TY - JOUR A1 - Aichner, Bernhard A1 - Herzschuh, Ulrike A1 - Wilkes, Heinz A1 - Schulz, Hans-Martin A1 - Wang, Yongbo A1 - Plessen, Birgit A1 - Mischke, Steffen A1 - Diekmann, Bernhard A1 - Zhang, Chengjun T1 - Ecological development of Lake Donggi Cona, north-eastern Tibetan Plateau, since the late glacial on basis of organic geochemical proxies and non-pollen palynomorphs JF - Palaeogeography, palaeoclimatology, palaeoecology : an international journal for the geo-sciences N2 - Organic geochemical proxy data from surface sediment samples and a sediment core from Lake Donggi Cona were used to infer environmental changes on the northeastern Tibetan Plateau spanning the last 18.4 kyr. Long-chain n-alkanes dominate the aliphatic hydrocarbon fraction of the sediment extract from most surface sediment samples and the sediment core. Unsaturated mid-chain n-alkanes (nC(23:1) and nC(25:1)) have high abundances in some samples, especially in core samples from the late glacial and early Holocene. TOC contents, organic biomarker and non-pollen-palynomorph concentrations and results from organic petrologic analysis on selected samples suggest three major episodes in the history of Lake Donggi Cona. Before ca. 12.6 cal ka BP samples contain low amounts of organic matter due to cold and arid conditions during the late glacial. After 12.6 cal ka BP, relatively high contents of TOC and concentrations of Botryococcus fossils, as well as enhanced concentrations of mid-chain n-alkanes and n-alkenes suggest a higher primary and macrophyte productivity than at present This is supported by high contents of palynomorphs derived from higher plants and algae and was possibly triggered by a decrease of salinity and amelioration of climate during the early Holocene. Since 6.8 cal ka BP Lake Donggi Cona has been an oligotrophic freshwater lake. Proxy data suggest that variations in insolation drive ecological changes in the lake, with increased aquatic productivity during the early Holocene summer insolation maximum. Short-term drops of TOC contents or biomarker concentrations (at 9.9 cal ka BP, after 8.0 and between 3.5 and 1.7 cal ka BP) can possibly be related to relatively cool and dry episodes reported from other sites on the north-eastern Tibetan Plateau, which are hypothesized to occur in phase with Northern Hemisphere cooling events. KW - Biomarker KW - Holocene KW - n-alkanes KW - Total organic carbon KW - Organic matter KW - Macerals KW - Aquatic macrophytes Y1 - 2012 U6 - https://doi.org/10.1016/j.palaeo.2011.10.015 SN - 0031-0182 VL - 313 IS - 2 SP - 140 EP - 149 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Sachse, Dirk A1 - Dawson, Todd E. A1 - Kahmen, Ansgar T1 - Seasonal variation of leaf wax n-alkane production and delta H-2 values from the evergreen oak tree, Quercus agrifolia JF - Isotopes in environmental and health studies N2 - In order to understand the timing of leaf wax synthesis in higher plants, we analysed the variability in leaf wax n-alkane concentration, composition (expressed as average chain length (ACL)), and delta H-2(wax) values as well as plant source water delta H-2 values (xylem and leaf water) in the evergreen tree Quercus agrifolia over a period of 9 months, beginning with leaf flush. We identified three distinct periods of leaf development with the first month following leaf flush being characterized by de novo synthesis and possibly removal of n-alkanes. During the following 3 months, n-alkane concentrations increased sevenfold and delta H-2(wax) and ACL values increased, suggesting this period was the major leaf wax n-alkane formation period. During the remaining 4 months of the experiment, stable values suggest cessation of leaf wax n-alkane formation. We find that n-alkane synthesis in Q. agrifolia takes place over 4 months, substantially longer than that observed for deciduous trees. KW - leaf wax KW - oak tree KW - hydrogen-2 KW - isotope ecology KW - n-alkanes Y1 - 2015 U6 - https://doi.org/10.1080/10256016.2015.1011636 SN - 1025-6016 SN - 1477-2639 VL - 51 IS - 1 SP - 124 EP - 142 PB - Routledge, Taylor & Francis Group CY - Abingdon ER - TY - JOUR A1 - Saini, Jeetendra A1 - Guenther, Franziska A1 - Aichner, Bernhard A1 - Mischke, Steffen A1 - Herzschuh, Ulrike A1 - Zhang, Chengjun A1 - Maeusbacher, Roland A1 - Gleixner, Gerd T1 - Climate variability in the past similar to 19,000 yr in NE Tibetan Plateau inferred from biomarker and stable isotope records of Lake Donggi Cona JF - Quaternary science reviews : the international multidisciplinary research and review journal N2 - We investigated 4.84-m-long sediment record spanning over the Late Glacial and Holocene from Lake Donggi Cona to be able to reconstruct circulation pattern on the Tibetan Plateau (TP). Presently, Lake Donggi Cona is located at the boundaries of Westerlies and Asian monsoon circulations in the northeastern TP. However, the exact timing and stimulating mechanisms for climatic changes and monsoon shifts in this region are still debated. We used a 19-ka-long stable isotope record of sedimentary n-alkanes to address this discrepancy by providing insights into paleohydrological conditions. The SD of nC(23) is influenced by lake water evaporation; the BD. values of sedimentary nC(29) are mainly controlled by moisture source and temperature changes. Long-chain n-alkanes dominate over the core whereas three mean clusters (i.e. microbial, aquatic and terrestrial) can be inferred. Multi-proxies suggest five major episodes in the history of Lake Donggi Cona. The Lake Donggi Cona record indicates that the Late Glacial(18.4-14.8 cal ka BP) was dominated by low productivity of mainly microbial and aquatic organisms. Relatively low delta D values suggest low temperatures and moist conditions eventually caused by stronger Westerlies, winter monsoon and melt-water influence. Likely, the shift (similar to 17.9 cal ka BP) from microbial to enhanced aquatic input suggests either a change from deep to shallow water lake or a break in local stratification. Between 14.8 and 13.0 cal ka BP, variable climatic conditions prevailed. Although the Westerlies weekend, the increase in temperature enhanced the permafrost and snow melting (displayed by a high sedimentary accumulation rate). Higher delta D values indicate increasingly arid conditions with higher temperatures which eventually lead to high evaporative conditions and lowest lake levels. Low vegetation cover and high erosion rates led to high sediment accumulation resulting in stratification followed by anoxia in the terminal lake. From 13.0 to 9.2 cal ka BP, lowered values of 813 along with high contents of terrestrial organic matter marked the early-Holocene warming indicating a further strengthening of summer precipitation and higher lake levels. A cooling trend was observed in the mid-Holocene between 9.2 and 3.0 cal ka BP accompanied by higher moisture availability (displayed by lowered SD values) caused by reduced evaporative conditions due to a drop in temperature and recovering Westerlies. After 3.0 cal ka BP, a decrease in lake productivity and cold and semi-arid conditions prevailed suggesting lower lake levels and reduced moisture from recycled air masses and Westerlies. We propose that the summer monsoon was the predominant moisture source during the Belling-Allered warm complex and early -Holocene followed by Westerlies in mid-to-late Holocene period. Stable carbon isotope values-32%o indicate the absence of C-4 -type vegetation in the region contradicting with their presence in the Lake Qinghai record. The 81) record from lake Donggi Cona highlights the importance of the interplay between Westerlies and summer monsoon circulation at this location, which is highly dynamic in northeastern plateau compared to the North Atlantic circulation and insolation changes. Consequently lake Donggi Cona might be an important anchor point for environmental reconstructions on the Tibetan Plateau. (C) 2017 The Authors. Published by Elsevier Ltd. KW - n-alkanes KW - Hydrogen isotopes (delta D) KW - Carbon isotopes (delta C-13) KW - Carbon preference index (CPI) KW - Westerlies KW - Continental air masses KW - Precipitation KW - Late Glacial and Holocene Y1 - 2017 U6 - https://doi.org/10.1016/j.quascirev.2016.12.023 SN - 0277-3791 VL - 157 SP - 129 EP - 140 PB - Elsevier CY - Oxford ER - TY - THES A1 - van der Veen, Iris T1 - Defining moisture sources and (palaeo)environmental conditions using isotope geochemistry in the NW Himalaya N2 - Anthropogenic climate change alters the hydrological cycle. While certain areas experience more intense precipitation events, others will experience droughts and increased evaporation, affecting water storage in long-term reservoirs, groundwater, snow, and glaciers. High elevation environments are especially vulnerable to climate change, which will impact the water supply for people living downstream. The Himalaya has been identified as a particularly vulnerable system, with nearly one billion people depending on the runoff in this system as their main water resource. As such, a more refined understanding of spatial and temporal changes in the water cycle in high altitude systems is essential to assess variations in water budgets under different climate change scenarios. However, not only anthropogenic influences have an impact on the hydrological cycle, but changes to the hydrological cycle can occur over geological timescales, which are connected to the interplay between orogenic uplift and climate change. However, their temporal evolution and causes are often difficult to constrain. Using proxies that reflect hydrological changes with an increase in elevation, we can unravel the history of orogenic uplift in mountain ranges and its effect on the climate. In this thesis, stable isotope ratios (expressed as δ2H and δ18O values) of meteoric waters and organic material are combined as tracers of atmospheric and hydrologic processes with remote sensing products to better understand water sources in the Himalayas. In addition, the record of modern climatological conditions based on the compound specific stable isotopes of leaf waxes (δ2Hwax) and brGDGTs (branched Glycerol dialkyl glycerol tetraethers) in modern soils in four Himalayan river catchments was assessed as proxies of the paleoclimate and (paleo-) elevation. Ultimately, hydrological variations over geological timescales were examined using δ13C and δ18O values of soil carbonates and bulk organic matter originating from sedimentological sections from the pre-Siwalik and Siwalik groups to track the response of vegetation and monsoon intensity and seasonality on a timescale of 20 Myr. I find that Rayleigh distillation, with an ISM moisture source, mainly controls the isotopic composition of surface waters in the studied Himalayan catchments. An increase in d-excess in the spring, verified by remote sensing data products, shows the significant impact of runoff from snow-covered and glaciated areas on the surface water isotopic values in the timeseries. In addition, I show that biomarker records such as brGDGTs and δ2Hwax have the potential to record (paleo-) elevation by yielding a significant correlation with the temperature and surface water δ2H values, respectively, as well as with elevation. Comparing the elevation inferred from both brGDGT and δ2Hwax, large differences were found in arid sections of the elevation transects due to an additional effect of evapotranspiration on δ2Hwax. A combined study of these proxies can improve paleoelevation estimates and provide recommendations based on the results found in this study. Ultimately, I infer that the expansion of C4 vegetation between 20 and 1 Myr was not solely dependent on atmospheric pCO2, but also on regional changes in aridity and seasonality from to the stable isotopic signature of the two sedimentary sections in the Himalaya (east and west). This thesis shows that the stable isotope chemistry of surface waters can be applied as a tool to monitor the changing Himalayan water budget under projected increasing temperatures. Minimizing the uncertainties associated with the paleo-elevation reconstructions were assessed by the combination of organic proxies (δ2Hwax and brGDGTs) in Himalayan soil. Stable isotope ratios in bulk soil and soil carbonates showed the evolution of vegetation influenced by the monsoon during the late Miocene, proving that these proxies can be used to record monsoon intensity, seasonality, and the response of vegetation. In conclusion, the use of organic proxies and stable isotope chemistry in the Himalayas has proven to successfully record changes in climate with increasing elevation. The combination of δ2Hwax and brGDGTs as a new proxy provides a more refined understanding of (paleo-)elevation and the influence of climate. N2 - Die Auswirkungen des menschgemachten Klimawandels wirken sich auch auf den Wasserkreislauf aus. Während manche Regionen höhere Niederschlagsmengen zu erwarten haben, werden andere mit stärkeren und häufigeren Trockenperioden zu konfrontiert sein. Diese Veränderungen haben einen unmittelbaren Einfluss auf Evaporation, Langzeit-Wasserreservoire, Grundwasserbildung, Schneefall und Gletscher. Da Gebirge und Hochplateaus überdurchschnittlich von den Auswirkungen des Klimawandels betroffen sind, ist die Wasserversorgung der Menschen entlang der dort entspringenden Flüsse gefährdet. Insbesondere der Himalaya gilt als instabile Region, dessen Abflüsse die Wasserversorgung von annähernd einer Milliarde Menschen gewährleisten. Um zu erwartende Veränderungen des Wasserbudgets in Abhängigkeit von verschiedenen möglichen Klimawandelszenarien abschätzen zu können, ist ein detaillierteres Verständnis des Wasserkreislaufs in Hochgebirgen und -plateaus erforderlich. Neben dem globalen Klimawandel gibt es weitere Faktoren, die sich auf den Wasserkreislauf auswirken. Das Wechselspiel zwischen Gebirgsbildung und klimatischen Bedingungen beeinflusst den Wasserkreislauf auf geologischen Zeitskalen. Entsprechende Veränderungen und ihre Auswirkungen lassen sich jedoch nur eingeschränkt bestimmen. Mittels geeigneter Proxies für höhenbedingte Änderungen der Hydrologie lassen sich der Orogeneseverlauf sowie dessen klimatische Auswirkungen allerdings genauer rekonstruieren. In der vorliegenden Arbeit werden die Verhältnisse stabiler Isotope (als δ2H und δ18O ausgedrückt) von meteorischen Wassern sowie von organischem Material mit Methoden der Satellitenfernerkundung als Indikator für atmosphärische und hydrologische Prozesse kombiniert, um ein besseres Verständnis der verschiedenen Wasserquellen des Himalaya zu erlangen. Darüber hinaus wurde der Link zwischen modernen klimatischen Bedingungen und verbindungsspezifischen stabilen Isotopen von Blattwachsen (δ2Hwax) sowie von brGDGTs (branched Glycerol dialkyl glycerol tetraethers) rezenter Bodenproben aus den Einzugsgebieten vierer Flüsse des Himalaya hergestellt, um sie als Paläo-Klima- und Paläo-Höhenproxy verwenden zu können. Zu guter Letzt wurden hydrologische Veränderungen auf einer Zeitskala von 20 Mio. Jahren anhand von δ13C- and δ18O-Werten von Bodencarbonat und organischem Material aus Sedimentschnitten der pre-Siwalik und Siwalik-Einheiten nachvollzogen. Die Erkenntnisse dieser tragen zu einer deutlich genaueren Rekonstruktion von Vegetationsänderungen und der Entwicklung der Monsun-Intensität sowie -Saisonalität bei. Die Isotopenzusammensetzung der Oberflächenwasser der untersuchten Flüsse wird hauptsächlich durch Rayleigh-Destillation der im Wesentlichen vom Indischen Sommer Monsun eingetragenen Feuchtigkeit bestimmt. Der durch Satellitenfernerkundungsdaten bestätigte Anstieg des Deuterium-Exzesses (d-excess) im Frühjahr verdeutlicht den signifikanten Einfluss von Schnee- und Gletscherschmelze, der auch in Zeitreihen von Oberflächenwasserproben erkennbar ist. Sowohl brGDGT als auch δ2Hwax können potentiell die absolute Höhe zum Zeitpunkt ihrer Synthese abbilden, da sie stark mit der Lufttemperatur, bzw. mit Oberflächenwasser δ2H und somit indirekt auch mit der Höhe korreliert sind. Im direkten Vergleich der mittels brGDGT und δ2Hwax rekonstruierten Höhen ergaben sich insbesondere in ariden Teilen der Höhenprofile große Unterschiede. Diese sind hauptsächlich auf verstärkte Evapotranspiration und deren Auswirkung auf Pflanzenwasser und -wachse zurückzuführen. Basierend auf den Erkenntnissen der vorliegenden Arbeit können weitere vergleichende Untersuchungen beider Proxies genauere Paläo-Höhenstudien ermöglichen. Diese Arbeit zeigt, dass die Isotopie von Oberflächenwassern genutzt werden kann, um den sich ändernden Wasserhaushalt des Himalya im Kontext voraussichtlich weiter ansteigender Temperaturen zu beobachten. Unsicherheiten bei der Rekonstruktion von Paläo-Höhen konnten durch eine vergleichende Analyse zweier organischer Proxies (δ2Hwax and brGDGTs) aus Paläo-Bodenproben des Himalayas minimiert werden. Verhältnisse stabiler Isotope von Blattwachsen aus diesen Bodenproben spiegeln die Entwicklung der Vegetation unter dem Einfluss des Monsuns im späten Miozän wider. Zusammenfassend wurde erfolgreich gezeigt, dass organische Proxies und stabile Isotope höhenabhängige Änderungen des Klimas im Himalaya aufzeichnen können. Die Kombination von δ2Hwax and brGDGTs als neuer Proxy ermöglicht eine deutlich differenziertere Betrachtung von rekonstruierten Paläo-Höhen sowie Paläo-Klima. KW - stable isotope KW - Himalaya KW - n-alkanes KW - d-excess KW - biomarker KW - paleohydrology KW - GDGT KW - GDGT KW - Himalaya KW - Biomarker KW - Deuterium Exzesses KW - n-alkane KW - Paläohydrologie KW - stabilen Isotopen Y1 - 2021 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-514397 ER -