Late Miocene atmospheric CO2 concentrations and the expansion of C-4 grasses

The global expansion of C-4 grasslands in the late Miocene has been attribu ted to a large-scale decrease in atmospheric carbon dioxide (CO,) concentra tions. This triggering mechanism is controversial, in part because of a lac k of direct evidence for change in the partial pressure of CO2 (pCO(2)) and because other factors are also important determinants in controlling plant -type distributions. Alkenone-based pCO(2) estimates for the Late Miocene i ndicate that pCO(2) increased from 14 to 9 million years ago and stabilized at preindustrial values by 9 million years ago. The estimates presented he re provide no evidence for major changes in pCO(2) during the Late Miocene. Thus, C-4 plant expansion was Likely driven by additional factors, possibl y a tectonically related episode of enhanced Low-latitude aridity or change s in seasonal precipitation patterns on a global scale (or both).