Abstract
Compound-specific hydrogen and oxygen isotope analyzes on leaf wax-derived n-alkanes (δ2Hn–alkane) and the hemicellulose-derived sugar arabinose (δ18Oara) are valuable, innovative tools for paleohydrological reconstructions. Previous calibration studies have revealed that δ2Hn–alkane and δ18Oara 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 δ2Hn–alkane and δ18Oara in topsoils collected along a transect through Mongolia, and we compared these values with the isotopic composition of precipitation (δ2Hp–WM and δ18Op–WM, modeled data) and various climate parameters. δ2Hn–alkane and δ18Oara are more positive in the arid south-eastern part of our transect, which reflects the fact that also the precipitation is more enriched in 2H and 18O along this part of the transect. The apparent fractionation εapp, i.e., the isotopic difference between precipitation and the investigated compounds, shows no strong correlation with climate along the transect (ε2H n–C29/p = −129 ± 14‰, ε2H n–C31/p = −146 ± 14‰, and ε18O ara/p = +44 ± 2‰). Our results suggest that δ2Hn–alkane and δ18Oara 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 δ2Hn–C29, 12.6 for δ2Hn–C31, and 2.2‰ for δ18Oara, so our findings corroborate the great potential of compound-specific δ2Hn–alkane and δ18Oara analyzes for paleohydrological research in Mongolia.
Highlights
Leaf wax-derived n-alkanes and hemicellulose-derived sugars are produced by higher terrestrial plants and stay wellpreserved in soils and sediments, because of their resistance against biochemical degradation (Eglinton and Hamilton, 1967)
These compounds and their compound-specific stable hydrogen (δ2Hn-alkane) and oxygen (δ18Osugar) isotopic composition get incorporated into soils through above-ground and root litter, abrasion, as well as grazing, and they have a mean residence time of ∼40 years, while pentoses average over ∼20 years (Schmidt et al, 2011)
Since δ2Hn-alkane and δ18Osugar are not strongly affected by degradation effects (Zech et al, 2011, 2012), they are increasingly used for paleohydrological reconstructions (Aichner et al, 2015; Hepp et al, 2015, 2019; Thomas et al, 2016; Rach et al, 2017; Schäfer et al, 2018; Bliedtner et al, 2020)
Summary
Leaf wax-derived n-alkanes and hemicellulose-derived sugars are produced by higher terrestrial plants and stay wellpreserved in soils and sediments, because of their resistance against biochemical degradation (Eglinton and Hamilton, 1967). The apparent fractionation (ε2H n-alkane/p, ε18O sugar/p), i.e., the difference between the isotopic signature of precipitation/source water (δ2Hp and δ18Op) and δ2Hn-alkane and δ18Osugar, respectively, basically integrates over εSW, εEt and εbio and results in 2H-depleted leaf wax-derived n-alkanes but 18Oenriched hemicellulose-derived sugars relative to δ2Hp and δ18Op (Sachse et al, 2012; Tuthorn et al, 2015; Daniels et al, 2017; Liu and An, 2019; Strobel et al, 2020)
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