scholarly journals Validation of a coupled δ<sup>2</sup>H<sub><i>n</i>-alkane</sub>-δ<sup>18</sup>O<sub>sugar</sub> paleohygrometer approach based on a climate chamber experiment

2019 ◽  
Author(s):  
Johannes Hepp ◽  
Bruno Glaser ◽  
Dieter Juchelka ◽  
Christoph Mayr ◽  
Kazimierz Rozanski ◽  
...  
Keyword(s):  
Relative Humidity ◽  
Isotopic Composition ◽  
Vicia Faba ◽  
Isotope Composition ◽  
Leaf Water ◽  
Source Water ◽  
Leaf Wax ◽  
Hydrogen Isotopic Composition ◽  
Water Isotope ◽  
Isotopic Signal

Abstract. The hydrogen isotopic composition of leaf wax-derived biomarkers, e.g. long chain n-alkanes (δ2Hn-alkane), is widely applied in paleoclimatology research. However, a direct reconstruction of the isotopic composition of paleoprecipitation based on δ2Hn-alkane alone can be challenging due to the overprint of the source water isotopic signal by leaf-water enrichment. The coupling of δ2Hn-alkane with δ18O of hemicellulose-derived sugars (δ18Osugar) has the potential to disentangle this effect and additionally allow relative humidity reconstructions. Here, we present δ2Hn-alkane as well as δ18Osugar results obtained from leaves of the plant species Eucalyptus globulus, Vicia faba var. minor and Brassica oleracea var. medullosa, which were grown under controlled conditions. We addressed the questions (i) do δ2Hn-alkane and δ18Osugar values allow precise reconstructions of leaf water isotope composition, (ii) how accurately does the reconstructed leaf-water-isotope composition enables relative humidity (RH) reconstruction in which the plants grew, and (iii) does the coupling of δ2Hn-alkane and δ18Osugar enable a robust source water calculation? For all investigated species, the alkane n-C29 was most abundant and therefore used for compound-specific δ2H measurements. For Vicia faba, additionally the δ2H values of n-C31 could be evaluated robustly. With regard to hemicellulose-derived monosaccharides, arabinose and xylose were most abundant and their δ18O values were therefore used to calculate weighted mean leaf δ18Osugar values. Both δ2Hn-alkane and δ18Osugar yielded significant correlations with δ2Hleaf-water and δ18Oleaf-water, respectively (r2 = 0.45 and 0.85, respectively; p 

scholarly journals Validation of a coupled δ<sup>2</sup>H<sub><i>n</i>-alkane</sub>-δ<sup>18</sup>O<sub>sugar</sub> paleohygrometer approach based on a climate chamber experiment

2020 ◽  
Author(s):  
Johannes Hepp ◽  
Christoph Mayr ◽  
Kazimierz Rozanski ◽  
Imke Kathrin Schäfer ◽  
Mario Tuthorn ◽  
...  
Keyword(s):  
Relative Humidity ◽  
Isotopic Composition ◽  
Vicia Faba ◽  
Isotope Composition ◽  
Leaf Water ◽  
Source Water ◽  
Leaf Wax ◽  
Hydrogen Isotopic Composition ◽  
Water Isotope ◽  
Isotopic Signal

Abstract. The hydrogen isotopic composition of leaf wax-derived biomarkers, e.g. long chain n-alkanes (δ2Hn-alkane), is widely applied in paleoclimatology research. However, a direct reconstruction of the isotopic composition of source water based on δ2Hn-alkane alone can be challenging due to the alteration of the soil water isotopic signal by leaf-water heavy-isotope enrichment. The coupling of δ2Hn-alkane with δ18O of hemicellulose-derived sugars (δ18Osugar) has the potential to disentangle this effect and additionally to allow relative humidity reconstructions. Here, we present δ2Hn-alkane as well as δ18Osugar results obtained from leaves of the plant species Eucalyptus globulus, Vicia faba var. minor and Brassica oleracea var. medullosa, which grew under controlled conditions. We addressed the questions (i) do δ2Hn-alkane and δ18Osugar values allow precise reconstructions of leaf water isotope composition, (ii) how accurately does the reconstructed leaf-water-isotope composition enables relative humidity (RH) reconstruction in which the plants grew, and (iii) does the coupling of δ2Hn-alkane and δ18Osugar enable a robust source water calculation? For all investigated species, the alkane n-C29 was most abundant and therefore used for compound-specific δ2H measurements. For Vicia faba, additionally the δ2H values of n-C31 could be evaluated robustly. With regard to hemicellulose-derived monosaccharides, arabinose and xylose were most abundant and their δ18O values were therefore used to calculate weighted mean leaf δ18Osugar values. Both δ2Hn-alkane and δ18Osugar yielded significant correlations with δ2Hleaf-water and δ18Oleaf-water, respectively (r2 = 0.45 and 0.85, respectively; p 

scholarly journals Validation of a coupled <i>δ</i><sup>2</sup>H<sub><i>n</i>-alkane</sub>–<i>δ</i><sup>18</sup>O<sub>sugar</sub> paleohygrometer approach based on a climate chamber experiment

2021 ◽  
Vol 18 (19) ◽  
pp. 5363-5380
Author(s):  
Johannes Hepp ◽  
Christoph Mayr ◽  
Kazimierz Rozanski ◽  
Imke Kathrin Schäfer ◽  
Mario Tuthorn ◽  
...  
Keyword(s):  
Relative Humidity ◽  
Vicia Faba ◽  
Isotope Composition ◽  
Leaf Water ◽  
Source Water ◽  
Climate Chamber ◽  
Chamber Experiment ◽  
Leaf Wax ◽  
Hydrogen Isotope Composition ◽  
Water Isotope

Abstract. The hydrogen isotope composition of leaf-wax-derived biomarkers, e.g., long-chain n-alkanes (δ2Hn-alkane), is widely applied in paleoclimate. However, a direct reconstruction of the isotope composition of source water based on δ2Hn-alkane alone is challenging due to the enrichment of heavy isotopes during evaporation. The coupling of δ2Hn-alkane with δ18O of hemicellulose-derived sugars (δ18Osugar) has the potential to disentangle this limitation and additionally to allow relative humidity reconstructions. Here, we present δ2Hn-alkane as well as δ18Osugar results obtained from leaves of Eucalyptus globulus, Vicia faba, and Brassica oleracea, which grew under controlled conditions. We addressed the questions of (i) whether δ2Hn-alkane and δ18Osugar 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 δ2Hn-alkane and δ18Osugar enables a robust source water calculation. For all investigated species, the n-alkane n-C29 was most abundant and therefore used for compound-specific δ2H measurements. For Vicia faba, additionally the δ2H values of n-C31 could be evaluated robustly. Regarding hemicellulose-derived monosaccharides, arabinose and xylose were most abundant, and their δ18O values were therefore used to calculate weighted mean leaf δ18Osugar values. Both δ2Hn-alkane and δ18Osugar yielded significant correlations with δ2Hleaf water and δ18Oleaf water, respectively (r2=0.45 and 0.85, respectively; p<0.001, n=24). Mean fractionation factors between biomarkers and leaf water were found to be −156 ‰ (ranging from −133 ‰ to −192 ‰) for εn-alkane/leaf water and +27.3 ‰ (ranging from +23.0 ‰ to 32.3 ‰) for εsugar/leaf water, respectively. Modeled RHair values from a Craig–Gordon model using measured Tair, δ2Hleaf water and δ18Oleaf water as input correlate highly significantly with modeled RHair values (R2=0.84, p<0.001, RMSE = 6 %). When coupling δ2Hn-alkane and δ18Osugar values, the correlation of modeled RHair values with measured RHair values is weaker but still highly significant, with R2=0.54 (p<0.001, RMSE = 10 %). Finally, the reconstructed source water isotope composition (δ2Hs and δ18Os) as calculated from our coupled approach matches the source water in the climate chamber experiment (δ2Htank water and δ18Otank water). This highlights the great potential of the coupled δ2Hn-alkane–δ18Osugar paleohygrometer approach for paleoclimate and relative humidity reconstructions.

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

2020 ◽  
Vol 17 (3) ◽  
pp. 741-756 ◽  
Author(s):  
Johannes Hepp ◽  
Imke Kathrin Schäfer ◽  
Verena Lanny ◽  
Jörg Franke ◽  
Marcel Bliedtner ◽  
...  
Keyword(s):  
Isotopic Composition ◽  
Soil Ph ◽  
Isotope Composition ◽  
Leaf Water ◽  
Source Water ◽  
Central European ◽  
Ph Values ◽  
Stable Isotopic Composition ◽  
Stable Isotopic ◽  
Water Isotope

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.

The dominant environmental driver of leaf water stable isotope enrichment differs for H-2 compared to O-18

2020 ◽  
Author(s):  
Matthias Cuntz ◽  
Lucas A Cernusak ◽  
Keyword(s):  
Stable Isotope ◽  
Relative Humidity ◽  
Air Temperature ◽  
Gordon Equation ◽  
Isotope Composition ◽  
Environmental Variable ◽  
Leaf Water ◽  
Source Water ◽  
Stable Isotope Composition ◽  
Water Stable Isotope

&lt;p&gt;Several important isotopic biomarkers derive at least part of their signal from the stable isotope composition of leaf water (e.g., leaf wax &amp;#948;&lt;sup&gt;2&lt;/sup&gt;H, cellulose &amp;#948;&lt;sup&gt;2&lt;/sup&gt;H and &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O, lignin &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O). In order to interpret these isotopic proxies, it is therefore helpful to know which environmental variable most strongly controls a given leaf water stable isotope signal. We collated observations of the stable isotope compositions of leaf water, xylem water, and atmospheric vapour, along with air temperature and relative humidity, to test whether the dominant driver of leaf water &lt;sup&gt;2&lt;/sup&gt;H concentration could differ from that of &lt;sup&gt;18&lt;/sup&gt;O concentration. Our dataset comprises 690 observations from 35 sites with broad geographical coverage. We limited our analysis to daytime observations, when the photosynthetic processes that incorporate the leaf water isotopic signal primarily take place. The Craig-Gordon equation was generally a good predictor for daytime bulk leaf water stable isotope composition for both &amp;#948;&lt;sup&gt;2&lt;/sup&gt;H (R&lt;sup&gt;2&lt;/sup&gt;=0.86, p&lt;0.001) and &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O (R&lt;sup&gt;2&lt;/sup&gt;=0.63, p&lt;0.001). It showed about 10% admixture of source water was caused by unenriched water pools such as leaf veins or the P&amp;#233;clet effect. Solving the Craig-Gordon equation requires knowledge of relative humidity, air temperature, and the stable isotope compositions of source water and atmospheric vapour. However, it is not possible to invert the Craig-Gordon equation to solve for one of these parameters unless the others are known. Here we show that the two isotopic signals of &amp;#948;&lt;sup&gt;2&lt;/sup&gt;H and &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O are predominantly driven by different environmental variables: leaf water &amp;#948;&lt;sup&gt;2&lt;/sup&gt;H correlated most strongly with the &amp;#948;&lt;sup&gt;2&lt;/sup&gt;H of source water (R&lt;sup&gt;2&lt;/sup&gt;=0.68, p&lt;0.001) and atmospheric vapour (R&lt;sup&gt;2&lt;/sup&gt;=0.63, p&lt;0.001), whereas leaf water &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O correlated most strongly with air relative humidity (R&lt;sup&gt;2&lt;/sup&gt;=0.46, p&lt;0.001). We conclude that these two isotopic signals of leaf water are not simply mirror images of the same environmental information, but carry distinct signals of different climate factors, with crucial implications for the interpretation of downstream isotopic biomarkers.&lt;/p&gt;

scholarly journals Coupling δ<sup>2</sup>H and δ<sup>18</sup>O biomarker results yields information on relative humidity and isotopic composition of precipitation – a climate transect validation study

2015 ◽  
Vol 12 (12) ◽  
pp. 3913-3924 ◽  
Author(s):  
M. Tuthorn ◽  
R. Zech ◽  
M. Ruppenthal ◽  
Y. Oelmann ◽  
A. Kahmen ◽  
...  
Keyword(s):  
Fatty Acids ◽  
Isotopic Composition ◽  
Validation Study ◽  
Leaf Water ◽  
Deuterium Excess ◽  
Air Humidity ◽  
Leaf Wax ◽  
Relative Air Humidity ◽  
Paleoclimate Reconstructions ◽  
Hydrogen Isotopic Composition

Abstract. The hydrogen isotopic composition (δ2H) of leaf waxes, especially of n-alkanes (δ2Hn-alkanes), is increasingly used for paleohydrological and paleoclimate reconstructions. However, it is challenging to disentangle past changes in the isotopic composition of precipitation and changes in evapotranspirative enrichment of leaf water, which are both recorded in leaf wax δ2H values. In order to overcome this limitation, Zech M. et al. (2013) proposed a coupled δ2Hn-alkanes–δ18Osugar biomarker approach. This coupled approach allows for calculating (i) biomarker-based "reconstructed" δ2Hδ18O values of leaf water (δ2Hδ18Oleaf water), (ii) biomarker-based reconstructed deuterium excess (d-excess) of leaf water, which mainly reflects evapotranspirative enrichment and which can be used to reconstruct relative air humidity (RH) and (iii) biomarker-based reconstructed δ2Hδ18Oprecipitation values. Here we present a climate transect validation study by coupling new results from δ2H analyses of n-alkanes and fatty acids in topsoils along a climate transect in Argentina with previously measured δ18O results obtained for plant-derived sugars. Accordingly, both the reconstructed RH and δ2Hδ18Oprecipitation values correlate highly significantly with actual RH and δ2Hδ18Oprecipitation values. We conclude that compared to single δ2Hn-alkane or δ18Osugar records, the proposed coupled δ2Hn-alkane–δ18Osugar biomarker approach will allow more robust δ2Hδ18Oprecipitation reconstructions in future paleoclimate research. Additionally, the proposed coupled δ2Hn-alkane–δ18Osugar biomarker approach allows for the establishment of a "paleohygrometer", more specifically, the reconstruction of mean summer daytime RH changes/history.

scholarly journals Revisiting Mt. Kilimanjaro: Do n-alkane biomarkers in soils reflect the δ<sup>2</sup>H isotopic composition of precipitation?

2014 ◽  
Vol 11 (6) ◽  
pp. 7823-7852 ◽  
Author(s):  
M. Zech ◽  
R. Zech ◽  
K. Rozanski ◽  
A. Hemp ◽  
G. Gleixner ◽  
...  
Keyword(s):  
Relative Humidity ◽  
Isotopic Composition ◽  
Soil Samples ◽  
Leaf Water ◽  
Isotopic Enrichment ◽  
Altitude Effect ◽  
Organic Layers ◽  
Leaf Wax ◽  
Soils And Sediments ◽  
Transect Study

Abstract. During the last decade compound-specific deuterium (δ2H) analysis of plant leaf wax-derived n-alkanes has become a promising and popular tool in paleoclimate research. This is based on the widely accepted assumption that n-alkanes in soils and sediments generally reflect δ2H of precipitation (δ2Hprec). Recently, several authors suggested that δ2H of n-alkanes (δ2H,sub>n-alkanes) can also be used as proxy in paleoaltimetry studies. Here we present results from a δ2H transect study (~1500 to 4000 m a.s.l.) carried out on precipitation and soil samples taken from the humid southern slopes of Mt. Kilimanjaro. Contrary to earlier suggestions, a distinct altitude effect in δ2Hprec is present above ~2000 m a.s.l., i.e. δ2Hprec values become more negative with increasing altitude. The compound-specific δ2H values of nC27 and nC29 do not confirm this altitudinal trend, but rather become more positive both in the O-layers (organic layers) and the Ah-horizons (mineral topsoils). Although our δ2Hn-alkane results are in agreement with previously published results from the southern slopes of Mt. Kilimanjaro (Peterse et al., 2009, BG, 6, 2799–2807), a major re-interpretation is required given that the δ2Hn-alkane results do not reflect the δ2Hprec results. The theoretical framework for this re-interpretation is based on the evaporative isotopic enrichment of leaf water associated with transpiration process. Modelling results show that relative humidity, decreasing considerably along the southern slopes of Mt. Kilimanjaro (from 78% at ~ 2000 m a.s.l. to 51% at 4000 m a.s.l.), strongly controls δ2Hleaf water. The modelled δ2H leaf water enrichment along the altitudinal transect matches well the measured 2H leaf water enrichment as assessed by using the δ2Hprec and δ2Hn-alkane results and biosynthetic fractionation during n-alkane biosynthesis in leaves. Given that our results clearly demonstrate that n-alkanes in soils do not simply reflect δ2Hprec but rather δ2Hleaf water, we conclude that care has to be taken not to over-interpret δ2Hn-alkane records from soils and sediments when reconstructing δ2H of paleoprecipitation. Both in paleoaltimetry and in paleoclimate studies changes in relative humidity and consequently in δ2Hn-alkane values can completely mask altitudinally or climatically-controlled changes in δ2Hprec.

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