vegetation and climate changes during the eemian and early weichselian in the upper volga region...

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Quaternary Science Reviews 26 (2007) 2574–2585

Vegetation and climate changes during the Eemian and EarlyWeichselian in the Upper Volga region (Russia)

O.K. Borisovaa, E.Yu Novenkoa, A.A. Velichkoa, K.V. Kremenetskib,F.W. Jungec,�, T. Boettgerd

aLaboratory of Evolutionary Geography, Institute of Geography, Russian Academy of Sciences, 109017 Moscow, Russian FederationbUniversity of California, Los Angeles, USA

cSaxon Academy of Sciences in Leipzig, D-04107 Leipzig, GermanydDepartment of Isotope Hydrology, UFZ Helmholtz Centre for Environmental Research, D-06120 Halle, Germany

Received 28 November 2005; accepted 28 June 2007

Abstract

Ples is the key-section that represents the Mikulino (Eemian) interglaciation and early part of the Valdai (Weichselian) glacial epoch in

the north-central Russia. Pollen profile of this section reflects spread of the broad-leaved forest dominated by Quercus, Ulmus, and Tilia

with participation of Acer, Fraxinus and Carpinus in the optimum phase of the interglaciation. A gradual cooling and increasing

humidity of climate brought about a decline in the broad-leaved forest and development of dark-coniferous communities (Picea-Abies-

Pinus sibirica) in the end of the interglaciation. It was followed by Scots pine and birch forest, and then replaced by communities of

tundra shrubs with patches of the open birch forest. During the first post-Eemian cooling the climatic conditions were not uniform, as a

short climatic amelioration separated it into two stages. This minor warming is indicated by an increase in birch woodland in complex

plant cover. This first Early Weichselian cold stage was followed by a warm interstade, when mixed spruce-birch, and later larch-pine and

spruce-pine forests occupied the region. This interstade, known as Verkhnevolzhskiy (Upper Volga) in the climatostratigraphic

subdivision of the Late Pleistocene in the Russian Plain (after Grichuk [1961. Fossil floras as the basis for the Quaternary stratigraphy.

In: Markov, K.K. (Ed.), Rel‘ef i Stratigrafiya Chetvertichnykh Otlozheniy Severo-zapada Russkoi Ravniny, Izdatel’stvo AN SSSR,

Moscow, pp. 25–71]), corresponds to the Brorup interstade in the northern Europe, or to Marine Oxygen Isotope Substage (MIS) 5c (e.g.

Mangerud [1989. Correlation of the Eemian and the Weichselian with deep sea oxygen isotope stratigraphy. Quaternary International 3,

1–4.]). On the whole, we correlate the sediment sequence at Ples to the MIS from 5e to 5c.

r 2007 Elsevier Ltd. All rights reserved.

1. Introduction

The Last Interglaciation attracts a close attention of awide range of Quaternary researchers, being a possibleanalogue to the advanced stage of the global warming. Inthe Russian literature, the Last (Mikulino) Interglaciation,commonly understood as an interval with climate as warmas or warmer than today, is correlated with the Eemianinterglaciation in western central Europe. The generallyagreed equivalent of the Last Interglaciation sensu stricto in

the deep-sea sediments is Marine Oxygen Isotope Substage(MIS) 5e (as in Zagwijn, 1996; Kukla et al., 2002).Subdivision of the Last Interglaciation in north-westEurope based on pollen assemblages was first proposedby Jessen and Milthers (1928), and modified by many since(among others: Menke and Tynni, 1984).Correlation of regional pollen zones for the Mikulino

interglaciation established by Grichuk (1961) with thosefor western and central Europe, as well as West SiberianPlain (Gurtovaya, 1987), is shown in Table 1 (see alsoVelichko et al., 1991, 2005). Short-term oscillationscharacteristic for the transitional parts of the climaticmacrocycle (transitions from glaciation to interglaciationand vice versa) reveal the highest natural rates of thelandscape components’ response to the climate change.

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0277-3791/$ - see front matter r 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.quascirev.2007.07.001

�Corresponding author. Tel.: +49341 7115318; fax: +49 341 7115344.

E-mail addresses: [email protected] (O.K. Borisova),

[email protected] (F.W. Junge),

[email protected] (T. Boettger).


Therefore, studies of such transitional phases are especiallyimportant for estimating possible consequences of theanthropogenically induced global warming in the 21stcentury.

Since 1950s more than 25 sections of the EemianInterglacial deposits were discovered and studied palyno-logically in the central region of the East-European Plain.Only in a few of these sections (Mikulino, NizhnyayaBoyarschina, Borkhov Rov, Domanovo) the Interglacialdeposits are directly overlain by lacustrine sediments of theEarly Weichselian glacial epoch (Fig. 1). Late PleistoceneInterglacial deposits in this region attracted a greatattention of researches since long ago (e.g. Markov,1940). Nevertheless, pollen studies were mainly focusedon the Interglacial epoch itself, remaining rather schematicfor the glacial part of the sequences.

Section Ples (571270N, 411320E) discussed in this paper isthe most complete Eemian/Early Weichselian sedimentsequence in the Upper Volga basin. It can be accepted asthe standard profile of Eemian/Early Weichselian time slice

in Eastern Europe. In the north-central East-EuropeanPlain the Ples section represents a unique opportunity tostudy short-term oscillations at the Eemian/Weichseliantransition. It was first studied palynologically by Grichukand Grichuk (1959). The present detail investigation of thePles section (Ples-2002) using the high-resolution pollensampling and calculations of pollen concentration wasconducted during the last 5 years.

2. Study site

The investigated area is situated at the southern flank ofthe Galich-Chukhloma Upland. A chain of low rollinghills, forming a watershed between the tributaries of theUpper Volga and Klyaz’ma, connects the upland to theeastern end of the Klin-Dmitrov morainic ridge, the highestpart of the large Smolensk-Moscow Upland (Fig. 1). In thePles area, the main features of topography were formed inthe marginal zone of the Moscow (Warta) stage ofthe Saalian Glaciation (Velichko et al., 2004). Glacial

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Table 1

Last Interglaciation biostratigraphic correlation

Oxygen Isotope

Substages

West Europe, Jessen,

Milthers (1928)

West and Central Europe,

Menke, Tynni (1984)

East European Plain,

Grichuk (1961)

West Siberian plain, Gurtovaya (1987)

5c Brorup Interstade Upper Volga

InterstadeWFIIb

WFIIa

5d Herning stade substages: The first post-

Mikulino cold stageWF1c

WF1b

WF1a

5e i—Picea-Pinus forests

with birch

E7 Pinus M8 Pinus-Picea-

Betula (transition to

the Valdai glaciation)

K5 Picea and Abies

forests with East

Siberian species

h—Abies-Picea forests

with Pinus

E6 Pinus-Picea-Abies M7 Picea K4 K4b—Picea-Abies

forests

g—Mixed Carpinus-

Tilia forests with

other broad-leaved

species, Picea and

Abies

E5 Carpinus-Picea M6 Carpinus K4a—southern taiga

dark coniferous

forests

f—Quercus forests

with Ulmus, Tilia,

Acer, Fraxinus and

Corylus

E4b Corylus-Taxus-Tilia M5 Tilia-Quercus-

Ulmus (the second

half of Corylus

maximum)

K3 K3b—southern taiga

dark coniferous and

pine forests

E4a Quercetum mixtum-

Corylus

M4 Quercus-Ulmus

(the first half of

Corylus maximum)

K3a—larch-pine and

dark coniferous

forests, forest steppe

e—Pinus-Betula

forests with Ulmus

and other broad-

leaved species

E3 Pinus-Quercetum

mixtum

M3 Pinus-Betula

(Quercus-Ulmus-

Corylus)

K2 Larch and dark

coniferous forests and

steppe communities

d—Pinus-Betula

forests with broad-

leaved species

c—Betula forests with

Pinus

E2 Pinus-Betula M2 Betula K1 Larch-pine forests

with birch, forest-

steppe

E1 Betula M1 Picea

O.K. Borisova et al. / Quaternary Science Reviews 26 (2007) 2574–2585 2575


accumulative relief is considerably reworked by subsequentfluvial processes. The Volga River valley in this part isdeeply incised and has steep slopes dissected by manyravines. River terraces are poorly developed within thisstretch of the valley. The region belongs to the southerntaiga sub-zone, with forests dominated by spruce (Picea

excelsa) and widespread woodlands of birch (Betula alba),aspen (Populus tremula), and alder (Alnus incana).

At the Ples site, 0.5 km downstream from the town ofPles, a deep gulley cuts through the layers of peat and lakedeposits filling in an elongated through in the ancientmorainic and glaciofluvial relief. The biggest outcrop ofthese peat and lake sediments is found in the deepestmiddle part of the ravine (15–20m). In this outcrop, twopeat layers separated by a layer of clayey gyttja, with thetotal thickness of 680 cm, are exposed (Fig. 2, Table 2).

3. Method

Sampling interval for pollen analyses was 5 cm inpeat layers in the lower and upper parts of the sectionand 10 cm in lacustrine sediments. Samples were processedusing the pollen extraction procedure developed byGrichuk (1940). The treatment included separation byheavy liquid (cadmium iodine) with the net weight of2.2 g/cm3. A minimum of 500 pollen grains and spores persample was counted. Relative frequency of pollen wascalculated based upon the total terrestrial pollen sum,arboreal pollen (AP) plus non-arboreal pollen (NAP;Fig. 3). Pollen of the aquatic plants and spores werealso calculated in relation to this sum (AP+NAP). Forthe analysis of the forest composition an additionaldiagram was made with the percentages based on the

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Fig. 1. The study area. (1). Location of Ples section; (2) other sections of the Mikulino interglaciation mentioned in the paper: (1) Borkhov Rov,

(2) Mikulino, (3) Nizhnyaya Boyarschina, (4) Plescheevo Lake, (5) Polovetsko-Kupanskoye, (6) Cheremoshnik, (7) Levina Gora, (8) Cheremukha River,

(9) Chermenino, (10) Dolgopolka, (11) Solonets, (12) Domanovo; Main glacial boundaries (after Velichko et al., 2004): (3). Late Valdai (Weichselian),

(4). Moscow (Late Saalian), and (5). Dnieper (Early Saalian) glaciation.

O.K. Borisova et al. / Quaternary Science Reviews 26 (2007) 2574–25852576


sum of tree pollen and shrub pollen percentages referredto the same sum (Fig. 4). This way of pollen diagramconstruction is widely accepted in Russian literature.For calculating the pollen concentrations (Fig. 5), Lyco-

podium tablets (Stockmarr, 1971) were added to eachsample prior to the pollen extraction. Pollen diagrams werecompiled using Tilia and TiliaGraph programs (Grimm,1987, 1990).

The sediments in layers 13 and 14 at the base of thesection contain only rare microfossils of Quaternary,Tertiary and Mesozoic age. The scarce palynologicaldata on these layers were not included in the pollendiagram.

4. Results of pollen analyses

The diagram has been divided into 10 local pollenassemblage zones (PAZ) on the basis of changes in thecomposition of both pollen and spores (Figs. 3–5).AP values in PAZ PL-1 (750–715 cm) are relatively high

(85–90% from the sum of terrestrial pollen). Total pollenconcentration rises gradually toward the top of this zone.Pollen spectra on the whole are dominated by Pinus

sylvestris, but its percentages gradually decrease from 95%of tree pollen sum (TPS) at the base of PL-1 to 80% at itsupper boundary. The concentration of pine pollen reacheshere 120 thousand grains/cm3. Numerous wood remainsand cones of Scots pine found in the sediments suggest ahigh abundance of pine in forest communities in thevicinity of the section. Pollen of tree and shrub birch(Betula alba, Betula nana, Betula humilis) is also present.The content of B. alba increases from 3% to 17% of TPSwithin zone PL-1. Rare grains of Siberian pine (Pinus

sibirica) and spruce are recorded. Pollen of willowconstantly occurs in the spectra. Pollen of other shrubs,such as Viburnum and, more rarely, Sambucus, Lonicera,and Frangula alnus, is registered. In zone PL-1 only rarepollen grains of Quercus, Ulmus, and Fraxinus (in theuppermost part) are found.Cyperaceae and Poaceae dominate the NAP group.

Pollen of Artemisia, Chenopodiaceae, and Ericales is lessabundant, as well as Asteraceae, Polygonaceae, andThalictrum. Pollen of plants growing both in the shallowwater and on the wet shores of the lake (generally namedhere ‘‘wetland plants’’) is registered in zone PL-1:Sagittaria, Typha latifolia, Sparganium. Of the aquaticspecies, Nuphar, Nymphaea, and Potamogeton occur.Spores are more abundant in the lower part of the zone(up to 50%) and represented mainly by Pteridium

aquilinum, Polypodiaceae, and also by brown mosses(Encalypta) and Sphagnum.Pollen spectra in PAZ PL-2 (715–655 cm) are domi-

nated by AP (80–90%), mainly by P. sylvestris. Pollencontents of tree birch do not exceed 15–20%. Picea pollenconstantly occurs in zone PL-2; rare grains of Larix andJuniperus are recorded. The frequencies of broad-leavedtree pollen (Quercus and Ulmus) increase progressively.Pollen of Fraxinus is a permanent component of spectra.Acer pollen is rare. Alnus, Corylus, and a relativelythermophilic species Thelycrania cf. sanguinea representshrubs, while dwarf birch (B. nana) disappears from thespectra. Pollen of shrub birch (B. humilis) is registered in allsamples. Its concentration rises significantly as comparedto the previous zone, reaching 6000 grains/cm3 at somelevels.Composition of NAP is generally similar to that in PAZ

PL-1, Poaceae and Cyperaceae being predominant. Ephe-

dra and Echinops appear here. Pollen values of meadowplants and their diversity increase in this zone, while theabundance of wetland plants decreases. Aquatic plants arerepresented by several taxa. Concentrations of this group

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Fig. 2. Sediment sequence at the Ples site (for description of the

lithological units see Table 2).



reach their maximum values for the entire section(1–2 thousand grains/cm3).

The peaks on pollen curves (e.g., Quercus, Ulmus) at thetop of subzone 2a correspond to the level where thesampling was transferred from one section of the outcropto another. Therefore it is possible that there is someoverlap between the lowermost part of Section 1 and theupper part of Section 2.

AP in zone PL-3 (655–615 cm) reaches 90–95%. Oak andelm are dominant forest-forming trees. Quercus pollencurve forms a high peak: up to 55% of AP+NAP and 65%of tree pollen sum. At the same time, both relativefrequencies and concentrations of broad-leaved tree pollen(Quercus, Ulmus, and Fraxinus) reach their maximums, theconcentrations of these taxa being about 200, 40, and20 thousand grains/cm3, respectively. Acer pollen con-stantly occurs in zone 3 in small quantities, Carpinus

pollen is registered as well. The contents of pine and birchpollen reduce significantly. PAZ PL-3 is characterised by arapid increase in Alnus and Corylus pollen values, theirconcentrations reaching 180 and 230 thousand grains/cm3

at the top of the zone. Such peaks are typical for theMikulino (Eemian) pollen diagrams in East Europe. Pollenconcentrations of all broad-leaved trees except for Carpinus

decrease in the upper part of PL-3. Herbaceous pollencontents are low. NAP group includes mainly Poaceae,Cyperaceae and wetland plants (Sagittaria, Menyanthes

trifoliata, Sparganium, T. latifolia). A permanent compo-nent of pollen spectra is Humulus lupulus, an herbaceousliana characteristic to the broad-leaved forests on wet soil.

In PAZ PL-4 (615–575 cm), pollen assemblages arecharacterised by the highest content of AP (up to 98%)and maximum values of the total pollen concentration forthe entire section (approx. 500 thousand grains/cm3). Tilia

pollen forms a conspicuous peak in this zone (up to 35% of

tree pollen). The role of Quercus reduces compare to zonePL-3, although its percentages are still relatively high(30–35% of tree pollen). Ulmus frequencies slightlydecrease as well. The important components of pollenspectra are Tilia platyphyllos and Carpinus betulus (1% ofAP+NAP sum or 5–8% of tree pollen), as these species donot occur in the central and northern Russian Plain atpresent. Pollen of Acer and Fraxinus is found in this zone inminor quantities. The later parts of alder and hazel peakscorrespond to zone PL-4. Their decline is much clearer onthe percentage diagram than on the concentration one.Alnus glutinosa keeps its position in the vegetation. Pineand birch contents remain low.NAP percentages in this zone are low. It belongs mainly

to Poaceae and at some levels to Ericales. The appearanceof Ericales probably reflects increasing humidity. Besidechanges in the forest communities, this tendency issuggested by the composition of spores. Percentages ofSphagnum increase in this zone, and Osmunda cinnamomea,a characteristic species of the Mikulino interglacial flora,occurs there. Pollen of Drosera, a typical species of the peatmires, was found at the same level with relatively highamount of Sphagnum.The composition of pollen spectra changes abruptly at

the boundary between PAZ PL-4 and PL-5. This sharptransition corresponds to the lithological boundary of peat(unit 10) and clay (unit 9). Probably, a hiatus insedimentation occurs at this level, and the final part ofthe Interglacial optimum is missing in this profile.In the lower part of zone PL-5 (575–535 cm) Picea pollen

reaches a conspicuous maximum (up to 45% of AP+NAP). Its concentration exceeds 100 thousand grains/cm3.Birch pollen values are close to those of spruce and at thedepth of 560 cm become even higher. P. sylvestris pollen isalso abundant (approx. 60 thousand grains/cm3). In the

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Table 2

Composition of the sediments in Ples-2002 section

Unit # Sediment description Thickness (cm) Depth (cm)

1 Brown-grey loam (modern topsoil) 70 70

2 Clayey brown peat with leafy texture, with interlayers of dark-brown peat 0.8–1.5 cm thick 123 193

3 Gray-brown sandy loam with high organic content, with thin (2–5mm) interlayers of fine-grain sand 44 237

4 Laminated thickness: alternating layers of grey-brown loam (up to 20mm) and sandy loam (3–4mm) 50 287

5 Brown-grey dense clay with rare interlayers of fine sand (2–5mm) 35 322

6 Dark grey clay with high organic content, faintly laminated. The layers are 2–3mm thick. 80 402

7 Uniform grey-brown clay with rare sub-horizontal slightly undulating interlayers of fine sand (1–2mm) 25 427

8 Dark-brown dense clay, medium granular (with angular units 4–5mm in diameter); contains mollusc

shells 2–3mm in size.

40 467

9 Dense grey clay, large granular (with units 2–3 cm in diameter); in the lower part of the layer the colour

is lighter due to a lower organic content.

68 535

10 Alternating layers of grey and brown clay rich in organic matter (2–3 cm thick). In the lower part of the

unit the darker interlayers contain organic detritus.

39 574

11 Dark grey to black peat with leafy texture and abundant plant remains. 105 679

12 Dark-grey organic clay with medium granular units 1–2 cm in diameter; at the base of the layer—

compressed tree trunks; in the lowermost 15 cm—sandy clay.

68 747

13 Gray medium dense clay with yellow-brown interlayers of sandy loam 1.5 cm thick, inclusions of gravel

(1–3 cm) and rare rubbles (up to 10–15 cm in diameter)

67 814

14 Gray loam with abundant gravel and rubbles (moraine) (56) (870)

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Fig. 3. Pollen and spores composition of Ples-2002 section. Selected taxa (AP+NAP ¼ 100%). Clear curves represent � 5 exaggeration of base curves. The arrow indicates the detected hiatus.

Lithological units are described in Table 2.

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Fig. 4. Composition of arboreal pollen of Ples-2002 section. Selected taxa (tree pollen ¼ 100%).

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upper part of this zone rare pollen grains of P. sibirica andAbies occur. Beginning from this zone, shrub birches(B. humilis and B. nana) become permanent components ofspectra. Alnaster fruticosus, a cold-tolerant shrub alder,appears in pollen assemblages. Although AP content inzone PL-5 is still relatively high (above 80%), these changesin pollen spectra indicate a significant cooling of climate. Itis confirmed by the presence of a typical arcto-alpineplant—Selaginella selaginoides. Rare grains of thermophi-lic taxa found in this zone are probably redeposited.

Poaceae pollen in PL-5 is still predominant, Cyperaceaeis important. The role of Artemisia increases considerablyin comparison with zone PL-4 (up to 25% of NAP). Theamount and variety of herbaceous pollen (Rosaceae,Ranunculaceae, Asteraceae, Apiaceae, Caryophyllaceae,Fabaceae, etc.) rise in this interval. The presence of plantstypical for birch forest is recorded (Polygonum bistorta,Thalictrum, Valeriana). Spores of clubmosses characteristicfor taiga (Lycopodium annotinum, L. clavatum, Lycopodium

selago) and pine forest (Lycopodium tristachyum) occur inthis layer. Concentration of Lycopodium spores reacheshere its maximum for the entire section (4000 grains/cm3).Spores of Equisetum and Botrychium lunaria are alsopresent.

Pollen spectra of PAZ PL-6 (535–480 cm) are character-ized by an increase in NAP content (50–60%) and a highdiversity of the herbaceous plants. Poaceae and Cyperaceaedominate in this group. Their concentrations reach 20 and10–12 thousand grains/cm3, respectively, while the totalpollen concentration decreases to app. 100 thousand -grains/cm3 (as in zone PL-1). Concentration of Artemisia

pollen remains the same as in zone PL-5 (app. 5000grains/cm3),though its percentages decrease slightly. Instead, bothabundance and diversity of meadow herbs (Ranuncula-ceae, Polygonaceae, Rosaceae, Rubiaceae, Lamiaceae, etc.)continue to increase. Pollen of the cold-tolerant plants(Polemonium, Saxifraga) and of a typical hypoarcticspecies Rubus chamaemorus appears in this zone. Concen-trations of AP fall abruptly in zone PL-6, with theexception of Salix and B. humilis. Pollen of B. alba

dominates among trees, that of P. sylvestris is alsorelatively abundant. Picea, P. sibirica and Larix pollenconstantly occur in zone 6 in small quantities. The peakof concentration of the typical arcto-alpine speciesS. selaginoides is registered in this zone.In PAZ PL-7 (480–427 cm) AP content decreases to

20%. In this group pollen of B. alba (a tree with anextremely high pollen productivity) prevails. Its concentration,

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Fig. 5. Pollen concentration of selected taxa.



nevertheless, remains approximately at the same level as inzone 6. The amount of coniferous tree pollen is very low(rare grains). Of shrubs, Salix, B. humilis, B. nana, andAlnaster occur, pollen concentrations of the two latterspecies being slightly higher than in zone PL-6. Concentra-tions of Artemisia and Chenopodiaceae rise considerably inthis zone (up to 40 and 13 thousand grains/cm3, respec-tively). Artemisia pollen values reach 50% of NAP. Pollenof Helianthemum spp. occurs permanently. The composi-tion of NAP is still relatively diverse here, althoughwetland plants become scarce. S. selaginoides, cold tolerantand sensitive to the moisture supply, disappears from theplant communities.

In zone PL-8 (427–380 cm) pollen spectra are charac-terised by a noticeable increase of AP content (up to70–85%), mainly due to the tree birch pollen. Pollen ofB. alba comprises 65–80% of AP+NAP, its concentrationreaching here the maximum for the entire section (approx.170 thousand grains/cm3 and over 400 thousand grains/cm3

in one sample). Other trees and shrubs are poorlyrepresented in this zone with the exception of B. nana. Itsconcentration achieves the highest values in zone 8: over4 thousandgrains/cm3 and even over 10 thousandgrains/cm3

at one level. Pollen of herbaceous plants typical for birchcommunities (P. bistorta, Sanguisorba officinalis, Thalic-

trum, Valeriana) often occur in this interval. The presenceof Nuphar pollen in zone 8, registered here for the first timesince the onset of the glacial epoch, indicates somewarming. Therefore, we can conclude that birch openforest with dense undergrowth of dwarf birch, somewillows and shrub alder was a zonal type of vegetation.Probably, heaths (Ericales) also grew in these communities.

Apparently, the climate amelioration, which gave animpulse to the development of birch woodlands, was ofminor magnitude and duration. B. alba, being a typical‘pioneer’ tree, responded to it quickly, while other treespecies more demanding to climate conditions did notspread over the area.

Zone PL-9 (380–200 cm) is characterised by the lowestAP content (15–20%) and its minimum concentration.Tree pollen is still dominated by birch, though itspercentages decrease from 95% to 75% of TPS towardthe top of the zone due to the rise of pine and sprucevalues. In relation to the total sum of terrestrial pollen,percentages of B. alba decrease sharply compare to zone 8:from 65–68% to 10–20% of total terrestrial pollen. Rarepollen grains of P. sibirica, Larix and Malaceae (probably,Sorbus and/or Padus) are recorded. Salix and B. humilis

become more abundant while B. nana contents reduce.Alnaster pollen does not occur in this zone. The amount ofNAP varies from 65% to 85%, with predominant Poaceae(up to 55% of NAP), Artemisia, and Cyperaceae.Apiaceae, Chenopodiaceae, Asteraceae, Rosaceae, Faba-ceae, Onagraceae, Ranunculaceae, and Ericales are pre-sent. Pollen of meadow and step plants—Bupleurum,Centaurea cyanus, Echinops, Convolvulus, Linum, Euphor-

bia, Ephedra, Eurotia ceratoides, and halophytes of the

Plumbaginaceae family constantly occur in this zone.Among spores, Botrychium, L. annotinum and Sphagnum

are often registered. Spores of S. selaginoides reappear.Pollen spectra in Pl-10 (200–80 cm) are marked by an

overall increase of AP content and concentrations (up to200–300 thousand grains/cm3). Among tree pollen B. alba

is still predominant, reaching 80% in the lower part of thezone (subzone 10a) and 50% in its upper part (subzone10b). Picea forms a peak in subzone 10a, and P. sylvestris

achieves maximum in subzone 10b. Comparison ofpercentage and concentration diagrams indicates that Scotspine, characterised by great pollen productivity, did notreplace spruce in the vegetation but penetrated into theforest communities. Although the percentages of sprucepollen decrease in subzone 10b, its concentrations remainapproximately the same as in subzone 10a, even increasingat some levels. P. sibirica also took part in the vegetation,its concentration reaching 3000 grains/cm3 in subzone 10b.Rare pollen grains of Larix are registered in this zone.Concentrations of birch pollen somewhat decreased. Thus,forest communities of this interval were rich in species andsimilar to the modern middle sub-zone of taiga inWest Siberia. In the forest understorey there were Salix,B. humilis, B. nana, Viburnum, and Alnaster. B. nana mightalso grow on peat bogs and in the riverine shrubcommunities.In zone 10 the proportion of mesophytes among non-

arboreal plants increases. Drought-tolerating and light-requiring plants, such as Bupleurum, Centaurea, Echinops,Euphorbia, Helianthemum, disappear, while pollen ofvarious aquatic and wetland plants (Nuphar luteum,Potamogeton, M. trifoliata, Sagittaria sagittifolia) is foundhere. Spores of L. annotinum, L. selago, B. lunaria,Ophioglossum vulgatum and Equisetum are recorded in thislayer as well.

5. Discussion

Palynological study of Ples profile enables us toreconstruct the history of vegetation development in theregion during the Eemian (Mikulino) Interglacial andEarly Weichselian (Early Valdai) glacial epoch under theimpact of climatic changes. Table 3 shows vegetationdynamics in the Upper Volga region inferred from pollendata of Ples section in comparison with biostratigraphiczonation in the East European Plain (Grichuk, 1961).During Stage I open pine and birch-pine forests with

well-developed shrub understorey (with Viburnum opulus,Sambucus racemosa, F. alnus, and Lonicera) occupied theterritory. Ferns (especially P. aquilinum), grasses andmesophilic herbs were abundant in the ground cover.Small patches of dark coniferous (P. sibirica-Picea) forestsof the middle taiga type occurred in the best-protectedlocalities. Hypo-arctic elements survived, probably, infavourable habitats, for example, dwarf birch (B. nana),might grow on the peat bogs. The climate then was colderand more continental than at present.

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During Stage II pine forests were still predominant in theplant cover, but oak, elm, and later also ash penetrated theforest communities. Alnus, Corylus and a relatively heat-requiring Thelycrania appeared in the forest understorey.Spruce forests were spread locally. Climatic conditions inthe end of the second stage were close to the modern onesin the Upper Volga region.

The zonal type of vegetation during Stage III was oakforest (Quercus robur) with elm (Ulmus laevis, Ulmus

scabra), ash (Fraxinus excelsior), and maple (A. plata-

noides). At the end of this stage hornbeam (C. betulus)appeared in the forests. The characteristic feature of thisstage is abundance of hazel (Corylus avellana). Generallyhigh pollen productivity of Corylus rises even more for theplants growing in the forest openings and margins, that is,in the habitats with more light available. Probably, hazelthickets became especially widespread at the transitionfrom the thermoxerotic phase of the Interglacial to itsthermohygrotic phase, when restructuring of forest com-munities took place, before oak and elm woods gave wayto more shady broad-leaved forests with linden andhornbeam. The important components of vegetation werecommunities of A. incana on the wet soil and swampedforests of A. glutinosa. The climate of this interval waswarmer than the modern one.

Stage IV: Broad-leaved forests formed by linden (Tilia

cordata), oak and elm occupied the territory, withFraxinus, Acer, C. betulus and T. platyphyllos participatingin their composition. Noteworthy is a presence of Acer

campestre (its seeds are recorded in the samples from thiszone; Zyuganova, 2005). At present it grows only south ofthe Oka River valley, while in the optimum phase of the

last interglacial its range stretched into the Upper Volgaregion. Macrofossils of some aquatic plants, now extinct inthis region but very typical for the optimum phase of theLast Interglaciation (Brasenia, Aldrovanda vesiculosa,Salvinia natans), were found here (Zyuganova, 2005).Hazel and alder remained prominent in the vegetation.The finds of plants, growing at present in the regions withmuch warmer and milder climate than that of theinvestigated area, allow us to correlate stage IV with thebeginning of the Mikulino Interglacial optimum.Unfortunately, sediments of the final part of the

optimum are absent in this section. Presumably, a positiveshift in the moisture balance in this period led to the rise ofthe lake level and submerging of its marshy shores. Peataccumulation then ceased, and the uppermost peat layerdestroyed. A similar discontinuity in sedimentation occursin other sections in the Yaroslavl’ region of the UpperVolga basin. For example, a sedimentation break isregistered in the later part of the Last Interglacial inPolovetsko-Kupanskoye (Lavrushin and Chistyakova,2001). An absence of a well-defined Carpinus phase and arapid decline of all thermophilic components at the lowerboundary of the spruce zone can be seen on the pollendiagrams of sections Dolgopolka, Chermenino, andSolonets located in the Upper Volga basin (Fig. 1,sites 9–11) (Novskiy, 1975). Apparent sedimentation ratesin these sections slowed down considerably at the later partof the Mikulino optimum phase.Within the region, deposits with pollen spectra of

Carpinus phase are preserved in sections Levina Gora(Gorlova, 1967) and Cheremukha River near Rybinsk(Grichuk, 1961). These data show that mixed broad-leaved

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Table 3

Vegetation dynamics during the Mikulino Interglaciation and the early stages of the Valdai glacial epoch

Biostratigraphic zones East

European Plain, Grichuk (1961)

PAZ in Ples-

2002 section

Stages of vegetation development (based on the

Ples section)

Inferred climatic

conditions

Upper Volga Interstade PL-10b Middle taiga forests (spruce, scots pine,

Siberian pine, and larch)

Interstadial warming,

relatively humid.

PL-10a Northern taiga forests (birch and spruce)

The first post-Mikulino cold stage PL-9 Birch forest-tundra with periglacial steppe-like

communities

Cold, relatively humid

PL-8 Birch open woodland with bush thickets and

mires

Minor warming

PL-7 Birch forest-tundra Cold, drier than before

M8 (transition to the Valdai

glaciation)

PL-6 Pine-birch forests with some spruce, bush

thickets, meadows, steppe-like communities,

and mires

Cool, humid

M7 PL-5 Spruce and birch-pine forests Beginning of cooling,

humid

M6 hiatus

M5 PL-4 Mixed broad-leaved forests (with Tilia cordata,

T. platyphyllos, Carpinus)

Corylus

maximum

Warmest part of the

sequence, more humid

than before

M4 PL-3 Broad-leaved forests (Quercus and Ulmus

dominated)

Warm, relatively dry

M3 PL-2 Pine forests (with an admixture of broad-leaved

species)

Developing warming, dry

M2 PL-1 Pine and birch-pine forests Cool, dry



forests with high proportion of hornbeam occupied theterritory in the second half of the climatic optimum. Thenspruce communities began to expand signalising the end ofthe warmest stage of the interglaciation.

Stage V: Decreasing AP contents at this stage indicate ageneral reduction of the forested area. At the same time,spruce continued to spread over the area, the concentrationof Picea pollen reaching its maximum. Along with dark-coniferous forests (with Abies and P. sibirica), open birchand pine-birch woodlands appeared. The occurrence ofcryophilic plants reflects a progressive cooling. Alnaster

fruticosus and shrub birches (B. nana and B. humilis) grewon the mires and in shrub thickets. The peaks of theirpollen concentrations in zone PL-5 confirm their increasingrole in the vegetation. Similar composition of vegetation istypical for the modern southern taiga in the sub-Uralsregion and in West Siberia, under cool and relativelyhumid climate.

Stage VI (PAZ PL-6): Changes in the composition ofpollen spectra (an increase in NAP percentages andconcentrations accompanied by reduction in AP contentsand total pollen concentration) indicate degradation offorest communities. Complex vegetation of that timeincluded open dark-coniferous, birch and larch-pineforests, steppe-like communities (with Poaceae, Artemisia,Chenopodiaceae, Ephedra, and Eurotia ceratoides), cryo-philic shrub thickets (B. nana, B. humilis, and Alnaster),meadows and mires. A cold-resistant mesophyte S.

selaginoides occurred in wet meadows and bogs. This coldand relatively humid stage corresponds to the transitionfrom Interglacial to glacial epoch and to the beginning ofthe Early Valdai (Weichselian) glacial epoch.

Stage VII (PAZ PL-7): The role of forest communitiesin the plant cover sharply decreased. Even B. alba, acold-resistant pioneer tree, did not form dense forests.Cryophilic shrubs became widespread. Vegetation thenwas similar to the contemporary forest-tundra. Spreadof open landscapes is indicated by the presence of typicalheliophytes (Helianthemum spp.) and abundance ofChenopodiaceae and Artemisia among herbaceous plants.A cold tolerant but demanding to moisture supply species,S. selaginoides, disappeared from the plant communities.These changes reflect further cooling along with increasingaridity and continentality of climate.

Stage VIII (PAZ PL-8): The composition of flora at thisstage is very close to that of the previous one. Nevertheless,in response to a minor warming some changes in thevegetation took place. Birch open woodlands spread overthe area, while the proportion of grassland slightly reduced.Shrub birches took a significant part in the vegetation cover:concentrations of dwarf birch pollen form distinctivemaximums in this interval. Heaths also became moreabundant. Probably, the territory was still occupied bybirch forest-tundra (or open woodland) with a thick shrubcover of dwarf birch, willow, and bush alder, though forestcommunities covered larger areas than during stage VII.This warming was short and had a small magnitude.

Stage IX (PAZ PL-9): The composition of pollenspectra suggests that open landscapes similar to themodern birch forest-tundra with periglacial elements werespread over the landscape. Forest communities occupiedsmall areas in the most protected habitats. Herbaceousplans were highly diverse and dominated by Poaceae,Artemisia and Cyperaceae. Species of meadow and stepcommunities were abundant in this interval. Spores ofS. selaginoides and pollen of Dryas, typical arcto-alpineplants, registered in zone PL-9, indicate development oftundra-like communities.According to palynological data, the climate at stage IX

was cold and relatively humid. Other researchers recon-structed similar conditions for the beginning of the EarlyValdai glacial epoch, based on the palynological studies ofthe sections in the Upper Volga region, for example, at thePlescheevo Lake and near the village of Cheremoshnik(Lavrushin and Chistyakova, 2001). Later in stage IX, agradual warming has started. At the transition betweenstages IX and X lacustrine accumulation at Ples ceased,and peat aggradation started once again.

Stage X (PAZ PL-10): Changes in pollen assemblages atthis stage reflect a rapid afforestation of the area. Birchwoodlands with participation of spruce became widespreadduring the first half of the interval. Larch, pine and spruceforests with P. sibirica, similar to the contemporary middletaiga in West Siberia, were predominant during its secondhalf. The role of shrub communities also increased. Highconcentrations of herb pollen imply that meadows werewidespread during this interval. Probably these vegetationchanges were brought about by a climate warming of theinterstadial rank.

6. Conclusions

Section Ples is the most complete Eemian/Early Weich-selian sediment sequence in the Upper Volga basin. Pollenanalyses show that sediments exposed in this standardsequence accumulated during a long time, starting from theSaalian termination and well into the Early Weichselianglaciation, with the exception of a short break insedimentation at the second half of the Eemian (Mikulino)Interglacial optimum. In the lower part of the profile(zones PL-1–PL-5), a succession of forest communities,typical for the Mikulino interglaciation in the East-European Plain, as described by Grichuk (1961), isrecorded (see Table 3). Zone PL-6 of Ples profilecorresponds to zone M8 in this biostratigraphic scheme,indicating transition to the Early Valdai glacial epoch. Theresults obtained are in a good agreement with the data ofprevious investigations of the section (Grichuk andGrichuk, 1959).Phases VII–X in the Ples section belong to the

Early Weichselian (Valdai) glacial epoch. Within thisinterval, two cold stages and two warm periods can bedistinguished. Of the two warm intervals, the former onewas much smaller both in the time-span and in the

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magnitude. By the composition of pollen spectra and theflora, the latter of the two warm intervals in Ples-2002section (zone PL-10) is attributed to the Verkhnevolzhsky(Upper Volga) interstade, corresponding to the Brorupinterstadial in West Europe. The Upper Volga interstadewas earlier described by Grichuk and Grichuk (1959) basedon the Ples pollen sequence. This warm period was alsodistinguished in the stratotype section Mikulino (Grichuk,1961), and in the sections Nizhnyaya Boyarschina andBorkhov Rov (Chebotareva and Makarycheva, 1982),where the Early Valdai deposits overlay the Mikulinosediments in the continuous sequences. The first minorwarming (phase VIII—birch open woodland) was muchsmaller and shorter, and thus might passed unnoticed inthe above-mentioned sections, where the main attention ofthe researchers was concentrated on the interglacial epoch.Analyses of pollen concentrations were not performedthen, while they provide important information, whichhelps to distinguish minor-scale climatic oscillations, suchas the ‘‘birch interphasial warming’’ in Ples section.

There are certain data indicating existence of similarsmall-scale warm episodes inside the Herning cooling inWest and Central Europe. For example, there are ‘birchintervals’ in the sections Rederstall-I (Menke and Tynni,1984) and Grobern (Litt, 1990), where the Early Weichse-lian deposits are most fully represented and studied indetail. Probably, the climate of the initial part of the glacialepoch was characterized by inner instability resulting in thesequence of second-, and even third-order climatic oscilla-tions expressed against the background of the overall trendtoward cooling. Nevertheless, the amount of the availabledata is still insufficient for the large-scale correlations ofsuch events.

Acknowledgements

This work was supported financially by the subproject‘‘EEM’’ of the DEKLIM program (Grant number01LD0041) of the German Ministry of Education andResearch (BMBF). Our thanks go to St. Knetsch (Halle),Dr. D. Degering, Dr. M. Krbetschek (both Freiberg),T. Samborski, Yu. Kononov (both Moscow) for your helpduring the fieldwork campaign for sampling in 2002.

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