Plio Pleistocene Variations in East African Moisture Availability

Figure 13.1 illustrates that late Cenozoic tectonic activity in the EARS led to the production of isolated basins within

Cenozoic Tectonic Events

Fig. 13.1 Compilation of tectonic features and prominent lake periods for the eastern branch of the East African Rift System. Tectonic features and events complied from Baker et al. (1988), Strecker et al. (1990), Ebinger et al. (2000), Williams et al. (1983) and Foster et al. (l997). Paleoenvironmental and radiometric age data for the Olduvai Basin from Walter et al. (1991) and Ashley and Hay (2002); for the Magadi-Natron and Olorgesailie Basins from Potts (1998,1999), and Behrensmeyer et al. (2002). Natron has one persistent lacustrine interval (a Member of the Monik Formation, called the Moinik Clays) dated to 1.1-1.0 Ma (Deino, A., pers. comm., 2008). Paleoenvironmental and radiometric age data for the Gicheru Basin from Baker et al. (1988), Strecker (1991), Boven (1992) and this work; for the Naivasha Basin from Strecker et al. (1990) and Trauth et al. (2003, 2005); for the Nakuru-Elmenteita Basin from Evernden and Curtis (1965), Strecker (1991), Boven (1992) and Trauth et al. (2005); for the Baringo-Bogoria Basin from Owen et al. (2002) and Deino et al.(2006); for the Suguta Basin from Butzer et al.(1969), HillarieMarcel et al. (1986) and Sturchio et al. (1993); for the Omo-Turkana Basin from McDougall and Watkins (1988) and Brown and Feibel (1991); for the Ethiopian Rift from Williams et al. (1979), Gasse (1990) and WoldeGabriel et al. (2000); for the Afar Basin from Gasse (1990).

which lakes could form. Southward propagation of rifting and magmatic activity resulted in formation of lake basins first in the northern parts of the EARS. For example, the flu-violacustrine history of the Afar, Omo-Turkana and Baringo-Bogoria Basins in the north began in the Middle and Late Miocene, whereas the oldest lacustrine sequences in the central and southern segments of the rift in Kenya and Tanzania occur in the Early Pliocene (Tiercelin and Lezzar, 2002). In general, palaeo-lakes first appear in the EARS earlier in the north than in the south, due to the progressive formation of separate basins. If tectonics were the sole control over lake formation, then either a North to South or Northwest to Southeast temporal trend would be expected. However, what is observed is the appearance of large, deep lakes synchronously across large geographical areas at specific points in time (Trauth et al., 2005, 2007), suggesting that regional climatic control is operative.

Carbon isotope records from both soil carbonates (Levin et al., 2004; Wynn, 2004; Segalen et al., 2007) and biomark-ers (n-alkanes) extracted from deep-sea sediments (Feakins et al., 2005) provide clear evidence a progressive vegetation shift from C3 (-trees and shrubs) to C4 (-tropical grasses) plants during the Plio-Pleistocene. This shift has been ascribed to increased aridity that arose from the progressive rifting of East Africa (deMenocal, 2004; Sepulchre et al., 2006). Superimposed on this regime of subdued moisture availability, three periods characterized by the occurrence of large and deep lakes have been broadly identified in East Africa at 2.7-2.5, 1.9-1.7 and 1.1-0.9 Ma (Trauth et al., 2005, 2007), indicating consistency in the moisture history of the Kenyan and Ethiopian Rifts. Although preservation of East African lake records prior to 2.7 Ma is patchy, there is limited evidence for lake phases at -3.20-2.95, -3.4-3.3, 4.0-3.9, and -4.7-4.3 Ma (Fig. 13.1). The lake phases correspond to drops in the East Mediterranean marine dust abundance (Larrasoana et al., 2003), which are thought to reflect the aridity of the eastern Algerian, Libyan, and western Egyptian lowlands located north of the central Saharan watershed (Fig. 13.2). The lake phases also correspond to an increased occurrence of sapropels in Mediterranean Sea, which are thought to be caused by increased Nile River discharge (Lourens et al., 2004). The correspondence of the Mediterranean marine records with lake records of East Africa suggest a consistent moisture record for a region encompassing much of central and northern Africa over the last 3-5 million years.

In contrast, these East African wet phases correlate with significant intermediate-term increases in the dust records from ocean sediment cores adjacent to West Africa and Arabia (deMenocal, 1995, 2004). While, at first, this seems contradictory, examination of these data in chronologic detail demonstrates that both the lake and dust records are responding to precessional forcing, and that they are in-phase.

Fig. 13.2 Comparison of eccentricity variations (Berger and Loutre, 1991) with high latitude climate transitions (St John and Krissek, 2002; Cowan, 2001) and Mediterranean dust flux (Larrasoana et al., 2003). Soil carbonate carbon isotopes: (yellow dots = Levin et al., 2004; red dots = Wynn et al., 2004). Data for East African lake occurrences from Trauth et al. (2005, 2007). Hominin species appearances and durations from Reed (1997), Dunsworth and Walker (2002), McHenry (2002), White (2002) and White et al. (2006).

Fig. 13.2 Comparison of eccentricity variations (Berger and Loutre, 1991) with high latitude climate transitions (St John and Krissek, 2002; Cowan, 2001) and Mediterranean dust flux (Larrasoana et al., 2003). Soil carbonate carbon isotopes: (yellow dots = Levin et al., 2004; red dots = Wynn et al., 2004). Data for East African lake occurrences from Trauth et al. (2005, 2007). Hominin species appearances and durations from Reed (1997), Dunsworth and Walker (2002), McHenry (2002), White (2002) and White et al. (2006).

Deino et al., (2006) and Kingston et al., (2007) found that the major lacustrine episode of the Baringo Basin between 2.72.55 Ma actually consisted of five paleo-lake phases separated by a precessional cyclicity of 23 kyr. The lakes occurrences are in-phase with increased freshwater discharge, and therefore sapropel formation in the Mediterranean Sea (Lourens et al., 2004), and are out of phase with the dust records from the Indian Ocean (deMenocal, 1995, 2004). Hence, the lake records from East Africa and the Indian Ocean dust records document extreme climate variability with precession-forced wet and dry phases. Precessional forcing of vegetation change also occurred at this time in southwest Africa, independent of glacial-interglacial cycles (Denison et al., 2005). There is also emerging evidence for precessional forcing of the 1.9-1.7 Ma lake phase in the KBS

Member of the Koobi Fora Formation in the northeast Turkana Basin of Kenya (Lepre et al., 2007). During the same period, an oxygen isotope record from the Buffalo Cave flowstone (Makapansgat Valley, Limpopo Province, South Africa) shows clear evidence of precessionally-forced changes in rainfall in South Africa (Hopley et al., 2007).

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