عنوان مقاله [English]
Introduction: The study of the northern Iranian loess is important since they are located in the middle of the Asian and European loess belt. In addition, presence of a climatic gradient i.e. increases in rainfall from north to south and from west to east, and the constant temperature, provide a unique area to study climate change and environmental conditions. There are many recent studies on loess-palaeosol sequences in this region, that show the grain size variation and clay mineralogy changed with increasing precipitation. The purpose of this study was to investigate the trend of grain size variations and clay mineralogy in this climatic gradient to infer origin of the surface loess in this region.
Methods and Materials: Grain size and XRD analyses were carried out on six soil profiles extending from low precipitation in DashliBorun (200mm) to high precipitation in SeyyedMiran (670mm) regions. The grain size analysis was carried out with Beckman-Coulter LS 13320 PIDS. The device uses the principle of the forward scattering of monochromatic light and its diffraction under a certain angle. The angle of diffraction is inversely proportional to particle size. That means coarser particles cause a smaller angle of diffraction compared to finer particles. This method is quick, its results offer a good reproducibility and the data are digital and direct. Necessity for only small amounts of sample material, and covering a wide range from 0.4 to 2000 mm in diameter are other advantages of this method. Nevertheless, the amount of clay percentage may be subjected to certain underestimations because particles smaller than 0.4 μm diffract light in all directions that can cause problems with detecting the signal correctly. Hence the sum of particles with less than 5.5 mm in diameter were chosen as an estimate of clay percentage. Clay fractions were separated based on the methodology outlined by Kittrick and Hope (1963) and Jackson (1975). The carbonates were initially removed using 1 N sodium acetate buffered at pH 5. The organic matter was then oxidized by treating the carbonate-free soils with 30% H2O2, and digestion in a water bath. Free iron oxides were removed from samples by the citrate dithionate method of Mehra and Jackson (1960). The clay separates were removed by centrifuge and studied by a Bruker D8 Advance X-ray diffractometer. Clay minerals were estimated semi-quantitatively from the relative x-ray peak areas of glycol-treated samples.
Result and Discussion: The grain size analysis by laser diffraction method showed that the amount of clay (12.09 %) and fine silt (7.03%) in the soil located in Dashlibron (200 mm/yr) profile had the lowest amount and the U-ratio (6.35) and the medium grain-sized particle (48.28 µm) had the highest amount during this climatic gradient. Increasing precipitation had clear impact on theses parameters, so that the maximum amount of clay (36.56 %) and fine silt (28.02%) and the minimum amount of U-ratio (1.00) and the average medium grain size (8.78 µm) were observed in SayedMiran profile with the highest precipitation (670 mm/yr). Clay mineralogical study of soil parent material showed mica, chlorite, kaolinite and smectite as dominant minerals in the soils. As the amount of precipitation increases along the climate gradient, the amount of pedogenic vermiculite and smectite increase. Silt minerals included quartz, plagioclase, potassium feldspar, mica, chlorite and calcite. The amount of quartz, plagioclase or potassium feldspar minerals did not change in parent materials. The average medium grain size and U-ratio are function of the maximum wind speed and distance from the source. In addition, weathering can affect the size of the particles and cause changes in the mineralogy and amount of minerals in the size of the silt and clay fractions. On the other hand, understanding the changes of clay minerals can provide origin-dependent changes, transport processes, and climatic variations as they are recorded in these minerals.
Conclusion: In general, it can be concluded that the medium grain size and U-ratio decreased from low rainfall regions to higher rainfall and clay and fine silt contents showed a reverse trend. Clay minerals included mica, chlorite, and kaolinite and dominant silt minerals were quartz and plagioclase in all studied soil profiles along the climatic gradients. The results showed that changes in grain size reflect the distance from the loess material sources and also indicate intensity of pedogenic processes. Mineralogical analysis showed the similar mineral types in all loess parent materials of different regions, probably indicating the similar sources for the loess. The change in minerals abundances in the upper horizons indicate the pedogenic processes affected by climatic conditions along the climate gradient.