The Late Miocene Rise of C4 Vegetation in Eastern Africa Documented by Terrestrial Plant Waxes in Marine Cores

Abstract:

C₄ plants are predominantly grasses and they account for ~20% of global net primary productivity, serve as important sources of food, and are the dominant plant type in non-forested tropical ecosystems. Yet the reasons behind their rise to such a globally significant component of the terrestrial biosphere within the last 10 million years are not well understood. In eastern Africa, the expansion of C₄ grasslands led to long-term changes in faunal distributions and resulted in major dietary shifts in mammalian lineages. Potential mechanisms leading to the rise of C₄ plants include a decrease in atmospheric CO₂, ecosystem perturbations by fire or large herbivores, and increased aridity or seasonality of precipitation. Improvement of the temporal and spatial coverage of vegetation records in the Late Neogene of East Africa may help elucidate the mechanisms responsible for regional and global C₄ grassland expansion. It will also improve our ability to assess the relationship between vegetation change and mammalian evolution.

To evaluate the evolution of C₄ grasslands in East Africa, we measured carbon isotope ratios of n-alkanes from four DSDP cores stretching from the Red Sea (19.1° N) to the Somali Basin (2.4° S) that range in age from ~24 Ma to 0.5 Ma. Carbon isotope data from Somali Basin sites 235 and 241 indicate the appearance of C₄ vegetation by ca. 10 Ma, followed by a relatively steady increase through the late Pleistocene. Odd numbered n-alkane homologues (C_{29 ­}to C₃₅) exhibit up to a 10‰ increase in δ¹³C. We also established end member molecular distributions of n‑alkanes and tracked changes in their proportional contributions through time. Changes in molecular distribution are broadly synchronous with increases in carbon isotope ratios, suggesting that n-alkane distributions reflect changes in C₃ and C₄ vegetation types.