Abstract. The hydrogen isotope composition of leaf-wax-derived biomarkers, e.g., long-chain n-alkanes (δ2Hn-alkane), is widely applied in
paleoclimate. However, a direct reconstruction of the isotope composition of
source water based on δ2Hn-alkane alone is
challenging due to the enrichment of heavy isotopes during evaporation. The
coupling of δ2Hn-alkane with δ18O of
hemicellulose-derived sugars (δ18Osugar) has the potential
to disentangle this limitation and additionally to allow relative humidity
reconstructions. Here, we present δ2Hn-alkane as well
as δ18Osugar results obtained from leaves of Eucalyptus globulus, Vicia faba, and
Brassica oleracea, which grew under controlled conditions. We addressed the questions of (i) whether δ2Hn-alkane and δ18Osugar 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 δ2Hn-alkane and δ18Osugar enables a robust source water calculation. For all investigated species, the n-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. Regarding hemicellulose-derived monosaccharides, arabinose and
xylose were most abundant, and their δ18O values were therefore
used to calculate weighted mean leaf δ18Osugar values.
Both δ2Hn-alkane and δ18Osugar
yielded significant correlations with δ2Hleaf water and
δ18Oleaf 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 RHair values from a
Craig–Gordon model using measured Tair, δ2Hleaf water
and δ18Oleaf water as input correlate highly significantly
with modeled RHair values (R2=0.84, p<0.001, RMSE = 6 %). When coupling δ2Hn-alkane and δ18Osugar values, the correlation of modeled RHair values
with measured RHair values is weaker but still highly significant, with
R2=0.54 (p<0.001, RMSE = 10 %). Finally, the
reconstructed source water isotope composition (δ2Hs and
δ18Os) as calculated from our coupled approach matches the
source water in the climate chamber experiment (δ2Htank water and δ18Otank water). This
highlights the great potential of the coupled δ2Hn-alkane–δ18Osugar paleohygrometer
approach for paleoclimate and relative humidity reconstructions.
A late-Holocene multiproxy fire record from a tropical savanna, eastern Arnhem Land, Northern Territory, Australia
