Geochronology of Taung and other southern African australopiths

Pieter Vermeesch, Philip Hopley, Nick M. W. Roberts , Randall Parrish

One century after the Taung juvenile’s discovery, its age and that of many other southern African australopiths remains unknown. This lack of accurate geochronological constraints leaves important questions unanswered, including the temporal relationship between the South African fossil sites and their well dated eastern African counterparts. Previous age estimates for the Taung fossil range from 942 ka to 3.03 Ma. Radiometric age estimation is difficult in the absence of the volcanic marker beds that are so useful in eastern Africa. The advent of carbonate U–Pb geochronology has been heralded as the long-awaited solution to this conundrum. However, some notes of caution are necessary, because the 206Pb/238U isochron method requires correction for any initial 234U/238U disequilibrium. In the case of southern African carbonates, this correction sometimes exceeds 50%. We show that, beyond ∼ 1.5 Ma, the uncertainty of the disequilibrium correction may exceed the correction itself, degrading the value of the 206Pb/238U method. We propose the 207Pb/235U method as an alternative to the 206Pb/238U method, because it is largely unaffected by initial disequilibrium. We use this alternative U–Pb dating approach to obtain a new minimum age estimate for the Taung juvenile, by analysing a flowstone situated above, and in stratigraphic succession with, the fossil-bearing deposit. The uncorrected 206Pb/238U isochron age for the sample is 3.30±0.12 Ma. Disequilibrium correction lowers this value to 2.05 + 0.11/ − 0.08 Ma, a correction of 40%. The 207Pb/235U isochron age of 2.961 ± 0.058 Ma is close to the uncorrected 206Pb/238U age. Unfortunately, the two age estimates do not overlap within uncertainty. We attribute this disagreement to open system behaviour of the uranium in the sample, which leads to an overestimation of the 234U/238U disequilibrium correction and an underestimation of the 207Pb/235U isochron age. Consequently, the 207Pb/235U isochron age of ∼ 3 Ma is a minimum age estimate for the flowstone and for the Taung juvenile, placing both firmly in the Pliocene epoch. Disappointingly, LA-ICP-MS screening of borehole samples below the Taung discovery site have not yielded any further datable carbonates. Therefore, we have not been able to pair our minimum age estimate with an equally robust upper age bracket. The search for this additional age constraint continues.