ميكرومورفولوژي خاك هاي لسي اوايل پليستوسن و مقايسه آن با لس هاي مدرن

Comparison of the micromorphology of the early Pleistocene paleosols with modern loess-derived soils

وجود تناوبي از خاك‌هاي قديمي در رسوبات لسي شمال ايران، اشاره به دوران سرد و خشك در زمان رسوب­گذاري لس و دوران گرم و مرطوب در زمان تشكيل خاك‌هاي قديمي دارد. از اين رو، اين پژوهش با هدف پژوهش ميكرومورفولوژيك خاك­هاي لسي اوايل پليستوسن و مقايسه آن با لس­هاي مدرن در مناطق آق­بند، يلي­بدراق و قره­آغاچ استان گلستان انجام شد. پس از مشاهدات مقدماتي و صحرايي و تعيين افق‌ها براي هر خاك­رخ در منطقه، از هر يك از افق­ها به مقدار كافي، نمونه­ خاك براي انجام آزمايش­هاي فيزيكوشيميايي و همچنين نمونه­ دست نخورده، به‌منظور پژوهش مقاطع نازك خاك جمع­آوري شد. يكي از روش­هاي مهم براي شناسايي و تفسير لس­هاي مدرن و خاك­هاي قديمي در راستاي مطالعات اقليم شناسي گذشته، بررسي ميكرومورفولوژيكي خاك است. نتايج به­دست آمده از مقاطع نازك خاك­هاي پالئوسول و مدرن نشان مي­دهد كه زمان و تغييرات آب و هوا، باعث تغيير در ويژگي­هاي ميكرومورفولوژي خاك (مانند نوع و تعداد حفرات، ريزساختمان خاك، بي­فابريك و پدوفيچر و ...) مي­شود. مشاهده ­هاي ميكرومورفولوژيكي، شواهد مختلفي از وقوع فرآيندهاي پدوژنيك را در اين خاك­ها نشان داد. بي فابريك غالب خاك­ها به‌دليل آبشويي آهك در افق­هاي آرجيليك، لكه­اي و در افق­هاي كلسيك، بلورين مي­باشد. از مهم‌ترين شاخص­هاي خاك­سازي، وجود پوشش­هاي رسي دور حفرات در برخي از افق­هاي خاك­هاي قديمي است كه شواهدي از شرايط اقليمي مرطوب­تر براي آب­شويي و انتقال رس است. تشكيل لس و خاك­هاي قديمي مورد پژوهش احتمالاً به ترتيب مربوط به دوره ­هاي يخچالي و بين يخچالي با شرايط اقليمي متفاوت بوده است.

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 Early Pliestocene loess (Reddish loess) 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. Micromorphology is an important technique to identify and interpret the loess- paleosol for paleoclimate studies. Microscopy is a method of studying undisturbed soil samples with the help of microscopic techniques (and sometimes with ultramicroscopic ones), in order to identify their constituents, determine their mutual relations in space and time and interpret their formation conditions. Micromorphology uses these characteristics to make interpretations, generally on the soil formation processes. This study aimed to conduct a micromorphological investigation on the early Pleistocene loess and to compare it with the modern loess derived soils in Agh-Band, Yelli-Badrag and Qareh-Agach in loess plateau of eastern Golestan. Materials and Methods The study area is located in semiarid climate in loess Plateau east Golestan. Six profiles were selected and studied. 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. Then, soil samples were prepared from each horizon for micromorphology studies. For micromorphological studies, thin sections were prepared from undisturbed, oriented and dry clods by standard methods and described under a polarizing optical microscope. Results and Discussion Comparing the results of physicochemical properties (such as color, carbonate percentage, the cation exchange capacity, etc.) in paleosol and modern loess soils indicates that the in paleosols, soil forming processes have passed several stages. The existence of the argillic horizons and the evolved calcic in paleosols and their absence in the modern soils in which they are present, indicates 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. Reducing annual precipitation decrease soil pedogenesis. Conclusion Comparison of the results obtained from paleosols of early Pleistocene with modern soils indicates that the time and climate change caused alterations in the soil micromorphology features (such as the type and amount of pores, soil structure and b-fabric and pedofeatures etc.). One of the most important pedofeatures was clay coating around void, presented only in buried paleosols, which is the evidence for moist climate conditions and subsequently enough leaching for clay translocation. Further, the presence of planar void caused by shrink and swell of clay is evidence for evolution in the paleosols. In argillic horizons of paleosols, dominant b-fabric is speckled due to carbonate leaching while in calcite horizon, it is crystallitic b-fabric. The micromorphological index of soil development calculated, showed that these red-colored deposits are formed under an annual precipitation of about 450- 650mm which represents more humid conditions at the time of their formation than the modern loess soils. In modern soils derived from recent loess, lack of clay coating can be a reason for weakly developed soil formation.