Late Miocene Rise of C4 Vegetation in NW Africa from Leaf Wax Biomarkers

Abstract:

When and why did NW Africa become dry? The answers to these important questions have proven elusive. Strong climate controls on African vegetation today make knowledge of past changes a valuable proxy for understanding NW Africa’s climate evolution. Various lines of geologic and paleobotanical evidence indicate that NW African landscapes changed from more humid conditions in the late Oligocene/early Miocene to arid/hyper-arid environments by the late Pliocene. As proxies for the paleohydrological and paleovegetation signatures of this event, we analyzed leaf wax n-alkane stable isotopes (δD_wax and δ¹³C_wax) at Ocean Drilling Program Site 659 (20°N), offshore West Africa, from 0 – 25 Ma. Between 25 to 10 Ma, n-alkane δ¹³C_wax values were persistently very low (-31‰) suggesting that C₃ vegetation dominated NW African landscapes over this interval. Between 10-7 Ma there is a marked, positive secular δ¹³C_wax shift (in excess of 4‰) suggesting the initial growth and establishment of C₄ Sahel grasslands. δ¹³C_wax shows a sustained positive trend (>7‰ total) until 1 Ma. The 10-7 Ma date for the establishment of NW African C₄ grasslands is earlier than comparable records from South Africa and South Asia. We will also present low-resolution isotope data from equatorial ODP Site 959 (3°N) and compare these data with Site 659 (20°N) to reconstruct the development of the modern vegetation and hydrological gradients in NW Africa over this time span.