Validation of a coupled δ2Hn-alkane-δ18Osugar paleohygrometer approach based on a climate chamber experiment.

The hydrogen isotopic composition of leaf wax-derived biomarkers, e.g. long chain 푛-alkanes (d²-H_푛-alkane), is widely applied in paleoclimatology research. However, a direct reconstruction of the isotopic composition of source water based on d²H_푛-alkane alone can be challenging due to the alteration of the soil water isotopic signal by leaf-water heavy-isotope enrichment. The coupling of d²H_푛-alkane with d¹⁸O of hemicellulose-derived sugars (d¹⁸O_sugar) has the potential to disentangle this effect and additionally to allow relative humidity reconstructions. Here, we present d²H_푛-alkane as well as d¹⁸O_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 d²H_푛-alkane and d¹⁸O_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 d²H_푛-alkane and d¹⁸O_sugar enable a robust source water calculation? For all investigated species, the alkane 푛-C₂₉ was most abundant and therefore used for compound specific d²H measurements. For Vicia faba, additionally the d²H values of 푛-C₃₁ could be evaluated robustly. With regard to hemicellulose-derived monosaccharides, arabinose and xylose were most abundant and their d¹⁸O values were therefore used to calculate weighted mean leaf d¹⁸O_sugar values. Both d²H_푛-alkane and d¹⁸O_sugar yielded significant correlations with d²H_leaf-water and d¹⁸O_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 e_{n-alkane/leaf-water} and +27.3‰ (ranging from +23.0 to 32.3‰) for e_{sugar/leaf-water}, respectively. Modelled RH_air values from a Craig-Gordon model using measured T_air, d²H_leaf-water and d¹⁸O_leaf-water as input correlate highly significantly with measured R_Hair values (R² = 0.84, p < 0.001, RMSE = 6%). When coupling d²H_푛-alkane and d¹⁸O_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 (d²H_s and d¹⁸O_s) as calculated from the coupled approach matches the source water in the climate chamber experiment (d₂H_tank-water and d¹⁸O_tank-water). This highlights the great potential of the coupled d²H_푛-alkane-d¹⁸O_sugar paleohygrometer approach for paleoclimate and relative humidity reconstructions. [ABSTRACT FROM AUTHOR]