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Environmental change in eastern Africa

Following early break-up of pan-Equatorial African rainforests during the middle Miocene, regional palaeoclimatic records from the East Africa late Miocene are poorly known due to a relative paucity of sedimentary records in East Africa during the crucial interval between 14 and 4 Ma. However, there is increasing evidence for major regional climatic change during this interval and now becoming more evident from strata of Lothagam and the Baringo Basin in Kenya. Following this early but currently poorly resolved palaeoclimatic record of the Miocene, the dynamic development of savanna ecosystems in East Africa is best known from the widely deposited and continuous Plio-Pleistocene sediments of the eastern branch of the East African Rift.

Through the last 4 m.y., East African climate evolved from moist and warm to arid and perhaps slightly cooler. This trend in the terrestrial record is best documented in the Turkana Basin, where Plio-Pleistocene palaeosols provide a high-resolution record of aridification during the past 4.3 Ma. This aridification trend is marked by several punctuations at 3.58 - 3.35 Ma, 2.52 - 2.32 Ma, and 1.81 - 1.58 Ma. The most marked shift in climate appears to have occurred between 2.4 - 2.5 Ma. By 2.34 Ma, riverine and forest taxa had diminished, and xeric species dominated. Then, by just after 2.32 Ma, forest taxa were gone, for the fossil record contains only "dry savannas/open savanna woodland" and "arid/semiarid steppe" taxa. Palaeontological and palynological data can neither resolve short-term climate changes at most localities nor define unambiguous spatial differences in the pattern of increasing aridification. The aridification events identified at ca. 3.4 Ma, ca. 2.4 Ma and ca. 1.7 Ma correspond to intervals of increased faunal turnover (speciation and extinction events), but more importantly, increased diversity of bovid taxa, known to be the hallmark indicators of arid, open environments. Although the Turkana Basin hominid record lacks the temporal resolution and diversity of the bovid record, the aridification intervals identified are marked by similar increases in the diversity and turnover of hominids during these intervals of aridification, suggesting that homininds were also forced to adapt to increasingly variable habitats.

Early global climate model studies noted the temporal coincidence between landmarks in Earth's most recent global cooling trend and East African aridity, leading to the hypothesis of a forcing relationship between Pliocene to Pleistocene aridification in Africa and cold North Atlantic sea surface temperatures, which are in turn forced by Northern Hemisphere glaciation. However, more recent modelling studies have suggested that precipitation patterns in East Africa are more sensitive to Indian Ocean sea surface temperatures, which cooled during the Neogene by progressive tectonic closing of the Indonesian Seaway, leaving East Africa increasingly arid, independent of the North Atlantic trend. However, in order to validate this or other climatic models with palaeoclimatic data from East Africa much remains to be demonstrated as to the rate and geographical patterns of East African climate change - a primary purpose of the terrestrial palaeoclimatic research proposed here. The Albertine and Turkana basins provide excellent opportunities to examine the regional response to global climate and local rift-flank uplift at the evolving boundary between the east and west African climatic systems.