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<p>The hydrogen isotope composition of leaf-wax-derived biomarkers, e.g., long-chain <span class="inline-formula"><i>n</i></span>-alkanes (<span class="inline-formula"><i>δ</i>²</span>H<span class="inline-formula">_{<i>n</i>-alkane}</span>), is widely applied in paleoclimate. However, a direct reconstruction of the isotope composition of source water based on <span class="inline-formula"><i>δ</i>²</span>H<span class="inline-formula">_{<i>n</i>-alkane}</span> alone is challenging due to the enrichment of heavy isotopes during evaporation. The coupling of <span class="inline-formula"><i>δ</i>²</span>H<span class="inline-formula">_{<i>n</i>-alkane}</span> with <span class="inline-formula"><i>δ</i>¹⁸</span>O of hemicellulose-derived sugars (<span class="inline-formula"><i>δ</i>¹⁸</span>O<span class="inline-formula">_sugar</span>) has the potential to disentangle this limitation and additionally to allow relative humidity reconstructions. Here, we present <span class="inline-formula"><i>δ</i>²</span>H<span class="inline-formula">_{<i>n</i>-alkane}</span> as well as <span class="inline-formula"><i>δ</i>¹⁸</span>O<span class="inline-formula">_sugar</span> results obtained from leaves of <i>Eucalyptus globulus</i>, <i>Vicia faba</i>, and <i>Brassica oleracea</i>, which grew under controlled conditions. We addressed the questions of (i) whether <span class="inline-formula"><i>δ</i>²</span>H<span class="inline-formula">_{<i>n</i>-alkane}</span> and <span class="inline-formula"><i>δ</i>¹⁸</span>O<span class="inline-formula">_sugar</span> 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 <span class="inline-formula"><i>δ</i>²</span>H<span class="inline-formula">_{<i>n</i>-alkane}</span> and <span class="inline-formula"><i>δ</i>¹⁸</span>O<span class="inline-formula">_sugar</span> enables a robust source water calculation.</p> <p><span id="page5364"/>For all investigated species, the <span class="inline-formula"><i>n</i></span>-alkane <span class="inline-formula"><i>n</i></span>-C<span class="inline-formula">₂₉</span> was most abundant and therefore used for compound-specific <span class="inline-formula"><i>δ</i>²</span>H measurements. For <i>Vicia faba</i>, additionally the <span class="inline-formula"><i>δ</i>²</span>H values of <span class="inline-formula"><i>n</i></span>-C<span class="inline-formula">₃₁</span> could be evaluated robustly. Regarding hemicellulose-derived monosaccharides, arabinose and xylose were most abundant, and their <span class="inline-formula"><i>δ</i>¹⁸</span>O values were therefore used to calculate weighted mean leaf <span class="inline-formula"><i>δ</i>¹⁸</span>O<span class="inline-formula">_sugar</span> values. Both <span class="inline-formula"><i>δ</i>²</span>H<span class="inline-formula">_{<i>n</i>-alkane}</span> and <span class="inline-formula"><i>δ</i>¹⁸</span>O<span class="inline-formula">_sugar</span> yielded significant correlations with <span class="inline-formula"><i>δ</i>²</span>H<span class="inline-formula">_{leaf water}</span> and <span class="inline-formula"><i>δ</i>¹⁸</span>O<span class="inline-formula">_{leaf water}</span>, respectively (<span class="inline-formula"><i>r</i>²=0.45</span> and 0.85, respectively; <span class="inline-formula"><i>p</i>&lt;0.001</span>, <span class="inline-formula"><i>n</i>=24</span>). Mean fractionation factors between biomarkers and leaf water were found to be <span class="inline-formula">−156</span> ‰ (ranging from <span class="inline-formula">−133</span> ‰ to <span class="inline-formula">−192</span> ‰) for <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M55" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi mathvariant="italic">ε</mi><mrow><mi>n</mi><mtext>-alkane</mtext><mo>/</mo><mtext>leaf water</mtext></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="78pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="59d14d7d1d5c48452dca99270f9ed0e8"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-5363-2021-ie00001.svg" width="78pt" height="12pt" src="bg-18-5363-2021-ie00001.png"/></svg:svg></span></span> and <span class="inline-formula">+</span>27.3 ‰ (ranging from <span class="inline-formula">+</span>23.0 ‰ to 32.3 ‰) for <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M58" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi mathvariant="italic">ε</mi><mrow><mi mathvariant="normal">sugar</mi><mo>/</mo><mtext>leaf water</mtext></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="67pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="e949386fa312f22aa9b5d2e430396377"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-5363-2021-ie00002.svg" width="67pt" height="12pt" src="bg-18-5363-2021-ie00002.png"/></svg:svg></span></span>, respectively. Modeled RH<span class="inline-formula">_air</span> values from a Craig–Gordon model using measured <span class="inline-formula"><i>T</i>_air</span>, <span class="inline-formula"><i>δ</i>²</span>H<span class="inline-formula">_{leaf water}</span> and <span class="inline-formula"><i>δ</i>¹⁸</span>O<span class="inline-formula">_{leaf water}</span> as input correlate highly significantly with modeled RH<span class="inline-formula">_air</span> values (<span class="inline-formula"><i>R</i>²=0.84</span>, <span class="inline-formula"><i>p</i>&lt;0.001</span>, RMSE <span class="inline-formula">=</span> 6 %). When coupling <span class="inline-formula"><i>δ</i>²</span>H<span class="inline-formula">_{<i>n</i>-alkane}</span> and <span class="inline-formula"><i>δ</i>¹⁸</span>O<span class="inline-formula">_sugar</span> values, the correlation of modeled RH<span class="inline-formula">_air</span> values with measured RH<span class="inline-formula">_air</span> values is weaker but still highly significant, with <span class="inline-formula"><i>R</i>²=0.54</span> (<span class="inline-formula"><i>p</i>&lt;0.001</span>, RMSE <span class="inline-formula">=</span> 10 %). Finally, the reconstructed source water isotope composition (<span class="inline-formula"><i>δ</i>²</span>H<span class="inline-formula">_s</span> and <span class="inline-formula"><i>δ</i>¹⁸</span>O<span class="inline-formula">_s</span>) as calculated from our coupled approach matches the source water in the climate chamber experiment (<span class="inline-formula"><i>δ</i>²</span>H<span class="inline-formula">_{tank water}</span> and <span class="inline-formula"><i>δ</i>¹⁸</span>O<span class="inline-formula">_{tank water}</span>). This highlights the great potential of the coupled <span class="inline-formula"><i>δ</i>²</span>H<span class="inline-formula">_{<i>n</i>-alkane}</span>–<span class="inline-formula"><i>δ</i>¹⁸</span>O<span class="inline-formula">_sugar</span> paleohygrometer approach for paleoclimate and relative humidity reconstructions.</p>