Heritage Cases

Bushman Rock Shelter, located on the edge of the Highveld in Limpopo Province, is a key Later and Middle Stone Age site, which, despite previous careful excavations conducted by Prof. Eloff in the 60’s/70’s, still lacks detailed published analyses of its archaeological content and chronological context (Porraz et al., 2015). Radiocarbon dating has demonstrated that the Later Stone Age layers document the transition from the Pleistocene to the Holocene, but the chronology of the Middle Stone Age sequence is more difficult to establish. First results of recent Optically Stimulated Luminescence (OSL) dating (Porraz et al., in prep.) suggest that a significant chronological and sedimentary gap separates the Later Stone Age sequence from the Middle Stone Age one, the latter most likely predating MIS 4 (> 71 ka). Interestingly, the lithic industry recovered from the Middle Stone Age layers of Bushman Rock Shelter, both from previous and ongoing excavations (Porraz et al., 2015), has been attributed to the Pietersburg techno-complex. This techno-complex, poorly described and somewhat fallen into anonymity since its first description in the 50’s, has no precise chronological attribution so far. Besides Bushman Rock Shelter, several major archaeological sites south of the Limpopo River have yielded artefacts interpreted as Pietersburg (e.g. Cave of Hearths and Border Cave), but only Border Cave has been tentatively dated (Grün and Beaumont, 2001). Dating the Middle Stone Age sequence of Bushman Rock Shelter using new and accurate dating techniques therefore provides us with the opportunity to pin point, the Pietersburg industry within southern African prehistory. While luminescence dating methods are well adapted to the study of Middle Stone Age sequences because of the time range covered and the quasi ubiquity, in archaeological deposits, of quartz and feldspar grains used by such methods, they also present several drawbacks such as a poor resolution (5-10% uncertainty at one sigma) or parameters that are difficult to handle (e.g. stability of the dose rate over time). Hence, crossing different dating methods can only be of benefit for an improved knowledge of the chronology of Middle Stone Age sites. Here, we intend to apply a new U (Uranium)-series dating methodology on shells. Previous U-series methodologies implied full disintegration of the sample for analysis of the U and Th (Thorium) content. However, in this case, late (post-depositional) input of U and Th could not be avoided, leading either to minimum age estimates or to a complete failure of the dating. More recent developments use standard laser ablation system in order to evaluate the U and Th incorporation (e.g. Sharp et al., 2016) but the dating still rest on modeling of this incorporation. The new methodology that has been developed by our team (collaboration between the IRAMAT-CRP2A, Bordeaux, IPREM-LCABIE, Pau and LSCE, Gif-sur-Yvette, France) takes advantage of the femto-laser ablation system build at the Pau University (Donard et al., 2015; Pecheyran et al., 2017). This laser system allows virtual beam shaping by combining a high repetition rate with a fast galvanometer movement. This is of great interest in order to adjust the apparent laser beam shape to the structure of the sample, striae for instance (Fernandez et al., 2007), while common lasers only allow circular craters at low repetition rate: this results in a much better signal sensitivity (with limits of detection as low as a few attograms, (Donard et al., 2015, Aramendia et al., 2015) and a better spatial resolution (Fernandez et al., 2007). Therefore, the femto-laser ablation system available in Pau university allows: 1) mapping of the U and Th content at high resolution and detecting parts of the shells that have not been polluted by late U and Th incorporations, and 2) reaching the level of sensitivity required for analyzing the very low amount of U and Th present in these parts of the sample. In consequence, no more modeling of U input is theoretically necessary. This laser facility is unique in the world: these analyses can therefore only be conducted in France. The few Achatina snail shell (n = 32) and ostrich eggshell (n = 3) fragments that we are planning to date and for which we are applying for this export permit are small (<2 cm2). Besides, such remains are abundant at the sequence; in other words, their export and partial destruction for analysis will be of no consequence for the general study of the faunal material. references Aramendia, M., Rello, L., Bérail, S., Donnard, A., Pécheyran, C., Resano, M. Direct analysis of dried blood spots by femtosecond-laser ablation-inductively coupled plasma-mass spectrometry. Feasibility of split-flow laser ablation for simultaneous trace element and isotopic analysis. Journal of Analytical Atomic Spectrometry (2015), 30, 296-309. Donard, A., Pottin A. -C., Pointurier F., and Pécheyran C. Determination of relative rare earth element distributions in very small quantities of uranium ore concentrates using femtosecond UV laser ablation – SF-ICPMS coupling. Journal of Analytical Atomic Spectrometry (2015), 30, 2420-2428. Fernandez, B., Claverie, F., Pécheyran, C., Donard, O.F.X., Claverie, F. Direct analysis of solid samples by fs-LA-ICPMS. TrAC - Trends in Analytical Chemistry (2007), 26, 951-966. Grün, R. and Beaumont, P.. Border Cave revisited: a revised ESR chronology. Journal of Human Evolution (2001), 40(6), 467-482. Pecheyran, C., Claverie, F., Tabouret, H., Bareille, G., Grijalba, N., Unceta, N., Bérail, S., Mercier, N., Martin, L., Tribolo, C., Daverat, F., Garnier, D. L’ablation Laser ICPMS: de l’élémentaire à l’isotopie, de l’environnement à la lutte contre la contrefaçon, des premières expressions artistiques de l’homme moderne à la poésie contemporaine. Journée IPGP consacrée aux développements de la mesure Isotopique. 28 avril 2017, Paris. Porraz G., Val, A., Dayet, L., de la Peña, P., Douze, K., Miller, C.E., Murungi, M.L., Tribolo C, Schmid VC, Sievers C. Bushman Rock Shelter (Limpopo, South Africa): A perspective from the edge of the Highveld. The South African Archaeological Bulletin (2015), 70, 166-179. Sharp ,W., Fylstra, N.D., Tryon, C.A., Faith, J.T., Peppe, D.J. U-Th burial dating of ostrich eggshell: A new geochronometer for African archaeological sites. Quaternary International (2016), 404, 198-199.