No abstract

Proxy estimates of atmospheric CO2 are necessary to reconstruct Earth's climate history. Confidence in paleo‐CO2 estimates can be increased by comparing results from multiple proxies at a single site, but so far this strategy has been implemented only for marine‐based techniques. Here we present CO2 estimates for the well‐studied early Paleocene Castle Rock site in Colorado using four paleobotanical proxies. Median estimates range from 470 to 813 ppm, demonstrating fair correspondence. The synthesis yields a median of 616 ppm (352–1110 ppm at 95% confidence), considerably higher than previous early Paleocene CO2 estimates (~300 ppm). Ash bed geochronology by the high‐precision U‐Pb method places the Castle Rock assemblage at 63.844 ± 0.097 Ma (fully propagated 2σ error). When these results are placed into the broader context of other Cenozoic CO2 estimates from plant‐gas‐exchange approaches and coeval estimates of global mean surface temperature, a pattern emerges of an Earth system sensitivity around 3 °C per CO2 doubling during the Paleocene and Eocene, a time with little land ice, then steepening to >7 °C after the Eocene once land ice was present on Antarctica.

First, and most of all, I want to thank Dr. Dana Royer for giving me the opportunity to work on this exciting project and for being a wonderful advisor and mentor. I have learned an incredible amount in the past two years and I really appreciate all of your help! Thank you also to my thesis committee members: Dr. Ian Miller, who helped me select fossil specimens and has answered numerous questions about Castle Rock; and Dr. Barry Chernoff, who has provided valuable statistics advice. I would also like to thank Dr. Sonia Sultan and Jacob Herman for allowing me to use the LI-COR gas exchange analyzer and Dr. Tim Ku for providing lab space. Wesleyan undergraduates Alex (Jed) Siebert and Melissa McKee prepared and analyzed one of the fossil morphotypes used in this study-thank you both! Thank you to Dr. David Beerling for provided useful advice regarding the BRYOCARB code and to Drs. Michaela Grein and Anita Roth-Nebelsick for counsel on running the Konrad gas exchange model. Thank you also to Drs. Regan Dunn and Beth Miller for answering questions about Castle Rock, and to Dr. Peter Franks for helpful discussion about the gas exchange model. Finally, thank you to Dr. Paul Renne for providing a spreadsheet to calculate updated Ar/Ar ages. Ginny Harris and Joel LaBella-thank you for all of your help around the department. Finally, thank you to my family for cheering me on!