Quantifying pedogenic carbonate accumulations using stable carbon isotopes
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Four pedons from a late Quaternary chronosequence developed in calcareous alluvium in central Texas were investigated to assess the ability of the stable C isotope method to partition and quantify pedogenic carbonate accumulations. To quantify pedogenic carbonate accumulations with this method, 13C values of bulk, pedogenic, and parent carbonate must be known. For each pedon, 13C values of bulk carbonate were measured on a horizon by horizon basis. The parent carbonate end-member for all pedons was approximated by averaging the bulk 13C values for all horizons from the weakly developed floodplain soil. The diffusion model of Cerling (1984) and Quade et al. (1989) was used to estimate the pedogenic carbonate end-member. Quantification of pedogenic carbonate accumulations by the isotopic method was compared to quantitative estimates conducted by field morphology, binocular light microscope point counts, and thin-section point counts. Results suggest that the isotopic method is superior to other methods of quantifying pedogenic carbonate accumulations. Whole-soil pedogenic carbonate accumulations as calculated by the isotopic method for the chronosequence were: (1) 1 to 4 vol.% by 2000 years; (2) 4 to 15 vol.% by 5000 years; and (3) 1 to 12 vol.% after 15,000 years. These results reveal net soil carbonate loss through time for central Texas climates. This corroborates similar conclusions drawn for the same chronosequence using mass balance analysis to calculate the flux of carbonate. Net carbonate loss through time also indicates that these soils do not serve as long-term carbonate C sequesters.