Palaeoclimatology of Central Asia and the northeastern Tibetan Plateau on basis of compound-specific isotope analysis of biomarkers.

• Applicant: Dr. Bernhard Aichner
• Subject Area: Palaeontology ; Mineralogy, Petrology and Geochemistry
• Term: from 2014 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 259668396

• Final Report Year: 2021

Final Report Abstract

For this project, more than 500 samples from three sediment cores have been analysed for concentrations, hydrogen and carbon isotopic values of biomarkers. The three lakes represent two study areas: the northeastern Tibetan Plateau (Lake Donggi Cona and Hala Hu) and the Central Asian Pamirs (Lake Karakul). Dating back to 19, 23 and 31 kyrs BP, the used sedimentary archives provided paleoclimatic and -ecological information about the last glacial to interglacial transition-periods and the Holocene. Hydrogen isotope values (δD) of terrestrial biomarkers showed similar trends in all three lakes, with most negative values from the late glacial to the mid-Holocene. This reflects the changes of vapor source to the study areas, caused by the strengthening and weakening of the Asian monsoonal branches and consequent relocation of the westerlies pathways. Compound-specific radiocarbon dating of those biomarkers revealed no significant lag-time due to pre-aging in soils before being transported to the lake sediment. Ecological and hydrological responses were complex in all studied lake ecosystems and their catchments. Distinct expansion phases of alpine grasses 31-30, 27-26 and 17-14 kyrs BP were observed in the catchment of Lake Karakul, which could be related to short-term Northern Hemispheric rapid climate events. Synchronous positive shifts of aquatic δD values gave evidence for changes in lake hydrology, resulting in lake levels up to 35 m higher than at present. Multi-proxy data and outputs of climate models suggested that changes in seasonality of the precipitation-evaporation balance, and specifically wetter summers during mentioned episodes, were the most likely drivers behind such observations. On the northeastern Tibetan Plateau, pronounced phases of appearance and disappearance of aquatic biomarkers, and shifts in theirs δD values, showed distinct changes in lake hydrology and in composition of aquatic communities. The data from Hala Hu further suggest an asynchronous response of the lake ecosystem to monsoonal strengthening. This gave evidence that not shifts of vapour source or increased precipitation amount were the major triggers of ecosystem responses. Instead warmer temperatures during the Holocene optimum, receding glaciers and consequently increased meltwater discharge into the lake, caused the pronounced changes in phytoplankton communities. Other than in the near Lake Qinghai there is no indication from alkenone indices for a pronounced cooling event 5 - 3 kyrs BP. Instead, the termination of the mid-Holocene optimum at Hala Hu is characterized by a gradual decline of terrestrial vegetation and a rapid decrease of phytoplankton abundance in combination with changes in lake hydrology. The results illustrate how sensitive high-altitude lake ecosystems react to changes of hydroclimatic triggers. Here, independent from monsoonal intensity, temperature rise and related meltwater influx had the strongest impact on aquatic communities. This is of relevance when assessing future lake ecosystem responses in context of global warming.

Publications

• Biosynthetic hydrogen isotopic fractionation factors during lipid synthesis in submerged aquatic macrophytes: Effect of groundwater discharge and salinity. Organic Geochemistry 113, 10-16, 2017
  Aichner, B., Hilt, S., Perillon, C., Gillefalk, M., Sachse, D.
  
• Climate variability in the past ~19,000 yr in NE Tibetan Plateau inferred from biomarker and stable isotope records of Lake Donggi Cona. Quaternary Science Reviews 157, 129-140, 2017
  Saini, J., Günther, F., Aichner, B., Mischke, S., Herzschuh, U., Zhang, C., Mäusbacher, R., Gleixner, G.
  
• Radiocarbon and optically stimulated luminescence dating of sediments from Lake Karakul, Tajikistan. Quaternary Geochronology 41, 51-61. 2017
  Mischke, S., Lai, Z., Aichner, B., Heinecke, L., Mahmoudov, Z., Kuessner, M., Herzschuh, U.
  
• Hydroclimate in the Pamirs was driven by changes in precipitation‐evaporation seasonality since the last glacial period. Geophysical Research Letters 46, 2019 GL085202
  Aichner, B., Makhmudov, Z., Rajabov, I., Zhang, Q., Pausata, F. S. R., Werner, M., Heinecke, L, Kuessner, M., Feakins, S., Sachse, D., Mischke, S.
  
• Do radiocarbon ages of plant wax biomarkers agree with 14C-TOC/OSL-based age models in an arid high-altitude lake system? Radiocarbon, Vol. 63. 2021, Issue 6, pp. 1575 - 1590.
  Aichner, B., Gierga, M., Stolz, A., Mętrak, M., Wilk, M., Suska-Malawska, M., Mischke, S., Sachse, D., Rajabov, I., Rajabov, N., Rethemeyer, J.
  
• Local effects on soil leaf wax hydrogen isotopes along a west to east transect through the Pamirs, Tajikistan. Organic Geochemistry 160, 104272, 2021
  Aichner, B., Rajabov, N., Shodmonov, M., Mętrak, M., Suska-Malawska, M., Strecker, M., Sachse, D.
  
• Asynchronous responses of aquatic ecosystems to hydroclimatic forcing on the Tibetan Plateau. Communications Earth & Environment, Vol. 3 2022, Article number: 3.
  Aichner, B., Wünnemann, B., Callegaro, A., van der Meer, M.T.J., Yan, D., Zhang, Y., Barbante, C., Sachse, D.