Stunning claims have recently been made concerning a site in
the Keep River region, eastern Kimberley, Northern Territory.
Dr Richard Fullagar and Dr Lesley Head, who have been excavating
in the Jinmium rockshelter for some years, have with the help of
David Price obtained a series of thermoluminescence age estimates
from the sediment stratigraphy. Although their report still remains
unpublished, they have, together with rock art specialist Dr Paul
Tašon, made spectacular claims about the early occupation of Australia,
and most particularly about the antiquity of the rock art at Jinmium.
There is a large panel of cupules above the excavation, and a small
rock fragment bearing similar cup marks was found in the excavation.
It was therefore reasoned that this stratified fragment would provide
a minimum age for the petroglyphs above ground.
These claims have appeared in the printed and electronic media around
the world, considerably embellished by the imagination of any number
of journalists. That this scoop was made available to only one newspaper
initially has not helped the journalistic treatment either, leading to
thinly disguised rivalry between news groups.
Bearing in mind the certainly provisional nature of the findings, many
archaeologists are concerned about the effects these premature claims
and their stage-managed media release might have on Australian archaeology.
According to the claims, the exfoliated rock art was found between layers
dated at 58000 years and 75 000 years respectively, which is of course
well before the first occupation of the continent as generally recognised.
Moreover, occupation evidence in the form of stone tools still occurs at
levels 'dated' between 116 000 and 176 000 years BP.
This would more than double the duration of human occupation in Australia,
but it also raises the credibility of the evidence.
There are numerous points to consider, but the main ones appear to be that,
firstly, rock art is not generally datable by excavation of a sediment
below it, but direct dating methodology is to be preferred.
Its application was not attempted.
The presence of what appears to be an exfoliated fragment is of little
relevance: we know of other sites where such fragments were found in
inappropriate contexts, e.g. Pedra Furada in Brazil.
Rock fragments can be displaced by many postdepositional processes,
ranging from rodent or termite activity to various taphonomic agents.
Moreover, even if the fragment does occur in its correct stratigraphic
context, it may refer to art that has long since disappeared from the rock
above, where the cupule tradition continued to be copied until more recent times.
The second major problem with the Jinmium claims concerns the dating method
used and its limitations, particu-larly when it is applied to regolithic or
saprolithic sandstones as is the case there.
Most of the quartz grains in exfoliated rock pieces will not be subjected to
daylight before they become buried, so their TL 'clock' will not be 'reset'.
Hence the sediment will appear to be of a greater age than it really is.
Once the fragments have become decomposed, grains become mixed and the TL
result is merely a combined average of those that were exposed to light and
those that were not.
Such a result is not relevant to the age of the sediment in question.
To check it it is essential to use the optically stimulated luminescence
(OSL) method on single grains.
One can then identify two distinctive clusters of 'ages', and discard the
older group entirely.
Dr Bert Roberts, La Trobe University, has used this method at northern
Australian shelter sites and believes that it yields valid age estimates.
These, of course, are expected to be younger than the average values
derived from TL analysis.
Australian TL specialists such as Dr Nigel Spooner and Professor John
Prescott have pronounced the 1996 TL results as meaningless, and as
indicating that the sediments are probably under 20 000 years old.
Recently, charcoal has been found in the basal layers of the Jinmium site,
and has yielded very recent radiocarbon dates from a layer first claimed
to be much older than 100 000 years.
The original Jinmium results as reported in Antiquity, against the
advice of the referees, need to be disregarded, and Antiquity is obliged
to publish an appropriate clarification.
Robert G. Bednarik