Uranium-Lead dating of hominid fossil sites in South Africa

Abstract

Discoveries of hominid, or apeman, fossils in the twentieth century have brought human evolution into the scientific spotlight. It is important that ages are assigned to such hominids so that their place in human evolution can be established. South African hominids are found in cave sites that are complex in terms of their stratigraphy. Dating methods previously applied at the caves include faunal, palaeomagnetic, electron-spin resonance and cosmogenic isotope methods. These have been unable to give conclusive ages to the fossils. South African hominids are therefore poorly dated in comparison to their East African counterparts, which lie in volcanic deposits that are more simply stratified and more simply dated. U-Pb dating is a radiogenic method proven to be applicable to carbonate deposits. More recently it has been applied to young speleothem deposits with sufficiently high concentrations of U and low levels of common Pb. In this study U-Pb dating is applied to speleothern deposits in stratigraphic context with the South African hominid fossils. Samples were taken from the Silberberg Grotto at Sterkfontein. The hominid fossil, StW 573, was found here in 1998. Three layers of flowstone were sampled, two from above the skeleton and one from below. The combined results indicate an age for StW 573 of ≈ 2.2Ma. This is considerably younger than previous age estimates have inferred. Samples from three other caves were analysed - Kromdraai, Swartkrans and the Limeworks - but these did not produce reliable ages. Within the samples both U and Pb were found to vary spatially over small scales in quantity, and Pb in composition too. Published studies in the area have revealed a major 234U excess in groundwater and speleothem. Where conventional age calculations are used this effect could result in an age much greater than the true age. After the sample analyses presented here, 234U excess analyses of some samples became available and these were used to calculate corrected ages. Variations in initial Pb composition introduced scatter on the age plots but this was not investigated further here. The results are presented firstly as maximum ages and following correction for 234U excess, as best estimates. The flowstone layers immediately above - layer 2C - and below - layer 2B - StW 573 gave maximum abg es of 3.04 ± 0.08Ma (SK3 result) and 2.97 ± 0.13Ma respectively. Layer 2C was corrected for disequilibrium and gave a best estimate age of 2.17 ± 0.17Ma using a weighted average of three results. The corrected age for Layer 2B was 2.24 + 0.09/ - 0.07. There are important implications for the U-Pb age of StW 573. Firstly it provides evidence as to the age and formation of the Sterkfontein cave and therefore other hominid bearing caves with comparable faunal assemblages. The cave sediments exhibit complex stratigraphic relationships that render chronostratigraphy uncertain. More importantly it lends further information on how South African hominids fit into the family tree. At 2.2Ma StW 573 may come under the classification of Australopithecus africanus and be contemporaneous with the fossil Sts 5, also from Sterkfontein, which had previously been considered to postdate StW 573. It also confirms that the South African branch of this genus is probably not as ancient as the East African and that these hominids were not widespread in Africa at 4Ma. With the first evidence for the genus Homo at 2.5Ma, StW 573 is unlikely to be a direct ancestor of modern humans. Beyond their implications for human evolution, these results confirm the applicability of U-Pb dating to carbonates, and more specifically to young carbonates