title: Beyond single biomarker-isotope studies in paleoclimatology : potential and limitations of a coupled δ²Hn-alkane-δ¹⁸Osugar paleohygrometer approach
Creators: name: Hepp, Johannes
Publisher: name: University of Bayreuth
Year: 2019
Subjects: subject: hydroclimate reconstruction
subject: paleoclimate proxies
subject: biomarker
subject: leaf water
subject: evapo(trans)pirative enrichment
subject: deuterium-excess
subject: relative humidity
subject: 500
subject: 540
subject: 550
subject: 580

description: Stable water isotopes have significantly contributed to the reconstruction of climate history qualitatively and quantitatively during the last decades. The hydrogen isotopic composition (δ2H) of terrestrial leaf wax-derived n-alkanes is used as source water δ2H recorder, and is often interpreted as δ2Hprecipitation. However, δ2Hn-alkane is not only influenced by δ2Hprecipitation changes, but also by the incorporation of the leaf water enrichment signal caused by evapotranspiration. Therefore, single n-alkane δ2H-based climate proxies are often interpreted only quantitatively. Oxygen isotopic composition (δ18O) of hemicellulose-derived sugars can be interpreted comparable to the δ2H of n-alkanes. By combining δ2H n-alkane with δ18Osugar results, potentially a powerful tool is available for disentangling between source water and evapotranspirative enrichment changes. Such a coupled δ2Hn-alkane-δ18Osugar approach was shown to derive quantitative hydroclimate records, i.e. past δ2Hsource-water, δ18Osource-water and relative air humidity (RHair) values, respectively. In previous studies, this coupling was therefore introduced as paleohygrometer approach in analogy to the often used paleothermometer approaches in Quaternary research. Within this PhD thesis I aim at contributing to the validation and the broader application of the coupled δ2Hn-alkane-δ18O sugar paleohygrometer approach, because a detailed validation and particularly the application to lake sediments is still missing. The first two studies are therefore addressing the validation of the coupled δ2Hn-alkane-δ18Osugar (paleohygrometer) approach. Firstly, leaf sample material from a climate chamber experiment conducted with three different plant species was analyzed. The climate chamber experiment showed that leaf δ2Hn-alkane and δ18Osugar are well correlated with δ2Hleaf-water, δ18Oleaf-water (r2 = 0.45 and 0.85, respectively, p &lt; 0.001, n = 24). RHair was robustly reconstructed based on a simplified Craig-Gordon model. The second validation approach is a European topsoil transect study. It revealed that the coupled δ2Hn-alkane-δ18Osugar approach allows the reconstruction of δ2Hsource-water, δ18Osource-water and mean RH during day-time and vegetation period (RHMDV). However, systematic offsets between biomarker-based (reconstructed) δ2Hsource-water, δ18Osource-water and RHMDV values and a clear larger range compared to δ2Hprecipitation, δ18Oprecipitation and climate station RHMDV, respectively, were observed. The application of the coupled δ2Hn-alkane-δ18Osugar approach to the terrestrial climate archive Maundi (Mt. Kilimanjaro, Tanzania) was successful, allowing the reconstruction of δ2Hsource-water, δ18Osource-water and day-time RH (RHD) throughout the last ≈ 100 ka for East African region. The observed strong positive relationship between the biomarker-based δ2Hsource-water, δ18Osource-water and deuterium-excess of leaf water values indicates that an amount effect in precipitation isotope composition seems not to be present on long time scales. In order to provide the backbone for applying the coupled δ2Hn-alkane-δ18Osugar approach, a source identification study of terrestrial versus aquatic sugar biomarkers in lake sediments was conducted. For Late Glacial-Early Holocene sediments of Lake Gemündener Maar (Western Eifel region, Germany), the results show that arabinose is primarily of terrestrial origin while fucose and xylose stem predominantly from aquatic sources. This allows for using δ18Oarabinose and terrestrial δ2Hn-alkane results from Lake Gemündener Maar sediments in a coupled δ2Hn-alkane-δ18Osugar paleohygrometer approach in order to derive a day-time vegetation period RH (RHdv) record. The results challenge the paradigmatic view that the Younger Dryas is characterized by dry climatic conditions. They rather suggest that a relatively wet phase at the beginning of the Younger Dryas prevailed, which is followed by a more drier ending of the Younger Dryas. Also, large RHdv changes during the Early Holocene were obvious which are more pronounced than the variations during the Allerød-Younger Dryas transition phase. Finally, I aimed to apply the coupled δ2Hn-alkane-δ18Osugar approach also to the Late Glacial-Early Holocene sediments of Lake Bergsee (Southern Black Forest, Germany) in order to validate or falsify the findings from Lake Gemündener Maar. However, the source identification strongly suggest that the sugar biomarkers in that lake are primarily of aquatic origin. Hence, a coupling of terrestrial δ2Hn-alkane with δ18Osugar records was not feasible. At the same time, the coupling of aquatic δ2Hn-alkane with δ18Osugar records was not possible either, because n-C23 could not be considered as robust aquatic biomarker. Overall, the findings of the studies I conducted for this thesis highlight the large potential to derive quantitative hydroclimate information from the coupled δ2Hn-alkane-δ18Osugar (paleohygrometer) approach. At the same time, the reconstruction of δ2Hleaf-water from δ2Hn-alkane turned out to be a major uncertainty, representing the limitation regarding the reconstruction of rather small variability of δ2Hsource-water, δ18Osource-water and RH. Furthermore, a clear differentiation between terrestrial or aquatic origins of the n-alkane and sugar biomarkers seems to be of fundamental importance for a successful application of the coupled δ2Hn-alkane-δ18Osugar approach to lacustrine archives.