Northeast African vegetation change over 12 m.y.

Intense debate surrounds the evolution of grasses using the C4 (Hatch-Slack) photosynthesis pathway and the emergence of African grasslands, often assumed to be one and the same. Here, we bring new insights with the combination of plant leaf wax carbon isotopic composition (δ13Cwax) and pollen data from marine sediments of the Gulf of Aden (northeast Africa), which show that C4 biomass increases were not necessarily associated with regional grassland expansion. We find broadly opposing trends toward more enriched δ13Cwax values and decreased grass pollen proportions between 12 and 1.4 Ma. This apparently contradictory evidence can be reconciled if a greater proportion of the Late Miocene northeast African landscape were covered by C3 grasses than previously thought, such that C4 grasses and shrubs replaced a C3 ecosystem including trees and productive grasslands. In addition, δ13Cwax and pollen both indicate that true rainforests were unlikely to have been extensive in northeast Africa at any time in the last 12 m.y., although seasonally dry forests were a significant component of the regional landscape since the Late Miocene. Here, we extend regionally integrative marine archives of terrestrial vegetation back to 12 Ma, and we evaluate them in the context of an updated compilation of pedogenic carbonate δ13C values from East African Rift strata. We identify two distinct phases of increasing C4 biomass between 11 and 9 Ma (with a reversal by 4.3 Ma) and then a re-expansion between 4.3 and 1.4 Ma; surprisingly, neither was associated with grassland expansion.