Jafar Sharifi Garmdareh; farhad khormali; Christian Rolf; Martin kehl; Ali shahriari; Manfred Frechen
Abstract
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 ...
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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.
A. Shahriari; F. Khormali; Martin kehl; Ali reza Karimi; M. Mousavidastenaei; E. Lehndorff
Abstract
Introduction: Knowledge about palaeoenviroment and palaeovegetation provides information about how vegetation reacts on climate fluctuations in the past, what will help understanding current and future developments caused by e.g. climate change. Northern Iranian Loess-Plateau forms a strongly dissected ...
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Introduction: Knowledge about palaeoenviroment and palaeovegetation provides information about how vegetation reacts on climate fluctuations in the past, what will help understanding current and future developments caused by e.g. climate change. Northern Iranian Loess-Plateau forms a strongly dissected landscape with steeply sloping loess hills. This loess record reflects numerous cycles of climate change and landscape evolution for the Middle to Late Quaternary period. therefore, this study was done for reconstruction of palaeoenvironment (climate and vegetation) in loess-palaeosol sequences in northern Iran. Therefore, this study aims at a preliminary reconstruction of palaeovegetation and palaeoenvironment, in loess-palaeosol sequences along a cliomosequnce in Northern Iran.
Materials and Methods: Two loess-palaeosol sequences (Agh Band and Nowdeh sections) were chosen in Golestan province, in northern Iran and step-wise profiles were prepared. Agh Band section is located in the western most part of the Northern Iranian loess plateau and has about 50 m thickness of loess deposits. Nowdeh loess-palaeosol sequence is located about 20 km southeast of Gonbad-e Kavus, in the vicinity of the Nowdeh River. Soil sampling was done in several field campaigns in spring 2012. More than 30cm of the surface deposits were removed in order to reach for undisturbed loess and palaeosols and one mixed sample was taken from each horizonA comparison of palaeosols with modern soils formed under known Holocene climatic conditions, which are derived from substrates with similar granulometric and mineralogical composition are suited for reconstructing past climate and environment. Hence, six modern soil profiles were prepared along the climosequnce and the vegetation cover changed from grassland in the dry area to dense shrub land and forest in the moist part of the ecological gradient. For reconstruction of palaeoenvironment (climate and vegetation) some basic physico-chemical properties, clay mineralogy and n-alkane biomarkers were used.
Results and Discussion: Results of soil texture analysis showed silt particles were dominant (more than 50 %) in the modern soil profiles and loess-paleosol sequences which confirmed aeolian source of loess deposit. Clay content increased while silt content decrease in more strongly developed palaeosol horizons which it may reflected weathering processes of clay and/or its translocation. The modern soil profiles were classified as Entisols, Inceptisols, Mollisols and Alfisols which shows impact of climate as an important soil formation factor in the studied area. Clay mineralogy results in two loess-palaeosol sequences showed that illite, chlorite, kaolinite and smectite are dominant in these deposits. Mineralogical changes in the soil horizons are consistent with morphology and soil evaluation, so smectite, illite-smectite (mixed layer) and vermiculite minerals were dominant minerals in more strongly developed palaeosol horizons indicating to high precipitation and good vegetation cover (e.g., forest). The n-alkane biomarker results in the modern soil profiles showed, the average chain length (ACL) and (nC31+nC33)/(nC27+nC29) ratio are very efficient parameters for reconstruction of vegetation, therefore these parameters were used to unravel the palaeovegation in loess-palaeosol sequences. In both sections n-alkane biomarkers studies showed vegetation changes in different periods. These changes were most intense in Nowdeh loess-palaeosol sequence, so grassland and shrub in profil1 (Bk horizon) and profile 2 (ABk horizon) palaeosols (with illite dominance) changes to forest in profile 2 (AB horizon with smectite dominance) and profile 3 (Btky horizon with smectite dominance and vermiculite presence) palaeosols. Agh Band section had one palaeosol including two horizons (Bw and Bk) which based on n-alkane specifications the Bw-horizon indicates grass/shrub vegetation (alsosmectite presence). It could indicate favorable environmental conditions promoting the growth of more dense vegetation.
Conclusions: Results showed that clay mineralogy changes are in line with n-alkane biomarkers results and both analyses reflect climate and environment conditions in soil formation periods and they are more effective for the accurate reconstruction of palaeoenviroment. According to chronological data for Nowdeh and Agh Band loess-palaeosol sequences, Nowdeh section had more suitable environment (more precipitation, more dense vegetation and suitable conditions for formation and development of soil, pedologically) compared with Agh Band section at the same times. Clay mineralogy and n-alkane biomarker resulted in the modern soil profiles and loess-palaeosol sequences showed that the modern ecological gradient (especially for precipitation) existed during the time and climate was an important soil formation factor in the studied region.
