1Isfahan University of Technology, Department of Soil Science, Isfahan, Iran, and
2Saskatchewan Centre for Soil Research, Department of Soil Science, 51 Campus Drive, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5A8, Canada
Tóm tắt
SummaryPedogenic carbonates in arid and semi‐arid regions of the world have a great significance as palaeoecological and palaeoclimatological indicators and form a major pool in the carbon cycle. We analysed the ultra‐microfabric and the stable isotope composition of C and O in pedogenic carbonates in colluvial soils derived from limestone in an arid region of central Iran. Our objective was to determine the conditions for the formation of soft pedogenic carbonate nodules and their co‐existence with palygorskite in the palaeo‐argillic horizon. Scanning electron microscopy revealed that the calcite aggregates were matted with palygorskite. Ultra‐microtome cuts, examined using transmission electron microscopy, provided more detailed information about the fundamental particle association of secondary carbonates and palygorskite. Although less abundant, other silicate clays were detected in both the acid‐insoluble clay fractions and in ultra‐cuts, mostly in fine clay size, suggesting the engulfing of palygorskite by growing calcite or illuviation of palygorskite during or after formation of the calcite. Coatings of illuvial clays on calcite crystals support the hypothesis that palygorskite was trapped by pedogenic carbonate when the climate was wetter than it is today to form an argillic horizon. However, electron microscopic evidence of the occurrence of fibres on the immediate pedogenic carbonate particle surfaces suggests the in situ formation of palygorskite. The δ13C and δ18O values of pedogenic carbonates suggest that these carbonates were formed in an environment with more available moisture and more C4 plants than now.
