Micromorphology of pedogenic carbonates on loess and aeolian affected residual soils

Introduction: Accumulation of secondary calcium carbonates in arid and semiarid regions is one of the most prominent processes of soil formation. Pedogenic carbonates are valuable indicators of soil evolution (Durand et al., ۲۰۱۰; Zamanian et al., ۲۰۱۶; Silva et al., ۲۰۱۷). The morphology of pedogenic carbonates and calcretes are highly diverse and are controlled by environmental factors (Kovda et al., ۲۰۰۳; Zhou and Chafetz, ۲۰۰۹). The objective of this study was to identify and describe the pedogenic carbonates features at the macro- and micro-scale in semiarid soils developed on various parent materials in northeastern Iran as a proxy for the reconstruction of paleoenvironments. Methods and Materials: The study area is located in the Binalood zone (۳۶° ۲′ to ۳۶° ۴۲′ N, and ۵۹° ۶′ ۵۹° ۴۸′ E) in northeastern Iran. Mean annual precipitation and temperature are ~۲۶۰ mm and ۱۳.۷ °C, respectively (Sahebjam, ۲۰۰۲). Granitic, ophiolitic, and metamorphic rocks as well as discontinuous loess deposits are the main geological constituents of the Binaloud zone (Karimi et al., ۲۰۰۹). Ten pedons on various parent materials were described and sampled. Thin sections were prepared from undisturbed samples and examined under a petrological microscope (plane and cross polarized lights). For submicroscopic studies, carbonates nodules and pendants were analyzed using a scanning electron microscope (Zeiss EVO HD۱۵) equipped with an energy dispersive X-ray (EDX) spectrometer for semi-quantitative elemental analysis. Results and Discussions: Light and dark microlaminae were observed in the thin section of calcite pendants (Fig ۱, A-B). Calcite layers in granitic pendants were thick and regular. The layers in ultramafic pendants had, however, a poor lamination. The light microlaminae, composed of purer calcium carbonate having well-formed and parallel-oriented crystals seem to be indicative of drier periods that were less favorable for biological activity. The dark microlaminae have a high content of detrital particles and are composed of less developed and randomly oriented calcite crystals. This presumably indicates wetterperiods characterized by an activation of soil biota and amelioration of climatic conditions. Our findings are supported by those of Pustovoytov (۲۰۰۳) and Sharp et al (۲۰۰۳). Carbonates are in early stages of development. They include clay coatings and nodules in limestone- and loess-derived soils, respectively. The morphology of calcite nodules indicates the impregnation of the groundmass by carbonates which have either diffuse boundaries or distinct boundaries. An impregnation of the groundmass by micritic calcite is well detectable.The SEM images of the pendants and nodules of the ultamafic pedon show rough surfaces of calcite crystals with needle-shaped and fibrous calcite. Carbonate nodules in the Crk۱ layer of granitic pedon consist mainly of star-shaped crystals which probably indicates calcium oxalate (Fig ۱, C-D). Intertwined strands of carbonate were observed in carbonate nodules of the Crk۲ horizon of the ultramafic pedon. Pedogenic carbonates of the studied pedons are of Holocene age (Karimi et al., ۲۰۰۹) and do not show evidence of dissolution and recrystallization.

کلیدواژه ها:

Pedogenic carbonates ، Holocene ، soil evolution ، ultramafic ، granitic