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Response of the East African Climate to Orbital Forcing During the Last Interglacial (130–117 Ka) and the Early Last Glacial (117–60 Ka)

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Response of the East African Climate to Orbital Forcing During the Last Interglacial (130–117 Ka) and the Early Last Glacial (117–60 Ka) Response of the East African climate to orbital forcing during the last interglacial (130±117 ka) and the early last glacial (117±60 ka) Martin H. Trauth* Institut fuÈr Geowissenschaften, UniversitaÈt Potsdam, Postfach 601553, Potsdam, Germany Alan Deino Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, USA Manfred R. Strecker Institut fuÈr Geowissenschaften, UniversitaÈt Potsdam, Postfach 601553, Potsdam, Germany ABSTRACT Variations in the temporal and spatial distribution of solar radiation caused by changes in Earth's orbit provide a partial explanation for observed long-term ¯uctuations in Af- rican lake levels. The understanding of causal links between insolation changes and lake- level ¯uctuations is essential for the design of models predicting future changes in the hydrological budgets and water supply in Africa. Here we present a record of climate change in East Africa between 175 and 60 ka. This time span includes the last interglacial (the Eemian, 130±117 ka), which may provide the closest analogue to the present inter- glacial. Assessments of the nature and timing of East African climate changes are based on lake-level ¯uctuations of Lake Naivasha (Kenya) inferred from sediment characteris- tics, diatom assemblages, and 40Ar/39Ar dating. Our results show dramatic alternation between deep, freshwater and shallow, highly alkaline lake conditions. The Lake Naivasha record demonstrates that periods of increased humidity in East Africa mainly follow pre- cessional insolation forcing in spring, causing more intense April±May rains every 23 k.y. Keywords: East Africa, lake sediments, Milankovitch, climate. INTRODUCTION forcing (Street and Grove, 1979; Gasse et al., vide a detailed chronology of lake-level Present African climate is mainly controlled 1989; Bonneville et al., 1990). A series of changes and East African climate between 175 by the strength of the African-Asian mon- abrupt events of increased precipitation and and 60 ka (Fig. 1) (Trauth and Strecker, 1996). soonal circulation and the position and sea- temperature may re¯ect complex interactions Together with the two most relevant African sonal migration of the Intertropical Conver- between orbital forcing, atmosphere, ocean, records from Lake Abhe in northern Ethiopia gence Zone (Nicholson, 1996). Whereas most and land-surface conditions (Gasse, 2000). (ca. 75 ka to present; Gasse, 1977) and the of the northern and southern part of the con- Marine records (e.g., Rossignol-Strick, 1983; Pretoria Saltpan in South Africa (ca. 200 ka tinent is characterized by monsoonal climates Clemens and Prell, 1990; deMenocal et al., to present; Partridge et al., 1997), the Naiva- with summer rains and winter droughts, the 1993; Zahn and Pederson, 1991) as well as sha lake history for the ®rst time provides pa- main source of rainfall in equatorial East Af- continental records (Gasse, 1977; Partidge et leoclimate data in East Africa extending this rica is the Intertropical Convergence Zone al., 1997) suggest antiphase changes in sum- far back in time. In addition, this record pro- (McGregor and Nieuwolt, 1998). The zone of mer insolation and enhanced rainfall in the maximum rainfall follows the latitudinal po- northern and southern monsoonal belts of Af- sition of the overhead sun with a time lag of rica as predicted by orbital precession geom- ;4±6 weeks resulting in two rainy seasons, etry (Berger, 1978). Detailed and well-dated the long rains around April±May and the short records from equatorial Africa documenting rains around October±November. pre±23 ka ¯uctuations of the intertropical con- During the late Pleistocene, African climat- vergence and convective rainfall on the equa- ic changes on time scales of thousands of tor are rare. These records suggest a period of years apparently were paced by periodic (23± 19 k.y.) variations in insolation resulting from high lake levels and increased humidity ca. Earth's orbital precession (Rossignol-Strick, 135 ka, well before the onset of the monsoon 1983; Kutzbach and Street-Perrott, 1985; system (Butzer et al., 1969; Sturchio et al., Clemens et al., 1991). Low lake levels in 1993; deMenocal et al., 1993; Zahn and Ped- northern and eastern Africa during the last gla- erson, 1991). cial maximum (23±18 ka) that followed high- New high-resolution paleohydrological and radiometric age data from lake deposits and stands ca. 15 ka are consistent with orbital Figure 1. Map of Kenya Rift showing loca- pyroclastic intercalations in the closed Nai- tions of Lake Turkana, paleo±Lake Suguta, *E-mail: [email protected]. vasha basin (central Kenya Rift Valley) pro- Lake Naivasha, and Lake Magadi. q 2001 Geological Society of America. For permission to copy, contact Copyright Clearance Center at www.copyright.com or (978) 750-8400. Geology; June 2001; v. 29; no. 6; p. 499±502; 3 ®gures. 499 Bulk mineralogy was obtained from X-ray dif- shallow freshwater lake. The diatomite bed is fraction using standard techniques. Anortho- overlain by several beds of unaltered pyro- clase and sanidine phenocryst concentrates clastic material, 107 6 7 ka, re¯ecting sus- from several tuff beds (ignimbrites, air-fall tained freshwater conditions. However, in the tuffs, and reworked tuffs) and one lava ¯ow upper part of this unit, perlitic cracks in glass were dated by the laser total-fusion (Deino shards, abundant phytoliths, and erosional un- and Potts, 1990; Deino et al., 1998) and laser conformities herald a major lake regression 40 39 (CO2) incremental-heating Ar/ Ar dating and a return to more alkaline conditions. Sub- methods (Sharp and Deino, 1996). Silicic py- aerial conditions appear to have been attained roclastic deposits in the pro®le allow an eval- at least along the southern periphery of the uation of lake alkalinity by means of the state basin before an ignimbrite dated as 106 6 4 of alteration of volcanic glass and the presence ka covered this unit. Waterlaid and partly re- of authigenic silicates (Utada, 1966; Surdam worked pyroclastic deposits containing vol- and Sheppard, 1978). Authigenic silicates canic glass shards with perlitic cracks and such as zeolites and potassium feldspar are montmorillonite rims indicate deposition in a common in ephemeral water bodies in arid or freshwater lake with a pH between 8 and 9, semiarid regions, where high pH conditions equated with lake-level highstand III. The age result from concentration of sodium carbon- of this highstand is de®ned by three interca- ate-bicarbonate due to evapotranspiration lated and relatively unaltered tuffs dated as 92 (Surdam and Sheppard, 1978). Vertical varia- 6 3, 90 6 2, and 88 6 4 ka. A subsequent tions of these authigenic mineral phases in the lake-level lowstand and short periods of inter- Naivasha sediment sequence thus de®ne lake- vening subaerial conditions are indicated by level ¯uctuations and alkalinity changes mud cracks and impact marks of air-fall pum- through time. Freshwater phases are charac- ice lapilli in overlying strata, 91 6 5 ka. To- terized by diatomite, unaltered volcanic glass, ward the top, three diatomite beds, to 16 cm and absence of authigenic silicates. In con- thick and intercalated by 80 6 4 ka pyroclas- trast, silicic glass with perlitic cracks, glass tic material, document highstand II. The dia- shards with montmorillonite rims, occasional tom assemblages are characterized by Fragi- chabazite, and phillipsite represent the transi- laria construens, F. brevistriata, and F. tion to alkaline conditions with a pH of ;9. pinnata, re¯ecting shallow freshwater condi- Even higher alkalinity resulted in the forma- tions, but with a possible connection to a tion of clinoptilolite, and the most alkaline deeper lake in the north. However, the occur- pore waters led to the precipitation of rence of Thalassiosira faurii, Mastogloia smi- analcime. thi, M. elliptica, and Anomoenoneis sphaero- Figure 2. Sediment pro®le of Ol Njorowa phora contrasts with the older diatomites, Gorge showing major lithologic units and RESULTS suggesting slightly alkaline conditions with a inferred lake-level ¯uctuations based on pH just above 8. The deposits of highstand II diagenetic mineral phases and diatomites. Five lake-level highstands and intermittent lowstands can be distinguished in the pro®le, are overlain by several waterlaid tuff layers, above silicic lava dated as 320 6 20 ka and dated as 73 6 3 and 72.8 6 1.8 ka, that con- vides data for a longitudinal transect of late ca. 400 ka deposits of Longonot volcano tain various amounts of zeolites indicating Pleistocene climate-proxy records, now per- (Scott, 1980; Clarke et al., 1990), which higher alkalinity and lower lake levels. A re- mitting evaluation of the timing of tropical cli- formed the southern boundary of the Naivasha turn to prevailing freshwater conditions is mate response to orbitally induced insolation basin. The ®rst manifestation of a larger lake documented by a succession of waterlaid py- forcing across the equator. (highstand V) in this basin is indicated by 145 roclastic deposits containing unaltered glass 6 2 ka ¯uvial deposits that grade into a 3.3- shards, indicating a pH , 9. These units are METHODS m-thick diatomite bed. A tuff within this di- interpreted to correspond to highstand I. Over- Situated at an elevation of 1890 m, Lake atomite yields an age of 140 6 3 ka. Predom- lying gray air-fall tuffs indicate subsequent Naivasha is the highest freshwater lake of the inant diatom taxa as Aulacoseira granulata subaerial conditions at 59 6 4 ka. An increas-
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