Sapropels and the Age of Hominins Omo I and II, Kibish, Ethiopia

Home , Lake Turkana, Omo River, Turkana Basin

Journal of Human Evolution 55 (2008) 409–420

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Sapropels and the age of hominins Omo I and II, Kibish, Ethiopia

Ian McDougall a,*, Francis H. Brown b, John G. Fleagle c a Research School of Earth Sciences, The Australian National University, Canberra ACT 0200, Australia b Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112, USA c Department of Anatomical Sciences, Stony Brook University, Stony Brook, NY 11794, USA article info abstract

Article history: The provenance and age of two Homo sapiens fossils (Omo I and Omo II) from the Kibish Formation in Received 24 April 2007 southern Ethiopia have been much debated. Here we conﬁrm that Omo I and the somewhat more Accepted 15 May 2008 primitive-looking Omo II calvariae are from similar stratigraphic levels in Member I of the Kibish For- mation. Based on 40Ar/39Ar age measurements on alkali feldspar crystals from pumice clasts in the Keywords: Nakaa’kire Tuff, a tuffaceous bed in Member I just below the hominin levels, we place an older limit of 40 39 Ar/ Ar dating 198 14 ka (weighted mean age ¼ 196 2 ka) for the hominins. A younger limit of 104 7 ka (weighted Ethiopia mean age ¼ 104 1 ka) is provided by feldspars separated from pumice clasts in the Aliyo Tuff in Kibish Formation Member III. Geological evidence indicates rapid deposition of each member of the Kibish Formation, Modern hominins 40 39 Mediterranean sapropels concurrent with deposition of sapropels in the Mediterranean Sea. The Ar/ Ar age measurements, Tephrostratigraphy together with correlations with sapropels, indicate that the hominin fossils are close in age to the older limit. Our preferred estimate of the age of the hominins is 195 5 ka, making them the earliest well- dated anatomically modern humans yet described. Ó 2008 Elsevier Ltd. All rights reserved.

Introduction from small pumice clasts in a tuffaceous bed, the Nakaa’kire Tuff, of Member I, and from the Aliyo Tuff in Member III of the Kibish Among the finds of the International Omo Research Expedition Formation, providing secure, numerical age limits for the hominin to southern Ethiopia (1967–1974) were two hominin calvariae, fossils. The present paper is a more detailed account of results given Omo I and Omo II, together with parts of the postcranial skeleton of in McDougall et al. (2005) and includes additional age data from Omo I (Day, 1969; Day and Stringer, 1982, 1991; Day et al., 1991). pumice clasts in the Aliyo Tuff in Member III of the Kibish Forma- Although both calvariae were considered to belong to Homo sapi- tion overlying the Mursi Formation. Coordinates quoted in the text ens, Omo I appears to be more modern in aspect than Omo II. The and in the tables are from GPS measurements using WGS84 as the localities of these hominin fossils are about 3 km apart on either datum. side of the Omo River, but both were thought to be from Member I of the Kibish Formation (Butzer, 1969). Although there has been Stratigraphy much debate (e.g., Howell, 1978; Smith et al., 1989), our evidence indicates that Omo I and Omo II are derived from similar strati- The Kibish Formation, defined by Butzer and Thurber (1969), graphic levels in Member I, as was originally reported. There has consists of a cumulative thickness of about 100 m of essentially also been much interest in, and controversy about, the age of these flatlying, tectonically undisturbed, relatively unconsolidated sedi- fossils, which possibly represent some of the earliest modern ments deposited mainly in deltaic environments. It is the youngest humans. Butzer et al. (1969) quoted a U-series disequilibrium age of exposed significant sedimentary sequence in the Omo-Turkana 130 5kaon Etheria from a layer near the Omo I site, but they Basin, with the youngest unit extending into the Holocene and doubted the reliability of the age. Here we summarize the stratig- deposited during the most recent highstand of Lake Turkana. The raphy of the Kibish Formation, which is covered in much greater Kibish Formation was divided into Members I–IV, recording four detail in a companion paper by Brown and Fuller (2008), and report 40 39 distinct intervals of deposition at times when the northern margin results of Ar/ Ar age measurements on alkali feldspar crystals of Lake Turkana lay about 100 km north of its current position. Lake Turkana lies in a closed basin, with the Omo River, draining the highlands of Ethiopia, providing the main inflow to the lake (Fig. 1). Current deposition occurs principally within the lake, * Corresponding author. E-mail addresses: [email protected] (I. McDougall), [email protected] where active subsidence continues (Dunkelman et al., 1988, 1989), utah.edu (F.H. Brown), jfl[email protected] (J.G. Fleagle). and in the delta complex of the Omo River (Butzer, 1970).

Fig. 1. Map showing the distribution of the Kibish Formation (shaded) in the Lower Omo Valley, southern Ethiopia, after Davidson (1983). Inset on lower left shows locations of Omo I and Omo II, measured sections, and dated samples.

Outcrops of the Kibish Formation are confined mainly to the members by as much as 25 m prior to deposition of the next valley eroded by the Omo River as it skirts the Nkalabong Range younger member. The Kibish Formation is deposited un- (Fig. 1). The plain west of the outcrops has an elevation of about conformably or disconformably upon the Nkalabong Formation or 445 m, 10–15 m below that of the watershed between the Omo and unconformably on the underlying Mursi Formation (Butzer and Pibor-Sobat drainages of the Nile River (Fuchs, 1939; Butzer et al., Thurber, 1969). The Nkalabong Formation correlates with the 1972), providing a limit to the maximum level of Lake Turkana, the Shungura Formation (de Heinzelin, 1983; Brown and Fuller, 2008). present surface of which is at an altitude of about 360 m (Brown The youngest member of the Mursi Formation is a basalt lava and Fuller, 2008). Intermittent hydrologic links between the Omo- (Butzer, 1969, 1976), which yields a mean K/Ar age of 4.1 0.1 Ma Turkana and Nile systems are apparent from Nilotic molluscs, fish, (Fitch and Miller, 1976; Brown et al., 1985; Haileab et al., 2004). and reptiles in the Turkana Basin sedimentary sequence (Ara- Alkali feldspars from pumice clasts in the Chibele Tuff of the mbourg, 1935, 1943, 1947; Fuchs, 1939; Butzer, 1971). In the Kibish Nkalabong Formation yield concordant ages of 1.72 0.02 Ma area, the Omo River is at an altitude of about 400 m, so that the (McDougall and Brown, 2006). Current information indicates that Kibish Formation outcrops are confined to a vertical height of only the unconformity or disconformity between the Nkalabong For- about 45 m (Brown and Fuller, 2008). mation and the Kibish Formation represents about 1.5 Ma of time. Each member of the Kibish Formation is separated from the A composite stratigraphic section, made up of the thickest-mea- next overlying member by an erosional break (disconformity) sured examples of each member of the Kibish Formation is shown produced by downcutting of the Omo River into the underlying in Fig. 2. Download English Version: https://daneshyari.com/en/article/4557025

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