Radiocarbon Determinations, Luminescence Dating and Australian Archaeology

Article excerpt

The September 1993 editorial (ANTIQUITY 65: 44-5) made comment on recent Australian dates, by luminescence techniques, significantly older than radiocarbon determinations from elsewhere in Australia and New Guinea, which formed a single continent in the late Pleistocene. (There are hints also from the Americas of a discrepancy between dates by the two methods.) The period involved, c. 30-60,000 years ago, is crucial also in the Old World mainland, where the beginning of the European Upper Palaeolithic is usually set at c. 35,000 years ago by radiocarbon determinations.

A new view of the chronology of the initial colonization of Australia

In 1989, reviewing the available radiocarbon determinations from Australia (including Tasmania), New Guinea and the nearer Melanesian Islands, I observed that there was no appreciable difference in the oldest radiocarbon ages from this region, from north to south and east to west, with the exception of the centre of Australia where the oldest desert date was then c. 22,000 years ago (Allen 1989). A general antiquity, based on radiocarbon, of 35-40,000 years for the initial colonization of Australia had not been extended during the previous decade, and I concluded that current evidence gave little reason to go beyond a date of c. 40,000 years. However, within a few months Roberts, Jones & Smith (1990a) had published thermoluminescence (TL) dates for the Northern Territory site of Malakunanja II suggesting that it was first occupied by humans between 50,000 and 60,000 years ago. Nine TL dates in stratigraphic order span the period 200[+ or -]1,300 years (KTL-156) from the very top of the site to 107,000[+ or -]21,000 years (KTL-163) at a depth of 4.58 m, about 2 m below the point where artefacts cease. The vital dates towards the bottom of the archaeological deposit are 45,000[+ or -]9000 years (KTL-164) from a depth of 2.30-2.36 m; 52,000[+ or -]11,000 years (KTL-158) at 2.41-2.54 m; and 61,000[+ or -]13,000 years (KTL-162) at 2.54-2.59 m. This latter date corresponds with the lowest artefact recovered during the 1988 re-excavation of the site (Roberts et al. 1990a: 154). There is a marked peak in artefact density from 2.3 to 2.5 m and thus the date of 45,000[+ or -]9,000 years is seen as a minimal date for human occupation; a linear least-squares regression of the nine dates suggests that 50,000 years is a conservative age for the sediments surrounding the lowest artefacts (Roberts et al. 1990b: 127). Note that the errors are not standard deviations in the usual sense, but rather are 'total uncertainties' (see Roberts et al. 1990b: 126; Roberts et al. in press). As Roberts et al. (1990b: 126) note, while these wide error margins pertain, greater confidence can be given to the central tendencies of luminescence dates if multiple, closely spaced samples disclose a pattern of steadily increasing age with depth. While true, at the same time there is a logical constraint to this general argument of stratigraphic superpositioning; it will still work if there is a constant error in the data, such as an inappropriate dose rate in the equation. By itself depth-age correlation is no demonstration of real age, but merely of consistency between samples.

This increase in the accepted antiquity for humans in Australia of between 25% and 50% not unexpectedly drew questions, comments and criticisms (Bowdler 1990; 1991; Frankel 1990; Hiscock 1990) and responses (Roberts et al. 1990b; 1990c). The questions followed two main lines, firstly concerning the technique and especially the large standard deviations associated with the TL dates, and secondly about the association of the human artefacts and the dated sand sediments. The possibility of artefacts moving down into older deposits, raised by both Hiscock and Bowdler, was rejected by Roberts et al. on the grounds that the mixing of artefacts and previously deposited sands in a 'kick zone' would re-set the TL 'clock' of these sediments. …