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EXPORT OF LUMINESCENCE DATING SAMPLES FROM BOOMPLAAS CAVE FOR THE PURPOSE OF DEVELOPING AGE MODEL

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Case Type: 

ProposalDescription: 

This application is to export geological samples from the archaeological site Boomplaas to the United Kingdom for the purpose of Luminescence dating. The goal of the analysis is to provide the first accurate ages for Boomplaas Cave’s archaeological deposits, especially the site’s lower 3m deposits. This project is taking place at ERF 30/33, Boomplaas, Cango Valley, Eden District, Western Cape (HWC permit case number 18021501AS0223E).

Expanded_Motivation: 

Luminescence dating is an umbrella term for a family of dosimetric dating techniques that usually determine the time elapsed since grains of sand were last exposed to the sunlight (i.e. burial age). In southern Africa the use of optically stimulated luminescence (OSL) dating, primarily applied to grains of sand-sized quartz has revolutionized our understanding of the chronologies of archaeological sites exceeding the upper limits of radiocarbon dating methods (approximately 40,000 years). As the upper age limit of the OSL method is typically greater than 100,000 years, OSL dating allowed significant refinements in our understandings of the Middle Stone Age in general, and the ages of specific stone tool industries (e.g. Jacobs et al., 2008). In the vicinity of Boomplaas Cave, OSL dating with quartz has been extensively used, and has underpinned the chronologies of several key coastal sites, including the Pinnacle Point complex (Jacobs, 2010; Smith et al. 2018), Blombos Cave (Jacobs et al., 2013) and Klipdrift rockshelter (Henshilwood et al., 2014). These results, as well as other OSL analyses and other contexts have consistently demonstrated the suitability for OSL dating methods in this region (Carr et al., 2019; Helm et al. 2022). The sand sized quartz fraction will be analyzed using single grain luminescence methods. This involves the analysis of several thousand individual sand grains to determine their burial doses, and to isolate only those grains that exhibit suitable luminescence characteristics. By analyzing at the single grain level we can also assess the degree to which the grains represent a population associated with a single burial event, the degree (or not) of post depositional mixing/disturbance and/or assess the likelihood of incomplete signal resetting prior to grain burial. From this various mathematical modeling approaches are then used to estimate the most likely burial dose to enter into the final age equations (e.g. Galbraith and Roberts, 2012). Therefore, for this project, we propose to use Luminescence dating to place Boomplaas’ lower stratigraphic horizons more accurately on a reliable and comparable timeline with surrounding sites. Additionally, we will use luminescence dating to test the current age model that has been established using radiocarbon dating. While radiocarbon dating is essential for developing an age model at Boomplaas, it is only reliable for deposits < 40 ka. However, luminescence dating can be used to provide another dating technique that will improve age estimates at Boomplaas. Samples necessary for this project only consist of sand of geological antiquity – NOT soils. No archaeological material was included. All samples were piece-plotted and recorded. Samples will be analyzed by the Dr. Andy Carr at the School of Geography, Geology and the Environment at the University of Leicester. Boomplaas Cave We are applying for an export permit to undertake the first luminescence dating of samples from Boomplaas (BPA) Cave. This project is taking place at ERF 30/33, Boomplaas, Cango Valley, Eden District, Western Cape (HWC permit case number 18021501AS0223E). BPA is an important site to study because it is located along the northern margin of the Little Karoo Basin within a year-round rainfall regime and can provide insight into how environmental change in this rainfall regime may have influenced past populations, specifically during the Middle Stone Age-Later Stone Age transition. The deposits date back to 80 ka; however, the chronology mainly relies on radiocarbon dating which is inconclusive for deposits > 40 ka (Pargeter et al. 2018). Pargeter et al. (2018) redated Boomplaas’ deposits using radiocarbon dating of charcoal assemblages from Hilary Deacon’s excavations, however, this study solely focused on the late Pleistocene deposits. Currently, there is no other reliable dating method establishing the chronology at this site. Five U-series and two AAR ages on dripstone and ostrich eggshell provide the only dates for the site’s lower deposits. It is suspected that the deposits extend back to 80 ka, if not older, due to the presence of a Howieson’s Poort backed tool near the base and the extrapolation of the radiocarbon dates (Pargeter et al. 2018). Regardless, there is no other reliable dating method to independently verify these hypotheses. Field Methods In August/September 2022, we collected one continuous luminescence column from Boomplaas, beginning at the base of the previously excavated stratigraphic section and covering deposits dating to 80-19 ka. Before sampling, the exposed stratigraphic section is cleaned using a trowel and brush to limit potential contamination. High-resolution photography is taken before sampling so that sample locations can be directly placed on the stratigraphic profile in ArcGIS. Starting at the base of the section, 10-20 grams of sediment are collected using a trowel and mini-shovel and stored in glass vials. Between each sample, all tools are cleaned using water and a rag to limit cross-contamination. Every sample is piece potted with a total station so it can be placed on a 3D grid following analyses. All the samples are currently stored at the Palaeoecology Laboratory at Nelson Mandela University in Port Elizabeth. We propose to export for analysis 20 geological samples of about 50 grams each. These luminescence samples are essential for building a high-resolution chronological profile at Boomplaas and building an isochron that can be used to compare deposits from other sites. Figure 1 shows an example of the samples on a rectified (=corrected to grid space) image. Laboratory Methods In detail, the samples are prepared using a wet chemical workup to removal carbonates (dilute HCl) and organic matter (H2O2), then wet sieved to the desired size fraction and density separated to isolate the quartz-rich (<2.70 and >2.62 g cm-3) fraction. The samples are etched in concentrated hydrofluoric acid, and then rinsed and re-sieved leaving a purified quartz sample in the 180-212 µm range. The samples will be analyzed using a Risoe DA20 TL/OSL reader equipped with a single grain laser system. Sub-samples of the sampled material will also be taken in order to determine the environmental dose rate (required for the final age equation). This will involve: 1) the estimation of the concentrations of uranium, thorium and potassium using ICP-MS methods (performed in house at the University of Leicester); 2) low level GM beta counting carried out in the at University of Leicester luminescence dating laboratory. We also performed several in-situ, non-destructive assays of site gamma dose rate during excavation using a portable gamma spectrometer. All sample preparation and analysis will be conducted by ASC.

ApplicationDate: 

Tuesday, January 3, 2023 - 22:00

CaseID: 

20433

OtherReferences: 

ReferenceList: 

CitationReferenceType
Carr, A.S., Bateman, M.D., Cawthra, H., Sealy, J., 2019. First evidence for on-shore Marine Isotope Stage 3 aeolianite formation on the southern Cape coastline of South Africa. Marine Geology 407, 1-15.
Galbraith, R.F. and Roberts, R.G., 2012. Statistical aspects of equivalent dose and error calculation and display in OSL dating: an overview and some recommendations. Quaternary Geochronology 11, 1-27.
Helm, C.W., Carr, A.S. Cawthra, H.C., Vynck. J.C., Dixon, M.A., Lockley, M.G., Stear, W., Venter, J.A. 2022. Large Pleistocene tortoise tracks on the Cape south coast of South Africa. Quaternary Research, in press
Henshilwood, C.S., van Niekerk, K.L., Wurz, S., Delagnes, A., Armitage, S.J., Rifkin, R.F., Douze, K., Keene, P., Haaland, M.M., Reynard, J. and Discamps, E., 2014. Klipdrift shelter, southern Cape, South Africa: preliminary report on the Howiesons Poort layers. Journal of Archaeological Science 45, 284-303.
Jacobs, Z., Roberts, R.G., Galbraith, R.F., Deacon, H.J., Grun, R., Mackay, A., Mitchell, P., Vogelsang, R. Wadley, L., 2008. Ages for the Middle Stone Age of southern Africa: implications for human behavior and dispersal. Science, 322, 733-735.
Jacobs, Z., 2010. An OSL chronology for the sedimentary deposits from Pinnacle Point Cave 13B—a punctuated presence. Journal of Human Evolution 59, 289-305.
Jacobs, Z., Hayes, E.H., Roberts, R.G., Galbraith, R.F. Henshilwood, C.S., 2013. An improved OSL chronology for the Still Bay layers at Blombos Cave, South Africa: further tests of single-grain dating procedures and a re-evaluation of the timing of the Still Bay industry across southern Africa. Journal of Archaeological Science 40, 579-594.
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