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 -