Miocene to Pliocene changes in South African hydrology and vegetation in relation to the expansion of C4 plants

The Miocene expansion of C-4 plants (mainly tropical grasses) between 8 and 4 million years (Ma) remains an enigma since regional differences in the timing of the expansion rules out decreased CO2 (PCO2) as a dominant forcing [e.g. Tipple and Pagani, 2007. The early origins of terrestrial C-4 photosynthesis. Annu. Rev. Earth Planet. Sci. 35, 435-4611. Other environmental factors, such as low-latitude aridity and seasonality have been proposed to explain the low tree versus grass ratio found in savannahs and tropical grasslands of the world, but conclusive evidence is missing. Here we use pollen and stable carbon (delta C-13) and hydrogen (delta D) isotope ratios of terrestrial plant wax from a South Atlantic sediment core (ODP Site 1085) to reconstruct Miocene to Pliocene changes of vegetation and rainfall regime of western southern Africa. Our results reveal changes in the relative amount of precipitation and indicate a shift of the main moisture source from the Atlantic to the Indian Ocean during the onset of a major aridification 8 Ma ago. We emphasize the importance of declining precipitation during the expansion of C-4 and CAM (mainly succulent) vegetation in South Africa. We suggest that the C-4 plant expansion resulted from an increased equator-pole temperature gradient caused by the initiation of strong Atlantic Meridional Overturning Circulation following the shoaling of the Central American Seaway during the Late Miocene. (C) 2013 Elsevier B.V. All rights reserved.