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 paleoprecipitation based on δ²H_n-alkane alone can be challenging due to the overprint of the source water isotopic signal by leaf-water enrichment. The coupling of δ²H_n-alkane with δ¹⁸O of hemicellulose-derived sugars (δ¹⁸O_sugar) has the potential to disentangle this effect and additionally 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 were grown 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. Using rearranged Craig-Gordon equations with either T_air or T_leaf and measured δ²H_leaf-water or δ¹⁸O_leaf-water as input variables, we furthermore modeled climate chamber RH_air and RH_leaf values. Modelled RH_air values, from the more simplified Craig-Gordon model, turned out to be most accurate and correlate highly significantly with measured RH_air values (R² = 0.84, p < 0.001; RMSE = 6 %). When combining δ²H_leaf-water and δ¹⁸O_leaf-water values that are calculated from the alkane and sugar biomarkers instead of actually measured δ²H_leaf-water and δ¹⁸O_leaf-water as input variables, the correlation of modelled RH_air values with measured RH_air 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 δ²H_n-alkane-δ¹⁸O_sugar paleohygrometer approach for suitable relative humidity reconstructions. 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).