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Dating the Naisiusiu Beds, Olduvai Gorge, by electron spin resonance
Quaternary Science Reviews 22 (2003) 1361–1366
Dating the Naisiusiu Beds, Olduvai Gorge, by electron spin resonance A.R. Skinnera,*, R.L. Hayb, F. Masaoc, B.A.B. Blackwella a
Department of Chemistry, Williams College, Williamstown, MA 01267, USA Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA c Paleo-Cultural and Environmental Research, P.O. Box 70566, Dar es Salaam, Tanzania b
Abstract The lower beds at Olduvai Gorge are well known for containing early hominid fossils and Oldowan stone tools, and their ages have been established by 40Ar/39Ar dating and paleomagnetic stratigraphy. Ages are generally less certain for the upper deposits at Olduvai Gorge because of the scarcity of datable tuffs. The youngest archaeologically significant site at Olduvai is microlithic LSA, which lies in the type section of the Naisiusiu Beds. The age for the site is controversial, with 14C dates of 17,000–17,550 (Hay, R.L., 1976 Geology of Olduvai Gorge, University of California Press, Berkeley) and >42,000 BP (Manega, P.C., 1993. Geochronology, geochemistry, and isotopic study of the Plio–Pleistocene Hominid sites and the Ngorongoro Volcanic Highland in Northern Tanzania. Unpublished Ph.D. Thesis, University of Colorado, Boulder, CO). The tuff bed in the zone with artifacts does not contain materials datable by 40Ar/39Ar, and some other dating method was needed. In the summer of 2001, five equid teeth were collected from the type Naisiusiu site. Another tooth had previously been collected. ESR ages have been determined for three teeth from the archaeological level and their ages cluster around 6275 ka, assuming linear uranium uptake. Another tooth from a level without artifacts and believed to be significantly younger dated to 3975 ka, again assuming LU. These dates are considerably older than previous estimates and suggest that the East African MSA/LSA transition occurred very early. r 2003 Elsevier Science Ltd. All rights reserved.
1. Introduction Paleoanthropologists and many members of the general public are aware of the importance of Olduvai Gorge’s lower beds to early hominid evolution studies. The gorge’s upper beds are less known, but also important. The Naisiusiu Beds are the most recent of these, deposited after the gorge had eroded nearly to its present depth. They are particularly interesting because they contain a Late Stone Age microlith-bearing deposit with both obsidian and chert material. L.S.B. Leakey collected tools from these beds in 1931, which he grouped within the Upper Kenya Capsian, but the materiel was later divided among several museums. A more comprehensive excavation by M.D. Leakey in 1969 yielded both tools and lithic waste. The excavation also produced fragmentary bones, teeth identified as Equus burchelli, and ostrich egg shell (Leakey et al. (1972)). The excavators found no hominid remains or *Corresponding author. Tel.: +1-413-597-2285; fax: +1-413-5974116. E-mail address: [email protected] (A.R. Skinner).
signs of hominid occupation other than the tools, which have sharp edges and lack evidence of fluvial reworking. Attempts to obtain ages for the Naisiusiu Beds, using radiocarbon, 40Ar/39Ar and amino acid racemization, have been largely inconclusive. Manega (1993) obtained an AMS 14C date of >42 ky BP for the Naisiusiu archaeological site, in sharp contrast to those reported by Leakey et al. (1972), 17 ka BP and 17.671.0 (not calibrated). The latter dates are very probably traceable to contamination by modern carbon, and the former date is simply a lower bound, not a precise age. Manega also dated biotite grains by 40Ar/39Ar, yielding an age of 42710 ka. He dated one grain to 90730 ka and stated that the beds might be this old. However, the 40Ar/39Ar method would not be expected to give good results on a tuff created by lithification of reworked aeolian sediments. Aspartic acid racemization on bones and teeth (Bada, 1981) resulted partly in contradictory dates, in which the upper Ndutu bed, laid down prior to the Naisiusiu, yielded ages both younger (32,000 years) and older (56,000 years) than the Naisuisiu Beds (39,000 years). These results may be attributable to incorrect 14C calibration. Although no specific date for the artifacts
0277-3791/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0277-3791(03)00015-5
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can be inferred from these results, they can be presumed to be too old for 14C dating. Calcrete from Naisiusiu outcrops at the gorge rim, above where the Cattle Trail teeth were collected, yielded uncalibrated 14C ages of 15.870.3– 22.570.5 ky BP (Hay, 1976). Contamination with young carbon was regarded as improbable because these calcretes are quite impermeable, but the dated deposit may not be contemporaneous with that in which the teeth were found. Bone collagen 14C ages from the Cattle Trail site of this study, again uncalibrated, clustered between 10 and 15 ky BP (Hay, 1976), and may well reflect modern contamination. Electron spin resonance (ESR) dating is particularly well suited for sites that are older than radiocarbon limits. While many materials are potentially datable by ESR, including ostrich eggshell, fossil teeth are both common and good for dating. Published results on teeth (Rink, 1997) include ages ranging from a few thousand years to several million years. ESR measures radiation damage in a sample, not radioactive decay, and is therefore dependent on the environment. An early effort to use ESR dating on the tooth fragments collected by Bada was incomplete because the fragments did not have enough dentine to provide a trustworthy internal dose estimate. In 1999, Dr. Richard Hay provided an equid tooth from the Naisiusiu Bed type section for a test of ESR dating. The preliminary result, an age of 60,000 years, is ‘consistent’ with Manega’s (1993) 14C date of >42,000 BP. However, no single sample should be considered definitive. Therefore a new excavation for fossil mammal teeth was conducted in August 2001.
2. Site geology Most samples in the present study were collected at the Naisiusiu type section, now known as MDL Hill after the 1969 excavator. The type site is at the bottom of the gorge, 110 m W of the Second Fault on the north side midway between localities 29 and 5 of Hay (1976). The total height of the hill, on the east side where the trenches were dug, is approximately 8 m. The base of the section rests on the basalt at the bottom of the gorge. Immediately above this are sands and gravels indicating a fluvial environment. Above the fluvial unit are 2 m of fine-grained tuffaceous sandstone, capped by a yellow marker tuff. Detailed stratigraphy of the trench (Fig. 1) subdivided the fine-grained sandstone into two units of differing color, and the main sample layer is approximately 40 cm below the interface of the two, and about 20 cm above the fluvial unit. Above the marker tuff the deposits are exclusively aeolian. No faunal material has been recovered from these aeolian deposits although a few tools were found in the lowermost section.
Fig. 1. Stratigraphy of Trench 1, MDL Hill. Levels with teeth are shown by xxxx.
The most extensive excavation was an E–W trench running parallel to and approximately 3 m north of the original MDL trench. Sediments were cut down in 10 cm steps beginning 1.25 m b.d., in the grey and brown sandstones of Fig. 1. While some bones and lithic material were found in all steps, the majority came from the first step, just below the marker tuff, and from a step 1 m below. The second artifactual layer corresponds to that found by Mary Leakey. The sample locations are indicated both by notation and by ‘xxxx’ in Fig. 1. Below that step the sediments again contained few bones or tools. Additional tooth samples were collected on the surface, and in material scraped off the profile surface before excavation began (‘surface scrape’ in Table 1). Since no teeth were found at the top step of the first trench, a second trench was cut 50 cm to the south, just below the marker tuff. One tooth was recovered there. Naisiusiu aeolian tuffs are found discontinuously around the rim and along the sides of the gorge, generally underlain and overlain by laminated calcrete. Tooth samples were collected from aeolian tuff along a cattle trail, on the north side of the gorge, west of Hoopoe Gully. The Cattle Trail site is typical of the discontinuous Naisiusiu exposures. Teeth were collected from exposed faces in deposits approximately 1 m thick, capped by
A.R. Skinner et al. / Quaternary Science Reviews 22 (2003) 1361–1366 Table 1 Sample locations for ESR-datable teeth
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4. Methodology
yields a time-averaged dose rate (Blackwell and Schwarcz, 1993; Skinner, 2000; Blackwell et al., 2001), would provide a more accurate age in these cases. For this project, with one exception, the teeth were too small to provide an isochron, and even for that one no isochron has been calculated because the internal dose rates are too similar to give a statistically significant result. Fortunately the geology of the area suggests strongly that the climate over the last 50 ka has been largely similar to present-day conditions (Hay, 1976). Wetter and slightly cooler conditions during the period 24–27 ka are suggested by AAR results (J.L. Bada, 1974, pers. commun.), but as shown in Fig. 2, changes of a few percent in environmental water would not have a significant impact on the ages. Secondly, the internal dose rate in tooth enamel varies with time because uranium is absorbed by both enamel and dentine during burial. Blackwell et al. (2001), discuss a mathematical function for this uptake, but given the complexities of the fossilization process, we generally assume one of three simple models. For samples in this time range we would expect the uptake model to be somewhere between Early Uptake (EU) and Linear Uptake (LU). Recent Uptake (RU) ages were calculated for these samples, but are not shown because they are neither methodologically nor paleontologically reasonable. U-series measurements on a few samples would help confirm this. After separating dentine and enamel, enamel samples were powdered to 38–90 mm and divided into aliquots of approximately 20 mg each. Irradiation at the McMaster University 60Co source followed, with an average dose rate of 0.1 Gy/s. If only a few aliquots could be obtained from a given subsample, the aliquots were re-irradiated until we had at least 12 ESR intensity values with the maximum artificial dose at or near ten times the AD. Spectra were measured on a JEOL RE1X spectrometer, at 2 mW power, a time constant of 0.1–0.3 s, a scan rate of 1.2 mT/min and modulation amplitude of 0.1 mT.
In ESR dating, we measure the accumulated damage in the tooth caused by environmental radiation from sources both within and without the tooth itself. A good summary of the method can be found in Rink (1997). Once the total radiation damage has been quantified, called here the AD or accumulated dose, the radioisotope concentrations in the enamel, associated dentine, and surrounding sediment are measured to determine the time-integrated dose rate to the tooth. Relating the accumulated dose to the dose rate yields the age. Standard ESR dating requires an external dose calculation. The most reliable results have been obtained from well-stratified cave deposits. Open-air sites, such as the Naisiusiu Beds, have potentially experienced greater environmental fluctuations. Isochron dating, which
Fig. 2. Effect on calculated ages of changing sedimentary water concentration.
Sample
Site
Location
CT24 CT40 CT41 CT42
MDL Hill MDL Hill MDL Hill Cattle Trail
Surface Surface scrape, Trench 1 Surface scrape, Trench 1 In situ
calcrete. All teeth were still embedded in the tuffs with a small portion exposed to the surface. In addition to the teeth, samples of sediment surrounding the teeth were collected to determine the external dose. For CT24, collected (RLH) from the surface of MDL hill, at about the level of tools in Mary Leakey’s excavation, the sediments from the lower archaeological unit (sample layer 2 of the brown sandstone in Fig. 1) were presumed to be appropriate, as teeth and other material were by far most abundant there.
3. Samples Altogether, nine teeth were found in 5 days of excavation. Five were chosen, in consultation with the Tanzanian Department of Antiquities representative, for ESR analysis. All were small equid molars, fragmented and non-specific, although they could well be the same Equus burchelli previously identified at the site. With the tooth supplied earlier by Richard Hay, the suite of material was thus six teeth. To date four have been examined. Two are still in preparation; on one the dentine and enamel were virtually impossible to separate, and another on further examination had very little enamel. Details are given in Table 1.
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The ESR results were fitted to a saturating exponential using Vfit, and the results usually had less than 5% uncertainty in the AD. Neutron activation analysis (NAA) provided radioisotope concentrations in sediment, enamel and dentine. The program Data-HPS was used to calculate the ages presented here. ROSY, the most recent age-calculation program, is based on a better model for b attenuation in tooth enamel (Brennan et al., 1997). However, ages calculated (ARS) for a few subsamples using ROSY gave values indistinguishable from Data-HPS.
Fig. 3. Effect on calculated ages of changing assumed cosmic radiation dose.
5. Results Table 2 shows the uranium concentrations for these teeth, and average values for sediments from within 30 cm of the teeth. The tooth-bearing sediments of MDL Hill in the area sampled are quite uniform in composition and consistency, lacking large cobbles or other ‘lumps’. The doses from bulk sediment should therefore accurately represent those experienced by the teeth. The Cattle Trail site is compositionally similar with respect to radioisotopes, as would be expected since all Table 2 Radioisotope concentrations (a) Tooth dentine and enamel Sample [U] (den) (ppm) CT24 CT24 CT24 CT24 CT24 CT40 CT41 CT41 CT41 CT41 CT42 CT42
EN1 EN2 EN6 EN7+3 Den 4 EN1 EN1 Den3I EN 3 Den4 EN1 EN 2
6.53 5.59 6.54 6.63 4.56 1.36 2.09 1.88 2.27 1.47 4.21 4.46
[U] (en) (ppm) 1.43 1.17 1.18 2.53 0.14 0.19 0.20 0.07 1.52 1.51
(b) Naisiusiu Bed sediments Sample Tooth [U] (ppm) [Th] (ppm) [K] (%) Dext (mGy/a) OLD1A CT24/40 1.47 OLD1C CT24/40 1.95 Avg. 1.71
18.30 20.90 19.15
2.02 2.06 2.04
OLD3A CT41 OLD3C CT41 OLD3F CT41 Avg.
2.46 1.80 2.23 2.16
18.20 19.80 24.00 20.19
1.66 1.69 2.97 2.11
17067115
OLD8A CT42 OLD8B CT42 Avg.
1.23 1.15 1.19
N/A 19.40 19.40
3.20 2.25 2.72
19567123
Naisiusisu sediments are believed to derive from the same source. Calculated ages assume alpha efficiency (k)=0.15, initial 234U/238U ratio=1.20, and enamel density=2.95 g/cm3. Radon loss was assumed to be zero. Although the climate is extremely dry, the dentine moisture content was left at 575%, since in our experience dentine has noticeable moisture especially in teeth this young. The sedimentary water concentration was 572%, based on the moisture content of sediment samples. A cosmic dose of 200 mGy/yr was assumed based on the low level of cover over the samples and the altitude of the site. Figs. 2 and 3 show that the latter two assumptions little affect calculated ages. While a trend is clear, the variation over reasonable changes in environmental parameters is within or close to experimental error. Table 3 shows the calculated ages. For CT24, the tooth provided by Richard Hay, one subsample appears substantially younger than the others. The uranium content of that enamel is still unknown and it may be that when that figure is available, the age will be more conformable. However, Table 3 also shows that the average of all CT24 subsamples does not differ within experimental error whether this subsample is included (avg.) or not (rev. avg).
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6. Discussion
Typical uncertainties are: [U]=70.02 ppm; [Th]=70.5 ppm; and [K]=0.05%.
Previous studies in East Africa indicate that the transition from Middle to Late Stone Age occurred well before 46 ka (Ambrose, 1998). Because the dates proposed in Table 3 would push the age back beyond those suggested by Ambrose, they require very careful scrutiny. Overall, the agreement between three teeth collected from different areas at the type site is very encouraging.
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Table 3 Ages for Naisiusiu samples Sample
Site
AD (Gy)
EU age (ka)
EU Dint (mGy/a)
LU age (ka)
LU Dint (mGy/a)
CT24en1 CT24en2 CT24en4 CT24en6 CT24en7 Avg., CT24 Rev. avg., CT24
MDL MDL MDL MDL MDL
Hill Hill Hill Hill Hill
144.4710.9 153.075.7 101.378.9 149.475.6 14977.0 137.077.8 148.977.4
6275 6774 4675 6474 5874 5974 6374
538743 478745 406749 530742 780769 546749 582750
7176 7675 5175 7374 6975 6875 7275
246723 219719 187717 243719 360737 251723 298724
CT40en1 CT40en2 CT40en3 CT40en4 Avg., CT40 CT41en3 MDL Hill avg.
MDL MDL MDL MDL
Hill Hill Hill Hill
106.179.1 114.974.5 113.577.6 118.474.4 113.276.4 107.274.1
5776 6174 6175 6374 6075 5576 5975
8179 8479 8379 123715 93710 130712
5876 6374 6275 6574 6275 5877 6275
3873 3872 3873 5575 4273 6075
CT42en1 CT42en2 Cattle Trail avg.
Cattle Trail Cattle Trail
86.4473.07 90.0773.71 88.2673.39
3672 3872 3772
261716 205713 233714
3872 4073 3972
12277 9877 11077
MDL Hill
For calculation assumptions see text. Uncertainties are 2s:
It seems unlikely that any arrived at the site through reworking, either from older deposits or from recent burials. Three factors affect calculated ESR ages: the internal dose rate, the external dose rate, and possible changes in the paleoenvironment with time. For teeth, concern about the internal dose rate generally focuses on the uranium uptake model. Here we see that the ages do not depend, within experimental error, on a choice between EU and LU models, partly because the overall uranium concentration is fairly low. This is hardly surprising in such a dry climate. Figs. 2 and 3 show that uncertainty over paleoenvironmental changes is unlikely to affect the validity of these results. The external dose provides about 2/3 of the total dose rate for most samples. Because the sediments are derived from volcanic materials, the relatively high concentrations of thorium and potassium are not surprising. A concern, however, is that if all the samples come from the same sediments, and the sedimentary dose dominates the calculations, the statistical variance computed for the ages may overstate the precision of the results. If we consider the variance in AD shown in Table 3, corrected for variance in dentine uranium concentration shown in Table 2a, the overall precision appears closer to 15% than 8%. Therefore we suggest the age of the MDL Hill deposits and the associated artifacts is 60710 ka. The Cattle Trail site is not archaeologically interesting in itself. While it provided faunal material not found in other aeolian tuffs, no artifacts have been recovered there and very few from similar outcrops. However, its age should provide a minimum age for MDL Hill. The date obtained for CT42 is, we note, consistent with the
geological assumption that aeolian Naisiusiu deposits are substantially younger than the levels containing LSA material. Although the aeolian tuffs of the cattle trail are lithologically similar to the upper and lithified aeolian tuffs at MDL Hill, no basis presently exists for more detailed correlation as individual beds of aeolian tuff and their associated calcretes are discontinuous, and the Naisiusiu marker tuff of Hay (1976) has not been found at this locality.
Acknowledgements Excavation and collection was carried out under Tanzanian Commission for Science and Technology (COSTECH) permit 2001-23 and a Department of Antiquities Excavator and Collector License, issued to ARS and supervised by Peter Abwole of the Tanzanian Antiquities Department. Funding for this project was obtained from the National Science Foundation (NSF Grants ILI 9151111 and SBR 9904376), the Stearns Foundation, and Williams College. Additional support was provided to RLH by OLAPP (Olduvai Landscape Paleoanthropology Project) and the Leakey Foundation. ARS thanks OLAPP and the Tanzanian Antiquities Department for the use of Olduvai Camp and its facilities. For sample preparation, we are indebted to Emily Hogeland of Williamstown. Some irradiations were performed by Janet Welch at McMaster University, and NAA analyses by Jean Johnson. Francis Mach of Williams College provided additional assistance in preparation and ESR measurements.
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References Ambrose, S.H., 1998. Chronology of the Later Stone Age and food production in East Africa. Journal of Archaeological Science 25, 377–392. Bada, J.L., 1981. Racemization of amino acids in fossil bones and teeth from the Olduvai Gorge region, Tanzania, East Africa. Earth and Planetary Science Letters 13, 292–298. Blackwell, B.A., Schwarcz, H.P., 1993. ESR isochron dating: a brief demonstration in solving the external dose calculation problem. Applied Radiation and Isotopes 44, 243–252. Blackwell, B.A.B., Skinner, A.R., Blickstein, J.I.B., 2001. ESR isochron exercises: how accurately do modern dose rate measurements reflect paleodose rates? Quaternary Science Reviews (Quaternary Geochronology) 20, 1031–1039. Brennan, B.J., Rink, W.J., McGuirl, E.L., Schwarcz, H.P., Prestwich, W.V., 1997. Beta doses in tooth enamel by ‘‘one-group’’ theory and
the rosy ESR dating software. Radiation Measurements 27, 307–314. Hay, R.L., 1976. Geology of the Olduvai Gorge. University of California Press, Berkeley. Leakey, M.D., Hay, R.L., Thurber, R., Protsch, R., Berger, R., 1972. Stratigraphy, archaeology and age of the Ndutu and Naisiusiu beds, Olduvai Gorge, Tanzania. World Archaeology 3, 328–341. Manega, P.C., 1993. Geochronology, geochemistry, and isotopic study of the Plio–Pleistocene Hominid sites and the Ngorongoro Volcanic Highland in Northern Tanzania. Unpublished Ph.D. Thesis, University of Colorado, Boulder, CO. Rink, W.J., 1997. Electron spin resonance (ESR) dating and ESR applications in quaternary science and archaeometry. Radiation Measurements 27, 975–1025. Skinner, Anne R., 2000. ESR dating: is it still an ‘experimental’ technique? Applied Radiation and Isotopes 52, 1311–1316.
Dating the Naisiusiu Beds, Olduvai Gorge, by electron spin resonance A.R. Skinnera,*, R.L. Hayb, F. Masaoc, B.A.B. Blackwella a
Department of Chemistry, Williams College, Williamstown, MA 01267, USA Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA c Paleo-Cultural and Environmental Research, P.O. Box 70566, Dar es Salaam, Tanzania b
Abstract The lower beds at Olduvai Gorge are well known for containing early hominid fossils and Oldowan stone tools, and their ages have been established by 40Ar/39Ar dating and paleomagnetic stratigraphy. Ages are generally less certain for the upper deposits at Olduvai Gorge because of the scarcity of datable tuffs. The youngest archaeologically significant site at Olduvai is microlithic LSA, which lies in the type section of the Naisiusiu Beds. The age for the site is controversial, with 14C dates of 17,000–17,550 (Hay, R.L., 1976 Geology of Olduvai Gorge, University of California Press, Berkeley) and >42,000 BP (Manega, P.C., 1993. Geochronology, geochemistry, and isotopic study of the Plio–Pleistocene Hominid sites and the Ngorongoro Volcanic Highland in Northern Tanzania. Unpublished Ph.D. Thesis, University of Colorado, Boulder, CO). The tuff bed in the zone with artifacts does not contain materials datable by 40Ar/39Ar, and some other dating method was needed. In the summer of 2001, five equid teeth were collected from the type Naisiusiu site. Another tooth had previously been collected. ESR ages have been determined for three teeth from the archaeological level and their ages cluster around 6275 ka, assuming linear uranium uptake. Another tooth from a level without artifacts and believed to be significantly younger dated to 3975 ka, again assuming LU. These dates are considerably older than previous estimates and suggest that the East African MSA/LSA transition occurred very early. r 2003 Elsevier Science Ltd. All rights reserved.
1. Introduction Paleoanthropologists and many members of the general public are aware of the importance of Olduvai Gorge’s lower beds to early hominid evolution studies. The gorge’s upper beds are less known, but also important. The Naisiusiu Beds are the most recent of these, deposited after the gorge had eroded nearly to its present depth. They are particularly interesting because they contain a Late Stone Age microlith-bearing deposit with both obsidian and chert material. L.S.B. Leakey collected tools from these beds in 1931, which he grouped within the Upper Kenya Capsian, but the materiel was later divided among several museums. A more comprehensive excavation by M.D. Leakey in 1969 yielded both tools and lithic waste. The excavation also produced fragmentary bones, teeth identified as Equus burchelli, and ostrich egg shell (Leakey et al. (1972)). The excavators found no hominid remains or *Corresponding author. Tel.: +1-413-597-2285; fax: +1-413-5974116. E-mail address: [email protected] (A.R. Skinner).
signs of hominid occupation other than the tools, which have sharp edges and lack evidence of fluvial reworking. Attempts to obtain ages for the Naisiusiu Beds, using radiocarbon, 40Ar/39Ar and amino acid racemization, have been largely inconclusive. Manega (1993) obtained an AMS 14C date of >42 ky BP for the Naisiusiu archaeological site, in sharp contrast to those reported by Leakey et al. (1972), 17 ka BP and 17.671.0 (not calibrated). The latter dates are very probably traceable to contamination by modern carbon, and the former date is simply a lower bound, not a precise age. Manega also dated biotite grains by 40Ar/39Ar, yielding an age of 42710 ka. He dated one grain to 90730 ka and stated that the beds might be this old. However, the 40Ar/39Ar method would not be expected to give good results on a tuff created by lithification of reworked aeolian sediments. Aspartic acid racemization on bones and teeth (Bada, 1981) resulted partly in contradictory dates, in which the upper Ndutu bed, laid down prior to the Naisiusiu, yielded ages both younger (32,000 years) and older (56,000 years) than the Naisuisiu Beds (39,000 years). These results may be attributable to incorrect 14C calibration. Although no specific date for the artifacts
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can be inferred from these results, they can be presumed to be too old for 14C dating. Calcrete from Naisiusiu outcrops at the gorge rim, above where the Cattle Trail teeth were collected, yielded uncalibrated 14C ages of 15.870.3– 22.570.5 ky BP (Hay, 1976). Contamination with young carbon was regarded as improbable because these calcretes are quite impermeable, but the dated deposit may not be contemporaneous with that in which the teeth were found. Bone collagen 14C ages from the Cattle Trail site of this study, again uncalibrated, clustered between 10 and 15 ky BP (Hay, 1976), and may well reflect modern contamination. Electron spin resonance (ESR) dating is particularly well suited for sites that are older than radiocarbon limits. While many materials are potentially datable by ESR, including ostrich eggshell, fossil teeth are both common and good for dating. Published results on teeth (Rink, 1997) include ages ranging from a few thousand years to several million years. ESR measures radiation damage in a sample, not radioactive decay, and is therefore dependent on the environment. An early effort to use ESR dating on the tooth fragments collected by Bada was incomplete because the fragments did not have enough dentine to provide a trustworthy internal dose estimate. In 1999, Dr. Richard Hay provided an equid tooth from the Naisiusiu Bed type section for a test of ESR dating. The preliminary result, an age of 60,000 years, is ‘consistent’ with Manega’s (1993) 14C date of >42,000 BP. However, no single sample should be considered definitive. Therefore a new excavation for fossil mammal teeth was conducted in August 2001.
2. Site geology Most samples in the present study were collected at the Naisiusiu type section, now known as MDL Hill after the 1969 excavator. The type site is at the bottom of the gorge, 110 m W of the Second Fault on the north side midway between localities 29 and 5 of Hay (1976). The total height of the hill, on the east side where the trenches were dug, is approximately 8 m. The base of the section rests on the basalt at the bottom of the gorge. Immediately above this are sands and gravels indicating a fluvial environment. Above the fluvial unit are 2 m of fine-grained tuffaceous sandstone, capped by a yellow marker tuff. Detailed stratigraphy of the trench (Fig. 1) subdivided the fine-grained sandstone into two units of differing color, and the main sample layer is approximately 40 cm below the interface of the two, and about 20 cm above the fluvial unit. Above the marker tuff the deposits are exclusively aeolian. No faunal material has been recovered from these aeolian deposits although a few tools were found in the lowermost section.
Fig. 1. Stratigraphy of Trench 1, MDL Hill. Levels with teeth are shown by xxxx.
The most extensive excavation was an E–W trench running parallel to and approximately 3 m north of the original MDL trench. Sediments were cut down in 10 cm steps beginning 1.25 m b.d., in the grey and brown sandstones of Fig. 1. While some bones and lithic material were found in all steps, the majority came from the first step, just below the marker tuff, and from a step 1 m below. The second artifactual layer corresponds to that found by Mary Leakey. The sample locations are indicated both by notation and by ‘xxxx’ in Fig. 1. Below that step the sediments again contained few bones or tools. Additional tooth samples were collected on the surface, and in material scraped off the profile surface before excavation began (‘surface scrape’ in Table 1). Since no teeth were found at the top step of the first trench, a second trench was cut 50 cm to the south, just below the marker tuff. One tooth was recovered there. Naisiusiu aeolian tuffs are found discontinuously around the rim and along the sides of the gorge, generally underlain and overlain by laminated calcrete. Tooth samples were collected from aeolian tuff along a cattle trail, on the north side of the gorge, west of Hoopoe Gully. The Cattle Trail site is typical of the discontinuous Naisiusiu exposures. Teeth were collected from exposed faces in deposits approximately 1 m thick, capped by
A.R. Skinner et al. / Quaternary Science Reviews 22 (2003) 1361–1366 Table 1 Sample locations for ESR-datable teeth
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4. Methodology
yields a time-averaged dose rate (Blackwell and Schwarcz, 1993; Skinner, 2000; Blackwell et al., 2001), would provide a more accurate age in these cases. For this project, with one exception, the teeth were too small to provide an isochron, and even for that one no isochron has been calculated because the internal dose rates are too similar to give a statistically significant result. Fortunately the geology of the area suggests strongly that the climate over the last 50 ka has been largely similar to present-day conditions (Hay, 1976). Wetter and slightly cooler conditions during the period 24–27 ka are suggested by AAR results (J.L. Bada, 1974, pers. commun.), but as shown in Fig. 2, changes of a few percent in environmental water would not have a significant impact on the ages. Secondly, the internal dose rate in tooth enamel varies with time because uranium is absorbed by both enamel and dentine during burial. Blackwell et al. (2001), discuss a mathematical function for this uptake, but given the complexities of the fossilization process, we generally assume one of three simple models. For samples in this time range we would expect the uptake model to be somewhere between Early Uptake (EU) and Linear Uptake (LU). Recent Uptake (RU) ages were calculated for these samples, but are not shown because they are neither methodologically nor paleontologically reasonable. U-series measurements on a few samples would help confirm this. After separating dentine and enamel, enamel samples were powdered to 38–90 mm and divided into aliquots of approximately 20 mg each. Irradiation at the McMaster University 60Co source followed, with an average dose rate of 0.1 Gy/s. If only a few aliquots could be obtained from a given subsample, the aliquots were re-irradiated until we had at least 12 ESR intensity values with the maximum artificial dose at or near ten times the AD. Spectra were measured on a JEOL RE1X spectrometer, at 2 mW power, a time constant of 0.1–0.3 s, a scan rate of 1.2 mT/min and modulation amplitude of 0.1 mT.
In ESR dating, we measure the accumulated damage in the tooth caused by environmental radiation from sources both within and without the tooth itself. A good summary of the method can be found in Rink (1997). Once the total radiation damage has been quantified, called here the AD or accumulated dose, the radioisotope concentrations in the enamel, associated dentine, and surrounding sediment are measured to determine the time-integrated dose rate to the tooth. Relating the accumulated dose to the dose rate yields the age. Standard ESR dating requires an external dose calculation. The most reliable results have been obtained from well-stratified cave deposits. Open-air sites, such as the Naisiusiu Beds, have potentially experienced greater environmental fluctuations. Isochron dating, which
Fig. 2. Effect on calculated ages of changing sedimentary water concentration.
Sample
Site
Location
CT24 CT40 CT41 CT42
MDL Hill MDL Hill MDL Hill Cattle Trail
Surface Surface scrape, Trench 1 Surface scrape, Trench 1 In situ
calcrete. All teeth were still embedded in the tuffs with a small portion exposed to the surface. In addition to the teeth, samples of sediment surrounding the teeth were collected to determine the external dose. For CT24, collected (RLH) from the surface of MDL hill, at about the level of tools in Mary Leakey’s excavation, the sediments from the lower archaeological unit (sample layer 2 of the brown sandstone in Fig. 1) were presumed to be appropriate, as teeth and other material were by far most abundant there.
3. Samples Altogether, nine teeth were found in 5 days of excavation. Five were chosen, in consultation with the Tanzanian Department of Antiquities representative, for ESR analysis. All were small equid molars, fragmented and non-specific, although they could well be the same Equus burchelli previously identified at the site. With the tooth supplied earlier by Richard Hay, the suite of material was thus six teeth. To date four have been examined. Two are still in preparation; on one the dentine and enamel were virtually impossible to separate, and another on further examination had very little enamel. Details are given in Table 1.
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The ESR results were fitted to a saturating exponential using Vfit, and the results usually had less than 5% uncertainty in the AD. Neutron activation analysis (NAA) provided radioisotope concentrations in sediment, enamel and dentine. The program Data-HPS was used to calculate the ages presented here. ROSY, the most recent age-calculation program, is based on a better model for b attenuation in tooth enamel (Brennan et al., 1997). However, ages calculated (ARS) for a few subsamples using ROSY gave values indistinguishable from Data-HPS.
Fig. 3. Effect on calculated ages of changing assumed cosmic radiation dose.
5. Results Table 2 shows the uranium concentrations for these teeth, and average values for sediments from within 30 cm of the teeth. The tooth-bearing sediments of MDL Hill in the area sampled are quite uniform in composition and consistency, lacking large cobbles or other ‘lumps’. The doses from bulk sediment should therefore accurately represent those experienced by the teeth. The Cattle Trail site is compositionally similar with respect to radioisotopes, as would be expected since all Table 2 Radioisotope concentrations (a) Tooth dentine and enamel Sample [U] (den) (ppm) CT24 CT24 CT24 CT24 CT24 CT40 CT41 CT41 CT41 CT41 CT42 CT42
EN1 EN2 EN6 EN7+3 Den 4 EN1 EN1 Den3I EN 3 Den4 EN1 EN 2
6.53 5.59 6.54 6.63 4.56 1.36 2.09 1.88 2.27 1.47 4.21 4.46
[U] (en) (ppm) 1.43 1.17 1.18 2.53 0.14 0.19 0.20 0.07 1.52 1.51
(b) Naisiusiu Bed sediments Sample Tooth [U] (ppm) [Th] (ppm) [K] (%) Dext (mGy/a) OLD1A CT24/40 1.47 OLD1C CT24/40 1.95 Avg. 1.71
18.30 20.90 19.15
2.02 2.06 2.04
OLD3A CT41 OLD3C CT41 OLD3F CT41 Avg.
2.46 1.80 2.23 2.16
18.20 19.80 24.00 20.19
1.66 1.69 2.97 2.11
17067115
OLD8A CT42 OLD8B CT42 Avg.
1.23 1.15 1.19
N/A 19.40 19.40
3.20 2.25 2.72
19567123
Naisiusisu sediments are believed to derive from the same source. Calculated ages assume alpha efficiency (k)=0.15, initial 234U/238U ratio=1.20, and enamel density=2.95 g/cm3. Radon loss was assumed to be zero. Although the climate is extremely dry, the dentine moisture content was left at 575%, since in our experience dentine has noticeable moisture especially in teeth this young. The sedimentary water concentration was 572%, based on the moisture content of sediment samples. A cosmic dose of 200 mGy/yr was assumed based on the low level of cover over the samples and the altitude of the site. Figs. 2 and 3 show that the latter two assumptions little affect calculated ages. While a trend is clear, the variation over reasonable changes in environmental parameters is within or close to experimental error. Table 3 shows the calculated ages. For CT24, the tooth provided by Richard Hay, one subsample appears substantially younger than the others. The uranium content of that enamel is still unknown and it may be that when that figure is available, the age will be more conformable. However, Table 3 also shows that the average of all CT24 subsamples does not differ within experimental error whether this subsample is included (avg.) or not (rev. avg).
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6. Discussion
Typical uncertainties are: [U]=70.02 ppm; [Th]=70.5 ppm; and [K]=0.05%.
Previous studies in East Africa indicate that the transition from Middle to Late Stone Age occurred well before 46 ka (Ambrose, 1998). Because the dates proposed in Table 3 would push the age back beyond those suggested by Ambrose, they require very careful scrutiny. Overall, the agreement between three teeth collected from different areas at the type site is very encouraging.
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Table 3 Ages for Naisiusiu samples Sample
Site
AD (Gy)
EU age (ka)
EU Dint (mGy/a)
LU age (ka)
LU Dint (mGy/a)
CT24en1 CT24en2 CT24en4 CT24en6 CT24en7 Avg., CT24 Rev. avg., CT24
MDL MDL MDL MDL MDL
Hill Hill Hill Hill Hill
144.4710.9 153.075.7 101.378.9 149.475.6 14977.0 137.077.8 148.977.4
6275 6774 4675 6474 5874 5974 6374
538743 478745 406749 530742 780769 546749 582750
7176 7675 5175 7374 6975 6875 7275
246723 219719 187717 243719 360737 251723 298724
CT40en1 CT40en2 CT40en3 CT40en4 Avg., CT40 CT41en3 MDL Hill avg.
MDL MDL MDL MDL
Hill Hill Hill Hill
106.179.1 114.974.5 113.577.6 118.474.4 113.276.4 107.274.1
5776 6174 6175 6374 6075 5576 5975
8179 8479 8379 123715 93710 130712
5876 6374 6275 6574 6275 5877 6275
3873 3872 3873 5575 4273 6075
CT42en1 CT42en2 Cattle Trail avg.
Cattle Trail Cattle Trail
86.4473.07 90.0773.71 88.2673.39
3672 3872 3772
261716 205713 233714
3872 4073 3972
12277 9877 11077
MDL Hill
For calculation assumptions see text. Uncertainties are 2s:
It seems unlikely that any arrived at the site through reworking, either from older deposits or from recent burials. Three factors affect calculated ESR ages: the internal dose rate, the external dose rate, and possible changes in the paleoenvironment with time. For teeth, concern about the internal dose rate generally focuses on the uranium uptake model. Here we see that the ages do not depend, within experimental error, on a choice between EU and LU models, partly because the overall uranium concentration is fairly low. This is hardly surprising in such a dry climate. Figs. 2 and 3 show that uncertainty over paleoenvironmental changes is unlikely to affect the validity of these results. The external dose provides about 2/3 of the total dose rate for most samples. Because the sediments are derived from volcanic materials, the relatively high concentrations of thorium and potassium are not surprising. A concern, however, is that if all the samples come from the same sediments, and the sedimentary dose dominates the calculations, the statistical variance computed for the ages may overstate the precision of the results. If we consider the variance in AD shown in Table 3, corrected for variance in dentine uranium concentration shown in Table 2a, the overall precision appears closer to 15% than 8%. Therefore we suggest the age of the MDL Hill deposits and the associated artifacts is 60710 ka. The Cattle Trail site is not archaeologically interesting in itself. While it provided faunal material not found in other aeolian tuffs, no artifacts have been recovered there and very few from similar outcrops. However, its age should provide a minimum age for MDL Hill. The date obtained for CT42 is, we note, consistent with the
geological assumption that aeolian Naisiusiu deposits are substantially younger than the levels containing LSA material. Although the aeolian tuffs of the cattle trail are lithologically similar to the upper and lithified aeolian tuffs at MDL Hill, no basis presently exists for more detailed correlation as individual beds of aeolian tuff and their associated calcretes are discontinuous, and the Naisiusiu marker tuff of Hay (1976) has not been found at this locality.
Acknowledgements Excavation and collection was carried out under Tanzanian Commission for Science and Technology (COSTECH) permit 2001-23 and a Department of Antiquities Excavator and Collector License, issued to ARS and supervised by Peter Abwole of the Tanzanian Antiquities Department. Funding for this project was obtained from the National Science Foundation (NSF Grants ILI 9151111 and SBR 9904376), the Stearns Foundation, and Williams College. Additional support was provided to RLH by OLAPP (Olduvai Landscape Paleoanthropology Project) and the Leakey Foundation. ARS thanks OLAPP and the Tanzanian Antiquities Department for the use of Olduvai Camp and its facilities. For sample preparation, we are indebted to Emily Hogeland of Williamstown. Some irradiations were performed by Janet Welch at McMaster University, and NAA analyses by Jean Johnson. Francis Mach of Williams College provided additional assistance in preparation and ESR measurements.
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the rosy ESR dating software. Radiation Measurements 27, 307–314. Hay, R.L., 1976. Geology of the Olduvai Gorge. University of California Press, Berkeley. Leakey, M.D., Hay, R.L., Thurber, R., Protsch, R., Berger, R., 1972. Stratigraphy, archaeology and age of the Ndutu and Naisiusiu beds, Olduvai Gorge, Tanzania. World Archaeology 3, 328–341. Manega, P.C., 1993. Geochronology, geochemistry, and isotopic study of the Plio–Pleistocene Hominid sites and the Ngorongoro Volcanic Highland in Northern Tanzania. Unpublished Ph.D. Thesis, University of Colorado, Boulder, CO. Rink, W.J., 1997. Electron spin resonance (ESR) dating and ESR applications in quaternary science and archaeometry. Radiation Measurements 27, 975–1025. Skinner, Anne R., 2000. ESR dating: is it still an ‘experimental’ technique? Applied Radiation and Isotopes 52, 1311–1316.