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 southeastern 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 = +41 ± 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 1.2 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.

Organic Molecular Paleohypsometry: A New Approach to Quantifying Paleotopography and Paleorelief

“…In addition, the magnitude of change in n-alkane δ 2 H observed on the SE Tibetan Plateau margin mirrors that observed in other regional studies from the SE Asian region (Jia et al, 2008;Bai et al, 2015). Some studies suggest that the relationship between plant wax δ 2 H and ambient water may vary as a function of climate and/or elevation in some circumstances, however this remains a subject of debate with recent studies showing no climatic effect on apparent fractionation (Struck et al, 2020), and is similar to questions surrounding uncertainties in water of formation for authigenic minerals and glasses that are commonly used for paleoelevation reconstruction. Plant waxes, including long-carbon chain normal alkanes, can thus provide a geochemical record of ambient water δ 2 H with similar or less uncertainty than other commonly used paleoprecipitation isotope proxies such as soil carbonates, clays, or volcanic glass.…”

“…There are a number of factors that can influence the apparent fractionation between plant waxes and ambient water, including plant-specific differences in evapotranspiration and biosynthesis (e.g., Sachse et al, 2012), and some data suggests potential changes in apparent fractionation with elevation (Bai et al, 2017). However, other data suggests no clear correlation between climate and apparent fractionation (Struck et al, 2020) and numerous studies of biomarkers in soils show a generally consistent fractionation between water and wax (Jia et al, 2008;Tipple and Pagani 2013;Bai et al, 2015;Zhuang et al, 2015;Liu et al, 2016;Vogts et al, 2016;Bai et al, 2017;Wang et al, 2017;Liu and An 2019). Here we present new organic molecular biomarker and water isotope data from soils and rivers of Taiwan to test whether organic biomarker isotopes can reflect catchment integrated signatures of catchment hypsometry.…”

Precipitation and Lake Water Evaporation Recorded by Terrestrial and Aquatic n‐Alkane δ2H Isotopes in Lake Khar Nuur, Mongolia

“…Terrestrial leaf waxes incorporate the δ 2 H signal of the local precipitation (δ 2 H P ) during the growing season through the uptake of soil water by the plants (Sachse et al, 2012). The observed n-alkane δ 2 H values of topsoils in the Khar Nuur catchment are in good agreement with previous findings of Struck et al (2020), who investigated δ 2 H C31 in Mongolian topsoils. Struck et al (2020) found that the apparent fractionation, that is, the isotopic difference of δ 2 H P and δ 2 H C31 , is constant at −146 ± 14‰.…”

Leaf Wax Hydrogen Isotopes as a Hydroclimate Proxy in the Tropical Pacific

“…Tropical Pacific n-C29-alkane δ 2 H values ranged from −177 to −139‰, while those of n-C28-acid ranged from −175 to −119‰ (Table 1; Figure 3). Adding these new measurements to an updated global compilation of δ 2 HWax values from all available surface sediment and soil data sets in non-marine settings (compilations from McFarlin et al, 2019, as well as data from Nelson, 2013;Bakkelund et al, 2018;Feng et al, 2019;Goldsmith et al, 2019;Li et al, 2019;Wu et al, 2019;Daniels et al, 2020;Lu et al, 2020;Struck et al, 2020;van der Veen et al, 2020) has minimal impact on the slope, y-intercept, or correlation coefficients for the global linear regression (Figure 3).…”