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Comment on the age of Late Pleistocene cave deposits associated with the "Howiesons Poort" stone tool industry in South Africa. J. Francis Thackeray January, 2002 Late Pleistocene cave deposits such as those from Klasies River Mouth (Singer and Wymer, 1982; Deacon, H.J. 1995) have yielded hominid fossils and artefacts that are relevant to questions concerning where and when "anatomically modern" Homo sapiens developed "modern" behaviour. Of particular interest is the dating of a Middle Stone Age industry named the "Howiesons Poort", with relatively large stone segments and flake-blades (A.I. Thackeray, 1992). It is generally accepted that the Howiesons Poort industry is greater than 40 kya (1 kya = 1000 years ago), but precise dating is difficult since it is beyond the range of conventional radiocarbon dating. Thorium-uranium (Th-U) analyses of limestone formations, at Klasies River Mouth and Boomplaas caves (Vogel, 2001), have been used to place the Howiesons Poort Industry in South Africa between 70 and 60 kya. The possibility that the industry relates to a younger period, between 58 and 48 kya, has been suggested on the basis of analyses of mammalian microfauna, marine molluscs, and oxygen isotope ratios that reflect global changes in past climate (J.F. Thackeray, 1992). In this article, thorium-uranium results reported by Vogel (2001) are re-examined, focussing on standard deviations and variability around mean values for particular dates, rather than on mean values alone. Using the Th-U dating technique, Vogel (2001) obtained a mean age of 65.6 +/- 5.3 kya for a Howiesons Poort sample from Klasies River Mouth. This compares favourably with mean Th-U dates of 64.0 (+/- 3.2) kya for sample U417, 66.1 (+/- 13) kya for sample U414, 58.6 (+/- 3.7) kya for sample U365, and 62.4 +/- 2.0 kya for a "composite" sample, all associated with the Howiesons Poort from Boomplaas cave. However, these results need to be examined carefully in relation to standard deviations associated with mean values. Notably, the 95% confidence limits (two standard deviations on either side of the mean value) associated with the Howiesons Poort sample from Klasies River Mouth span a period from 55.0 - 76.2 kya. The 95% confidence limits associated with Boomplaas samples U 417, U414, U365 and the "composite" sample are respectively 57.6 - 70.4 kya, 40.1 - 92.1 kya, 51.2 - 66.0 kya, and 58.4 - 66.4 kya. At least potentially, the Howiesons Poort could be circa 55 kya at Klasies River Mouth, and the industry could be as recent as 51 kya at Boomplaas (based on sample U 365). The results obtained from sample U414 suggest that the Howiesons Poort at Boomplaas could be even younger than 51 kya. Radiocarbon and Th-U dates, from a sequence of deposits at Boomplaas (Vogel, 2001), can be used to establish age-depth relationships. Least-squares linear regression analysis has been undertaken to generate an equation whereby the age of samples may be related to corresponding depth in the cave sequence, for samples that are between 2,000 and 40,000 years old (within the range of the conventional radiocarbon dating technique). For the Boomplaas sequence, a strong correlation ( r > 0.9) has been found between age and depth of these samples, suggesting an almost constant rate of deposition. When the regression equation is applied to estimate the age of samples of known depth, an age estimate of circa 51 kya is obtained for the Howiesons Poort industry at Boomplaas. The estimated date of circa 51 kya based on an age-depth relationship is of interest in the context of the suggestion that part if not all of the Howiesons Poort at Klasies River Mouth can be dated between 48 - 58 kya (J.F. Thackeray, 1992). This suggestion was based on relative abundances of small mammals (Avery,1987; Thackeray and Avery, 1990) and marine molluscs (Thackeray, 1988) from Klasies River Mouth, compared to deuterium isotope ratios from the Vostok ice core in Antarctica (Jouzel et al, 1987). The results were used in such a way to allow estimation of the age of the Howiesons Poort industry at Klasies River Mouth. Dates of between 48 and 58 kya for the Howiesons Poort (J.F. Thackeray, 1997) are not incompatible with results obtained from Th-U analyses of samples from Howiesons Poort layers at Klasies River Mouth and Boomplaas, when these are assessed in the context of 95% confidence limits rather than on mean values of Th-U dates, and in the context of age-depth relationships for the Late Pleistocene sequence at Boomplaas. The chronology of the Howiesons Poort in South Africa is important in the light of discoveries of bone points, incised bone and engraved ochre at Blombos Cave in the western Cape Province, where the "Stillbay" industry is represented (Henshilwood and Sealy, 1997; dErrico, Henshilwood and Nilssen, 2001). If the minimum date for the Stillbay industry is based in part on the date for the inception of the Howiesons Poort industry, it is clear that there is a need to obtain reliable dates for the latter, recognising that dates such as those based on uranium and thorium are associated with standard deviations around mean values, which are estimates of the actual age. This study has been supported by the National Research Foundation, South Africa. 23 January, 2002 References
Avery, D.M. 1987. Late Pleistocene coastal environment of the southern Cape Province of South Africa: micromammals from Klasies River Mouth. Journal of Archaeological Science 14, 405-421. Deacon, H.J. 1995. Two Late Pleistocene-Holocene archaeological depositories from the southern Cape, South Africa. South African Archaeological Bulletin, 50, 121-131. dErrico, F., Henshilwood, C. and Nilssen, P. 2001. An engraved bone fragment from c. 70,000-year-old Middle Stone Age levels at Blombos Cave, South Africa: impications for the origin of symbolism and language.. Antiquity 75, 309-318. Henshilwood, C. and Sealy, J. 1997. Bone artefacts from the Middle Stone Age at Blombos Cave, southern Cape, South Africa. Current Anthropology 38, 890-895. Jouzel, J.C., Lorius, C., Petit, J.R., Genthon, C., Barkov, N.I., Kotlyakov, V.M., and Petrov., V.M. 1987. Vostok ice core: a continuous isotope temperature record over the last climatic cycle (160,000 years). Nature 329, 403-408. Singer, R. and Wymer, J. 1982. The Middle Stone Age at Klasies River Mouth in South Africa. University of Chicago Press. Thackeray. A.I. 1992. The Middle Stone Age south of the Limpopo River. Journal of World Prehistory 6, 385-440. Thackeray, J.F. 1988. Molluscan fauna from Klasies River Mouth, South Africa. South African Archaeological Bulletin 43, 27-30. Thackeray, J.F. 1992. Chronology of Late Pleistocene deposits associated with Homo sapiens at Klasies River Mouth, South Africa. Palaeoecology of Africa and the Surrounding Islands 23, 177-191. Thackeray, J.F. and Avery, D.M. 1990. A comparison between temperature indices for Late Pleistocene sequences at Klasies River Mouth and Border Cave, South Africa. Palaeoecology of Africa and the Surrounding Islands 21, 311-316. Vogel, J.C. 2001. Radiometric dates for the Middle Stone Age in South Africa. In Humanity from African Naissance to Coming Millennia. P.V. Tobias, M.A. Raath, J. Moggi-Cecchi, and Doyle, G.A. (Eds). Pp 261-268. Firenze University Press and Witwatersrand University Press.
FTHP: January 12, 2002
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