Radiocarbon Chronology of Pedogenesis on Floodplains in the Upper Angara Region (South of Eastern Siberia)

Fluvial relief formation plays a significant role in the development of the environment in the south of Eastern Siberia, but remains poorly studied for the last 15 000 years. The response of fluvial processes to changes in the environmental conditions is recorded in the structure of floodplains, an integral part of which is soil formation periodically renewed under conditions of a stable surface. The paper presents an analysis of the chronostructure of soil formation stages based on statistical processing of an array of 119 radiocarbon dates obtained from the organic matter of the in-situ soils buried within different-aged generations of floodplain and low terraces of the left-bank tributaries of the Angara River. The chronology of the formation of these surfaces has been determined: 9.2–8.6 kyr BP for the first terraces, 5.0–4.5 kyr BP for high floodplains, and 3.0–2.8 kyr BP for low floodplains. Mass data on the carbonate content, particle-size distribution, and Corg content (341, 407, and 182 samples, respectively) attest to significant spatial heterogeneity of alluvial deposits and soils. Temporal variations in these indicators allow us to assess the Late Glacial period, the Early Holocene, and the first half of the Middle Holocene as periods of the greatest fluvial activity, an important factor of which was the degradation of mountain glaciers. The predominance of stable stages of floodplain development is characteristic of the Middle Holocene/Late Holocene boundary with the maximum duration of pedogenesis on stable floodplain surfaces. The stages of increasing fluvial activity and pedogenesis in the valleys over the last millennium appear to be much shorter, and their frequency increases. The identified stages of river valley development and their temporal relationships correspond to landscape and climate changes recorded over the past 15 000 years in the studied region. This enables us to consider the climatic factor as the leading driver in the formation of floodplain-channel complexes, soil formation being an important component of floodplain development in the Holocene.