Quaternary calcretes of Qazvin Plain (N Iran) based on multi-story geochemical and petrographic signatures

Striking examples of calcretes have been formed in alluvial and aeolian sediments of the Qazvin Plain in Iran. In the present research, eight profiles were studied, varying from recent proximal alluvial fans to adjacent alluvial plains and sand dunes. Macroscopic, microscopic, isotopic, and elemental composition studies show that both pedogenic and non-pedogenic processes have been responsible for the formation of these horizons. The remarkable features of the calcretes in this study are the prevalence of spores, nanofibers, and pisoids. This research provides clues for organisms involved, and the interface of soil with the atmosphere or the rhizosphere in the calcretization process. Nanofibers only occur around root traces in the pisoides and gypcretes. The shape of nanofibers reflects the contribution of roots and their associated microorganisms in the formation of calcrete and gypcrete. According to the results of the present study, the main factors controlling the calcretization are climate, calcrete position in the landscape, a host material, and topography. The stable isotope analysis of two alluvial fan profiles show that δ 13 C and δ 18 O values range for the groundwater calcretes in the phreatic zone from −4.34 to −1.98‰ and −9.26 to −6.97‰, respectively. These values suggest a high contribution of soil-derived light isotopes in meteoric water. These ranges in the vadose zone vary from −1.98 to −7.32 ‰ and −9.26 to −7.32‰, respectively. This result can be attributed to the density and types of vegetation, host detrital sediments, meteoric water, and evaporation that have controlled the isotopic composition of the calcretes.