ESR DATING HUMAN PHYSICAL AND CULTURAL EVOLUTION IN MEMBER 5 AT STERKFONTEIN, SOUTH AFRICA

At Sterkfontein, Member 5 yielded fossil remains from an early Homo. Up to six Australopithecine species and Homo ergaster may occur in Members 3-5. Several technologically distinct lithic assemblages also occur there. Some of the tools from Member 5 retain preserved residues on tools. Sedimentological analyses suggest that successive karsting/precipitation cycles have affected many of the units since their initial deposition. Recent excavations and archaeological investigations suggested that several members as originally defined may actually represent composites of two or more units with very similar lithologies. Because the site is very “lumpy” (sedimentologically and mineralogically inhomogeneous) with a complex history, in situ external dose rate, D_ext(t), values may not provide an accurate estimate for the time-averaged D_ext(t).

In order to date Member 5, more than 25 independent standard ESR dates have been completed for five bovid teeth. To assess the external dose rate, several independent NAA analyses were completed for the associated sediment to allow volumetrically averaged D_ext(t) to be derived (Blackwell & Blickstein, 2000, Quaternary International 68-71: 329-343), but isochron analyses were also completed (Blackwell & Schwarcz, 1993, Applied Radiation & Isotopes 44: 243-252). Since the tooth acts as its own dosimeter, an isochron date can provide both the external dose rate and the age for the tooth.

Assuming linear uptake (LU), two of the teeth date to 110 ± 5 ka and 147 ± 7 ka. Their isochrons indicate D_ext(t)=0.26 ± 0.12 mGray/y, as indicated by the volumetric analyses. The remaining teeth give standard LU ages ranging from 820 ??? 50 ka to 1.6 ± 0.1 Ma. Their isochrons, however, clearly indicated that they have experienced a second diffusional U uptake event, which may have partially equilibrated U concentrations across the teeth (cf. Blackwell et al., 2001, Quaternary Science Reviews 20: 1031-1039). These data suggest that the karstic dissolution that opened fissures in Member 5 which later saw deposition of the younger teeth, occurred some time before 150 ka. This same event may have caused the secondary U uptake in the older teeth. The older teeth’s ages are best modeled with an uptake model intermediate between LU and RU (recent uptake), indicating the correct ages are probably closer to 2 Ma than 1 Ma.