LATE GLACIAL AND HOLOCENE ENVIRONMENTAL CHANGES IN THE NORTH OF CENTRAL MONGOLIA (DARKHAN-SELENGE AREA)

A study of landscape and climate changes over the last 14000 cal year BP in Northern Mongolia is presented in the paper, based on a comprehensive analysis of friable sediments and seventeen AMS 14C dates from two sections in the Orkhon River basin. The Orkhon and Darkhan sections are located in similar geomorphological conditions on the first above-floodplain river terrace but have some differences. The Orkhon section is located directly at the riverbed, not far from the mountain range, while the Darkhan section is located in a ravine that cuts the terrace surface, at a distance from the river and mountains. This geomorphological difference causes the completeness and complexity of the paleoarchives in the studied sections. Both sections have a thick soil-sediment sequence with several heterochronous paleosols separated by sediments of varying genesis (mainly aeolian and fluvial). The paleosols were formed during periods of slow relief transformation, reflecting the most optimal bioclimatic conditions. The interlaying sediments (sandy-silty layers) display the phases of soil degradation due to frequent droughts and increased aeolian processes, with sporadic and catastrophic rainfalls (horizontally and lenticularly layered strata with a large amount of detritus and gravel). According to our data, the soil profile that was formed in the Late glacial period is characterized by a humid type of soil formation, and according to palynological data, meadow vegetation was widespread. In the Early Holocene, there was one stage of optimal conditions (increased heat and moisture) proper for soil development in Northern Mongolia. In the Middle Holocene, there are at least three stages. Finally, there were two stages with increased climate humidity in the Late Holocene. In the Early Holocene, Northern Mongolia had more humid forest-steppe conditions, which were replaced by steppe conditions in the Middle Holocene, when the climate changed more significantly. An expansion of forest vegetation (pine forests), probably on the northern slopes, is noted in the pollen spectra of Middle Holocene soils. Steppe and semi-desert landscapes predominated. Several stages of enhanced aridization were reconstructed: about 3800-4000, between 8400-8000 years BP. In the Darkhan section, the period of 8394-2775 years was a significant hiatus in soil formation. About 50 cm of sand accumulated over 5600 years, and there are no soils in this layer. Plausibly, the surface was essentially denudated before the second paleosol formation. At the same time, the Orkhon section accumulated aeolian sediment more than 70 cm thick. It suggests the existence of a period of severe droughts at the end of the Middle Holocene (after 3000 years BP). In the Late Holocene, climate humidification was reconstructed, when meadow steppes predominated again. The uppermost paleosol in the Darkhan section was formed in the second half of the Late Holocene.