R EPORTS 17. T. C. Onstott, M. L. Miller, R. C. Ewing, G. W. Arnold, Research Council (NERC) Ocean Margins Project re- Materials and Methods D. S. Walsh, Geochim. Cosmochim. Acta 59, 1821 (1995). search grant number 3220-GL021-GRA0782. We thank Figs. S1 to S3 18. M. A. Kendrick, R. Burgess, R. A. D. Pattrick, G. Turner, A. Craven for access to the TEM, G. Sherwood for com- Tables S1 to S5 Econ. Geol. 97, 435 (2002). ments, and J. Still for technical assistance. References 19.N.J.L.Bailey,P.Walko,M.J.Sauer,inPetroleum Geology of Northwest Europe, J. Brooks, K. Glennie, Eds. Supporting Online Material (Graham and Trotman, London, 1987), p. 711. www.sciencemag.org/cgi/content/full/309/5743/2048/ 13 June 2005; accepted 18 August 2005 20. This work is supported by the Natural Engineering DC1 10.1126/science.1116034 tectonic and magmatic processes have to be Late Cenozoic Moisture History present to accommodate the lakes; second, the climate has to sustain a positive precipitation/ evaporation balance for a substantial period of of East Africa time. Here we elucidate East African climate Martin H. Trauth,1 Mark A. Maslin,2 Alan Deino,3 changes in the Late Cenozoic, using detailed Manfred R. Strecker1 sedimentary records from 10 basins of the east- ern branch of the EARS. The large geographic Lake sediments in 10 Ethiopian, Kenyan, and Tanzanian rift basins suggest dispersion of these basins along a north-south that there were three humid periods at 2.7 to 2.5 million years ago (Ma), 1.9 transect helps to separate the effects of volcanic- to 1.7 Ma, and 1.1 to 0.9 Ma, superimposed on the longer-term aridification of tectonic and climatic influences on rift sedimen- East Africa. These humid periods correlate with increased aridity in northwest tation. Synchronous changes in the hydrological and northeast Africa and with substantial global climate transitions. These balance inferred from sediment characteristics episodes could have had important impacts on the speciation and dispersal of and silica algae (diatom) assemblages that mammals and hominins, because a number of key events, such as the origin of contrast with the volcanic-tectonic history are the genus Homo and the evolution of the species Homo erectus, took place in attributed to climate change. These relations this region during that time. suggest possible links between climate change and mammalian and hominin evolution during Recent investigations of both terrestrial and Although much smaller than the lakes in the theLateCenozoic. marine paleoclimate archives have led to a western branch and often subaerially exposed, The EARS has a great diversity of sedi- concerted debate regarding the nature of Late these basins host a rich sedimentary record, mentary environments. Structurally and mag- Cenozoic environmental changes in East Africa with intercalated volcaniclastic deposits that matically controlled processes have created and their influence on mammalian and hominin permit high-precision 40Ar/39Ar age calibra- complex relief and drainage conditions that are evolution (1–3). Because terrestrial records of tion of lake-level highstands (5, 6) (Fig. 2). highly variable over time, beginning at about East African environmental change are typically For rift lakes to form, two basic conditions 45 million years ago (Ma) and continuing into rare, geographically dispersed, and incomplete, have to be satisfied. First, basins defined by the present (7–9). Volcanism and faulting Indian and Atlantic Ocean sediment records have been used to reconstruct climatic changes in the region (3). However, because of the unique tec- AFAR Central Afar tonic and magmatic evolution of the East African BASIN Rift System (EARS) and resulting changes in topography and drainage patterns, marine sed- iment records may not reflect contemporaneous environmental changes in East Africa. It is, there- Ethiopian Rift fore, important to reach a better understanding of the processes changing the habitat of mammals ETHIOPIAN RIFTRIF and hominins before suggesting possible links between climate and faunal changes. WESTERN The Rift Valley lakes are excellent record- 6 BRANCH Omo Areas > 1000 m ers of past climate changes in East Africa (4, 5). OF THE EARS above sea level The western branch of the EARS contains Lake Turkana Normal Fault several large and deep lakes that formed dur- 4 ing the past 10 million years, and a series of EASTERN Turkana small lakes that are presently partly alkaline BRANCH has developed in the eastern branch since the 2°N Suguta Valley OF THE EARS Pliocene (5). The lake history in the Ethiopian, Lake Mt. Elgon KENYA RIFT Kenyan, and Tanzanian rifts is complex and Albert Baringo-Bogoria closely tied to the volcanic and tectonic evo- Nakuru- Equator lution of the area, leading to the formation of Lake Elmenteita Mt. Kenya internally drained basins with fluctuating river Edward Turasha Lake Naivasha TANZANIAN RIFT networks and catchment sizes (5)(Fig.1). Victoria Gicheru Magadi- Indian Ocean Lake Natron Olorgesailie Kivu 1Institut fu¨r Geowissenschaften, Universita¨t Potsdam, Post Office Box 601553, D-14415 Potsdam, Germa- Olduvai Mt. Kilimanjaro ny. 2Environmental Change Research Center, Depart- 30°E 32 34 38 40 42 44 ment of Geography, University College London, UK. 3Berkeley Geochronology Center, 2455 Ridge Road, Fig. 1. Map of East Africa, showing topography, rift faults, and sites of lake sediment sequences Berkeley, CA 94709, USA. discussed in the paper. www.sciencemag.org SCIENCE VOL 309 23 SEPTEMBER 2005 2051 R EPORTS were diachronous and progressed from north to be affected by normal faulting, leading to five major diatomite beds occurs at precession- to south (7–9). In the Ethiopian Rift, volcan- further structural segmentation (4). al intervals, as calibrated by 40Ar/39Ar ages on ism started between 45 and 33 Ma; in north- In contrast, sedimentation in the Tanzanian intercalated ash layers (13). Contemporaneous ern Kenya, it started at about 33 Ma and sector of the rift began within isolated basins at lake deposits from adjacent basins have not continued to about 25 Ma; and the magmatic È5Ma(11). Major normal faulting in the yet been found (14–16). The possibility cannot activity of the central and southern segments Magadi-Natron and Olduvai basins occurred be excluded, however, that sediments of that of the rifts in Kenya and Tanzania started at 1.2 Ma and produced the present-day rift age are buried and downfaulted in the central between 15 and 8 Ma (9). escarpments (11). Late Quaternary structural Kenya Rift. In this area, no evidence for lakes Major faulting in Ethiopia from 20 to 14 Ma en echelon segmentation during WNW-ESE– exists between a 4.7-to-4.3-million-year-old and was followed by the evolution of the Turkana oriented extension created numerous sub-basins È90-m-thick diatomite sequence at Turasha on Rift zone in northern Kenya (9), whereas east- in the individual rift sectors that commonly the Kinangop Plateau and the È1-million-year- dipping faults developed between 12 and 6 Ma hosted smaller lakes (4). The southward prop- old lake deposits at Kariandusi. In contrast, in the Kenya Rift south of 3-N(4, 9). The early agation of rifting, including the formation of diatomitesupto30mthickontheeastern halfgrabens of the Kenya Rift were subsequently faults and magmatic activity, led to the forma- shoulder of the Ethiopian Rift and the Afar faulted antithetically between about 5.5 and 3.7 tion of lake basins in the northern part of the Basin record an important lacustrine period at Ma, generating a full-graben morphology (4). rift. The fluvio-lacustrine deposition within the Gadeb between 2.7 and 2.4 Ma (17). Before the full-graben stage, the large Aberdare Afar, Omo-Turkana, and Baringo-Bogoria After 2 Ma, the sedimentary record be- volcanic complex, with elevation in excess of Basins began in the Middle and Upper Miocene, comes more complete in the eastern branch of 4000 m, developed and is now an important whereas the oldest lacustrine sequences in the the EARS, particularly in the Kenya Rift, and topographic barrier on the eastern shoulder of central and southern segments of the rift in provides strong evidence for several deep lakes the central Kenya Rift (10). By 2.6 Ma, the Kenya and Tanzania are Early Pliocene (5). In between 1.9 and 1.7 Ma. The Plio-Pleistocene central sector was further segmented by west- the following, we provide a compilation of im- Konso-Gardula sedimentary sequence suggests dipping faults, creating the 30-km-wide intrarift portant lake periods in the Ethiopian, Kenyan, that large lakes existed in the southern sector of Kinangop Plateau and the tectonically active and Tanzanian Rifts since the Pliocene (12). the Ethiopian Rift at least temporarily between 40-km-wide inner rift (4). The inner rift was Evidence has been found for deep lakes 1.9 and 1.7 Ma (18). Contemporaneously, sev- subsequently covered by trachytic, basaltic, between 2.7 and 2.5 Ma in the western eral large lakes are also documented in the and rhyolitic lavas and tuffs and continues Baringo-Bogoria Basin, where a sequence of central Afar Basin. Lacustrine deposits are Fig. 2. Compilation of lake and riverine records based on sediment including the Gicheru, Naivasha and Nakuru-Elmenteita Basins, from characteristics and diatom assemblages in the Ethiopian, Kenyan, and (2, 4, 32) and this work; for the Baringo-Bogoria Basin from (13, 33); Tanzanian Rifts. Global climate transitions are from (29–31). Paleo- for the Suguta Basin from (34–36); for the Omo-Turkana Basin from environmental and radiometric age data are given in millions of years (19, 22); for the Ethiopian Rift from (14, 17, 18); and for the Afar Basin for the Olduvai Basin from (20); for the Magadi-Natron and from (14).