Devonian paleoclimate and its drivers: A reassessment based on a new conodont δ18O record from South China

A new Devonian oxygen isotope record based on 180 measurements of conodont apatite is reported from South China. The comparison with published Devonian δ18Oapatite data shows a considerable offset between records from different paleocontinents. This difference can be interpreted by regional variations in salinity, with the epicontinental seas having a distinctly lower salinity and δ18Oseawater than the open ocean due to the influence of fresh-water runoff. Our findings suggest that the oxygen isotope record from open ocean settings is the preferred archive for reconstructing the history of ocean temperature and/or ice volume over the Phanerozoic.

Despite regional differences, the South China and European records show similar long-term trends characterized by a pronounced cooling during the Pragian to Eifelian followed by significant warming over the Eifelian/Givetian to Frasnian, and a further cooling starting in the Famennian, accelerating in the earliest Carboniferous. The Early Devonian cooling coincided with early vascular plant root − soil interactions and a significant diversity increase in both spores and megaplant fossils, suggesting that the rise of rooted vascular plants may have played a key role in triggering climate cooling. Subsequent climatic warming over the Middle to Late Devonian transition may have been linked to metamorphic CO2 input from the Acadian Orogeny, while Famennian cooling occurred in a context of exposure and weathering of basalts and ophiolites and coincided with the advent of seed plants. We conclude that climate changes during the Devonian were likely driven by a combination of plant evolutionary advances and changes in tectonics.

We further test these interpretations by running the COPSE (Carbon, Oxygen, Phosphorus, Sulfur and Evolution) biogeochemical model. The model prediction is capable of reproducing the pCO2 record under these scenarios, although the model is not capable of reproducing the degree of temperature variation, likely due to its simplicity.