2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 4
Presentation Time: 2:15 PM


FRICKE, Henry, Department of Geology, Colorado College, 14 E. Cache La Poudre, Colorado Springs, CO 80903 and BOWEN, Gabriel, Biology Department, University of Utah, Salt Lake City, UT 84112, hfricke@coloradocollege.edu

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.