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Obsidian types from Holocene sites around Lake Turkana, and other localities in northern Kenya
Journal of Archaeological Science 38 (2011) 1371e1376
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Obsidian types from Holocene sites around Lake Turkana, and other localities in northern Kenya Barbara P. Nash a, *, Harry V. Merrick b, Francis H. Brown a a b
Department of Geology and Geophysics, University of Utah, 115 S. 1460 E, Salt Lake City, UT 84112-0111, USA Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Boston, MA, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 August 2010 Received in revised form 4 January 2011 Accepted 6 February 2011
Obsidian has been widely used by early Holocene hunter-gatherers and succeeding Pastoral Neolithic peoples in northern Kenya. Here we report the results of over 2000 electron microprobe analyses of artifactual and non-artifactual obsidian from the greater Lake Turkana region. Of the 15 compositional types of obsidian observed, a preponderant type is widespread across the region from the Barrier in the south to Ileret in the north and east as far as Kargi. This obsidian is the principal type at Lowasera and most Pastoral Neolithic sites, including the Jarigole Pillar site and Dongodien (GaJi4). The source of this obsidian is not known, but based on its distribution the source may be located on the Barrier or in the Suguta Valley immediately to the south of Lake Turkana. Although there are several possible sources of local obsidian identified for minor types, in stark contrast to the central part of the Kenyan Rift, major sources of obsidian available for artifact manufacture are not known in the Lake Turkana region. The lack of obsidian from demonstrable Ethiopian Rift and central Kenyan Rift sources, and the absence of obsidian with compositions found at the Turkana area sites in assemblages in the central part of the Kenyan Rift suggests that the earlier Pastoral Neolithic peoples around Lake Turkana interacted with each other, but perhaps not as strongly with people farther south along the Rift Valley, even as herding practices were expanding to the southward into central Kenya. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Kenya Lake Turkana Jarigole Obsidian Artifact Electron microprobe Pastoral Neolithic
1. Introduction For many years, obsidian has been known to have been occasionally utilized for tool manufacture by the early Holocene huntergatherers and the succeeding Pastoral Neolithic peoples in northern Kenya (Barthelme, 1977, 1985; Phillipson, 1977; Nelson, 1995), but little was known of its chemical composition and source. Merrick and Brown (1984a,b), based on the analysis of a very limited number of elements, reported preliminary results on a small number of obsidian artifacts from several sites in the Lake Turkana region and suggested on the then available information about sources and their chemistries, that only local sources were likely being used. The purpose of this paper is to report the results of a much more extensive investigation of the chemical composition of obsidian pieces, artifactual and non-artifactual, from the greater Lake Turkana region, and to suggest possible areas for the geologic source of some of the obsidians used for artifact manufacture. Unlike some areas of the central Ethiopian and central Kenyan Rifts where geological sources of obsidian are numerous and yield * Corresponding author. Tel.: þ1 801 581 8587; fax: þ1 801 581 7065. E-mail address: [email protected] (B.P. Nash). 0305-4403/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jas.2011.02.001
abundant high quality obsidian for artifact manufacture, the Lake Turkana region in northern Kenya does not appear to contain many sources of obsidian. Geological survey in the region suggests that sources of obsidian are few, relatively localized geographically and associated with Miocene through Pleistocene pyroclastic flows and deposits, the contained obsidian for the most part being in the form of small pebble sized lapilli. To date no obvious ‘quarry’ localities have been identified for any potential northern Kenyan source of obsidian. The local scarcity of obsidian has several effects on the Turkana region’s Holocene archaeological assemblages. Typically, multiple materials may be used for artifact manufacture including chert, chalcedony, fossil wood, quartzite, lavas and obsidian. Among obsidian artifacts, due to the small size of the source material regionally available, size tends to be microlithic and the artifacts often show traces of cortex indicative of being created from pebble size nodules. Finally, artifacts of obsidian, even when they may be proportionally the dominant type, are not absolutely numerically abundant. Because obsidian sources appear to be so few in the region and are, as yet, not well documented nor their chemistries well characterized, we have chosen to examine and characterize a sample of artifactual and non-artifactual obsidians in hopes of identifying
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potential sources in the region as a first step in examining patterns of source utilization within the region during Later Stone Age and Pastoral Neolithic times.
Table 1 Sampled Turkana region sites and localities and obsidian types present. Arranged by probable cultural association and location (north to south). Site and locality
2. The obsidian sample The majority of analyses reported here are of obsidian artifacts sampled from excavations at Holocene hunter-gatherer/fisher and Neolithic pastoralist sites on the eastern side of Lake Turkana. The artifact collections from these sites, including the sampled artifacts, are housed in the National Museums of Kenya, Nairobi. The sampled hunting/gathering/fishing sites include GaJj1 (Barthelme, 1985), and Lowasera (Phillipson, 1977). Sampled Pastoral Neolithic sites associated with the Nderit ceramic tradition (includes Barthelme’s (1985) Ileret tradition) include the Jarigole Pillar site (GbJj 1) south of Allia Bay (Nelson, 1995), the Dongodien area sites, GaJi 4 and GaJi 2, and the Ileret Stone Bowl site, FwJj 5 (Barthelme, 1985). We also report analyses of a few artifacts of uncertain cultural association and unmodified pieces of obsidian collected from the surface at scattered localities in the Lake Turkana region. Table 1 lists the sites and localities sampled for archaeological and non-archaeological obsidians and Fig. 1 illustrates their locations in the Turkana area. 3. Analysis For electron microprobe analysis small fragments were removed from each of the artifacts, and a white spot was painted onto each artifact held in the National Museums of Kenya to indicate where the fragment was removed. The fragments were mounted in resin plugs, ground flat, polished, and coated with a thin layer of carbon. Analysis was done on a Cameca SX-50 electron microprobe at the University of Utah using techniques described in Nash (1992). Standards used are identical to those used for analysis of glass shards from volcanic ash layers in the Turkana Basin given by Brown and Fuller (2008). Three to eight analyses for 13 elements were made on each of the obsidian samples; averaged analyses are presented in Table 2. All 2079 individual analyses are given in Supplementary Table 1. Barium was measured, but because none of the samples contains as much as 0.01%, it is not reported. Geologically younger obsidian, either in the form of artifacts or loose material on the surface, has low water contents and thus analytical totals are close to 100%. Older hydrated obsidians (Il Derati and the Suregei samples) have totals ranging from 92 to 97% that are also typical for hydrated glass shards in volcanic tuffs (cf., Brown and Fuller, 2008). 4. Results and discussion 4.1. Obsidian types present in the Lake Turkana region Fifteen distinct chemical types of obsidian can be provisionally recognized among the sampled artifact and non-artifactual specimens (Table 2). For convenience of reference we have labeled these 15 obsidian types with the designations Northern Kenya types A to O (NK-A to NK-O) and only when we are reasonably confident of the geographic source have we added a geographic name as an identifier. Eight of these obsidian types, including the two most commonly used types (NK-A and NK-B) are known from artifacts only. Seven of the types are known from samples of unmodified obsidian, often collected from eroded exposures on or near tuffaceous deposits, and thus these localities might be considered to represent potential ‘sources’. However at present only one of these types (NK-N ‘Suregei B’) is thought to have been potentially used for artifact manufacture.
# Sampled
Obsidian type present and number of pieces
Early/Middle Holocene hunting and fishing GaJj1 2 NK-A Lowasera 16 Unit 2 ¼ 8 NK-A (5) NK-B (3) Unit 4 ¼ 8 NK-A (5) NK-B (3) Pastoral Neolithic (Nderit tradition sensu lato) FwJj 5 Ileret Stone 5 NK-A (2) NK-C (1) NK-E (1) Bowl Site GaJi 2 (just west of 3 NK-A (1) NK-C (1) NK-D (1) Dongodien) GaJi 4 Dongodien 20 NK-A GbJj1 Jarigole Pillar 228 NK-A (226) NK-B (2) Site Pieces of uncertain artifactual and/or cultural association (all surface finds) Omo (ETH86-01) 1 NK-N (?) Ileret (K85-2557) 1 NK-A Karari 1 NK-A GaJi 16 Lorenyang 1 NK-F Lokalalei 3 NK-A (2) NK-H (1) Lomekwi 1 NK-I Area 206, K. Fora 1 NK-A Sibilot 2 NK-A Il Moiti 1 NK-A Kalodirr 1 NK-A Moruarot 2 NK-A (1) NK-B (1) Kalatum 1 NK-A Lowasera Gorge 1 NK-A Loiyengalani 2 NK-A Gatab airstrip 3 NK-A Gatab Junction 7 NK-A Serima 3 NK-A Kargi 4 NK-A Barrier 5 NK-A Unmodified obsidian chunks (nodules, pebbles, etc.) (potential “source” material) Suregei 5 NK-M (2) NK-N (2) NK-O (1) North Island 1 NK-J Il Derati (K82-892) 2 NK-G Suguta (MER-93) 1 NK-L Samburu Hills 2 NK-K (1) NK-L (1)
Table 2 is arranged so that the preponderant types of obsidian are listed first. The most common type, NK-A, recognized in 21 locations, is a well-defined compositional type that is found in archeological sites and as surface occurrences from the Barrier, the volcanic ridge separating the southern end of the Turkana basin from the Suguta valley, north to Ileret east of Lake Turkana, and as far north as Lokalalei west of Lake Turkana. Its distribution extends eastward to Gatab on the southern slopes of Mt. Kulal and at least as far east as Kargi (Fig. 1). Not only is it the principal obsidian type (99%) at the Jarigole Pillar site, but it is also the principal type at GaJi 4 and both excavated levels at Lowasera. It is also present at GaJi 1, GaJi 2, and FwJj 5. The source of this obsidian is not certainly known, but a strong case may be made for a southern Turkana region location. This type was recognized as the regionally dominant obsidian type in Merrick and Brown’s (1984b) pioneering study (their “Petrological Group 1”) and, in hindsight, was erroneously attributed to a Suregei area source. Their assignment of artifacts to this source, whose chemistry was then known only from Watkins’ (1981) single analysis, was based on characterization using only three elements (Fe, Ca and Ti) that clearly proved inadequate to discriminate between northern Kenyan area sources. None of our analyses of unmodified obsidian since sampled from the Suregei region (Types NK-M, NK-N and NK-O) match this type. A more
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Fig. 1. Index map of sample locations in the Lake Turkana region. The shaded area represents elevations between the current lake level and 460 m within which the majority of the sample localities lie. Brown and Fuller (2008) estimate the elevation of the outlet to the Nile River to be 457 m, so 460 m is near the maximum late Pleistocene and Holocene level of Lake Turkana.
southerly location for the source appears likely on the basis of several observations. First, it appears that NK-A is the dominant or exclusive type present in all southern localities. Only in the more northerly areas do the minor obsidians appear with any regularity (Fig. 1). Second, the density of obsidian fragments exposed on the modern regional land surface is strikingly different from one area to another. Whereas one can spend an entire day in the field and see only a fragment or two in the more northerly greater Koobi Fora region,
the abundance is markedly greater toward the south end of the lake, and is quite high on the Barrier, a linear east-west volcanic structure formed by multiple volcanic centers, which separates Lake Turkana from the Suguta Valley. Obsidian artifacts are also apparently encountered with regularity to the south of the Barrier in the Suguta Valley (Garcin et al., 2009). From these admittedly unsystematic observations we suggest that the source of the dominant obsidian is likely to be located on the Barrier or in the Suguta Valley.
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Table 2 Average electron microprobe analyses of artifact obsidians (values in weight percent). Locality
# of pcs
N
SiO2
TiO2
ZrO2
Al2O3
Fe2O3a
MnO
MgO
CaO
Na2O
K2O
F
Cl
Totalb
Area 102, K. Fora (GaJj 1) Area 206, K. Fora Barrier Dongodien area (GaJi 2) Dongodien (GaJi 4) Gatab airstrip Gatab Junction Il Moiti Ileret (FwJj 5) Ileret (KBS-2557) Jarigole (GbJj 1) Kalatum Kalodirr Karari Kargi Loiyengalani Lokalalei Lowasera 1 Unit 4 Lowasera 1 Unit 2 Lowasera Gorge Moruarot Serima Sibilot Laikipia (Kisima)
2 1 5 1 20 3 7 1 2 1 226 1 1 1 4 2 2 5 5 1 1 2 2 1
16 3 59 5 214 24 56 3 16 3 1372 3 3 12 39 6 6 40 39 3 3 6 6 5
73.3 73.9 74.6 73.6 72.7 73.3 73.4 74.1 73.4 73.8 73.6 74.4 74.2 74.6 74.3 74.6 74.2 73.1 74.3 74.2 74.0 73.2 73.9 73.2
0.28 0.25 0.27 0.28 0.29 0.28 0.28 0.26 0.29 0.27 0.28 0.27 0.28 0.25 0.26 0.28 0.28 0.27 0.28 0.28 0.26 0.31 0.28 0.27
0.11 0.10 0.14 0.09 0.12 0.12 0.11 0.05 0.15 0.08 0.10 0.08 0.06 0.12 0.11 0.07 0.06 0.12 0.10 0.09 0.07 0.07 0.06 0.08
11.1 11.1 11.3 11.1 11.1 11.0 11.1 11.3 11.1 11.1 11.1 11.2 11.2 10.8 10.9 11.1 11.1 11.1 11.0 11.2 11.1 11.2 11.1 11.4
4.36 4.42 4.39 4.39 4.44 4.38 4.39 4.50 4.31 4.37 4.48 4.47 4.46 4.34 4.39 4.41 4.39 4.35 4.42 4.46 4.46 4.42 4.40 4.25
0.13 0.13 0.12 0.12 0.12 0.12 0.13 0.11 0.13 0.13 0.12 0.12 0.11 0.11 0.11 0.12 0.12 0.13 0.12 0.11 0.10 0.10 0.12 0.11
0.02 0.00 0.01 0.03 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.00 0.00 0.01 0.00 0.01 0.01 0.00 0.01 0.01 0.00 0.01 0.01 0.00
0.35 0.32 0.34 0.37 0.36 0.34 0.35 0.34 0.35 0.03 0.35 0.32 0.33 0.33 0.34 0.32 0.31 0.34 0.35 0.33 0.31 0.34 0.32 0.32
5.03 4.92 4.67 5.17 5.01 4.96 4.93 4.81 5.03 5.03 5.07 4.92 5.04 4.89 4.90 4.89 4.77 5.00 5.02 5.00 4.98 4.94 5.09 4.91
4.89 4.66 4.92 4.92 4.89 4.90 4.95 4.71 4.89 4.58 4.93 4.86 4.64 4.89 4.89 4.86 4.63 4.95 4.93 4.61 4.67 4.60 4.67 4.82
0.31 0.36 0.24 0.33 0.26 0.33 0.30 0.37 0.30 0.38 0.27 0.37 0.39 0.23 0.22 0.35 0.37 0.32 0.30 0.36 0.36 0.38 0.37 0.24
0.16 0.12 0.16 0.19 0.16 0.16 0.16 0.12 0.16 0.13 0.16 0.16 0.12 0.16 0.15 0.16 0.12 0.16 0.16 0.13 0.10 0.11 0.13 0.17
99.8 100.2 101.0 100.5 99.3 99.7 99.9 100.5 100.1 100.2 100.3 101.0 100.7 100.3 100.2 101.0 100.2 99.7 101.1 100.7 100.3 99.5 100.2 99.7
Most widespread minor obsidian type NK Type B Jarigole (GbJj 1) Lowasera 1 Unit 4 Lowasera 1 Unit 2 Moruarot Serima (K91-3541)
2 3 3 1 1
11 23 24 3 3
72.9 72.2 73.4 72.6 73.4
0.32 0.31 0.32 0.31 0.31
0.14 0.13 0.14 0.11 0.08
10.6 10.6 10.5 10.6 10.8
5.42 5.32 5.35 5.40 5.19
0.15 0.14 0.15 0.13 0.14
0.00 0.00 0.01 0.00 0.01
0.32 0.31 0.34 0.31 0.34
5.23 5.40 5.44 5.08 5.16
4.81 4.86 4.80 4.42 4.45
0.28 0.36 0.37 0.41 0.46
0.20 0.21 0.19 0.14 0.13
100.2 99.6 100.9 99.3 100.2
Other minor types from sites and surface collections NK Type C Dongodien area (GaJi 2) Ileret (FwJj 5) NK Type D Dongodien area (GaJi 2) NK Type E Ileret (FwJj 5) NK Type F Ileret (FwJj 5) Lorenyang (GaJi 16) NK Type G Il Derati (K82-892) Il Derati (K82-893) NK Type H Lokalalei (K87-3068C) NK Type I Lomekwi (K85-2667A) NK Type J North Islandc “North Island” North Islandd North Islandd NK Type K Samburu Hills NK Type L Samburu Hills Suguta (MER-93) NK Type M Suregei 1 “Suregei A” Suregei 4 NK Type N Omo (ETH86-01) “Suregei B” Suregei 2 Suregei 3 NK Type O “Suregei C” Suregei 5
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
8 8 8 13 8 3 3 3 2 3 1 1 1 8 8
72.6 72.0 72.6 71.1 70.9 71.5 68.7 70.6 70.0 74.5 63.6 63.1 62.8 68.8 68.5 68.1 71.7 72.3 75.3 72.6 70.8 72.6
0.44 0.44 0.38 0.28 0.49 0.49 0.35 0.34 0.32 0.24 0.66 0.64 0.67 0.39 0.48 0.50 0.22 0.20 0.22 0.21 0.23 0.20
0.24 0.22 0.24 0.48 0.14 0.11 0.12 0.11 0.31 0.20 0.11 e e 0.55 0.38 0.33 0.08 0.08 0.12 0.19 0.16 0.17
9.8 10.0 10.5 8.2 9.8 10.0 10.4 10.6 7.2 9.8 15.4 15.7 15.7 6.9 6.8 6.6 11.3 11.4 10.6 10.6 11.5 10.5
5.85 5.67 5.70 7.75 7.25 7.36 5.36 5.54 10.93 5.46 5.88 6.27 6.26 9.72 11.12 11.00 2.85 2.87 3.76 3.69 3.65 3.81
0.28 0.27 0.26 0.23 0.35 0.36 0.14 0.17 0.33 0.14 0.15 0.13 0.13 0.40 0.41 0.38 0.08 0.10 0.06 0.07 0.07 0.09
0.21 0.23 0.16 0.01 0.21 0.19 0.01 0.02 0.00 0.02 1.15 0.93 1.06 0.03 0.08 0.07 0.01 0.00 0.01 0.01 0.00 0.00
0.22 0.24 0.20 0.21 0.24 0.24 0.38 0.30 0.50 0.20 3.14 3.15 3.16 0.30 0.56 0.58 0.27 0.33 0.21 0.22 0.24 0.15
5.74 5.67 5.74 6.64 6.33 6.21 4.91 4.06 7.27 5.53 5.92 5.59 5.47 7.71 7.17 6.87 2.09 2.24 4.62 1.93 1.35 2.03
4.82 4.83 4.86 4.54 4.87 4.47 4.63 4.87 4.14 4.45 4.00 3.96 3.82 4.41 4.45 4.14 6.31 5.70 4.47 5.20 4.35 5.41
0.41 0.39 0.33 0.92 0.34 0.44 0.34 0.26 0.94 0.60 e e e 1.04 0.98 0.27 0.25 0.37 0.45 0.33 0.20 0.34
0.22 0.23 0.14 0.45 0.18 0.18 0.10 0.09 0.11 0.28 e e e 0.53 0.37 0.29 0.09 0.07 0.13 0.10 0.12 0.10
100.9 99.9 101.1 100.4 100.9 101.3 95.3 96.8 101.6 101.1 100.0 99.0 98.9 100.2 100.8 99.0 95.0 95.5 99.7 94.9 92.5 95.3
Obsidian type Principal obsidian type NK Type A
a b c d
3 2 3 3 3 3
Total Fe as Fe2O3. Total corrected for O ¼ F, Cl. Whole rock, wet chemical analysis (Brown and Carmichael, 1971). Whole rock, instrumental analysis (Furman et al., 2004).
The Barrier is constructed of several individual Pleistocene volcanoes (Likayu East, Likayu West, Kakorinya, and Kalolenyang), with Likayu East being the oldest and Kakorinya the youngest (Dunkley et al., 1993). The only obsidian reported from these volcanoes is a welded pyroclastic flow with obsidian clasts exposed in the wall of the inner ring fault on the western side of Kakorinya (Dunkley et al., 1993 Fig. 11.6). All of the rocks analyzed from this volcano are trachytes or phonolites with 60e62% SiO2 and total alkali (Na2O þ K2O) contents between 12 and 15% (Dunkley et al., 1993). This information is probably sufficient to eliminate Kakorinya as a source because the artifactual obsidians are of rhyolitic
composition with 74% SiO2 and a total alkali content near 10%. No obsidians were observed in the trachytes of Likayu East, Likayu West, or the eastern flank of Kakorinya during a traverse of this territory. We have not visited Kalolenyang. It is worth noting that of all the volcanic rocks analyzed by Dunkley et al. (1993) from Lake Baringo northward only Namurunu has two which plot in the compositional field of rhyolites on a silica vs. total alkali diagram. Unfortunately no details of the analyzed material are given, nor are the original analyses published. The second most widely distributed obsidian type, NK-B, was recognized among artifacts at four locations. The northernmost
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occurrence is at the Jarigole Pillar site where it is represented by just two artifacts. In more southerly sites it is found in both excavated units at Lowasera, in surface occurrences at Moruarot, west of Lake Turkana, and at Serima Gorge, southeast of the lake. The source of this material is unknown, although the geographic distribution may provisionally suggest a southern Turkana area source. The NK-B type is the “unknown D” type recognized in Merrick and Brown (1984b). There is a strong similarity between this type and the northerly Il Derati source (NK-G). The Il Derati unmodified pebbles are from an ephemeral stream that drains into the basin through rhyolites that are about 13.1 Ma old (McDougall and Watkins, 1988). Compositionally the obsidians from Il Derati are similar to NK-B. The principal differences in composition are that the obsidians from Il Derati are lower in SiO2. Thus the differences may be due to hydration of the Il Derati obsidians. For other elements the correspondence is remarkably good, so that it would be worth collecting and analyzing additional samples from these Miocene flows. In particular, the interiors of larger fragments of these potential source obsidians would be of interest because they may be less hydrated than the outer surfaces. Seven other compositional types of obsidian are also known now among artifacts from this region (Table 2). Their appearance at archaeological sites is summarized in Table 1. These less common types tend to be found in the northern and western portions of the Turkana region. For example the Ileret Stone Bowl site has 3 minor types (NK-C, NK-E and NK-F), the Dongodien area site GaJi 2 has two (NK-C and NK-D), and west of Lake Turkana, types unknown elsewhere are present at Lokalalei (NK-H) and at Lomekwi (NK-I), and the same is true of the single flake analyzed from the lower Omo Valley in southern Ethiopia (NK-N). Potential sources for these minor types have not been identified, except perhaps for type NK-N (see below). The geographic distribution of artifacts assigned to these minor types suggests likely sources in the northern portion of the Turkana area. Merrick and Brown (1984b) previously suggested that the Suregei area in the northeastern corner of the Turkana basin might be a major source area, and analysis of 5 unmodified obsidian pebbles (Table 2) reveals that at least three compositional groups are present in this region. These sampled obsidian fragments from the Suregei derive from ash flow tuffs of the Langaria Formation which are 26.8 0.3 Ma old (McDougall and Watkins (1988, 2006)). Two of these groups Suregei A (NK-M) and Suregei C (NK-O) have not been recognized among the artifacts, but Suregei B (NK-N) may have been. The single piece from the lower Omo Valley is very similar compositionally to the Suregei B obsidian (NK-N; Table 2), but the Suregei obsidian has higher water content and lower Na2O and SiO2. Clearly further characterization of this potential source is needed, but the Suregei area is apparently not a major source of obsidian for artifact manufacture as has been previously suggested (Merrick and Brown, 1984b). The remaining compositional types (NK-J, NK-K and NK-L) recognized in this study are known only from unmodified pieces. The samples from the Samburu Hills south of the lake are of compositions that have not been seen north of the Barrier, but one appears similar to an obsidian nodule (MER-93) collected from the top of a small knoll on the west side of the Suguta valley, that may be near the source of this type. The occurrence is a surface lag concentrate overlying basalts. Obsidian present on North Island (NK-J) has not been identified in any locale beyond the island. On current physical and chemical evidence, it appears that all nine of the obsidian types identified among the Turkana region artifacts are likely to be from Turkana area sources. In addition to the small artifact size and occasional appearance of cortex on the obsidian suggesting manufacture utilizing small pebbles, there appear to be no close chemical matches with any of the more northerly Ethiopian Rift sources currently known (Negash and
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Shackley, 2006; Negash et al., 2011, in press), and none with the somewhat better documented central Kenyan sources (Merrick and Brown, 1984b; Merrick et al., 1994). 4.2. Implications for the regional prehistory Although obsidian from only a handful of Turkana region sites has been examined thus far, the pattern of the predominant use of Turkana region obsidians appears the norm for the earlier Holocene hunter/fisher folk and the Pastoral Neolithic peoples in the region. While the locations of most of these obsidian sources remain unknown, it is likely that a substantial portion of the obsidian appearing in many sites around the lake is derived from a source near the southern end of Lake Turkana. The distances that small quantities of this obsidian may have traveled from source to site may be upwards of 200 km, a distance in keeping with distances previously reported for Later Stone Age and Pastoral Neolithic movement of obsidian in Kenya (Merrick and Brown, 1984b). The pattern of use of local Turkana area obsidian in Pastoral Neolithic times, and the absence of evidence for either northern Ethiopian, or southern central Kenyan obsidians also seems noteworthy. The sampled Pastoral Neolithic sites, the Jarigole Pillar site (GbJj1), the Ileret Stone Bowl site (FwJj5), and the Dongodien areas sites, GsJi 4 and GsJi 2 are all associated with the broad Nderit ceramic tradition, if one regards Barthelme’s (1985) “Ileret Ware” as simply the later part of this tradition. This ceramic tradition is widely regarded as associated with the first pastoral peoples to occupy northern Kenya. This ceramic tradition and presumably its producers subsequently expanded into central Kenya and northern Tanzania. The origins of the Nderit tradition remain obscure but are thought to lie to the north in the southern Sudanic region, or less likely in southwestern Ethiopia. The absence of obsidian from Ethiopian sources may support the former origin rather than the latter. The absence of obsidian from central Kenyan sources to the south may also be of interest regarding the arrival timing and interactions among early pastoralists during their expansion into central Kenya and northern Tanzania. The dating of sites with Nderit ware in the Turkana basin appears to place this tradition between about 5500 and 4000 BP, and research at the Jarigole Pillar Site (Nelson, 1995) and the Il Lokeridede site (GaJi 23) (Koch et al., 2002) documents the association of Nderit ceramics with a distinctive mortuary tradition involving communal burial mounds marked with stone pillars. Similarly structured sites with Nderit ceramics present are also known from several localities on the west side of Lake Turkana, including one at Kalakol [i.e., Namoratunga of Lynch and Robbins (1978) and Soper (1982)], and one at Lothagam (Fig. 1). Although the distinctive mortuary tradition seems largely restricted to northern Kenyan locales, pottery of the Nderit ware tradition is found in central Kenya and northern Tanzania by the mid 5th millennium BP, where it is often correlated with the first appearance of pastoral subsistence practices (Gifford-Gonzalez, 1998). Despite the presumed cultural connections or interactions implied by the pottery similarities, there is not much other direct evidence for contact between the Turkana region’s pastoralists and the folk in central Kenya. At present there is no evidence at any Turkana area Nderit tradition sites of the distinctive obsidians (e.g., Njorowa, Sonanchi, Eburru; Merrick and Brown, 1984b) from central Kenya that were so widely distributed in the south during Pastoral Neolithic times. Eastward or perhaps southward connections of the people who utilized the Jarigole Pillar site for burials are known from the presence of rare beads made of marine shell (Nelson, 1993), but at least as far as obsidian is concerned, we have only limited evidence for cultural connections to the south. Significantly, to date obsidian artifacts with Turkana area compositions have only been recognized at one site south of the
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Turkana basin, the Kisima Farm KFR-A5 site on the Laikipia Plateau, some 180 km south of Lake Turkana. This was the most northerly of the central Kenya sites studied in Merrick and Brown’s earlier survey of Kenyan sites and sources (Merrick and Brown, 1984b). At the KFRA5 site in the LSA with pottery/Pastoral Neolithic levels were found one piece attributable to the NK-A source and one to the NK-E source (Supplemental Table 1). Obviously, there remains a major gap in our knowledge of both sources and artifactual obsidians between Lake Turkana and the Laikipia Plateau/Lake Baringo region, and it will be of great interest to see how the patterns of obsidian source utilization are distributed in future work. Acknowledgments Financial support was provided by the Leakey Foundation, the National Science Foundation (grant BCS06-21543) and the College of Mines and Earth Sciences at the University of Utah. Sampling of the artifactual obsidians was conducted by one of us (HM) as part of the research program of the Division of Archaeology of the National Museums of Kenya and as part of the research of the NMK/Harvard Koobi Fora Palaeoanthropological Field School between 1982 and 1996. Appendix. Supplementary material Supplementary data related to this article can be found online at doi:10.1016/j.jas.2011.02.001. References Barthelme, J.W., 1977. Holocene sites northeast of Lake Turkana: a preliminary report. Azania 12, 33e41. Barthelme, J.W.,1985. Fisherehunters and Neolithic Pastoralists in East Turkana, Kenya. In: BAR International Series, vol. 254. British Archaeological Reports, Oxford. Brown, F.H., Carmichael, I.S.E., 1971. Quaternary volcanoes of the Lake Rudolf region: II. The lavas of north island, south island and the Barrier. Lithos 4, 305e323. Brown, F.H., Fuller, C.R., 2008. Stratigraphy and tephra of the Kibish formation, southwestern Ethiopia. J. Hum. Evol. 55, 366e403. Dunkley, P.N., Smith, M., Allen, D.J., Darling, W.G., 1993. The Geothermal Activity and Geology of the Northern Sector of the Kenya Rift Valley. British Geological Survey Research Report SC/93/1. NERC, Nottingham, pp. 185.
Furman, T., Bryce, J.G., Karson, J., Iotti, A., 2004. East African rift system (EARS) plume structure: insights from Quaternary mafic lavas of Turkana, Kenya. J. Petrol. 45, 1069e1088. Garcin, Y., Junginger, A., Melnick, D., Olago, D.O., Strecker, M.R., Trauth, M.H., 2009. Late PleistoceneeHolocene rise and collapse of Lake Suguta, northern Kenya rift. Quaternary Sci. Rev. 28, 911e925. Gifford-Gonzalez, D., 1998. Early pastoralists in East Africa: ecological and social dimensions. J. Anthropol. Archaeol. 17, 166e200. Koch, C.P., Pavlish, L.A., Farquhar, R.M., Hancock, R.G.V., Beukens, R.P., 2002. INAA of pottery from Il Lokeridede and Jarigole, Koobi Fora region, Kenya. In: Jerem, E., Biro, K.T. (Eds.), Archaeometry 98, proceedings of the 31st Archaeometry Symposium, Bar International Series 1043 (II). Oxford, pp. 587e592. Lynch, B.M., Robbins, L.H., 1978. Namoratunga: the first archeoastronomical evidence in Sub-Saharan Africa. Science 200, 766e768. McDougall, I., Watkins, R.T., 1988. Potassium-argon ages of volcanic rocks from northeast of Lake Turkana, northern Kenya. Geol. Mag. 125, 15e23. McDougall, I., Watkins, R.T., 2006. Geochronology of the Nabwal Hills: a record of earliest magmatism in the northern Kenya rift valley. Geol. Mag. 143, 25e39. Merrick, H.V., Brown, F.H., 1984a. Rapid chemical characterization of obsidian artifacts by electron microprobe analysis. Archaeometry 26, 230e236. Merrick, H.V., Brown, F.H., 1984b. Obsidian sources and patterns of source utilization in Kenya and Tanzania: some initial findings. Afr. Archaeological Rev. 2, 129e152. Merrick, H.V., Brown, F.H., Nash, W.P., 1994. Use and movement of obsidian in the early and middle stone ages of Kenya and northern Tanzania. MASCA Research Papers in Science and Archaeology. In: Childs, S.T. (Ed.), Society, Culture, and Technology in Africa, pp. 29e44. Supplement to Vol. 11, 1994. Nash, W.P., 1992. Analysis of oxygen with the electron microprobe: applications to hydrated glass and minerals. Am. Mineral 77, 453e457. Negash, A., Shackley, M.S., 2006. Geochemical provenance of obsidian artifacts from the MSA site of Porc Epic, Ethiopia. Archaeometry 48, 1e12. Negash, A., Alene, M., Brown, F.H., Nash, B.P., Shackley, M.S., 2007. Geochemical provenance of the terminal Pleistocene/early Holocene obsidian artifacts from the site of Beseka, central Ethiopia. J. Archaeol. Sci. 34, 1205e1210. Negash, A., Brown, F.H., Nash, B.P., 2011. Varieties and sources of artifactual obsidian in the Middle Stone Age of the Middle Awash, Ethiopia. Archaeometry 53, in press. Nelson, C.M.,1993. Evidence for early trade between the coast and interior of East Africa. In: Paper Prepared for the WAC Mombassa Intercongress Conference. Electronic Document. http://www.chaz.rg/Arch/Turkana/Jarigole/TERADEW>HTM Accessed 2010. Nelson, C.M., 1995. The work of the Koobi Fora field school at the Jarigole pillar site. Kenya Past and Present 27, 49e63. Phillipson, D.W., 1977. Lowasera. Azania 12, 1e32. Soper, R., 1982. Archaeo-astronomical cushites: some comments. Azania 17, 145e162. Watkins, R.T., 1981. The Geochemistry of Rhyolitic and Tholeiitic Rocks From East of Lake Turkana, Northern Kenya, MSc. Thesis, University of Leeds, pp. 155.
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Journal of Archaeological Science journal homepage: http://www.elsevier.com/locate/jas
Obsidian types from Holocene sites around Lake Turkana, and other localities in northern Kenya Barbara P. Nash a, *, Harry V. Merrick b, Francis H. Brown a a b
Department of Geology and Geophysics, University of Utah, 115 S. 1460 E, Salt Lake City, UT 84112-0111, USA Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Boston, MA, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 August 2010 Received in revised form 4 January 2011 Accepted 6 February 2011
Obsidian has been widely used by early Holocene hunter-gatherers and succeeding Pastoral Neolithic peoples in northern Kenya. Here we report the results of over 2000 electron microprobe analyses of artifactual and non-artifactual obsidian from the greater Lake Turkana region. Of the 15 compositional types of obsidian observed, a preponderant type is widespread across the region from the Barrier in the south to Ileret in the north and east as far as Kargi. This obsidian is the principal type at Lowasera and most Pastoral Neolithic sites, including the Jarigole Pillar site and Dongodien (GaJi4). The source of this obsidian is not known, but based on its distribution the source may be located on the Barrier or in the Suguta Valley immediately to the south of Lake Turkana. Although there are several possible sources of local obsidian identified for minor types, in stark contrast to the central part of the Kenyan Rift, major sources of obsidian available for artifact manufacture are not known in the Lake Turkana region. The lack of obsidian from demonstrable Ethiopian Rift and central Kenyan Rift sources, and the absence of obsidian with compositions found at the Turkana area sites in assemblages in the central part of the Kenyan Rift suggests that the earlier Pastoral Neolithic peoples around Lake Turkana interacted with each other, but perhaps not as strongly with people farther south along the Rift Valley, even as herding practices were expanding to the southward into central Kenya. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Kenya Lake Turkana Jarigole Obsidian Artifact Electron microprobe Pastoral Neolithic
1. Introduction For many years, obsidian has been known to have been occasionally utilized for tool manufacture by the early Holocene huntergatherers and the succeeding Pastoral Neolithic peoples in northern Kenya (Barthelme, 1977, 1985; Phillipson, 1977; Nelson, 1995), but little was known of its chemical composition and source. Merrick and Brown (1984a,b), based on the analysis of a very limited number of elements, reported preliminary results on a small number of obsidian artifacts from several sites in the Lake Turkana region and suggested on the then available information about sources and their chemistries, that only local sources were likely being used. The purpose of this paper is to report the results of a much more extensive investigation of the chemical composition of obsidian pieces, artifactual and non-artifactual, from the greater Lake Turkana region, and to suggest possible areas for the geologic source of some of the obsidians used for artifact manufacture. Unlike some areas of the central Ethiopian and central Kenyan Rifts where geological sources of obsidian are numerous and yield * Corresponding author. Tel.: þ1 801 581 8587; fax: þ1 801 581 7065. E-mail address: [email protected] (B.P. Nash). 0305-4403/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jas.2011.02.001
abundant high quality obsidian for artifact manufacture, the Lake Turkana region in northern Kenya does not appear to contain many sources of obsidian. Geological survey in the region suggests that sources of obsidian are few, relatively localized geographically and associated with Miocene through Pleistocene pyroclastic flows and deposits, the contained obsidian for the most part being in the form of small pebble sized lapilli. To date no obvious ‘quarry’ localities have been identified for any potential northern Kenyan source of obsidian. The local scarcity of obsidian has several effects on the Turkana region’s Holocene archaeological assemblages. Typically, multiple materials may be used for artifact manufacture including chert, chalcedony, fossil wood, quartzite, lavas and obsidian. Among obsidian artifacts, due to the small size of the source material regionally available, size tends to be microlithic and the artifacts often show traces of cortex indicative of being created from pebble size nodules. Finally, artifacts of obsidian, even when they may be proportionally the dominant type, are not absolutely numerically abundant. Because obsidian sources appear to be so few in the region and are, as yet, not well documented nor their chemistries well characterized, we have chosen to examine and characterize a sample of artifactual and non-artifactual obsidians in hopes of identifying
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potential sources in the region as a first step in examining patterns of source utilization within the region during Later Stone Age and Pastoral Neolithic times.
Table 1 Sampled Turkana region sites and localities and obsidian types present. Arranged by probable cultural association and location (north to south). Site and locality
2. The obsidian sample The majority of analyses reported here are of obsidian artifacts sampled from excavations at Holocene hunter-gatherer/fisher and Neolithic pastoralist sites on the eastern side of Lake Turkana. The artifact collections from these sites, including the sampled artifacts, are housed in the National Museums of Kenya, Nairobi. The sampled hunting/gathering/fishing sites include GaJj1 (Barthelme, 1985), and Lowasera (Phillipson, 1977). Sampled Pastoral Neolithic sites associated with the Nderit ceramic tradition (includes Barthelme’s (1985) Ileret tradition) include the Jarigole Pillar site (GbJj 1) south of Allia Bay (Nelson, 1995), the Dongodien area sites, GaJi 4 and GaJi 2, and the Ileret Stone Bowl site, FwJj 5 (Barthelme, 1985). We also report analyses of a few artifacts of uncertain cultural association and unmodified pieces of obsidian collected from the surface at scattered localities in the Lake Turkana region. Table 1 lists the sites and localities sampled for archaeological and non-archaeological obsidians and Fig. 1 illustrates their locations in the Turkana area. 3. Analysis For electron microprobe analysis small fragments were removed from each of the artifacts, and a white spot was painted onto each artifact held in the National Museums of Kenya to indicate where the fragment was removed. The fragments were mounted in resin plugs, ground flat, polished, and coated with a thin layer of carbon. Analysis was done on a Cameca SX-50 electron microprobe at the University of Utah using techniques described in Nash (1992). Standards used are identical to those used for analysis of glass shards from volcanic ash layers in the Turkana Basin given by Brown and Fuller (2008). Three to eight analyses for 13 elements were made on each of the obsidian samples; averaged analyses are presented in Table 2. All 2079 individual analyses are given in Supplementary Table 1. Barium was measured, but because none of the samples contains as much as 0.01%, it is not reported. Geologically younger obsidian, either in the form of artifacts or loose material on the surface, has low water contents and thus analytical totals are close to 100%. Older hydrated obsidians (Il Derati and the Suregei samples) have totals ranging from 92 to 97% that are also typical for hydrated glass shards in volcanic tuffs (cf., Brown and Fuller, 2008). 4. Results and discussion 4.1. Obsidian types present in the Lake Turkana region Fifteen distinct chemical types of obsidian can be provisionally recognized among the sampled artifact and non-artifactual specimens (Table 2). For convenience of reference we have labeled these 15 obsidian types with the designations Northern Kenya types A to O (NK-A to NK-O) and only when we are reasonably confident of the geographic source have we added a geographic name as an identifier. Eight of these obsidian types, including the two most commonly used types (NK-A and NK-B) are known from artifacts only. Seven of the types are known from samples of unmodified obsidian, often collected from eroded exposures on or near tuffaceous deposits, and thus these localities might be considered to represent potential ‘sources’. However at present only one of these types (NK-N ‘Suregei B’) is thought to have been potentially used for artifact manufacture.
# Sampled
Obsidian type present and number of pieces
Early/Middle Holocene hunting and fishing GaJj1 2 NK-A Lowasera 16 Unit 2 ¼ 8 NK-A (5) NK-B (3) Unit 4 ¼ 8 NK-A (5) NK-B (3) Pastoral Neolithic (Nderit tradition sensu lato) FwJj 5 Ileret Stone 5 NK-A (2) NK-C (1) NK-E (1) Bowl Site GaJi 2 (just west of 3 NK-A (1) NK-C (1) NK-D (1) Dongodien) GaJi 4 Dongodien 20 NK-A GbJj1 Jarigole Pillar 228 NK-A (226) NK-B (2) Site Pieces of uncertain artifactual and/or cultural association (all surface finds) Omo (ETH86-01) 1 NK-N (?) Ileret (K85-2557) 1 NK-A Karari 1 NK-A GaJi 16 Lorenyang 1 NK-F Lokalalei 3 NK-A (2) NK-H (1) Lomekwi 1 NK-I Area 206, K. Fora 1 NK-A Sibilot 2 NK-A Il Moiti 1 NK-A Kalodirr 1 NK-A Moruarot 2 NK-A (1) NK-B (1) Kalatum 1 NK-A Lowasera Gorge 1 NK-A Loiyengalani 2 NK-A Gatab airstrip 3 NK-A Gatab Junction 7 NK-A Serima 3 NK-A Kargi 4 NK-A Barrier 5 NK-A Unmodified obsidian chunks (nodules, pebbles, etc.) (potential “source” material) Suregei 5 NK-M (2) NK-N (2) NK-O (1) North Island 1 NK-J Il Derati (K82-892) 2 NK-G Suguta (MER-93) 1 NK-L Samburu Hills 2 NK-K (1) NK-L (1)
Table 2 is arranged so that the preponderant types of obsidian are listed first. The most common type, NK-A, recognized in 21 locations, is a well-defined compositional type that is found in archeological sites and as surface occurrences from the Barrier, the volcanic ridge separating the southern end of the Turkana basin from the Suguta valley, north to Ileret east of Lake Turkana, and as far north as Lokalalei west of Lake Turkana. Its distribution extends eastward to Gatab on the southern slopes of Mt. Kulal and at least as far east as Kargi (Fig. 1). Not only is it the principal obsidian type (99%) at the Jarigole Pillar site, but it is also the principal type at GaJi 4 and both excavated levels at Lowasera. It is also present at GaJi 1, GaJi 2, and FwJj 5. The source of this obsidian is not certainly known, but a strong case may be made for a southern Turkana region location. This type was recognized as the regionally dominant obsidian type in Merrick and Brown’s (1984b) pioneering study (their “Petrological Group 1”) and, in hindsight, was erroneously attributed to a Suregei area source. Their assignment of artifacts to this source, whose chemistry was then known only from Watkins’ (1981) single analysis, was based on characterization using only three elements (Fe, Ca and Ti) that clearly proved inadequate to discriminate between northern Kenyan area sources. None of our analyses of unmodified obsidian since sampled from the Suregei region (Types NK-M, NK-N and NK-O) match this type. A more
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Fig. 1. Index map of sample locations in the Lake Turkana region. The shaded area represents elevations between the current lake level and 460 m within which the majority of the sample localities lie. Brown and Fuller (2008) estimate the elevation of the outlet to the Nile River to be 457 m, so 460 m is near the maximum late Pleistocene and Holocene level of Lake Turkana.
southerly location for the source appears likely on the basis of several observations. First, it appears that NK-A is the dominant or exclusive type present in all southern localities. Only in the more northerly areas do the minor obsidians appear with any regularity (Fig. 1). Second, the density of obsidian fragments exposed on the modern regional land surface is strikingly different from one area to another. Whereas one can spend an entire day in the field and see only a fragment or two in the more northerly greater Koobi Fora region,
the abundance is markedly greater toward the south end of the lake, and is quite high on the Barrier, a linear east-west volcanic structure formed by multiple volcanic centers, which separates Lake Turkana from the Suguta Valley. Obsidian artifacts are also apparently encountered with regularity to the south of the Barrier in the Suguta Valley (Garcin et al., 2009). From these admittedly unsystematic observations we suggest that the source of the dominant obsidian is likely to be located on the Barrier or in the Suguta Valley.
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Table 2 Average electron microprobe analyses of artifact obsidians (values in weight percent). Locality
# of pcs
N
SiO2
TiO2
ZrO2
Al2O3
Fe2O3a
MnO
MgO
CaO
Na2O
K2O
F
Cl
Totalb
Area 102, K. Fora (GaJj 1) Area 206, K. Fora Barrier Dongodien area (GaJi 2) Dongodien (GaJi 4) Gatab airstrip Gatab Junction Il Moiti Ileret (FwJj 5) Ileret (KBS-2557) Jarigole (GbJj 1) Kalatum Kalodirr Karari Kargi Loiyengalani Lokalalei Lowasera 1 Unit 4 Lowasera 1 Unit 2 Lowasera Gorge Moruarot Serima Sibilot Laikipia (Kisima)
2 1 5 1 20 3 7 1 2 1 226 1 1 1 4 2 2 5 5 1 1 2 2 1
16 3 59 5 214 24 56 3 16 3 1372 3 3 12 39 6 6 40 39 3 3 6 6 5
73.3 73.9 74.6 73.6 72.7 73.3 73.4 74.1 73.4 73.8 73.6 74.4 74.2 74.6 74.3 74.6 74.2 73.1 74.3 74.2 74.0 73.2 73.9 73.2
0.28 0.25 0.27 0.28 0.29 0.28 0.28 0.26 0.29 0.27 0.28 0.27 0.28 0.25 0.26 0.28 0.28 0.27 0.28 0.28 0.26 0.31 0.28 0.27
0.11 0.10 0.14 0.09 0.12 0.12 0.11 0.05 0.15 0.08 0.10 0.08 0.06 0.12 0.11 0.07 0.06 0.12 0.10 0.09 0.07 0.07 0.06 0.08
11.1 11.1 11.3 11.1 11.1 11.0 11.1 11.3 11.1 11.1 11.1 11.2 11.2 10.8 10.9 11.1 11.1 11.1 11.0 11.2 11.1 11.2 11.1 11.4
4.36 4.42 4.39 4.39 4.44 4.38 4.39 4.50 4.31 4.37 4.48 4.47 4.46 4.34 4.39 4.41 4.39 4.35 4.42 4.46 4.46 4.42 4.40 4.25
0.13 0.13 0.12 0.12 0.12 0.12 0.13 0.11 0.13 0.13 0.12 0.12 0.11 0.11 0.11 0.12 0.12 0.13 0.12 0.11 0.10 0.10 0.12 0.11
0.02 0.00 0.01 0.03 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.00 0.00 0.01 0.00 0.01 0.01 0.00 0.01 0.01 0.00 0.01 0.01 0.00
0.35 0.32 0.34 0.37 0.36 0.34 0.35 0.34 0.35 0.03 0.35 0.32 0.33 0.33 0.34 0.32 0.31 0.34 0.35 0.33 0.31 0.34 0.32 0.32
5.03 4.92 4.67 5.17 5.01 4.96 4.93 4.81 5.03 5.03 5.07 4.92 5.04 4.89 4.90 4.89 4.77 5.00 5.02 5.00 4.98 4.94 5.09 4.91
4.89 4.66 4.92 4.92 4.89 4.90 4.95 4.71 4.89 4.58 4.93 4.86 4.64 4.89 4.89 4.86 4.63 4.95 4.93 4.61 4.67 4.60 4.67 4.82
0.31 0.36 0.24 0.33 0.26 0.33 0.30 0.37 0.30 0.38 0.27 0.37 0.39 0.23 0.22 0.35 0.37 0.32 0.30 0.36 0.36 0.38 0.37 0.24
0.16 0.12 0.16 0.19 0.16 0.16 0.16 0.12 0.16 0.13 0.16 0.16 0.12 0.16 0.15 0.16 0.12 0.16 0.16 0.13 0.10 0.11 0.13 0.17
99.8 100.2 101.0 100.5 99.3 99.7 99.9 100.5 100.1 100.2 100.3 101.0 100.7 100.3 100.2 101.0 100.2 99.7 101.1 100.7 100.3 99.5 100.2 99.7
Most widespread minor obsidian type NK Type B Jarigole (GbJj 1) Lowasera 1 Unit 4 Lowasera 1 Unit 2 Moruarot Serima (K91-3541)
2 3 3 1 1
11 23 24 3 3
72.9 72.2 73.4 72.6 73.4
0.32 0.31 0.32 0.31 0.31
0.14 0.13 0.14 0.11 0.08
10.6 10.6 10.5 10.6 10.8
5.42 5.32 5.35 5.40 5.19
0.15 0.14 0.15 0.13 0.14
0.00 0.00 0.01 0.00 0.01
0.32 0.31 0.34 0.31 0.34
5.23 5.40 5.44 5.08 5.16
4.81 4.86 4.80 4.42 4.45
0.28 0.36 0.37 0.41 0.46
0.20 0.21 0.19 0.14 0.13
100.2 99.6 100.9 99.3 100.2
Other minor types from sites and surface collections NK Type C Dongodien area (GaJi 2) Ileret (FwJj 5) NK Type D Dongodien area (GaJi 2) NK Type E Ileret (FwJj 5) NK Type F Ileret (FwJj 5) Lorenyang (GaJi 16) NK Type G Il Derati (K82-892) Il Derati (K82-893) NK Type H Lokalalei (K87-3068C) NK Type I Lomekwi (K85-2667A) NK Type J North Islandc “North Island” North Islandd North Islandd NK Type K Samburu Hills NK Type L Samburu Hills Suguta (MER-93) NK Type M Suregei 1 “Suregei A” Suregei 4 NK Type N Omo (ETH86-01) “Suregei B” Suregei 2 Suregei 3 NK Type O “Suregei C” Suregei 5
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
8 8 8 13 8 3 3 3 2 3 1 1 1 8 8
72.6 72.0 72.6 71.1 70.9 71.5 68.7 70.6 70.0 74.5 63.6 63.1 62.8 68.8 68.5 68.1 71.7 72.3 75.3 72.6 70.8 72.6
0.44 0.44 0.38 0.28 0.49 0.49 0.35 0.34 0.32 0.24 0.66 0.64 0.67 0.39 0.48 0.50 0.22 0.20 0.22 0.21 0.23 0.20
0.24 0.22 0.24 0.48 0.14 0.11 0.12 0.11 0.31 0.20 0.11 e e 0.55 0.38 0.33 0.08 0.08 0.12 0.19 0.16 0.17
9.8 10.0 10.5 8.2 9.8 10.0 10.4 10.6 7.2 9.8 15.4 15.7 15.7 6.9 6.8 6.6 11.3 11.4 10.6 10.6 11.5 10.5
5.85 5.67 5.70 7.75 7.25 7.36 5.36 5.54 10.93 5.46 5.88 6.27 6.26 9.72 11.12 11.00 2.85 2.87 3.76 3.69 3.65 3.81
0.28 0.27 0.26 0.23 0.35 0.36 0.14 0.17 0.33 0.14 0.15 0.13 0.13 0.40 0.41 0.38 0.08 0.10 0.06 0.07 0.07 0.09
0.21 0.23 0.16 0.01 0.21 0.19 0.01 0.02 0.00 0.02 1.15 0.93 1.06 0.03 0.08 0.07 0.01 0.00 0.01 0.01 0.00 0.00
0.22 0.24 0.20 0.21 0.24 0.24 0.38 0.30 0.50 0.20 3.14 3.15 3.16 0.30 0.56 0.58 0.27 0.33 0.21 0.22 0.24 0.15
5.74 5.67 5.74 6.64 6.33 6.21 4.91 4.06 7.27 5.53 5.92 5.59 5.47 7.71 7.17 6.87 2.09 2.24 4.62 1.93 1.35 2.03
4.82 4.83 4.86 4.54 4.87 4.47 4.63 4.87 4.14 4.45 4.00 3.96 3.82 4.41 4.45 4.14 6.31 5.70 4.47 5.20 4.35 5.41
0.41 0.39 0.33 0.92 0.34 0.44 0.34 0.26 0.94 0.60 e e e 1.04 0.98 0.27 0.25 0.37 0.45 0.33 0.20 0.34
0.22 0.23 0.14 0.45 0.18 0.18 0.10 0.09 0.11 0.28 e e e 0.53 0.37 0.29 0.09 0.07 0.13 0.10 0.12 0.10
100.9 99.9 101.1 100.4 100.9 101.3 95.3 96.8 101.6 101.1 100.0 99.0 98.9 100.2 100.8 99.0 95.0 95.5 99.7 94.9 92.5 95.3
Obsidian type Principal obsidian type NK Type A
a b c d
3 2 3 3 3 3
Total Fe as Fe2O3. Total corrected for O ¼ F, Cl. Whole rock, wet chemical analysis (Brown and Carmichael, 1971). Whole rock, instrumental analysis (Furman et al., 2004).
The Barrier is constructed of several individual Pleistocene volcanoes (Likayu East, Likayu West, Kakorinya, and Kalolenyang), with Likayu East being the oldest and Kakorinya the youngest (Dunkley et al., 1993). The only obsidian reported from these volcanoes is a welded pyroclastic flow with obsidian clasts exposed in the wall of the inner ring fault on the western side of Kakorinya (Dunkley et al., 1993 Fig. 11.6). All of the rocks analyzed from this volcano are trachytes or phonolites with 60e62% SiO2 and total alkali (Na2O þ K2O) contents between 12 and 15% (Dunkley et al., 1993). This information is probably sufficient to eliminate Kakorinya as a source because the artifactual obsidians are of rhyolitic
composition with 74% SiO2 and a total alkali content near 10%. No obsidians were observed in the trachytes of Likayu East, Likayu West, or the eastern flank of Kakorinya during a traverse of this territory. We have not visited Kalolenyang. It is worth noting that of all the volcanic rocks analyzed by Dunkley et al. (1993) from Lake Baringo northward only Namurunu has two which plot in the compositional field of rhyolites on a silica vs. total alkali diagram. Unfortunately no details of the analyzed material are given, nor are the original analyses published. The second most widely distributed obsidian type, NK-B, was recognized among artifacts at four locations. The northernmost
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occurrence is at the Jarigole Pillar site where it is represented by just two artifacts. In more southerly sites it is found in both excavated units at Lowasera, in surface occurrences at Moruarot, west of Lake Turkana, and at Serima Gorge, southeast of the lake. The source of this material is unknown, although the geographic distribution may provisionally suggest a southern Turkana area source. The NK-B type is the “unknown D” type recognized in Merrick and Brown (1984b). There is a strong similarity between this type and the northerly Il Derati source (NK-G). The Il Derati unmodified pebbles are from an ephemeral stream that drains into the basin through rhyolites that are about 13.1 Ma old (McDougall and Watkins, 1988). Compositionally the obsidians from Il Derati are similar to NK-B. The principal differences in composition are that the obsidians from Il Derati are lower in SiO2. Thus the differences may be due to hydration of the Il Derati obsidians. For other elements the correspondence is remarkably good, so that it would be worth collecting and analyzing additional samples from these Miocene flows. In particular, the interiors of larger fragments of these potential source obsidians would be of interest because they may be less hydrated than the outer surfaces. Seven other compositional types of obsidian are also known now among artifacts from this region (Table 2). Their appearance at archaeological sites is summarized in Table 1. These less common types tend to be found in the northern and western portions of the Turkana region. For example the Ileret Stone Bowl site has 3 minor types (NK-C, NK-E and NK-F), the Dongodien area site GaJi 2 has two (NK-C and NK-D), and west of Lake Turkana, types unknown elsewhere are present at Lokalalei (NK-H) and at Lomekwi (NK-I), and the same is true of the single flake analyzed from the lower Omo Valley in southern Ethiopia (NK-N). Potential sources for these minor types have not been identified, except perhaps for type NK-N (see below). The geographic distribution of artifacts assigned to these minor types suggests likely sources in the northern portion of the Turkana area. Merrick and Brown (1984b) previously suggested that the Suregei area in the northeastern corner of the Turkana basin might be a major source area, and analysis of 5 unmodified obsidian pebbles (Table 2) reveals that at least three compositional groups are present in this region. These sampled obsidian fragments from the Suregei derive from ash flow tuffs of the Langaria Formation which are 26.8 0.3 Ma old (McDougall and Watkins (1988, 2006)). Two of these groups Suregei A (NK-M) and Suregei C (NK-O) have not been recognized among the artifacts, but Suregei B (NK-N) may have been. The single piece from the lower Omo Valley is very similar compositionally to the Suregei B obsidian (NK-N; Table 2), but the Suregei obsidian has higher water content and lower Na2O and SiO2. Clearly further characterization of this potential source is needed, but the Suregei area is apparently not a major source of obsidian for artifact manufacture as has been previously suggested (Merrick and Brown, 1984b). The remaining compositional types (NK-J, NK-K and NK-L) recognized in this study are known only from unmodified pieces. The samples from the Samburu Hills south of the lake are of compositions that have not been seen north of the Barrier, but one appears similar to an obsidian nodule (MER-93) collected from the top of a small knoll on the west side of the Suguta valley, that may be near the source of this type. The occurrence is a surface lag concentrate overlying basalts. Obsidian present on North Island (NK-J) has not been identified in any locale beyond the island. On current physical and chemical evidence, it appears that all nine of the obsidian types identified among the Turkana region artifacts are likely to be from Turkana area sources. In addition to the small artifact size and occasional appearance of cortex on the obsidian suggesting manufacture utilizing small pebbles, there appear to be no close chemical matches with any of the more northerly Ethiopian Rift sources currently known (Negash and
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Shackley, 2006; Negash et al., 2011, in press), and none with the somewhat better documented central Kenyan sources (Merrick and Brown, 1984b; Merrick et al., 1994). 4.2. Implications for the regional prehistory Although obsidian from only a handful of Turkana region sites has been examined thus far, the pattern of the predominant use of Turkana region obsidians appears the norm for the earlier Holocene hunter/fisher folk and the Pastoral Neolithic peoples in the region. While the locations of most of these obsidian sources remain unknown, it is likely that a substantial portion of the obsidian appearing in many sites around the lake is derived from a source near the southern end of Lake Turkana. The distances that small quantities of this obsidian may have traveled from source to site may be upwards of 200 km, a distance in keeping with distances previously reported for Later Stone Age and Pastoral Neolithic movement of obsidian in Kenya (Merrick and Brown, 1984b). The pattern of use of local Turkana area obsidian in Pastoral Neolithic times, and the absence of evidence for either northern Ethiopian, or southern central Kenyan obsidians also seems noteworthy. The sampled Pastoral Neolithic sites, the Jarigole Pillar site (GbJj1), the Ileret Stone Bowl site (FwJj5), and the Dongodien areas sites, GsJi 4 and GsJi 2 are all associated with the broad Nderit ceramic tradition, if one regards Barthelme’s (1985) “Ileret Ware” as simply the later part of this tradition. This ceramic tradition is widely regarded as associated with the first pastoral peoples to occupy northern Kenya. This ceramic tradition and presumably its producers subsequently expanded into central Kenya and northern Tanzania. The origins of the Nderit tradition remain obscure but are thought to lie to the north in the southern Sudanic region, or less likely in southwestern Ethiopia. The absence of obsidian from Ethiopian sources may support the former origin rather than the latter. The absence of obsidian from central Kenyan sources to the south may also be of interest regarding the arrival timing and interactions among early pastoralists during their expansion into central Kenya and northern Tanzania. The dating of sites with Nderit ware in the Turkana basin appears to place this tradition between about 5500 and 4000 BP, and research at the Jarigole Pillar Site (Nelson, 1995) and the Il Lokeridede site (GaJi 23) (Koch et al., 2002) documents the association of Nderit ceramics with a distinctive mortuary tradition involving communal burial mounds marked with stone pillars. Similarly structured sites with Nderit ceramics present are also known from several localities on the west side of Lake Turkana, including one at Kalakol [i.e., Namoratunga of Lynch and Robbins (1978) and Soper (1982)], and one at Lothagam (Fig. 1). Although the distinctive mortuary tradition seems largely restricted to northern Kenyan locales, pottery of the Nderit ware tradition is found in central Kenya and northern Tanzania by the mid 5th millennium BP, where it is often correlated with the first appearance of pastoral subsistence practices (Gifford-Gonzalez, 1998). Despite the presumed cultural connections or interactions implied by the pottery similarities, there is not much other direct evidence for contact between the Turkana region’s pastoralists and the folk in central Kenya. At present there is no evidence at any Turkana area Nderit tradition sites of the distinctive obsidians (e.g., Njorowa, Sonanchi, Eburru; Merrick and Brown, 1984b) from central Kenya that were so widely distributed in the south during Pastoral Neolithic times. Eastward or perhaps southward connections of the people who utilized the Jarigole Pillar site for burials are known from the presence of rare beads made of marine shell (Nelson, 1993), but at least as far as obsidian is concerned, we have only limited evidence for cultural connections to the south. Significantly, to date obsidian artifacts with Turkana area compositions have only been recognized at one site south of the
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Turkana basin, the Kisima Farm KFR-A5 site on the Laikipia Plateau, some 180 km south of Lake Turkana. This was the most northerly of the central Kenya sites studied in Merrick and Brown’s earlier survey of Kenyan sites and sources (Merrick and Brown, 1984b). At the KFRA5 site in the LSA with pottery/Pastoral Neolithic levels were found one piece attributable to the NK-A source and one to the NK-E source (Supplemental Table 1). Obviously, there remains a major gap in our knowledge of both sources and artifactual obsidians between Lake Turkana and the Laikipia Plateau/Lake Baringo region, and it will be of great interest to see how the patterns of obsidian source utilization are distributed in future work. Acknowledgments Financial support was provided by the Leakey Foundation, the National Science Foundation (grant BCS06-21543) and the College of Mines and Earth Sciences at the University of Utah. Sampling of the artifactual obsidians was conducted by one of us (HM) as part of the research program of the Division of Archaeology of the National Museums of Kenya and as part of the research of the NMK/Harvard Koobi Fora Palaeoanthropological Field School between 1982 and 1996. Appendix. Supplementary material Supplementary data related to this article can be found online at doi:10.1016/j.jas.2011.02.001. References Barthelme, J.W., 1977. Holocene sites northeast of Lake Turkana: a preliminary report. Azania 12, 33e41. Barthelme, J.W.,1985. Fisherehunters and Neolithic Pastoralists in East Turkana, Kenya. In: BAR International Series, vol. 254. British Archaeological Reports, Oxford. Brown, F.H., Carmichael, I.S.E., 1971. Quaternary volcanoes of the Lake Rudolf region: II. The lavas of north island, south island and the Barrier. Lithos 4, 305e323. Brown, F.H., Fuller, C.R., 2008. Stratigraphy and tephra of the Kibish formation, southwestern Ethiopia. J. Hum. Evol. 55, 366e403. Dunkley, P.N., Smith, M., Allen, D.J., Darling, W.G., 1993. The Geothermal Activity and Geology of the Northern Sector of the Kenya Rift Valley. British Geological Survey Research Report SC/93/1. NERC, Nottingham, pp. 185.
Furman, T., Bryce, J.G., Karson, J., Iotti, A., 2004. East African rift system (EARS) plume structure: insights from Quaternary mafic lavas of Turkana, Kenya. J. Petrol. 45, 1069e1088. Garcin, Y., Junginger, A., Melnick, D., Olago, D.O., Strecker, M.R., Trauth, M.H., 2009. Late PleistoceneeHolocene rise and collapse of Lake Suguta, northern Kenya rift. Quaternary Sci. Rev. 28, 911e925. Gifford-Gonzalez, D., 1998. Early pastoralists in East Africa: ecological and social dimensions. J. Anthropol. Archaeol. 17, 166e200. Koch, C.P., Pavlish, L.A., Farquhar, R.M., Hancock, R.G.V., Beukens, R.P., 2002. INAA of pottery from Il Lokeridede and Jarigole, Koobi Fora region, Kenya. In: Jerem, E., Biro, K.T. (Eds.), Archaeometry 98, proceedings of the 31st Archaeometry Symposium, Bar International Series 1043 (II). Oxford, pp. 587e592. Lynch, B.M., Robbins, L.H., 1978. Namoratunga: the first archeoastronomical evidence in Sub-Saharan Africa. Science 200, 766e768. McDougall, I., Watkins, R.T., 1988. Potassium-argon ages of volcanic rocks from northeast of Lake Turkana, northern Kenya. Geol. Mag. 125, 15e23. McDougall, I., Watkins, R.T., 2006. Geochronology of the Nabwal Hills: a record of earliest magmatism in the northern Kenya rift valley. Geol. Mag. 143, 25e39. Merrick, H.V., Brown, F.H., 1984a. Rapid chemical characterization of obsidian artifacts by electron microprobe analysis. Archaeometry 26, 230e236. Merrick, H.V., Brown, F.H., 1984b. Obsidian sources and patterns of source utilization in Kenya and Tanzania: some initial findings. Afr. Archaeological Rev. 2, 129e152. Merrick, H.V., Brown, F.H., Nash, W.P., 1994. Use and movement of obsidian in the early and middle stone ages of Kenya and northern Tanzania. MASCA Research Papers in Science and Archaeology. In: Childs, S.T. (Ed.), Society, Culture, and Technology in Africa, pp. 29e44. Supplement to Vol. 11, 1994. Nash, W.P., 1992. Analysis of oxygen with the electron microprobe: applications to hydrated glass and minerals. Am. Mineral 77, 453e457. Negash, A., Shackley, M.S., 2006. Geochemical provenance of obsidian artifacts from the MSA site of Porc Epic, Ethiopia. Archaeometry 48, 1e12. Negash, A., Alene, M., Brown, F.H., Nash, B.P., Shackley, M.S., 2007. Geochemical provenance of the terminal Pleistocene/early Holocene obsidian artifacts from the site of Beseka, central Ethiopia. J. Archaeol. Sci. 34, 1205e1210. Negash, A., Brown, F.H., Nash, B.P., 2011. Varieties and sources of artifactual obsidian in the Middle Stone Age of the Middle Awash, Ethiopia. Archaeometry 53, in press. Nelson, C.M.,1993. Evidence for early trade between the coast and interior of East Africa. In: Paper Prepared for the WAC Mombassa Intercongress Conference. Electronic Document. http://www.chaz.rg/Arch/Turkana/Jarigole/TERADEW>HTM Accessed 2010. Nelson, C.M., 1995. The work of the Koobi Fora field school at the Jarigole pillar site. Kenya Past and Present 27, 49e63. Phillipson, D.W., 1977. Lowasera. Azania 12, 1e32. Soper, R., 1982. Archaeo-astronomical cushites: some comments. Azania 17, 145e162. Watkins, R.T., 1981. The Geochemistry of Rhyolitic and Tholeiitic Rocks From East of Lake Turkana, Northern Kenya, MSc. Thesis, University of Leeds, pp. 155.