Leaf Waxes and Hemicelluloses in Topsoils Reflect the δ2H and δ18O Isotopic Composition of Precipitation in Mongolia

Abstract

Compound-specific hydrogen and oxygen isotope analyzes on leaf wax-derived n -alkanes (δ 2 H n–alkane ) and the hemicellulose-derived sugar arabinose (δ 18 O ara ) are valuable, innovative tools for paleohydrological reconstructions. Previous calibration studies have revealed that δ 2 H n–alkane and δ 18 O ara reflect the isotopic composition of precipitation, but – depending on the region – may be strongly modulated by evapotranspirative enrichment. Since no calibration studies exist for semi-arid and arid Mongolia so far, we have analyzed δ 2 H n–alkane and δ 18 O ara in topsoils collected along a transect through Mongolia, and we compared these values with the isotopic composition of precipitation (δ 2 H p–WM and δ 18 O p–WM , modeled data) and various climate parameters. δ 2 H n–alkane and δ 18 O ara are more positive in the arid south-eastern part of our transect, which reflects the fact that also the precipitation is more enriched in 2 H and 18 O along this part of the transect. The apparent fractionation ε app , i.e., the isotopic difference between precipitation and the investigated compounds, shows no strong correlation with climate along the transect (ε 2H n–C29/p = −129 ± 14‰, ε 2H n–C31/p = −146 ± 14‰, and ε 18O ara/p = +44 ± 2‰). Our results suggest that δ 2 H n–alkane and δ 18 O ara in topsoils from Mongolia reflect the isotopic composition of precipitation and are not strongly modulated by climate. Correlation with the isotopic composition of precipitation has root-mean-square errors of 13.4‰ for δ 2 H n–C29 , 12.6 for δ 2 H n–C31 , and 2.2‰ for δ 18 O ara , so our findings corroborate the great potential of compound-specific δ 2 H n–alkane and δ 18 O ara analyzes for paleohydrological research in Mongolia.