This article presents the results of a comprehensive geomorphological and paleogeographic study of the central part of the Oka Plateau (Eastern Sayan). The relief structure of the region is characterized in detail. A special place in it is occupied by elements of the paleohydronetwork: basins of drained moraine-dammed water bodies and abandoned fragments of valleys, including marginal channel. A comprehensive record has been obtained from the bottom sediments of Lake Sagan-Nur, which makes it possible to reconstruct a consistent picture of changes in the natural environment in the lake basin in the Holocene. It is shown that, about 8600-7100 BP, the lake was fed by glaciers, large flood events occurred in its basin, the water level in the lake was higher than today, and it was probably connected to a moraine-dammed paleowater body (traces of which are recorded in the estuary part of the Aynak River valley). Later, about 7100-5500 BP, a sharp drop in the rate of lacustrine sedimentation and a gradual attenuation of the relative amplitude of flood events occurred. After 5500 BP, the warming of the regional climate, especially in winter, and the disappearance of the remnants of the late Pleistocene glaciers caused a gradual expansion of dry areas at lower hypsometric levels favorable for the regional development of pine forests and the rise of the upper boundary of pine on the slopes. At the end of this time interval, the last restructuring of the local hydrographic network occurred and the modern river system was finally formed. Reconstructions for the last 5500 years show a much calmer hydrological regime and sedimentation pattern. Palynological indices also suggest a warming of the regional climate and a decrease in soil moisture, which took place following a change in the influx of solar radiation in the temperate latitudes of the Northern Hemisphere.

1 To explore the role of edaphic controls in the response of arctic tundra to climate change, we analysed Holocene pollen records from lakes in northern Alaska located on glaciated surfaces with contrasting soil texture, topography and tundra communities. Using indicator taxa, pollen accumulation rates (PARs) and multivariate comparison of fossil and modern pollen assemblages, we reconstructed the vegetational changes at Upper Capsule Lake (Sagavanirktok surface) and Red Green Lake (Itkillik II surface) in response to increased effective moisture between the early and middle Holocene. 2 In the Red Green record, low PARs and the continuous presence of taxa indicative of prostrate-shrub tundra (PST; Equisetum, Polypodiaceae, Thalictrum and Rosaceae) indicate that the vegetation resembled PST throughout the Holocene. During the warm, dry early Holocene (11 300-10 000 cal years BP), PST also occurred on Sagavanirktok surfaces, as evidenced by PST indicators (Bryidae, Polypodiaceae, Equisetum and Rosaceae) in this interval of the Upper Capsule record. However, PARs increased, suggesting increased vegetation cover, PST taxa declined and taxa indicative of dwarf-shrub tundra (DST; Rubus chamaemorus and Lycopodium annotinum) increased between 10 000 and 7500 cal years BP. 3 We hypothesize that between the early and middle Holocene the fine-textured soils and smooth topography of Sagavanirktok surfaces led to increased soil moisture, greater vegetation cover, permafrost aggradation, anoxic and acidic soil conditions, slower decomposition and the development of a thick organic layer. In contrast, soil moisture remained low on the better-drained Itkillik II surface, and vegetational changes were minor. 4 Landscape-scale substrate variations have an effect on how tundra responds to climate change, suggesting that the response of arctic ecosystems to future variability may be spatially heterogeneous.