04 Nov 2019
 | 04 Nov 2019
Status: this discussion paper is a preprint. It has been under review for the journal Biogeosciences (BG). The manuscript was not accepted for further review after discussion.

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

Johannes Hepp, Bruno Glaser, Dieter Juchelka, Christoph Mayr, Kazimierz Rozanski, Imke Kathrin Schäfer, Willibald Stichler, Mario Tuthorn, Roland Zech, and Michael Zech

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.

Status: closed
Status: closed
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
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Status: closed
Status: closed
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
Printer-friendly Version - Printer-friendly version Supplement - Supplement

Johannes Hepp et al.

Johannes Hepp et al.


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