Introduction

A complex post-glacial history in Central Russia is recorded by a combination of periods that were strongly influenced by changing climatic conditions. At first, stagnant ice and permafrost were widespread in Northwestern and Central Russia during the Late Glacial and the Early Holocene periods. Later, warmer air temperatures triggered permafrost degradation, resulting in subsurface drainage of the lakes, recorded as asynchronous sedimentary hiatuses in its sediments (Wohlfarth et al. 2007). During the Holocene, the northern part of Central Europe (above 47°N) experienced a significantly slower temperature increase in comparison to its southern counterpart (below 50°N) (Feurdean et al. 2014). The Early and Middle Holocene was relatively cool and dry, followed by warming during the Holocene thermal optimum. This latter period is marked in the Central European Russia (54–58°N) between ca. 8600 and 6900 cal. yr BP. Human impact occurred later and is registered as two events at ca. 4200–3700 and ca. 1400–1000 cal. yr BP as a result of forest clearance at that time (Tarasov et al. 2019).

In general, key forest-forming trees such as birch, pine, and spruce migrated from Southeast Russia to the west, and later spread to the northeast and northwest, paralleling the direction of ice retreat, with B. pubescens immigrating first, followed by P. sylvestris and Picea abies. The first P. abies and B. pubescens macrofossils around Lake Terebenskoye in the Valdai Hills appeared at ca. 11,200 cal. yr BP (Wohlfarth et al. 2007). Pollen data from Lake Terebenskoye suggests an occurrence of Picea at 11,200–11,820 cal. yr BP (T. Sapelko, unpublished data). Mixed broad-leaved coniferous forests were widespread in the Rostov-Jaroslavl’ area between 8200 and 6100 cal. yr BP and developed into dense, species-rich forests between 6100 and 2500 cal. yr BP, thus marking the warmest interval (Wohlfarth et al. 2006). Around Lake Terebenskoye in the Valdai Hills (T. Sapelko, unpublished data) and in the Upper Volga region (Borisova 2019), the first broad-leaved trees appeared around 10,300–10,670 cal. yr BP.

Former geomorphological studies revealed that the relief of Central Russia was formed during the Weichselian/Valdanian and the Saale/Moscow Glacial periods, mainly during their degradation and under neotectonic conditions that both formed a variety of landscape types (Markov 1977; Velichko et al. 1997). Two of them are especially interesting and refer to the vast and low-located East European Plain and the prominent Valdai Hills. The Hills extend to nearly 600 km in length, have a maximum elevation of 346 m a.s.l., and have a northeast–northwest extension (Astakhov et al. 2016), and thus preserves an important palaeogeographical record for the region. Affected by several older glaciations and the last glaciation, the Hills are considered a well-preserved ice marginal belt of the Scandinavian ice sheet (Rinterknecht et al. 2018), with multiple fronts (Gorlach et al. 2017; Kalm 2012). Numerous chronologies from sediments north of the Valdai Hills were obtained and established between 17,000 yr and 18,000 yr BP (i.e. Mangerud et al. 2008). The latest cosmogenic surface exposure dating positioned the timing of the last local glacial limit at 20,100 ± 400 yr BP (Rinterknecht et al. 2018). At this time, several proglacial lakes were formed along the ice margin (Gorlach et al. 2017). After this, deglaciation took place no earlier than 19,100 cal. yr BP and by 13,300 cal. yr BP (Lasberg and Kalm 2013), and further resulted in lake drainage (Rinterknecht et al. 2018). Lake Seliger is traditionally considered a relict lake (Kvasov 1975) that remained after degradation of a huge proglacial lake, where sand, mud and gyttja alternate (Konstantinov et al. 2018). This latter combination of sediments is a response of the aquatic and terrestrial ecosystems to the Holocene climate fluctuations.

Here, we present a palaeoecological study from the Valdai Hills to examine the Holocene (a time span between 11,700 and 2100 cal. yr BP) dynamics of lake and forest ecosystems developed on the island of Lake Seliger. This is the first multiproxy Holocene data on the lake deposits in this region. The Holocene palaeoenvironmental development of the East European Plain in Northern Eurasia has been intensively studied in Russia since the 1950s, but mostly published in Russian and without any reliable chronology (Neustadt 1957, Neustadt et al. 1965; Sukachev 1968; Khotinsky 1977). Late post-glacial deposits from various archives were studied, but until now, mostly peat deposits have been used for reconstruction of climate and vegetation changes (Aleshinskaya et al. 1992; Tarasov et al. 1996, 2019; Kremenetski et al. 2000; Davydova et al. 2001; Gunova et al. 2001; Wohlfarth et al. 2006; 2007; Borisova 2019). However, in recent years, new studies re-examining old sites have begun to appear in publications (Borisova 2019; Tarasov et al. 2019). Nevertheless, a knowledge gap still exists, and our new and multiproxy study aims to close this gap.

This study examines Lake Chernoye, located in the Khachin Island of Lake Seliger. Studies on small lakes provide not only the history of the lake itself along with the island in which it is located, but also the regional changes of the Lake Seliger basin. During the Late Glacial Period, Lake Seliger was a part of the larger Lake Tverskoye (Kvasov 1975), with a water level of ca. 140–145 m a.s.l. during the Late Glacial Period followed by its later drop. At its highest level, the lake occupies the entire lowlands with a flow threshold east of the modern watershed. Due to the shallow depths of Lake Tverskoye varved clay, usually present in Late Glacial lakes, did not accumulate here.

The main aims of our study are: (1) to reconstruct the small lake ecosystem that developed on the island of the bigger lake, (2) to assess forest vegetation response to Holocene climate change at Khachin Island and in the catchment of the Lake Seliger basin, and (3) to compare the development of the isolated lake and forest ecosystems with general patterns of vegetation shifts in the region.

Study site

Lake Chernoye (57° 17,039′ N; 33° 02,765′) is located at an elevation of 213 m a.s.l. in the middle East European Plain. The lake is situated in the biggest island, Khachin Island, within the Lake Seliger system (Fig. 1). Lake Chernoye covers an area of 10,136 m2 with a maximum depth of 15.2 m (Naumenko et al. 2014).

Fig. 1
figure 1

Location map with the last glacial limit (LGM after Svendsen et al. 2004) (1), study area, where red cross marks the location of Lake Chernoye on Khachin Island (2) and details on Lake Chernoye: bottom depths (in m) (a) with slopes (in °) (b) (3). 1—Karelian Isthmus; 2—Lake Terebenskoye. Hatching shows the Valdai Hills

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The climate of the study region is temperate continental, mild and humid. According to the weather station in Ostashkov near Lake Seliger (57°11′N; 33° 10′E; 214 m a.s.l.), the average annual temperature is +4 °C (mean July temperature of 17 °C). The average annual precipitation is about 600 mm and most of it falls in the summer. The prevailing winds are in a west and southwest direction. The territory of the district is located in the subzone of the southern taiga. Sod-podzolic soils are mostly widespread. Forests with a dominance of evergreen coniferous trees (spruce and pine) dominate the natural vegetation cover. Among deciduous tree species, Betula, A. incana, A. glutinosa, and Populus tremula are common, but Quercus, Ulmus, Tilia, Acer, and Fraxinus also occur in the region. Common and waterlogged meadows are also widespread. The most predominant types of meadow grasses include Poaceae, Cyperaceae, Fabaceae, etc.

Materials and methods

Coring and sampling

A manual Russian corer with a diameter of 5 cm and length of 1 m was used for drilling. Overlapping sediment sequence was transported to the laboratory, described, and sub-sampled for detailed palaeoecological studies using pollen, plant macrofossils combined with quartz grains, loss on ignition and metals analyses. The sampling interval in the lake sediments sequence was 2 cm at the bottom and 5 cm at the top of the sequence.

Morphometric model

The data for the morphometric model of Lake Chernoye was obtained during a field trip, when echo-sounding depth measurements were carried out. To measure depth, a Lowrance echo sounder with a GPS coordinate system was used (determining the depth at an accuracy of up to 0.1 m, coordinates up to 5 m). To obtain all these, an inflatable boat in calm weather was used for 1–2 days. Digital models of depth and slope distribution were created with help from precise survey by echo sounding (Naumenko et al. 2014). Spatial resolution of the models is 10 m.

Chronology

Six accelerator mass spectrometry (AMS) radiocarbon dates measured on selected terrestrial plants at the Poznań Radiocarbon Laboratory were used to generate the age–depth (Table 1). Calibration of the radiocarbon dates and construction of the age–depth were performed with OxCal 4.1 software (Bronk Ramsey 2009) and the IntCal13 curve (Reimer et al. 2013), applying a P_Sequence function with a k parameter of 1 cm−1 and 1-cm resolution. The most distinct changes in deposit composition, which might be a signal of changes in the sedimentation accumulation rate, were introduced using the boundary command (Fig. 3). The modelled age is expressed as calibrated calendar years before present (cal. yr BP) where present equals 1950 AD.

Table 1 Radiocarbon dates of sediment sequence from the Lake Chernoye of the Khachin Island, Lake Seliger, Valdai Hills
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Pollen analysis

Sub-samples of 2 cm3 for each 2 cm horizon were prepared for pollen analysis using a standard procedure (Grichuk 1940; Berglund and Ralska-Jasiewiczowa 1986) that includes heavy liquid separation (CdI2 + KI). 400–800 pollen grains were counted for most samples, except for the bottom sandy layer, where low pollen concentrations (about 100 pollen grains) were counted. Pollen identification was based on a proposal by Kuprianova and Alyoshina (1972), Moore et al. (1991), and by comparison with pollen reference slides at the Institute of Limnology Russian Academy of Sciences (RAS) in St. Petersburg. The results were plotted in diagrams using TILIA, TILIA GRAPH and TGView programs (Grimm 1991, 2007). The total pollen sum was calculated as the sum of tree, shrub, herb pollen and spores.

Plant macrofossil analysis

Plant macrofossils were analysed at 1-cm intervals in contiguous samples of approximately 10–12 cm3, totalling 335 samples. Samples were washed and sieved under warm tap water using 0.20-mm mesh screens. Initially, the entire sample was analysed with a stereoscopic microscope. In this way, the percentage of individual fossils of vascular plants and brown mosses were obtained. Fossil carpological remains and vegetative fragments (leaves, rootlets, epidermis) were identified using identification keys (Smith 2004; Velichkevich and Zastawniak 2006, 2008) and were compared with recent collection materials. The numbers of seeds, fruits, needles, bud scales and leaves were counted separately. Macroscopic charcoal pieces were also counted during plant macrofossils analysis, and their presence in the sediment provides information of past fire occurrence (Mooney and Tinner 2011; Robin et al. 2013).

Quartz grain analysis

Sand quartz grains were visually detected in the samples taken from a depth of 700–705 cm, 792–796 cm and 807–809.5 cm (six samples altogether; Table 2), and these samples were subjected to a quartz grain inspection. This inspection allows us to define grain surface and its three-dimensionality, and such a combination of both grain rounding and the type of its surface may be offered by Cailleux (1942) method, further modified by Mycielska-Dowgiałło and Woronko (2004). Prior to the analysis, a portion of sediment was treated by 10% HCl to remove any carbonates, washed thoroughly with distilled water to remove any excess and dried at room temperature. Seven grain groups were selected: (1) well rounded with a whole matte surface; (2) partially rounded, matte only on the most convex part of the grains; (3) well rounded and shiny; (4) partially rounded and shiny; (5) fresh, with sharp edges and corners; (6) broken with at least 30% of the original grain surface affected, and (7) with intensively weathered surface. Approximately, 100–125 quartz grains from a 0.2–0.355 mm fraction were randomly selected into one of the seven groups under binocular microscopy. Normally, coarser sand fractions (0.5–1.0 mm) are considered in the analysis. However, in this study, a finer fraction was chosen for grain observation due to its abundance and lack of coarser material.

Table 2 Data on quartz grain analysis of the 0.2–0.355 mm fraction
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Loss on ignition

The content of organic material and terrigenous matter was determined by a loss-on-ignition (LOI) survey. To ascertain the LOI, sediments covering a 2–5 cm interval were dried at 105 °C for 24 h and combusted at 500 °C (4 h) to calculate organic matter, carbonate and mineral compounds, respectively (Heiri et al, 2001).

Geochemical investigations

The total content of major elements (Si, Al, Fe, Ca, Mg) in weight percentages and trace elements (Mn, Ni, Co, Cr, Cu, Pb) in mg/kg were determined by atomic optical emission spectrophotometry (AOES) in the laboratory of the Institute of Limnology RAS, St. Petersburg, Russia.

Results

Morphometry, lithostratigraphy and chronology

The Lake Chernoye depression has a regular shape and forms a conical shape. Its southeastern part reveals a maximum slope inclination of up to 28°, whereas the northern part is gentler ca. 16°–18°. The central part of the depression has the least inclined slope (6°) with an average value of about 18° (Fig. 1).

The core excavated at sampling point 3 generally consists of two parts (Fig. 2). The bottom part (8.05–8.15 m depth) of the section consists of light grey silty clay, brown gyttja sand and dark brown organic layers. The upper part (4.45–8.05 m) is represented by brown and dark brown gyttja with plant macrofossils and sandy gyttja at the bottom. The borders between the units are distinct. At the base of the section, light grey sand appears. A sandy organic layer with wood fragments (bark, small branches) appears between depths 7.02–7.05 m.

Fig. 2
figure 2

Sediment lithology and LOI (%) of Lake Chernoye

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Based on the depth–age model (Fig. 3), accumulation of organic sediments began at ca. 11,100 cal. yr BP. The bottom part of the studied deposits accumulated between 11,100 and 10,800 cal. yr BP at depths 815–805 cm, and consists of light grey silty clay, brown gyttja sand and dark brown organic layers. Above this layer, and between 10,800 and 2100 cal. yr BP at depth 805–445 cm, brown and dark brown gyttja with plant detritus was deposited. Sediments at a depth of 705–702 cm were dated back at ca. 7000 cal. yr BP.

Fig. 3
figure 3

Age–depth model of the lake sediments profile in Lake Chernoye. Colours refer to main stages of the sediment accumulation

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Palaeobotanical results

Trees, shrubs and selected terrestrial plant succession

Six zones in trees, shrubs and selected herbs plants development were visually delimited in the pollen and macrofossil diagram (Fig. 4). Zone CL-tr-1 (11,100–10,800 cal. yr BP) is characterised by high abundances of Picea, Pinus, Artemisia, Chenopodiaceae, and Asteraceae pollen. One P. abies needle and B. section albae macrofossils in this zone were also found. In zone CL-tr-2 (10,800–9600 cal. yr BP), the highest abundance of Betula pollen (up to 60%) and macrofossils (fruits and fruit scales) were recorded. High abundances of B. nana and Poaceae pollen were also recorded. In zone CL-tr-2 (9600–6700 cal. yr BP), an increase in deciduous trees such as Ulmus (the highest value in the entire period), Tilia, Quercus and Alnus, as well as high abundances of Pinus and Betula (ca. 50%) took place. Zone CL-tr-4 (6700–4200 cal. yr BP) is dominated by deciduous trees such as Quercus, Tilia, and Alnus, and is also characterised by an increased presence of Picea (up to 20%). Numerous Picea abies and P. sylvestris macroremains were also recorded. In zone CL-tr-5 (4200–2700 cal. yr BP), Picea, Pinus, Betula and A. glutinosa pollen were dominant. A constant presence of P. sylvestris macrofossils and Cerealia pollen in this zone were also documented. Zone CL-tr-6 (2700–2100 cal. yr BP) is characterised by high abundances of coniferous trees and low abundances of deciduous trees. An increase in B. nana pollen (up to 6%) and a peak of A. incana pollen also took place in this zone.

Fig. 4
figure 4

Pollen and plant diagram presenting local and regional vegetation (mainly trees) changes of Lake Chernoye. Pollen percentages are shown in black, five times exaggeration in grey. Plant macrofossils are given in counted numbers. CL-tr-1–6 refer to six zones that mark the development of trees, shrubs and selected herbs between 11,100 cal. yr BP and 2100 cal. yr BP

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Local wetland plants succession

Five zones in the local vegetation development were visually delimited in the pollen and macrofossil diagram (Fig. 5). Zone CL-lo-1 (11,100–9700 cal. yr BP) representing a shallow lake with aquatic submerged plants such as Chara sp., Myriophyllum spicatum, C. submersum and P. perfolitus. High values of rush plant pollen such as Carex, Equisetum, and Sparganium, as well as the presence of plant macrofossils such as Schoenoplectus lacustris and Carex pseudocyperus were also documented. One macrocharcoal piece in this zone was found. Zone CL-lo-2 (9700–8500 cal. yr BP) is characterised by the abundant presence of Isöetes microspores, Polypodiaceae spores, Alisma pollen and Pediastrum. Nymphaea cf. candida, P. cf. lucens and Menyantes trifoliata macroremains were also recorded. In zone CL-lo-3 (8500–6600 cal. yr BP), spreading of Chara sp., N. marina, N. flexilis, and P. pusillus took place. Two macrocharcoal pieces were also found. In zone CL-lo-4 (6600–3300 cal. yr BP), Myriophyllum sp., Potamogeton sp. and Nuphar sp. pollen were common. The first leaves of Sphagnum mosses were found here. In zone CL-lo-5 (3300–2100 cal. yr BP), P. pusillus reappeared and Carex lasiocarpa and Sphagnum teres appeared for the first time. Myriophyllum sp. pollen disappeared in this zone.

Fig. 5
figure 5

Pollen and plant diagram, presenting local and regional vegetation changes of Lake Chernoye. Pollen percentages are shown in black, five times exaggeration in grey. Plant macrofossils are given in counted numbers. The CL-lo-1–5 refer to five zones that mark an occurrence of local wetland plants between 11,100 cal. yr BP and 2100 cal. yr BP

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Mineral and geochemical analysis

No clear grain group prevalence can be seen in the investigated part of the profile, but rather three groups of quartz grains contribute roughly equally in all investigated samples (Table 2). These are: (1) the partially rounded grains with a shiny surface (19–42%), (2) fresh grains with sharp edges (22–34%), and (3) partially rounded grains with a matte surface (19–43%). These are followed by broken grains (7–19%) and well-rounded grains with a matte surface (0–7%). The remaining types of grains were either absent or minor. Importantly, almost half of the investigated grains in the lowermost part of the profile (807–809.5 cm) have a matte surface, followed by an instant interlayer where shiny grains prevail.

The LOI results show stable high organic content for most of the core (ca. 90%) and large fluctuations in the bottom section. Minimum LOI values (2–7%) are marked ca. 11,000 cal. yr BP for non-organic deposits (Fig. 6). Further up, there is a sharp increase in the LOI values to 76% ca. 10,800 fluctuations with a range 10–83% between 10,700 and 9800 cal. yr BP. From ca. 9800 cal. yr BP (depth of 776 cm) to 2100 cal. yr BP, LOI values are 88–92%. The exception is the sandy peat layer accumulated between 7500/7000 cal. yr BP (depth 705–700 cm), where LOI values decreased to 37%. In zone CL-ge-1 (11,100–9600 cal. yr BP), the majority of elements had elevated concentrations. Zone CL-ge-2 (9600–8000 cal. yr BP) is characterised by constant low values of elements such as Si, Al, Mn, Fe, Pb. In CL-ge-3 zone (8000–6600 cal. yr BP), elements such as Si, Al, Ni, Cr, Pb and Co peaked. In zone CL-ge-4 (6600–2100 cal. yr BP), elements such as Ca, Mn and Cu had unstable values; elements such as Si, Al, Mg, Ni, Cr and Pb had low and nearly constant values. At ca. 5700 cal. yr BP, Fe, Cr and Pb values peaked.

Fig. 6
figure 6

Changes in the LOI and metals of Lake Chernoye. CL-ge-1–4 refer to four zones with LOI and major element fluctuations

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Discussion

Palaeobotanical, geochemical and quartz grains data obtained from a sediment log of Lake Chernoye along with radiocarbon dating allow us to determine the local and regional vegetation history and island lake development in the Holocene. This is discussed in the following sections (Fig. 7).

Fig. 7
figure 7

Comparison of chosen taxa from two botanical data sets: plant macrofossils and pollen, and LOI presenting vegetation and lake development with marked main climatic events

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Forest development

Picea abies is the main forest species in the region, and appeared at ca. 11,000 cal. yr BP on the island as apparent from the macroremains and pollen study. Between ca. 10,400 and 9700 cal. yr BP, Betula pollen dominates the forest. At ca. 9500, the climate must have warmed, as the population of B. nana declined and was replaced by Ulmus, which was one of the first deciduous trees in the region. Its Early Holocene spreading is synchronised with other data from Central and Eastern Europe (Feurdean et al. 2014). The period between ca. 9600 and 6700 cal. yr BP was dominated by a pine and birch forest, with small percentages of Ulmus, Tilia, Quercus and Corylus. With gradually improving climate conditions and a possible increase of groundwater at ca. 9000 cal. yr BP, swamp forests of black alder (A. glutinosa) developed around the lake. Increasing humidity might have also contributed to the spread of grey alder (A. incana). In the Upper Volga region, an increase in A. glutinosa and A. incana was observed in sediments deposited at about 9000–4700 cal. yr BP (Borisova 2019). In the Rostov-Jaroslavl’ area, mixed deciduous–coniferous forests were common between 8200 and 6100 cal. yr BP and became dense and species rich between 6100 and 2500 cal. yr BP (Wohlfarth et al. 2006). P. abies most likely re-appeared on the studied island at ca. 6500 cal. yr BP, because macroremain (bud scale) was recorded at that time. Starting from this time, the role of spruces gradually increased; it might have been one of the most important trees next to Pinus in the forest that occupied the island (Fig. 3). Climate cooling, noted in Scandinavia and the Baltic countries from ca. 4500 cal. yr BP (e.g. Heikkilä and Seppä 2003; Seppä and Poska 2004), is clearly observed in our study at ca. 4300 cal. yr BP through a decline of deciduous trees and a spread of Picea. This regional cooling signal is supported by changes in the main forest components in many sites in Eastern and Central Europe (Seppä and Poska 2004; Gaidamavičius et al. 2011; Gałka et al. 2014; Stivrins et al. 2014).

The first traces of Cerealia pollen were found approximately 4200 cal. yr BP. The presence of Cerealia, Plantago lanceolata and Centaurea cyanus pollen (Fig. 4) suggests that the open spaces in Khachin Island were used as fields or pastures. A ca. 2800–2600 cal. yr BP decline of Ulmus, Quercus, and Corylus responds to the 2.8 ka cool event (cf. Wanner et al. 2011).

Our data clearly correspond with other palaeobotanical date of this type in the East European Plain, and therefore our island case seems to not record specific isolated conditions, but rather contribute to a general regional tree succession pattern.

Lake ecosystem development in the western part of the boreal zone

Our study area is located in relative proximity to the LGM ice sheet limit (Svendsen et al. 2004), which deglaciated asynchronously and is dated back in this area to 20,000–19,000 yr BP (Hughes et al. 2016). The studied lake basin formed about 11,000 cal. yr BP, possibly due to melting of stagnant glacier ice. The great lake depth combined with its small area may indicate this along with a general geological situation. The high water level at the studied lake correlated with the Late Glacial and Early Holocene periods and was most likely the result of melting of most of the dead-ice blocks. At this time, variations in Ca correlate well with the Si elements in the lake sediment. This indicates that calcite is of siliciclastic origin (Pleskot et al. 2018).

Several Late Glacial ice marginal formations, termed as the Veps, Krestsy, Luga and Neva, along with intervening interstadial sediments, have been identified in the area between the Valdai Hills and the Karelian Isthmus (Subetto et al. 2002). During the recession of the ice sheet, blocks of dead ice remained in the ground. Later, ice-block preservation in the landscape depended on various aspects, such as air temperature, hydrology and geomorphological conditions (Stivrins et al. 2017). Thermokarst processes were extensive throughout Central and the Northern Europe, and also in Northwest Russia due to the Last Glacial deglaciation over these areas (Subetto et al. 2002; Stančikaite et al. 2009; Stivrins et al. 2017; Gałka et al. 2019). Climatic conditions at 11,100–10,800 cal. yr BP were cold according to our pollen data. Open landscapes with Poaceae, Artemisia, Chenopodiaceae and Cyperaceae were widespread around the lake at this time. Heavy metal distribution in the sediment core points to a high amount of elements in the lake formation and clearly reveals that lake conditions were relatively stable during the Holocene. The sharp decline in LOI in the bottom part of the core is not associated with a change in lithology, as depressions of organic matter were short term and visible layers in the lithological sequence were not represented.

A distinct increase of partially rounded, matte grains is observed in the bottom part of the sediment profile, where almost half of the investigated grains showed a matte outline. These matte grains carry an entirely matte surface, including in grain depressions. This clearly demonstrates that grains were mobilised under colder climate conditions and the sediment underwent long-term aeolian abrasion. The material of aeolian qualities was deposited slightly after 11,091–10,685 cal. yr BP, likely representing Early Holocene aeolian activity as known throughout Western and Central Europe (Kolstrup 2007; Vandenberghe et al. 2013; Zieliński et al. 2015) and in the neighbouring Baltic States (Kalińska et al. 2019, 2020a). Aeolian grains are also not a surprise in lake sediment record, since it has been observed in numerous lake profiles (Kalińska-Nartiša et al. 2018; Kalińska-Nartiša and Gałka 2018). In such cases, transportation is rather short and comes from adjacent aeolian sources. In this study, shiny quartz grains, which, in general, are known as a typical record of fluvial action (Kalińska et al. 2020b), also dominate in lacustrine deposition at ca. 11,000 cal. yr BP, which corresponds well with a high stand water in the lake in the end of the Late Glacial.

After 10,800 cal. yr BP, the climate became warmer. The area of boreal forest with Pinus and Betula development increased (Fig. 7). At ca. 10,700 cal. yr BP, one of the first documented aquatic plants that occurred in the studied lake was C. demersum, which is an indicator of warmer conditions, and its presence suggests that temperatures in July oscillated between approximately 15 °C (Litt 1994) or even 18 °C (Mai 1985). However, C. demersum appeared in Northeast Europe earlier than recorded in our study. Its occurrence was documented before 11,000 cal. yr BP in Lake Kojle, Northeast Poland (Gałka et al. 2015). In addition, in the Kaliska Basin, Northeast Poland, C. demersum fruits were documented in sandy gyttja deposits, dated to ca. 9700 cal. yr BP (Stachowicz-Rybka and Obidowicz 2013). The later presence of C. demersum fruits in gyttja accumulated similarly atop of the peat layer, which was dated back to ca. 8000 cal. yr BP in sediment cores of Lake Ķikuru, Western Latvia (Stivrins et al. 2017). In our study, Typha pollen and Carex pseudocyperus occurred at ca. 10,500 cal. yr BP, which are typical species for the Early Holocene, indicating warming climate. The presence of these species in the sediment suggests that the mean minimum July temperature reached 13–16 °C, depending on the particular Typha species (Kolstrup 1979).

The area of the lake was probably larger than it is currently. At ca 9500–8500 cal. yr BP, the presence of Isoetes and Nymphaea cf. candidata indicate oligotrophic conditions in the lake. The gradual lowering of the water level, the formation of a shoreline close to the coring site and evidence for a hiatus between 8500 and 7500 cal. yr BP show that the level of the lake decreased considerably and remained low for a longer time. During this period, there was a sharp decrease in organic matter and an increase in the supply of mineral precipitation, according to the peaks of the all elements studied (Fig. 6). Combined with the number of broken quartz grains, the investigated grain mineral matter resembles glacial-driven conditions. Under ice conditions, either fracturing or abrasion occurred (Whalley and Langway 1980), thus producing either highly angular or partially abraded grains (Hart 2006), which can be found in numerous glacial-origin sediments (Immonen 2013; Mahaney and Kalm 1995; Vos et al. 2014). In our study, these two types of grains dominate. However, the number of fresh and broken grains increased in the 7.03–7.05 m sediment sample. Assuming the sediment deposition at 7950–7800 cal. yr BP, quartz grains of this glacial suite cannot mark a glaciation, but must originate, for example, from the adjacent moraine sources, being delivered to the lacustrine system, and further resulting into lake shallowing. A similar mechanism is known from lacustrine deposition, for example in Northeast Poland (Kalińska-Nartiša and Gałka 2018).

The warm period at about 6900–5000 cal. yr BP could be linked to the Holocene thermal maximum (Fig. 7), which, in Northwest Europe and Russia, was characterised by warm and dry conditions (Birks and Seppä 2010; Borisova 2019, respectively), and is marked by a reduced groundwater and lake level along with increased peat decomposition in peatlands in numerous sites during this period (Wohlfarth et al. 2007; Gałka et al. 2014; Stivrins et al. 2017). Similarly, the final meltdown of dead ice was at the Holocene thermal maximum (8500–7400 cal. yr BP) in Western Latvia, Eastern Baltic (Stivrins et al. 2017). Abiotic and biotic processes following the ice-block meltdown suggest abrupt development of thermokarst from 8500 to 7400 cal. yr BP. The Holocene thermal maximum in the Western Dvina River region started ca. 8600–6900 cal. yr BP and ended some centuries earlier ca. 8100–5600 cal. yr BP (Tarasov et al. 2019) than suggested for the neighboring Baltic region.

The decreased water level favoured the re-appearance of Chara and allowed development of submerged plant communities with N. flexilis, N. marina and P. pussilus. N. flexilis seeds are especially common and are noted in Eurasia and North America in Early and Middle Holocene sediments, but the presence of this species is usually short term in comparison to other submerged macrophytes (cf. Swinehart and Parker 2000; Gałka et al. 2012). From ca. 5000 cal. yr BP, leaves from Sphagnum mosses (S. sect. subsecunda, S. medium/divinum and later S. teres) and Carex lasiocarpa fruits were recorded in the sediments that suggest gradual development of floating mats around the lake.

Conclusions

The results of our study showed:

  1. 1.

    At ca. 11,000 cal. yr BP, development of Lake Chernoye began after the melting of buried dead-ice blocks, which was intensified by the Early Holocene warming. As pioneer plants, Chara sp., Isöetes and C. demersum appeared in the water with high Ca and Mg content. High values of Pinus, Artemisia, B. nana and Chenopodiaceae pollen may indicate remains of cold flora that occurred in the Younger Drays in this region.

  2. 2.

    Between 8000 and 6500 cal. yr BP, the water level decreased, as apparent from a sediment hiatus. This may have been related to the Holocene thermal maximum and final melting of the buried dead-ice block. Lower water level at that time is documented by the spreading of N. marina, N. flexilis and P. pusillus, which are all submerged plants growing in eutrophic shallow water. Significant changes in the forest ecosystem at that time also took place. An increase in Quercus, Corylus and Alnus was accompanied by a decrease in Pinus.

  3. 3.

    Ca. 4200 cal. yr BP and ca. 2700 cal. yr BP are associated with climate cooling, documented by multiproxy records in many sites in Europe. A decrease in deciduous tree pollen (Ulmus, Quercus, Tilia) and increase in coniferous tree pollen (Picea and Pinus) mark this cooling period.

  4. 4.

    At ca. 4200 cal yr BP, climate deterioration was marked by the contemporaneous appearance of cereal pollen.

  5. 5.

    Location of the lake on the isolated island did not lead to differ in the vegetation pattern development, since this latter is similar to the other sites in the East European Plain.