R. Grün, P. Beaumont1, P.V. Tobias1 and S. Eggins
1 McGregor Museum, South Africa
2 Sterkfontein Research Unit, School of Anatomical Sciences,
University of the Witwatersrand Medical School, South Africa
The archaeological
site of Border Cave, Kwazulu Natal, South Africa, has yielded
a number of modern hominid remains, labelled BC1 to BC8. BC5
is a fairly complete lower jaw (Figure 1) which was recovered
by C. Powell in 1974 from the northwest edge of square T20, while
she and one of us (PBB) were collecting sediment samples, at
the request of K.W. Butzer, from the south face of Excavation
Area 3A. It came from the 3 WA (the site shows an succession
of white and brown sediments which have been termed "white
ashes" (WA) and "brown soils" (BS), numbered from
top to bottom), about 0.25 m below its intact surface, and immediately
adjacent to a previously mapped and photographed depression,
the base of which cut by up to 0.15 m into the upper part of
the underlying 4BS.
Border Cave has been the subject of several
dating studies, by radiocarbon on charcoal, ESR on faunal teeth,
amino acid racemisation of ostrich eggshells and thermoluminescence
on burnt flint. Comparison of the dating results found slightly
younger mean ESR dates (in the range of 5 to 10%) than those
of the other dating techniques. This has been attributed to a
small fading component (see Grün and Ward 2002). There are
unfortunately no definite amino acid and TL results available
on layers 3WA and 4BS. If BC5 was contemporaneous with the faunal
teeth found in 3WA its age would be about 66±2 ka (average
ESR age of layer 3WA) and younger than 82±2 ka (average
ESR age of Layer 4BS).
Sillen and Morris (1996) have published and
provided one of us (PBB) with splitting factor (SF) and nitrogen
(N) assays on BC1 to BC7 and on faunal cortical fragments from
the Excavations 3A and 4A sequence that were submitted to Sillen
between 1990 and 1993. The interpretation by Sillen and Morris
(1996) of the Border Cave data (Figure 3) was based on Elands
Bay Cave (EBC) results where SF increases with depth to a limit
at about 20 ka BP. However, no comparable trend is evident at
Border Cave, except for increases from about 2.9 to 3.7 in the
1BS.UP (Iron Age) spits and from about 3.4 to 5.3 in the basal
6BS (arrows in Figure 3). Sillen and Morris (1996) concluded
that "until the differences [of SF factors] between BC3
and BC5 on the one hand, and the MSA fauna on the other can be
explained, these hominids cannot be connected to the MSA period
with confidence".
The following questions arise from the study
of Sillen and Morris (1996):
* Are BC3 and BC5 significantly younger than the layers in which
they were found?
* Is the measurement of lower SF factors in hominid material
proof of their claimed Holocene provenance? |
However, all arguments above are circumstantial
and conclusive proof for the age of BC5 can come only from the
direct dating of this specimen (see also Grün and Beaumont
2001). In 2002, we obtained permission to sample a small tooth
fragment of BC5 for ESR dating. Using a small screwdriver, a
4.6 mg fragment was detached from the loose, partial crown of
the mandibular right third molar and can be fitted back on to
the stump of that tooth which is still in the alveolar part of
the mandible. The tooth from which the chip was detached by Dr.
R.J. Clarke is moderately worn with only a small island of dentine
exposure, in contrast with the more heavily attrited second molar
of the same side, which is in position in the jaw. The degree
of wear of the second and third molars is compatible with this
jaw having belonged to an adult individual. The average thickness
of the fragment was 600±100 µm. First, we used laser
ablation ICP-MS on the mirror surface of the remaining tooth
to obtain U-profiles of the enamel and dentine (for details of
this technique, see Eggins et al. in press). The uranium concentration
in the enamel, 1 to 10 ppb, is close to modern values, whereas
the uranium concentration in the dentine, 220±20 ppb,
is slightly elevated and very uniformly distributed. For measurement
of the dose value we used the non-destructive procedures developed
at ANU and have been reported in earlier annual reports. A 4.6
mg tooth fragment yielded an average dose value of 150±5
Gy. Using the gamma dose rate values of an earlier detailed survey
as well as neutron activation results of a representative sediment
sample, we obtain an age of 76±5 ka. Because of the low
U-concentrations in enamel and dentine, the age estimate is not
sensitive to the mode of U-uptake. When compared to the results
of the previous ESR dating study on faunal material, the age
of BC5 fits exactly into the sequence of previously obtained
age estimates (Figure 2).
Our dating result demonstrates conclusively
that BC5 was buried at the beginning of the deposition of layer
3 WA. Its best age estimate is 76±5 ka. We conclude from
our dating result on BC5 and the above discussion that measurements
of SF and N assays are not particularly well suited to derive
age assignments for the fossil hominids at Border Cave. The age
of the other specimens are also best obtained by chronometric
studies. At present, it seems, ESR cannot provide any further
age information for the Border Cave hominids, as the only other
specimen with teeth, BC3, has vanished. It may, however, be possible
to date enigmatic fossils BC1 and BC2 by U-series dating. On
the other hand, non destructive ESR dating may help to establish
the ages of other important fossils whose age is debatable, e.g.
the mandible of Banyoles.
References
Eggins, S., Grün, R., Pike, A., Shelley, A. and Taylor,
L. (in press) 238U, 232Th profiling and U-series isotope analysis
of fossil teeth by laser ablation ICPMS. Quaternary Science Reviews.
Grün, R. and Beaumont, P. (2001) Border Cave revisited:
A revised ESR chronology. Journal of Human Evolution 40: 467-482.
Sillen, A. and Morris, A.G. (1996). Diagenesis of bone from Border
Cave: implications for the age of the Border Cave hominids. Journal
of Human Evolution 31, 499-506.
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