Regional grain size variations in aeolian sediments along the transition between Tibetan highlands and north‐western Chinese deserts – the influence of geomorphological settings on aeolian transport pathways

Abstract

Sediment distribution is investigated applying grain size analysis to 279 surface samples from the transitional zone between high mountains (Qilian Shan) and their arid forelands (Hexi Corridor) in north-western China. Six main sediment types were classified. Medium scale (10 3 m) geomorphological setting is carefully considered as it may play an important role concerning sediment supply and availability. A tripartite distribution of sedimentological landscape units along the mountain to foreland transition is evident. Aeolian sediments (e.g. loess and dune sands) are widespread. They are used to identify aeolian transport pathways. The mU/fS-ratio (5-11 μm/48-70 μm) among primary loess opposes the two grain size fractions being most sensitive to varying accumulation conditions. The first fraction is attributed to long-distance transport in high suspension clouds whereas the latter represents local transport in saltation mode. The ratio shows strong correlation with elevation (R 2 = 0.77). Thus, it indicates a relatively higher far-traveled dust supply in mountainous areas (>3000 m above sea level [a.s.l.]) compared to the foreland. The contribution of westerlies to high mountain loess deposits is considered likely. Hereby, the influence of the geomorphological setting on grain size composition of aeolian sediments becomes apparent: the contribution from distant dust sources is ubiquitous in the study area. However, the far-distance contribution may be reduced by the availability of fine sand provided in low topography settings. Plain foreland areas support fine sand deflation from supplying river beds, allowing the formation of sandy loess in foreland areas and intramontane basins. In contrast, high mountain topography inhibits strong sand deflation into loess deposits. Eastern parts of the Hexi Corridor show higher aeolian sand occurrence. In contrast, the western parts are dominated by gravel gobi surfaces. This is attributed to higher sand supply in eastern parts provided by the Badain Jaran Desert and fluvial storages as sand sources.

Eolian dust dispersal patterns since the last glacial period in eastern Central Asia: insights from a loess-paleosol sequence in the Ili Basin

“…It lies towards the finer end of the range of subgroup 1.b.2 (Vandenberghe, 2013). Comparable loess of the same grain size has been identified in loess from the northern Qilian Shan and the Hexi Corridor (EM2: 33 µm) in northern China, which was also interpreted as depositing from short-term suspension (Nottebaum et al, 2015). Loess of this grain size has been attributed to dust fallout (Muhs and Bettis, 2003;Pye, 1995) and fallout from low-altitude suspension clouds (Sun et al, 2003), as measured from modern depositional events.…”

“…Using the sediment groups identified in Vandenberghe (2013), some studies interpreted multiple sources for loess sediments (Nottebaum et al, 2014(Nottebaum et al, , 2015Yang et al, 2016). In this study, we apply the end-member analysis of NLK loess to the sediment groups of Vandenberghe (2013) in an effort to reconstruct dominant eolian processes.…”

“…On the one hand, researchers have suggested that grains of this size can be lifted by strong vertical air movement and subsequently incorporated into the high-level westerly air streams (Pye, 1995;Pye and Zhou, 1989). This process would link EM3 with long-term suspension transport driven by high-level westerlies (Nottebaum et al, 2014;Vandenberghe, 2013;Vriend et al, 2011). Conversely, Zhang et al (1999) argued that < 20 µm particle fractions derive from "non-dust storm processes" associated with northwesterly surface winds.…”

Late Quaternary aeolian sand deposition sustained by fluvial reworking and sediment supply in the Hexi Corridor — An example from northern Chinese drylands

“…Sediment supply and availability might have profound effects on aeolian sedimentation (Kocurek and Lancaster, 1999;Stauch et al, 2012Stauch et al, , 2014Telfer and Hesse, 2013;Nottebaum et al, 2014;Halfen et al, 2015). The role of fluvial activity has been shown to be an important factor for providing source material for aeolian transport (Langford, 1989;Clarke and Rendell, 1998;Bullard and Livingstone, 2002;Field et al, 2009).…”

“…Hence, as Yang and Scuderi (2010) pointed out: Understanding the age and triggering factors leading to Quaternary change in arid regions of China and Central Asia […] is a particularly challenging task due to uncertainties in both dating and interpretation of the geomorphic and sedimentologic evidence. Thus, reconstructions of palaeoenvironmental and palaeoclimatic conditions from sedimentological data require a detailed understanding of depositional processes as part of the sedimentary system (Lu et al, 2011a(Lu et al, , 2011bIJmker et al, 2012;Stauch et al, 2012;Nottebaum et al, 2014), which are controlled by geomorphologic, climatic, and hydrologic factors. Therefore, considering regional and local boundary conditions for aeolian depositional areas are a central part of this study.…”