CARBON ISOTOPIC VARIABILITY IN PALEOSOLS AND ITS EFFECTS ON RECONSTRUCTING PCO2: INSIGHTS FROM THE UPPER TRIASSIC CHINLE FORMATION
To investigate this problem further, we sampled calcic paleosols from the upper Chinle Formation of southeastern Utah, sampling every 10 cm along the soil profile. Multiple carbonate nodules were taken from each depth and were analyzed for δ13Ccarb and δ18Ocarb. Mudstone matrix from each depth was also analyzed for δ13Corg. Down-profile δ13Ccarb values rarely fit well to a theoretical vertical slope representing equilibrium, with variation of up to 2.7‰, often below the 50 cm depth. At any single sampling horizon within a paleosol, we observe variation of up to 1.5‰ between individual carbonate nodule δ13Ccarb values. This range of variation can significantly affect the pCO2 estimations; a difference of 1.5‰ in δ13Ccarb values (assuming the same δ13Corg value) can result in up to ~900 ppm difference in reconstructed pCO2. Thus, we suggest that characterization of down-profile and within-horizon variability is critical for obtaining more accurate atmospheric pCO2 values from paleosols.