Aeolian sediments on the northern Tibetan Plateau

Aachen (2017) [Habil / Postdoctoral Thesis (Non-german Habil)]

Page(s): 1 Online-Ressource (III, 185 Seiten) : Illustrationen, Diagramme, Karten

Abstract

In the frame of this thesis a new conceptual model was developed for the application of aeolian sediments for palaeoclimate reconstruction on the northern Tibetan Plateau (TP). It was highlighted that the previous state of the environment is an important parameter for the interpretation of aeolian sediments. Late glacial to early Holocene sediments indicate increasing moisture, while reactivation in the late Holocene is caused by a reduction in precipitation. The model was used to reconstruct the palaeoclimate in three basins on the northern TP (Gonghe Basin, Donggi Cona Basin and Heihai Basin). However, the complex sediment cascade with varying source areas through time, different topography, and frequent erosion requires a detailed basin-wide analysis of the transportation pathways and sediment deposits. Erosion is frequently occurring in the different terrestrial archives on the northern TP, resulting in the need for a relatively large number of sections. In the three different basins the entire sediment cascade of the aeolian system was reconstructed. Each system has a slightly different configuration, enabling a detailed understanding of the systems behaviour. The Gonghe Basin is dominated by large fluvial terraces of the Yellow River, providing vast amounts of material for aeolian processes. In combination with the width of the basin, high aeolian activity results in the formation of active dune systems and a well-developed spatial separation of different types of sediments. The Donggi Cona Basin is dominated by a large lake. Here, lake level variations have a profound influence on sediment availability. High accumulation rates of aeolian landforms are not directly related to climate variations in this case. However, the timing of sediment mobilisation and accumulation is a good indicator of climatic changes. A clear separation in the local geology in the Heihai Basin enabled a detailed reconstruction of different local transportation pathways. Furthermore, all three study areas are located on an east-west transect, following a reduction in present day precipitation in the area. An analysis of the aeolian sediments in all three areas showed a time-transgressive penetration of the Asian Summer monsoon (ASM) on the northern TP from the east during the late glacial and the early Holocene. A reverse movement was observed for the retreat of the ASM in the late Holocene.Besides the local case studies, two meta-analyses for aeolian sediments were conducted. The first one is including all published OSL (optical stimulated luminescence) ages from the TP for the last 21 ka and identified several millennial scale climate changes. Dry and cold glacial times were followed by climate amelioration during the late glacial as a consequence of the post-glacial strengthening of the Asian summer monsoon. The mid Holocene was characterized by highest moisture conditions during a fully developed monsoonal system. Aeolian processes were strongly reduced and fluvial reworking occurred. During the late Holocene, aeolian activity increased again, first in the western part of the study area and later at the eastern side of the TP. A second study analysed multi-decadal climate fluctuation on the north-eastern TP. Six phases of enhanced aeolian activity with a duration of 80 to 200 years could be linked to phases of low temperature of the northern hemisphere, reduced total solar irradiance and a reduced ASM. The results from this study show that aeolian sediments are a suitable proxy for palaeoclimate reconstruction on the northern TP despite that most of the studied archives were influenced by erosional processes.