Chemotaxonomic patterns of vegetation and soils along altitudinal transects of the Bale Mountains, Ethiopia, and implications for paleovegetation reconstructions – Part II: lignin-derived phenols and leaf-wax-derived <i>n</i>-alkanes

Abstract

How to cite:Lemma, B., Mekonnen, B., Glaser, B., Zech, W., Nemomissa, S., Bekele, T., Bittner, L., and Zech, M.: Chemotaxonomic patterns of vegetation and soils along altitudinal transects of the Bale Mountains, Ethiopia, and implications for paleovegetation reconstructions -Part II: lignin-derived phenols and leaf-wax-derived n-alkanes, E&G Quaternary Sci. J., 68, 189-200, https://doi.org/10.5194/egqsj-68-189-2019, 2019. Abstract:Erica is a dominant vegetation type in many sub-afroalpine ecosystems, such as the Bale Mountains in Ethiopia. However, the past extent of Erica is not well known and climate versus anthropogenic influence on altitudinal shifts are difficult to assign unambiguously, especially during the Holocene.The main objective of the present study is to chemotaxonomically characterize the dominant plant species occurring in the Bale Mountains using lignin phenols and n-alkane biomarkers and to examine the potential of those biomarkers for reconstructing vegetation history. Fresh plant material, organic layer and mineral topsoil samples were collected along a northeastern and a southwestern altitudinal transect (4134-3870 and 4377-2550 m a.s.l., respectively). Lignin-derived vanillyl, syringyl and cinnamyl phenols were analyzed using the cupric oxide oxidation method. Leaf-wax-derived nalkanes were extracted and purified using Soxhlet and aminopropyl columns. Individual lignin phenols and n-alkanes were separated by gas-chromatography and detected by mass spectrometry and flame ionization detection, respectively.We found that the relative contributions of vanillyl, syringyl and cinnamyl phenols allow us to chemotaxonomically distinguish contemporary plant species of the Bale Mountains. Erica in particular is characterized by relatively high cinnamyl contributions of > 40 %. However, litter degradation strongly decreases the lignin phenol concentrations and completely changes the lignin phenol pat-Published by Copernicus Publications on behalf of the Deutsche Quartärvereinigung (DEUQUA) e.V. 190 B. Lemma et al.: Lignin-derived phenols and leaf-wax-derived n-alkanes terns. Relative cinnamyl contributions in soils under Erica were < 40 %, while soils that developed under Poaceae (Festuca abyssinica) exhibited relative cinnamyl contributions of > 40 %.Similarly, long-chain n-alkanes extracted from the leaf waxes allowed for differentiation between Erica versus Festuca abyssinica and Alchemilla, based on lower C 31 / C 29 ratios in Erica. However, this characteristic plant pattern was also lost due to degradation in the respective O layers and A h horizons. In conclusion, although in modern-day plant samples a chemotaxonomic differentiation is possible, soil degradation processes seem to render the proxies unusable for the reconstruction of the past extent of Erica on the Sanetti Plateau, Bale Mountains, Ethiopia. This finding is of high relevance beyond our case study. Kurzfassung:Erica prägt als dominante Pflanzengattung viele Subafro-alpine Ökosysteme, so auch die Bale Berge in Äthiopien. Das Ausmaß d...

Chemotaxonomic patterns of vegetation and soils along altitudinal transects of the Bale Mountains, Ethiopia, and implications for paleovegetation reconstructions – Part 1: stable isotopes and sugar biomarkers

“…As a result, soils under Alchemilla and Festuca yield (G + M) / (A + X) ratios similar to those of Erica leaves. This has severe implications for paleovegetation reconstructions based on sugar biomarkers and resembles degradation problems reported for lignin-derived phenol and leaf-waxderived n-alkane biomarkers (Lemma et al, 2019a;Zech et al, 2012). In the case of the Bale Mountains, only fresh leaves or leaf material that has undergone little degradation (as it may hold true in anoxic lacustrine sediments) allow a chemotaxonomic differentiation between Erica and other dominant vegetation types, such as Alchemilla, Festuca and Helichrysum.…”

“…Given that pollen preservation is often poor in soils due to oxidation (Brewer et al, 2013;Hevly, 1981;Li et al, 2007) there have been large efforts during the last decades toward developing proxies based on organic molecules that are specific to certain plant and vegetation types (chemotaxonomy) in order to contribute to the reconstruction of vegetation changes. This is mostly done with lipid biomarkers (Jansen et al, 2006a(Jansen et al, , b, 2008; see our companion paper, Lemma et al, 2019a, for further details). In addition, Kebede et al (2007) have used amplified fragment length polymorphisms (AFLPs) to infer the phylogeographical history of the afromontane species (Lobelia giberroa) in the Bale Mountains.…”

“…The objectives of this study were to evaluate (i) whether the dominant plant and vegetation types of the afroalpine region of the Bale Mountains can be distinguished chemotaxonomically based on their stable carbon and nitrogen isotopic composition and/or sugar biomarkers and (ii) whether the isotope and biomarker patterns of plants are reflected in the soils below correspond to contemporary plants. Note that in our companion paper (Lemma et al, 2019a) we address the same questions but instead focusing on other biomarkers, namely lignin-derived phenols and leaf-wax-derived nalkanes. Overall the results of our two companion studies were meant to form a modern-day calibration for reconstructing the Late Quaternary vegetation history (mainly of Erica) on the Sanetti Plateau of the Bale Mountains.…”

Revisiting the subalpine Mesolithic site Ullafelsen in the Fotsch Valley, Stubai Alps, Austria – new insights into pedogenesis and landscape evolution from leaf-wax-derived &lt;i&gt;n&lt;/i&gt;-alkanes, black carbon and radiocarbon dating

“…For instance, black carbon (BC) based on the analyses of benzene polycarboxylic acids (BPCAs) as organic marker molecules can serve as a proxy for firederived carbon and aromatic-structured molecules in general even if char particles are not preserved (Glaser et al, 1998(Glaser et al, , 2001Glaser and Birk, 2012;Lehndorff et al, 2015). Similarly, plant leaf-wax-derived n-alkane biomarkers are preserved in soils and sediments over at least glacialinterglacial timescales, have potential in chemotaxonomic studies and thus allow us to reconstruct vegetation changes from palaeoenvironmental archives even if the preservation of pollen is poor (Zech et al, 2012;Lemma et al, 2019;Trigui et al, 2019). Moreover, compound-class radiocarbon dating of bulk n-alkanes was developed during the last years and allows us to address both potential pre-ageing and potential post-depositional rejuvenation of n-alkanes in sedimentary archives (Zech et al, 2017;Lerch et al, 2018;Bliedtner et al, 2020).…”

“…Similarly, plant leaf-wax-derived n-alkane biomarkers are preserved in soils and sediments over at least glacialinterglacial timescales, have potential in chemotaxonomic studies and thus allow us to reconstruct vegetation changes from palaeoenvironmental archives even if the preservation of pollen is poor (Zech et al, 2012;Lemma et al, 2019;Trigui et al, 2019). Moreover, compound-class radiocarbon dating of bulk n-alkanes was developed during the last years and allows us to address both potential pre-ageing and potential post-depositional rejuvenation of n-alkanes in sedimentary archives (Zech et al, 2017;Lerch et al, 2018;Bliedtner et al, 2020). Last but not least, other molecular lipid biomarkers allow us to infer, for instance, dietary information in geoarchaeological studies (Grillo et al, 2020), and the stable nitrogen isotopic composition (δ 15 N) of soils allows us to infer the human-induced opening of the N cycle (Zech et al, 2011a).…”