ETH Zuerich, Switzerland

Intense debate surrounds the evolution of grasses using the C (sub 4) (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 (delta (super 13) C (sub wax) ) and pollen data from marine sediments of the Gulf of Aden (northeast Africa), which show that C (sub 4) biomass increases were not necessarily associated with regional grassland expansion. We find broadly opposing trends toward more enriched delta (super 13) C (sub wax) 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 C (sub 3) grasses than previously thought, such that C (sub 4) grasses and shrubs replaced a C (sub 3) ecosystem including trees and productive grasslands. In addition, delta (super 13) C (sub wax) 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 delta (super 13) C values from East African Rift strata. We identify two distinct phases of increasing C (sub 4) 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.