The origin and development of the Miocene northwest African savanna.

Like many of Earth's subtropical open ecosystems, the African savanna originated in the Miocene Epoch (23.0 to 5.3 Ma), one of the best historical analogs for future climate states. The first C4 grass, C3 tree, and shrub savanna developed in the subtropics after ~10 Ma in northwest Africa, but the terrestrial sedimentary record of the region is not well preserved, making it difficult to determine when, how, and why the African landscape opened. Here, we leverage plant-wax biomarkers extracted from the marine sedimentary archive to document the origination and history of open ecosystems through Miocene northwest Africa. We show that a C3 grass-rich savanna with no modern analog replaced closed woodlands and forests during the middle Miocene (~15 to 14 Ma), a period of global cooling and aridification. This nonanalog ecosystem transitioned into the C4 savanna in stages in the late Miocene (most rapidly between 7.4 to 6.4 Ma) as global temperatures cooled and aridity increased, roughly coincident with several C4 expansions in other subtropical regions. Our new vegetation history, put in the context of other records, links initial ecosystem opening and C4 expansion to periods of global cooling and drying. It is consistent with a nonlinear system of feedbacks between climate, consumers (herbivory and fire), and plant-trait interactions that allows small changes in one parameter to have dramatic effects on regional vegetation structure and composition. The geologic record indicates that the response of subtropical open ecosystems to future climate perturbations will depend on the nature and strength of these feedbacks.