كاني‌شناسي رسي پالئوسول‌هاي لسي اوايل پليستوسن و مقايسه آن با خاك‌هاي لسي مدرن

Comparison of the clay mineralogy of the early Pleistocene paleosols With modern loess-derived soils

رسوبات لسي شمال ايران، حاوي اطلاعات ارزشمندي از شرايط اقليمي و محيطي گذشته منطقه­ مي­باشند. مطالعات سن­يابي و رسوب‌شناسي گسترده­اي بر روي لس­هاي مياني و اواخر پليستوسن در دهه‌هاي گذشته صورت گرفته است؛ ولي نياز به مطالعه­ مقايسه­اي در مورد رسوبات لسي قديمي­تر، ضروري است. از اين رو، اين پژوهش با هدف مطالعه كاني­شناسي رسي و خصوصيات فيزيكوشيميايي پالئوسول­هاي لسي اوايل پليستوسن و مقايسه آن با خاك­ هاي لسي مدرن در مناطق آق­ بند، يلي­بدراق و قره آغاچ استان گلستان انجام شد. پس از بررسي­هاي كامل گزارش­ هاي خاك­شناسي و تفسير عكس­هاي هوايي، در مجموع شش خاك­رخ حفر و مطالعه گرديد. پس از مشاهده‌هاي مقدماتي و صحرايي و تعيين افق‌ها براي هر پروفيل در منطقه، رده‌بندي خاك­ها براساس سيستم­هاي جهاني خاك صورت گرفت. سپس از هر يك از افق­ ها به مقدار كافي نمونه­ خاك براي انجام آزمايش ­هاي فيزيكوشيميايي و كاني­شناسي رس تهيه گرديد. مقايسه نتايج آزمايش­ هاي فيزيكوشيميايي (نظير رنگ، درصد آهك، ميزان ظرفيت تبادل كاتيوني، نسبت آهن و ...) در خاك­هاي لسي قديمي و مقايسه آن با خا ك­هاي لسي مدرن، نشان ­دهنده اين است كه خاك­ هاي لسي قديمي، چندين مرحله خاك­سازي را گذرانده ­اند. يكي از شاخص­هاي بررسي تغيير اقليم گذشته در لس­ ها، كاني­ هاي رسي است. نتايج كاني‌شناسي رس نشان داد در خاك ­هاي لسي اوايل پليستوسن، وجود رس­هاي هواديده­اي، مانند اسمكتيت در مقادير بيش‌تر بيانگر شرايط مرطوب­تر و مساعد هواديدگي در عصر بين يخچالي است و حضور رس ­هاي اوليه و بدون تحول مانند ايليت و كلريت در خاك­ هاي لسي اواخر پليستوسن، با اقليم حال حاضر در منطقه مطابقت داشته، محيط خشك­تر كنوني را تأييد مي­نمايد.

Introduction Loess sediments of northern Iran represent several cycles of climate change and evolution of the landform for the mid-to-late Quaternary. Climate change in elevations of Iran and its surrounding areas is very controversial in the mid and late Quaternary, and has been discussed in the past according to rainfall and rainfall periods and between rainfall, glacial and inter-glacial. Paleomegnatic results also indicate that these sediments have accumulated between, 1.8 to 2.4 million years ago. However, pedogenic processes and the effects of past climate in these soils still have not been fully investigated. The loess deposits in northern Iran are a valuable archive of regional paleoclimatic and paleoenvironmental information. Extensive sedimentological and chronological studies have been carried out on the middle to upper Pleistocene loess during the past decades, but it is necessary to do a comparative research on the older loess deposits. So, this study aimed to conduct a mineralogical and physicochemical investigation on the early Pleistocene loess and to compare it with modern loess soils in Agh-Band, Yelli-Badrag and Qareh-Agach in loess plateau of eastern Golestan. Materials and Methods The study area is located in a hot and dry climate in loess Plateau east Golestan. According to the previous studies, a total of six profiles were excavated and studied. Then, physicochemical properties such as soil texture, acidity (pH), electrical conductivity (EC), saturation moisture (SP), organic carbon (OM), cationic exchange capacity (CEC) and calcium carbonate equivalent (CCE) were measured in the laboratory. Clay separation was carried out with a specific method to separate the clay as well as identification of clay minerals. After preliminary field observations and determining the horizons for each profile in the region, soil classification was done based on soil taxonomy and WRB. Then, soil samples were prepared from each horizon for physicochemical and mineralogical studies in sufficient quantities. Results and Discussion Comparing the results of physicochemical properties (such as color, lime percentage, the cation exchange capacity and the ratio of iron, etc.) in paleosol and modern loess soils indicates that in paleosol soils, soil forming processes have passed several stages. Clay mineralogy is a good indicator for past climate change studies in loess.The existence of the arglic horizons and the evolved calcic in paleosols and their absence, in comparison with the modern soils in which they are present, indicate the change in soil formation conditions. The change in the color of paleosols also represents the soil moisture and the more suitable conditions of the past climate (temperature, and especially rainfall) in comparison with the present climate of the region, this color change was due to activation of soil formation processes in paleosols. All paleosol samples had a higher clay content than the late modern loess soils of the Pleistocene, suggesting favorable climatic conditions for soil formation processes and the development of more ancient soil than parent materials. Decrease in the amount of annual precipitation in the region, compared to the past, has led to decreased smectit and increased chlorite. Therefore, presence of smectit cannot be attributed to the present situation of the region. The presence of these clay minerals in paleosols can be due to wet weather conditions as well as weathering of clay mineral deposits. On the other hand, the dominance of less weathered clay minerals such as illite and chlorite in the late Pleistocene modern loess soils is correlated with the present dry climatic conditions. Conclusion The simultaneous presence of modern and old loess soils in the studied areas demonstrates the general evolution of geographical and climatic conditions during the Pleistocene period which has altered the properties of these layers and ultimately left out the effects of high clay conditions, which is a combination of climatic evidence and intermittent pedogenic soil formation processes. The presence of early Pleistocene loess soils between late Pleistocene loess sediments in Golestan province and the conditions of the study provided pedological and mineralogical comparisons of modern and paleosols in these areas and the results clarified a part of the climate change in northern Iran. The past climate study allows for prediction of the current and future climate change process. Therefore, a more accurate study of clay minerals as the key to all soil behaviors and past climate change in different parts of the eastern Golestan plateau can be very useful in completing studies of evidence of past climate change in paleosol soils