Late Quaternary Dynamics of Landscape and Climate in the North of the West Siberian Plain Revealed by Paleoecological Studies of Peat and Lake Sediments

West Siberian mires covering more than 50% of area in the subarctic are still poorly investigated despite their thick peat sediments suitable for paleogeographic research of past long-term landscape and climatic changes. In this research, a combination of paleoecological methods were used, including the analysis of pollen, spores, diatoms, NPPs, and macrofossils, the measurement of peat humification, and quantitative paleoclimate reconstruction. This multi-proxy approach was applied to study a palsa bog (frost peat heave mound) located in the north of western Siberia on the border of the northern taiga and forest–tundra (65°18′56″ N, 72°52′27″ E). Chronology is based on 21 radiocarbon dates, which were calibrated in CLAM. Studies have shown that sediments of palsa bog Nadym of a 1050 cm thickness were formed both in the Holocene and earlier periods of the Quaternary. Radiocarbon dating worked well for peat sequences (610 cm thick), but failed for underlying lacustrine and mineral sediments (440 cm thick). Numerous remains of salt-water diatoms and exotic Neogene pollen were found in the lacustrine sediments (650–850 cm). The oldest sediments (850–1050 cm) have signs of secondary epicryogenic diagenesis in the form of cryogenic iron-enriched granules. Both lacustrine and bottom sediments contain abundant coniferous pollen. At the same time, spore–pollen complexes dated to the Last Glacial Age were not found in low sediments because of failed dates. To explain this, the authors turn to the hypothesis of glyacioisostatic compensation, according to which the study area was uplifted during the Last Glacial Age and the ancient deposits underwent secondary diagenesis in subaerial conditions. Holocene lacustrine sedimentation began to form about 9800 cal. a BP. These lacustrine sediments turned out to be enriched in redeposited Neogene pollen and diatoms. It was interpreted as an influence of excess humid climate in combination with geological subsidence of landscape in the study area during the Early Holocene. This caused lake formation and introduction of exotic microfossils via surface run-off from higher-relief areas in the catchment. Syngenetic sedimentation in the Nadym section is associated only with peat-mire deposits covering the last 8400 cal. a BP. For this time, the dynamic of vegetation cover and quantitative changes in paleoclimate were reconstructed using spore–pollen, macrofossil, humus, and NPP data as well as the information–statistical method of V.A. Klimanov. The spore–pollen analysis revealed four main phases in the development of vegetation cover: 1. Spruce–birch forests with open meadows and lakes (8400–7600 cal. a BP); 2. Dominance of spruce forests and thawed eutrophic (minerotrophic) mires (7600 to 6500 cal. a BP); 3. Coniferous–birch forests and thawed mesotrophic mires (6500 to 4500 cal. a BP); 4. Birch–pine forests and oligotrophic (ombrotrophic) bogs with permafrost mounds—palsa bogs (approx. the last 4500 years). Quantitative reconstructions of paleoclimate based on pollen data show that in most cases the periods of a sharp decrease in mean January and mean July temperatures coincided with episodes of low solar activity. The assumption was made about the determining influence of solar activity on the formation of permafrost in the soils and mires of the study area. Sun minima caused permafrost formation in the mire periodically since 8400 cal. a BP in study peatland, but complete freezing of the peat mire and formation of the palsa bog occurred at c. 2800 cal. a BP.