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Recent debate over the initial occupation
of the Jinmium rock shelter, in the Kimberley
region in far northwestern Australia, has
highlighted some of the challenges involved
in using thermoluminescence (TL) to date
sediments in sandstone rockshelters. The
original dating of the Jinmium site published
by Fullagar, Price and Head (1996) suggested
the possibility of initial site occupation
over 100,000 years ago. However, Spooner’s
(1998) alternative interpretation of data
from this site and Roberts et al.’s
(1998) new ‘single-grain’ optically
stimulated luminescence (OSL) dates suggest
an initial occupation date of less than
10,000 years BP.
The younger figure conforms with middle
range theory (Flood 1995) and the conventional
understanding of the Indigenous colonisation
of Australia (e.g. Allen and Holdaway 1995).
The Holocene OSL date is disputed by Price
(1998), the TL expert who dated the original
samples, owing to OSL sampling procedures
and the interpretation of a single selected
grain, the statistics used in OSL, and the
apparent reliability and consistency of
TL. Another indication of Jinmium's antiquity
is the scientifically-ascertained age (30,000
years) assigned to rock art in the site(Tacon
et al. 1997), which was based upon dating
bees-wax figures adhering to the walls of
the rockshelter.
Jinmium's TL was based on analysis of 28
aliquots for each sample removed from the
ground; an aliquot comprised 2,900 quartz
grains (Fullagar et al. 1996:756). However,
there are problems involved with using TL
to determine age for sediments in sandstone
rockshelters. This paper will discuss how
some of these problems may have biased the
original TL dates.
Possible Errors
Fullagar et al. (1996:760) recognise the
possibility that some of the samples at
Jinmium may have been contaminated. In particular
they point out that W1646 might "thus
include grains from in situ disintegrated
rubble and saprolite". However, as
argued by O'Connell and Allen (1998), examination
of Jinmium’s stratigraphy reveals that
other samples may also be open to ‘re-interpretation’
on the same grounds, namely W1645, W1648
and W1752, which were taken from trenches
C1/III, C1/III and Auger hole 5 respectively.
Contamination may occur as part of the
sampling procedure. Ideally, samples should
be taken from a single, homogeneous stratigraphic
layer. Wagner et al. (1983:19) state the
"important consideration in this context
is to ensure that the surroundings of each
sample are homogeneous to a distance of
30cm so that a reliable assessment can be
made of the soil gamma dose-rate reaching
each sample."
At Jinmium, however, the stratigraphy is
such that some of the samples could not
have been acquired under these ideal conditions.
W1645 is directly adjacent to a large, ground,
mudstone feature. W1646 was immediately
on top of an un-excavated area of sandstone,
below a different sandstone pavement. Sample
W1648 was taken from an area located directly
beneath the rockshelter, within 10cm of
a sandstone pavement. In short, all of these
samples were removed from inside the recommended
30cm buffer zone. It is therefore possible
that they were all adversely affected by
varying levels of alpha, beta or gamma radiation
emanating from the non-homogeneous rockshelter
walls, or the ground mudstone or sandstone
pavement features.
Another possible difficulty with the dates
concerns the vital ‘radiation-flux’
measurement (see Aitken 1990:143). Aitken
(1985) states that gamma spectroscopy readings
are vital for obtaining a correct sample
age. It is essential that this reading be
made for each and every individual sample
hole owing to local gamma variation between
all soil samples. Wagner et al. (1983:6)
state that the problem of determining dose
rate can lead to samples being discarded
from a dating program. At Jinmium, though,
level calibration for the TL sample holes
was not measured (Fullagar et al. 1996:755).:
For TL samples taken from Auger hole
A4 and sample numbers W1645-W1648 [that
includes all the dates in question here]
taken from excavation trench C1/III,
in situ gamma spectrometer readings
were made. Because of a technical problem
that developed in the field, these values
were not used in the final age calculations.
Because of the sand sheet depth it was
not feasible at the auger holes, to
make in situ environmental radiation
measurements using the gamma spectrometer.
On these grounds, it is possible that an
inaccurate dose-rate value has been used
in the calculation of these samples ages.
The natural stratigraphy at Jinmium prevented
a large number of samples being removed
from each stratigraphic layer. The content
of the samples may also have placed an upper
limit on the dateable range of sample ages.
Wagner et al. (1983:19) state that while
"some of the factors that cause error
are general to a site, others vary from
sample to sample and it is advantageous
to test half a dozen or more samples from
each context". Although dating accuracy
was increased by multiple analysis of each
sample (Fullagar et al. 1996:756), it was
only possible to remove one sample from
each stratigraphic layer, thereby not addressing
the problem of sample variation within each
layer.
With reference to the limitations of TL,
Aitken (1990:173-174) states that the "furthest
age that can be reached with quartz is limited
by the onset of saturation. In round terms
the limit should be put at about 50,000
years BP, more if the annual dose [of gamma
radiation levels] is low." The Jinmium
TL samples contained quartz. All of the
dates under consideration have values greater
than 50,000 years BP, but the specific gamma
level of each sample remains unknown. Therefore,
it is possible that the dates have been
biased owing to the limitations inherent
in TL dating.
Conclusion
A combination of the difficulties described
above may account for the differences obtained
at Jinmium using various dating techniques.
New OSL evidence obtained by Roberts et
al. (1998) could provide a date younger
than 10,000 BP, but this only indicates
the most recent exposure of the sand grains
to light. OSL is also a relatively new technique
which has yet to be substantiated as a solid
dating procedure. While Roberts and his
colleagues’ initial results are interesting,
it is still possible that initial human
occupation at Jinmium dates well back into
the Pleistocene.
The Jinmium debate questions the importance
and reliability pertaining to different
methods of analytical dating. The relentless
pursuit of scientific authentication in
archaeology has greatly expanded the discipline
over the past fifty years. However education
of archaeologists regarding the theory behind
dating techniques and exact sampling procedures
is currently lacking. Advancements in teaching
and an increase in the number of ‘on-site’
specialists (such as Price) will allow future
research to combine accurate scientific
methodology, with precise excavation, observation
and ethnography. This should hopefully reduce
the chances that future projects will become
mired in the sort of ‘heated’
debate and disagreement that arose over
Jinmium.
References cited
Aitken, M.J. 1985. Thermoluminescence
dating. Academic Press, London.
Aitken, M.J. 1990. Science-based dating
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Allen, J. and Holdaway, S. 1995. The contamination
of Pleistocene radiocarbon determination
in Australia. Antiquity 69:101-112.
Flood, J. 1995. Archaeology of the Dreamtime.
The Story of Prehistory and its People.
(3rd edition). Angus and Robertson. Sydney.
Fullagar, R.L.K., Price, D.M. and Head,
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dating of Jinmium rockshelter, Northern
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O'Connell, J., and Allen, J. 1998. When
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The Sydney Morning Herald June 1,
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The Sydney Morning Herald June 1,
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Spooner, N.A. 1998. Human occupation at
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Tacon, P.S.C., Garde, M., Nelson, E., and
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