Reconstruction of Bioclimatic Changes during the Late Glacial and the Holocene as Evidenced by Buried Soils and Sedimentary Records from the Middle Reaches of the Orkhon River (Northern Mongolia)

The conditions of sedimentation and pedogenesis in northern Mongolia during the last 14 000 years have been reconstructed based on a comprehensive study of the pedosediment archive of the cover deposits on the young floodplain terrace of the Orkhon River (upper part of the Selenga River basin). The chronology was established using twelve radiocarbon dates. Paleosols were formed during the periods of stable landscape evolution under optimal bioclimatic conditions. Alternating sand and clay strata reflect phases of soil degradation driven by climate aridization and intensified aeolian and slope processes. Radiocarbon dating indicates that the earliest stage of soil formation corresponds to the Bølling–Allerød interstadial (from ~15.0 to ~12.5 ka BP). During that time, the conditions were favorable for the formation of chernozem-like soils under predominantly open steppe landscapes. At the end of the Late Glacial, a stage of accelerated accumulation of loesslike sediments followed, pointing to increasing climate aridity. Steppe and desert-steppe vegetation dominated the region. The next phase of pedogenesis occurred at the beginning of the Holocene, when humid conditions were re-established in northern Mongolia and resulted in the formation of meadow-steppe soils. Between 8.5 and 3.0 ka BP, climatic conditions remained unstable: humid meadow steppes of the Early Holocene were replaced by drier steppes in the Middle Holocene. Nevertheless, short-term episodes of overmoistening are recorded against the intensified aeolian activity and climate aridization and are most likely associated with the thawing of seasonally frozen ground. After 3.5 ka BP, the climate became warmer and wetter. The Late Holocene was characterized by the dominance of meadow steppes under conditions of increased humidity, while erosion–accumulation processes continued, but with a much lower intensity compared to earlier periods