DECLINING ATMOSPHERIC CO2 DURING THE LATE MIDDLE EOCENE CLIMATE TRANSITION (~40 MYRS AGO)

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).