Palaeoecological reconstructions in the Tobol-Ishim interfluve: combination of on-site pollen data from cultural layers and off-site peatland records

Abstract

The study, based on uniformed criteria, summarizes a large series of palynological data and is aimed at re-construction of the vegetation history of the second half of the Holocene based on pollen data from peatlands. The main objective of this work is to compare the results of palynological studies from cultural layers of 30 Neo-lithic-Medieval archaeological sites (on-site data) and five natural archives (off-site records) in the sub-taiga and forest-steppe areas of Tobol River region and on the territory of the Ishim Plain (south of Western Siberia). The main analytical and correlation tools were the calculated indicators of pollen indices and biomes. Baseline pa-laeoecological changes have been analysed by the pollen index of openness/forestness (the ratio of wood vs grass pollen), the aridity index (the ratio of the summed amount of wormwood and chenopod pollen to that of ce-reals) and the dynamics of forest and steppe biome of peatlands; similar pollen indices have been calculated for the cultural layers. All indicators have been synchronized by age and fixed on a single timeline for comparison. The increasing role of forest vegetation in the forest-steppe has been considered as an indicator of increased effective hydration, forest degradation has been associated with the lack of it. The aridity index has been used as an indirect argument, reflecting the condition of grass communities outside the forest, it increases with the syn-chronous aridization and warming, but is also subject to strong distortion under the influence of human activity. Six stages of vegetation change have been identified: 6.0–4.2 ka — increase in proportion of forests in the northern forest steppe; 4.2–3.3 ka — minimal forestation of the territory; 3.3–2.5 ka — gradual regeneration of forest areas; 2.5–1.9 ka — reduction of the proportion of forests; 1.9–0.7 ka — the most pronounced forestation of the territory; 0.7–0 ka — reduction of the proportion of forests. In general, the outlined stages correspond with the overall scheme of development of landscape and climatic conditions in the southern regions of Western Siberia, though have regional specifics. Comparison of palynological data from natural archives (peatlands) and cultural layers by pollen indices demonstrates, that the anthropogenic disturbance of the composition of grass vegetation near the sites in most cases prevents objective assessment of natural conditions, but characterizes the appearance of the settlement landscape near the sites during its functioning and is associated with economic activity.