Evaluation of bacterial glycerol dialkyl glycerol tetraether and 2H– 18O biomarker proxies along a central European topsoil transect

Hepp, Johannes; Schäfer, Imke Kathrin; Lanny, Verena; Franke, Jörg; Bliedtner, Marcel; Rozanski, Kazimierz; Glaser, Bruno; Zech, Michael; Eglinton, Timothy Ian; Zech, Roland

Molecular fossils, like bacterial branched glycerol dialkyl glycerol tetraethers (brGDGTs), and the stable isotopic composition of biomarkers, such as inline-formulaδ2H of leaf wax-derived inline-formulan-alkanes (inline-formulaδ2Hinline-formulan-alkane) or inline-formulaδ18O of hemicellulose-derived sugars (inline-formulaδ18Oinline-formulasugar), are increasingly used for the reconstruction of past climate and environmental conditions. Plant-derived inline-formulaδ2Hinline-formulan-alkane and inline-formulaδ18Oinline-formulasugar values record the isotopic composition of plant source water (inline-formulaδ2Hinline-formulasource-water and inline-formulaδ18Oinline-formulasource-water), which usually reflects mean annual precipitation (inline-formulaδ2Hinline-formulaprecipiation and inline-formulaδ18Oinline-formulaprecipiation), modulated by evapotranspirative leaf water enrichment and biosynthetic fractionation (inline-formulaεbio). Accuracy and precision of respective proxies should be ideally evaluated at a regional scale. For this study, we analysed topsoils below coniferous and deciduous forests as well as grassland soils along a central European transect in order to investigate the variability and robustness of various proxies and to identify effects related to vegetation. Soil pH values derived from brGDGTs correlate reasonably well with measured soil pH values but are systematically overestimated (inline-formulaΔpH inline-formula=inline-formula0.6±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 temperatures slightly (inline-formulaΔTMA=0.5inline-formulaC inline-formula± 2.4). Apparent isotopic fractionation (inline-formulaεn-alkane/precipitation and inline-formulaεsugar∕precipitation) is lower for grassland sites than for forest sites due to signal damping; i.e. grass biomarkers do not record the full evapotranspirative leaf water enrichment. Coupling inline-formulaδ2Hinline-formulan-alkane with inline-formulaδ18Oinline-formulasugar allows us to reconstruct the stable isotopic composition of the source water more accurately than without the coupled approach (inline-formulaΔδ2Hinline-formula=inline-formula M39inlinescrollmathml - normal 21 32pt10ptsvg-formulamathimge82a4d8b4a959976b462fc4efa2646e9 bg-17-741-2020-ie00001.svg32pt10ptbg-17-741-2020-ie00001.png  ‰ inline-formula± 22 ‰ and inline-formulaΔδ18Oinline-formula=inline-formula M43inlinescrollmathml - normal 2.9 35pt10ptsvg-formulamathimgfa28c201b7bdb30ed9d566e4ac581654 bg-17-741-2020-ie00002.svg35pt10ptbg-17-741-2020-ie00002.png  ‰ inline-formula± 2.8 ‰). Similarly, relative humidity during daytime and the vegetation period (RHinline-formulaMDV) can be reconstructed using the coupled isotope approach (inline-formula M46inlinescrollmathml normal Δ normal RH normal MDV = - normal 17 ± normal 12 107pt12ptsvg-formulamathimgd823af5a8ee2834d6d10a3ec7bc50932 bg-17-741-2020-ie00003.svg107pt12ptbg-17-741-2020-ie00003.png ). Especially for coniferous sites, reconstructed RHinline-formulaMDV values as well as source water isotope composition underestimate the measured values. This can likely be explained by understorey grass vegetation at the coniferous sites contributing significantly to the inline-formulan-alkane pool but only marginally to the sugar pool in the topsoils. Vegetation-dependent variable signal damping and inline-formulaεbio (regarding inline-formula2H between inline-formulan-alkanes and leaf water) along our European transect are difficult to quantify but likely contribute to the observed underestimation in the source water isotope composition and RH reconstructions. Microclimate variability could cause the rather large uncertainties. Vegetation-related effects do, by contrast, not affect the brGDGT-derived reconstructions. Overall, GDGTs and the coupled inline-formulaδ2Hinline-formulan-alkaneinline-formulaδ18Oinline-formulasugar approach have great potential for more quantitative paleoclimate reconstructions.

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Hepp, Johannes / Schäfer, Imke Kathrin / Lanny, Verena / et al: Evaluation of bacterial glycerol dialkyl glycerol tetraether and 2H–18O biomarker proxies along a central European topsoil transect. 2020. Copernicus Publications.

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