Aeolian silt transport processes as fingerprinted by dynamic image analysis of the grain size and shape characteristics of Chinese loess and Red Clay deposits

摘要

This study applied dynamic image analysis (DIA; Sympatec Qicpic) to characterize the grain size and shape of Chinese aeolian sediments in order to fingerprint their transportation processes. This is the first time this technique has been applied to late Neogene and Quaternary silt particles (2–63μm) from the Chinese Loess Plateau (CLP). We selected four well-studied Quaternary loess-palaeosol sequences along a north-south transect across the CLP and compared their grain size distribution obtained by DIA with that yielded by laser diffraction particle size analysis (LD; Fritsch Analysette 22 and Sympatec HELOS KR). This comparison demonstrated that DIA is successfully able to differentiate loess units from palaeosols, and to characterize clearly spatiotemporal variations in the grain size records of loess-palaeosol sequences formed during the last two glacial and interglacial periods. This is consistent with grain size results obtained using LD. DIA is also able to characterize spatial variations in the more fine-grained aeolian Red Clay deposits underlying the Quaternary loess, and allows the quantification of the fluvial contribution to Red Clay sequences. DIA of the characteristics of grain shapes in loess-palaeosol sequences and Red Clay deposits revealed a systematic pattern, whereby the aspect ratio decreased with increasing grain size, indicating that systematic shape sorting occurred during the aeolian transportation of these dust particles. It could be inferred from our study that particles in a certain grain size range correspond to a specific aspect ratio range, and may in turn be aerodynamically distinguishable from each other and further correlated with the wind velocity/strength. Also evident from analysis of our DIA data was a subtle but systematic downwind decrease in the aspect ratio of the particles in the loess units. This observation suggests that elongated and/or flat particles (with a low aspect ratio) were transported further downwind than more symmetrically shaped particles (with a high aspect ratio). This study indicates that DIA of grain size and shape characteristics can be an additional powerful tool for identifying grain size and shape sorting trends, determining the dominant mode of transport, and reconstructing transport pathways of silt-sized aeolian sediments.