CARBON ISOTOPIC VARIABILITY IN PALEOSOLS AND ITS EFFECTS ON RECONSTRUCTING PCO2: INSIGHTS FROM THE UPPER TRIASSIC CHINLE FORMATION

Carbon isotope values from paleosols are an important proxy for atmospheric pCO₂, providing cludes to deep time paleoclimate. In modern soils, δ¹³C_carb values typically reach equilibrium between 30-50 cm depth from the top of the soil profile (usually in the B horizon), so workers have assumed this to be true for paleosols as well. As a result, most paleosol carbonate pCO₂ proxy data derive from point samples taken from the B horizon. However, few paleosol studies have examined the variability of δ¹³C down profile and within a single sampling horizon. Therefore, it is unclear whether taking a point sample at or below 50 cm is sufficient to recover the isotopic value at equilibrium for every paleosol, and how this sampling strategy affects subsequent reconstructed pCO₂ values.

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 δ¹³C_carb and δ¹⁸O_carb. Mudstone matrix from each depth was also analyzed for δ¹³C_org. Down-profile δ¹³C_carb 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 δ¹³C_carb values. This range of variation can significantly affect the pCO₂ estimations; a difference of 1.5‰ in δ¹³C_carb values (assuming the same δ¹³C_org 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.