Pliocene palaeoclimate off SE Africa: Insights from IODP Expedition 361

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It has been suggested that East Africa has undergone strong aridification between 3-5 million years ago as a consequence of changes in the source water distribution through the Indonesian seaway into the Indian Ocean [deMenocal, 1995; Cane and Molnar, 2001]. Results from an ocean circulation model showed that the northward displacement of New Guinea, about 5 Myr ago, may have switched the source of flow through Indonesia from warm South Pacifc to relatively cold North Pacifc waters. In the modern climate there is a strong link between east African rainfall and Indian Ocean SST, with warmer temperatures associated with more rain [Goddard and Graham, 1999]. Assuming that the modern ocean circulation system worked in a similar way in the Pliocene, the consequence of a tectonically driven northward move of New Guinea would have caused a reduction in sea surface temperatures in the Indian Ocean ultimately leading to reduced rainfall over eastern Africa. Why is it important to study climate changes in East/southeast Africa over that time period? We all come from Africa. We know that major steps in the evolution of African hominids and the emergence of the genus Homo in East Africa occurred in the Pliocene [Bobe and Behrensmeyer, 2004]. These developments seem to fall into times of major global and regional climatic changes especially with shifts to more arid, open environmental conditions in Africa near 2.8 Ma [deMenocal, 1995]. The change from humid to more arid conditions in East Africa has been linked mainly to remote forcing by cold North Atlantic sea-surface temperatures associated with the onset of Northern Hemisphere glacial cycles[deMenocal, 1995]. Here, however, we want to test if instead a cooling in the southwest Indian Ocean temperature due to a change of water through the archipelago of Indonesia drove changes in East and southeast Africa rainfall variability.