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

The hydrogen isotope composition of leaf-wax-derived biomarkers, e.g., long-chain n-alkanes (delta H-2(n-alkane)), is widely applied in paleoclimate. However, a direct reconstruction of the isotope composition of source water based on delta H-2(n-alkane) alone is challenging due to the enrichment of heavy isotopes during evaporation. The coupling of delta H-2(n-alkane) with delta O-18 of hemicellulose-derived sugars (delta O-18(sugar)) has the potential to disentangle this limitation and additionally to allow relative humidity reconstructions. Here, we present delta H-2(n-alkane) as well as delta O-18(sugar) results obtained from leaves of Eucalyptus globulus, Vicia faba, and Brassica oleracea, which grew under controlled conditions. We addressed the questions of (i) whether delta H-2(n-alkane) and delta O-18(sugar) values allow reconstructions of leaf water isotope composition, (ii) how accurately the reconstructed leaf water isotope composition enables relative humidity (RH) reconstruction, and (iii) whether the coupling of delta H-2(n-alkane) and delta O-18(sugar) enables a robust source water calculation. For all investigated species, the n-alkane n-C-29 was most abundant and therefore used for compound-specific delta H-2 measurements. For Vicia faba, additionally the delta H-2 values of n-C-31 could be evaluated robustly. Regarding hemicellulose-derived monosaccharides, arabinose and xylose were most abundant, and their delta O-18 values were therefore used to calculate weighted mean leaf delta O-18(sugar) values.Both delta H-2(n-alkane) and delta O-18(sugar) yielded significant correlations with delta H-2(leaf water) and delta O-18(leaf water), respectively (r(2)=0.45 and 0.85, respectively; p<0.001, n=24). Mean fractionation factors between biomarkers and leaf water were found to be -156 parts per thousand (ranging from -133 parts per thousand to -192 parts per thousand) for epsilon(n-alkane/leaf water) and +27.3 parts per thousand (ranging from +23.0 parts per thousand to 32.3 parts per thousand) for epsilon(sugar/leaf water), respectively. Modeled RHair values from a Craig-Gordon model using measured Tair, delta 2Hleaf water and delta 18Oleaf water as input correlate highly significantly with modeled RHair values (R2=0.84, p<0.001, RMSE = 6 %). When coupling delta H-2(n-alkane) and delta O-18(sugar) values, the correlation of modeled RHair values with measured RHair values is weaker but still highly significant, with R-2=0.54 (p<0.001, RMSE = 10 %). Finally, the reconstructed source water isotope composition (delta H-2(s) and delta O-18(s)) as calculated from our coupled approach matches the source water in the climate chamber experiment (delta H-2(tank water) and delta O-18(tank water)). This highlights the great potential of the coupled delta H-2(n-alkane)-delta O-18(sugar) paleohygrometer approach for paleoclimate and relative humidity reconstructions.

Automated summary

The study demonstrates that using leaf-wax-derived biomarkers and isotopic composition of sugars from hemicellulose can reconstruct leaf water isotopic composition, and has the potential for relative humidity reconstructions.