Climate Changes Reflected in the Carbon and Oxygen Isotope Composition of Holocene Carbonates of Lake Tere-Khol, Tyva (Southern Siberia)

Bottom sediments of Lake Tere-Khol located in the southeast of the Sayan–Tuva Highland retained a detailed archive of landscape and climatic changes throughout the Holocene. This region is situated at the boundary between South Siberia and Central Asia and marks a transitional zone in terms of the factors that determined the hydroclimatic changes during the Holocene. The climate was mainly controlled by the western transport of Atlantic air masses north and west from the region and by the Asian-Pacific monsoon circulation to the south and east. The reconstruction of Holocene climatic changes in this region is crucial for understanding the dynamics of atmospheric circulation across the Eurasian continent’s interior. Stable isotope analyses in lacustrine carbonates were conducted to evaluate the Holocene humidity fluctuations. The water of Lake Tere-Khol is notably enriched in 18O and 2H by 6–8 and 50–60‰, respectively, compared to streams and rivers flowing in it, thus indicating a strongly evaporative water body. This suggests that the δ18O variations (14.1–20.0‰ SMOW) and their positive correlation with δ13C variations (–5.8 to 4.2‰ PDB) in the dispersed carbonate material of Holocene lacustrine sediments primarily reflect shifts in the hydrological regime. The positive δ18O and δ13C excursions correspond to the periods of aridification, while the negative excursions indicate phases of relative humidity. Three main humidity epochs were identified in the Holocene: relatively dry epochs from the initial Holocene to ~9.8 ka BP and from ~4.4 ka BP to the present, and a humid epoch between ~9.8 and ~4.4 ka BP. These phases were accompanied by a change of the second-order humidity. Thereby, the variability and amplitude of humidity fluctuations significantly increased in the second half of the Holocene, after approximately 6 ka BP. The most humid interval in the Holocene occurred between ~5.2 and ~4.4 ka BP. The sharp and significant aridification, the major hydroclimatic event in the Holocene, occurred at ~4.4 ka BP. The driest periods were observed between ~4.2 and ~3.1 ka BP and from ~1.9 to ~0.1 ka BP. At the turn of the eras around 2 ka BP and in the past century, these dry conditions were interrupted by short episodes of relative humidity. This Late Holocene aridification points to the weakening of the Pacific monsoon and the decrease of its penetration into the Eurasia’s interior, which aligns with cooling trends observed in the second half of the Holocene.