LATE MIOCENE RISE OF MODERN VEGETATION AND HYDROLOGICAL GRADIENTS IN NW AFRICA FROM LEAF WAX BIOMARKERS

When did NW Africa become dry? A precise answer to this important question has proven elusive. 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) at 0.25-0.5 Myr resolution 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‰) documenting the initial growth and establishment of C₄ Sahel grasslands reflecting seasonally arid conditions. 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 East and South Africa and South Asia, implying a different causes for regional vegetation shifts. We will also present low-resolution isotope data from equatorial ODP Site 959 (3°N) and compare these data with Site 659 to reconstruct the development of the modern vegetation and hydrological gradients in NW Africa over this time span.