A cautionary tale from down under: Dating the BlackCreek Swamp megafauna site on Kangaroo Island, South Australia

The extinction of the Australian megafauna is presently one of the most hotly contested debates in Australian Quaternary sciences. [Roberts et al., 2001. U-series and ESR analyses of bones and teeth relating to the human burials from Skhul. Journal of Human Evolution. 49, 316-334.] proposed contentiously that the megafauna went extinct within a short time period somewhere in the range of 39,000-52,000 years ago. Being tucked away at the continental fringe, Kangaroo Island offers an ideal refuge for the megafauna for survival. Initial radiocarbon analyses of soil organic matter, ESR of teeth and OSL of quartz provided consistent age assessments, which strongly suggested that the site could be as young as 20,000 years. However, it turned out that the sediments contained extreme disequilibria in the U-decay chain, with ²³⁰Th/²³⁸U ratios in the range of 0.3 and ²¹⁰Pb/²³⁸U ratios around 0.1. Furthermore, in situ laser ablation analysis revealed that uranium migrated into the teeth at a very late stage during the Holocene. Contrary to expectations, insoluble organic matter was considerably younger than the soluble fraction. After combining all analytical results, a complex geochemical history can be reconstructed, implying that organic matter and large amounts of uranium were injected into the megafauna-bearing layers around the Last Glacial Maximum. When combined U-series/ESR dates are calculated, they all turn out older than the proposed extinction window. This was confirmed by a subsequent OSL study. The Black Creek Swamp site is another example that the parametric early (EU) and linear U-uptake (LU) ESR age models, particularly when applied to teeth with high-U concentrations, may provide completely unreliable age results.