Leaf wax <i>n</i>-alkanes in modern plants and topsoils from eastern Georgia (Caucasus) – implications for reconstructing regional paleovegetation

Abstract

Abstract. Long-chain n-alkanes originate from leaf waxes of higher terrestrial plants, they are relatively resistant against physical and chemical degradation and are preserved in sediment archives for at least millennial timescales. Since their homologue patterns discriminate between vegetation forms, they were increasingly used for paleovegetation reconstructions during the last years. However, before any robust interpretation of the long-chain n-alkane patterns in sediment archives, reference samples from modern vegetation and topsoil material should be investigated at a regional scale. Apart from some temperate and tropical regions, such systematic regional studies on modern plant and topsoil material are still largely lacking.To test the potential of leaf wax-derived n-alkane patterns for paleoenvironmental studies in the semi-humid to semiarid central southern Caucasus region, we investigated the influence of different vegetation forms on the leaf wax n-alkane signal in modern plants and topsoil material (0-5 cm) from eastern Georgia. We sampled (i) sites with grassland/herbs that included steppe, cultivated grassland and meadows, and (ii) sites that are dominated by deciduous hornbeam forests.The results show that long-chain n-alkanes originate from leaf waxes of higher terrestrial plants and that their homologue pattern allow to discriminate between different vegetation forms: n-Alkanes derived from sites with grassland/herbs are mainly dominated by C 31 , while n-alkanes derived from sites with deciduous trees/shrubs show high abundances of C 29 . Thus, long-chain n-alkanes have a great potential when used for regional paleovegetation reconstructions. Moreover, the n-alkane distributions of the topsoils do not show correlations with mean annual temperatures and precipitation along the investigated transect. As degradation of organic matter can affect the leaf wax n-alkane distribution, we further present an updated end-member model that includes our results, accounts for degradation effects and enables semi-quantitative reconstructions of past vegetation changes in the central southern Caucasus region.

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

“…is dominated by n-C 29 . Those findings are in line with previous regional studies, which report n-C 31 being mainly produced by grasses/herbs and n-C 29 by deciduous trees and shrubs (Cheung et al, 2015;Wang et al, 2018c;Liu et al, 2018;Bliedtner et al, 2018a). Although Artemisia spp.…”

“…The average chain length (ACL) was determined after Poynter et al (1989) and is commonly used to distinguish between leaf waxes predominantly produced by deciduous trees and shrubs (n-C 27 and n-C ) and grasses and herbs (n-C 31 and n-C 33 ) (Bliedtner et al, 2018a).…”

“…1 Total lipids of the topsoils (∼ 35 g) from transect I were extracted at the University of Bern, Switzerland, using accelerating solvent extraction (Dionex ASE 200: 6.9 MPa, 100 • C) with 60 ml dichlormethane (DCM):methanol (MeOH) (9:1, v/v) over three extraction cycles as described by Schäfer et al (2016). Total lipids of topsoils (∼ 10 g) and plants (∼ 1 g) from transect II were extracted at the Friedrich-Schiller-University of Jena, Germany, using ultrasonic extraction with 20 ml DCM:MeOH (9:1, v/v) over three cycles as described by Bliedtner et al (2018a).…”

“…The need for regional calibrations has been emphasized by the fact that Bush and McInerney (2013) questioned whether leaf wax n-alkane homologue pattern can discriminate between modern vegetation forms on a global scale while several regional studies found them discriminating on a regional scale. Although the most abundant homologues differ from region to region, a good discrimination power has been reported from Europe (Schäfer et al, 2016;Zech et al, 2009Zech et al, , 2010, the Caucasus region (Bliedtner et al, 2018a), North-and South America (Diefendorf et al, 2015;Feakins et al, 2016;Lane, 2017) and the African rain forest and savanna (Vogts et al, 2009).…”

“…The relative homologue distribution of leaf wax n-alkanes has been used as a chemotaxonomic marker to differentiate among vegetation forms and thus reconstruct paleovegetation: the n-alkanes n-C 27 and n-C 29 are thought to be mainly produced by deciduous trees/shrubs, whereas n-C 31 and n-C 33 are mainly produced by grasses/herbs (Bliedtner et al, 2018a;Schäfer et al, 2016;Vogts et al, 2009). The compound-specific δ 13 C isotopes of leaf wax n-alkanes have also been used for reconstructing changes in the vegetation composition of C 3 (−20 to −35‰) and C 4 (−10 to −14‰) plants (Castañeda et al, 2009;Lane, 2017;Rao et al, 2016;Rommerskirchen et al, 2006), and give additional paleoclimatic information for C 3 plants (Aichner et al, 2010b, a;Schäfer et al, 2018).…”