Evaluation of bacterial glycerol dialkyl glycerol tetraether and 2H–18O biomarker proxies along a central European topsoil transect
Authors: Hepp Johannes , Schäfer Imke Kathrin , Lanny Verena , Franke Jörg , Bliedtner Marcel , Rozanski Kazimierz , Glaser Bruno , Zech Michael , Eglinton Timothy Ian , Zech Roland 
Published: February 2020

Abstract. Molecular fossils, like bacterial branched glycerol dialkyl glycerol
tetraethers (brGDGTs), and the stable isotopic composition of biomarkers,
such as δ2H of leaf wax-derived n-alkanes (δ2Hn-alkane) or δ18O of hemicellulose-derived
sugars (δ18Osugar), are increasingly used for the
reconstruction of past climate and environmental conditions. Plant-derived
δ2Hn-alkane and δ18Osugar values
record the isotopic composition of plant source water (δ2Hsource-water and δ18Osource-water), which
usually reflects mean annual precipitation (δ2Hprecipiation and δ18Oprecipiation), modulated
by evapotranspirative leaf water enrichment and biosynthetic fractionation
(εbio). 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 (ΔpH = 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 temperatures
slightly (ΔTMA=0.5 ∘C ± 2.4). Apparent
isotopic fractionation (εn-alkane/precipitation and
εsugar∕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 δ2Hn-alkane with δ18Osugar allows us to
reconstruct the stable isotopic composition of the source water more
accurately than without the coupled approach (Δδ2H = ∼-21 ‰ ± 22 ‰ and Δδ18O = ∼-2.9 ‰ ± 2.8 ‰).
Similarly, relative humidity during daytime and the vegetation period
(RHMDV) can be reconstructed using the coupled isotope approach
(ΔRHMDV=∼-17±12). Especially for
coniferous sites, reconstructed RHMDV 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 n-alkane pool but only marginally to the
sugar pool in the topsoils. Vegetation-dependent variable signal damping
and εbio (regarding 2H between n-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
δ2Hn-alkane–δ18Osugar approach have
great potential for more quantitative paleoclimate reconstructions.