Project OC4

The evolution of African hominids has been linked to the retreat of the tropical rainforest between 8 and 2 million years as reconstructed from paleovegetation data (e.g., Micheels et al. 2007). Several processes are cited as a cause for this biosphere shift. Isotopic studies (Cerling et al. 1997) have attributed a late Miocene transition from woodlands to grasslands to an atmospheric CO2 decrease. However, proxy data for CO2 do not reveal a larger change in the concentration of this greenhouse gas (Pagani et al. 2005). Other studies cite the uplift of the Tibetan Plateau and the consequent reduction in subtropical precipitation as a cause for the spreading of the grasslands (e.g., Fluteau et al. 1999). However, the timing of this orogeny is not well constrained, estimates range between 10-8 Ma and >15 Ma (see the recent review by Gupta et al. 2004). Further studies discuss an Indian Ocean sea surface temperature cooling in cause of the narrowing of the Indonesian Gateway (Cane and Molnar 2001) or topographic changes related to east African rift processes (Sepulchre et al. 2006) as first order forcings on African climate. Finally, the aridification occurs parallel with the onset of the modern thermohaline circulation between 8 and 6 Ma (e.g., Bickert et al. 2004).
Within OC4, we aim to reconstruct effects of changes in ocean circulation and climate on the Neogene development of the tropical rainbelt of the African continent. We intend to analyze sea surface temperatures, input of terrigenous organic carbon, changes in the vegetation cover and precipitation over land by evaluating marine and terrestrial organic proxies in sediment cores from locations off Northwest Africa and in the equatorial Indian Ocean. Estimates will be derived from proxy measurements using lipid biomarkers and microfossils. The analysis of the proxy records will be accompanied by global climate modeling in order to obtain insight into the physical mechanisms that drive changes in the intensity and structure of the Late Neogene tropical rainbelt. Sensitivity studies with the comprehensive Community Climate System Model, version 3 (CCSM3), will be performed. The model comprises interactively coupled components for atmosphere, ocean, land surface, and sea ice dynamics. The land surface component includes a dynamic vegetation module. The sensitivity studies will systematically explore the role of orography (e.g., uplift of the Tibetan plateau and eastern Africa), polar ice-sheets, atmospheric pCO2, final retreat of the Paratethys, and ocean circulation changes (e.g., through the closure of the Indonesian gateway) in shaping the tropical rainbelt and the Mio-Pliocene evolution of the African vegetation cover.