Late miocene atmospheric CO₂ concentrations and the expansion of C₄ grasses

The global expansion of C₄ grasslands in the late Miocene has been attributed to a large-scale decrease in atmospheric carbon dioxide (CO₂) concentrations. This triggering mechanism is controversial, in part because of a lack of direct evidence for change in the partial pressure of CO₂ (pCO₂) and because other factors are also important determinants in controlling plant-type distributions. Alkenone-based pCO₂ estimates for the late Miocene indicate that pCO₂ increased from 14 to 9 million years ago and stabilized at preindustrial values by 9 million years ago. The estimates presented here provide no evidence for major changes in pCO₂ during the late Miocene. Thus, C₄ plant expansion was likely driven by additional factors, possibly a tectonically related episode of enhanced low-latitude aridity or changes in seasonal precipitation patterns on a global scale (or both).