Age estimates for Willandra Lakes human
bones
by Richard Gillespie dizzy@better.net.au
Human bones from more than 100 burials have been found in many locations around the margins of now dry lakes in the World Heritage Willandra Lakes region of southwestern NSW, Australia. The regional geomorphology, environmental reconstruction and archaeological overprint is summarised in Bowler (1998) and the dating in Gillespie (1998). In this discussion, the WLH numbering system for skeletons employed by Webb (1989) will be used, except for 2 skeletons which have become better known under alternative names: Mungo 1 (WLH-1) and Mungo 3 or LM3 (WLH-3).
Bone has been notoriously difficult for all dating methods, particularly
in open semi-arid locations like the Willandra Lakes, because
the material is fragmented and often badly degraded. This usually
means a loss of the organic components (mainly the protein collagen),
alterations to the mineral components and contamination with organics
and inorganics from groundwater and sediments.
Radiocarbon dating (14C)
In well-preserved bone the best fraction for 14C dating is
collagen or it's constituent amino acids. The carbon in these
molecules was originally part of the living body and can be isolated
in high purity. A good overview of bone dating progress is in
Stafford et al (1991). Of 56 Willandra human bone samples analysed,
only 2 contained more than 0.2% nitrogen and are known to contain
well-preserved collagen: WLH-15 & 55 are probably Late Holocene
(Gillespie, 1998).
The next best choice for 14C dating (if collagen is absent) comes
from the carbon preserved by burning or cremation of the skeleton.
Again this carbon was originally part of the living body, but
it is difficult to identify a particular molecular structure to
isolate. Early workers used simple acid washing to remove carbonates,
preferably an alkaline wash is also used to remove humic acids,
with final measurements made on the insoluble residue.
Bones from burials Mungo 1, WLH-9, 23, 24, 28, 44 & 122 were
burnt and contained sufficient carbon to perform acid/base/acid
pretreatment on small fragments for AMS 14C analysis. In all samples
more than 90% of the total carbon present was contained in the
base-soluble humic acids fraction, WLH-44 was completely
base-soluble (Gillespie, 1997).
This acid/base/acid chemistry was originally designed for 14C
dating of charcoal and assumes that strongly acidic and basic
solutions will remove all contamination. For charcoal, dates on
base-soluble humic acids fractions are generally younger than
the residual charcoal, so we can say that younger contamination
has been removed and the insoluble residue date is closer to the
real age.
When measurements on both soluble and insoluble fractions for
Mungo 1 and WLH- 122 were made, the dominant base-soluble fractions
were both significantly older than their insoluble residues.
The original date of ~25,000 BP was based on the acid insoluble
residue (Bowler et al, 1972) and contained mostly humic acids
of that age. As it turns out, many of the so-called charcoal samples
from the Willandra also exhibit this contrary behaviour of humic
acids older than insoluble residue dates. These hearth or fireplace
samples contain mostly base-soluble material with little or no
real charcoal and the insoluble residues contain unburnt fine
organic particulates.
It is also possible to get 14C dates from the carbonate fraction
of bone apatite, although there is evidence that this is unreliable
because of ion exchange with groundwater carbonates. The apatite
date (Bowler et al, 1972) on Mungo 1 of 19,000 BP is between the
insoluble residue date (17,000 BP) and the humic acids at 25,000
BP.
Should we believe the older dates (which always seem curiously
more desirable to archaeologists) measured on base-soluble humic
acids fractions, or the younger dates on insoluble residues? The
unpalatable choice for both black sediment and burnt bone dates
seems to be between soluble or insoluble organic humic substances,
both of which are of unknown composition and dubious origin.
Webb (1989) argued for the older soluble humic acids dates on
these burnt bones, on my advice at the time. I subsequently changed
my view (Gillespie, 1997; 1998) and supported the younger insoluble
dates, on the grounds that humic acids of whatever age are not
likely to represent the burnt carbon we seek in charcoal or burnt
bone. Not much of a choice, and neither may be the real age.
Other dating methods
Have the results from other direct dating measurements on
skeletal remains helped to resolve the Willandra chronology? An
added confusion here is that calibration of radiocarbon dates
is necessary for comparison with other methods supposedly operating
on calendar time, an approximate calibration curve used here is
described in Gillespie (1998) with units of ka (thousands of years
ago).
Electron spin resonance (ESR) and uranium/thorium series (U/Th)
dating methods are secondary in the sense that they rely on measurement
of isotopes not originally present in the living body and the
modelling of processes taking place after burial. Thermoluminescence
(TL) and optically stimulated luminescence (OSL) dating methods
are applied to quartz grains in sediments containing skeletal
remains but not directly on the bones. These methods have advantages
in a longer useful time range than 14C, sometimes with a signal
increasing with time, and can sometimes be non-destructive.
On WLH-50, an ESR estimate of 29 ka on
bone by Caddie et al (1987) is contradicted by a very consistent
set of dates centred around 14 ka from TIMS & Gamma spec U/Th
series measurements (Simpson & Grün, 1998). This would
place the 'robust' WLH-50 burial in a similar age bracket with
'gracile' burials in the Garnpung/Leaghur area such as WLH-23,
24 & 122.
More controversial are the results from LM3, which was originally
estimated from regional stratigraphic evidence at about 30-35
ka (Bowler & Thorne, 1977). A combined age estimate of 62±6
ka based on TIMS & Gamma spec U/Th on bone, ESR on tooth enamel
and OSL on quartz in underlying sediments was proposed by Thorne
et al (1999). These results have been strongly challenged on sampling,
methodology and modelling grounds by Bowler & Magee (2000)
and Gillespie & Roberts (2000), who favour the fairly robust
14C, TL and OSL regional chronology suggesting that LM3 is less
than 43±3 ka. On the available evidence, with LM3 still
of undecided age (and sex, according to Brown, 2000), 9 skeletons
from the Willandra Lakes with direct 14C or U/Th dates (including
Mungo 1 & WLH-50) are younger than 20 ka.
References
Bowler, J M (1998) Willandra Lakes revisited: environmental
framework for human occupation. Archaeology in Oceania,
33, 120-155.
Bowler, J M & Magee, J W (2000) Redating Australia's oldest human remains: A sceptics' view. Journal of Human Evolution 38, 719-726.
Bowler, J M, Thorne, A G & Polach, H A (1972) Pleistocene man in Australia: age and significance of the Mungo skeleton: Nature 240, 48-50.
Brown, P (2000) Pleistocene variation and the sex of Lake Mungo 3. Journal of Human Evolution 38, 743-750.
Gillespie, R (1997) Burnt and unburnt carbon: dating charcoal and burnt bone from the Willandra Lakes, Australia: Radiocarbon 39, 225-236.
Gillespie, R (1998) Alternative timescales: a critical review of Willandra Lakes dating. Archaeology in Oceania, 33, 169-182.
Gillespie, R & Roberts, R G (2000) On the reliability of age estimates for human remains at Lake Mungo. Journal of Human Evolution 38, 727-732.
Simpson, J.J & Grün, R (1998) Non-destructive gamma spectrometric U-series dating. Quaternary Geochronology 17, 1009-1022.
Stafford, T W Jr, Hare, P E, Currie, L Jull, A J T & Donahue, D J (1991). Accelerator radiocarbon dating at the molecular level. Journal of Archaeological Science 18, 35-72.
Thorne, A, Grün, R, Mortimer, G, Simpson, J J, McCulloch, M, Taylor, L and Curnoe, D (1999) Australia's oldest human remains: age of the Lake Mungo 3 skeleton. Journal of Human Evolution, 36, 591-692.
Webb, S G (1989) The Willandra Lakes Hominids, Monograph, Dept. of Prehistory, RSPAS, Australian National University, Canberra.
Willandra Lakes bone dates
| Burial | Lab. No | Fraction dated | 14C Age BP |
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| WLH-1 | ANU-618A | Apatite (carbonate) | 19,030 ± 1300 |
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| (Mungo 1) | ANU-618B | Acid insoluble organics | 24,710 ± 1200 |
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| NZA-246 | Humic acids | 24,750 ± 2400 |
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| NZA-230 | Humic acids | 25,120 ± 1380 |
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| NZA-231 | Humic-free residue | 16,940 ± 635 |
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| WLH-9 | NZA-160 | Humic-free residue | 15,780 ± 430 |
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| WLH-23 | NZA-165 | Humic-free residue | 11,690 ± 580 |
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| WLH-24 | NZA-163 | Humic-free residue | 11,910 ± 340 |
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| WLH-122 | NZA-194 | Humic-free residue | 11,095 ± 370 |
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| NZA-164 | Humic acids | 16,540 ± 540 |
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| WLH-44 | NZA-159 | Humic acids | 18,640 ± 530 |
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| WLH-50 | Postcranial fragments |
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| Partial cranium |
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| Postcranial fragments |
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| WLH-3 | Long bone shavings |
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| LM3) |
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| Partial cranium |
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| Tooth enamel fragments |
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References and dating methods used:
1. Bowler, Thorne & Polach (1972); radiocarbon.
2. Web (1989), Gillespie (1997); AMS radiocarbon.
3. Simpson & Grun (1998)
(a) TIMS 230Th/234U
(b) Gamma spec 230Th/234U
(c) Gamma spec, 231Pa/235U
4. Caddie et al (1987); ESR
5. Thorne et al (1999)
(a) TIMS 230Th/234U
(b) Gamma spec 230Th/234U
(c) Gamma spec, 231Pa/235U
(d) ESR
This page last updated 21 May, 2000.
