Syngenetic ice wedges, isotopic composition, radiocarbon age and pollen spectra of the surrounding deposits have been analyzed during longterm study of “Bely Klyuch” key site on the first terrace of the Chara River (720 m a.s.l.) in Northern Transbaikalia. It has been revealed that ice wedges are of Holocene age and formed between 10 to 7.5 ka 14C BP. This period is identified as the first half of Holocene optimum for south Siberia. The isotopic composition of ice wedges, river and lake waters has been specified. The isotopic composition (δ18О, δD) of relict ice wedges is the lightest and amounts to –23 ‰ and –180 ‰, respectively. The isotopic composition of structured ice from sandy loam above ice wedges is –15.7 ‰ and –133.0 ‰; and that of small ice wedge in peat and sand is –15.3 ‰, –117.9 ‰, respectively.
Interpretation of the ice wedge isotope composition has shown that mean winter temperatures during cold stages of Holocene optimum were by 2–3 °С lower than today and during warm stages they were close to modern temperatures. Total winter temperatures during the coldest stages of Holocene optimum were –5100…–5700 °C·days, i. e. by 300–600 °C·days colder than in extremely severe modern winters. Total summer temperatures varied from 1300 to 1800 °C·days, i. e. it was usually slightly higher and only occasionally somewhat lower than modern ones.