A COMPARISON OF MULTIPLE PROXY ESTIMATES OF ATMOSPHERIC CO2 FROM AN EARLY PALEOCENE RAINFOREST

Proxy estimates of atmospheric pCO₂ are critical for reconstructing Earth’s climate history and for understanding long-term climate sensitivity. Because each proxy method has underlying assumptions and inherent limitations, our confidence in estimated pCO₂ will be increased by comparing results from multiple proxies for a given site. The well-studied and unusually diverse early Paleocene (63.8 Ma) Castle Rock fossil site from Colorado’s Denver Basin allows for such a comparison. Using fossil leaf cuticle from multiple Castle Rock taxa (Ginkgo sp., a liverwort, and angiosperms including two Lauraceae morphotypes), I compare pCO₂ estimates from three leaf-based proxies: stomatal index, a new proxy based on a gas exchange model of C₃ photosynthesis, and the BRYOCARB model for liverworts. Preliminary estimates from Ginkgo give pCO₂ values in the range of 470 – 560 ppm, slightly elevated over previous stomatal index and paleosol estimates for the early Paleocene, which are closer to present-day pCO₂. Reconstructions of mean annual temperature and precipitation from Castle Rock flora show a climate similar to modern tropical or paratropical rainforests; the existence of this climate in the Denver Basin during the early Paleocene at pCO₂ values only moderately elevated over the present-day implies a high climate sensitivity.