterior part of the Zhukovskoye fen using the Russian peat corer.The core included 725 cm of peat and 155 cm of underlying lakedeposits (gyttja and sandy clay). The core was systematically sub-sampled (1 cm3) for pollen, macrofossil and LOI analyses at 10 cmintervals. Additional cores were taken within 20 cm from theoriginal one to collect sufficiently large samples for conventionalradiocarbon dating at 20e100 cm intervals.

Radiocarbon analysis of 15 samples from 12 intervals of depth(Table 1) was performed in the Radiocarbon Laboratory of theInstitute of Geography RAS. The dates are given in the text asuncalibrated 14C years B.P. to facilitate the comparison with earlierpublished radiocarbon-dated pollen sequences. The age model forpollen diagrams is based on linear interpolation. LOI was measuredfollowing procedures outlined in Dean (1974).

Samples were processed for pollen analysis using the pollenextraction technique of Grichuk (1940): the processing includedheavy liquid (cadmium iodine) separation. A minimum of 500pollen grains and spores per sample was counted, with theexception of 12 samples with very high contents of spores; in suchsamples a minimum of 200 pollen grains was achieved, whilespores were tallied in addition. Relative frequency of pollen wascalculated based upon the total terrestrial pollen sum, as well aspercentages of spores. To calculate the pollen concentrations,Lycopodium tablets (Stockmarr, 1971) were added to each sample.Pollen diagrams were compiled using Tilia and TiliaGraphprograms (Grimm, 1990).

4. Results

Core chronology is based on a total of twelve radiocarbon dates,which are reported along with sample depths and non-calibratedages in Table 1. The pollen diagram of the core reveals a completesequence of the Holocene deposits underlain by the Lateglacialsediments beginning from c. 12,000 BP (Fig. 2). Sediment stratig-raphy, as described in the field and summarised from the study ofthe peat composition, performed by E.Ya. Muldiyarov, is presentedwith the age-depth and loss-on-ignition curves in Fig. 2, as well aswith the percentage pollen diagram (Fig. 3).

The diagram has been divided into 6 local pollen assemblagezones (LPAZ) on the basis of changes in the composition of bothpollen and spores, aided by results of a constrained cluster analysis(CONISS; Grimm, 1987). Time limits of LPAZ were estimated usingthe depth-age curve (Fig. 2).

4.1. LPAZ Zhuk-1, 880e815 cm (app. 12,000e11,200 BP)

The layer is represented by lake deposits: 10 cm of sandy clay atthe base of the section and the lower part of the gyttja horizon.

Zone Zhuk-1 is characterised by relatively high arboreal pollen(AP) content (up to 70%), although AP concentrations within thezone are low compared to the upper part of the section (Fig. 4). AP isdominated by larch pollen. Its concentrations within this layerreach the maximum values for the entire section. Tree birch andspruce pollen are also abundant in this interval. Spruce pollenforms a peak on the percentage diagram, though its concentrationis relatively low. P. sylvestris and P. sibirica, as well as shrub birch arepoorly represented in this interval. As the pollen production ofpines is very high, and pine pollen is easily transported by wind, itslow content in zone 1 probably indicates the absence of bothspecies of pine in the local vegetation. Shrubs, such as Juniperus,Alnaster, and B. humilis, on the contrary, probably occurred in thevicinity of the lake, though they were not abundant.

In the group of non-arboreal pollen (NAP) Artemisia andCyperaceae are predominant, Chenopodiaceae and Poaceae are lessabundant. Artemisia percentages increase substantially towards theend of zone 1. Pollen of xerophytes, typical for periglacial condi-tions (Ephedra distachya, Ephedra sp.) and that of the plants, char-acteristic for the Lateglacial floras of northern Eurasia on the whole(Helianthemum, Pleurospermum), also occur in zone 1.

Aquatic plants are represented by rare pollen grains of Myr-iophyllum. Helophytes, such as Typha latifolia and M. trifoliata, aremore abundant. Spores of Polypodiaceae and Equisetum are poorlyrepresented at the base of the zone, but Polypodiaceae content

Fig. 2. Peat composition, age-depth curve, and % loss-on-ignition (LOI) at 500 �C for the Zhukovskoye site. 1 e sandy clay; 2 e gyttja; 3 e Menyanthes-sedge-brown moss peat; 4 e

brown moss peat; 5 e sedge-brown moss peat; 6 e brown moss-sedge peat.

O.K. Borisova et al. / Quaternary International 237 (2011) 65e7368

increases sharply towards the top of zone 1, reflecting a rapidspread of ferns on the marshy lake shores.

4.2. Zone Zhuk-2, 815e755 cm (11,200e10,000 BP), gyttja

In the pollen composition, NAP strongly prevails over AP (up to80%). Artemisia, Poaceae and Chenopodiaceae, as well as

Fig. 3. Zhukovskoye mire: percentage pollen diagram. Pollen sum: AP

herbaceous pollen on the whole, achieve their maximum concen-trations for the entire section. Pollen of xerophytes, relativelyindifferent to cold climatic conditions (E. distachya, Eurotia cera-toides), is registered in this zone. Cyperaceae pollen content isespecially high in the lower part of zone 2 but rapidly decreasesbeginning from its middle part. High frequencies of Thalictrumpollen are characteristic for this interval. Thalictrum species, being

þ NAP. Clear curves represent �10 exaggeration of base curves.

Fig. 4. Zhukovskoye mire: pollen and spores concentrations for selected taxa, grains per cm3.

O.K. Borisova et al. / Quaternary International 237 (2011) 65e73 69

mainly meadow plants, do not form zonal communities. Theirpercentages in zone 2 are nevertheless comparable with those ofPoaceae. Of the plants growing both on the waterlogged soil and inshallow water, T. latifolia is the most abundant, and M. trifoliata isalso present.

In the AP group, B. alba prevails, its content increasing from 20%to c. 60% of AP within the zone. Both shrub (B. humilis) and dwarfbirch (B. nana) reach their maximums in zone 2, the former beingmore abundant in the upper part of the zone. Of coniferous trees,Larix remains the most important. Willow pollen occurs more oftenthan in zone 1, while larch and spruce frequencies decreaseconsiderably. Alnaster pollen is present.

The content of Polypodiaceae spores in the lower part of zone 2sharply decrease compare to zone 1 to increase again at the top ofthe zone. Other spores are rare, although Equisetum reaches itsmaximum concentrations (over 150 grains per cm3) in this zone.

4.3. Zone Zhuk-3, 755e635 cm (10,000e7800 BP), upper part ofgyttja and basal peat layer (Menyanthes-sedge-brown moss)

Pollen spectra of zone 3 are characterised by an overall increaseof AP content, interrupted at the depths of 720e680 cm by a sharprise of the NAP curve. As this peak corresponds to the initial stage ofpeat accumulation and is formed almost entirely by Cyperaceaepollen, it can be attributed to a rapid spread of sedges over thenewly emerged marshy area surrounding the lake. Therefore, thistemporary recess of AP curve reflects local changes in the compo-sition of vegetation, while in the broader area a process of affor-estation continued.

Pollen of B. alba (tree birch) dominates not only the AP group butalso the pollen assemblages on the whole, alternating withCyperaceae. Concentrations of B. alba pollen reach their maximumwithin the zone (over 12,000 grains per cm3) but vary over a broadrange. Larix makes up to 7% of AP. At the top of zone 3 pollencontent of P. sylvestris and, to a lesser extent, that of Picea, increases.Pollen of shrub and dwarf birch and juniper seldom occur. Rarepollen grains of Abies and P. sibirica are registered in most samples.

The NAP group is dominated by Cyperaceae. Artemisia pollencontent gradually decreases throughout zone 3. Rare pollen grainsof E. distachya, E. sp., Bupleurum and Pleurospermum are present.Spores content declines sharply, mainly due to that of Poly-podiaceae. Spores of Pteridium aquilinum are typical for the upperpart of the zone. Equisetum spores are found in minor quantity.

Pollen of T. latifolia, being abundant in the lake sediments in thissection, is not registered in the peat, while M. trifoliata constantlyoccurs in zone 3.

4.4. Zone Zhuk-4, 635e305 cm (7800e4000 BP), sedge-brownmoss and Hypnum peat

Beginning from zone 4, pollen spectra are dominated by AP(from 60% at the bottom to 85% at the top of the zone), mainly bythat of P. sylvestris. Pollen contents of tree birch vary in a broadrange, but remain generally lower than in zone 3. Although Piceapollen percentages are relatively low, they increase compared tozone 3. The concentration of spruce pollen increases two to fourtimes, reaching approximately 2000 grains per cm3. P. sibiricapollen percentages become close to those of spruce. Abies pollenconstantly occurs in zone 4 in small quantities. Pollen concentra-tions of both fir and Siberian pine in zone 4 are considerably higherthan in zone 3, the calculated pollen influxes of both species beingclose to the maximum values for the entire section. Larix pollenseldom occurs in zone 4. Pollen of shrub birches, juniper and wil-low are registered in minor quantities.

In the upper part of zone 4 (from the depth of c. 500 cm, whichcorresponds to 6400 BP) Alnus pollen grains were found, moreoften at the top of the zone. In the uppermost part of zone 4 rarepollen grains of Ulmus were also identified. These pollen grainswere presumably transported by wind from long distances.

NAP content in zone 4 is low, especially in the upper part of thezone. NAP is mainly represented by Cyperaceae and, to a lesserextent, by Artemisia. Pollen of Poaceae, Chenopodiaceae, Rosaceae,Fabaceae, Apiaceae, Thalictrum, and other herbaceous plants isscarce but relatively diverse. Pollen of Drosera (a characteristicspecies of the peat mires) occurs sporadically. Spores are slightlymore abundant in the lower part of the zone and representedmainly by P. aquilinum, Polypodiaceae, and at some levels also byEquisetum.

4.5. Zone Zhuk-5, 305e165 cm (4000e2800 BP), sedge-brown mosspeat

Pollen spectra are generally similar to those of zone 4: treepollen dominated by P. sylvestris strongly prevails in the spectra(90e95%). Abies pollen forms a low but distinctive peak, itsconcentrations reaching their maximum values for the entire

O.K. Borisova et al. / Quaternary International 237 (2011) 65e7370

section (over 1000 grains per cm3). Both relative frequencies andconcentrations of Picea pollen are reduced compared to zone 4.B. alba percentages continue to decrease. Rare pollen grains of Alnusstill occur in most samples, while those of Ulmus are not registered.At the very top of the zone, pollen of shrub alder (Alnaster fruti-cosus) reappear in the spectra.

A decrease in the relative abundance of NAP in zone 5 resultsmainly from Cyperaceae decline. Percentages of Artemisia pollenalso decrease towards the end of the zone. At the boundarybetween zones 4 and 5, a conspicuous change in the composition ofNAP occurs: from this level upward pollen of Cichoriaceae, Aster-aceae, Brassicaceae, and Polygonaceae is constantly present. Manyspecies of these families grow on fresh or eroded ground withdisturbed natural vegetation communities.

Spores percentages in zone 5 are low. Spores belong mainly toPolypodiaceae and Sphagnum. The latter occurs more often in theend of the zone. Spores of P. aquilinum and Equisetum are seldomfound.

4.6. Zone Zhuk-6, 165e0 cm (2800epresent), brown moss-sedgepeat

Zone 6 is characterised by a substantial rise of P. sibirica andPicea percentages and concentrations along with high values ofP. sylvestris (up to 70% of AP þ NAP). Abies pollen frequencies arelower than in zone 5. Birch pollen contents are the lowest for theentire section, except for the topmost part of the zone. Alnasterpollen is constantly present, while that of Alnus is not found inzone 6.

Within the group of NAP, a slight increase in Poaceae pollencontent takes place. Cichoriaceae, Asteraceae, Brassicaceae, andPolygonaceae pollen also occur in slightly higher quantities than inzone 5. Rare Ericales pollenwas identified in zone 6. Spores (mainlythose of Polypodiaceae and Sphagnum) are scarce. P. aquilinumspores are not found in this zone.

5. Discussion

Palynological study of Zhukovskoye peat sequence and itscomparison with other high-resolution sections within the borealforest (taiga) zone of West Siberia enable reconstruction of thehistory of vegetation development in the region, strongly influ-enced by climatic changes.

Specific features of the pollen assemblages in zone Zhuk-1, suchas the peaks formed by Larix and Picea pollen curves, high contentsof NAP and the presence of heliophytes and xerophytes, as well asthe estimated age of the zone from c. 12 to 11.2 ka BP, correlate thisinterval with the Alleröd Interstadial of the Blytt-Sernander strati-graphical scale. Previously published data (Neustadt and Selikson,1971) show that larch pollen maximum is typical for the Alleröddeposits inWest Siberia, whereas the so-called “lowermaximum ofspruce” is characteristic for this time interval all over northernEurasia (e.g. Neustadt, 1957; Piavchenko, 1957; Khotinsky, 1977).

Comparison between the composition of recent pollen spectraand that of the vegetation at the sample sites in West Siberia(Piavchenko, 1966; Piavchenko et al., 1973) shows that Larix isusually strongly underrepresented in the pollen spectra. Therefore,relatively high percentages of larch pollen (up to 40%) in zone Zhuk-1 indicate a predominance of larch in the forest communitiesduring the Alleröd interval. Larix stomata were found at the depthof 830 cm, thus confirming a local presence of larch.

Picea also tends to be underrepresented in pollen assemblages ofthe northern taiga and forest tundra, although to a lesser extent thanlarch (Piavchenko, 1966). On the other hand, relatively low concen-trations of spruce pollen in zone 1 suggest a possibility of its long

distanceorigin.Nevertheless, theactual presenceof spruceat the siteis shown by finds of Picea stomata at the depths of 830e840 cm.

Composition of pollen spectra in zone Zhuk-1 reflects a complexvegetation pattern of the Alleröd Interstadial. Patches of larch forestwith birch and spruce onwetter ground co-existed with dry steppecommunities, which probably occupied mainly watershed areas.Such Artemisia-dominated communities with Poaceae and Cheno-podiaceae species as co-dominants included periglacial steppeelements (Ephedra,Pleurospermum) andheliophytes (Helianthemum),characteristic for the Lateglacial vegetationofnorthernEurasia on thewhole. Low percentages of shrub pollen (that of Juniperus, B. humilis,Salix and Alnaster) indicate a minor role of shrubs in the zonalplant communities. Sedges and ferns dominated the marshvegetationnear the lake.Menyanthes and Typha grewboth in shallowwater and on the waterlogged soil along the lake shores. Presence ofT. latifolia indicates relatively warm summer conditions, as themean July temperatures within its present-day range exceed 14 �C.

Following the warm Alleröd interval, pollen assemblages ofzone Zhuk-2 reflect a substantial cooling, correspondent to theYounger Dryas. During the Younger Dryas cold stage, xerophyllousherbaceous communities with periglacial steppe elements, domi-nated by Artemisia in association with Poaceae and Chenopodia-ceae, became more widespread than they were in the Alleröd.Maximum of Artemisia pollen is a characteristic feature of theYounger Dryas cold stage in the pollen sequences all over Europe, aswell as in Siberia. Typical xerophytes tolerant to low wintertemperatures, such as E. distachya and E. ceratoides, occurred in theperiglacial steppe communities.

In the composition of woodlands, larch gave place to birch as thedominant tree species. A prominent role of tree birch (B. alba) in thewoodlands during the Younger Dryas can be explained by itstolerance both to low winter temperatures and to summerdraughts, and therefore to high concentrations of mineral salts inthe ground water as well.

Picea pollen curve shows more rapid decline in the end of theAlleröd, than that of Larix. Larch, being a deciduous tree, is moreresistant against low winter temperatures and desiccation by windthan spruce. At present, Siberian larch (L. sibirica) forms thenorthern tree line in West Siberia (Arealy derev’ev i kustarnikovSSSR, 1977). Therefore, the decline of spruce in response to theYounger Dryas cooling was more rapid, than that of larch. The roleof the cold-tolerant shrubs (B. nana, B. humilis, A. fruticosus andSalix) in the plant communities increased with the onset of colderclimate. On the whole, woodlands became more open than theywere during the Alleröd, as indicated by the decrease in tree pollenconcentrations and by higher frequencies of herbaceous pollen inthe spectra. Concentrations of Artemisia, Chenopodiaceae, andThalictrum pollen form distinctive maximums in this interval(Fig. 4). Both the sedimentation rate of the lake deposits andorganic content in them also reduced substantially (Fig. 2).

Under dry climatic conditions of the Lateglacial, in a regionwith widespread sandy soils, spruce could grow only in locationswith high ground moisture supply. It is possible that the groundmoisture sufficient for spruce was then preserved in the topsoil bythe permafrost. Although the distance from the site to thepresent-day boundary of the permafrost zone (discontinuouspermafrost) is over 400 km (Trofimov et al., 1980), the isolatedlocations with permafrost are found in West Siberia at similarlatitudes, e.g., Uzhur frozen peat mire at 55�330N (Piavchenkoet al., 1973) and Bugristoye palsa bog at 58�150N (Blyakharchukand Sulerzhitsky, 1999). At present, the mean annual air temper-ature at the site is �0.6 �C, while the southern limit of discon-tinuous permafrost largely coincides with the isotherms of �3 or�4 �C. Consequently, if the mean annual air temperature duringthe Lateglacial was by 3� lower than at present, the climatic

Fig. 5. Distribution of radiocarbon dates based on: (a) fossil wood remains near thenorthern forest line; basal peat layers (b) in the north and middle taiga, and (c) in thesouth taiga on the West Siberian Plain.

O.K. Borisova et al. / Quaternary International 237 (2011) 65e73 71

conditions in the region would be favourable then for thepermafrost development.

Being generally cold and dry, the climate during the YoungerDryas was nevertheless characterised by warm summers, as indi-cated by presence of T. latifolia pollen in zone Zhuk-2. Therefore,during the Younger Dryas an annualmagnitude of air temperatures,indicating a degree of the continentality of climate, was greaterthan at present. The floodplain lake at this stage became shallowerand was partly overgrown by wetland vegetation.

Pollen assemblages of zone Zhuk-3, correspondent to the Pre-boreal and Boreal periods of the Holocene, reflect a dramaticchange in the regional vegetation caused by the climatewarming atthe beginning of the Holocene. AP percentages within the zoneincrease from 30 to 80% thus reflecting a rapid afforestation of thearea. Birch forest quickly expanded over the area.

In the Preboreal time, the process of lake terrestrialisation wascompleted, so that about 9 ka BP peat accumulation has begun in therich fen covered by sedge-Menyanthes-brown moss communities.The peat has a relatively highmineral content due to the flooding ofthe marsh by the river waters in spring. At approximately the sametime, peat initiation occurred inmany localities all over the southerntaiga zone (Fig. 5), mainly also due to the terrestrialisation of smalllakes on the river floodplains and in the depressions of relief(Neustadt, 1977; Liss and Berezina, 1981; Firsov et al., 1982; Glebov,1988; Kremenetski et al., 2003). It was probably caused by thedevelopment of warming under relatively dry climatic conditions.

The data on arboreal vegetation spread north of themodern treeline (MacDonald et al., 2000) also indicate that a substantialwarming occurred in early Holocene all over West Siberia. Themodern thermal level was achieved at its northern part by thebeginning of the Boreal (see Fig. 5). The majority of larch macro-fossils found near the boundary of the present range of larch aredated to 9e4 14C ka BP (MacDonald et al., 2000). All available datesof spruce macrofossils belong to the same time interval, sprucebeing more heat-demanding than larch.

At the end of the Boreal the role of pine in the forest compositionincreased, indicating further warming. At the Zhukovskoye site theraise of P. sylvestris pollen curve corresponds to c. 8 ka BP. A similarraise of the pine curve is registered about 500 years earlier at Gorno-Slinkino peat section, situated over 1000 km to the west of Zhu-kovskoye (Volkov et al., 1973), at about the same time at theBugristoye section, 200 km to the north (Blyakharchuk andSulerzhitsky, 1999), at least 500 years later at Novy Tevriz, about500 km to theWNW(Liss and Berezina,1981), and 1000e1500 yearslater at Entarny (Arkhipov et al., 1980) and Nizhnevartovsk(Neustadt and Selikson,1971), situated about 500 and 600 km to thenorth-east from Zhukovskoye, respectively (see Fig. 1). Such time-transgressive spread of P. sylvestris over the region suggests thepermafrost degradation in the area, as pine does not grow on frozensoil with a shallow active layer. In the recent pollen spectra of WestSiberia (Levkovskaya, 1973), P. sylvestris dominates the AP grouponly south of 63�N in the middle and southern taiga sub-zones,although it is strongly over-represented in the pollen assemblages ofthe northern taiga and forest tundra (Piavchenko, 1966) due to thehigh production of pollen easily transportable by wind.

At the end of the Boreal, spruce participation in the forestincreased again, indicating an increase in the humidity. Abies, themost demanding of warm and humid climatic conditions of all theconiferous trees in West Siberia, appears in the forests at the sametime. Accordingly, by the beginning of the Atlantic the process ofpaludification was widespread over the boreal forest zone of WestSiberia (Neustadt, 1977; Liss and Berezina, 1981).

During the Boreal, the role of dry steppe communities with theperiglacial elements reduced rapidly, but they probably still occu-pied dry slopes and well-drained watersheds with sandy soil until

mid-Boreal time. A typical xerophyte, Ephedra, occurred in suchcommunities until the end of the Boreal. Comparison with palyno-logical data on other peat sections (Neustadt and Selikson, 1971;Volkov et al., 1973; Glebov et al., 1974; Blyakharchuk andSulerzhitsky, 1999) shows that NAP percentages remain high untilthe end of the Boreal all over the taiga zone ofWest Siberia. Presenceof light-demanding forest plants, such as Juniperus and P. aquilinum,indicates that the forest canopy remained relatively open.

Since the beginning of the Atlantic, the area became denselyforested, as indicated by increasingly high concentrations of tree andshrub pollen in zone Zhuk-4. Zhukovskoye peat mire itself wascovered by open pine forest. On the surrounding territory, B. alba,Picea and P. sibirica were the main forest-forming trees. P. sylvestrisgrewalsoonsandysoils.Abiesand Larix seldomoccurred in the forestcommunities, as well as shrub Salix and Juniperus. Shrub birches(B. nana and B. humilis) probably grew mainly on the mire. It ispossible that the sharp peaks of B. alba pollen in this zone are con-nectedwithepisodes of rapidexpansionof tree birchafter forestfires.

Alnus pollen is first registered in Zhukovskoye peat section atthe depth of 470 cm corresponding to approximately 6 ka BP The

O.K. Borisova et al. / Quaternary International 237 (2011) 65e7372

present-day isolated locations of alder (Alnus glutinosa) in WestSiberia occur in the Irtysh River valley at approximately 55�N and74�E (about 500 km from Zhukovskoye), while the distance fromthe modern range of elm (Ulmus laevis) to the site is over 1000 km.In the end of the Atlantic, rare pollen grains of Ulmus are also foundin the peat. These pollen grains of relatively thermophilous specieswere presumably transported by wind from long distance at thetimewhen alder and elm hadmigrated into areas further to the eastcompare to their present-day ranges, so that they occurred nearerto the site. Therefore, their presence marks the warmest(“optimum”) part of the Atlantic, as well as of the Holocene on thewhole. The apparent peat accumulation rate, the highest for thesection during the early and middle Atlantic, decreases consider-ably in the late Atlantic, which can be explained by better peatdecomposition and a decrease of effective moisture (an excess ofprecipitation over evaporation) due to the warmer climate. Thisassumption is supported by an increase in the organic content ofthe peat by almost 30% during the period from 6.5 to 5 ka BP (Fig. 2).Since the end of the Atlantic the fen was not flooded by the river,which indicates a substantial change in the hydrological regime.

Comparison with other pollen sequences in the taiga zone ofWest Siberia shows that the time interval from about 6 to 5 ka BPwas the warmest part of the Holocene in the entire region. In everycase, the most thermophilous of local arboreal species eitherappeared in the section for the first time or increased conspicuouslyin relative abundance. For example, Ulmus pollen is registered inthe peat section Gorno-Slinkino (Volkov et al., 1973) beginningfrom 6.5 ka BP, and at about 5 ka BP pollen of Tilia also appearedthere. Further to the east, in the Bugristoye peat section(Blyakharchuk and Sulerzhitsky, 1999) the warmest interval ismarked by the rise of Abies, beginning from approximately 6.5 kaBP. In the Nizhnevartovsk section, in the middle taiga zone, the lateAtlantic warming is reflected by a rapid increase of P. sibirica andP. sylvestris and a constant presence of Abies, as well as of rare pollengrains of Alnus and Ulmus (Neustadt and Selikson, 1971). At Entarny,Abies and P. sibirica appear in the spectra, along with the rapid raiseof P. sylvestris pollen curve at about 7 ka BP. Rare pollen grains ofUlmus are also registered at this time interval (Arkhipov et al.,1980). Pollen of broad-leaved tree species is often registered inthe sediments of the late Atlantic and early Subboreal over theWestSiberian Plain, especially in its south-western regions (Volkova andBelova, 1980). Far in the north of the Plain, on the Taz-Pur interfluve(Peteet et al., 1998), Abies pollen (presumably, wind-blown) firstoccur in the section at the same time interval (about 6 ka BP), andthe pollen curve of A. fruticosus raises slightly around 6.5 ka BP todecline again after 5 ka BP.

Certain features indicating drier climatic conditions in the lateAtlantic can also be distinguished in the above peat sections. Thus,interlayers of woody peat with tree trunks and stumps in situ werediscovered both in Nizhnevartovsk (at app. 6.5 ka BP) and in theGorno-Slinkino peat profile (about 5 ka BP). In the section on thePur-Taz interfluve, peat in the age interval from 6 to 5 ka BP showsvery high proportion of sandy grains (Peteet et al., 1998). Activationof wind erosion, which caused transportation of sand in the peatbog, can be explained by drier land surface and more open vege-tation cover at that period. After this episode, accumulation of peatwith high organic content resumed at the site.

LPAZ Zhuk-5 generally corresponds to the Subboreal period ofthe Holocene. The character of vegetation during the Subboreal wassimilar to that of the Atlantic, although certain changes took placein the forest composition: fir became more abundant in the forests.An increase of the density of forests is indicated by virtual absenceof Juniperus pollen, which constantly occurred during the Atlantic(zone 4). Rare finds and low values of P. aquilinum spores alsosuggest that the forest canopy became closer. Shrub birches, poorly

represented, probably grew mainly on the mire, but possiblyoccurred also in the forest understorey. At the end of the zoneAlnaster re-appeared at the site. Since the beginning of the Sub-boreal, the wind-blown pollen of Ulmus did not occur at the site.These changes reflect a gradual cooling, as well as an increase of thehumidity of climate. Peat accumulation rate increased again andreached the values typical for the earlier part of the Atlantic.

As the river did not flood the fen any more, certain changes inthe composition of the local plant communities took place. Speciesof Cichoriaceae, Asteraceae, Brassicaceae, and Polygonaceae fami-lies, which usually occur in intrazonal conditions, penetrated at thistime the margins of the peat mire and eroded slopes, where theoriginal vegetation was disturbed. Drosera almost entirely dis-appeared from the local plant communities on the fen. Presence ofSphagnum spores in the late Holocene layers possibly indicatesa spread of oligotrophic bogs in the broader area, as the peatcomposition of Zhukovskoye mire does not show the transition tothe oligotrophic stage of development.

The process of change in the composition of the forest, whichbegan in the Subboreal, continued during the Subatlantic, whichprobably indicates further cooling of the climate. The participationof fir decreased in favour of Siberian pine and spruce, thus reflectinga further expansion of the dark coniferous taiga forest in the region.Changes in the AP group show a gradual decline of tree birch(B. alba) and a constant presence of Alnaster. Alnus pollen is notregistered in the Subatlantic deposits. Within NAP group the rela-tive abundance of Poaceae pollen increased. An increase in theconcentrations of Sphagnum spores shows that the process of theoligotrophic bogs development continued in the area adjacent tothe site. The finds of Ericales pollen indicate a spread of the dwarfshrubs over the bogs.

The apparent peat accumulation rate in the Subatlantic wasreduced almost two-fold compared to the Subboreal. Presumably,the actual decrease of the peat accumulation rate was evengreater, as both the degree of decomposition and the densitydecrease in the uppermost layer of peat. These changes in thevegetation and peat accumulation suggest that the further coolingof climate took place during the Subatlantic period. Similarfeatures can be found in other high-resolution pollen sequences ofthe region. In Nizhnevartovsk (Neustadt and Selikson, 1971),although the apparent accumulation rate is rather constant, Ulmusand Abies pollen does not occur after c. 2 ka BP. At Gorno-Slinkino(Volkov et al., 1973) the apparent peat accumulation ratedecreased slightly after 4 ka BP, while at the Bugristoye(Blyakharchuk and Sulerzhitsky, 1999) the process of peat aggra-dation virtually stopped at about 3.5 ka BP. An old age of theuppermost peat layer (4570 � 60 BP at the depth of 25e30 cm) inthe above-mentioned section in the Pur-Taz region (Peteet et al.,1998) also suggests a possible lack or very low rates of peataccumulation in the late Holocene.

6. Summary and conclusions

Palynological investigation of the Zhukovskoye peat mire, situ-ated in the south-east of the taiga zone of West Siberia, recordedsignificant changes in the vegetation during the Lateglacial and theHolocene, caused by climatic change.

In the Alleröd the area was occupied by periglacial steppe withpatches of larch, birch and spruce open forest in better protectedlocations. The climate of this interval was relatively cold and conti-nental, with moderately warm summers. In the Younger Dryas thewooded area reduced, and the role of Artemisia-dominatedcommunities increased due to the onset of even colder and drierclimate. Permafrost was widespread in the region during theLateglacial.

O.K. Borisova et al. / Quaternary International 237 (2011) 65e73 73

Warming in the beginning of the Holocene caused degradationof permafrost and spread of birch forest over the area. The role ofperiglacial steppe communities was reduced substantially. Theclimate remained dry and continental. The shallow floodplain lakeat the site was filled in, and peat accumulation began. At the end ofthe Boreal, a rapid expansion of P. sylvestris took place. Increasedmoisture caused the spread of Picea forest. Abies and P. sibiricagradually penetrated the forest communities during the Boreal.

Since the beginning of the Atlantic, the participation of fir andSiberian pine in the forests became greater due to the furtherwarming and increasing humidity of climate. At 6e5 ka BP, theclimate was the warmest of the entire Holocene, as indicated byspread of Alnus and Ulmus to the north and to the east of theirpresent-day ranges. Due to an increase in potential evaporation, thehumidity of climate temporarily reduced. Spring flooding of themire by river waters stopped.

The cooling that followed after 5 ka BP along with increasinghumidity brought about a greater role of Abies in forest composi-tion. Oligotrophic Sphagnum bogs began to spread over the area.With further cooling in the beginning of the Subatlantic, Abiesretreated and denser Picea and P. sibirica forests occupied theterritory. P. sylvestris grew both on sandy soil and on the peat mires,as at present. Peat accumulation in the Zhukovskoye fen sloweddown.

Acknowledgements

The research was carried out within the scope of INTAS Project99-1718 “Climate in relation to carbon accumulation: spatial andtemporal analyses of West Siberian peat ecosystems (CIRCA)” Ourthanks go to Dr. S.P. Efremov for helps in the field campaign and Dr.E.Ya. Muldijarov for information about the peat composition.

References

Arealy derev’ev i kustarnikov SSSR. In: (Trees and Shrubs Distributions of USSR),vol. 1, 1977. Nauka, Leningrad, 164 pp. (in Russian).

Arkhipov, S.A., Votakh, M.R., 1980. Palinologicheskaya kharacteristika i absolyutnyivozrast torfyanika v ust’ye r. Tomi (Palynological characteristics and radio-carbon age of a peat section at the Tom’ River mouth). In: Sachs, V.N., Volko-va, V.N., Khlonova, A.F. (Eds.), Paleopalinologiya Sibiri. Nauka, Moscow, pp.118e122 (in Russian).

Arkhipov, S.A., Levina, T.P., Panychev, V.A., 1980. Palynologicheskaya kharakteristikadvukh golotsenovykh torfyanikov iz doliny Srednei i Nizhnei Obi (Palynologicalcharacteristics of two Holocene peat sequences in the Middle and Lower Ob’valley). In: Sachs, V.N., Volkova, V.S., Khlonova, A.F. (Eds.), PaleopalinologiyaSibiri. Nauka, Moscow, pp. 123e127 (in Russian).

Blyakharchuk, T.A., Sulerzhitsky, L.D., 1999. Holocene vegetation and climaticchanges in the forest zone of Western Siberia according to pollen records fromthe extrazonal palsa bog Bugristoye. The Holocene 9, 621e628.

Bronzov, A.Ya, 1930. Verkhovye bolota Narymskogo kraya. In: Trudy Nauchno-Issled. Torfyanogo Instituta, vol. 3, Moscow, 100 pp. (in Russian).

Dean Jr., W., 1974. Determination of carbonate and organic matter in calcareoussediments and sedimentary rocks by loss on ignition: comparison with othermethods. Journal of Sedimentary Petrology 44, 242e248.

Firsov, L.V., Volkova, V.S., Levina, T.P., Nikolayeva, I.V., Orlova, L.A., Panychev, V.A.,Volkov, I.A., 1982. Stratigrafiya, geokhronologiya i standartnaya sporovo-pyl’t-sevaya diagramma golotsenovogo torfyanika Boloto Gladkoye v Novosibirske(Pravye Chemy). (Stratigraphy, geochronology and standard pollen diagram ofthe Holocene peat section Boloto Gladkoye in Novosibirsk (Pravye Chemy)). In:Archipov, S.A., Volkova, V.S., Troitskaya, T.S. (Eds.), Problemy stratigrafii i pale-ogeografii pleistotsena Sibiri. Nauka, Novosibirsk, pp. 96e117 (in Russian).

Glebov, F.Z., 1988. Vzaimootnosheniya lesa i bolota v tayozhnoi zone. Nauka,Novosibirsk, 183 pp. (in Russian).

Glebov, F.Z., Toleiko, L.S., Starikov, E.V., Zhidovenko, V.A., 1974. Palinologicheskayakharakteristika I datirovanie po C14 torfyanika v Aleksandrovskom raioneTomskoi oblasti (Palynological characteristics and radiocarbon dating of thepeatland in the Aleksandrovskiy district of the Tomsk region). In: Lavrenko, E.M.(Ed.), Tipy bolot i printsipy ikh klassifikatsii. Nauka, Leningrad, pp. 194e200 (inRussian).

Grichuk, V.P., 1940. Metodika obrabotki osadochnykh porod, bednykh organ-icheskimi ostatkami, dlya tselei pyl’tsevogo analiza (Method of treatment of thesediments poor in organic remains for the pollen analysis). Problemy fiziche-skoi geografii 8, 53e58 (in Russian).

Grimm, E., 1987. CONISS: a FORTRAN 77 program for stratigraphically constrainedcluster analysis by the method of incremental sum of squares. Computers andGeosciences 13, 13e35.

Grimm, E.C., 1990. TILIA and TILIA*GRAPH.PC spreadsheet and graphics software forpollen data. INQUA, Working Group on Data-Handling Methods. Newsletter 4,5e7.

Il’ina, I.S., Lapshina, E.I., Lavrenko, N.N., et al., 1985. Rastitel’nyi Pokrov Zapadno-Sibirskoi ravniny. Nauka, Novosibirsk, 251 pp. (in Russian).

Katz, N.Ya, 1971. Bolota Zemnogo shara. Nauka, Moscow, 295 pp. (in Russian).Khotinsky, N.A., 1977. Golotsen Severnoy Evrazii (The Holocene of Northern Eura-

sia). Nauka, Moscow, 200 pp. (in Russian).Kind, N.V., 1969. Pozdne- i poslelednikovye Sibiri (novye materialy po absolyutnoi

khronologii) (Late- and postglacial of Siberia: new data on radiocarbon chro-nology). In: Neustadt, M.I. (Ed.), Golotsen (Holocene). Nauka, Moscow, pp.195e201 (in Russian).

Kremenetski, K.V., Velichko, A.A., Borisova, O.K., MacDonald, G.M., Smith, L.C.,Frey, K.E., Orlova, L.A., 2003. Peatlands of the Western Siberian lowlands:current knowledge on zonation, carbon content and Late Quaternary history.Quaternary Science Reviews 22, 703e723.

Levina, T.P., 1980. Razvitiye rastitel’nosti v nizov’yakh Yeniseya i Srednei Obi vgolotsene (The development of vegetation in the Lower Yenisei and Middle Ob’areas in the Holocene). In: Sachs, V.N., Volkova, V.S., Khlonova, A.F. (Eds.),Paleopalinologiya Sibiri. Nauka, Moscow, pp. 128e132 (in Russian).

Levkovskaya, G.M., 1973. Zonal’nye osobennosti sovremennoi rastitel’nosti iretsentnykh sporovo-pyl’tsevykh spektrov Zapadnoi Sibiri (Zonal peculiaritiesof the modern vegetation and recent spore-pollen spectra of West Siberia). In:Medvedeva, A.M. (Ed.), Metodicheskiye voprosy palinologii. Nauka, Moscow,pp. 116e120 (in Russian).

Liss, O.L., Berezina, N.A., 1981. Bolota Zapadno-Sibirskoi ravniny. Izdatel’stvo Mos-kovskogo Universiteta, Moscow, 208 pp. (in Russian).

MacDonald, G.M., Velichko, A.A., Kremenetski, C.V., Borisova, O.K., Goleva, A.A.,Andreev, A.A., Cwynar, L.C., Riding, R.T., Forman, S.L., Edwards, T.W.D.,Aravena, R., Hammarlund, D., Szeicz, J.M., Gattaulin, V.N., 2000. Holocenetreeline history and climate change across Northern Eurasia. QuaternaryResearch 53, 302e311.

Neustadt, M.I., 1957. Istoriya lesov i paleogeografiya SSSR v golotsene. Izdatel’stvoAN SSSR, Moscow, 404 pp. (in Russian).

Neustadt, M.I., 1967a. Ob absolyutnom vozraste torfyanykh bolot Zapadnoi Sibiri(On the radiocarbon age of the peat mires of West Siberia). Revue Roumaine deBiologie, ser. botanique 12 (2e3), 181e186.

Neustadt, M.I., 1967b. Stratigrafiya torfyanykh mestorozhdeniy v svete dannykhabsolyutnogo vozrasta (Stratigraphy of the peat deposits as based on theradiocarbon dating). In: Neustadt, M.I. (Ed.), Priroda bolot i metody ikh issle-dovaniy. Nauka, Leningrad, pp. 90e95 (in Russian).

Neustadt, M.I., 1977. Vozniknoveniye i skorost’ razvitiya protsessa zabolachiva-niya (Initiation and speed of the paludification process). In: Neustadt, M.I.(Ed.), Nauchnye predposylki osvoyeniya bolot Zapadnoi Sibiri. Nauka,Moscow, pp. 39e48 (in Russian).

Neustadt, M.I., Selikson, E.M., 1971. Neue Angaben zur Stratigraphie der TorfmooreWestsibiriens. Acta Agralia Fennica 123, 27e32.

Peteet, D., Andreev, A., Bardeen, W., Mistretta, F., 1998. Long-term Arctic peatlanddynamics, vegetation and climate history of the Pur-Taz region, Western Siberia.Boreas 27, 115e126.

Piavchenko, N.I., 1957. Nizhnyaya el’ v torfyanikakh (Lower spruce in thepeatlands). In: Trudy Instituta Lesa AN SSSR, 36. Krasnoyarsk, pp. 178e186(in Russian).

Piavchenko, N.I., 1966. Rezul’taty palinologicheskogo izucheniya torfyanikov Yeni-seiskoi polosy Sibiri (Results of the palynological study of the peats of thesub-Yenisei stretch of Siberia). In: Neustadt, M.I. (Ed.), Znacheniye pal-inologicheskogo analiza dlya stratigrafii i paleofloristiki. Nauka, Moscow, pp.232e238 (in Russian).

Piavchenko, N.I., 1983. O vozraste torfyanikov i smenakh rastitel’nosti na yugeZapadnoi Sibiri v golotsene (On the age of peatlands and Holocene changes ofvegetation in the south of West Siberia). Bulleten komissii po izucheniyuchetvertichnogo perioda 52, 164e170 (in Russian).

Piavchenko, N.I., Metel’tseva, E.P., Kozlovskaya, L.S., Gorlova, R.N., 1973. Paleo-geograficheskiye usloviya golotsena na yuge Zapadnoy Sibiri po dannymkompleksnogo izucheniya torfyanikov (Paleogeographical conditions in thesouth of West Siberia during the Holocene as based on the complex studies ofpeatlands). In: Tikhomirov, B.A. (Ed.), Problemy biogeotsenologii, geobotaniki ibotanicheskoi geografii. Nauka, Leningrad, pp. 207e218 (in Russian).

Stockmarr, J., 1971. Tablets with spores used in absolute pollen analysis. Pollen etSpores 13, 615e621.

Trofimov, V.T., Badu, Yu.B., Dubikov, G.I., 1980. Kriogennoye stroyeniye i l’distost’mnogoletnemerzlykh porod Zapadno-Sibirskoi plity. Izdatel’stvo MoskovskogoUniversiteta, Moscow, 246 pp. (in Russian).

Volkov, I.A., Gurtovaya, Ye.Ye., Firsov, L.V., Panychev, V.A., Orlova, L.A., 1973.Stroyeniye, vozrast i istoriya formirovaniya golotsenovogo torfyanika u s.Gorno-Slinkina na Irtyshe (Stratigraphy, age and history of the Holocene peatsection on the Irtysh River near Gorno-Slinkino). In: Sachs, V.N. (Ed.), Pleis-totsen Sibiri i smezhnykh oblastei. Nauka, Moscow, pp. 34e39 (in Russian).

Volkova, V.S., Belova, V.A., 1980. O roli shirokolistvennykh porod v rastitel’nostigolotsena Sibiri (On the role of broad-leaved species in the Holocene vegetationin Siberia). In: Sachs, V.N., Volkova, V.S., Khlonova, A.F. (Eds.), PaleopalinologiyaSibiri. Nauka, Moscow, pp. 112e117 (in Russian).