2009 Portland GSA Annual Meeting (18-21 October 2009)

Paper No. 11
Presentation Time: 10:45 AM


DORIA, Gabriela1, ROYER, Dana L.2, WOLFE, Alexander P.3, FOX, Andrew4 and BEERLING, David J.4, (1)Smithsonian Tropical Research Institute, Unit 0948 APO 34002, Balboa, Ancon, Panama, 0843-03092, Panama, (2)Department of Earth and Environmental Sciences, Wesleyan University, Exley Science Center 445 (265 Church St.), Middletown, CT 06459-0139, (3)Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2E3, Canada, (4)Department of Animal and Plant Sciences, University of Sheffield, Sheffield, S10 2TN, United Kingdom, gabriela.doria@yale.edu

The transition from the extreme greenhouse world of the early Paleogene (~55 Myrs ago) to the present-day icehouse represents one of the most prominent changes in Earth’s climate evolution. However, there is substantial controversy over the history of climate and atmospheric CO2 during part of the Middle Eocene “doubthouse” interval (42-38 Myrs ago), including evidence for pulses of global warmth and ice sheet growth. Here we estimate the concentration of atmospheric CO2 during the late Middle Eocene (~40 Myrs ago) using stomatal indices of mummified Metasequoia needles from ten levels in an exceptionally-preserved core from the Giraffe Pipe kimberlite in northwestern Canada (62 °N paleolatitude). Reconstructed atmospheric CO2 values lie between 600-1000 ppm but with a secular decline to 400-500 ppm towards the top of the studied core. Because the CO2-threshold for nucleating large ice sheets during the Cenozoic is ~500 ppm, our CO2 record is most compatible with a transition from warm, largely ice-free conditions to cooler climates and the existence of ice sheets. Our fossils also unequivocally demonstrate that high-latitude deciduous forests thrived in the geologic past under atmospheric CO2 concentrations that will likely be reached within the current century (500-1000 ppm).