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

Abstract

Molecular fossils, like bacterial branched glycerol dialkyl glycerol tetraethers (brGDGTs), and the stable isotopic composition of biomarkers, such as δ 2 H of leaf waxderived n-alkanes (δ 2 H n-alkane ) or δ 18 O of hemicellulosederived sugars (δ 18 O sugar ), are increasingly used for the reconstruction of past climate and environmental conditions. Plant-derived δ 2 H n-alkane and δ 18 O sugar values record the isotopic composition of plant source water (δ 2 H source-water and δ 18 O source-water ), which usually reflects mean annual precipitation (δ 2 H precipiation and δ 18 O precipiation ), modulated by evapotranspirative leaf water enrichment and biosynthetic fractionation (ε bio ). 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 ( 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 temperatures slightly ( T MA = 0.5 • C ± 2.4). Apparent isotopic fractionation (ε n-alkane/precipitation and ε sugar/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 δ 2 H n-alkane with δ 18 O sugar allows us to reconstruct the stable isotopic composition of the source water more accurately than without the coupled approach ( δ 2 H = ∼ −21 ‰ ± 22 ‰ and δ 18 O = ∼ −2.9 ‰ ± 2.8 ‰). Similarly, relative humidity during daytime and the vegetation period (RH MDV ) can be reconstructed using the coupled isotope approach ( RH MDV =∼ −17 ± 12). Especially for coniferous sites, reconstructed RH MDV 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 ε bio (regarding 2 H 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 δ 2 H n-alkane -δ 18 O sugar approach have great potential for more quantitative paleoclimate reconstructions.

Leaf Waxes and Hemicelluloses in Topsoils Reflect the δ2H and δ18O Isotopic Composition of Precipitation in Mongolia

“…and Artemisia spp., whereas n-C 31 is the most abundant homolog in grasses. Since shrubs and dicotyledonous plants in general are more sensitive to evapotranspirative enrichment than grasses (Sachse et al, 2012;Kahmen et al, 2013b;Hepp et al, 2020), the observed offset might indicate (i) plant-physiological differences affecting the evapotranspirative enrichment of different plants, and (ii) plantphysiological differences affecting ε bio .…”

“…Assuming constant ε bio values of −160 for leaf wax n-alkanes (Sessions et al, 1999;Hepp et al, 2020), evapotranspirative enrichment would be ∼31 for δ 2 H n-C29 and ∼15 for δ 2 H n-C31 (Figure 4). While n-C 29 is the most abundant homolog in shrubs, and n-C 31 is the most abundant homolog in grasses (Bliedtner et al, 2018;Struck et al, 2020) the observed offset in evapotranspirative enrichment most likely results from plant physiological differences described above, particularly the dampening effect.…”

“…Since δ 2 H n-alkane and δ 18 O sugar are not strongly affected by degradation effects (Zech et al, 2011(Zech et al, , 2012, they are increasingly used for paleohydrological reconstructions (Aichner et al, 2015;Hepp et al, 2015Hepp et al, , 2019Thomas et al, 2016;Rach et al, 2017;Schäfer et al, 2018;Bliedtner et al, 2020). Usually, they are interpreted to record the isotopic composition of precipitation (Sachse et al, 2006(Sachse et al, , 2012Tuthorn et al, 2015;Hou et al, 2018;Hepp et al, 2020), which in turn is long acknowledged as a valuable proxy for paleoclimate reconstructions and controlled by e.g., the temperature and amount effect, continentality, and altitude (Dansgaard, 1964). The isotopic signal of precipitation can be altered by isotopic fractionation at the soil-plantatmosphere interface, including evaporative enrichment of soil water (ε SW ), transpirative enrichment of leaf water (ε Et ) and biosynthetic fractionation (ε bio ) (Schimmelmann et al, 2006;Sachse et al, 2012;Liu et al, 2016;Cormier et al, 2018;Liu and An, 2019).…”

“…While some studies suggest that ε 2H n-alkane/p in plants and topsoils remains relatively constant over latitudinal distances (Liu et al, 2016, Liu andAn, 2019;Vogts et al, 2016), many others show correlations of ε 2H n-alkane/p with relative humidity, mean annual precipitation (MAP) and the aridity index (AI) (Hou et al, 2008(Hou et al, , 2018Douglas et al, 2012;Tipple and Pagani, 2013;Berke et al, 2015;Herrmann et al, 2017;Li et al, 2019). This can be related to more evapotranspirative enrichment under more arid conditions, but plant physiological adaptations might also play a role, as well as changes in vegetation, as e.g., monocotyledons have more negative δ 2 H n-alkane values than dicotyledons (Sachse et al, 2012;Kahmen et al, 2013b;Hepp et al, 2020). For relatively arid regions in the United States and South Africa, Hou et al (2008), Feakins and Sessions (2010), and Strobel et al (2020) found no correlations of ε 2H n-alkane/p with climate, and they reported constant, but quite different values (−94 ± 21 , -99 ± 8 , and −133 ± 12 , respectively).…”

“…So far, only very few calibration studies applied compoundspecific δ 18 O sugar analyzes (Tuthorn et al, 2015;Hepp et al, 2016Hepp et al, , 2020Strobel et al, 2020). For semi-arid and arid regions in South America and South Africa, Tuthorn et al (2015) and Strobel et al (2020) suggest enhanced evapotranspirative enrichment and higher ε 18O sugar/p values with increasing aridity.…”