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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2019-197
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2019-197
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 29 May 2019

Submitted as: research article | 29 May 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Biogeosciences (BG).

Evaluation of bacterial glycerol dialkyl glycerol tetraether and 2H-18O biomarker proxies along a Central European topsoil transect

Johannes Hepp1,2, Imke K. Schäfer3, Verena Lanny4, Jörg Franke3, Marcel Bliedtner3,a, Kazimierz Rozanski5, Bruno Glaser2, Michael Zech2,6, Timothy I. Eglinton4, and Roland Zech3,a Johannes Hepp et al.
  • 1Chair of Geomorphology and BayCEER, University of Bayreuth, 95440 Bayreuth, Germany
  • 2Institute of Agronomy and Nutritional Sciences, Soil Biogeochemistry, Martin-Luther-University Halle-Wittenberg, 06120 Halle, Germany
  • 3Institute of Geography and Oeschger Centre for Climate Change Research, University of Bern, 3012 Bern, Switzerland
  • 4Department of Earth Science, ETH Zurich, 8092 Zurich, Switzerland
  • 5Faculty of Physics and Applied Computer Science, AGH University of Science and Technology, 30-059 Kraków, Poland
  • 6Institute of Geography, Faculty of Environmental Sciences, Technical University of Dresden, 01062 Dresden, Germany
  • anow at: Institute of Geography, Chair of Physical Geography, Friedrich-Schiller University of Jena, 07743 Jena, Germany

Abstract. Molecular fossils, like bacterial branched glycerol dialkyl glycerol tetraethers (brGDGTs), and the stable isotopic composition of biomarkers, such as δ2H of leaf wax-derived n-alkanes (δ2Hn-alkane) or δ18O of hemicellulose-derived sugars (δ18Osugar) are increasingly used for the reconstruction of past climate and environmental conditions. Plant-derived δ2Hn-alkane and δ18Osugar values record the isotopic composition of plant source water (δ2H/δ18Osource-water), which usually reflects mean annual precipitation (δ2H/δ18Oprecipiation), modulated by evapotranspirative leaf water enrichment and biosynthetic fractionation. Accuracy and precision of respective proxies should be ideally evaluated at a regional scale. For this study, we analysed topsoils below coniferous and deciduous forests, as well as grassland soils along a Central European transect in order to investigate the variability and robustness of various proxies, and to identify effects related to vegetation. Soil pH-values derived from brGDGTs correlate reasonably well with measured soil pH-values, but systematically overestimate them (ΔpH = 0.6 ± 0.6). The branched vs. isoprenoid tetraether index (BIT) can give some indication whether the pH reconstruction is reliable. Temperatures derived from brGDGTs overestimate mean annual air temperatures slightly (∆TMA = 0.5 °C ± 2.4). Apparent isotopic fractionation (εn-alkane/precipitation and εsugar/precipitation) is lower for grassland sites than for forest sites due to "signal damping", i.e. grass biomarkers do not record the full evapotranspirative leaf water enrichment. Coupling δ2Hn-alkane with δ18Osugar allows to reconstruct the stable isotopic composition of the source water more accurately than without the coupled approach (Δδ2H = ~-21 ‰ ± 22 and Δδ18O = ~-2.9 ‰ ± 2.8). Similarly, relative humidity during daytime and vegetation period (RHMDV) can be reconstructed using the coupled isotope approach (ΔRHMDV = ~-17 ± 12). Especially for coniferous sites, reconstructed RHMDV values as well as source water isotope composition underestimate the measured values. This can be likely explained by understory grass vegetation at the coniferous sites contributing significantly to the n-alkane pool but only marginally to the sugar pool in the topsoil. The large uncertainty likely reflect the fact that biosynthetic fractionation is not constant, as well as microclimate variability. Overall, GDGTs and the coupled δ2Hn-alkane18Osugar approach have great potential for more quantitative paleoclimate reconstructions.

Johannes Hepp et al.
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Status: final response (author comments only)
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Johannes Hepp et al.
Johannes Hepp et al.
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