Joint 118th Annual Cordilleran/72nd Annual Rocky Mountain Section Meeting - 2022

Paper No. 9-18
Presentation Time: 8:30 AM-6:00 PM


BEUKEMA, Aiden, Department of Geology and Geophysics, University of Utah, Salt Lake City, UT 84112-0102, IRMIS, Randall, Natural History Museum of Utah and Department of Geology & Geophysics, University of Utah, 301 Wakara Way, Salt Lake City, UT 84108-1214 and MILNER, Andrew R.C., St. George Dinosaur Discovery Site at Johnson Farm, 2180 East Riverside Dr, St. George, UT 84790

Carbon isotope values from paleosols are an important proxy for atmospheric pCO2, providing cludes to deep time paleoclimate. In modern soils, δ13Ccarb 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 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.