Pliocene northeast African vegetation change

Plant leaf wax molecular fossils have yielded key insights into forest-grassland transitions in northeast Africa. The stable carbon isotopic composition (δ¹³C) of sedimentary leaf waxes (including n-alkanoic acids) records variations in the proportions of plants using the C₃ (Calvin-Benson) pathway including trees, most shrubs and cool-season grasses versus the C₄ (Hatch-Slack) pathway including warm-season grasses and sedges. Here we apply this technique to marine sediments from the Gulf of Aden to characterize northeast African environmental variability during the Pliocene. We sample DSDP Site 231 integrating 3kyr intervals to generate a high-resolution reconstruction spanning 3.7-5.3Ma. The δ¹³C compositions of downcore C₃₀ n-alkanoic acid analyses average -27.5‰ (σ=1.2, n=177), ranging between -30.2 to -24.3‰ with repeated oscillations in the proportions of C₄ biomass, presumably corresponding to the eccentricity and precessional modulation of insolation acting on monsoonal precipitation. Although δ¹³C values cannot be uniquely translated to C₄ biomass given the spread in the C₃ 'endmember', we find complimentary evidence for proportions of C₄ biomass from the isotopic spread of the n-alkanoic homologues (n-C28 to n-C32). Comparison to pollen data indicates that at times of low δ¹³C values, complete forest cover remains unlikely given the presence of grass pollen - taken together these records indicate the presence of C₃ grasses. By evaluating our longer record from this site, as well as the terrestrial soil carbonate record, we find the Pliocene has low C₄ proportions in comparison to higher proportions in Pleistocene and even late Miocene times. Intriguingly this drop in C₄ plant biomass may correspond to a period of elevated atmospheric pCO₂. Our ongoing reconstructions are of interest both for constraining environments of hominin evolution and tropical climate variability prior to the onset of Northern Hemisphere Glaciation, in particular during this Pliocene time period of interest as a 'modest greenhouse' world.