Ancient CO2 calculations from stable isotopes of fossil soils

Specific Goals:
-Learn how calculations of CO₂ concentration can be made from real data.
-Learn how calculations of mean annual precipitation can be made from real data
-Learn possible limitations and potential sources of error to such calculations

Content/Concept goals: paleoclimate proxies
Higher-order thinking skills goals: data analysis; sensitivity testing
Other skills: Mathematics and Excel skills

This homework assignment was designed for an upper level undergraduate and graduate 3 credit hour paleoclimatology class at a 4-year public university. Students should have been introduced to the concept of paleoclimate proxies and the paleosol carbonate proxy for atmospheric CO₂ concentration. Students should have some working knowledge of Microsoft Excel.

This homework assignment includes an introductory file that includes a description of the assignment and instructions.
It is recommended that the students read through this file first before starting to work on the Excel Homework file. It is also helpful to read Cotton and Sheldon, 2012 (see references).

The Excel file takes students step by step to calculate ancient atmospheric CO₂ concentration from two contemporaneous paleosol carbonates from the Cretaceous Period. The Word document provides a description of the two localities.

The task of the students will be to enter the correct equations into the appropriate cells to generate the calculations. Once finished, there are three short answer questions in the last tab to stimulate thought and discussion for students. The questions reference the paper by Cotton and Sheldon, 2012 so students should at least have access to this paper. 
Introduction and Instructions (Microsoft Word 2007 (.docx) 2MB Sep15 25) 
Excel Homework File (Excel 2007 (.xlsx) 130kB Sep15 25) 
Excel Answer Key (Excel 2007 (.xlsx) 131kB Sep15 25) 

Even though the students have all of the information needed within the Excel file to complete the assignment, they often struggle to know what to do. The important thing for students to know is that their task is to enter the appropriate equations in the appropriate cells. Even though these two samples are thought to be contemporaneous, they give very different estimates. This is purposeful so that students can learn the potential pitfalls of such a proxy.
For those that have the time and resources, the assignment could be modified to include one's own data.

There is a grading rubric on the last tab of the assignment.
Assessment is based on whether the students calculate the various variables correctly and whether they identify some potential issues with the samples based on the guidelines provided in Cotton and Sheldon, 2012

https://paleo-co2.org/co2pip

Cotton, J.M., and Sheldon, N.D. 2012. New constraints on using paleosols to reconstruct atmospheric pCO2. GSA Bulletin, 124, 1411-1423.

Romanek, C. R., Grossman, E. L., and Morse, J. W. 1992. Carbon isotope fractionation in synthetic aragonite and calcite: Effects of temperature and precipitation rate. Geochimica et Cosmochimica Acta, 56, 419-430.

Nordt, L. C., and Driese, S. D. 2010. New weathering index improves paleorainfall estimates from Vertisols. Geology, 38, no. 5, 407-410.

Ekart, D. D., Cerling, T. E., Montanez, I. P., and Tabor, N. J. 1999. A 400 million year carbon isotope record of pedogenic carbonate: Implications for paleoatmospheric carbon dioxide. American Journal of Science, 299, no. 10, 805-827.

Cerling, T. E., and Quade, J. 1993. Stable Carbon and Oxygen Isotopes in Soil Carbonates, Climate Change in Continental Records, Volume Geophysical Monograph 78, American Geophysical Union.

Sheldon, N. D., Retallack, G. J., and Tanaka, S. 2002. Geochemical climofunctions from North American soils and application to paleosols across the Eocene-Oligocene boundary in Oregon. Journal of Geology, 110, no. 6, 687-696.

Harper, D.T., Suarez, M.B., Uglesich, J., You, H.-L., Li, D.-Q., Dodson, P. 2021, Aptian-Albian clumped isotopes from northwest China: Cool temperatures, variable atmospheric pCO2, and regional shifts in hydrologic cycle. Climate of the Past. v. 17, p. 1607-1625. https://doi.org/10.5194/cp-17-1607-2021.

Suarez, M.B., Knight, J., Snell, K.E., Ludvigson, G.A., Kirkland, J.I., Murphy, L. 2020, Multiproxy paleoclimate estimates of the continental Cretaceous Ruby Ranch Member of the Cedar Mountain Formation. In: Bojar, A.-V., Pelc., A., and Lecuyer, C. (eds.) Stable Isotopes Studies of Water Cycle and Terrestrial Environments. Geological Society, London, Special Publications, v. 507. https://doi.org/10.1144/SP507-2020-85