Sediment transport processes across the Tibetan Plateau inferred from robust grain-size end members in lake sediments

Abstract

Abstract. Grain-size distributions offer powerful proxies of past environmental conditions that are related to sediment sorting processes. However, they are often of multimodal character because sediments can get mixed during deposition. To facilitate the use of grain size as palaeoenvironmental proxy, this study aims to distinguish the main detrital processes that contribute to lacustrine sedimentation across the Tibetan Plateau using grain-size end-member modelling analysis. Between three and five robust grain-size end-member subpopulations were distinguished at different sites from similarly-likely end-member model runs. Their main modes were grouped and linked to common sediment transport and depositional processes that can be associated with contemporary Tibetan climate (precipitation patterns and lake ice phenology, gridded wind and shear stress data from the High Asia Reanalysis) and local catchment configurations. The coarse sands and clays with grain-size modes > 250 µm and < 2 µm were probably transported by fluvial processes. Aeolian sands (∼ 200 µm) and coarse local dust (∼ 60 µm), transported by saltation and in near-surface suspension clouds, are probably related to occasional westerly storms in winter and spring. Coarse regional dust with modes ∼ 25 µm may derive from near-by sources that keep in longer term suspension. The continuous background dust is differentiated into two robust end members (modes: 5-10 and 2-5 µm) that may represent different sources, wind directions and/or sediment trapping dynamics from long-range, upperlevel westerly and episodic northerly wind transport. According to this study grain-size end members of only fluvial origin contribute small amounts to mean Tibetan lake sedimentation (19 ± 5 %), whereas local to regional aeolian transport and background dust deposition dominate the clastic sedimentation in Tibetan lakes (contributions: 42 ± 14 % and 51 ± 11 %). However, fluvial and alluvial reworking of aeolian material from nearby slopes during summer seems to limit end-member interpretation and should be crosschecked with other proxy information. If not considered as a stand-alone proxy, a high transferability to other regions and sediment archives allows helpful reconstructions of past sedimentation history.

Vegetation state changes in the course of shrub encroachment in an African savanna since about 1850 CE and their potential drivers

“…Assuming a significant contribution of trees to the sedimentary n-alkanes, the overall shift of ACL toward lower values throughout the profile is thus in agreement with the vegetation shifts inferred from pollen analyses and sedaDNA as discussed in detail below.In addition, grain-size distribution lets us infer changes in landscape openness via the type of sediment transport reflected in the grain-size rEM(Dietze et al, 2014). The two types of sand deposits represented by rEM4 and rEM5 indicate short-term, high energetic sediment input from run-off during rain events or sand storms in an open landscape with reduced grass cover(Dietze et al, 2014;Walther & Neumann, 2011). In particular, within the error of the age model, rEM5 can be associated with the main flood events documented in the region from the 1920s until 2000…”

Environmental history recorded in aeolian deposits under stone pavements, Mojave Desert, USA

“…Shape and mode of EM 1 ( Figure 4C) is also similar to the fluvial sand EM of Dietze et al (2014) who compiled robust EM from lakes across the Tibetan Plateau. EM 2 has a mode at 2.71 φ.…”

“…The explained variance per EM loading is roughly equal, between 10 and 19 % ( Figure 4C). Figure 4C also comprises the ranges of common grain-size EM found in lacustrine sedimentary archives across the Tibetan Plateau (Dietze et al, 2014) and the ranges of loess modes as described by (Vandenberghe, 2013), which were intensively discussed in terms of general sorting trends due to sediment sources, transport and deposition characteristics. Figure 5 shows the correlation of the inferred stratigraphic units and their associated soil sections across the study area.…”

“…Dietze and Dietze introduced a comprehensive package for the statistical software R which allows open-source comprehensive modelling. The technique has been successfully tested ) and applied to a wide variety of environmental archives (Stuut et al, 2002;Prins et al, 2007;Dietze et al, 2014).…”

“…EM 3 shows similarities to a material termed "proximal desert dust" (recruited from e.g. local washed, Sweeney et al (2013)) with a primary mode around 4.08 φ and a secondary mode between 5.6 and 9.0 φ. EM 4 with a mode at 6.63 φ is interpreted as "regional dust", which represents episodic background deposition from high-level suspension cloud taking up sediment from larger distances (100-1000 km; Vandenberghe (2013); Dietze et al (2014)). EM 5 (mode at 8.78 φ) represents "remote dust", generated from even longer travel distances in atmospheric levels up to 7 km (Vandenberghe, 2013).…”