The aeolian sedimentary system along the transition between the Qilian Shan and northern Chinese drylands during the late Quaternary
Nottebaum, Veit; Lehmkuhl, Frank (Thesis advisor); Diekmann, Bernhard (Thesis advisor)
Dissertation / PhD Thesis
Atmospheric circulation patterns and resulting winds are responsible for moisture transport and climatic conditions in given regions. Furthermore, resulting winds entrain, transport, and accumulate particulate matter. Thus, whenever conditions are suitable for long-term deposition and accumulation of aeolian sediments, promising archives are constructed to study past environmental conditions. In central Asia, different large-scale wind systems (i.e., Westerlies, the Asian monsoon system) interact along the transition between the Tibetan Plateau, the Chinese Loess Plateau, and north-western Chinese Deserts. This study focuses on the distribution of (mainly aeolian) sediments, the different involved geomorphological processes, and palaeoenvironmental implications. A set of almost 800 sedimentological samples and 58 optically stimulated luminescence samples was obtained from the research area along the northern margin of the Qilian Shan mountain range. A variety of geomorphological settings was sampled in order to assess spatial and temporal distribution patterns and evaluate geomorphological influences and palaeoenvironmental changes among sediment properties. Between the partly glaciated Qilian Shan (<5700 m asl) and the foreland basins (i.e., Hexi Corridor; <2000 m asl), a sediment classification based on grain size analysis yields six major sediment types which are distributed in three sedimentological landscape units along an altitudinal cross section: I) Periglacial debris and glaciofluvial sediments dominate in altitudes above 3800 m asl. II) Primary loess dominates between 3800 and 2000 m asl. III) Below 2000 m asl aeolian sands alternate with reworked loess, lacustrine deposits, and gravel gobi surfaces. Focusing on loess grain size in the Qilian Shan, a significant fining trend is evident with increasing altitude. This is explained by a relatively increased contribution from far distant dust sources caused by stronger Westerly influence in higher altitudes. Additionally, restricted sand source areas compared to flat foreland topography, such as narrow mountain valleys, dominate in higher elevations. The same phenomenon is observed in the lateral perspective: The sand and coarse silt content of primary loess decreases with increasing distance to fluvial storages. Loess deposition started in the area during the Pleistocene-Holocene transition at around 12 ka. Statistical decomposition of loess grain size distributions resulted in three contributing end-members, which were used to reconstruct the relative contribution from different transport processes. These represent I) short-distance transport from mainly fluvial storages, II) saltation and suspension in dust storm events, and III) long distance transport in high suspension clouds. As the relative proportion of each of these transport processes is depending on the surrounding geomorphological and environmental setting, a reconstruction of landscape conditions can be approached. Although bulk grain size variations were only slight, a consistent reconstruction of environmental conditions, with respect to spatial geomorphologic variability, is presented. The interpretation yields a threefold separation of the last ca. 12 ka: During the onset of loess deposition the foreland areas exhibited vast alluvial fan surfaces providing large sand amounts for deflation, which is found in the local transport component of loess deposits. This observation reflects a rather instable landscape state, still influenced by the glacial time’s sediment supply. The mid-Holocene loess exhibits the highest proportion of fine materials, indicative for far-travelled dust. This implies that surrounding surfaces were relatively stable and sand availability was low as a result of the yet developed loess cover on alluvial fan surfaces. As prerequisite for further dust-trapping, a stabilizing vegetation cover likely existed and implies comparably moist climatic conditions. The late Holocene records exhibit a declining trend of the fine particle proportion and an enhanced dust storm contribution. This reflects higher availability of dust particles in dust storm source- and bypassing areas (e.g., lacustrine and fluvial deposits of northerly deserts and lower Hexi Corridor). Aeolian sand accumulation frequently occurred during the Holocene in the central Hexi Corridor, while deflation dominated in its western part. The comparison of OSL ages shows high accumulation in the western part during the Pleistocene-Holocene transition. In contrast, the central Hexi Corridor exhibits aeolian sand ages throughout the entire Holocene. The difference in this sedimentologic behaviour is attributed to a shortage in sediment supply in western parts, while the central Hexi Corridor receives higher precipitation and is crossed by a several perennial streams providing material from fluvial storages for deflation. A comparison of loess and aeolian sand deposits exhibits considerable differences in their reaction to environmental change, also because the hosting geomorphologic settings differ. This fact is often neglected in palaeoenvironmental studies on aeolian archives. This study presents an approach of studying the comprehensive aeolian sedimentary system. As such, loess deposits are preserved on stable terrace surfaces and in upper regions during periods of relatively moist climatic conditions — exhibiting river incision and fluvial reworking of storages). In contrast, aeolian sands located in lowland regions may be fed and reworked by enhanced fluvial activity. Sediment supply is identified as the limiting factor for the formation of aeolian sand deposits in the Hexi Corridor. The same applies to the sand fraction in loess deposits. For loess accumulation, however, a stable geomorphologic surface and a dust-trapping steppe-like vegetation cover are required. Dust supply has likely been sufficient at least since the last Glacial. Moreover, the investigation of adjacent geomorphological units exhibiting loess and aeolian sand is promising, because both archives reflect different behaviours in a geomorphologic and temporal context, depending on previous environmental conditions. A diverse set of surface samples allows a better evaluation of medium-scale geomorphologic influences on sedimentary processes, useful for the interpretation of long vertical sections. Comparing sedimentological and geochronological observations in aeolian sediments is found promising for the reconstruction of I) sedimentary processes as part of a greater sediment cascade and II) for the reconstruction of palaeoenvironmental conditions.