ESTIMATING PCO2 ACROSS THE PETM USING CARBON ISOTOPE RATIOS OF PALEOSOL CARBONATES AND ORGANIC MATTER FROM THE BIGHORN BASIN, WYOMING

Warming during the PETM has been documented worldwide, and is usually attributed to an increase in the concentration of greenhouse gases such as CO₂. To date, only a few proxies for the partial pressure of CO₂ in the atmosphere (pCO₂) have provided records for the Paleogene, and only a single data point represents the PETM interval in particular. Thus the exact role of CO₂ in this period of rapid warming remains ambiguous.

One area that has the potential to provide high-resolution records of pCO₂ for the PETM interval is the Bighorn basin of Wyoming, where this time is represented by tens of meters of sediment. Carbon isotope records from paleosol carbonates and from dispersed organic matter have been obtained from these rocks, thus it should be possible to estimate pCO₂ using a diffusion-reaction model for the carbon isotope ratio of soil CO₂ and soil carbonate. Unfortunately, making useful estimates is easier said than done.

The primary difficulty is that the diffusion model approach is characterized by a number of different variables, and that estimates of pCO₂ obtained using it vary drastically depending on input parameters (e.g. temperature, isotope ratio of vegetation and atmosphere, partial pressure of CO₂ in soils). For example, if ‘typical' assumptions are made (MAT = 25°C, carbon isotope ratio of vegetation inferred from that of marine carbonate, soil pCO₂ = 5000 ppmv), pCO₂ decreases during the PETM to negative values. Even estimates made using independent records of MAT and carbon isotope data from sedimentary organic matter result in low estimates of pCO₂, with a negligible/negative change in pCO₂ across the PETM. The best-constrained estimates are those that also account for isotopic degradation of organic matter in soils, and these indicate that pCO₂ increased during the PETM. The magnitude of the increase, however, is dependent on assumed partial pressure of CO₂ in soils. Thus lack of specific information regarding soil pCO₂ ultimately results in equivocal estimates of pCO₂ using this method.

Models of soil carbon dynamics during the Eocene, studies of modern soil systems, and/or independent estimates of pCO₂ obtained using other proxy records may allow better estimates of soil pCO₂ to be made, thus overcoming these problems.