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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-2019-427
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2019-427
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 04 Nov 2019

Submitted as: research article | 04 Nov 2019

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

Validation of a coupled δ2Hn-alkane18Osugar paleohygrometer approach based on a climate chamber experiment

Johannes Hepp1,2,a, Bruno Glaser2, Dieter Juchelka3, Christoph Mayr4,5,b, Kazimierz Rozanski6, Imke Kathrin Schäfer7, Willibald Stichler8, Mario Tuthorn3,c, Roland Zech7,9,d, and Michael Zech2,10,e Johannes Hepp et al.
  • 1Chair of Geomorphology and BayCEER, University of Bayreuth, Universitätsstrasse 30, 95440 7Bayreuth, Germany
  • 2Institute of Agronomy and Nutritional Sciences, Soil Biogeochemistry, Martin-Luther-University Halle-Wittenberg, Von-Seckendorff-Platz 3, 06120 Halle (Saale), Germany
  • 3ThermoFisher Scientific, Hanna-Kunath-Str. 11, 28199 Bremen, Germany
  • 4Institute of Geography, Friedrich-Alexander-University Erlangen-Nürnberg, Wetterkreuz 15, 91058 12Erlangen, Germany
  • 5GeoBio-Center & Earth and Environmental Sciences, Ludwig-Maximilian University Munich, Richard-Wagner-Str. 10, 80333 München, Germany
  • 6Faculty of Physics and Applied Computer Science, AGH University of Science and Technology, Al. Mickiewicza 30, 30-059 Kraków, Poland
  • 7Institute of Geography and Oeschger Centre for Climate Research, University of Bern, Hallerstrasse 12, 3012 Bern, Switzerland
  • 8Helmholtz Zentrum München, German Research Center for Environmental Health, Ingolstädter Landstrasse 1, 85764 Neuherberg, Germany
  • 9Institute of Geography, Chair of Physical Geography, Friedrich-Schiller University of Jena, Löbdergraben 32, 07743 Jena, Germany
  • 10Institute of Geography, Heisenberg Chair of Physical Geography with focus on paleoenvironmental research, Technical University of Dresden, Helmholtzstrasse 10, 01062 Dresden, Germany
  • apresent address: Chair of Geomorphology and BayCEER, University of Bayreuth, Universitätsstrasse 30, 95440 Bayreuth, Germany
  • bpresent address: Institute of Geography, Friedrich-Alexander-University Erlangen-Nürnberg, Wetterkreuz 15, 91058 Erlangen, Germany
  • cpresent address: ThermoFisher Scientific, Hanna-Kunath-Str. 11, 28199 Bremen, Germany
  • dpresent address: Institute of Geography, Chair of Physical Geography, Friedrich-Schiller University of Jena, Löbdergraben 32, 07743 Jena, Germany
  • epresent address: Institute of Geography, Heisenberg Chair of Physical Geography with focus on paleoenvironmental research, Technical University of Dresden, Helmholtzstrasse 10, 01062 Dresden, Germany

Abstract. The hydrogen isotopic composition of leaf wax-derived biomarkers, e.g. long chain n-alkanes (δ2Hn-alkane), is widely applied in paleoclimatology research. However, a direct reconstruction of the isotopic composition of paleoprecipitation based on δ2Hn-alkane alone can be challenging due to the overprint of the source water isotopic signal by leaf-water enrichment. The coupling of δ2Hn-alkane with δ18O of hemicellulose-derived sugars (δ18Osugar) has the potential to disentangle this effect and additionally allow relative humidity reconstructions. Here, we present δ2Hn-alkane as well as δ18Osugar results obtained from leaves of the plant species Eucalyptus globulus, Vicia faba var. minor and Brassica oleracea var. medullosa, which were grown under controlled conditions. We addressed the questions (i) do δ2Hn-alkane and δ18Osugar values allow precise reconstructions of leaf water isotope composition, (ii) how accurately does the reconstructed leaf-water-isotope composition enables relative humidity (RH) reconstruction in which the plants grew, and (iii) does the coupling of δ2Hn-alkane and δ18Osugar enable a robust source water calculation?

For all investigated species, the alkane n-C29 was most abundant and therefore used for compound-specific δ2H measurements. For Vicia faba, additionally the δ2H values of n-C31 could be evaluated robustly. With regard to hemicellulose-derived monosaccharides, arabinose and xylose were most abundant and their δ18O values were therefore used to calculate weighted mean leaf δ18Osugar values. Both δ2Hn-alkane and δ18Osugar yielded significant correlations with δ2Hleaf-water and δ18Oleaf-water, respectively (r2 = 0.45 and 0.85, respectively; p < 0.001, n = 24). Mean fractionation factors between biomarkers and leaf water were found to be −156 ‰ (ranging from −133 to −192 ‰) for εn-alkane/leaf-water and +27.3 ‰ (ranging from +23.0 to 32.3 ‰) for εsugar/leaf-water, respectively. Using rearranged Craig-Gordon equations with either Tair or Tleaf and measured δ2Hleaf-water or δ18Oleaf-water as input variables, we furthermore modeled climate chamber RHair and RHleaf values. Modelled RHair values, from the more simplified Craig-Gordon model, turned out to be most accurate and correlate highly significantly with measured RHair values (R2 = 0.84, p < 0.001; RMSE = 6 %). When combining δ2Hleaf-water and δ18Oleaf-water values that are calculated from the alkane and sugar biomarkers instead of actually measured δ2Hleaf-water and δ18Oleaf-water as input variables, the correlation of modelled RHair values with measured RHair values is getting worse, but is still highly significant with R2 = 0.54, p < 0.001; RMSE = 10 %. This highlights the potential of the coupled δ2Hn-alkane18Osugar paleohygrometer approach for suitable relative humidity reconstructions. Finally, the reconstructed source water isotope composition (δ2Hs and δ18Os) as calculated from the coupled approach matches the source water in the climate chamber experiment (δ2Htank-water and δ18Otank-water).

Johannes Hepp et al.
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