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Late Cenozoic Moisture History of East Africa

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Science  23 Sep 2005:
Vol. 309, Issue 5743, pp. 2051-2053
DOI: 10.1126/science.1112964

Abstract

Lake sediments in 10 Ethiopian, Kenyan, and Tanzanian rift basins suggest that there were three humid periods at 2.7 to 2.5 million years ago (Ma), 1.9 to 1.7 Ma, and 1.1 to 0.9 Ma, superimposed on the longer-term aridification of East Africa. These humid periods correlate with increased aridity in northwest and northeast Africa and with substantial global climate transitions. These episodes could have had important impacts on the speciation and dispersal of mammals and hominins, because a number of key events, such as the origin of the genus Homo and the evolution of the species Homo erectus, took place in this region during that time.

Recent investigations of both terrestrial and marine paleoclimate archives have led to a concerted debate regarding the nature of Late Cenozoic environmental changes in East Africa and their influence on mammalian and hominin evolution (13). Because terrestrial records of East African environmental change are typically rare, geographically dispersed, and incomplete, Indian and Atlantic Ocean sediment records have been used to reconstruct climatic changes in the region (3). However, because of the unique tectonic and magmatic evolution of the East African Rift System (EARS) and resulting changes in topography and drainage patterns, marine sediment records may not reflect contemporaneous environmental changes in East Africa. It is, therefore, important to reach a better understanding of the processes changing the habitat of mammals and hominins before suggesting possible links between climate and faunal changes.

The Rift Valley lakes are excellent recorders of past climate changes in East Africa (4, 5). The western branch of the EARS contains several large and deep lakes that formed during the past 10 million years, and a series of small lakes that are presently partly alkaline has developed in the eastern branch since the Pliocene (5). The lake history in the Ethiopian, Kenyan, and Tanzanian rifts is complex and closely tied to the volcanic and tectonic evolution of the area, leading to the formation of internally drained basins with fluctuating river networks and catchment sizes (5) (Fig. 1). Although much smaller than the lakes in the western branch and often subaerially exposed, these basins host a rich sedimentary record, with intercalated volcaniclastic deposits that permit high-precision 40Ar/39Ar age calibration of lake-level highstands (5, 6) (Fig. 2).

Fig. 1.

Map of East Africa, showing topography, rift faults, and sites of lake sediment sequences discussed in the paper.

Fig. 2.

Compilation of lake and riverine records based on sediment characteristics and diatom assemblages in the Ethiopian, Kenyan, and Tanzanian Rifts. Global climate transitions are from (2931). Paleoenvironmental and radiometric age data are given in millions of years for the Olduvai Basin from (20); for the Magadi-Natron and Olorgesailie Basins from (15, 16, 21); for the Central Kenya Rift Basins, including the Gicheru, Naivasha and Nakuru-Elmenteita Basins, from (2, 4, 32) and this work; for the Baringo-Bogoria Basin from (13, 33); for the Suguta Basin from (3436); for the Omo-Turkana Basin from (19, 22); for the Ethiopian Rift from (14, 17, 18); and for the Afar Basin from (14). The sedimentary evidence for large lakes is discussed in more detail in (12).

For rift lakes to form, two basic conditions have to be satisfied. First, basins defined by tectonic and magmatic processes have to be present to accommodate the lakes; second, the climate has to sustain a positive precipitation/evaporation balance for a substantial period of time. Here we elucidate East African climate changes in the Late Cenozoic, using detailed sedimentary records from 10 basins of the eastern branch of the EARS. The large geographic dispersion of these basins along a north-south transect helps to separate the effects of volcanic-tectonic and climatic influences on rift sedimentation. Synchronous changes in the hydrological balance inferred from sediment characteristics and silica algae (diatom) assemblages that contrast with the volcanic-tectonic history are attributed to climate change. These relations suggest possible links between climate change and mammalian and hominin evolution during the Late Cenozoic.

The EARS has a great diversity of sedimentary environments. Structurally and magmatically controlled processes have created complex relief and drainage conditions that are highly variable over time, beginning at about 45 million years ago (Ma) and continuing into the present (79). Volcanism and faulting were diachronous and progressed from north to south (79). In the Ethiopian Rift, volcanism started between 45 and 33 Ma; in northern Kenya, it started at about 33 Ma and continued to about 25 Ma; and the magmatic activity of the central and southern segments of the rifts in Kenya and Tanzania started between 15 and 8 Ma (9).

Major faulting in Ethiopia from 20 to 14 Ma was followed by the evolution of the Turkana Rift zone in northern Kenya (9), whereas east-dipping faults developed between 12 and 6 Ma in the Kenya Rift south of 3°N(4, 9). The early halfgrabens of the Kenya Rift were subsequently faulted antithetically between about 5.5 and 3.7 Ma, generating a full-graben morphology (4). Before the full-graben stage, the large Aberdare volcanic complex, with elevation in excess of 4000 m, developed and is now an important topographic barrier on the eastern shoulder of the central Kenya Rift (10). By 2.6 Ma, the central sector was further segmented by west-dipping faults, creating the 30-km-wide intrarift Kinangop Plateau and the tectonically active 40-km-wide inner rift (4). The inner rift was subsequently covered by trachytic, basaltic, and rhyolitic lavas and tuffs and continues to be affected by normal faulting, leading to further structural segmentation (4).

In contrast, sedimentation in the Tanzanian sector of the rift began within isolated basins at ∼5 Ma (11). Major normal faulting in the Magadi-Natron and Olduvai basins occurred at 1.2 Ma and produced the present-day rift escarpments (11). Late Quaternary structural en echelon segmentation during WNW-ESE–oriented extension created numerous sub-basins in the individual rift sectors that commonly hosted smaller lakes (4). The southward propagation of rifting, including the formation of faults and magmatic activity, led to the formation of lake basins in the northern part of the rift. The fluvio-lacustrine deposition within the Afar, Omo-Turkana, and Baringo-Bogoria Basins began in the Middle and Upper Miocene, whereas the oldest lacustrine sequences in the central and southern segments of the rift in Kenya and Tanzania are Early Pliocene (5). In the following, we provide a compilation of important lake periods in the Ethiopian, Kenyan, and Tanzanian Rifts since the Pliocene (12).

Evidence has been found for deep lakes between 2.7 and 2.5 Ma in the western Baringo-Bogoria Basin, where a sequence of five major diatomite beds occurs at precessional intervals, as calibrated by 40Ar/39Ar ages on intercalated ash layers (13). Contemporaneous lake deposits from adjacent basins have not yet been found (1416). The possibility cannot be excluded, however, that sediments of that age are buried and downfaulted in the central Kenya Rift. In this area, no evidence for lakes exists between a 4.7-to-4.3-million-year-old and ∼90-m-thick diatomite sequence at Turasha on the Kinangop Plateau and the ∼1-million-year-old lake deposits at Kariandusi. In contrast, diatomites up to 30 m thick on the eastern shoulder of the Ethiopian Rift and the Afar Basin record an important lacustrine period at Gadeb between 2.7 and 2.4 Ma (17).

After 2 Ma, the sedimentary record becomes more complete in the eastern branch of the EARS, particularly in the Kenya Rift, and provides strong evidence for several deep lakes between 1.9 and 1.7 Ma. The Plio-Pleistocene Konso-Gardula sedimentary sequence suggests that large lakes existed in the southern sector of the Ethiopian Rift at least temporarily between 1.9 and 1.7 Ma (18). Contemporaneously, several large lakes are also documented in the central Afar Basin. Lacustrine deposits are exposed on the floors of the Dikhil and Abhé-Gobaad Basins or are interbedded with the latest basaltic lava flows of the present-day plateaus (14). Dominated by fluvial conditions for most of its history, the Omo-Turkana Basin provides strong evidence for a large lake fed from the north by the Omo River between 1.9 and 1.7 Ma (15, 16, 19). At Munyu wa Gicheru, an ∼30-m-thick sequence of diatomaceous sediments suggests that a major lake existed between 1.96 and 1.65 Ma in a trough on a platform along the eastern flank of the southern Kenya Rift. In the Tanzanian Rift, the Olduvai Gorge exposes a 2-million-year sedimentary record in an incised river valley draining eastward from the Serengeti Plains (20). The ∼100-m-thick sequence suggests that a major lake existed between 1.92 and 1.7 Ma (20).

Deep lakes also existed between 1.1 and 0.9 Ma in East Africa. The Olorgesailie Formation in the southern Kenya Rift records the formation of a lake shortly before 0.992 Ma, with subsequent alterations between lacustrine and subaerial environments through the next ∼500,000 years but no episodes of major erosion (15, 16, 21). The most important lake period occurs between 0.992 and 0.974 Ma, as documented by the deposition of a 31-m-thick main diatomite bed (15, 16, 21). An important lake period at about 1 Ma has also been identified in the Turkana Basin (22). Comparing the flora contained in lake sediments older than 0.8 million years with Late Pleistocene units (∼135,000 years before the present) exposed in the Naivasha and Elmenteita-Nakuru Basins indicating lakes 100 to 150 m deep (2), the Early and Mid-Pleistocene lakes were much deeper; that is, several hundreds of meters deep, like the modern lakes in the western branch of the EARS. This prominent lake period recorded in the Olorgesailie and Nakuru-Elmenteita Basins also correlates with a period of deep lakes in the Afar Basin between 1.1 and 0.9 Ma, registered by fresh water diatom species, some of which still live in large temperate lakes today (14).

These synchronous changes in the water balance inferred from fluvio-lacustrine deposits contrast with the predominantly southward propagating and diachronous volcanic-tectonic history of the EARS and can therefore be attributed to regional climate change. It is evident that East Africa experienced three major Late Cenozoic lake periods at 2.7 to 2.5 Ma, 1.9 to 1.7 Ma, and 1.1 to 0.9 Ma. With the exception of the Baringo lacustrine sequence at 2.7 to 2.5 Ma, we cannot conclude at present whether the deep lakes in the eastern rifts were characterized by relatively stable lacustrine conditions for a long period of time (∼100,000 years) or if these lakes fluctuated on shorter orbital or suborbital time scales, although preliminary evidence (13) supports the latter.

On time scales of more than 100,000 years, rift-related volcanic-tectonic processes shaped the landscape of East Africa and thus profoundly influenced local climates and surface hydrology through the development of relief features. The uplifts of the Kenyan and Ethiopian Plateaus, with attendant changes in topography and rain shadow effects, are believed to be the major driving force for increased variability of surface moisture throughout eastern and southern Africa. Soil carbonate stable-isotope studies provide clear evidence of long-term aridification of the continent (23) and perhaps the spread of the savannah mosaic in East Africa (24). However, regions with high relief have more complex climates and respond differently to changes in the dominant forcing factors as compared to other African regions, and may result in both decreased and increased water availability. This fact helps to explain the anticorrelation between the East African lake levels and dust records from ocean sediment cores adjacent to West Africa and Arabia (3). Obviously, important differences exist in the moisture history of equatorial East Africa, subtropical Africa, Arabia, and Southeast Asia (13, 2527). These differences can best be explained by regional responses to global climate change, combined with the influence of local variations in insolation (2, 4, 28).

The periods of deep lakes correlate with important global climatic changes. The period between 2.7 and 2.5 Ma corresponds to intensification of the Northern Hemisphere Glaciation (29), 1.9 to 1.7 Ma to an important intensification and shift in the east-west zonal atmospheric circulation referred to as the Walker circulation (30), and the interval from 1.1 to 0.9 Ma to the initiation of the Mid-Pleistocene Revolution: the shift from glacial/interglacial cycles every 41,000 years to every ∼100,000 years (31). If these lakes are ephermal features of the landscape forced by precession, that strongly supports the Variability Hypothesis of human evolution (16), because the environment inside the East African Rift Valley would have varied rapidly between sustained humid and arid periods, providing the stress required to initiate speciation.

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