IDENTIFICATION OF PEDOGENIC CARBONATES USING STABLE CARBON ISOTOPE AND MICROFABRIC ANALYSES

In soils formed from limestone, it is often difficult to distinguish between inherited and pedogenic carbonate. In nature, both thermodynamic and kinetic factors have caused fractionation of C isotopes. While marine carbonates usually have .delta.13C values near zero, the processes of pedogenic carbonate formation result in considerably lower values, which are dependent only on the .delta.13C of the soil CO2 gas. Therefore, a simple proportionality may be employed to quantitatively estimate pedogenic carbonates in the soil. Seven pedons formed over Cretaceous limestone in Central and West Texas [USA] were studied. The .delta.13C values were determined for the parent carbonates, soil organic matter and for the various carbonate-enriched horizons. Both finely disseminated carbonates in A horizons and massive indurated petrocalcic materials were confirmed by isotopic analyses to be primarily of pedogenic origin. Characteristic microfabrics were correlated with isotopic assay of massive petrocalcic materials which may be diagnostic for identification. Although generally useful, isotopic analysis underestimates the pedogenic component in certain carbonate forms, including laminar caps and some concretions and pendants. A different mode or environment of formation for these forms is probably responsible for discrepancies with theoretical values.