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Abstract:  
Introduction
 
Particle Size Distribution (PSD) is one of the important physical properties of sediments that affects many other physical and chemical properties. Fractals are objects or processes that represent the same appearance or behavior on several large, spatial or temporal scales. Each fractal can be divided into parts each of which resemble the main body; many natural phenomena and processes are based on fractal models. PSD loess is a natural fractal. After being modified by pedogenesis, the loess particles still have good self-similar properties, so the fractal dimension of the particle size of these types of sediments are suggested as a new indicator of particle size. The loess sequences, which were produced from aeolian influenced by past weather changes, were transported and deposited and have undergone many changes by pedogenesis. This information is recorded in loess particles. By studying the PSD, one can discover the past environment and PSD changes can be used as an indicator of the intensity of pedogenesis or soil age and the pedogenesis process.
Post-depositional pedogenesis, including chemical and biological weathering, causes further particle crushing. The amount of crushing at different locations may vary due to the PSD of the deposition, the time and intensity of the pedogenesis, or other factors. Typically, intense pedogenesis or poor to very poor sorting occurs in warm and humid climates and poor pedogenesis in cold and dry climates. The changes in loess texture reflect the conditions and developments that have affected the loess after being deposited. In this research, for the first time in Iran, the loess texture developments in Golestan province is investigated and analyzed by fractal PSD. The results of fractal PSD are then compared with the fractal geometry obtained from electron microscope images. Fractal can interpret the extent of tissue changes at different points.
 
Materials and methods:
This study was conducted in Golestan province in the southeast of the Caspian Sea between latitude 38° 8’ to 36° 30’ North and longitude 53° 57’ to 56° 22’ East. Sampling was done from three zones and 16 samples based on three types of loess textures including sand loess, silt loess and clay loess. In this paper, fractal is measured in two methods of differential box counting and PSD. The PSD fractal is calculated by two sieve-hydrometric (DbH) and laser (DbL) methods.
 
Discussion and conclusion
The results of examining DbH dimensions in loess of Golestan province show that with increasing DbH, percentage of sand decreases, and clay content increases. In the comparison of fractal with sediment textual parameters, with increasing DbH, the number of sorting increases and the sorting of particle decreases. This means that there is a better gradation of PSD and a larger volume of particle size classes in the samples studied. As DbH increases, the kurtosis index decreases and the curvature broadening increases since the size of the particle size classes increases.
The results of examining DbL fractal dimensions in loess of Golestan province show that with increasing DbL, the percentage of sand decreases and silt and clay content increases. Therefore, based on the laser sizing results, increased silt and clay content, results in better gradation of sediments. The trend of particle sorting is negative versus DbL, meaning that with increasing DbL, the sorting index decreases and particle sorting increases. The kurtosis trend versus DbL is positive meaning that with increasing DbL, the kurtosis index increases and curvature broadening decreases.
The three stations of AlmaGol and AlaGol and Agh Ghala Belt in zone 3 had lower DbH and DbL, better sorting than other areas, and the median size in them was the highest. This difference may be due to differences in their origin or forming environment and their retransfer. This indicates that fractal values ​​can be useful for identifying the transfer mechanism of different sediments.
As the loess texture changes, the fractal geometry changes. So higher fractal dimension content indicates higher soil formation and higher fine particle ratios; and particle distribution is well graded, so given the results, it could be claimed that fractal geometry can observe the changes after loess deposition.
According to the fractal results obtained from electron microscope images in Golestan loess, the fractal dimensions of the grain increased as diameter increased, indicating that it increased with increasing diameter, confirming the fact that near the source, grains are deposited with higher order and as it distances from the source, with decreased grain size, the fractal number becomes smaller.
With increasing roundness from zone 3(near the source) to1 (away from the source) the fractal dimensions of the grain decreases which means that with increasing roundness of the particles, the sediment’s order decreases and the texture is less subjected to changes.
On the other hand, the fractal grain dimensions increases with increasing sphericity. Since the sphericity decreases from zone3 to1, so the fractal number of grain dimensions decreases, meaning that the higher the sphericity, the higher the initial order of the sediment is and the texture is less subjected to change.
The fractal geometry values ​​of the grain fabric density of the fabric in different parts of Golestan province are not equal. Zone 3 and 1 have higher order than zone 2. Zone 3 has high order during deposition due to its proximity to the source, so its fractal number is close to 2. In zone 2, with further transfer, the particles were highly subjected to changes in size and way of arrangement, thus the fractal number and order are subjected to changes and decrease. The highest fractal number is seen in zone 1. This is due to the humid climate of zone 1, which causes the formation of secondary clay and increases fractal numbers and sediment order. These results also show that the content of clay determines the order and homogeneity of sediment texture. In the final summation, fractal and its related parameters, as an efficient tool in loess sediment, can justify the zone of texture changes, distance with the major source, pedogenesis and climate and it can determine the model of changes after deposition.
     
Type of Study: Research | Subject: Special
Received: 2020/05/8 | Accepted: 2021/02/2