South
African Hominid Sites South
African Hominid Sites Numerous sites in South Africa have yielded australopithecine and early hominid remains. Unlike East African sites such as Olduvai Gorge, this region has no volcanic history so K-Ar dating is not possible. ESR appears to be the best hope for absolute ages in this area.
The hominid site, Swartkrans, occupies a cavern in dolomitic limestone containing at least five calcite-cemented breccia units. Many Australopithecine (Paranthropus) robustus and early rare Homo cf. habilis remains, along with Lower Stone Age bone and stone artifacts, have come from the older Members 1, 2, and 3. Member 4, which contains Middle Stone Age artifacts and many fossils, represents a human occupation site, but Member 5, which lacks artifacts, probably accumulated due to other agents. Attempts to date Swartkrans have been equivocal, because the sediment was added to the cave as a talus cone, which was later eroded, infilled, and re-cemented cyclically. Earlier ESR studies have shown that some teeth in Members 3 and 5 have been reworked from other units.
ESR (electron spin resonance) can provide accurate dates for tooth enamel, the paleontological artifact itself, rather than the associated sediment. The equivalent radiation dose needed to produce the observed ESR signal in fossil enamel integrates over time the total environmental dose rate experienced by the tooth after its deposition. Since the age depends on the uranium (U) history assumed, three limiting cases are calculated assuming early U uptake (EU), continuous (linear) uptake (LU), and recent U uptake (RU), respectively the minimum, median and maximum ages. In isochron analysis, multiple subsamples with variable U concentrations eliminate the need for external dose rate measurements. The slope of isochron equals the age, while the y-intercept gives the externally generated component of the accumulated dose, Aext. Two teeth from Member 3 at Swartkrans have been ESR-dated using isochron analysis.
The two teeth date at 580 ± 150 ka (EU) and 1150 ± 300 ka (LU) with 83-92% confidence in the isochrons. With Aext = 0.050 ± 0.110 Gray, the postulated external dose rates are respectively 0.070 ± 0.200 and 0.050 ± 0.110 mGray/y, which are significantly lower than those measured for the sediments, 0.650 ± 0.166, averaged from 12 sediment analyses. This could indicate that late secondary U deposition in the sediment accompanying secondary calcite deposition. Alternatively, these teeth may have been reworked from older units where the dose rates differ from those measured in the surrounding sediment. If the ages for other tooth from Member 3 are recalculated using the isochron dose rates, two other teeth also give the same ages. Isochron dates from Members 1 and 2 now in progress will help to clarify the ages for all the members.
The Gladysvale site in Transvaal, South Africa recently yielded a hominid tooth from the breccia exposed at the cave mouth. Using a radiation-sensitive signal and the total environment radiation dose rate, electron spin resonance (ESR) dating can calculate the ages for mammal teeth. To model U uptake into the teeth over time, three models are used: early uptake (EU) giving a minimum age, linear uptake (LU) producing a median age, and recent uptake (RU) yielding a maximum age. Two teeth from the upper breccia layer at Gladysvale averaged 660 ± 110 ka (EU), 940 ± 120 ka (LU), or 1.4 ± 0.2 Ma (RU), while the tooth from the underlying breccia cone dated to 1.1 ± 0.1 Ma (EU), 1.8 ± 0.2 Ma (LU), or 3.9 ± 0.5 Ma (RU). Isochron ages for QT83 are 990 ± 120 ka (EU) and 1830 ± 290 ka (LU). Because the cave has been relatively dry throughout its history, the EU model is probably the best model. Therefore, the cave began to fill with the breccia cone ≥1.1 ± 0.1 Ma, while the breccia layer containing the hominid formed at ≥660 ± 110 ka.
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