Year

2011

Degree Name

Doctor of Philosophy

Department

centre for Archaeological Science, School of Earth and Environmental Sciences, Faculty of Science

Abstract

The Late Pleistocene was a period during which the behaviour and technology of Middle Stone Age (MSA) and Later Stone Age (LSA) modern humans in East Africa changed considerably. The East African climate shifted between periods of humidity and aridity, and populations of modern humans expanded, contracted and dispersed within, and out of, Africa. During this time, symbolic behaviours proliferated, technological change occurred at a rapid rate and innovative tool types, such as backed microliths, became prevalent. The timing and reasons for these changes in behaviour and technologies remain largely unresolved, however, due to the difficulty of obtaining reliable age estimates for the East African MSA and LSA cultures. Constraining the timing of behavioural and technological changes in the archaeological record can allow an interpretation of why these changes occurred, through comparisons with palaeoclimatic and genetic records. To this end, the main aim of this thesis is to resolve when various changes in behaviour and technology occurred during the East African MSA and LSA. Two sites were chosen to address this issue: Mumba rockshelter in Tanzania and Moche Borago rockshelter in Ethiopia. Sediment samples for optically stimulated luminescence (OSL) and infrared stimulated luminescence (IRSL) dating were collected to construct improved, numerical-age chronologies for the archaeological deposits at both of these sites. OSL dating is a technique that has been used to estimate accurate and precise depositional ages for Late Pleistocene sediments in other geographic regions and archaeological contexts and, thus, has the potential to construct reliable chronologies for the MSA and LSA in East Africa. OSL investigations were conducted on individual sand-sized grains of quartz using the single-aliquot regenerative-dose (SAR) procedure. Substantial grain-to-grain variability in OSL signal behaviour was observed and characterised. Dose recovery tests were used to determine the measurement conditions and data analysis procedures most appropriate for objectively isolating grains dominated by the most light-sensitive component of quartz OSL, for which the SAR procedure can be used to obtain reliable estimates of the equivalent dose (De). De values were estimated for individual grains and the distributions of these values were analysed using well-established statistical models. For samples affected by beta dose rate heterogeneity and post-depositional mixing, corrections were applied and the De populations that best represented the burial ages of the sediments and associated artefacts were used to obtain OSL ages for samples from Mumba rockshelter. Potassium (K) feldspars were also investigated at Mumba to extend the luminescence chronology to the older deposits at this site. Quartz grains from the latter deposits yielded only minimum ages, because the OSL signal was in saturation. The IRSL signal was found to be associated with a high-temperature TL peak at 430°C and the IRSL signal intensity and decay curve shape were largely dependent on stimulation temperature. The IRSL signal measured at an elevated temperature (225°C) after an initial low-temperature (50°C) IR bleach (the ‘post-IR IRSL’ signal) was shown to be bleachable by sunlight, suitable for De estimation using the SAR procedure and suffer only minimally from the malign phenomenon of anomalous fading. Using this signal, De values and fading rates were measured, and the fading-corrected IRSL ages were consistent with the single-grain OSL ages for the younger deposits at Mumba, where both techniques could be applied. Finally, volcanic sediments from Moche Borago were investigated. Quartz was absent or rare and had a poorly-behaved OSL signal that was unsuitable for the SAR procedure. K-feldspars were also investigated and were shown to have substantially different TL and IRSL properties than those from Mumba. Although the IRSL signal passed the standard tests of SAR suitability, the combination of high fading rates, weak luminescence signals and few grains prevented the determination of reliable IRSL ages for the Moche Borago samples. The OSL and post-IR IRSL ages obtained for the archaeological sequence at Mumba rockshelter are in correct stratigraphic order and provide temporal constraints on the significant behavioural and technological changes recorded at the site. First, the revised chronology constrains the timing of the point-based Kisele Industry (~74–63 ka), the backed piece-based Mumba Industry (~57–49 ka) and the scraper-based Nasera Industry (~37 ka). Second, the timing of the emergence of ornamental OES beads associated with the Mumba Industries is shown to have occurred ~49 ka ago. Third, ages for deposits that contain obsidian from distant sources provide a minimum estimate of the duration of likely long-distance exchange networks between the occupants of Mumba and southern Kenya. The improved chronology for Mumba allows the archaeological record at this site to be compared with existing archaeological, palaeoenvironmental, genetic and demographic records for East Africa and the continent more broadly. In doing so, possible reasons for the observed technological and behavioural changes that occurred in the Late Pleistocene can be inferred, and such inferences are discussed in the penultimate chapter of this thesis.

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