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Identification of Pedogenic Carbonates using Stable Carbon Isotope and Microfabric Analyses¹

ABSTRACT

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 ¹³C values near zero, the processes of pedogenic carbonate formation result in considerably lower values, which are dependent only on the 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 were studied. The 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. It is postulated that a different mode or environment of formation for these forms is responsible for discrepancies with theoretical values.

¹ Contribution from Dep. of Soil and Crop Sciences, Texas A&M University and the Texas Agric. Exp. Stn. Presented in part before Div. S-5, Soil Science Society of America, Atlanta, Ga, 2 Dec. 1981. Texas Agric. Exp. Stn. Paper no. TA-19005.

² Formerly Research Assistant, Dep. of Soil and Crop Sciences, Texas A&M Univ., currently Assistant Professor of Pedology, Dep. of Agronomy, Univ. of Maryland, College Park; Professor of Pedology, and Research Associate, Dep. of Soil and Crop Sciences, Texas A&M Univ., College Station, respectively.

Received for publication January 27, 1983. Accepted for publication October 5, 1983.