Late-Holocene dynamics of vegetation cover and humidity of climate in the southeastern sector of the West Siberian Plain according to the data of palynological and rhizopod research of peat deposits

The study of climatic dynamics that affects the territories with the main concentration of agricultural production in Siberia is complicated by the lack of sufficiently long-term instrumental observations of changes in climatic parameters. Investigations of such natural archives as lake and peat sediments, which are widespread in Western Siberia help to solve this problem. Paleopalynological studies, accompanied by the results of additional paleoecological research methods, namely multi proxy paleoecological studies, provide valuable information on the dynamics of the biogeocoenotic cover of the Earth at various levels of its organization, allowing to link geomorphology, soils, vegetation, air currents, climate, as well as biocenoses at different levels of their organization from the microscopic world to geographic zoning. One of the least paleogeographically studied regions is the vast territory of Siberia. We performed a multi-proxy paleoecological investigation of isolated peat massif Krugloe mire (56°53'01.44''N, 84°34'43.08''E) located in the eastern outskirts of the Great Vasyugan mire on the terrace of the Bakhchar river (See Fig. 1) by methods of spore-pollen, testate amoebae (rhizopod), macrofossil peat analyses and microcharcoal analysis to study past environmental and climatic changes. Using the results of performed spore-pollen analysis of the peat section, a spore-pollen diagram was constructed (See Fig. 3). Two radiocarbon dates were obtained for study section (See Table 1). In this research we revealed that sedimentation (first, mineral and, then, peat deposition) on the peat massif began 5320 cal. yrs ago and had continued to the present day. We visually distinguished nine local pollen zones in the spore-pollen diagram reflecting nine stages in the development of regional vegetation (See Table 2). Based on available radiocarbon dates using the Bacon program (See Fig. 2), we calculated the age of pollen zones, and that of the corresponding changes in regional and local vegetation. Also based on pollen data using Bukreeva's formula (1991), calculated by multivariate statistical analysis of a large number of modern pollen spectra and corresponding climatic parameters, we reconstructed the dynamics of the annual amount of precipitation (See Fig. 5). Due to the performed research, we found out that during the existence of the mire, the vegetation cover of the study area changed from steppe mixed-wormwood communities to birch forest-steppe with areas of rich-herb meadows and then to thick southern taiga with cedar-birch and birch-cedar forests. We discovered that a significant decline in the role of cedar, fir and spruce in the study area started about 800 years ago. During the Little Ice Age (LIA) on the pollen diagram of Krugloe mire, an increase in the abundance of wormwood (Artemisia) pollen was noted. It is possible that it reflects the global increase in the role of steppe vegetation in the region due to the forming of a more continental climate in the central regions of Eurasia. An alternative hypothesis supposes that an increase in Artemisia pollen may indicate the beginning of anthropogenic impact on the vegetation cover in the study area intensified during the LIA. At the same time, we found out that more arid conditions in the second half of the LIA were noted both in pollen and in testate amoebae complexes (See Fig. 4), which supports the climatic hypothesis. We show that the mire in its endogenous development passed through eutrophic, mesotrophic and oligotrophic stages. The mire development is reflected in macrofossil composition of peat and in pollen of local mire plants. We found out that these stages in the development of the mire coincide with specific changes in regional vegetation surrounding the mire. For example, transition from the eutrophic stage to the mesotrophic one took place 3130 cal. yrs ago, simultaneously with the spread of thick southern taiga forests on the area of the former birch foreststeppe. In the period of the maximum distribution of Siberian cedar (Pinus sibirica) forests in the investigated territory 1300 calendar years ago, the mire passed to the oligotrophic stage of development. We hypothesize that the revealed change in local and regional vegetation was caused by a general increase in the climate humidity in the late Holocene. Our investigation of testate amoebae complexes from the same section showed that their abundance and species composition are largely determined by the endogenous stage of the mire development. But during the oligotrophic stage of the mire development, the complexes of testate amoebae were more sensitive to the centennial and decadal fluctuations in the moisture content of the climate than the local mire vegetation and regional terrestrial vegetation. We established that the content of micro charcoal in peat is maximal in the lower layers of peat. Probably, the abundance of micro charcoal increased largely due to the influence of fires on the mire with peat thickness less than 45 cm. We discovered that the effect of post-fire plant communities on pollen spectra is indicated by the increase in the abundance of Artemisia, Poaceae, Urtica, Filipendula and Thalictrum pollen synchronous to microcharcoal maxima. We assume that an increase in the frequency of fires during initial period of the mire development was also stimulated by a more arid climate at the beginning of the mire development, since the course of the curve of the abundance of micro charcoals mirrors the reconstructed annual precipitation. Paleoecological evidence from Krugloe mire (pollen and testate amoebae) agree in general with the data of other authors for the territory of southern Western Siberia, but show a more detailed picture of the reaction of local and regional elements of biogeocenoses to climatic changes. In some cases, these reactions do not coincide with the data of other researchers. This is observed when we compare our Krugloe mire paleoreconstructions with more distant points of research, located in the steppe zone of West Siberia. We hypothesize that such cases reflect heterochronic moistening of the forest and steppe zones of Siberia. A mismatch occurs more often with less prolonged climatic variations. When more prolonged climatic changes are recorded, both regional and local bioindicators react synchronously in the study area. In general, the dynamics of climate humidity revealed by biological indicators (pollen complexes, testate amoebae complexes, botanical composition of peat, pollen types diversity, microcharcoal), coincides well with the historical evidence given in the work of LG Gumilev (1993), and confirms the hypothesis of heterochronic moistening of the steppe and forest zones. The paper contains 5 Figures, 2 Tables and 37 References. Acknowledgments: The authors are grateful to Anatoly Grigor'evich Dyukarev, Dr. Sci. (Geogr.) for providing peat samples and radiocarbon dates of Krugloe mire section and to Natalia Aleksandrovna Chernova, Cand. Sci. (Biol.) for performing the botanical analysis of peat. We are also very thankful to anonymous Reviewers for their attentive writing of the text and efforts to improve the manuscript by remarks and advice.

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