Optically and infrared stimulated luminescence investigations of the Middle and Later Stone Age in East Africa

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

Refining late Quaternary plunge pool chronologies in Australia\u27s monsoonal \u27Top End\u27

Plunge pool deposits from Australia\u27s \u27Top End\u27 are considered as important archives of past monsoonal activity in the region. The available chronology of these deposits was so far based on thermoluminescence (TL) dating and indicated maximum flood magnitudes during the Last Glacial Maximum in contrast with more arid conditions as deduced from other archives of the region. This study revisits plunge pool deposits at Wangi Falls by applying multiple and single-grain Optically Stimulated Luminescence (OSL) dating of quartz and high-resolution gamma spectrometry, supported by radiocarbon dating of organic material. The aim is to reappraise the existing chronology and investigate if the deposits are affected by partial bleaching, post-depositional mixing and/or problems related to annual dose determination. The latter seems to have a minor impact on the ages at most. Equivalent Dose (De) distributions are broad, in particular for single grains, but apparently not result from partial bleaching or post-depositional mixing. Rather, microdosimetry caused by radiation hotspots in the sediment and zircon inclusions in the quartz grains is considered problematic for these sediments. The results presented here imply that the previous TL chronology overestimated the real deposition age of the sediments

Luminescence characteristics and dose distributions for quartz and feldspar grains from Mumba rockshelter, Tanzania

Mumba rockshelter is a key archaeological site for the Middle and Later Stone Age occupation of East Africa, but its chronology has remained unresolved. We report the results of a dating study, focussing on the characterisation of the optically stimulated luminescence (OSL) and infrared stimulated luminescence (IRSL) properties of quartz- and potassium-rich feldspar grains. Quartz has previously proven troublesome for dating in East Africa. We identified five general types of quartz grain behaviour, based on the shapes of their OSL decay and dose–response curves. Dose recovery experiments and measurement of the natural samples revealed particular problems with quartz grains that had hyperbolic dose–response curves, low characteristic saturation doses and anomalously large changes in sensitivity between regenerative-dose cycles. The use of pulsed irradiation during measurement and standard and additional quality-assurance criteria during data analysis reduced the number of grains that suffered from these problems. We applied these procedures to individual grains of quartz from eight samples and, using appropriate statistical models, identified post-depositional mixing in three samples. The remaining samples had scattered equivalent-dose distributions typical of quartz grains that had received variable beta dose rates during burial. Stratigraphically consistent OSL ages were obtained for all but the deepest sample. Feldspar ages were estimated for the latter sample and three other samples, using the elevated temperature IRSL signal that exhibited low rates of anomalous fading. Fading-corrected feldspar ages for the latter three samples are consistent with their single-grain quartz ages, but single-grain dating is needed to address problems of post-depositional mixing

Chronostratigraphy, site formation, and palaeoenvironmental context of late pleistocene and holocene occupations at grassridge rock shelter (Eastern Cape, South Africa)

© 2020 The Author(s). Grassridge rock shelter is located in the high elevation grassland foothills of the Stormberg Mountains in the Eastern Cape of South Africa. This places Grassridge at an important biogeoclimatic intersec-tion between the Drakensberg Mountains, the South African coastal zone, and the interior arid lands of southern Africa. First excavated in 1979, the approximately 1.5 m stratigraphic sequence was divided into two major occupational components: a 50–70 cm thick Later Stone Age component dating between 7–6 ka and an underlying 50–80 cm thick Middle Stone Age component dated to 36 ka at the base. Here we present a reanalysis of the Grassridge stratigraphic sequence that combines new optically stimulated luminescence and radiocarbon age estimates with sedimentological and microbotanical analyses to evalu-ate site formation processes and the palaeoenvironmental context of human occupations. Results indicate a complex history of geogenic, anthropogenic, and biogenic inputs to the depositional sequence that are interspersed with pulsed human occupation from 43–28 ka, 13.5–11.6 ka, and 7.3–6.8 ka. Microbotanical remains indicate a cooler, drier grassland environment in MIS 3 that transitions to a warmer, moister grassland environment dominated by summer rainfall in the middle of MIS 1. The pulsed occupational sequence identified at Grassridge is characteristic of the Pleistocene and Holocene record across the greater high elevation grassland region of South Africa, which, based on comparison with other currently available evidence, seems linked to a complex system of forager mobility entwined with rapidly fluctuating palaeoenvironments across the last glacial to interglacial transition

Variations in luminescence properties of quartz and feldspar from modern fluvial sediments in three rivers

Studies of modern sediments, their sedimentology and depositional processes are important for understanding the behaviour of the luminescence characteristics of quartz and feldspar in fluvial settings. Previous studies have shown large variations in OSL characteristics of quartz from different fluvial systems, while the IRSL and pIRIR signals from K-feldspar have been understudied. We test the effects of fluvial setting on luminescence characteristics by collecting modern (old) bedload sediments down the courses of three river systems with very different hydrological characteristics, geologic contexts, and catchment lithologies. The single grain (SG) and multi-grain aliquot (MGA) OSL (quartz) and IRSL and pIRIR (K-feldspar) properties of samples were measured and compared to better understand intra- and inter-fluvial system patterns in sensitivity, bleaching, and equivalent dose (De) distribution skewness and kurtosis. The quartz OSL and K-feldspar IRSL and pIRIR signal sensitivities increase with downstream transport distance of sediments, confirming previous studies (quartz) and showing that IRSL signals from K-feldspar also increase in response to reworking cycles. Increasing transport distance also results in better bleaching of the OSL signal from quartz samples (MGA and SG) due to more grains being exposed to sunlight. By contrast, the IRSL and pIRIR signals retain significant residuals in all samples, though 5¿15% of grains yield zero-dose De values and age modelling of SG data yields accurate burial dose estimates. Additionally, the skewness and kurtosis of SG OSL De datasets from one river increase with transport distance, with the best bleached samples exhibiting the highest skewness, thereby questioning the applicability of the skewness-value of a De dataset as an accurate indicator for partial-bleaching. Our data shows marked variability between (i) different river systems and (ii) the measured minerals, however consistent use of statistical models allows accurate De estimation in all contexts. Age modelling of SG data from K-feldspar, thus, provides a valuable tool for future fluvial research in regions where poor OSL characteristics prevent the use of quartz as a dosimeter

Sediment mixing in aeolian sandsheets identified and quantified using single-grain optically stimulated luminescence

Post-depositional mixing processes are extremely common and often obscure a record of deposition in dune and sand sheet deposits. We show that the upper half metre of a dune in southeastern Australia is currently being turned over through bioturbation, but that single-grain OSL dating and contextual knowledge can be used to identify and model these modern mixing processes. In the sandy deposits investigated, mixing processes were observed to be acting to a predicable depth of ~50-60 cm. This observation was used to develop a conceptual framework that can be applied to buried deposits and used to temporally constrain the evolution of the landform and quantify rates of mixing. When our mixing zone conceptual framework was combined with the MAM we show that phases of significant dune aggradation occurred at ~29.9, ~18.3, ~10.3 ka, and continued through the Holocene. We also present an approach using single-grain OSL data to estimate downward mixing rates, which show a strong depth dependency and are coherent with previously reported mixing rates. Modern downward mixing rates indicate that the upper ~50 cm (Zone 1) will be completely turned over on millennial time scales. While caution needs to be used when interpreting archaeological and OSL data from bioturbated sandy environments, our results demonstrate that contextual knowledge and single-grain OSL data can resolve mixing processes and contribute to an understanding of landscape evolution

New ages for middle and later Stone Age deposits at Mumba rockshelter, Tanzania: optically stimulated luminescence dating of quartz and feldspar grains

The archaeological deposits at Mumba rockshelter, northern Tanzania, have been excavated for more than 70 years, starting with Margit and Ludwig Köhl-Larsen in the 1930s. The assemblages of Middle Stone Age (MSA) and Later Stone Age (LSA) artefacts collected from this site constitute the type sequences for these cultural phases in East Africa. Despite its archaeological importance, however, the chronology of the site is poorly constrained, despite the application since the 1980s of several dating methods (radiocarbon, uranium-series and amino acid racemisation) to a variety of materials recovered from the deposits. Here, we review these previous chronologies for Mumba and report new ages obtained from optically stimulated luminescence (OSL) and infrared stimulated luminescence (IRSL) measurements on single grains of quartz and multi-grain aliquots of potassium (K) feldspar from the MSA and LSA deposits. Measurements of single grains of quartz allowed the rejection of unrepresentative grains and the application of appropriate statistical models to obtain the most reliable age estimates, while measurements of K-feldspars allowed the chronology to be extended to older deposits. The seven quartz ages and four K-feldspar ages provide improved temporal constraints on the archaeological sequence at Mumba. The deposits associated with the latest Kisele Industry (Bed VI-A) and the earliest Mumba Industry (Bed V) are dated to 63.4 ± 5.7 and 56.9 ± 4.8 ka (thousands of years ago), respectively, thus constraining the time of transition between these two archaeological phases to ~60 ka. An age of 49.1 ± 4.3 ka has been obtained for the latest deposits associated with the Mumba Industry, which show no evidence for post-depositional mixing and contain ostrich eggshell (OES) beads and abundant microlithics. The Nasera Industry deposits (Bed III) contain large quantities of OES beads and date to 36.8 ± 3.4 ka. We compare the luminescence ages with the previous chronologies for Mumba, and briefly discuss how the revised chronology fits in the context of existing archaeological records and palaeoclimatic reconstructions for East Africa

Drilling 1100-km-long seafloor ridges reveals how palaeoshorelines control carbonate shelf morphologies (North West Shelf, Australia)

The Rowley Shelf, the southern half of the tropical, carbonate-dominated, North West Shelf of Australia, is covered with linear ridges that can be tracked parallel to the coast over 1100 km between the modern coast and the 140 m isobath. Here, we investigate the origin and nature of these ridges based on the integration of extensive borehole data, high-resolution geophysical data, age dating and compaction analysis. Our investigation reveals that each ridge consists of now-submerged relict coastal deposits that were formed over the last 200,000 years through wave, tidal, fluvial and aeolian processes. These features were dominantly preserved through early diagenesis and illustrate the longest continuous submerged palaeoshorelines reported to date. The distribution of relict coastal features, and therefore early diagenesis, controls the morphology of the continental shelf. First, relict coastal features tend to be stacked on top of each other, resulting in the formation of composite diachronous coastal deposits that form distinctive steps on the seabed, up to 10s m high. Second, relict coastal features form a hard substrate that controls the location of coral reefs along the shelf. Reefs, including a 20-m-thick MIS 3 drowned coral reef described here for the first time, as well as modern reefs of the Muiron Islands, Montebello Islands and Dampier Archipelago, are all developed on top of relict coastal features. The distribution, size and mineralogy of relict coastal features highlight climatic changes along the Rowley Shelf. High relative sea levels (RSL) are associated with low carbonate production and up to 50% of terrigenous grains, while low RSL deposits exhibit increased carbonate production and reduced terrigenous grain content. This asymmetry is interpreted to reflect the onset of the monsoon associated with increased fluvial runoffs during interglacial periods. Lastly, our work shows that compaction analyses are critical for palaeoenvironment and RSL reconstructions. Indeed, the accumulation of 50 m of sediment on a RSL indicator can result in an offset of the measurement by 12.7 m