C Values from Koobi Fora, Kenya: Implications for Floral Compositions

Journal of Human Evolution 53 (2007) 560e573

Paleogeographic variations of pedogenic carbonate d13C values from Koobi Fora, Kenya: implications for ﬂoral compositions of Plio-Pleistocene hominin environments

Rhonda L. Quinn a,b,*, Christopher J. Lepre a, James D. Wright b, Craig S. Feibel a,b

a Department of Anthropology, Rutgers University, 131 George Street, New Brunswick, NJ 08901, USA b Department of Geological Sciences, Rutgers University, 610 Taylor Road, Piscataway, NJ 08854, USA Received 4 November 2005; accepted 22 January 2007

Abstract

Plio-Pleistocene East African grassland expansion and faunal macroevolution, including that of our own lineage, are attributed to global climate change. To further understand environmental factors of early hominin evolution, we reconstruct the paleogeographic distribution of veg- 13 etation (C3-C4 pathways) by stable carbon isotope (d C) analysis of pedogenic carbonates from the Plio-Pleistocene Koobi Fora region, northeast Lake Turkana Basin, Kenya. We analyzed 202 nodules (530 measurements) from ten paleontological/archaeological collecting areas spanning environments over a 50-km2 area. We compared results across subregions in evolving fluviolacustrine depositional environments in the Koobi Fora Formation from 2.0e1.5 Ma, a stratigraphic interval that temporally brackets grassland ascendancy in East Africa. Significant differences in d13C values between subregions are explained by paleogeographic controls on floral composition and distribution. Our results indicate grassland expansion between 2.0 and 1.75 Ma, coincident with major shifts in basin-wide sedimentation and hydrology. Hypotheses may be correct in linking Plio-Pleistocene hominin evolution to environmental changes from global climate; however, based on our results, we interpret complexity from proximate forces that mitigated basin evolution. An w2.5 Ma tectonic event in southern Ethiopia and northern Kenya exerted strong effects on paleography in the Turkana Basin from 2.0e1.5 Ma, contributing to the shift from a closed, lacustrine basin to one dominated by open, fluvial conditions. We propose basin transformation decreased residence time for Omo River water and expanded subaerial floodplain landscapes, ultimately leading to reduced proportions of wooded floras and the establishment of habitats suitable for grassland communities. Ó 2007 Elsevier Ltd. All rights reserved.

Keywords: Paleogeography; Savannas; East Africa; Carbon isotopes

Introduction argues the need for a middle ground, tethering the ‘‘global- scale climatic phenomena’’ to ‘‘environmental change, habitat Global climate is often elected as a catalyst for environmen- shift, and biotic evolution’’, and offers the sedimentary basin as tal changes acting as selective pressures on Plio-Pleistocene a scale for analysis. Environments within individual basins re- African hominins (Stanley, 1992; Vrba, 1995, 1999; deMeno- spond to climate with different sensitivities and thresholds cal, 2004). Habitat theory of Vrba (1992) and variability selec- inﬂuenced by basin size, topography, depositional and tectonic tion hypothesis of Potts (1998) credit mammalian evolutionary regime, and water availability (Carroll and Bohacs, 1999; pattern and process to climate change. Feibel (1999: 276) Withjack et al., 2002). Stable carbon isotope (d13C) records from pedogenic carbonates are interpreted as reﬂecting the spread of C4 grasses * Corresponding author. in East Africa beginning as early as the Miocene (Cerling, E-mail address: [email protected] (R.L. Quinn). 1992). From these data, environmental change and increased

0047-2484/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jhevol.2007.01.013 R.L. Quinn et al. / Journal of Human Evolution 53 (2007) 560e573 561 aridity associated with global climate are temporally correlated with faunal macroevolution including the branching pattern of the hominin lineage and the origins of genus Homo (see review in deMenocal, 2004). Unlike proxies of global climate from the marine realm, pedogenic carbonate isotopes offer di- rect and local environmental information from the habitats of hominins and other members of the mammalian community. Isotopic studies of pedogenic carbonates in the Turkana Basin have interpreted terrestrial East Africa as responsive to global climate over approximately the last four million years (Cerling et al., 1988; Wynn, 2004). Here we employ stable isotopic values of paleosol carbonates from hominin-bearing sediment in the Koobi Fora region of the basin to examine paleoenvironmental change. We focus on the interval 2.0e1.5 Ma, which brackets a marked shift in aridity (Wynn, 2004) and the ap- pearance of early African Homo erectus in the area (Antoń and Swisher, 2004; Wood and Strait, 2004). We enlarge the current database to place our isotopic measures of floral community structure into a paleogeographic framework and inte- grate climatic and tectonic influences on a basin-wide scale.

Study region

Geographic setting

The Turkana Basin of northern Kenya and southern Ethio- pia lies within the eastern branch (or Gregory Rift) of the East African Rift System between the Kenyan and Ethiopian domes (Ebinger et al., 2000). The basin presently contains one of the largest rift lakes, Lake Turkana, with an area of 7,500 km2 (Frostick, 1997). Today, the lake is a saline-alkaline and closed-basin lake that receives over 90% of its water from rainfall over the Ethiopian Highlands, via the Omo River (Fig. 1), with minor inputs from the Turkwell and Kerio River systems (Yuretich, 1979). The general climate within this rift bottom setting is arid to semi-arid and receives 250e 500 mm of rainfall annually (Nicholson, 1996). Bushland grasslands dominate the landscape, with gallery forests clus- Fig. 1. Location map of Lake Turkana and the Koobi Fora Formation (stippled area). Inset map shows shaded collecting areas used in this study (after Brown tered along perennial and ephemeral river channels (Lind and Feibel, 1991; Feibel et al., 1991; Feibel, 1999). and Morrison, 1974). The Koobi Fora region, situated within the northeast Tur- reconstruction of a range of habitats across paleolandscapes kana Basin of Kenya (Fig. 1), is one of the richest fossil and at discrete temporal intervals. archaeological localities in East Africa (Leakey and Leakey, 1978; Isaac and Isaac, 1997). Plio-Pleistocene sediments ex- Stratigraphy and paleogeography posed in the region are attributed to the Koobi Fora Formation (Brown and Feibel, 1986), which preserves a record of homi- Although outcrops of the Koobi Fora Formation are discon- nin evolution and environmental change for the period w4.0e tinuous, stratigraphic control has been determined by radiomet- 1.0 Ma (Feibel et al., 1989, 1991; Brown and Feibel, 1991). rically dated and correlated tuffs, aerially extensive bioclastic Lake Turkana constrains the western margin of the Koobi lacustrine marker beds, and an established geomagnetic polar- Fora Formation, while Miocene-Pliocene volcanics to the ity stratigraphy (McDougall, 1985; Brown and Feibel, 1986, west deﬁne the eastern border (Watkins, 1986). The formation 1991; Hillhouse et al., 1986; Feibel et al., 1989; McDougall is exposed in geographic subregions of the Koobi Fora region, et al., 1992; Brown et al., 2006; McDougall and Brown, including Ileret, Il Dura, Karari Ridge, and Koobi Fora Ridge 2006; Fig. 3). Here we focus on the upper Burgi, KBS, and (Fig. 2A), which are segregated into numbered paleontological lower Okote Members of the formation, approximately repre- and archaeological collecting areas (Fig. 1). The extensive senting the period between 2.0 and 1.5 Ma (McDougall, geographic exposure of the Koobi Fora Formation, and its 1985; Brown and Feibel, 1986, 1991: *Age of Lorenyang well-documented stratigraphy and geochronology, affords Tuff is approximated by sedimentation rate (scaled age). 562 R.L. Quinn et al. / Journal of Human Evolution 53 (2007) 560e573

Fig. 2. Paleogeographic reconstructions of the Koobi Fora Formation between 2.0 and 1.5 Ma (after Feibel, 1988; Feibel et al., 1991; Rogers et al., 1994). A) Modern paleogeography, basemap area equals approximately 50 km2 (after Isaac and Behrensmeyer, 1997). B) Paleogeography at 1.7e1.5 Ma. C) Paleogeography at 1.8e1.7 Ma. D) Paleogeography at 1.9e1.8 Ma. E) Paleogeography at 2.0e1.9 Ma. R.L. Quinn et al. / Journal of Human Evolution 53 (2007) 560e573 563

Sedimentation of these members is inferred to be largely continuous, except for an unconformity (w500,000-year depositional pause) in the upper portion of the Burgi Member (Fig. 4) and the background hiatuses inherent to the sedimentation of the fluviolacustrine environments (Brown and Feibel, 1986; Feibel et al., 1989, 1991). A large lake formed in the Turkana Basin at about 2.0 Ma, as indicated by the widespread occurrence of lacustrine and associated facies just above the upper Burgi Member unconformity (Brown and Feibel, 1991). This precursor of Lake Turkana, Lake Lorenyang (Fig. 2E), occupied an area of approximately 9,000 km2, mainly fed by an ancestor of the modern Omo River (Feibel et al., 1991; Feibel, 1997). The deposition of the Lorenyang and KBS Tuffs in a deltaic facies (Feibel, 1988) suggests that the ancestral Omo River delta began prograding into the Koobi Fora region by about 1.9 Ma (Brown and Feibel, 1991; Fig. 2D). The river coursed through the region as indicated by fluvial channel and floodplain facies just above the stratigraphic level of the KBS Tuff (White et al., 1981). Channel and floodplain deposits are more widespread within the upper part of the Olduvai Subchron, suggesting an increase in the prevalence of ancestral Omo River environments at Koobi Fora near 1.8e1.7 Ma (Feibel et al., 1989; Brown and Feibel, 1991; McDougall et al., 1992). Additional channel systems em- anated from the highlands along the northeastern basin margin at this time (Feibel et al., 1991). The Omo River and basin margin channels emptied into a shallow lake margin/series of lake margins, over which there were frequent transgressions and regressions (Feibel et al., 1991; Feibel, 1994; Fig. 2C). After about 1.7 Ma, lake-related depositional environments were virtually absent from the region, except for a few shallow, aerially restricted, and transient examples (Brown and Feibel, 1991; Fig. 2B). For the majority of the period between 1.7 and 1.4 Ma, the main (axial) channels of the ancestral Omo were not readily active in the study area, rather the landscape was occupied by smaller distributaries that either derived from the main channel or from the northeastern margin of the basin (Feibel et al., 1991; Fig. 2B). These distributaries were respon- sible for the deposition of a series of correlative marker tuffs, referred to as the Okote Tuff Complex, Koobi Fora Tuff Com- plex, and Ileret Tuff, which have an age near 1.6e1.5 Ma (Brown and Feibel, 1985, 1986; Brown et al., 2006; McDou- gall and Brown, 2006). Large river channels, attributed to the ancestral Omo, reclaimed the Koobi Fora region near early Chari Member times, w1.4 Ma (Brown and Feibel, 1991).

East African and Turkana Basin paleoenvironments

Many studies attribute a marked increase in East African aridity and grassland expansion between 2.0 and 1.5 Ma to causes derived from global climate change (reviewed in deMe- nocal, 2004). Increased terrigenous sediment inputs, ca. 1.7 Ma, from the northern low-latitude areas of continental Fig. 3. Composite stratigraphic sections of Koobi Fora Formation by subre- Africa (deMenocal and Bloemendal, 1995) have been linked gions with radiometric ages of tuffs (after Brown and Feibel, 1991; McDougall and Brown, 2006). with an intensiﬁcation of Northern Hemisphere Glaciation at w1.8 Ma (e.g., Shackleton et al., 1990; Shackleton, 1995). 564 R.L. Quinn et al. / Journal of Human Evolution 53 (2007) 560e573

Fig. 4. Sedimentation rates and depositional hiatuses for the Koobi Fora Formation throughout its depositional history (after Feibel, 1988).

Indications of a response to high-latitude glacial conditions are that grass species dominated from lake and delta environments found in marine pollen records from off the coast of northwest at 2.0 Ma (Bonnefille and Vincens, 1985); however, cooler and Africa, which show wooded savanna to desert vegetation zones more humid conditions have been interpreted in areas of the appearing around 2.6e2.4 Ma and dominating by 1.8 Ma (Le- basin by the presence of highland and riverine pollen species roy and Dupont, 1994). East African palynological evidence (Bonnefille, 1976). Cerling and others (1988) and Wynn suggests drier conditions in rift valley lowlands at 2.35 Ma (2000) demonstrated tropical grass expansion in the Turkana and an intensification of aridity at around 1.8 Ma (Bonnefille, Basin over the last 4.3 Ma, with marked aridity events during 1995). East and South African mammalian species abundances the intervals between 3.58e3.35, 2.52e2.00, and 1.81e show a shift to more arid-adapted fauna culminating at 1.8 Ma 1.58 Ma. However, Wynn’s (2004: his figure 2B) compilation (e.g., Vrba, 1995; Reed, 1997). Pedogenic carbonate isotope re- illustrated a mean value trending toward wooded vegetation cords amassed from several Plio-Pleistocene East African lo- between 1.8e1.5 Ma. Wynn (2004) also reported a general ari- calities also display aridification with increased proportions dification trend with increasingly shallower calcic horizon of C4 grasses and heightened evaporation of soil water or depths in Turkana Basin paleosols, although he found an ex- change in isotopic composition of rainfall (Cerling, 1992; cursion to a humid period between 1.9 and 1.8 Ma with depth Levin et al., 2004; Wynn, 2004). to calcic horizon estimates. Other researchers, however, have found ambiguous or contrasting records of East African paleoenvironments. Kingston Materials and methods and others (1994) found no significant change in vegetation from the Baringo Basin, but a persistent mosaic environment Stable carbon isotopes (d13C) of pedogenic carbonates over the last 15 Ma. Plio-Pleistocene vegetation at Olduvai Gorge had a significant amount of wooded vegetation (Sikes, The C3 (Calvin-Benson) and C4 (Hatch-Slack) photosyn- 1994). Trauth et al. (2005, 2007) have suggested the presence thetic pathways have distinct differences in the fractionation of a humid period within the general pattern of East African of carbon isotopes (see Ehleringer, 1989). Woodland vegeta- aridification indicated by the formation of large lakes circa tion (trees, shrubs, temperate grasses) utilizing the C3 pathway 1.9e1.7 Ma. Suids found in South and East African hominin discriminate against the heavier and kinetically slower isotope 13 localities maintain postcranial morphologies for closed and in- of carbon, C; whereas tropical grasses using the C4 pathway termediate habitats (Bishop, 1999). also discriminate but allow the inclusion of more 13C into tis- Within the Turkana Basin, contrasting and complex proxy sues than do C3 flora. Since most low-latitude grasses use the records also confound environmental interpretations. Mamma- C4 pathway, East African vegetation shows a clear separation 13 lian species abundances show a gradual shift to more arid- in d C values (C3: 31.4 to 24.6&,C4: 14.1 to 11.5&; adapted fauna from 2.5 to 1.8 Ma rather than a ‘‘turnover Cerling et al., 2003). pulse’’ coinciding with glacial intensification (Behrensmeyer Following soil gas diffusion models of Cerling (1984) and et al., 1997). Additional studies on the same dataset proposed Cerling and Quade (1993) and the conditions of respiration a series of pulses occurring at periods of 100,000 years (Bobe rates developed by Quade et al. (1989), pedogenic carbonates and Behrensmeyer, 2004). Palynological evidence indicates at depths greater than 30 cm in soils with relatively high R.L. Quinn et al. / Journal of Human Evolution 53 (2007) 560e573 565 respiration rates incorporate CO2 of decaying organic matter example, with the KBS Tuff dated to 1.87 Ma (McDougall derived from surface vegetation during soil development. and Brown, 2006) and an average sedimentation rate of the d13C values of pedogenic carbonates from the savanna biome Karari Escarpment determined as 6.2 cm/kyr (Feibel, 1988), are intermediate between the C3-C4 end members and reflect we calculated a fossil soil located five meters above the the percentage of grasses versus woody vegetation present on KBS Tuff in collecting Area 105 to be approximately the land surface with an isotopic fractionation between 13.5e 1.79 Ma. All ages are expected to have an error of 0.03e 16.6& (Cerling and Quade, 1993). Pedogenic carbonates 0.05 Ma (e.g., Feibel et al., 1989). form under a negative water budget in regions with rainfall be- In addition to the sedimentological indicators of paleogeog- low 100 cm/yr during periods averaging hundreds to thousands raphy, we used paleopedology to interpret environmental con- of years (Jenny, 1941, 1980; Birkeland, 1984; Srivastava, text of the isotopic results. At the outcrop, we identified 2001). Fossil soils preserved in the Plio-Pleistocene Koobi pedogenic carbonate and paleosols by criteria set forth in Re- Fora Formation are dominated by paleovertisols, formed under tallack (2001) and refined for the Turkana Basin by Wynn a dry season of four or more months and 250e1,000 mm of (2000, 2001, 2004). We sampled pedogenic carbonates in annual moisture (Feibel, 1988; Wynn, 2000, 2004). the preserved calcic horizon of the paleosol at a minimum of 30 cm below the contact with the overlying stratum, as sug- Soil carbonate sampling strategy and analysis gested by Quade and others (1989), and excavated back from the vertically exposed surface by approximately 50 cm (e.g., We examined pedogenic carbonate isotope (d13C) values Sikes, 1994). Calcite nodules were extracted from within indi- from paleosols to reconstruct geographic distribution of vege- vidual peds. Since most Turkana fossil soils are paleovertisols, tation (C3-C4 pathways) through time to assess the nature, they show vertic features and slickensided surfaces; we chose timing, and controls on paleoenvironmental change in homi- calcite nodules that exhibited slickensides and/or were adjacent nin-associated habitats of Koobi Fora. We compared lake- to slickenslided surfaces. Although abundant in the formation margin, river floodplain, and distributary channel floodplain deposits, we did not include calcareous rhizoliths in this study depositional landscapes for the period of w2.0e1.5 Ma that due to isotopic alteration by shallow cementation (Driese and temporally brackets grassland ascendancy in East Africa as Mora, 1993) and groundwater (Liutkus et al., 2005). documented in Wynn’s (2004) third C4 expansion event. We In the laboratory, we sectioned pedogenic nodules and sam- enhanced the previous isotopic studies of pedogenic carbon- pled micritic portions with a 0.5 mm carbide drill bit (Foredom ates from the Turkana Basin (Cerling et al., 1988; Wynn, Series), avoiding surface and sparry calcite. We subsampled 2004) for the 2.0e1.5 Ma time interval by: 1) increasing the nodules 2e6 times depending on size in order to test for internal sample size (nodules, n ¼ 202; analyses, n ¼ 530), and 2) con- nodular variability and averaged the subsample values. Approx- ducting widespread sampling of synchronous lateral horizons imately 5% of the samples were analyzed by cathodolumines- in the Koobi Fora Formation. This lateral sampling approach cence (CL) to test for groundwater contribution (Marshall, has been suggested for attaining ecotonal variability and hab- 1988; Wynn, 2004). All isotopic analyses were conducted at itat gradient (e.g., Kingston et al., 1994; Levin et al., 2004; the Stable Isotope Laboratory at Rutgers University on a Micro- Wynn, 2004). With attention to paleohydrology (after Levin mass Optima Mass Spectrometer with an attached Multi-Prep et al., 2004), we partitioned the northeast Turkana Basin as device. Samples were reacted in 100% phosphoric acid at subregions by proximity to precursors of Lake Turkana and 90 C for 13 minutes. d13C values are reported versus the Pee to the ancestral Omo River system, which was the main con- Dee Belemnite reference standard (V-PDB) through analysis trol of basin-wide hydrology and deposition during the Plio- of the laboratory standard (NBS-19) with values of 1.95& for Pleistocene (e.g., Feibel et al., 1991). d13C(Coplen et al., 1983). Our study interval begins with the upper Burgi Member We used Wynn’s (2000, 2001) savanna environment cate- (2.0 to 1.9 Ma), spans the entire KBS Member (1.9 to 1.6e gories to interpret the d13C values as they reflect floral com- 1.5 Ma), and ends with the lower portion of the Okote Member munity structure and composition. We acknowledge the [1.6e1.5 Ma (e.g., Brown and Feibel, 1986, 1991; McDougall fluidity of ecotonal boundaries, but attempt to explain floral and Brown, 2006)]. We sampled exposures from collecting community change with statistically significant shifts in d13C area 1a in the Ileret subregion, area 41 in the Il Dura subre- values. As widespread sampling of fossil soils at one interval gion, areas 101, 102, 103, and 104 along the Koobi Fora potentially yields a wide range of isotopic ratios spanning the Ridge, and areas 105, 130, 131, and 133 on the Karari Ridge values of C3-C4 communities, we approached the question of (Fig. 1). Our study covers approximately 50 km2 in total area significant change in vegetation across space and through of exposure and examines evolving paleolandscapes along time in the following ways. In order to gauge change in veg- lake-margin areas and floodplain areas. The Koobi Fora Ridge etation across space, we compared results by subregion, due and Karari Ridge span three and four of the environments to position within the basin (e.g., proximal to the basin margin, through time, respectively, whereas the Il Dura and Ileret sub- proximal to the lake shore). For change in vegetation through regions only record one depositional regime (Fig. 2BeE). time, we delineated brackets of time that witnessed major Age control of pedogenic carbonate samples was deter- changes in paleogeography. Paleogeographic interpretations mined with the established chronostratigraphic framework are based on sedimentary evidence and provide the context and scaled with sedimentation rates for each subregion. For for our isotopic results. We present our results in 100-kyr 566 R.L. Quinn et al. / Journal of Human Evolution 53 (2007) 560e573 intervals that confine major changes of paleogeography and Table 2 deposition: 2.0e1.9 Ma, 1.9e1.8 Ma, 1.8e1.7 Ma, 1.7e Summary statistics by 100-kyr intervals 1.6 Ma, and 1.6e1.5 Ma. The last two intervals record the 2.0e1.9 1.9e1.8 1.8e1.7 1.7e1.6 1.6e1.5 same depositional environment. We also examined the results Ma Ma Ma Ma Ma by subregion and by subregion within 100-kyr intervals. In all Sample size 11 54 43 62 32 13 approaches, we treated our groups as populations and com- Mean d C value 7.10 6.22 4.98 4.85 5.48 d13 pared means and variances between groups with one-way AN- Median C value 7.16 5.94 4.90 4.66 5.51 Minimum d13C value 8.54 10.43 9.37 7.22 9.07 OVA, Tukey’s all pairs comparison, and Kruskal-Wallis rank Maximum d13C value 5.37 3.76 0.43 0.44 3.67 sum tests. We also interpolated change through time amongst Standard deviation 0.96 1.49 1.83 1.42 1.24 all datapoints with a 10% weighted smooth curve fit after Sti- Standard error 0.29 0.20 0.28 0.18 0.22 neman (1980). All summary statistics and statistical analyses Variance 0.92 2.21 3.36 2.02 1.53 were conducted with KaleidagraphÒ Software. Skewness 0.16 1.01 0.10 0.52 0.81 Kurtosis 0.68 1.01 0.87 0.85 0.65

Results and interpretation do not vary significantly from one another. Results from the one-way ANOVA and Kruskal-Wallis rank sum test indicate d13C values of pedogenic carbonates that the samples are likely from the same distribution. However, results from the following three 100-kyr intervals show that the d13C values of all pedogenic carbonates from Koobi Fora subregions reflect different populations (Fig. 8); results from yielded a mean of 5.5& and standard deviation of 1.6 (Table the one-way ANOVA and Kruskal-Wallis rank sum test corrob- 1). Summary statistics are shown by formation and subregions orate the smooth curve fits (p < 0.001). In each of the (Table 1) and by 100-kyr intervals (Table 2). Estimated with 1.8e1.7 Ma, 1.7e1.6 Ma, and 1.6e1.5 Ma intervals, data a 10% weighted smooth curve fit, the formation shows a 3& from the Karari subregion significantly vary from those of the depletion in 13C from 2.0 to 1.75 Ma. After 1.75 Ma, the curve Koobi Fora Ridge and Ileret subregions (p < 0.001). fluctuates within 1& (Fig. 5). Results from 100-kyr intervals representing different depositional environments show that all are not likely drawn from one population (p < 0.001). One-way ANOVA and Tukey’s all pairs comparison results are shown in Tables 3 and 5. Four of ten paired comparisons show significant differences between groups (p < 0.001); the first two 100-kyr intervals (2.0e1.9 Ma, 1.9e1.8 Ma) vary significantly from the next two 100-kyr intervals (1.8e1.7 Ma, 1.7e1.6 Ma) (Table 2; Fig. 6). Kruskal-Wallis rank sum test corroborates the results of the one-way ANOVA (Table 3). Results by subregion for all time intervals also show mean- ingful differences between groups. One-way ANOVA and Tu- key’s all pairs comparison results are shown in Tables 4 and 6. Kruskal-Wallis rank sum test yielded similar results (Table 4). The data from the Karari Ridge and Il Dura are significantly different from those of the Koobi Fora Ridge and Ileret (p < 0.001; Fig. 7). Furthermore, comparing the subregions within individual 100-kyr intervals yields significant differences between groups (Fig. 8). In the 1.9e1.8 Ma interval, data from the Karari Ridge, Il Dura, and Koobi Fora Ridge

Table 1 Summary statistics by subregions All Koobi Karari Ileret Il Dura subregions Fora Ridge Ridge Sample size 202 71 85 39 7 Mean d13C value 5.47 4.73 6.27 4.76 7.05 Median d13C value 5.46 4.69 6.35 4.60 6.31 Minimum d13C value 10.43 10.43 9.07 6.74 9.53 Maximum d13C value 0.43 0.43 3.30 3.38 5.79 Standard deviation 1.63 1.85 1.19 0.87 1.59 Standard error 0.11 0.22 0.13 0.14 0.60 Fig. 5. d13C(&) vs. age (Ma). Pedogenic carbonate d13C values from paleo- Variance 2.65 3.40 1.42 0.77 2.52 sols in the Koobi Fora Formation between 2.0 and 1.5 Ma. Black line repre- Skewness 0.04 0.47 0.06 0.52 0.89 sents 10% weighted smooth curve ﬁt of all data points (B). Savanna Kurtosis 1.31 2.31 0.18 0.63 1.08 categories are taken from Wynn (2000, 2001). R.L. Quinn et al. / Journal of Human Evolution 53 (2007) 560e573 567

Table 3 Table 5 Results of one-way ANOVA and Kruskal-Wallis rank sum test of 100-ky inter- Results of Tukey’s all pairs comparison by 100-kyr interval groups val groups Comparison Mean jqj P 95% CL One-way ANOVA DF SS MS F P difference Total 201 531.91 2.65 10.51 <0.0001 (1.7-1.6) vs. (2.0-1.9) 2.25 6.52 <0.0001 0.91e3.59 A 4 93.55 23.39 (1.7-1.6) vs. (1.9-1.8) 1.37 6.97 <0.0001 0.60e2.13 Error 197 438.35 2.23 (1.7-1.6) vs. (1.6-1.5) 0.63 2.74 0.2998 0.26e1.52 (1.7-1.6) vs. (1.8-1.7) 0.13 0.63 0.9916 0.68e0.95 Kruskal-Wallis K-W statistic P (1.8-1.7) vs. (2.0-1.9) 2.12 5.94 0.0004 0.73e3.50 rank sum test (1.8-1.7) vs. (1.9-1.8) 1.24 5.74 0.0007 0.40e2.08 37.02 <0.0001 (1.8-1.7) vs. (1.6-1.5) 0.50 2.02 0.6105 0.46e1.46 (1.6-1.5) vs. (2.0-1.9) 1.62 4.39 0.0183 0.18e3.06 (1.6-1.5) vs. (1.9-1.8) 0.74 3.14 0.1762 0.18e1.66 Interpretation of ﬂoral compositions (1.9-1.8) vs. (2.0-1.9) 0.88 2.52 0.3856 0.48e2.24

The Koobi Fora Formation preserves a range of habitats from forest (minimum d13C: 10.4&) to savanna grassland (after Wynn, 2004), is present at and after 1.8 Ma, and shows (maximum d13C: 0.4&) with the majority of environments distinct sand-filled surface cracks and a thick calcic horizon. falling in the mosaic compositions including savanna wood- This fossil soil has been interpreted to support seasonally sparse savanna grassland with little time for development land, thicket and scrub, and low-tree shrub savanna (mean 13 d13C: 5.5&; interpreted after Wynn, 2000, 2001). Based due to quick burial (Wynn, 2004). d C results from this subregion corroborate the previous interpretations of expansion of on Cerling’s (1984) model of C4 contribution to soil CO2, tropical grasses comprised 46% of the average landscape and C4 grasses after 1.8 Ma (Cerling et al., 1988; Wynn, 2004). 13 e ranged from 11e89%. Floral compositions by subregion are The weighted smooth curve of d C values at 1.9 1.8 Ma in- e presented below (Table 1; Figs. 7 and 8). creases from 6to4& (40 60% grasses), placing the area Il Dura. Sampled paleosols are moderately to well-developed within the low-tree shrub category. After 1.8 Ma, the curve clay vertisols, based on thickness, ped structure, and large dish- shifts to more open savanna grassland environments and shaped slickensides (e.g., Aberegaiya variant after Wynn, then returns to the low-tree shrub category at 1.65 Ma (Fig. 8). 2000), with a soil moisture estimate of >1000 mm/yr by calcic Karari Ridge. The Karari paleosols show morphological di- horizon at depth of approximately 600e800 cm (Wynn, 2000, versity. Below the KBS Tuff, the Karari shares the Aberegaiya 2004). We observe low d13C values prior to 1.8 Ma, with grasses paleosol with the Il Dura subregion. Directly underlying the w e comprising 35% (range: 18e44%) of the flora. We interpret Okote Tuff Complex ( 1.6 1.5 Ma), the Kimere pedotype woodland savanna and/or thicket and scrub environments in this subregion. Ileret. The sampled Ileret soils are thin (40e60 cm), imma- ture vertisols similar to Dite paleosols (Wynn, 2000) based on their crumb ped structures, shallow calcic horizons, and their stratigraphic positions between channelized tuffs. Samples are from one depositional environment and fluctuate in d13C values between 6 and 4& (Fig. 8), constituting approximately 40e60% of grassy vegetation. We interpret the savanna category based on isotopic results primarily as low-tree shrub, with few woodland, and thicket and scrub environments. Koobi Fora Ridge. Paleosols from the Koobi Fora Ridge vary markedly in morphology but generally show a high clastic content likely due to proximity to a fluctuating lake margin (Feibel, 1988). One soil in particular, the Lorenyang pedotype

Table 4 Results of one-way ANOVA and Kruskal-Wallis rank sum test of subregion groups One-way ANOVA DF SS MS F P Total 201 532.00 2.65 21.47 <0.0001 A 3 130.58 43.53 Error 198 401.42 2.03 Kruskal-Wallis K-W statistic P rank sum test Fig. 6. Nonparametric box and whisker plots of d13C values (&) by 100-kyr 63.20 <0.0001 interval groups. 568 R.L. Quinn et al. / Journal of Human Evolution 53 (2007) 560e573

Table 6 Results of Tukey’s all pairs comparison by subregion groups Comparison Mean Difference jqj P 95% CL Koobi Fora vs. Il Dura 2.32 5.83 0.0003 0.86e3.79 Koobi Fora vs. Karari 1.54 9.52 <0.0001 0.95e2.13 Koobi Fora vs. Ileret 0.03 0.16 0.9994 0.70e0.77 Ileret vs. Il Dura 2.29 5.54 0.0007 0.78e3.81 Ileret vs. Karari 1.51 7.75 <0.0001 0.80e2.22 Karari vs. Il Dura 0.78 1.98 0.5027 0.67e2.23 shows the presence of thick (w0.5 m) columnar and slickensided carbonate horizon, suggesting a stable land surface and pedogenesis for a substantial period of time (e.g., Wynn and Feibel, 1995). Within the Okote Tuff Complex, soils are poorly developed and characterized as Dite paleosols similar to those found in the Ileret subregion. d13C values from mor- phologically different pedotypes preserved on the Karari Ridge do not show the grassland expansion trend. Before 1.9 Ma, the smooth curve ﬁt of d13C values ﬂuctuates between 8 and 6& (30e40% grasses), placing the subregion within the savanna woodland and thicket and scrub categories (Fig. 8). We observe a 1& increase after 1.8 Ma showing an excursion to the low-tree shrub category. By 1.75 Ma environments have returned to the woodland and thicket savannas. Fig. 7. Nonparametric box and whisker plots of d13C values (&) by subregion groups.

Controls on d13C values and floral compositions Ileret paleosols are poorly developed and interpreted as indic- Paleogeographical, sedimentological, and paleohydrological ative of floodplain areas laden with small distributary channels changes related to the precursors of Lake Turkana and the Omo of the ancestral Omo River, supporting low-tree shrubs and River influenced floral compositions and distributions through grasses. Our section from the Koobi Fora Ridge begins just af- time. We suggest that differences in the isotopic ratios of pale- ter 1.9 Ma, when a deltaic system begins to form along north- osol carbonates were controlled by the context of paleosols in ern margins of Lake Lorenyang. Oscillations of lake-level relation to the inherent array of moisture and depositional after 1.9 Ma, coupled with high rates of clastic input from del- conditions found in fluviolacustrine systems. We discuss four taic sedimentation, may have reduced suitable lake-margin paleogeographic factors: water availability, sediment accumu- habitats for woodland floral communities, thus increasing the lation rates, ratios of subaqueous:subaerial land, and open and proportion of grasses. As compared with other subregions, closed basin conditions. We also view tectonics (subsidence the unique proximity to basin margin channel systems and rates) as an important force on paleogeographic change that paleogeographic history of the Karari Ridge may indicate a dif- can lead to differential distributions of the isotopic data. As ferent water availability regime and explain the exceptionally compared with paleoclimate, these tectonic-subsidence controls low d13C values. Marginal systems likely provided a consistent on paleogeographic change may have exerted an equal or supply of water to C3 floral communities, buffering against greater influence on the observed floral patterns. variations spurred by paleographic shifts from lacustrine- related environments at w1.9e1.8 Ma to fluvial channel and Paleogeographic controls on subregional d13C values floodplains after 1.8 Ma. Observed paleogeographic patterns influenced preservation We assume simple forcing relationships between paleogeo- of the soil carbonate and the manner in which vegetation is re- graphic changes and floral distribution. Wooded vegetation corded in isotopic values. Pedogenic carbonate formation is in- typifies gallery forests clustered along channel banks of the hibited near channel and lake-margin areas with high water modern Omo River. Tropical grasses are found in the lateral tables and sedimentation rates (van Breemen and Buurman, floodplain environments. Mixed communities are generally 2002). As a result, our ability to reconstruct vegetation is limited between the two areas in overbank environments with small to the drier subaerial portions of the landscape with carbonate channels dispersed within a flood basin (Carr, 1976). Results precipitation. Sampling strategies for isotopic records from pa- of C3-C4 patterns by subregion illustrate these predictions. leosol carbonates may be biased to strata of floodplain environ- Our results of the Il Dura subregion agree with the paleo- mentsdthe preferred habitats for tropical grasses. Landscapes geographic location near the axial meandering proto-Omo with a high proportion of floodplains to channels may skew pa- River, where a higher proportion of C3 vegetation is expected. leosol isotopic records toward a C4 signal. Consequently, R.L. Quinn et al. / Journal of Human Evolution 53 (2007) 560e573 569

associated floral compositions. Tectonism was a strong driver of paleogeographic changes in the Koobi Fora region during the Plio-Pleistocene (Brown and Feibel, 1991; Feibel et al., 1991). We propose an integrative explanation of paleogeographic evolution that incorporates climate and tectonics after Carroll and Bohacs (1999) and Withjack and colleagues (2002). These authors suggest that diachronic changes in the aerial distributions, facies, and environments of lake basins result from the combined effects of tectonic subsidence and climate and their control on the rate of potential accommodation space (mostly tectonic subsidence) and the delivery of water and sediment to a basin (mostly climate). Volcano-tectonic events at w2.5 Ma changed subsidence and basin accommodation rates and contributed to the transformation of the Turkana Basin from a hydrologically closed lacustrine-dominated basin to an open fluvial one over 2.0e1.5 Ma. We suggest that moisture and floral patterns are explained by resultant shifts in paleogeography, proportions of subaqueous landscapes relative to subaerial landscapes, and hydrologically open or closed basin conditions. Plio-Pleistocene grassland expansion at Koobi Fora is linked with expanded subaerial floodplains and expedi- ent basin water loss from the through-flowing ancestral Omo River, ultimately generated by tectonic-subsidence factors. According to the models of Carroll and Bohacs (1999) and Withjack and others (2002), a basindwhich is underfilled with water and sediment (subsidence-accommodation > sediment-water input)dcontains a closed lake system with inputs from rivers, runoff, and precipitation but outputs only by Fig. 8. d13C(&) vs. age (Ma). Pedogenic carbonate d13C values of the Koobi means of evaporation. A balance-filled basin (subsidence- Fora Formation between 2.0 and 1.5 Ma. Data points are separated by subre- accommodation ¼ sediment-water input) commonly contains B > , gion: Il Dura (), Karari Ridge ( ), Koobi Fora Ridge ( ), Ileret ( ). Black an open lake system with relatively equal amounts of sediment lines represent 10% weighted smooth curve fit of points by subregion: Karari Ridge (solid line), Koobi Fora Ridge (broken line), Ileret (broken line with and water input and output via rivers. An overfilled basin dots). Savanna categories are taken from Wynn (2000, 2001). (subsidence-accommodation < sediment-water input) is characterized by more water and sediment than the basin can accommodate and, consequently, is easily transformed into a through-flowing fluvial system. carbonate isotopic values will not capture the pure C3 values in the savanna environment, but rather combine values from fluc- The presence of major unconformities in the Koobi Fora tuating floral compositions as the meander belt avulses (Levin Formation (Fig. 4) is associated with volcano-tectonic et al., 2004). For comparison, lacustrine systems have a compar- down-warping of the Turkana Basin and associated uplift atively high proportion of subaqueous to subaerial landscapes, of the Ethiopian Plateau in southern Ethiopia (Brown and suggesting a low amount of suitable substrate for paleosol and Feibel, 1991; Feibel et al., 1991). Our stratigraphic study pedogenic carbonate formation. These systems often have rela- interval begins just above the level of the upper Burgi un- tively faster sedimentation rates, which are not conducive for conformity (w2.5e2.0 Ma). This hiatus was generated by the prolonged development of soils (Jenny, 1980; Retallack, tectonic disruption in the basin evident by the Stephanie 2001, van Breemen and Buurman, 2002). At 1.8 Ma, river avul- Uplift to the northeast and gentle tilting of the strata in sion away from the eastern basin (e.g., Brown and Feibel, 1991; the Koobi Fora region associated with an interval of synde- position (Feibel, 1997). These structural movements induced Feibel et al., 1991) may have dampened C3 contributions to soil carbonate isotopic values. Subsequent emplacement of smaller by volcano-tectonics increased subsidence rates and accom- fluvial-distributary systems and expanded floodplain environ- modation space in the Turkana Basin, leading to underfilled ments (Fig. 2) may have further reduced the signal of wooded conditions. Our interpretation of the Koobi Fora region as vegetation. an underfilled basin with a large degree of potential accommodation space at w2.5e2.0 Ma is supported by the plot of sedimentation rates for the entire Koobi Fora Formation Basin scale paleogeographic and tectonic factors (Fig. 4), which suggests an exponential and positive increase in sediment accumulation beginning just after Basin scale factors of paleogeographic change also influ- 1.9 Ma and ending around 1.7 Ma. Structural movements enced the character of paleosol and carbonate formation and at about 2.5 Ma resulted in increased subsidence rates and 570 R.L. Quinn et al. / Journal of Human Evolution 53 (2007) 560e573 ample space to accommodate a large influx of sediment into moisture and floral patterns are controlled by paleogeography, the basin between 1.9e1.7 Ma. proportion of subaqueous relative to subaerial landscapes, and Prior to about 1.9 Ma, the Turkana Basin had a fluvial out- hydrologically open or closed basin conditions. In half-graben let to the southeast but was occupied by a relatively deep and basins with lakes, there are high proportions of subaqueous broad lake, Lake Lorenyang (Feibel, 1994). The sedimentation landscapes relative to subaerial settings (cf., Gawthorpe rate curve (Fig. 4) may indicate that there were relatively few et al., 1994; Leeder et al., 1998; Gawthorpe and Leeder, sediments accumulating under a low proportion of sediment 2000). Our findings of more closed savanna environments supply relative to accommodation rate; sedimentation was out- are associated with the lake basin pooling water and a lack paced by the rate of subsidence generation. Evidence from pa- of suitable subaerial substrate for well-developed pedogenic leogeography and sedimentation rates suggests that the basin carbonates and grassland communities on which to form. In was balanced-filled or underfilled with sediment and overfilled comparison, we associate through-flowing fluvial intervals with water (cf., Carroll and Bohacs, 1999; Withjack et al., with lower proportions of subaqueous to subaerial landscapes 2002). and hydrologically open conditions (Gawthorpe et al., 1994; The period between 1.9 and 1.7 Ma was a highly variable Leeder et al., 1998; Gawthorpe and Leeder, 2000). Indications time in the basin with respect to depositional environments; of grassland expansion during these intervals may stem from lake waters became shallower, lake level frequently oscil- pronounced water loss in the basin under an open hydrology. lated, and deltaic and fluvial environments were active during The fluvial intervals are coeval with isotopic evidence of this interval (Fig. 2). Beginning at 1.9 Ma and continuing to grassland expansion and, by inference, increased aridity. We 1.7 Ma, the sedimentation rate curve shows a shift to a near interpret this finding as a result of a very low proportion of vertical trend that crosses to above the extrapolated mean subaqueous relative to subaerial landscapes coupled with ex- sedimentation rate line (Fig. 4). These patterns may indicate pedient basin water loss from the through-flowing ancestral an ‘‘instantaneous’’ rate of sediment accumulation under Omo River. The rise of the subaerial fluvial landscapes re- conditions of a more equal or lower proportion of accommo- sulted in expansion of preferred floodplain habitats for C4 dation rates relative to sediment supply rates as compared to vegetation. earlier times; sedimentation kept pace with or exceeded subsidence. The sedimentation rate curve and paleogeogra- Climatic factors phy support the interpretation that after 1.7 Ma the basin was progressing toward an infilled status, overfilled after Overall, our results show a shift from C3 vegetation and de- the models of Carroll and Bohacs (1999) andWithjackand creased evaporation to higher C4 proportions and evaporation others (2002). increase during 2.0e1.5 Ma, which we can ultimately link to After 1.7 Ma, the sedimentation rate curve takes a sharp tectonic-subsidence, accommodation space, basin infilling, turn back towards the mean sedimentation rate line (decrease and their control on paleohydrology. However, the establish- in the rate of sediment accumulation), which may suggest ment of grasslands recorded in the entire basin and East Africa a lower proportion of accommodation relative to sediment after 1.8 Ma, initially proposed by Cerling (1992), suggests an supply rates as compared with the 1.9e1.7 Ma period; sedi- overprint of a large-scale influence often attributed to global mentation outpaced subsidence. Brown and Feibel (1991) sug- climate change. Changes in the isotopic character of water gest that during 1.7e1.4 Ma the Turkana Basin had very few and vegetation in the Koobi Fora region during our study in- landscapes with lacustrine-related sedimentary environments, terval has been shown as a response to global climate change but there was a preponderance of fluvial channel and flood- due to glacial activity in the Northern Hemisphere (see deMe- plain environments. The protracted transformation of the basin nocal, 2004 for a recent review), and as a result, may explain from a lacustrine-dominated setting to one with a through- our observed shift in d13C values from 2.0 to 1.75 Ma. Based flowing river suggests infilling under a regime of progressively on the climate forcing model for the Turkana Basin proposed less tectonic subsidence (Carroll and Bohacs, 1999; Withjack by Lepre et al. (2007), we would expect to observe cycles in et al., 2002). water availability and, therefore, fluctuations in floral commu- Our examined stratigraphic interval brackets at least four nities coinciding with orbital periodicities. We do not observe successive times of differential sedimentation in the Plio- periodicities in our isotopic results. This is in part due to the Pleistocene Turkana Basin, including periods of lacustrine, preservation of soils though time (i.e., there are large gaps in deltaic, shallow/fluctuating lacustrine, and fluvial channel the stratigraphic record hindering continuous isotopic records). and floodplain deposition (Fig. 2BeE). Previous research If climate is producing recurring sedimentary packages (e.g., demonstrates that these periods are tracking the transformation deep lake, shallow lake, beach, paleosol), soil carbonates are of the basin from a hydrologically closed lake basin to one that recording only one segment of the orbital cycle. Moreover, is open with a through-flowing ancestral Omo River (Brown soil carbonate isotopes are sensitive to local vegetation change, and Feibel, 1991). Shifts in isotopic ratios and inferred floral which potentially masks floral changes on orbital timescales. patterns are coeval with these periods (Figs. 5 and 6). We in- Additional work with complementary isotopic proxy records terpret these data as indicating relatively wet conditions during of environmental change (e.g., lacustrine invertebrates) may lacustrine and deltaic intervals and dry conditions during shal- increase our understanding of climate’s role on the Turkana low/fluctuating lacustrine and fluvial intervals. We suggest Basin. R.L. Quinn et al. / Journal of Human Evolution 53 (2007) 560e573 571

Implications for hominin environments at Koobi Fora Fora from 2.0e1.5 Ma was ultimately related to volcano- tectonic events at w2.5 Ma in the northern portion of the Our d13C values indicate grassland expansion from 2.0 to eastern branch of the East African Rift. Down-warping of the 1.75 Ma, trending from more closed savanna woodlands to Turkana Basin and the associated uplift of the Ethiopian more open low-tree shrub savannas. Environments fluctuated Plateau, the subsequent changes in subsidence and accommo- within the low-tree shrub savanna category from 1.75 to dation, and basin infilling gradually transformed the basin 1.5 Ma. We propose that habitat fragmentation increased; from a lake system to a flow-through fluvial system between that is, wooded environments are maintained, and the mosaic 2.0 to 1.5 Ma. As a result, there was a through-time decrease and grassland pieces are expanded. Behrensmeyer and others in the residence time of Omo River water in the basin and an (1997) interpreted faunal turnover as a slow process in the Tur- expansion of subaerial landscapes. Climate may have served kana Basin between 2.5 and 1.8 Ma rather than a short-term to modify or exaggerate the effects of tectonic perturbations pulse coinciding with the onset of Northern Hemisphere Gla- by altering rainfall quantities delivered to the catchment areas ciation between 2.8 and 2.5 Ma. Our change in average floral of the ancestral Omo River. The combined effects of these composition between 2.0 and 1.75 Ma coincides with one of phenomena caused a decrease in wooded habitats by increasing the highest faunal turnover pulses in the Turkana Basin (Beh- basin water loss and expanding subaerial floodplain habitats rensmeyer et al., 1997; Bobe and Behrensmeyer, 2004). Based suitable for grassland communities. Habitat fragmentation re- on our interpretation of paleogeographic controls on floral dis- sulting from basin-wide evolution may have created additional tribution, the timing of faunal turnover is coeval with the trans- niche spaces and scavenging opportunities for sympatric formation of the basin from a closed lacustrine-dominated hominins. Based on our subregion environmental reconstruc- basin to an open fluvial setting. tions and the distribution of archaeological traces at Koobi We interpret our results to reflect habitat partitioning by Fora, we interpret that tool-using hominins preferred relatively subregion possibly due to differential water supply. Along more closed and well-watered environments. the Karari Ridge, the axial system and possibly marginal rivers sustained wooded environments when other subregions in the Acknowledgements basin were beginning to show grassland spread. Higher fre- quencies of archaeological evidence in the KBS and Okote We would like to thank Beth Christensen and Mark Maslin Members along the Karari Ridge have been suggested as indi- for organizing the AGU session that gave rise to this volume. cators of home range expansion and excursions into marginal We thank Dr. Idle Farah and members of the Kenya National and drier habitats by early African Homo erectus (Rogers Museums for logistical support of data collection. Dr. Jack et al., 1994). If preservational differences are negligible Harris and the Koobi Fora Field School staff provided vital in- amongst the subregions, the Karari Ridge as reconstructed frastructure for portions of fieldwork at Koobi Fora. Special by d13C values of pedogenic carbonates was a wetter and thanks to Jerry Delaney for help with CL work. The comments more wooded environment from 2.0 to 1.5 Ma. Rather than and suggestions of Naomi Levin and Martin Trauth contrib- the hominins venturing into marginal and drier habitats, the uted greatly to this paper. This work was supported by the abundance of archaeological sites and hominin paleontological Center for Human Evolutionary Studies (CHES) at Rutgers material may attest to habitat preference in well-watered areas University and by the National Science Foundation (BSC- with woodland savanna and low tree-shrub grassland compo- 0218511, C.S. Feibel). sitions. Habitat partitioning by subregion may have created additional niche spaces for sympatric Homo and Paranthropus (Bromage and Schrenk, 1995; Wood and Strait, 2004) and References possibly higher percentage of ecotonal boundaries between wooded and grassland environments, which may have afforded Antoń, S.C., Swisher III, C.C., 2004. Early dispersals of Homo from Africa. early Homo increased scavenging opportunities (after Blumen- Ann. Rev. Anthropol. 33, 271e296. schine, 1987). 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