The glacial origin of carbonates in the soils developed on glacial deposits in the southern Alborz Mountain slope

Introduction: The main goals of this research are to investigate the origin of soil-forming carbonates in soils formed in glacial sediments, to study the effect of glacial processes on the formation of pedogenic carbonates and the resulting micromorphological complications, and to explore the possibility of pedogenic carbonates forming directly through the dissolution of carbon dioxide in cold glacial waters.Methods and Materials: This study was conducted in the year ۲۰۲۱ in Alborz Province, and eight profiles located in the Karaj and Hashtgerd regions with glacial-alluvial parent materials were described and sampled. Physical and chemical characteristics, including soil texture before and after the removal of carbonates by the hydrometer method, pH, and EC in saturated extract, organic carbon by the Walkley-Black method, Calcium Carbonate Equivalent (CCE) measured by the calcimetric method, and Cation Exchange Capacity (CEC) measured by the ammonium acetate method, were carried out on ۲۷ samples. Soil description and classification were performed based on the American classification system. A micromorphological study of undisturbed samples, before and after the removal of carbonates, was carried out following their impregnation with polyester resin, cutting, sawing, mounting on glass slides, and reducing the thickness to about ۳۰ microns. Imaging was done with a polarizing microscope, and the analysis and interpretation of the results were conducted according to the guide for the analysis of thin sections.Results and Discussions: Examining the particle size distribution of the soils developed on the glacial sediments before and after the removal of carbonates revealed that most of the carbonates in these soils are in the clay fraction (and to some extent in the silt fraction). With the removal of carbonates, the percentage of clay decreased drastically, and the particle size distribution class changed from loam and clay-loam classes to coarser texture classes of loamy-sand, sandy-loam, and even sandy. The percentage of clay in the middle part of the profiles before and after the removal of carbonates determines the characteristics of the argillic horizons. The thin sections before the removal of carbonates did not show clay coating pedo-features. Examination of the thin sections after the removal of carbonates showed that the crystalline b-fabric in the micromass was removed and turned into an undifferentiated b-fabric. A speckled b-fabric with phyllosilicate clays and weak clay coatings on some parts of the sections appeared. Of course, the possibility of transporting phyllosilicate clays together with the water resulting from the melting of glaciers and theirco-precipitation with carbonates in the petrocalcic horizons is not far from expected. The petrocalcic horizons formed on glacial tills and moraines result from the long-term infiltration of cold water rich in dissolved carbonates into the soil. This model of secondary carbonate formation in the soil differs from other models that mainly consider the origin of carbonates to be the dissolution and recrystallization of primary carbonates or the biological respiration of roots and living organisms.

کلیدواژه ها:

Carbonates ، Carbon Dioxide ، Glacial sediments ، Micromorphology ، Removal of Carbonates