African climate changes during C4 plant evolution and expansion

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In this workpackage (TP 2) we propose to establish detailed records of vegetation changes on the African continent by investigating terrigenous plant lipid biomarkers and their molecular, carbon and hydrogen, isotopic signatures. These analyses will be conducted in combination with pollen associations on samples from marine sediment cores in the eastern equatorial and southern Atlantic. In the first phase (year 1-3) we focus on the middle to Late Miocene, covering the large-scale expansion of C4 grasslands. In the second phase (year 4-6) we will investigate the interval from the Late Eocene to the Early Oligocene, when C4 plants evolved. Both evolution and expansion of C4 plants have yet mainly been linked to decreasing atmospheric CO2 levels according to the CO2-treshold model. While for the Eocene/Oligocene interval falling C02 levels were reconstructed a mismatch exists between estimates of Miocene pCO2 levels and the detected expansion of C4 grasslands. This contrast, in turn, points to the significance of other environmental factors for large-scale vegetation changes, such as amount and seasonality of precipitation. In order to resolve this discrepancy, we will apply novel compound-specific hydrogen isotope analyses on terrestrial lipid biomarkers, which enable accurate assessment of continental hydrological conditions. A detailed comparison of lipid biomarker with pollen records will allow the distinction between C4 vegetations of different composition. In addition, we will establish organic-geochemical records of past sea-surface temperature (SST) changes in the south-eastern Atlantic to unravel climatic linkages between the continental environmental changes and oceanographic variations. In this respect, the results obtained by this TP will directly be comparable to the marine proxy data generated in TP 4. Within the entire Research Unit, this TP, in conjunction with TP 01, provides the data base of terrestrial vegetation and climate parameters to quantitatively validate the obtained modelling results on the global carbon and water cycle.