The challenges of direct dating of fossil teeth from the Australopithecus africanus, Taung Child type site, South Africa

This paper presents the analyses of Combined Uranium Series and Electron Spin Resonance (US-ESR) dating on enamel fragments from five fossil teeth from the Hrdlička Pinnacle of the Australopithecus africanus Type Site at the Buxton-Norlim Limeworks, near Taung, South Africa. The mammal teeth were recovered from two types of deposits, the A. africanus bearing Pink Claystone (PCS) deposit that formed as layers within the Thabaseek tufa, and the secondary Yellow Red Sandstone (YRSS) deposit that filled cavities eroded through the tufa and PCS. The teeth were dated using enamel fragments following the US-ESR procedure. Two software programs, USESR (Shao et al., 2014) and DATA (Grün et al., 1988) were employed for the age estimation, however, the USESR software encountered numerous challenges, especially in modelling uranium uptake history. This is the first attempt to date a Late Pliocene to Early Pleistocene tufa deposit using US-ESR dating. The complicated history, poor preservation of the samples, and the diagenetic process in the fossil teeth samples make the dating very challenging. Especially as all samples suffered from uranium leaching and three of the five samples had high uranium concentrations in the enamel. This appears to be related to the nature of the tufa environment where high uranium concentrations have also been identified in secondary flowstones formed in caves within the tufa deposits. The final age estimate for the PCS deposits, contemporary with the Taung skull, is a minimum of approximately 2.4 Ma, based on the EU model. Similarly, a minimum age of approximately 1.8 Ma is estimated for the YRSS deposits. The USESR ages (∼2.0 Ma for the PCS deposit and ∼1.3 Ma for the YRSS deposit) were less preferred due to being derived using an extrapolation function. Nevertheless, all results are associated with many uncertainties and complexities and it should be approached with great caution. Another possibility is that the deposit might be challenging to date using the USESR method. The difficulties of dating old sites like Taung in South Africa and in particular tufa environment have been highlighted.