A MIDDLE EOCENE ATMOSPHERIC CO2 RECONSTRUCTION FROM FOSSIL METASEQUOIA, AXEL HEIBERG ISLAND (NUNAVUT, CANADA)

The Eocene (55-34 Ma) has been well studied by paleoclimatologists primarily due to the dramatic climate shift from the early Eocene Climatic Optimum (EECO, ~52 Ma) greenhouse to the onset of Antarctic glaciation at the Eocene-Oligocene boundary. Reconstructions of paleoatmospheric carbon dioxide (CO₂) concentration are abundant for the late Paleocene and the EECO. However, relatively less is known about the middle Eocene (50-40 Ma) which represents an important time period between peak Cenozoic warmth and the transition to a global icehouse. The middle Eocene (46.1-37.2 Ma) fossil forests of the Buchanan Lake Formation on Axel Heiberg Island in the Canadian High Arctic provide an abundance of well-preserved remains of the deciduous conifer Metasequoia. Metasequoia is frequently used in reconstructions of paleoatmospheric CO₂ from stomatal density (SD) and stomatal index (proportion of stomatal to epidermal cells, SI) due to the high degree of evolutionary stasis within the genera. Both SD and SI reconstructions are based on species-specific calibration curves, which are laborious to construct and perform poorly at estimating high levels of CO₂. More recent gas-exchange based models, which emphasize stomatal conductance, known photosynthetic parameters, and carbon isotope data, show promise to improve stomatal based CO₂ reconstructions by removing dependence on extant calibrations and improving the precision of high CO₂ estimates. Here, we test gas-exchange models on extant M. glyptostroboides to determine their applicability in reconstructing CO₂ using fossil Metasequoia. Then, we apply these models to fossil M. occidentalis from six forest layers from Axel-Heiberg Island. Preliminary estimates appear to suggest that these models work well for estimating CO₂ from present-day M. glyptostroboides (reconstructed values ~470 ppmV) and estimates of middle Eocene CO₂ from Axel Heiberg range from 400-600 ppmV.