Leaf wax <i>n</i>-alkane patterns and compound-specific <i>δ</i><sup>13</sup>C of plants and topsoils from semi-arid and arid Mongolia

Abstract

Abstract. Leaf wax n-alkane patterns and their compound-specific δ13C signatures are valuable proxies for paleoenvironmental reconstructions. So far, their potential has not been investigated in semi-arid to arid Mongolia. We have therefore analysed the leaf wax n-alkanes and their compound-specific δ13C signature of five plant species (Poaceae, Cyperaceae, Artemisia spp., Caragana spp. and Larix sp.) and topsoils (0–5 cm) along two transects in central and southern Mongolia. Grasses show a distinct dominance of the n-C31 homologue, whereas the shrubs Caragana spp. and Artemisia spp. are dominated by n-C29. Larix sp. is characterised by the mid-chain n-alkanes n-C23 and n-C25. From plant to topsoil, n-alkane patterns show the potential to differentiate between grass-covered sites from those covered by Caragana spp. n-Alkane concentrations and odd-over-even predominance (OEP) of the topsoils are distinctly influenced by mean annual temperature, mean annual precipitation and aridity, likely reflecting the degree of n-alkane degradation and biomass production. In contrast, the average chain length (ACL) and the n-alkane ratio (n-C31∕n-C29+n-C31) are not affected by climatic parameters and, thus, are not biased by climate. The compound-specific δ13C signatures are strongly correlated to climate, showing a significant enrichment with increasing aridity, indicating the effect of water use efficiency. Our calibration results suggest that long-chain n-alkanes and their compound-specific δ13C signatures have great potential to reconstruct paleoenvironmental and paleoclimatic conditions when used in sediment archives from Mongolia.

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

“…Along the investigated transect δ 2 H n-C29 is on average ∼15 more positive than δ 2 H n-C31 . As described previously by Struck et al (2020), n-C 29 is the most abundant homolog in the woody shrubs Caragana spp. and Artemisia spp., whereas n-C 31 is the most abundant homolog in grasses.…”

“…and the woody shrub Caragana spp. Larix sibirica occurred at a few sites (for more details, see Struck et al, 2020). The investigated transect follows a west-east and north-south gradient with increasing aridity and 2 H p and 18 O p enrichment toward eastern and southern Mongolia ( Figures 1A,B, 2 -all sampling sites and respective climate parameters are listed in the Supplementary Material).…”

“…While the n-alkanes had been extracted and quantified in a previous study (Struck et al, 2020) and were available and ready for compound-specific δ 2 H analyzes, we have selected 28 sampling sites for additional sugar analyzes. Hemicellulosederived sugars were hydrolytically extracted from 0.1 to 1.4 g topsoil material using 10 ml of 4M trifluoroacetic acid at 105 • C for 4 h according to Amelung et al (1996).…”

“…dataset, (ii) the analytical errors of the δ 2 H n-alkane and δ 18 O ara measurements, (iii) micro-climatic effects along the investigated transect, and (iv) metabolic differences affecting ε bio (Sachse et al, 2012;Fick and Hijmans, 2017;Cormier et al, 2018;Hou et al, 2018;Bowen, 2019;Strobel et al, 2020;Struck et al, 2020).…”

“…Assuming constant ε bio values of −160 for leaf wax n-alkanes (Sessions et al, 1999;Hepp et al, 2020), evapotranspirative enrichment would be ∼31 for δ 2 H n-C29 and ∼15 for δ 2 H n-C31 (Figure 4). While n-C 29 is the most abundant homolog in shrubs, and n-C 31 is the most abundant homolog in grasses (Bliedtner et al, 2018;Struck et al, 2020) the observed offset in evapotranspirative enrichment most likely results from plant physiological differences described above, particularly the dampening effect. Assuming a constant ε bio factor of +27 for arabinose (Lehmann et al, 2017;Hepp et al, 2020) evapotranspirative enrichment would be ∼14 for δ 18 O ara .…”