Aeolian Sediments on the Northern Tibetan Plateau

Aeolian sediments on the northern Tibetan Plateau Von der Fakultät für Georessourcen und Materialtechnik der Rheinisch-Westfälischen Technischen Hochschule Aachen genehmigte Habilitationsschrift von Dr. rer. nat. Georg Stauch Gutachter: Univ.-Prof. Dr. rer. nat. F. Lehmkuhl Univ.-Prof. Dr. rer. nat. R. Mäusbacher Prof. Dr. L. Owen Tag der Habilitation: 21. Juni 2016 Diese Habilitation ist auf den Internetseiten der Universitätsbibliothek online verfügbar. In memory of Andreas Stauch (03.03.1977 – 16.09.2012). I Content Summary ................................................................................................................................................ III 1 Introduction ..................................................................................................................................... 1 2 Current state of research ................................................................................................................ 5 2.1 The Asian monsoon system ..................................................................................................... 5 2.2 Aeolian sediments and palaeoclimate reconstructions ........................................................ 11 3 Short summaries of the articles .................................................................................................... 16 4 Local case studies from the northern Tibetan Plateau ................................................................. 26 4.1 Environmental changes during the late Pleistocene and the Holocene in the Gonghe Basin, north-eastern Tibetan Plateau .......................................................................................................... 26 4.2 Aeolian sediments on the north-eastern Tibetan Plateau .................................................... 45 4.3 Interaction of geomorphological processes on the north-eastern Tibetan Plateau during the Holocene, an example from a sub-catchment of Lake Donggi Cona ................................................ 69 4.4 Landscape and climate during the late Quaternary on the northern Tibetan Plateau ......... 91 5 Synthesis of OSL ages from the Tibetan Plateau ......................................................................... 115 5.1 Geomorphological and palaeoclimate dynamics recorded by the formation of aeolian archives on the Tibetan Plateau ...................................................................................................... 115 5.2 Multi-decadal periods of enhanced aeolian activity on the north-eastern Tibet Plateau during the last 2ka ........................................................................................................................... 138 6 Conclusions .................................................................................................................................. 151 References ....................................................................................................................................... 156 II Summary 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. III 1 Introduction Aeolian sediments are an important geo-archive and are frequently used to reconstruct past geomorphological processes and related climate changes. Large variations in the type, composition and preservation of aeolian sediments exist. In contrast to many other archives, they can be directly dated by optical stimulated luminescence. The dating methods captures the last exposure to sunlight and therefore, with some limitations, the last transportation process. This is particularly valuable in an area like the northern Tibetan Plateau (TP). Geo-archives are in many cases only partly preserved on the TP and in the surrounding dry lands. Large uncertainties regarding the interpretation of the proxies from these archives exist. However, the area was identified as especially sensitive to climate change (Colman et al., 2007; Y. Wang et al., 2010; An et al., 2012). The northern TP is located at the interplay between the Asian summer monsoon (ASM), the Asian winter monsoon (AWM) and the mid-latitude westerlies (Böhner, 2006) (Fig. 1.1). Despite intensive research during the last two decades, considerable differences occur among reconstructed palaeoclimate variations. These are for example, uncertainties in the first increase in perception related to the onset of the ASM after the dry and cold glacial times. In the Qinghai Lake Shen et al. (2005) reconstructed an early onset at around 17 ka, while An et al. (2012) assumed increased precipitation only at the beginning of the Holocene. Divergent interpretations also exist regarding the maximum strength of the ASM in the area. Several reconstructions indicate an early Holocene moisture maximum (Jin et al., 2015; Wang et al., 2015; Thomas et al., 2016) while other show a mid-Holocene maximum (Shen et al., 2005; Mischke et al., 2008; Liu et al., 2013a). As a consequence there is a considerable debate if the area was/is influenced by the Indian summer monsoon (ISM) or the East Asian summer monsoon (EASM), the two most important subsystems of the ASM (Y. Wang et al., 2010; Liu et al., 2015). Similar uncertainties exist regarding the onset of the late Holocene dry period as well as the timing of short term climate change during the last 2ka. However, the timing of the late Holocene climate changes are of special interest, as they presumably had an impact on Chinese civilizations (Zhang et al., 2008; Dong et al., 2016). Fig. 1.1: The Tibetan Plateau and the main atmospheric systems (according to Böhner, 2006; Yao et al., 2012). The northern limit of ASM according to Chen et al. (2010; green line), An et al. (2012; red) and P. X. Wang et al. (2014; blue). The white box indicates the study area. 1 Some of the divergence in the interpretation of the archives might be related to different interpretation of the proxies. One of the main archives used on the TP are lake systems. Comparisons of different lakes

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