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

Thursday, 18 December 2014
Cassaundra Ashley Rose, Columbia University of New York, Palisades, NY, United States, Peter B deMenocal, Lamont-Doherty Earth Obs, Palisades, NY, United States and Pratigya J Polissar, Lamont-Doherty Earth Observato, Nyack, NY, United States
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 (δDwax and δ13Cwax) at Ocean Drilling Program Site 659 (20°N), offshore West Africa, from 0 – 25 Ma. Between 25 to 10 Ma, n-alkane δ13Cwax 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 δ13Cwax shift (in excess of 4‰) suggesting the initial growth and establishment of C4 Sahel grasslands. δ13Cwax 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.