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Journal of Human Evolution xx (2007) 1e11

Plio-Pleistocene facies environments from the KBS Member,Koobi Fora Formation: implications for climate controls

on the development of lake-margin hominin habitatsin the northeast Turkana Basin (northwest Kenya)

Christopher J. Lepre a,*, Rhonda L. Quinn a,c, Josephine C.A. Joordens b,Carl C. Swisher III c, Craig S. Feibel a,c

a Department of Anthropology, Rutgers The State University of New Jersey, New Brunswick, NJ 08901-1414, USAb Department of Paleoclimatology and Geomorphology, Vrije Universiteit Amsterdam, De Boelelaan 1085 1081 HV, Amsterdam, The Netherlands

c Department of Geological Sciences, Rutgers The State University of New Jersey, Piscataway, NJ 08854, USA

Received 4 November 2005; accepted 22 January 2007

Abstract

Climate change is hypothesized as a cause of major events of Plio-Pleistocene East African hominin evolution, but the vertically discontin-uous and laterally confined nature of the relevant geological records has led to difficulties with assessing probable links between the two. High-resolution sedimentary sequences from lacustrine settings can provide comprehensive data of environmental changes and detailed correlationswith well-established orbital and marine records of climate. Hominin-bearing deposits from Koobi Fora Ridge localities in the northeast TurkanaBasin of Kenya are an archive of Plio-Pleistocene lake-margin sedimentation though significant developmental junctures of northern Africanclimates, East African environments, and hominin evolution. This study examines alluvial channel and floodplain, nearshore lacustrine, and off-shore lacustrine facies environments for the approximately 136-m-thick KBS Member (Koobi Fora Formation) exposed at the Koobi Fora Ridge.Aspects of the facies environments record information on the changing hydrosedimentary dynamics of the lake margin and give insights intopotential climatic controls. Seasonal/yearly climate changes are represented by the varve-like laminations in offshore mudstones and the slick-ensides, dish-shaped fractures, and other paleosol features overprinted on floodplain strata. Vertical shifts between facies environments, however,are interpreted to indicate lake-level fluctuations deriving from longer-term, dry-wet periods in monsoonal rainfall. Recurrence periods for theinferred lake-level changes range from about 10,000 to 50,000 years, and several are consistent with the average estimated timescales of orbitalprecession (w20,000 years) and obliquity (w40,000 years). KBS Member facies environments from the Koobi Fora Ridge document thedevelopment of lake-margin hominin habitats in the northeast Turkana Basin. Environmental changes in these habitats may be a result ofmonsoonal rainfall variations that derive from orbital insolation and/or glacial forcing.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Depositional environments; Lacustrine sedimentation; Hominin evolution; East Africa

Introduction

Analyses of benthic d18O records from marine cores placethe onset of Northern Hemisphere Glaciation at w2.7 Ma(Shackleton et al., 1984; Lisiecki and Raymo, 2005). The

* Corresponding author.

E-mail address: clepre@rci.rutgers.edu (C.J. Lepre).

0047-2484/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jhevol.2007.01.015

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consequences of increased climate variability and aridityacross northern low-latitude Africa are evident in marinedust-flux (deMenocal et al., 1993; deMenocal, 1995) and pal-ynological (Dupont and Leroy, 1995) sequences. Theseincreases are roughly contemporary with significant periods ofgrassland ascension in East Africa, as suggested by terrestrialevidence from mammal fossil assemblages (Vrba, 1999; Bobeand Behrensmeyer, 2004; Fernandez and Vrba, 2006), isotopic

es environments from the KBS Member, Koobi Fora Formation: implications

ortheast Turkana Basin (northwest Kenya), J Hum Evol (2007), doi:10.1016/

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proxy records from paleosol carbonates (Cerling, 1992; Levinet al., 2004; Wynn, 2004), and pollen spectra (Bonnefille,1995) at hominin sites. Grassland expansions are a main selec-tive force behind open-habitat adaptations and the divergence ofthe human lineage from quadrupedal apes (Dart, 1925;Robinson, 1954; Jolly, 1970). Therefore, the onset and subse-quent intensifications of Northern Hemisphere Glaciation areconsidered to be potential causes of important East Africanhominin events (Brain, 1981; Potts, 1998; Vrba, 1999), suchas the origins of the genus Homo and material culture dated to2.5e2.3 Ma (Hill et al., 1992; Kimbel et al., 1996; Semawet al., 2003), and the evolution of African Homo erectus at1.9e1.8 Ma (Wood, 1991; Anton and Swisher, 2004).

Other perspectives, however, call into question the linkagebetween East African Plio-Pleistocene hominin evolution andNorthern Hemisphere Glaciation. For example, research onmarine cores (Denison et al., 2005) and lacustrine sequences(Trauth et al., 2003; Deino et al., 2006) highlight the importantinfluence of low-latitude insolation budgets on African climatechange (Kutzbach and Street-Perrott, 1985; Rossignol-Strick,1985; Pokras and Mix, 1987; Prell and Kutzbach, 1987;Molfino and McIntyre, 1990; Hilgen, 1991; Lourens et al.,1996; Tuenter et al., 2003; Clement et al., 2004). New datafrom tectonic models demonstrate the feasibility for rift valleyextension and uplift to restrict zonal atmospheric movements,leading to a damping of rainfall over eastern Africa (Sepulchreet al., 2006). Additionally, Cane and Molnar (2001) suggestthe temporal resolution of hominin paleoenvironmental re-cords does not permit the reconstruction of short-term climatechanges or the recognition of unambiguous spatial differencesin the pattern of increasing African aridity. These authorsargue that the present state of the data indicates a protracteddrying period induced by a gradual force, like changes inglobal oceanic and atmospheric circulation deriving from a tec-tonic closure of seaways. These last viewpoints highlightfundamental challenges because it is difficult to debate alter-nate environmental hypotheses of hominin evolution whenthe relevant terrestrial geological sequences are of lowerchronostratigraphic resolution than marine climate records(Feibel, 1997).

To overcome problems of geological resolution, East Afri-can studies are now turning attention to lacustrine sedimentarysequences from Plio-Pleistocene hominin sites (Ashley andDriese, 2002; Trauth et al., 2005; Deino et al., 2006). Thelacustrine sedimentology, stratigraphy, and depositionalenvironments of East African basins and other rift settingsare excellent tools for reconstructing past lake-level oscilla-tions and evaluating regional paleoclimatology, tectonicsetting, and the history of basin infilling (Butzer et al., 1972;Owen et al., 1982; Tiercelin, 1990; Feibel et al., 1991; Olsenand Kent, 1996; Cohen et al., 1997; Scholz et al., 1998;Keighley et al., 2003; Ilgar and Nemec, 2005; Gasse, 2006).Lacustrine deposits are ideal for obtaining detailed geologicalrecords because they typically have fast rates of sedimentationand may be punctuated with few depositional hiatuses (e.g.,Sadler, 1981). Lacustrine environments also have a highpotential to record ‘‘Milankovitch’’ climate changes caused

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by orbital cycles that operate at timescales on the order of104 to 105 years (de Boer and Smith, 1994). There is great pos-sibility for reconstructing East African climate and assessinglinks with hominin evolution through an understanding oflacustrine depositional systems.

Our study concerns the lake-margin sedimentary record forthe hominin-bearing KBS Member (w1.9e1.6 Ma) of theKoobi Fora Formation, and examines Koobi Fora Ridge local-ities in the northeast portion of the Turkana Basin (Figs. 1 and2). For these localities, studies by Bowen (1974), Feibel (1983,1988), and Tindall (1986) document sedimentary facies suc-cessions that archive information on lake-level oscillations.These authors note that many of the facies changes are predict-ably sequential, which may suggest a cyclic influence on depo-sition like Milankovitch climate forcing. However, apart fromthe exploratory discussions of Brown (1995), there has beenlittle detailed work on the deposits to understand connectionsbetween the paleoenvironmental changes and probable cli-matic controls. The KBS Member is also noteworthy becauseit brackets the period 1.8e1.7 Ma, which is thought to be aninterval of heightened aridity and grassland expansion inEast Africa, possibly due to global climate change (e.g., Cerl-ing, 1992; Bonnefille, 1995; deMenocal, 1995; Bobe and Beh-rensmeyer, 2004; Fernandez and Vrba, 2006). It also preservessome of the earliest fossils of African Homo erectus (Leakeyand Leakey, 1978; Feibel et al., 1989; Wood, 1991; Antonand Swisher, 2004).

The main objectives of this paper are twofold: examine thefacies and depositional environments for the KBS Member atthe Koobi Fora Ridge, and assess the probable controls on theformation of the lake-margin strata. The goal of this study is toestablish a basic paleoclimate framework from sedimentologyand stratigraphy to be refined by ongoing research. Our sub-divisions of the KBS Member sediments into discrete faciesenvironments and chronostratigraphic units, which reflectdistinctive phases of lake-level and climate conditions, providedetailed information on the origin and evolution of lake-margin hominin habitats of Koobi Fora.

Methods and geological setting

A lake formed in the Turkana Basin at �2.0 Ma and waslargely infilled by w1.5 Ma (Brown and Feibel, 1991). Itsorigination, subsequent phases, and timing are well studiedfrom the KBS Member deposits of the Koobi Fora Formationat Koobi Fora Ridge localities in the northeast Turkana Basin(Bowen, 1974; Feibel, 1983, 1988; Brown and Feibel, 1986,1991; Tindall, 1986; Brown et al., 2006; McDougall andBrown, 2006; Figs. 1 and 2). We use the stratigraphic patternsof facies in the KBS Member at the Koobi Fora Ridge to infersuccessive, short-term hydrological and sedimentary changesfor a marginal area of the lake. Sedimentary facies analysisis the fundamental approach used to resolve the depositionalsetting and paleogeography of the Koobi Fora Formation(e.g., Bowen, 1974; Feibel, 1983, 1988; Brown and Feibel,1986, 1991; Tindall, 1986). Our classification and interpreta-tive schemes are based on these prior studies, and incorporate

es environments from the KBS Member, Koobi Fora Formation: implications

ortheast Turkana Basin (northwest Kenya), J Hum Evol (2007), doi:10.1016/

Fig. 1. Location map of the study area. Upper left, map showing the distribution of the Koobi Fora Formation in the northeast Turkana Basin of Kenya (after Brown

and Feibel, 1991). Lower left, map of the Koobi Fora Region showing location of examined exposures of KBS Member facies and ABC mapping units (after

Feibel, 1983).

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Please cite this article in press as: Christopher J. Lepre et al., Plio-Pleistocene facies environments from the KBS Member, Koobi Fora Formation: implications

for climate controls on the development of lake-margin hominin habitats in the northeast Turkana Basin (northwest Kenya), J Hum Evol (2007), doi:10.1016/

j.jhevol.2007.01.015

Fig. 2. Stratigraphic section of the KBS Member, Koobi Fora Formation, at the

Koobi Fora Ridge (after Brown and Feibel, 1986). Dates of tuffs from McDou-

gall and Brown (2006). Position of the Olduvai-Matuyama transition from

Hillhouse et al. (1977), Feibel et al. (1989), and McDougall et al. (1992).

Age of the Olduvai-Matuyama transition from Lourens et al. (2004). See

Fig. 1 for aerial distribution of ABC mapping units in the study area.

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the categories of lithology, grain size, fossil content, primarystructure, bedding morphology, thickness, and lateral andvertical stratigraphic relationships. We also employ the previ-ously established geochronology and sediment rates (Hillhouseet al., 1977; Feibel et al., 1989; McDougall et al., 1992; Brownet al., 2006; McDougall and Brown, 2006) to calibrate the agesof facies and environmental changes for the period dating toabout 1.87e1.48 Ma. We then discuss feasible controls onthe lacustrine depositional system.

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Paleogeographic overview

We examined lake-margin sediments dated to 1.87e1.48 Ma, from the Koobi Fora Ridge in the northeast basin(Figs. 1 and 2). The lake-margin strata were deposited by abasin-wide lacustrine system that formed at �2.0 Ma. Thislake had an initial aerial extent in excess of 9,000 km2, anda depth on the order of 40 m (Feibel, 1988). The large extentof the lake lasted for some 100,000 years and reduced in areanear 1.9 Ma, possible due to piracy from Nile drainages and/orthe development of an outlet at the southeast margins of thebasin (Brown and Feibel, 1991; Feibel, 1994). Deltaic sedi-mentation was common to the areas adjacent to the northeastlake margin between 1.9 and 1.87 Ma (Brown and Feibel,1986).

The period 1.87e1.48 Ma was variable with respect to lakedynamics. The lake had a mostly open hydrology but frequentoscillations in lake level suggest that it periodically fell belowits outlet (Feibel, 1994). The lake reduced in overall aerialextent at about 1.87 Ma, and again near 1.8 Ma (Brown andFeibel, 1991). Salinity-alkalinity increased and/or lake watersbecame more concentrated with time, as suggested by the lim-ited lacustrine biota centered on algal communities that wasprogressively developed (Feibel et al., 1991).

A major channel drainage, which was largely perennial,formed in the northeast basin at w1.87 Ma (Feibel, 1988).This fluvial system, attributed to the ancestral Omo River, con-structed a network of distributary channels and floodplains atthe marginal areas of the lake. With time, these fluvial envi-ronments replaced subaerial deltaic settings as the main depo-sitional systems along lake margins (Brown and Feibel, 1991).

The lake was largely infilled by about 1.5 Ma; after which,the Turkana Basin was a predominantly fluvial basin witha through-flowing river (Brown and Feibel, 1991). This fluvialphase lasted for more than 100,000 years, but the precise tim-ing of its demise is poorly constrained.

Stratigraphy and age control

Brown and Feibel (1986, 1991) provide a synthetic descrip-tion and interpretation of the stratigraphy of the KBS Memberfrom the Koobi Fora Ridge (Fig. 2). These authors describe thePlio-Pleistocene sediments as terrigenous clastic, arenaceousbioclastic carbonate (ABC), and volcaniclastic beds froma lake-margin depositional setting. Fine- to coarse-grainedsandstones, siltstones, and claystones dominate the terrigenousclastics. The ABC beds consist of mollusk packstones, stro-matolites, and associated sandy deposits. Volcaniclasticsmainly consist of rhyolitic tuffs from source areas in Ethiopia(Cerling and Brown, 1982).

Two important tuffaceous markers in the section includethe KBS Tuff and the Koobi Fora Tuff Complex (Brown andFeibel, 1986, 1991; Fig. 2). The KBS Tuff is the basal levelof the KBS Member and the complex is an informal markerunit for the uppermost level of the KBS Member (Brownand Feibel, 1985, 1986). McDougall and Brown (2006) report40Ar/39Ar ages of 1.869� 0.021 Ma for the KBS Tuff and

es environments from the KBS Member, Koobi Fora Formation: implications

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Table 1

KBS Member facies environments at the Koobi Fora Ridge (see also Figs. 3

and 4)

Facies Environments/

processes

Facies environments

A) poorly to moderately

sorted, medium and

coarse pebbly sand;

massive, laminated,

and trough to planar

crossbeds; fines to

massive, clayey/silty

muds with sparse

laminations; muds

preserve abundant

color mottles and

slickensides

channels and

floodplains with soils;

subaerial areas

alluvial (Brown and

Feibel, 1991)

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1.476� 0.013 Ma for the Lower Koobi Fora Tuff. The latter isa basal tuff in the Koobi Fora Tuff Complex (Brown and Fei-bel, 1985, 1986; Brown et al., 2006).

Paleomagnetic studies recognize that the KBS Member atthe Koobi Fora Ridge reflects deposition during the OlduvaiSubchron and Matuyama Chron (Hillhouse et al., 1977). Fei-bel et al. (1989) place the top of the Olduvai Subchron atjust beneath the stratigraphic level of ABC mapping unitA2; the mapping unit is at w67 m above the base of theKBS Tuff (Feibel, 1983; Fig. 2). McDougall et al. (1992) placethe top of the Olduvai at about 60 m above the base of theKBS Tuff (Fig. 2). The top of the Olduvai Subchron dates tonear 1.778 Ma (Lourens et al., 2004).

Results

B) poorly to moderately

sorted, medium, and

beachface and

shoreface settings

nearshore (e.g.,

Renaut and Owen,

Facies and depositional environments coarse pebbly sand;

steeply-dipping planar

crossbeds, 25 to 50

centimeters in height,

ripples marks, soft

sediment deformation;

lenses with a shell hash

and reworked

stromatolite clasts

1991; Blair, 1999)

C) poorly to moderately

sorted, medium, and

subaqueous; shallow

lake waters; low

nearshore

(Williamson, 1982;

Alluvial facies environments. Channel and floodplain de-posits comprise the alluvial facies environments (Fig. 3; Table1). Alluvial sandstones are poorly to moderately sorted, andbegin with concave-up bases that typically show 5e10 m ofincision into underlying strata. Sedimentary structures consistof scour-and-fill features, well-developed trough and planarcrossbeds, and laminations. These coarse deposits verticallyfine into clayey to silty mudstones that are massive, but may

Fig. 3. Sedimentological features, facies environments (Table 1), and stratigra-

phy for the ABC mapping interval A1eA2. This section derives from the

lower-middle KBS Member, Koobi Fora Formation, at the Koobi Fora Ridge

(see Figs. 1 and 2). The mapping interval spans the top of the Olduvai Sub-

chron and, thus, probably has a late Pliocene/early Pleistocene age (e.g.,

Fig. 2 and Table 2). Note the pronounced interruption of the lacustrine sedi-

ments from channel erosion and deposition just above A1, which is thought

to be the result of a drop in lake level from climate change (e.g., Fig. 5).

coarse pebbly sand;

arenaceous bioclastic

carbonates; life and

death assemblages of

stromatolites and

mollusks; calcareous

sandstones; massive,

laminated, and ripple

beds, crossbeds

clastic input; lateral

and backbeach

settings; littoral areas

Casanova, 1986;

Renaut and Owen,

1991)

D) moderately to

well-sorted, medium to

fine, calcareous mud;

massive to laminated;

rare life and death

mollusk assemblages,

no stromatolites

subaqueous; deep and

open lake waters; high

clastic input, fallout

of sediment in

suspension; areas that

are ahead of

debauching stream or

delta channel

offshore (Bowen,

1974; Brown and

Feibel, 1991)

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show indistinct laminations. The mudstones also have green-ish-gray color mottles, and 10e100-cm thick horizons ofslickensides and dish-shaped fractures (Feibel, 1983).

Sandstones are channel deposits based upon their graincharacteristics, thicknesses, basal morphologies, sedimentarystructures, and fining-upward nature. Contrastingly, the fine li-thologies, the massive bedding, and the remnant laminationssuggest the mudstones are suspension deposits. The featuresoverprinted on the mudstone are typical indicators of paleosolsthat form under instances of shrinking and swelling, in repet-itive drying-wetting conditions (Feibel, 1988; Wynn, 2004).The color mottles also imply fluctuations in the local watertable. These lines of evidence suggest the mudstones are flood-plain strata. Since the sandstones vertically fine into the

es environments from the KBS Member, Koobi Fora Formation: implications

ortheast Turkana Basin (northwest Kenya), J Hum Evol (2007), doi:10.1016/

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mudstones, the deposits are likely penecontemporaneous com-ponents of the same alluvial depositional system.

Nearshore facies environments. ABC mapping units andsandstones form the nearshore facies environments (Fig. 4; Ta-ble 1). ABC mapping units are 10e150-cm thick, and the bestdeveloped ones cover areas greater than 250 km2 (Feibel,1983; Tindall, 1986). These units are biolithites consisting ofcryptalgal stromatolites and mollusks, in life and death posi-tions, and also include fossil fish nests (Feibel, 1987, 1988).A moderately sorted, calcareous sandy matrix surrounds thebiogenic deposits. This matrix is a medium to coarse quartzo-feldspathic sand that displays ripple marks, planar crossbeds,and massive bedding. ABC units vertically and laterally gradeinto 1e5-m-thick successions of sandstones and/or semi-consolidated sands. Such sandy deposits are distinctive fromchannel sandstones by their smaller overall thicknesses, flatbasal surfaces, and stratigraphic relationships with the ABCmapping units. These sandy beds are also distinguishablefrom channel sands by the features of ripple marks, softsediment deformation, and lenses with reworked bioclasticremains (Feibel, 1988). A notable set of structures for thesesands are steeply dipping crossbeds, typically 25 to 50 centi-meters in height that laterally interlayer with similar crossbedsof the ABC units.

The trace and body fossils indicate a low clastic input to thelake waters from which the ABC beds derive (Bowen, 1974).The depositional environments for the stromatolites have been

Fig. 4. Sedimentological features, facies environments (Table 1), and stratigra-

phy for the ABC mapping intervals A3eC6 and C6eA4. This section derives

from the KBS Member, Koobi Fora Formation, at the Koobi Fora Ridge (see

Figs. 1 and 2). The section is based from Feibel (1983, 1988) and new field

observations. The ages of these mapping intervals are early Pleistocene

(e.g., Fig. 2 and Table 2). Note the coarsening-up/shallowing-upward patterns,

which are thought to be a result of lake-level and climate changes.

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interpreted as shallow littoral areas, and nearshore photic zonesites with evaporative and hypersaline conditions (Johnson,1974; Abell et al., 1982; Casanova, 1986). Mollusks includethe gastropods Bellamya, Cleopatra, and Melanoides, andthe bivalve Mutela, which together indicate relatively freshwaters as compared with the stromatolites (Williamson,1982). The sandy beds intercalated with and adjacent to theABC units resemble deposits from the beachface/shorefacesettings of other well-studied lake margins (e.g., Renaut andOwen, 1991; Blair, 1999). All of these data suggest nearshoredepositional settings, with extensive and shallow shorelines oflittle relief, and beach areas (e.g., Feibel, 1983, 1988; Tindall,1986).

Offshore facies environments. A summary description andinterpretation of the offshore facies environments is listed inTable 1. Their vertical stratigraphic relations with other envi-ronments are schematically depicted in Figs. 3 and 4. Offshoremudstones are distinguishable from the fine-grain alluvial sed-iments through depositional features and secondary minerals.These deposits contain thin (1e20 cm) sandy lenses withostracod and bivalve remains, and consist of very fine to mediumsilts and clays, which are moderately to well sorted (Feibel,1988). They also characteristically preserve alternatingfine-coarse laminations that show dark-light color contrasts(Bowen, 1974). The offshore mudstones are also distinctive intheir secondary minerals of gypsum and limonite nodules(Cerling, 1979). Other features of the mudstones include thinand massive beds and calcareousness.

The sorting, grain sizes, and depositional structures of themudstones imply sedimentation from the fallout of grains insuspension. Excellent preservation of laminations indicateslimited erosion, weak currents, and a subaqueous depositionalenvironment below a wave base. The color-banded lamina-tions suggest deposition under serial contrasts in the influxof detritus, and a high rate of clastic input. Modern Lake Tur-kana has similar laminations in prodeltaic settings that receivesediment from seasonal/annual plumes of the Omo River(Yuretich, 1979). Similar controls on sedimentation are inter-preted for the laminated mudstones (Feibel, 1988; Feibelet al., 1991). The sedimentology of the mudstones suggestsdeposition in relatively deep offshore environments withhigh clastic input. These environments are thought to be aheadof delta channels or other streams debauching into unconfinedlake waters (e.g., Bowen, 1974; Brown and Feibel, 1991).

Origins of the lake-margin sediments

Tectonic factors

The importance of tectonics in forming sequences fromlacustrine basins is clear but overshadowed by the preponder-ance of studies that focus on climate (Carroll and Bohacs,1999). The geographic distribution of faults, tectonic move-ments, and the resulting basin geometry and structure areimportant controls on lake sedimentology and stratigraphyfrom East African settings (Tiercelin, 1990; Cohen et al.,1997; Scholz et al., 1998). Seismic data suggest that fault

es environments from the KBS Member, Koobi Fora Formation: implications

ortheast Turkana Basin (northwest Kenya), J Hum Evol (2007), doi:10.1016/

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movements can act with regularity and drive periodic changesof lake-level and deposition in East African basins (Scholz,1995; Morley et al., 2000).

The origin of the lake at �2.0 Ma was significantly medi-ated by tectonics. Brown and Feibel (1991) interpreted threeaspects of Turkana Basin geology as evidence of regionaltectonic activity at 2.5e2.0 Ma: 1) a disconformity in theKoobi Fora Formation, which increases in angularity towardthe northeast; 2) an influx of siliciclastics from basementsources in southwest Ethiopia; and 3) the formation of theMt. Kulal at the southeast margin of the Turkana Basin. It ishypothesized that Mt. Kulal acted as a damming sill, whilstnew depth to the Turkana Basin was generated by rift upliftin southwestern Ethiopia and associated down-warping innorthwestern Kenya (Feibel, 1988; Brown and Feibel, 1991;Feibel et al., 1991).

There is sparse evidence of tectonic activity subsequent tothese events; but the Plio-Pleistocene fault systems of the Tur-kana Basin are not extensively studied. The relevant geologi-cal sequences, however, contain fluvial deposits of tephra,which suggest episodes of explosive volcanism in Ethiopiaand the transport of ash by rivers entering the basin (Cerlingand Brown, 1982). There is also evidence from regional map-ping studies that demonstrates a few instances of local synde-positional faulting during the sedimentation of the lake stratafrom the Koobi Fora Ridge (Feibel, 1983; Tindall, 1986).

Lake level and climatic factors

Lake-level oscillations are of fundamental controls ondeposition at a lake margin (Collinson, 1978). In the strati-graphic record, regressions and transgressions of lake levelare often recognized by transitions between facies depositedin subaerial and subaqueous settings (e.g., Olsen and Kent,1996; Keighley et al., 2003; Ilgar and Nemec, 2005). For theKBS Member, the vertical stratigraphic distributions of faciesand corresponding depositional environments imply a dia-chronic and lateral movement of subaerial and subaqueoussedimentary settings across the ancient lake margin, resultingin alternate times of alluvial, nearshore, and offshore deposi-tion. The most obvious processes that could account for thesedepositional patterns are changes in lake level relative to theadjacent shoreline (e.g., Bowen, 1974; Feibel, 1988). Thus,the alluvial environments consisting of channel sandstones,floodplain mudstones, and paleosols are likely to be the resultof low phases in lake level. Offshore mudstones and ABCmapping units suggest periods of deposition when lake levelwas comparatively high.

For East African basins, the proximate climate-related pa-rameters that influence changes in lake level and depositionare the amount of direct rainout/evaporation over the waterbody, the rate of water and sediment delivery from sources,and the precipitation and sediment production regimes incatchment areas (e.g., Tiercelin, 1990; Cohen et al., 1997).Monsoon systems are the overall climate controls on modernand ancient lake-level changes in East Africa (Gasse, 2006).Monsoonal rainfall is a consequence of differential heating

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between marine and terrestrial areas, and the resulting land-ward convection of moisture-rich oceanic air (McGregor andNieuwolt, 1998). The timing of increased East African mon-soonal rainfall is mainly a product of the migration of theIntertropical Convergence Zone (ITCZ) that follows the sea-sonal and latitudinal movement of incoming solar insolation.The path of solar insolation moves from south to north duringthe rise of boreal summer, which causes northern African land-masses to warm and builds a low-pressure zone over the area.This progressively drags the ITCZ northward to where it even-tually settles in the northern low latitudes (Nicholson, 1996).The opposite occurs as the switch is made to austral summer,with the path of incoming sunlight trending southward. Thetwice-overhead passing of the sun and the associated move-ment of the ITCZ result in the characteristic biannual rainsof the East Africa monsoon.

On longer timescales, the fundamental pacemakers of pale-oclimate for East Africa are thought to be solar insolation bud-gets derived from variations in orbital eccentricity, obliquity,and most significantly, precession (Prell and Kutzbach, 1987;Clement et al., 2004). Precession is the orbital cycle(w20,000 year) that determines the position of the seasonswithin the Earth’s revolution (Berger et al., 1992). Preces-sional effects on the East African monsoon largely stemfrom the movement of Northern Hemisphere summer throughan elliptical orbit. Enhanced East African monsoonal rainfalloccurs as precession configures the season of maximum in-coming insolation (i.e., boreal summer) at perihelion, whichis when the Earth is closest to the sun during an elliptical orbit.The increase in direct solar radiation from perihelion aug-ments boreal summer insolation, resulting in more sensibleheating over northern Africa and strengthened summer mon-soon circulation. Boreal summer in perihelion, thus, leads toa greater disparity between boreal winter and summer temper-ature and precipitation, whilst boreal winter in perihelionlessens the disparity. Accordingly, studies of sapropelsequences from the Eastern Mediterranean Sea suggest thatintensified rainfall for Plio-Pleistocene East African occursevery w20,000 years, coinciding with insolation maximafrom precession (Hilgen, 1991). Sapropel deposition isthought to be related to anoxia induced by the stratificationbetween the dense, salty seawater and significant increasesin the freshwater discharge at the Nile Delta, deriving fromenhanced East African monsoonal rainfall (Rossignol-Strick,1985). Nile discharge is heavily dependent on precipitationover the Ethiopian highlands (Sutcliffe and Parks, 1999),which is where the headwaters of the Blue Nile River andthe Omo Riverdthe source of Lake Turkanadshare drainagedivides (Fig. 1). Because of the hydrological links betweenTurkana and the Nile, studies suggest that the two systemsrespond similarly to precessional insolation forcing (e.g., Butzeret al., 1972; Owen et al., 1982; McDougall et al., 2005). There isalso a latent but continuous w40,000-year cycle of obliquity-derived insolation to African monsoonal regimes, as indicatedby oxygen isotope data associated with the sapropel sequences(Lourens et al., 1996) and climate models (Tuenter et al.,2003). The sapropels additionally show thickening/thinning

es environments from the KBS Member, Koobi Fora Formation: implications

ortheast Turkana Basin (northwest Kenya), J Hum Evol (2007), doi:10.1016/

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patterns from obliquity insolation forcing, and at several insola-tion maximums there are ‘‘missing’’sapropels. These characterstend to be better developed every w100,000 years, wheneccentricity minimums cause a near circular orbit (Lourenset al., 1996) and, as a result, the precessional effects on theseasonal timing of perihelion are inconsequential (Bergeret al., 1992).

However, superimposed on changes in African monsoonrainfall and lake-level from orbital insolation are the effectsof Northern Hemisphere Glaciation. Evidence from Africanmarine sequences indicates increasing concentrations of eoliandust flux beginning near 2.5 Ma and peaking at w1.7 Ma(deMenocal et al., 1993; deMenocal, 1995). The variabilityin dust flux primarily follows the obliquity timescales ofNorthern Hemisphere Glacial cycles (deMenocal, 2004).Through the period 2.4e1.7 Ma, abundances of pollen typesin marine cores from northwest Africa also fluctuate on obliq-uity timescales (Dupont and Leroy, 1995). These marinepollen spectra demonstrate an Equatorward migration ofAfrican vegetation zones starting near 2.5 Ma and continuingto w1.8 Ma. Several lines of evidence from East African hom-inin sites complement the marine data. For example, studies ofpollen sequences (Bonnefille, 1995), isotopic data from paleo-sol carbonates (Cerling, 1992; Levin et al., 2004; Wynn,2004), and turnover rates for large mammal taxa (Bobe and

Fig. 5. Idealized facies successions for the C5eA4 ABC mapping interval in the KB

The development of the facies environments is related to lake-level changes, whic

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for climate controls on the development of lake-margin hominin habitats in the n

j.jhevol.2007.01.015

Behrensmeyer, 2004; Fernandez and Vrba, 2006) suggest aresponsiveness of paleoenvironments in East Africa to glacialactivity near 2.5 and 1.8 Ma. General circulation model sensi-tivity tests suggest that expansions of high-latitude glaciersresult in generally cooler and drier conditions over Plio-Pleis-tocene East Africa (deMenocal and Rind, 1996).

The sedimentary aspects of the KBS Member at theKoobi Fora Ridge suggest at least two orders of paleoclimatechange. Desiccation cracks in the alluvial paleosols and thecolor-banded laminations of the offshore mudstones indicatelow-magnitude changes. These features reflect moisture andsedimentary variations occurring with high frequency. Chang-ing weather patterns, seasonal/annual variations in sedimentdelivery from streams, and other episodic or short-term pro-cesses are likely controls. On the other hand, the stratigraphicshifts in facies environments indicate higher-amplitudesedimentary and hydrological fluctuations occurring at a rela-tively low frequency. The corresponding transgressions andregressions of lake level, respectively, imply wetter and drierclimatic conditions. The sequence of facies and inferred lakelevels suggests cyclic patterns in climate change (e.g.,Fig. 5). The nearshore environments of the ABC mappingunits, and the intermediate lake levels they represent, recurwith frequencies of 10,000e50,000 years, with several fallingin the range of 20,000e30,000 years and of 40,000e50,000

S Member, Koobi Fora Formation, at the Koobi Fora Ridge (see Figs. 1 and 2).

h may suggest climate cycles (on the order of 104 years).

es environments from the KBS Member, Koobi Fora Formation: implications

ortheast Turkana Basin (northwest Kenya), J Hum Evol (2007), doi:10.1016/

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years (Table 2). These periods are within or near the rangeof precession and obliquity timescales. For the time intervalof 0e50 Ma, the estimated values of the main astronomicalperiods range at 18,800 to 23,000 years for orbital precession,and 39,900 to 54,000 years for obliquity (Berger et al., 1992).Other noteworthy depositional changes include the recurrenceof alluvial-channel successions between the KBS Tuff to thetop of the Olduvai Subchron (e.g., Figs. 2 and 3). These suc-cessions suggest two intervals of low lake levels, one betweenABC units C4 and C5 (w1.85e1.83 Ma; Table 2) and theother constrained by A1eA2 (w1.80e1.75 Ma; Table 2).The bases of the alluvial-channel successions are separatedfrom each other by 40,000e50,000 years, implying that majordrops of lake levels recur with a timescale similar to obliquity.Collectively, the facies and sedimentation rates data may sug-gest the hydrosedimentary dynamics of the ancient lake mar-gin is responsive to changes of monsoonal rainfall that resultfrom orbital insolation and/or glacial forcing.

Summary and conclusions

Environmental hypotheses implicate global climate changeas a driving force of early hominin evolution in East Africa(Potts, 1998; Vrba, 1999). These hypotheses also predict thedevelopment of biological and behavioral advantages to bothrapid and long-term, climate-derived environmental changes,

Table 2

Thickness and age control data for ABC mapping units KBS Member, Koobi

Fora Ridge*

Level Position (m) Age (Ma) Interpolated ages (Ma)

LKFT 136 1.476� 0.013

A6 116 1.552

0.072 [D A5-A6]

A5 98 1.624

0.020 [D A4-A5]

A4 93 1.644

0.020 [D C6-A4]

C6 88 1.664

0.032 [D A3-C6]

A3 80 1.696

0.052 [D A2-A3]

A2 67 1.748

0.051 [D A1-A2]

tC2n 60 1.778�w0.003

A1 48 1.799

0.012 [D C5-A1]

C5 32 1.811

0.041 [D C4-C5]

C4 13 1.851

KBS 0 1.869� 0.021

* Abbreviations: LKFT¼ Lower Koobi Fora Tuff; tC2n¼ top of Olduvai

Subchron; KBS¼KBS Tuff. Other alphanumerical codes indicate ABC map-

ping units. Thickness data from Feibel (1983) and Brown and Feibel (1986).

Ages of the LKFT and KBS from McDougall and Brown (2006). Stratigraphic

position of tC2n after Hillhouse et al. (1977), Feibel et al. (1989), and McDou-

gall et al. (1992). Age of tC2n after Lourens et al. (2004). Interpolated ages of

ABC mapping units are derived from assuming a linear sedimentation rate of

w66 cm per 1,000 years for the interval between KBS and tC2n, and a rate of

w25 cm per 1,000 years between tC2n and LKFT. See Fig. 2 for stratigraphic

section.

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for climate controls on the development of lake-margin hominin habitats in the n

j.jhevol.2007.01.015

leading to the evolution and persistence of Homo specieswhen others hominin lineages went extinct. Milankovitchparameters of the Earth’s orbit and the resultant shifts in solarinsolation budgets and Northern Hemisphere ice volume wereinstrumental in generating Plio-Pleistocene environmentalchanges across low-latitude northern Africa (Hilgen, 1991;Lourens et al., 1996; deMenocal, 2004). Geological datafrom East African hominin sites are, thus, expected to revealperiods of environmental changes driven by climate (e.g.,Cerling, 1992; Bonnefille, 1995; Brown, 1995; Ashley andDriese, 2002; Bobe and Behrensmeyer, 2004; Levin et al.,2004; Wynn, 2004; Trauth et al., 2005; Deino et al., 2006;Fernandez and Vrba, 2006).

Our study focused on the Koobi Fora Ridge, in the north-east Turkana Basin of northwest Kenya, for the purpose of as-sessing probable controls on the development of lake-marginhominin habitats in the KBS Member (w1.9e1.6 Ma) of theKoobi Fora Formation. We documented sedimentary facies,reconstructed depositional environments, and interpretedlake-level changes as a proximate control on the lake-marginsystem. We hypothesized that the lake-level oscillationswere caused by variations in the intensity of monsoonal rain-fall. Evidence from sedimentation rates and the stratigraphicdistributions of facies environments suggested that there arelake-level changes recurring with frequencies similar to thetimescales of orbital precession (w20,000 years) and obliquity(w40,000 years). Collectively, the data may indicate an orbitaland/or glacial climate control on the Plio-Pleistocene environ-mental changes in the lake-margin hominin habitats from thenortheast Turkana Basin.

Acknowledgements

We would like to thank the National Science Foundation,Netherlands Organisation for Scientific Research (NWO),Catharine van Tussenbroek Foundation, Prins BernhardCultuurfonds, and the Center for Human Evolutionary Studies(CHES, Rutgers University) for financial support. Our grati-tude is extended to Mr. Kamoya Kimeu, Mr. Muthoka Kivingo,Mr. Ndolo Muthoka, and the Koobi Fora Research Project forlogistical support in the field. Andrew Cohen, Jim Russell,and several anonymous reviewers markedly improved earlierdrafts of this paper. We also thank Beth A. Christensen andMark Maslin for their solicitations and work with organizingthe 2004 American Geophysical Union sessions on AfricanContinental Paleoclimate and Hominid Evolution and thisspecial issue of the Journal of Human Evolution. Editorialcomments of Susan Anton are greatly appreciated. Finally,we thank the National Museums of Kenya for facilitating ourresearch endeavors in the Koobi Fora Region.

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