| Abstrakt: |
The hydrogen isotope composition of leaf-wax-derived biomarkers, e.g., long-chain n -alkanes (δ2 H n-alkane), is widely applied in paleoclimate. However, a direct reconstruction of the isotope composition of source water based on δ2 H n-alkane alone is challenging due to the enrichment of heavy isotopes during evaporation. The coupling of δ2 H n-alkane with δ18 O of hemicellulose-derived sugars (δ18 O sugar) has the potential to disentangle this limitation and additionally to allow relative humidity reconstructions. Here, we present δ2 H n-alkane as well as δ18 O 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 δ2 H n-alkane and δ18 O 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 δ2 H n-alkane and δ18 O 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 δ2 H measurements. For Vicia faba , additionally the δ2 H values of n -C 31 could be evaluated robustly. Regarding hemicellulose-derived monosaccharides, arabinose and xylose were most abundant, and their δ18 O values were therefore used to calculate weighted mean leaf δ18 O sugar values. Both δ2 H n-alkane and δ18 O sugar yielded significant correlations with δ2 H leaf water and δ18 O 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 εn-alkane/leaf water and + 27.3 ‰ (ranging from + 23.0 ‰ to 32.3 ‰) for εsugar/leaf water , respectively. Modeled RH air values from a Craig–Gordon model using measured Tair , δ2 H leaf water and δ18 O leaf water as input correlate highly significantly with modeled RH air values (R2=0.84 , p<0.001 , RMSE = 6 %). When coupling δ2 H n-alkane and δ18 O sugar values, the correlation of modeled 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 (δ2 H s and δ18 O s) as calculated from our coupled approach matches the source water in the climate chamber experiment (δ2 H tank water and δ18 O tank water). This highlights the great potential of the coupled δ2 H n-alkane – δ18 O sugar paleohygrometer approach for paleoclimate and relative humidity reconstructions. [ABSTRACT FROM AUTHOR] |
