Validation of a coupled δ<sup>2</sup>H<sub><i>n</i>-alkane</sub>-δ<sup>18</sup>O<sub>sugar</sub> paleohygrometer approach based on a climate chamber experiment

Abstract

<p><strong>Abstract.</strong> The hydrogen isotopic composition of leaf wax-derived biomarkers, e.g. long chain <i>n</i>-alkanes (δ<sup>2</sup>H<sub><i>n</i>-alkane</sub>), is widely applied in paleoclimatology research. However, a direct reconstruction of the isotopic composition of paleoprecipitation based on δ<sup>2</sup>H<sub><i>n</i>-alkane</sub> alone can be challenging due to the overprint of the source water isotopic signal by leaf-water enrichment. The coupling of δ<sup>2</sup>H<sub><i>n</i>-alkane</sub> with δ<sup>18</sup>O of hemicellulose-derived sugars (δ<sup>18</sup>O<sub>sugar</sub>) has the potential to disentangle this effect and additionally allow relative humidity reconstructions. Here, we present δ<sup>2</sup>H<sub><i>n</i>-alkane</sub> as well as δ<sup>18</sup>O<sub>sugar</sub> results obtained from leaves of the plant species <i>Eucalyptus globulus</i>, <i>Vicia faba</i> var. <i>minor</i> and <i>Brassica oleracea</i> var. <i>medullosa</i>, which were grown under controlled conditions. We addressed the questions (i) do δ<sup>2</sup>H<sub><i>n</i>-alkane</sub> and δ<sup>18</sup>O<sub>sugar</sub> 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 δ<sup>2</sup>H<sub><i>n</i>-alkane</sub> and δ<sup>18</sup>O<sub>sugar</sub> enable a robust source water calculation?</p> <p> For all investigated species, the alkane <i>n</i>-C<sub>29</sub> was most abundant and therefore used for compound-specific δ<sup>2</sup>H measurements. For <i>Vicia faba</i>, additionally the δ<sup>2</sup>H values of <i>n</i>-C<sub>31</sub> could be evaluated robustly. With regard to hemicellulose-derived monosaccharides, arabinose and xylose were most abundant and their δ<sup>18</sup>O values were therefore used to calculate weighted mean leaf δ<sup>18</sup>O<sub>sugar</sub> values. Both δ<sup>2</sup>H<sub><i>n</i>-alkane</sub> and δ<sup>18</sup>O<sub>sugar</sub> yielded significant correlations with δ<sup>2</sup>H<sub>leaf-water</sub> and δ<sup>18</sup>O<sub>leaf-water</sub>, respectively (r<sup>2</sup> = 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<sub><i>n</i>-alkane/leaf-water</sub> and +27.3 ‰ (ranging from +23.0 to 32.3 ‰) for ε<sub>sugar/leaf-water</sub>, respectively. Using rearranged Craig-Gordon equations with either T<sub>air</sub> or T<sub>leaf</sub> and measured δ<sup>2</sup>H<sub>leaf-water</sub> or δ<sup>18</sup>O<sub>leaf-water</sub> as input variables, we furthermore modeled climate chamber RH<sub>air</sub> and RH<sub>leaf</sub> values. Modelled RH<sub>air</sub> values, from the more simplified Craig-Gordon model, turned out to be most accurate and correlate highly significantly with measured RH<sub>air</sub> values (R<sup>2</sup> = 0.84, p < 0.001; RMSE = 6 %). When combining δ<sup>2</sup>H<sub>leaf-water</sub> and δ<sup>18</sup>O<sub>leaf-water</sub> values that are calculated from the alkane and sugar biomarkers instead of actually measured δ<sup>2</sup>H<sub>leaf-water</sub> and δ<sup>18</sup>O<sub>leaf-water</sub> as input variables, the correlation of modelled RH<sub>air</sub> values with measured RH<sub>air</sub> values is getting worse, but is still highly significant with R<sup>2</sup> = 0.54, p < 0.001; RMSE = 10 %. This highlights the potential of the coupled δ<sup>2</sup>H<sub><i>n</i>-alkane</sub>-δ<sup>18</sup>O<sub>sugar</sub> paleohygrometer approach for suitable relative humidity reconstructions. Finally, the reconstructed source water isotope composition (δ<sup>2</sup>H<sub>s</sub> and δ<sup>18</sup>O<sub>s</sub>) as calculated from the coupled approach matches the source water in the climate chamber experiment (δ<sup>2</sup>H<sub>tank-water</sub> and δ<sup>18</sup>O<sub>tank-water</sub>).</p>

The Holocene lake-evaporation history of the afro-alpine Lake Garba Guracha in the Bale Mountains, Ethiopia, based on δ18O records of sugar biomarker and diatoms

“…ϵ bio can be estimated as +29‰ between aquatic cellulose δ 18 O and δ 18 O lake water (Mayr et al, 2015). An ϵ bio of +29‰ may slightly underestimate the fractionation factor of aquatic hemicellulose, as is known for terrestrial cellulose and hemicellulose (Zech et al, 2014a, b;Hepp et al, 2019). The slight offset derives from the loss of the relatively 18 O-depleted oxygen atom attached to C-6 during 6-deoxyhexose (fucose) biosynthesis (C-6 decarboxylation; Altermatt and Neish, 1956;Harper and Bar-Peled, 2002;Burget et al, 2003).…”

“…Oxygen isotope composition has been determined for several Quaternary records in eastern Africa: for example, δ 18 O ice cores (Thompson et al, 2002), δ 18 O carbonate (Lamb et al, 2005), δ 18 O diatom (Barker et al, 2001(Barker et al, , 2007(Barker et al, , 2011Lamb et al, 2005), and δ 18 O sugar (Hepp et al, 2017). δ 18 O sugar has been successfully applied to several other paleolimnological archives (Zech et al, 2014b;Hepp et al, 2015Hepp et al, , 2019. In contrast to this relatively new proxy (Zech and Glaser, 2009), numerous δ 18 O diatom records have been published, proving the potential for reconstructing past climate changes from biogenic silica (δ 18 O diatom ) (Leng et al, 2001;Barker et al, 2004Barker et al, , 2007Barker et al, , 2011Lamb et al, 2005;Wilson et al, 2014;Narancic et al, 2016;Cartier et al, 2019;Kostrova et al, 2019).…”