Abstract
Pollen stratigraphy of an extrazonal palsa bog in the middle taiga of the West Siberian Plain is dated by radiocarbon at five levels. Local pollen assemblage zones (LPAZ) are the basis for palaeogeographical reconstructions. Tundra-steppe plant communities with shrub birch ( Betula nana) dominated in the latest Pleistocene. Warming after 10 000 14C yr BP caused the local thawing of permafrost, forming shallow lakes. Larixand then Picea spread along river valleys and depressions. Steppe plant communities existed on the dry interfluves. Further climatic warming and drying caused Picea to retreat, and Betula forest-steppe dominated 9500–8900 yr BP. Dense Pinus sylvestris and Pinus sibirica forests then spread over the whole area, and steppe communities decreased about 8300 yr BP. The Holocene climatic optimum (6000–5000 yr BP) was characterized by warm and wet conditions and Abies was widespread. Cooling then caused retreat of Abiesforests to the south and the expansion of Pinus sibirica forests on clay soils and Pinus sylvestris forests on sandy soils. Cooling about 4300 yr BP caused the peat to freeze and the palsa to form by bulging. Peat accumulation on the Bugristoye bog stopped at this time.
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