T210 Sedimentary Records of Neogene and Quaternary Environmental Change from Eastern Africa

Thursday, 29 October 2020
1:30 PM - 5:30 PM
Meeting Rooms

EARLY MIOCENE EVOLUTION OPEN ECOSYSTEMS AND C4 VEGETATION IN EQUATORIAL EAST AFRICA

Paper number: T210-2
Program: Technical Sessions
Time: 1:45 PM – 2:00 PM

Authors

Abstract

While grasses are known from Africa since the latest Cretaceous, regional vegetation models suggest that equatorial Africa was predominantly forest, rather than grassland prior to the late Miocene. Pollen and stable isotope data suggest evidence for more open habitats and local occurrences of both C3 grasslands and C4 vegetation beginning in the middle Miocene ~15-13 Ma. By the late Miocene (~10 Ma), terrestrial stable isotopes from pedogenic carbonate and mammalian tooth enamel, as well as plant-wax isotopes from marine cores, show a major expansion of C4 grasslands across East Africa. Although these records provide some evidence for open-dry vegetation during the Cenozoic, the onset of open habitats with abundant C4 biomass is poorly constrained, and more detailed data sets are needed to assess local and regional vegetation patterns. With rich stratigraphic archives, the rift valleys of equatorial East Africa are an ideal setting to examine these questions. Here we present ecosystem and vegetation reconstructions from several early Miocene localities across Kenya and Uganda that span ~5.5 myr (~22-16.5 Ma), using paleosols, fossil leaves, phytoliths, and isotopic data from paleosol carbonates, bulk organics, and plant waxes. We use these data to assess spatial and temporal patterns of vegetation change, with a focus on the emergence of ecosystems containing C4 biomass. Quantitative and qualitative observations demonstrate significant spatial and temporal heterogeneity in vegetation through the early Miocene. Reconstructed biomes range from wooded grasslands with abundant C4 vegetation to closed canopy forests. Isotopic data and phytolith analyses independently confirm that C4 grasses were common at some of these localities. In particular, C4 biomass is most common at sites interpreted to be wooded grasslands, shrublands, and open woodlands. These data demonstrate that during the early Miocene, equatorial East Africa was a mixture of open to closed habitats, and that locally, open environments with C4 biomass were an important component of East African ecosystems >5 myr earlier than previously documented. Further, these results indicate that a variety of factors influenced C4 grassland evolution and their increasing dominance in African ecosystems through the Miocene.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-355448

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