A terrestrial record of atmospheric CO2 across the Middle Eocene Climatic Optimum

The Middle Eocene Climatic Optimum (MECO; ca. 40.5–40.0 Ma) represents a prolonged interval of global warming that temporarily reversed long-term Eocene cooling. Yet terrestrial constraints on atmospheric CO2 across this event remain extremely sparse, limiting evaluation of carbon-cycle feedbacks during sustained greenhouse conditions. Here we present the first terrestrial atmospheric CO2 record across the MECO derived using the paleosol carbonate CO2 paleobarometer in the Escanilla Formation of the South Pyrenean Foreland Basin, Spain. Reconstructed CO2 values show pronounced stratigraphic variability, with mean values of ~400 ppmv prior to the MECO, a 2.75-fold increase to ~1100 ppmv at peak warming, and a return to near-background levels thereafter. Monte Carlo simulations confirm both the magnitude of the CO2 increase and enhanced variability during peak warming, indicating a robust atmospheric signal. The timing and magnitude of the terrestrial CO2 increase closely mirror marine boron-isotope reconstructions, supporting a transient CO2 perturbation concentrated near the MECO peak. These results demonstrate that relatively modest but focused increases in atmospheric CO2 were sufficient to drive sustained middle Eocene warming, and underscore the importance of terrestrial archives for constraining carbon-cycle feedbacks during prolonged greenhouse climates.