Calibration and paleohydrological application of compound-specific isotope analyses (δ13Cwax, δ2Hwax and δ18Osugar) in semi-arid/arid Mongolia

Several lake sediment studies have investigated the Holocene climate history in Mongolia using pollen, organic and inorganic elemental analyses. However, these studies come to very different conclusions. Isotope analyses, particularly compound-specific carbon, hydrogen and oxygen isotopic composition of leaf wax n-alkanes (δ¹³C_wax, δ²H_wax) and hemicellulose sugars (δ¹⁸O_sugar) are increasingly used for paleoenvironmental and -hydrological reconstructions and might have great potential to address the controversies in Mongolia.

Here we present a regional calibration of δ¹³C_wax, δ²H_wax and δ¹⁸O_sugar on topsoils along a distinct climate gradient in semi-arid/arid Mongolia. δ¹³C_wax significantly correlates with aridity indicating variations in water use efficiency. The apparent fractionation (Ɛ_app) of δ²H_wax and δ¹⁸O_sugar is nearly constant at -131 ± 13‰ for Ɛ_n-C29/p, -148 ± 11‰ for Ɛ_n-C31/p and 40.8 ± 1.9‰ for Ɛ_sugar/p, respectively. δ²H_wax (n-C₂₉ and n-C₃₁) and δ¹⁸O_sugar thus, reflect the isotopic composition of precipitation, which in turn is controlled by atmospheric circulation systems bringing moisture to continental Mongolia, i.e. the interaction between the Westerlies and the Asian Summer Monsoon. Therefore, we applied regionally calibrated δ¹³C_wax, δ²H_wax and δ¹⁸O_sugar isotopes, as well as well-established sedimentological and geochemical proxies and δ¹³C_org, δ¹³C_carb, δ¹⁸O_carb on a 160 cm long gravity core from Lake Telmen (Central Mongolia) that covers 4,110 ⁺³⁵⁰/_‑340 cal. a BP.

Low terrestrial input (e.g. low Al, Fe, K, Sr) suggests decreased runoff and points to overall dryer conditions in the area around Lake Telmen between 4,110 ⁺³⁵⁰/_‑340 and 3,040 ⁺⁶¹⁰/_‑400 cal. a BP. Those findings are in line with positive δ²H_n-C23, δ¹⁸O_sugar and δ¹⁸O_carb, which indicate enhanced lake water evaporation. From 3,040 ⁺⁶¹⁰/_‑400 to 1,360 ⁺²³⁰/_‑220 cal. a BP, high terrestrial input and more negative δ²H_n-C23, δ¹⁸O_sugar and δ¹⁸O_carb values indicating more humid conditions. This is in line with seismic results which reveal distinct subaqueous cliffs and an extreme lake level rise beginning at ~ 2,000 cal. a BP. Drier conditions and low lake levels occurred between 1,360 ⁺²³⁰/_‑220 and 700 ⁺²¹⁰/_‑180 cal. a BP and are indicated by low Ca/Mg ratios and a distinct enrichment in ¹³C_wax, ²H_wax,¹⁸O_sugar and ¹⁸O_carb. From 700 ⁺²¹⁰/_‑180 cal. a BP onwards, drier conditions continue but the terrestrial input increases possibly reflecting anthropogenic impact.