The good, the bad and the ugly: 20 years of U-series dating carbonates from South Africa.

South Africa has an extraordinary record of human evolution spanning the last ~3 million years, from the fossil remains of our early pre-human relatives in the Cradle of Humankind World Heritage site, through to more recent evidence for the emergence of modern humans. Common to almost all these sites are the presence of carbonate deposits, be they caves, rock shelters or open-air sites. Knowing how old the sites are, understanding the depositional environment and the potential to use the carbonates as records of past hydroclimate variability has motivated many years of research into them. While U-Th dating is a precise, robust, and very useful chronometer in human evolution, the biggest breakthroughs in the last decade have come from U-Pb dating. However, aspects of this chronometer remain a challenging analytical exercise, more-so as the technique becomes routinely applied to carbonates with less-than-ideal U/Pb ratios.

Here we present insights into both the U and Pb concentrations, distributions, and isotope ratios, from a relatively large U-Pb dataset ranging from ~3 Ma to just under 1 Ma. We divide the U-Pb ages into three categories defined by the % error on the U–Pb age (the good, the bad and the ugly), and use thin section petrography and laser ablation trace element transects through the flowstones to investigate the factors controlling the quality of the ages. Our data confirms the expected negative exponential relationship between U–Pb age and 234U/238U. There is no apparent relationship between U concentration and residual 234U/238U, suggesting U concentration patterns are controlled by 238U, not 234U. We show that variability in the amount of inherited Pb across the sampling layer (average variability of 63% relative to sample average) – not uranium (23% variability) or by extension radiogenic Pb – is a main factor controlling the resulting isochron’s quality.

The thin section petrography reveals all flowstones have undergone heavy diagenesis, the dominant fabric consisting of mosaic calcite with relic aragonite. However, we argue that the trace element signals, the abrupt, sympathetic step-like variation is Sr, U and in some cases Ba and Mg, indicate that this diagenesis is conservative. We do show what a completely recrystallized flowstone looks like, where the original trace element signals are completely obliterated, and it is impossible to resolve a U–Pb age. We identify a mixture of crystal and fluid dominated patterns, both of which are ultimately related to flow dynamics, in turn related to changes in external hydroclimate.

We look forward to future development in in situ laser ablation U-Pb dating, which will allow for even more material to be dated. There is great potential to extract valuable palaeoclimate records out of these old, U–Pb dated flowstones, which would be very interesting given their association with important early human evolution sites in South Africa. We predict these two areas will see rapid development in the coming years.