Richard Gillespie (dizzy@better.net.au)

Dizzy Heights, Ripps Road, Stokers Siding NSW 2484

Three strands of evidence, geology, archaeology and the dating programs based on this fieldwork, conspire in attempts to place significant events in the Willandra on a logical timescale. After 3 decades of research it seems that the conspiracy has failed, because protagonists from these 3 cultures still cannot agree on some of the important issues. Within each of these cultures there are sub-cultures using different methods and theoretical frameworks, which are as likely to be contrary as complementary in any analysis of a complex site. For most of the dating programs, sample chemistry rather than the physical measurement is central to understanding reported results, this will be illustrated by reference to dates on various materials from selected Willandra sites.

Most pretreatment chemistry employed for radiocarbon dating has been rudimentary and ineffective in removing contaminantion, with the resulting dates often unreliable. More recent work has focussed on component identification and selective separation science to isolate, in ideal cases, a single molecule or chemical class for isotopic measurement. For example, chlorite oxidation allows the isolation of cellulose from wood, permanganate oxidation can isolate oxalate from rock surfaces, and stronger dichromate oxidation can remove all unburnt carbon to isolate elemental burnt carbon. Carbonate dates on bone, shell, otolith and eggshell are sometimes unreliable because carbonate can exchange carbon with groundwater, better results are obtained by isolation of protein or amino acid components which can be purified by chromatography.

Chemistry is also important to U/Th, luminescence and ESR technique salthough the reasoning is different: life processes maintain a constant starting amount of cosmogenic 14C, but heavy radionuclides like U and Th not normally present in living organisms are aquired at some unknown time after death. This uncertainty in the boundary conditions requires modelling of radionuclide accumulation and loss, which leads to differing age estimations and interpretations. Similarly in the radiation exposure methods, the analytical chemistry of sediments is fundamental to estimating dose rates. Changing lake and groundwater levels, and deposition or erosion of sediments, produce changes in radionuclide chemistry and cosmic dose rates which affect age estimations.

In the period 45ka to the present, the radiocarbon record is strong: dates on mussel shells, fish otoliths & emu eggshells agree with each other and with charcoal where available. But there are no acceptable charcoal/shell pairs, in my view because there is almost no charcoal in the shell middens. Middens are just rubbish heaps containing lots of shells, sometimes with bones and other food debris, but the associated ‘black stuff’ is largely unburnt carbon – compost. Conversely, in hearths and fireplaces where there is good charcoal there are no shells. Black sediments usually labelled charcoal may be divided into 3 groups based on solubility: macro charcoal with younger humic acids, micro charcoal with no humic acids, and no charcoal with abundant compost that may be older than the insolubles. So there really is no shell/charcoal problem.

Bowler’s in situ black stump at the Lower Mungo/Upper Mungo Unit contact in the Outer Arumpo lunette has good macro charcoalat 40ka, identical in age with numerous shell & fish dates from stratigraphically equivalent locations, and compost slightly older.

The large excavations by Mulvaney and Shawcross near the Mungo 1 burial site have been largely ignored, except for quotation of the charcoal date ANU-1263 at >40,000 BP (which is incorrect due to inadequate measurement on an insufficient amount of a poorly decontaminated sample and should read >18,500 BP). Shawcross considers that sediments from his excavation above the beach gravels at 1.2m are Upper Mungo Unit, whereas Bowler places the same sediments in the Lower Mungo Unit. A fish otolith date from 1.1m of 39.5±1.6ka would appear to support an Upper Mungo interpretation.

This uncertainty in stratigraphic assignment clouds age estimates for the nearby human burials and bones from other Willandra locations, which have 14C, U/Th and ESR age estimates scattered from Modern to Last Interglacial. Possible reasons for this varibility are similar to those applicable to 14C dates on soil carbonates: there is no reliable, all-purpose model for accumulation and loss of the nuclides being measured. It seems likely that the commonly applied EU (early uptake) and LU (late uptake) models for uranium mobility do not place valid boundary conditions on the geochemistry of Willandra samples and some FU (fluctuating uptake) model needs to be developed.

The conspiracy between geological, archaeological and dating programs has not been particularly effective in the Willandra so far, but a few of the many significant events are fairly well defined. When appropriate methods are applied to suitably selected materials, there is generally good agreement between age estimates for Willandra samples, eg. luminescence dates and calibrated radiocarbon dates on the same hearths, or agreement between selective bleach TL, total bleach TL and OSL dates on identical material. By rejecting those age estimates for which considerable doubt exists it is possible to place most of the archaeology into a coherent chronology (Figure 1). General problems in the sequence appear to lie with questions concerning the timing of massive deflation in the Mungo lunette (which exposed the human bones) and of beach gravel deposition events (which shaped the sedimentary sequence).

Figure 1. Summary Willandra chronology based on reliable dates.

		Outer Arumpo	Mungo
Youngest middens	Shells & Fish		19 ± 2
Last Glacial Maximum		20–25 ka
	Shells & fish	26 ± 2
Palaeomagnetic site hearths	Charcoal		33 ± 3
	Luminescence		34 ± 5
Top Hut hearth sites	Charcoal	33 ± 3
Oldest middens	Shells & Fish	40 ± 2	40 ± 2
UM /LM Unit contact, Charcoal		40 ± 3 ka
	Emu Eggshells	41 ± 2
	Luminescence	42 ± 6
Beneath Mungo 3 burial			43 ± 6

Barbetti, M. & Polach, H.A. (1973). ANU Radiocarbon Datelist V: Radiocarbon 15, 241-251.