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

Abstract 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.