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Retorts

Rating the Dating?

May I comment on the Beavan and Sparks article "Rating the Dating" in your September issue? Their assessment of the controversial age estimations on Rattus exulans bone does not do justice to some of the data.

The two Oxford laboratory ages on Shag River samples, though older than the mean age of the site, still overlap at two standard deviations with the broad range of accepted radiocarbon ages, one of them considerably so when it is adjusted for marine reservoir effect, as recommended by the Oxford laboratory. The four Rafter laboratory determinations, however, are 600-1,400 years older than the range of accepted ages. This difference remains to be explained.

In relation to the Shag River samples, Beavan and Sparks state that "a feature of all of these bones is that they were highly degraded." No such opinion was advised to me by either laboratory, or it would have been reported in my 1996 paper (Archaeology in Oceania 31: 178-184), checked in advance of publication by Sparks and Beavan.

In recent correspondence with them, it is apparent that the statement does not arise from laboratory data on the dated samples but rather from an informal remark by me, earlier this year, about initial interpretation of FTIR (Fourier Transform Infrared Spectroscopy) assays on a different and later set of rat bone samples from Shag River, about half of which were then thought to be degraded (but see below). To extend that comment into a retrospective, blanket condemnation of the dated samples, all of them accepted as suitable by the laboratories concerned, goes beyond the limits of reasonable extrapolation.

In relation to the later samples of Shag River rat bone, referred to above, I have now received a report by John Head (Quaternary Dating Research Centre, ANU), which provides the results of a more detailed analysis of FTIR measurements, as well as crystallinity index, carbonate/phosphate ratio, collagen peak and phosphate peak values. He notes some variation between samples but concludes that "every indicator points to quite good structural preservation of the bone material. The collagen does not seem to have been altered to any extent and bone mineral diagenesis is minimal."

Of course, it is still possible that there has been chemical contamination which would show up in amino acid analyses, but short of that level of investigation, there is nothing to indicate that the Shag River Rattus exulans samples are, or were, incapable of providing satisfactory radiocarbon age measurements by reason of sample degradation.

I should point out that I do not subscribe to any particular view about the period of Rattus exulans colonisation in New Zealand. Insufficient has been published, especially about results from the natural bone deposits, to favour any one of the propositions on offer. Since 1995 I have sought a second opinion on the archaeological age measurements, by sending additional rat bone samples from several sites to dating laboratories in Oxford and Lucas Heights, as well as to Gracefield. The results of this project are not yet available.

Atholl Anderson, Professor of Prehistory, Institute of Advanced Studies, Australian National University