We reconstructed from sediments of different ocean sites phytoplankton carbon isotopic fractionation (ε_p), mainly linked to CO₂ variations, during the Oligocene to early Miocene. Data confirm Cenozoic long-term CO₂ record but show contrasting relationships with the sea surface temperatures evolution. We evaluate the role of non-CO₂ environmental factors such as temperature and nutrients at each site ε_p, highlighting the complexity of interpreting climate dynamics and CO₂ reconstructions.