Compilation of changes in vegetation cover, climate, landscape geochemistry and fires from multi-proxy data in the eastern sector of the southern taiga zone of Western Siberia

The comprehensive palaeoecological study of this lake sediment record from Shchuchye Lake, revealed Late Glacial and Holocene dynamics of climate, vegetation cover, geochemical changes in the landscape and its burning that occurred in the last 13,000 calendar years. We present a reconstruction of these changes based on the identified pollen zones with the events linked to the Blitt-Sernander geochronological scale (Figure 5), taking into account the large age uncertainties between 9 and 5.5 cal ka BP.

1. 1. Phase of spruce tundra-forest-steppe (LPZ-1a bottom). It is represented by only a few lower spore-pollen spectra extracted from the sandy sediments underlying the lake sediments. The phase characterizes the vegetation cover of the study area during the period of interstadial warming Allerød. The basis of the vegetation cover was shrub-moss-grass tundra-steppe with islands of spruce forest in river valleys, where the permafrost cover in the soils began to decrease, creating sufficient local moisture for spruce (forest-tundra-steppe). Probably, the local degradation of permafrost was started by the extension of frost-free summer periods caused by increased solar insolation of the northern hemisphere during the summer seasons in the post-glacial and early Holocene times (Berger 1978; Berger and Loutre Reference Berger and Loutre1991). The dry and relatively cold Allerød climate was not conducive to fires, which is confirmed by the small number of micro- and macro-charcoal particles concentration. The shrub cover was represented by thickets of dwarf birch, spirea and willows. A similar plant community of dwarf birch (Betula nana), spirea (Spirea alpina) currently forms a shrubby altitudinal belt in the Altai Mountains on the border between the mountain forest belt and the alpine vegetation belt (Kuminova Reference Kuminova1960), which could be seen as a modern analogue for Allerød vegetation in study area. The herb cover was represented by vast open spaces covered with meadow-grass vegetation in which, on dry areas the role of wormwood (Artemisia) and grasses (Poaceae) increased, and on more wet places mosses dominated. In general, such vegetation cover provided an excellent food base for glacial megafauna (Leshchinskiy and Burkanova Reference Leshchinskiy and Burkanova2022).

2. 2. Phase of larch tundra-forest-steppe (12.8–12.5 cal ka BP)(LPZ-1a). Judging by quantitative reconstructions of the late-glacial palaeoclimate, the climate of this phase was extremely unstable (Figure 6). Its severity increased sharply due to lower temperatures in July and January, but mainly due to the sharp drying of the climate. This led to the disappearance of spruce from forest patches. It was replaced by larch, which is capable of growing on permafrost soils, as is currently the case in vast areas of Eastern Siberia (Shumiliva Reference Shumiliva1962). The increase in permafrost in the soils made them drier and unsuitable for spruce, which disappeared, and the herbal cover was dominated by meadow-steppe species with a significant role of wormwood (Artemisia). Species from the families Asteraceae and Caryophyllaceae, Poaceae of the genus Androsace and the heliophyte Thalictrum were abundant. The absence of pollen of aquatic macrophytes indicates that the water table was extremely low in Shchuchye Lake during this period. It was just wetland hollow. The abundance of spores of the soil fungus Glomus also indicates the dry soil regime and widespread soil erosion (Van Geel et al. Reference Van Geel, Gelorini, Lyaruu, Aptroot, Rucina, Marchant, Sinninghe, Damsté and Verschuren2011). It is possible that during this dry and cold period (according to our reconstructions, atmospheric precipitation fell 400 mm less than today’s) the blowing of sand on the Ob’ River terraces intensified. The quartz particles of these deposits mostly have a rounded, rolled shape with traces of the influence of aeolian processes—micropits (Figure A.1). Numerous micropits on the surface of rounded quartz grains are consequences of the collision of sand grains during their transport by air (Velichko et al. Reference Velichko, Timireva, Kremenetski, MacDonald and Smith2011). They indicate dry and frozen conditions (cryoarid) that existed not only in the north and middle parts of Western Siberia, but also in the southeast of the plain in the study area. Strong winds and sand movement might also have ground larger macro-charcoal particles into micro-charcoal increasing number of the last (Figure A1). In addition, this phase is characterized by a large abundance of spores of the fungus Gelasinospora, which have also been used as indicator of local fires (Stivrins et al. Reference Stivrins, Aakala, Pasanen, Kuuluvainen, Vasanda, Gałka, Disbrey, Liepins, Holmström and Seppä2019; Van Geel Reference Van Geel1978). Finally, it is in these deposits that Epilobium angustifolium pollen is found, which is a typical post-fire disturbed soils colonizer (Prokop’ev Reference Prokop’ev2003). The presence of two indicators of local fires suggests that local fires occurred frequently during the YD cooling period but were not recorded by the abundance of macro-charcoal, possibly due to abrasion by sand and aeolian processes resulting in presence of micro-charcoal only, but at high abundances (Figure 5). Thus, palaeoecological data recorded the exact time frame of the cryoarid period in the development of landscapes in the southeastern sector of the southern taiga in the West Siberia, which took place ∼12.8–12.5 cal ka BP and was probably accompanied by the movement of dune sandy deposits. The arid climate in Western Siberia at this time was due to the fact, that a significant part of the Arctic and North Atlantic was covered with pack ice (Velichko et al. Reference Velichko, Timireva, Kremenetski, MacDonald and Smith2011). At the same time, the area covered by the influence of the high pressure of the Siberian anticyclone expanded significantly. As a result of these freezing and dry conditions, permafrost was distributed almost throughout the entire territory of Western Siberia (Baulin Reference Baulin1985; Velichko Reference Velichko, Velichko, Wright and Barnosky1984).

3. 3. Phase of spruce tundra-forest-steppe (12.5–12.2 cal ka BP, LPZ-1b). During this short period of ∼350 years, it is likely that the intensity of the YD cooling factor weakened, which resumed the warming effect of increased solar insolation during the summer in the northern hemisphere. Other studies have also observed regional variations with warmer summers during the YD period, explained by atmospheric blocking of cold Westerly winds by the Fennoscandian Ice Sheet (Schenk et al. Reference Schenk, Väliranta, Muschitiello, Tarasov, Heikkilä, Björck, Brandefelt, Johanssen, Naslung and Wohlfarth2018). At Shchuchye Lake, July and January temperatures and the duration of the frost-free period sharply increased during these 350 years, which led to degradation of permafrost. In places where permafrost thawed, local soil moisture increased, and spruce was able to settle along with larch. Spruce, unlike larch, is an evergreen species. Its crown produces more shade all year round. This created unfavorable conditions for heliophilous plants such as Betula nana and Thalictrum. As a result, their role in vegetation has sharply decreased. At the same time, one can see the synchronous spread of sedge (Carex) during the rapid change from mineral to organic sediment (Figure 5, Figure A.1). At the beginning of thawing, sedges spread in the thermocarst depression and spruce settled around the basin, while local soil moisture increased. However, the climate still remained cold and dry (cryo-arid) and xerophytic herbs and shrubs (Artemisia, Poaceae, Ephedra) dominated in the dry areas. This was a transitional stage in the transformation of the landscape from a situation with widespread permafrost (continuous permafrost in soil) to one where permafrost degraded and partly disappeared (discontinuous permafrost in soil).

4. 4. Phase of shrub birch forest-steppe (12.2–11.2 cal ka BP, LPZ-2a). This phase in the development of the landscape covers the last part of the YD and the first half of the PB. During this phase, which lasted approximately 1000 years, the landscape continued to actively change. The vegetation cover in the second half of the YD consisted of vast areas with dwarf birch shrub and islands of woody vegetation of tree birches (Betula pendula and Betula alba) in the floodplain of the Ob’ River and its tributaries, and in the depressions around newly formed lakes. In such areas the permafrost roof sank to greater depth and local moisture allowed woody vegetation to establish themselves. In higher watershed areas, thickets of dwarf birch alternated with patches of tundra (dominated by moss vegetation) and steppes (dominated by wormwood-grass vegetation). Despite the extremely harsh temperature conditions at the beginning of this time (July temperatures were by 7°C below modern temperatures, and January temperatures by 5–6°C), the annual amount of precipitation increased and this stimulated further degradation of permafrost with the accumulation of moisture in the formed thermokarst depressions, one of which developed into Lake Shchuchye. We suppose that numerous other lakes formed by similar way on the Ob’ River terraces during this time. Possibly the thawing of pingo formed in the ground during previous ice age period took important role in this process. Palynological data indicate that spruce and sedges were rapidly replaced by tree-like birches (Betula pendula and B. alba) and the proportion of heliophytes Betula nana and Thalictrum in the vegetation cover increased again. But now, instead of sedges, aquatic macrophytes Nuphar and Nymphaea appeared, which is also obvious from the many macroremains (spicula) of Nuphar. We interpret this as the start of this site as a lake, proper. With the increase in annual precipitation and rising temperatures, the tree birch spread to higher areas and a birch-shrub forest-tundra or forest-steppe with areas of wormwood meadow communities was formed. The progressive increase in January and July temperatures as well as longer duration of the frost- and ice-free period increased the productivity in the lake and its catchment, shown by the increase of organic matter content to 80%. Alternatively, the increased productivity of the young lake could be explained by high organic matter in un-leached soils, or both factors facilitated bioproductivity of lake in this period. It is noteworthy that with the formation of forest cover of tree birches, the fire intensity of the landscape increased sharply. During this phase, 3 local fire episodes of high intensity were recorded near Lake Shchuchye. It is possible the increased amount of fuel from the spread of tree and shrub vegetation combined with the (still) dry climate provided the conditions for high-intensity fires during 12.2–11.2 cal ka BP. To the north of study site, in the middle taiga of the Yenisei part of Western Siberia, a high frequency of fires was also noted in the same period of 12–10 cal ka BP (Karpenko et al. Reference Karpenko, Grenaderova, Mikhailova and Podubaeva2022) with large local fires of 11.6 and 11.2 cal ka BP.

   After 12.2 cal ka BP, the disappearance of spruce and larch indicates the loss of moist habitat locally, although local fires could also have contributed to the rapid disappearance of spruce. Annual precipitation, despite the increase after the end of the YD, was still 100–200 mm lower than modern values. With the beginning of the Holocene in PB, the temperatures rapidly increased in July (by 3°C) and January (by 2.5°C), and the frost-free period increased by 20 days. Because of this, the moisture balance of the territory apparently remained negative, which explains the loss of spruce on higher areas and the preservation of steppe areas with wormwood-grass vegetation until the end of the PB. Thus, our data confirm the hypothesis proposed by (Blyakharchuk and Sulerzhitsky Reference Blyakharchuk and Sulerzhitsky1999; Piavchenko Reference Piavchenko1957) that spruce followed the northward retreat of the permafrost boundary (with its increasing local soil moisture) in the early Holocene. That is why the early maximum of spruce pollen is transgressive in time and space with the gradual retreat of permafrost to the north and east, appearing earlier in the pollen sections of the southern taiga of Western Siberia. In Zhykovskoye mire (Fig. 1 point 1) the age of the early spruce maximum is >12 cal ka BP (Borisova et al. Reference Borisova, Novenko, Zelikson and Kremenetski2011), in Shchuchye Lake (this study) it is present at 12.5–12.2 cal ka BP. The “early spruce maximum” is later in pollen sections of the middle taiga: Entarny = 11 cal ka BP (Figure 1 point 2); Vodorazdel = 11–10.5 cal ka BP (Figure 1 point 3); Petropavlovka = 10 cal ka BP (Figure 1 point 4); Lukashkin Yar = 10.8–10 cal ka BP 9 (Figure 1 point 5); Bugristoye = 11–10.7 cal ka BP (Figure 1 point 6); Nizhnevartovskoye = 11.1-9.5 cal ka BP (Figure 1 point 7) (Arkhipov and Votakh Reference Arkhipov and Votakh1980; Arkhipov et al. Reference Arkhipov, Levina and Panychev1980; Blyakharchuk Reference Blyakharchuk2012; Blyakharchuk and Sulerzhitsky Reference Blyakharchuk and Sulerzhitsky1999; Glebov Reference Glebov1988; Glebov et al. Reference Glebov, Toleiko, Starikov, Zhidovlenko, Abramova, Boch and Galkina1974; Karpenko Reference Karpenko2000; Neustadt Reference Neustadt1971). This indicates that permafrost degradation in the southern taiga occurred earlier, towards the end of the YD period, and in the middle taiga later, in the PB in the BO periods. To the east, permafrost degradation and the decline of spruce appeared even later, in the AT, according to the Maksimkin Yar pollen diagram (∼200 km northeast of study site) (Blyakharchuk Reference Blyakharchuk2012). This picture reflects the gradual retreat of the permafrost boundary to the north and to the east. However, estimating the exact timing of events is hampered by uncertainties of 375–800 years in the age model.

5. 5. Phase of birch forests (11.2—10 cal ka BP, LPZ-2b). This phase, characterized by colonizing tree birch between 40–70% of the assemblage. It took place in the pre-boral (PB) period, and occurs in many pollen records in the taiga zone of Western Siberia. The phase is dated 12–8.5 cal ka BP in the Zhukovskoye mire; > 7.5 cal ka BP at the Tom’ River Mouth; at Vodorazdel between 10–7 cal ka BP; at Petropavlovka between 10–8 cal ka BP; at Nizhnevartovskye between 9–7 cal ka BP; at Lukashkin Yar 10–7 cal ka BP; and at Bugristoye 10.8–10 cal ka BP. Like the pattern of the early spruce maximum, the phase of birch forests occurs earlier in the southern taiga (PB-BO) and later in the middle taiga subzone (AT). Quantitative reconstructions of palaeoclimate by Shchuchye Lake section show a sharp increase in both average July and average January temperatures to modern and slightly higher (July) values with a sharp increase in duration of the frost-free period and annual precipitation (Figure 6). The higher amount of atmospheric precipitation favored the spread of birch forests, reducing open spaces with wormwood, grasses and dwarf birch, although open glades were still found in these forests, as evidenced by the constant finds of heliophilous pollen of Thalictrum. A sharp increase in precipitation likely increased leaching from the soils, evidenced by increased carbonate content (up to 30–40%, Figure 5) in lake sediments. The high carbonate concentrations in the lake coincide with the disappearance of Nuphar spicula and Nymphaea pollen from the record. This might be related to a decrease in lake productivity, or a higher water level, increasing the distance between coring site and macrophytes in the littoral zone of the lake. The established very humid and relatively warm climate contributed to the occurrence of fire episodes (Figure 5). The landscape’s fire frequency has decreased both regionally (micro-charcoal) and locally in the vicinity of Lake Shchuchye (macro-charcoal).

6. 6. Phase of pine-birch forests and the beginning of fir and cedar spreading (10–5.7 cal ka BP, LPZ-3). This phase covers the second part of the Boreal period (BO) and all of the Atlantic period (AT). Climatic parameters during this phase reached modern values. The intense leaching regime that existed in the previous phase ultimately leached all carbonates from the soil, which were not so abundant in the sandy and sandy loam soils of the region under study. This process is reflected in decreasing carbonate content of the sediments (Figure 5). The formation of podzols began and soils depleted of nutrients became less favorable for birch. As a result, birch was replaced by pine, which rapidly spread on the sandy soils of the terraces and ancient drainage gullies. Coniferous litter from pine forests increased acidity and further intensified the process of podzol formation in soils. The spread of pine forests coincides with a radical reduction of wormwood in the vegetation cover, reflecting the change of vegetation on the well-draining sandy soils and dune landscapes. However, distribution of a variety of soils in the southern taiga on the right bank of River Ob’ allowed the continued presence of birch and aspen-birch forests on more cohesive soils. Fir (Abies sibirica) appeared on clayey soils under the canopy of a birch forests, and later, from 8 cal ka BP, Siberian cedar (Pinus sibirica) began to spread rapidly. An increase in average July and average January temperatures, along with an extension of frost-free periods at the beginning of the BO-AT boundary caused an intensification of fires. During this phase, 5 large local fire episodes occurred. After a brief wet period of about 8.5–8.2 cal ka BP, the annual amount of precipitation stabilized around 30 mm higher than present day values, which could also stimulate the occurrence of fires. The dry cooling at the end of the AT, accompanied by an extension of the frost-free period coincides with increasing of local fires. But these change of palaeoclimate were very small and stay within the uncertainty band and hence not significant statistically. Nevertheless, some evidence of drier climate during this period we can find in the north in the middle taiga, where a “boundary layer” of woody peat was formed in Nizhnevartovskoye Bog peat section (Khotinskiy Reference Khotinskiy, Velichko, Wright and Barnosky1984; Neustadt Reference Neustadt1971). In general, the background level of natural fires in the modern southern taiga during BO and AT was lower than in the pre-Boreal period (PB), but higher than in the subsequent period of the late Holocene after 6 cal ka BP, based on the amount of charcoal fragments per peak event “peak magnitude” (the higher the values, the greater the fire severity and/or the closer to the site) (Rasmussen et al. Reference Rasmussen, Bigler, Blockley, Blunier, Buchardt, Clausen, Cvijanovic, Dahl-Jensen, Johnsen and Fischer2014). This trend could be explained by the development of the taiga zone in a humid climate. It can be noted that during this relatively stable phase in the development of southern taiga forests, the northern border of the forest zone in Western Siberia moved significantly northward (MacDonald et al. Reference MacDonald, Velichko, Kremenetski, Borisova, Goleva, Andreev, Cwynar, Riding, Forman and Edwards2000).

7. 7. Phase of birch-fir-cedar and pine forests (5.7—1.3 cal ka BP, LPZ-4), which covers the entire Subboreal period (SB) and most of the Subatlantic (SA). According to quantitative climate reconstructions at Shchuchye Lake the temperature regime and annual precipitation during this long period was relatively stable in the southern taiga subzone, only slightly deviating in a positive direction from modern values (Figure 6), unlike to reconstructions of mire wetness made by testate amoebae in peat mire Rybnaya (Feurdean et al. Reference Feurdean, Diaconu, Pfeiffer, Gałka, Hutchinson, Butiseaca, Gorina, Tonlkov, Niamir, Tantau, Zhang and Kapotin2022) located just ∼17 km north of Shchuchye Lake and covering last 8500 calendaric years. According to these authors a wet period existed between 8.5–7.5 cal ka BP and dry climatic condition between 7.5–4.5 cal ka BP. Our data does not support this dry climatic period, but the difference can be explained by different local hydrological settings. The Rybnaya mire is located on the watershed at a distance of about 8 km from the Ob’ riverbed, but Shchuchye Lake lies on the Ob’ terrace at a distance of no more than 4.8 km from the Ob’ riverbed. Possibly the moisturizing effect of the large river valley could completely neutralize the effect of climate drying in the vicinity of Shchuchye Lake, but not affect the more distant watershed area of the Rybnaya mire, suggesting the testate amoebae record is more sensitive to changes in moisture than vegetation (pollen) recorded in our lake sediment record. At the same time our palaeofire data demonstrate increased number of local fires in the vicinity of Shchuchye Lake (5 local fires) during this time, which agrees with the fire frequency in the Rybnaya mire (Feurdean et al. Reference Feurdean, Diaconu, Pfeiffer, Gałka, Hutchinson, Butiseaca, Gorina, Tonlkov, Niamir, Tantau, Zhang and Kapotin2022). During the period 3–4 cal ka BP, annual precipitation increased, exceeding modern values by 100 mm and this was accompanied by increase in the duration of the frost-free period. A similar wet period 4.5–2.5 cal ka BP was reconstructed by testate amoebae in the Rybnaya mire. At approximately 3–2.8 cal ka BP, a brief cooling was reconstructed by Shchuchye data with a reduction in the duration of the frost-free period. It was during this period that the Bugristoye peat bog, located at ∼130 km northeast, froze over (Blyakharchuk and Sulerzhitsky Reference Blyakharchuk and Sulerzhitsky1999). According both Shchuchye and Rybnaya pollen data during 7.5–4 cal ka BP Siberian cedar (Pinus sibirica) had maximal extent in the southern taiga and started to decline after 4 cal ka BP. In the first half of the phase 5.7–4 cal ka BP, fir (Abies sibirica) had maximal abundance in the surrounding forests, and after 4 cal ka BP, fir like cedar began to decline. With maximal spread of dark coniferous forests after 5.7 cal ka BP, the processes of podzolization and soil depletion intensified. This, in turn, contributed to the spread of moss and moss-lichen ground cover in forests, as seen in the increase in the proportion of spores in the pollen spectra of this phase (Figure 2). It is possible that the increase in ground low intensity fires stimulated the spread of moss-lichen ground cover in the forests of the southern taiga, bringing them closer to the type of vegetation of the middle taiga (Shumiliva Reference Shumiliva1962). The background level of charcoal particles in this phase became lower than in the previous phase and only 3 local fire episodes were identified in this period. A reduction in fires may be linked to higher annual precipitation as a result of increased Atlantic circulation.

8. 8. Phase of pine and Siberian cedar forests covers the last 1.3 cal ka BP. The climate parameters reconstructed for this phase are close to modern ones. Only annual precipitation decreased compared to the previous phase. We compared quantitative reconstructions of palaeoclimate based on pollen data from Shchuchye Lake with palaeo-reconstructions of climate based on pollen and water table fluctuations based on testate amoebae and plant macrofossil composition in peat deposits of the Plotnikovo Mire on the eastern edge of the Bolshoy Vasyugan swamp, ∼100 km southwest of the studied lake (Feurdean et al. Reference Feurdean, Gałka, Florescu, Diaconu, Tanţău, Kapotin and Hutchinson2019) (Figure 1 point 8), and the Rybnaya mire (Feurdean et al. Reference Feurdean, Diaconu, Pfeiffer, Gałka, Hutchinson, Butiseaca, Gorina, Tonlkov, Niamir, Tantau, Zhang and Kapotin2022). Both mires recorded drier conditions until ∼1.5 cal ka BP, after which conditions became wetter, with higher water tables. In contrast, the reconstructed annual precipitation at Shchuchye Lake in this period initially was 40 mm more than modern, but conditions gradually became drier until modern values were reached. The difference between the mire records and the Shchuchye Lake record can be explained by the local influence of the Ob’River on its floodplain landscapes, while the mires were more strongly influenced by the taiga on watershed areas and, possibly, by steppe located to the south of Plotnikovo Mire. The moistening effect of big river on floodplain vegetation is confirmed by modern geobotanical investigations in Kemerovo district 225 km south of study area (Lashchinskiy et al. Reference Lashchinskiy, Makunina and Gulyaeva2011). The effect of climate drying to modern time observed in Shchuchye Lake can be caused by stronger human influence on floodplain vegetation compare with bogged watershed areas in the study area. The cooling that occurred around ca 800 cal ka BP may reflect the cooling of the Little Ice Age (LIA), but better age control and/or more highly resolved data are required to confirm this. A suitable object for such study may be peat deposits of a mire close to the lake. Palaeoecological data from Rybnaya mire gives better control for the last millennium (Feurdean et al. Reference Feurdean, Diaconu, Pfeiffer, Gałka, Hutchinson, Butiseaca, Gorina, Tonlkov, Niamir, Tantau, Zhang and Kapotin2022). Our data suggests that changes in vegetation since 500 cal ka BP could be caused by both climate change and anthropogenic influence. The spread of weed species of wormwood (Artemisia) could have occurred in connection with the beginning of agriculture. The strengthening of the role of pine and dwarf birch in the vegetation cover may also be caused by the large-scale transition of bogs (which occupier about 50% of area in this region) to an oligotrophic stage of development. There has been a slight increase in the abundance of micro-charcoal in the last millennium, but after two local fire episodes at the start of this phase, no charcoal data is available after 1 cal ka BP because of insufficient material for charcoal analysis was present in the upper part of the unconsolidated top part of the lake core.

Correlation of palaeoecological data from Lake Shchuchye with nearby pollen sections and with known global climate changes

The pollen and spore data of Shchuchye Lake is very similar to the nearby pollen and spore records from Zhukovskoye bog (Borisova et al. Reference Borisova, Novenko, Zelikson and Kremenetski2011) and Rybnaya mire (Feurdean et al. Reference Feurdean, Diaconu, Pfeiffer, Gałka, Hutchinson, Butiseaca, Gorina, Tonlkov, Niamir, Tantau, Zhang and Kapotin2022) in the southern taiga zone (∼60 km to the south, and 17 km to the north, respectively; Figure 1b point 1) covering the Late Glacial and Holocene, as well as with an earlier record from a river bank expose at the mouth of the Tom’ River (Arkhipov and Votakh, Reference Arkhipov and Votakh1980) (Figure 1b point 9). The sequence of changes in the vegetation cover is also similar to that in the more northern section of the Bugristoye bog (Blyakharchuk and Sulerzhitsky Reference Blyakharchuk and Sulerzhitsky1999) (Figure 1b point 6). In the diagrams under consideration, similar pollen zones are identified: lower spruce, dominance of birch forest-steppe and birch forests; the spread of pine forests, and then fir and Siberian cedar, with a subsequent reduction in Siberian cedar. The spatial similarity in the evolution of vegetation indicates that similar factors drove the changes in the southeastern sector of Western Siberia. Among these factors mean temperatures of January and July, the duration of the frost-free period, and annual amount of precipitation are important as well as influence the evolution of soils, permafrost, water bodies and landscape geochemistry (Groisman et al. Reference Groisman, Blyakharchuk, Chernokulsky, Arzhanov, Marchesini, Bogdanova, Borzenkova, Bulygina, Karpenko, Karpenko and Knoght2013) The result of vegetation and soil development also impacted biological productivity of the lake. A maximum in biological productivity was noted in the early Holocene (highest LOI values of organic fraction from 12.2 to 11.2 cal ka BP) before the climate became more moist, leading to the spread forests, despite the relatively low mean temperatures of July and January at that time. This early peak in biological productivity of the lake was likely related to the high amount of nutrients still available in the steppe soil at that time, before they were removed by leaching and podzolization.

The currently known episodes of rapid Late-Glacial and Early Holocene climate changes of 11.4, 10.2, 9.3 and 8.2 cal ka BP (Fleitmann et al. Reference Fleitmann, Mudelsee, Burns, Bradley, Kramers and Matter2008) manifested themselves differently in the region under study. The cooling of the Younger Dryas is clearly expressed in palaeo-reconstructions of climate based on palynological data from Shchuchye Lake, occurring between 13.1 to 11.7 cal ka BP, which is consistent with the generally accepted framework. According to the data from Shchuchye Lake, however, the YD consisted of two phases, separated by a short (∼350-year) warming around ca. 12.4 cal ka BP. The warming could be an artifact in the quantitative reconstruction due to the local spread of spruce during the degradation of permafrost in the soils, but permafrost degradation itself is a consequence of climate warming and started during the Late Glacial period. It is also possible that the warming phase is real as warmer (summer) conditions during the YD have also been observed elsewhere in NW Eurasia (Schenk et al., Reference Schenk, Väliranta, Muschitiello, Tarasov, Heikkilä, Björck, Brandefelt, Johanssen, Naslung and Wohlfarth2018). In addition, a compilation of extensive palaeo pollen data located east of 40°E and transformed in plant functional types (PFT) by Binney et al. (Reference Binney, Edwards, Macias-Fauria, Lozhkin, Anderson, Kaplan, Andreev, Bezrukova, Blyakharchuk, Jankovska, Khazina, Krivonogov, Kremenetski, Nield, Novenko, Ryabogins, Silovieva, Willis and Zernitskaya2017) demonstrate the increased woody biomes vs. non-woody at ∼12 cal ka BP. The lowest July and January temperatures and extremely short frost-free periods occurred in the final phase of YD (12.1–11.7 cal ka BP) in the study area. This brings the southeastern region of Western Siberia closer to the easternmost territories of Eurasia, where the maximum YD cooling dates back to 12.2–11.25 cal ka BP instead of 12.9 cal ka BP as in the North Atlantic region (Ryabogina et al., Reference Ryabogina, Afonin, Ivanov, Li, Kalinin, Udaltsov and Nikolaenko2019).

A sharp warming at the beginning of the PB (11.6–11.5 cal ka BP) marked the end of the YD, and July temperatures increased by 3.5°C, January temperatures by 3°C, and the duration of the frost-free period increased by 35 days, in a period of about 100 years. Permafrost in soils completely disappeared in the study area during that time. This is consistent with the origin of the first swamps (12–11.5 cal ka BP) in Western Siberia precisely during this period of time (Kremenetski et al. Reference Kremenetski, Velichko, Borisova, MacDonald, Smith, Frey and Orlova2003; MacDonald et al. Reference MacDonald, Velichko, Kremenetski, Borisova, Goleva, Andreev, Cwynar, Riding, Forman and Edwards2000).

It is noteworthy that in the Altai Mountains (∼ 650 km south of the study area) in the period 10.9-8.9 cal ka BP a stalagmite growth occurred in the Kök-Tash cave as a result of permafrost degradation, (Li et al. Reference Li, Baker, Wang, Zhang, Wu, Li, Blyakharchuk, Yu, Shen and Cheng2021). The δ¹⁸O isotope data of this stalagmite and the analysis of the isotopic composition of atmospheric waters at all weather stations in southern Siberia allowed the authors to conclude that in the early Holocene there were significant changes in the atmospheric circulation in the territory of southern Siberia. Thus, 11.4–10.8 cal ka BP, as a result of shifting of atmospheric blocking from the southern Urals to Scandinavia and the weakening of the Siberian high pressure (SH), the territory of Altai and the south of Western Siberia found itself under the dominant influence of southwestern moisture currents, which contributed to sharp warming and humidification of the winter seasons, which caused degradation of permafrost and the beginning of stalagmite growth in the Kök-Tash cave (Li et al. Reference Li, Baker, Wang, Zhang, Wu, Li, Blyakharchuk, Yu, Shen and Cheng2021). Palaeoclimatic reconstructions of Shchuchye Lake indicate that the influence of this warming also extended to the territory of the southeast of West Siberian plain. The Kök-Tash isotope record indicates that during the transition from the YD to the Holocene, the presence of the Scandinavian Ice Sheet led to shifts in the pattern of atmospheric blocking. This, in turn, caused an earlier climate warming in Western Siberia and the Altai region compared to other continental regions of Eurasia (Baker et al. Reference Baker, Lachniet, Chervyatsova, Asmerom and Polyak2017; Li et al. Reference Li, Baker, Wang, Zhang, Wu, Li, Blyakharchuk, Yu, Shen and Cheng2021). This is exactly what we have observed in the Shchuchye Lake record as well, where permafrost degradation, marked by the spread of spruce on thermokarst subsidence, began at ∼12.5 cal ka BP in the southern taiga and ∼11.9 cal ka BP in the middle taiga (Leshchinskiy et al. Reference Leshchinskiy, Blyakharchuk, Vvedenskaya and Orlova2011).

Palaeoclimate reconstructions based on data from Shchuchye Lake also recorded short-term dry cooling periods of 10.2 and 9.3 cal ka BP (Fig. 6), but the warming at the boundary of BO/AT of about 9 cal ka BP was especially pronounced. With the final disappearance of the Scandinavian Ice Sheet, summer westerly currents intensified and shifted across the territory of Western Siberia, allowing inflow of Arctic air masses, making the summer seasons cool and wet (Li et al. Reference Li, Baker, Wang, Zhang, Wu, Li, Blyakharchuk, Yu, Shen and Cheng2021). The vegetation cover of the study area responded to increased climate humidity by the spread of coniferous forest. At the same time (9.5–8.8 cal ka BP), forest moved northward to their maximum extend on the Yamal Peninsula (Khotinskiy Reference Khotinskiy, Velichko, Wright and Barnosky1984; Lapteva and Korona Reference Lapteva and Korona2022).

The well-known episode of a sharp Holocene cooling of 8.2 cal ka BP (Fleitmann et al. Reference Fleitmann, Mudelsee, Burns, Bradley, Kramers and Matter2008; Rasmussen et al. Reference Rasmussen, Andersen, Svensson, Steffensen, Vinther, Clausen, Siggaard-Andersen, Johnsen, Larsen and Dahl-Jensen2006, Reference Rasmussen, Bigler, Blockley, Blunier, Buchardt, Clausen, Cvijanovic, Dahl-Jensen, Johnsen and Fischer2014) might also be recorded in the palaeo-reconstructions of Shchuchye Lake as a decrease in July and January temperatures to modern levels after the earlier warming at the BO/AT boundary. In general, a cooling around ∼8.2 cal ka BP in the study area occurred in a more humid climate than the modern one. This brings the southeastern region closer to the southwestern region of Western Siberia, where the cooling of 8.2 cal ka BP was also characterized by a wetter climate (Ryabogina et al. Reference Ryabogina, Afonin, Ivanov, Li, Kalinin, Udaltsov and Nikolaenko2019). In the rest of the Holocene, short-lived cold episodes occurred against a background of a wetter-than-present climate.

With regards to the influence of events around 8.2 and 4.2 cal ka BP on vegetation, we can stress that they took place during stable condition of taiga period in study area. The change of vegetation during these short periods of relative cooling were not as substantial as those in the late Glacial and Early Holocene. Wet cooling 8.2 cal ka BP caused a maximum shift of Abies sibirica in the forests, after which Pinus sibirica started to spread progressively on the territory. Wet cooling at 4.5-4.2 cal ka BP was even less well-expressed in the taiga vegetation. First of all, it caused sharp increase of moss cover in the forests followed by gradual decrease of Abies sibirica and Pinus sibirica in taiga forests. The short cooling episodes may just be coincidental with natural processes of migration of tree species and evolution of forest cover and soils in study area, but could also have acted as triggers for these changes.