A fractal evaluation of particle size distributions in an eolian loess-paleosol sequence and the linkage with pedogenesis

The changes of soil particle size distributions (PSDs) can be used as a proxy for soil development intensity and an indicator of soil formation processes. The fragmentation fractal dimensions can describe PSDs of soils in a single parameter. The objective of this research is to use mass-based fragmentation fractal dimension models to describe PSDs and evaluate the relationship between the fragmentation fractal dimension values (DF) and pedogenesis in loess-paleosol. Based on the theory of fragmentation fractal dimension, particle size data from 837 eolian loess samples from the lower part of Chaoyang section uniform loess deposits were analyzed with mass-based models using linear regression and piecewise linear regression. A single power-law exponent can characterize the PSDs across the domain of 0.1-54.4 μm. The trend of fragmentation fractal dimension values is consistent with chemical weathering indices, magnetic susceptibility, and <1 μm clay contents. The DF value of PSDs can be used to assess the degree of particle fragmentation and intensity of pedogenesis for loess-paleosols. A paleosol has a greater DF value than a loess; indicating that the greater the DF value, the greater the effect of energy events that have occurred. Loess particles in paleosols were fragmented to finer particles with an average median grain size (φ = 6.21 ± 0.24, CV = 3.94%) and were poorly sorted to very poorly sorted (So = 1.922 ± 0.128) due to strong pedogenesis under a prominent warm-wet climate. In contrast, particles in loess experienced fewer fragmentations and had a greater average median grain size (φ = 5.89 ± 0.35, CV = 5.89%) due to weak pedogenesis under prominent cold-dry climate. These results confirm the use of fractal characteristics to reflect the loess particle size reduction process. Two domains within the 0.1-54.4 μm domain, F1 and F2, were identified where power-law scaling was applicable. The close transition point from F1 to F2 is 7.64 ± 1.21 μm. The fragmentation fractal dimensions of the two domains decreased in the order: DF1 (close to 3) > DF2 (close to 2). The energy dissipation for fragmenting larger particles from coarse silt (CSI) to fine silt (FSI) is identified in the volume and for fragmenting smaller particles from fine silt (FSI) to clay (CL) it is substantially on the surface. This research indicated that fragmentation fractal model is a good descriptor for the PSDs and DF can be a value quantifying the intensity of loess-paleosol pedogenesis.