Reconstruction of atmospheric CO2 during the early middle Eocene by application of a gas exchange model to fossil plants from the Messel Formation, Germany

In this study, atmospheric CO2 during the early middle Eocene (~ 47 Ma) is reconstructed using fossil plants from the Messel Formation close to Darmstadt, Germany. CO2 concentration is calculated using a mechanistic model of gas exchange which optimizes CO2 uptake by photosynthesis against water vapor loss by transpiration, a strategy that is commonly realized in land plants. Input data for the model are stomatal data (density and pore length), paleoclimate data obtained by two different approaches and biochemical parameters of photosynthesis. Leaves from three taxa belonging to two different families were studied: Laurophyllum lanigeroides, Daphnogene crebrigranosa (both Lauraceae) and Rhodomyrtophyllum sinuatum (Myrtaceae). The results suggest that CO2 was between 853 and 1033 ppm during the development of the Messel Formation. Stomatal data therefore indicate a CO2 concentration that was substantially higher than today. The reconstructed range of CO2 concentration is close to data provided by geochemical modeling results and conforms to various data that indicate a warm and ice-free climate during this time interval.