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. Molecular fossils, like bacterial branched glycerol dialkyl glyceroltetraethers (brGDGTs), and the stable isotopic composition of biomarkers,such as δ2H of leaf wax-derived n -alkanes ( δ2H n-alkane ) or δ18O of hemicellulose-derivedsugars ( δ18O sugar ), are increasingly used for thereconstruction of past climate and environmental conditions. Plant-derived δ2H n-alkane and δ18O sugar valuesrecord the isotopic composition of plant source water ( δ2 H source-water and δ18O source-water ), whichusually reflects mean annual precipitation ( δ2 H precipiation and δ18O precipiation ), modulatedby evapotranspirative leaf water enrichment and biosynthetic fractionation( ebio ). Accuracy and precision of respective proxiesshould be ideally evaluated at a regional scale. For this study, we analysedtopsoils below coniferous and deciduous forests as well as grassland soilsalong a central European transect in order to investigate the variabilityand robustness of various proxies and to identify effects related tovegetation. Soil pH values derived from brGDGTs correlate reasonably wellwith measured soil pH values but are systematically overestimated ( Δ pH  = 0.6±0.6 ). The branched vs. isoprenoid tetraether index (BIT)can give some indication whether the pH reconstruction is reliable.Temperatures derived from brGDGTs overestimate mean annual air temperaturesslightly ( ΔTMA=0.5 ∘ C  ±  2.4). Apparentisotopic fractionation ( en-alkane/precipitation and esugar∕precipitation ) is lower for grassland sites thanfor forest sites due to signal damping; i.e. grass biomarkers do notrecord the full evapotranspirative leaf water enrichment. Coupling δ2H n-alkane with δ18O sugar allows us toreconstruct the stable isotopic composition of the source water moreaccurately than without the coupled approach ( Δδ2H = ∼ - 21  ‰  ±  22 ‰ and Δδ18O = ∼ - 2.9  ‰  ±  2.8 ‰).Similarly, relative humidity during daytime and the vegetation period(RH MDV ) can be reconstructed using the coupled isotope approach( Δ RH MDV = ∼ - 17 ± 12 ). Especially forconiferous sites, reconstructed RH MDV values as well as source waterisotope composition underestimate the measured values. This can likely beexplained by understorey grass vegetation at the coniferous sitescontributing significantly to the n -alkane pool but only marginally to thesugar pool in the topsoils. Vegetation-dependent variable signal dampingand ebio (regarding 2H between n -alkanes and leafwater) along our European transect are difficult to quantify but likelycontribute to the observed underestimation in the source water isotopecomposition and RH reconstructions. Microclimate variability could cause therather large uncertainties. Vegetation-related effects do, by contrast, notaffect the brGDGT-derived reconstructions. Overall, GDGTs and the coupled δ2H n-alkane – δ18O sugar approach havegreat potential for more quantitative paleoclimate reconstructions.

biogeosciences

Evaluation of bacterial glycerol dialkyl glycerol tetraether and &amp;lt;sup&amp;gt;2&amp;lt;/sup&amp;gt;H–&amp;lt;sup&amp;gt;18&amp;lt;/sup&amp;gt;O biomarker proxies along a central European topsoil transect