Late Miocene Atmospheric CO 2 Concentrations and the Expansion of C 4 Grasses

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Abstract
The global expansion of C 4 grasslands in the late Miocene has been attributed to a large-scale decrease in atmospheric carbon dioxide (CO 2 ) concentrations. This triggering mechanism is controversial, in part because of a lack of direct evidence for change in the partial pressure of CO 2 ( p CO 2 ) and because other factors are also important determinants in controlling plant-type distributions. Alkenone-based p CO 2 estimates for the late Miocene indicate that p CO 2 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 p CO 2 during the late Miocene. Thus, C 4 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).