عنوان مقاله [English]
Introduction During the last decade, considerable progress has been made in the study of known loess deposits and their paleoclimatic implications in Northern Iran, whereas little information is available about the red soils which are beneath the these loess. So, in this study, major and trace element concentrations were analyzed on the samples from a red sequence of Iranian Loess Plateau at Golestan province. The main objectives of this research are a) to address the origin of the red soils with compare to the other geochemical results such as upper Pleistocene loess-paleosol, upper continental crust and Jiaxian Red Clays in China, b)to examine the geochemical behaviors of certain elements and their ratios such as Al2O3/Na2O, Na2O/K2O, MgO/TiO2, Rb/Sr and Chemical Index of Alteration (CIA) during pedogenesis and finally, to reconstruct the early Pleistocene climate.
Materials and Methods This study was carried out on a 19-m-thick sequence of deposits exposed in a limestone quarry located near the Agh Band village of Golestan province in the east of the Iranian Loess Plateau (latitude 37.688889 N and longitude 55.158333 E). The so-called Agh Band red sequence underlies an upper Pleistocene loess-paleosol sequence and covers yellow limestone of the Akchagyl formation belonging to the Upper Pliocene of Kopet Dagh sedimentary basin. It is the first sequence one of red soils described for the loess plateau of Iran. Based on the paleomagnetically dating, this section is formed during ~2.4-1.8 Ma. The present-day climate of the study area is semi-arid, with mean annual precipitation and temperature of ca. 300 mm and 17◦ C, respectively. The soil moisture regime is Xeric-Aridic and the temperature regime is Thermic. In a field campaign in autumn 2014 the morphological characteristics of the section were recorded. Based on field observations, the sequence has been subdivided into 24 units, designated consecutively as U1-U24 from the top of limestone to the bottom of the Upper Pleistocene Loess. From each unit, representative samples were taken for color measurements, grain-size and geochemical analysis. Each air-dried sample was gently crushed, taking care not affect the grain size, and then measured using a Konica-Minolta CM-700 color meter. Grain size was measured using a Malvern Mastersizer 2000 laser grain-size analyzer following the pre-treatment procedures described in the text and the concentrations of major and trace elements were determined using a PANalytical PW2403/00 X-ray fluorescence spectrometer. All of the measurements were made in the Key Laboratory of Western China’s Environmental systems, Lanzhou University.
Results and Discussion The grain-size distribution of the red section is dominated by fine-grained silts with the average of 86.6 percent, in addition, the amount of clay and sand are 10.9 and 2.6 percent, respectively. Angular or sub-angular blocky structures are dominated in the red sequence. The section is mainly characterized by alternations of reddish yellow )10 YR 6/6) and brownish-red (7.5 YR 3/6) to reddish (5YR) layers. In general, the color of the soil horizons in the red deposits is much redder than that in the overlying loess (7.5YR vs.10YR, respectively), and this is one of the principal differences between the red soils and the overlying loess. Another different is the amount of carbonate nodules and the size of them (up to~20 cm diameter). These soils have been subjected to relatively intensive pedogenesis, as demonstrated by the presence of clay skins and Fe-Mn coatings. The high correlation of major and trace element compositions between Agh Band red soils section, upper loess and paleosol and the Jiaxian red clay in China supports the proposal that the Agh Band red soils was wind-blown in origin. The value of CIA index (69.6 for red soils versus 59.8 for the upper loess deposits), Al2O3/Na2O, K2O/Na2O and Rb/Sr ratios are higher in the red deposits than in the upper Pleistocene loess, also, the lower amount of MgO/TiO2 ratio in reddish soils, suggesting stronger chemical weathering and thus a wetter climate during the formation of red soils in early Pleistocene.
Conclusion Finally, our main findings are as follows: 1) The geochemical composition of the red-colored sedimentsis similar to the overlying upper Pleistocene loess suggesting a similar origin; 2) wind-blown origin of the red deposits and continuous atmospheric dust deposition in the Iranian Loess Plateau during the Early Pleistocene; 3) red soil sequence formed under wetter and more humid climate compared with the Upper Pleistocene loess.