Validation of a coupled δ²H_n-alkane-δ¹⁸O_sugar paleohygrometer approach based on a climate chamber experiment

Abstract. The hydrogen isotopic composition of leaf wax-derived biomarkers, e.g. long chain n-alkanes (δ²H_n-alkane), is widely applied in paleoclimatology research. However, a direct reconstruction of the isotopic composition of source water based on δ²H_n-alkane alone can be challenging due to the alteration of the soil water isotopic signal by leaf-water heavy-isotope enrichment. The coupling of δ²H_n-alkane with δ¹⁸O of hemicellulose-derived sugars (δ¹⁸O_sugar) has the potential to disentangle this effect and additionally to allow relative humidity reconstructions. Here, we present δ²H_n-alkane as well as δ¹⁸O_sugar results obtained from leaves of the plant species Eucalyptus globulus, Vicia faba var. minor and Brassica oleracea var. medullosa, which grew under controlled conditions. We addressed the questions (i) do δ²H_n-alkane and δ¹⁸O_sugar 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 δ²H_n-alkane and δ¹⁸O_sugar 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 δ¹⁸O_sugar values. Both δ²H_n-alkane and δ¹⁸O_sugar yielded significant correlations with δ²H_leaf-water and δ¹⁸O_leaf-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 ε_{n-alkane/leaf-water} and +27.3 ‰ (ranging from +23.0 to 32.3 ‰) for ε_{sugar/leaf-water}, respectively. Modelled RH_air values from a Craig-Gordon model using measured T_air, δ²H_leaf-water and δ¹⁸O_leaf-water as input correlate highly significantly with measured RH_air values (R² = 0.84, p < 0.001, RMSE = 6 %). When coupling δ²H_n-alkane and δ¹⁸O_sugar values the correlation of modelled RH_air values with measured RH_air values is weaker but still highly significant with R² = 0.54 (p < 0.001, RMSE = 10 %). Finally, the reconstructed source water isotope composition (δ²H_s and δ¹⁸O_s) as calculated from the coupled approach matches the source water in the climate chamber experiment (δ²H_tank-water and δ¹⁸O_tank-water). This highlights the great potential of the coupled δ²H_n-alkane-δ¹⁸O_sugar paleohygrometer approach for paleoclimate and relative humidity reconstructions.