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

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Resumo

The bottom sediments of Lake Tere-Khol, located in the southeast of the Sayano-Tuvinian Highlands, contain a detailed archive of landscape and climatic changes throughout the Holocene. Situated at the boundary between South Siberia and Central Asia, this region marks a transitional zone in terms of the factors driving hydroclimatic changes during the Holocene. To the north and west, the western transport of Atlantic air masses exerted a significant influence, while to the south and east, the Asian-Pacific monsoon circulation prevailed. Reconstructing Holocene climate changes in this region is crucial for understanding atmospheric circulation shifts across the Eurasian continent’s interior. Stable isotope analyses in lake carbonates were conducted to evaluate Holocene moisture fluctuations. The water of Lake Tere-Khol is notably enriched in 18O by 6–8‰ and ²H by 50–60‰ compared to its inflowing streams and rivers, indicating a strongly evaporative water body. This suggests that variations in δ18O (14.1 to 20.0‰ SMOW) and their positive correlation with δ¹³C variations (–5.8 to 4.2‰ PDB) in the dispersed carbonate material of Holocene lake sediments primarily reflect shifts in the hydrological regime. Positive δ18O and δ¹³C excursions correspond to periods of aridification, while negative excursions indicate phases of relative humidity. Three primary humidity epochs were identified in the Holocene. These include a relatively dry phase from the onset of the Holocene until 9.8 ka BP and from 4.4 ka BP to the present, with a humid phase in between, spanning from 9.8 to 4.4 ka BP. Superimposed on these major trends were second-order humidity changes, with variability and the amplitude of fluctuations notably intensifying in the latter half of the Holocene, after approximately 6 ka BP. The wettest interval occurred between 5.2 and 4.4 ka BP, while a sharp and substantial shift towards arid conditions around 4.4 ka BP stands out as the most significant hydroclimatic event of the Holocene. 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, short episodes of relative wetting interrupted these dry conditions. This late Holocene aridification trend points to a weakening of the Pacific monsoon and a reduction in its reach into Eurasia’s interior, which aligns with cooling trends observed in the latter part of the Holocene.

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Sobre autores

B. Pokrovskiy

Geological Institute, Russian Academy of Sciences

Autor responsável pela correspondência
Email: pokrov@ginras.ru
Rússia, Moscow

A. Panin

Institute of Geography, Russian Academy of Sciences

Email: pokrov@ginras.ru
Rússia, Moscow

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2. Fig. 1. Bathymetric map of the lake. A table with the location of the sampling points. 1 – well Pb-21; 2-4 – sampling points for isotopic composition of water: 2 – Lake. Tere-Khol, 3 – the Saldam river flowing out of the lake, 4 – rivers and streams flowing into the lake. The inset shows the location of the lake on the map of Asia. Tere-Khol (1) and some other lakes mentioned in the text: 2-4 – Baikal and Transbaikalia: 2 – Upper White, 3 – Tsagan-Tyrm, 4 – Kotokel, Buryatia; 5 – Gun Nuur, Northern Mongolia; 6-8 – Northern Tibet: 6 – Kukunor, or Qinghai (Qinghai), 7 – Genggahai (Genggahai), 8 – Harleg (Hurleg); 9 – Issyk-Kul, Northern Tien Shan; 10 – Van, Turkey, Asia Minor.

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3. Fig. 2. Paleohydrological and paleoclimatic reconstructions based on lake and coastal deposits of the lake. There is a hole. (a) – lithological column: 1 – loam, 2 – organomineral silt, 3 – organogenic carbonate silt (sapropel), 4 – carbonate silt; (b) is an indicator of the relative abundance of Kd diatoms (see explanations in the text); (c) – local ecozones according to paleoalgological analysis and their paleohydrological interpretation (Panin et al., 2012): 5-8 – lake flow rate: 5 – relatively high (zones II, IV, VI), 6 – relatively low (zones III, V), 7 – very low (zone VII), 8 – alluvial environment (zone I); (d) – paleoclimatic phases of the second half of the Holocene according to spore-pollen analysis (Borisova and Panin, 2019; Borisova et al., 2021): 9-12 – temperature: 9 – warm, 10 – relatively warm, 11 – cool, 12 – cold; 13-16 – humidity: 13 – humid, 14 – relatively humid, 15 – relatively dry, 16 – dry.

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4. Fig. 3. Age model of lake sediments of the Pb21 well according to Table 1, constructed in the R Bacon program.

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5. 4. Isotopic composition of oxygen and hydrogen in water. 1 – Lake Tere-Khol, 2 – the Saldam river flowing out of the lake, 3 – the Kungur-Tuk River flowing into the lake, 4 – small streams and rivers flowing into the lake. The solid line is the global trend of atmospheric precipitation: dD = 8 × d18O + 10 (Craig, 1961), the dotted line is the trend of lake evaporation. There is a hole.

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6. 5. Electron microscopic photographs of carbonate material from bottom sediments of the lake. Tere-Khol, well Pb-21.

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7. 6. Variations in the isotopic composition of carbon and oxygen in the context of Holocene lake sediments. There is a hole. (a) – lithological column (see symbols in Fig. 2); (b, c) – values of d13C and d18O in carbonates, respectively: 1a, 2a – gross samples; 1b, 2b – ostracod shells; (d) – Mg/Ca values in gross carbonate samples; (e) – hydroclimatic periodization according to isotopic data: 3, 5 – first–order events: 3 – wet, 5 – arid; 3-6 – second–order events: 3 – relatively wet, 4 – maximum moisture, 5 - relatively arid, 6 - maximum aridity.

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8. 7. The ratio of the isotopic composition of oxygen (a) and carbon (b) in dispersed carbonate and ostracod shells from one well core sample.

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9. 8. Changes in temperature (a) and precipitation (b) in Kyzyl over the past 80 years (http://www.pogodaiklimat.ru/climate.php .). 1 – average values for the warm season (April–September); 2 – average values for the cold season (October–March).

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10. Fig. 9. The ratio of carbon and oxygen isotopic composition in the gross carbonate samples from the bottom sediments of the lake. There is a hole. 1 is the base of the section, 2 is the Lower Holocene, 3 is the Middle Holocene, and 4 is the Upper Holocene. The arrows show: the values of d18O in calcite in equilibrium with modern lake and modern river water during fractionation of d18O (calcite–H2O) = 28 ± 1%, corresponding to ~25 ± 5 °C (Kim, O'Neil, 1997); the values of d13C in calcite in equilibrium with carbon dioxide of the pre-industrial atmosphere (Sundquist, Visser, 2003; Graven et al., 2017) with fractionation of d13C (CO2–calcite) = 10 ± 0.5% (Deines et al., 1974), corresponding to ~25 ± 5 °C.

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11. 10. Changes in oxygen isotopic composition in carbonates of some lakes of Central Asia in the Holocene. 1 – Tere-Khol (this work); 2 – Upper Beloe, Buryatia (Solotchina et al., 2012); 3 – Tsagan-Tyrm (Sklyarov et al., 2010b); 4 – Kotokel (Kostrova et al., 2012); 5 – Gun Nuur, Northern Mongolia (Zhang et al., 2012); 6 – Qinghai, Tibet, North China (Liu et al., 2007); 7 – Genggai, ibid. (Qiang et al., 2017); 8 – Harleg, ibid. (Ma et al., 2021); 9 – Issyk-Kul, Kyrgyzstan (Ricketts et al., 2001); 10 – Van, Turkey (Lemke and Sturm, 1997). The location of the lakes is shown in Fig. 1. All data has been converted to the SMOW standard. To construct curves 2 and 3, the original radiocarbon dates were calibrated, and the curves from the depth scale were projected onto the time scale by interpolating between the dates.

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