Submicroscopy and stable isotope geochemistry of carbonates and associatedpalygorskite in Iranian Aridisols

Pedogenic carbonates in arid and semi-arid regions of the world have a grea t significance as palaeoecological and palaeoclimatological indicators and form a major pool in the carbon cycle. We analysed the ultra-microfabric an d the stable isotope composition of C and O in pedogenic carbonates in coll uvial soils derived from limestone in an arid region of central Iran. Our o bjective was to determine the conditions for the formation of soft pedogeni c carbonate nodules and their co-existence with palygorskite in the palaeo- argillic horizon. Scanning electron microscopy revealed that the calcite ag gregates were matted with palygorskite. Ultra-microtome cuts, examined usin g transmission electron microscopy, provided more detailed information abou t the fundamental particle association of secondary carbonates and palygors kite. Although less abundant, other silicate clays were detected in both th e acid-insoluble clay fractions and in ultra-cuts, mostly in fine clay size , suggesting the engulfing of palygorskite by growing calcite or illuviatio n of palygorskite during or after formation of the calcite. Coatings of ill uvial clays on calcite crystals support the hypothesis that palygorskite wa s trapped by pedogenic carbonate when the climate was wetter than it is tod ay to form an argillic horizon. However, electron microscopic evidence of t he occurrence of fibres on the immediate pedogenic carbonate particle surfa ces suggests the in situ formation of palygorskite. The delta(13)C and delt a(18)O values of pedogenic carbonates suggest that these carbonates were fo rmed in an environment with more available moisture and more C4 plants than now.