Cyclic structure of the isotopic diagram of the lithalsa and its radiocarbon age, Sentsa River valley, Eastern Sayan

The lithalsa, located on the first terrace of the Sentsa River in Okinsky district of the Republic of Buryatia, were studied. The most typical features of isotopic diagrams, obtained from the ice core from the 20-meter borehole, is their stage structure: three isotope minima at depths of approximately: 3-5, 9-12, and 18-21 m are separated by two distinct isotope maxima. Most likely, this is due to the cycle of the triple flooding of the growing permafrost mound and the subsequent active evaporation of the lake water - ​the main source of water in the ice core of lithalsa. Based on the detailed radiocarbon dating of organic material from the upper layers of the highest and surrounding lithalsas, the time of their formation was established. The active accumulation of organic material in dried heaving soils occurred from 0,5 to 0,2 ka BP. This is the time of the beginning of the formation of permafrost, active ice formation and the associated heaving process and lithalsa growth. The age of at least 3 of 4 studied mounds is not older than 200 years, it coincides with the cooling in the early 19th century.