Shape Matters: Unlocking Transport Histories in Fine-Grained Aeolian Sediments With Endmember Modeling of Size–Shape Distributions

The transport medium, mode, energy, and distance are recorded in the grain-size and grain-shape distributions in a sedimentary deposit. While grain-size analysis has long been used in sedimentology, grain-shape analysis is increasingly recognized as a valuable tool for reconstructing sedimentary processes and palaeoenvironments. Using Dynamic Image Analysis, this study focuses on endmember modeling of combined grain-size-shape distributions as an additional and robust sedimentological tool. To refine the technique, the topmost 10 m of a sedimentary section from the Mangshan Loess Plateau, China, was analyzed. Endmember modeling of the size-shape distributions revealed three sediment populations indicating different transport modes: sandy silt via short-term modified saltation (decreasing convexity with increasing grain-size), coarse silt via short-term suspension (decreasing aspect ratio and Cox circularity with grain-size), and coarse silt via long-term suspension (relatively low decreasing aspect ratio with grain-size, relatively high Cox circularity and convexity). A strong negative correlation was found between the finest endmember and a loess microcodium oxygen isotope record (precipitation proxy) from a nearby site, indicating that analyzing shape of the particles may help distinguish between dry and wet deposition. The nature of shape sorting seems to change with grain-size, transport mode and transport distance. For silt-sized sediments, shape sorting mainly occurs during deposition and is dominated by overall shape of the particles, whereas for the sand-sized sediments predominant shape sorting occurs already during entrainment based on grain regularity. These findings highlight the significance of integrating grain-shape with grain-size analyses to better resolve sediment transport processes.