Beyond the Radiocarbon Barrier in Australian Prehistory

Article excerpt

The team that has been dating early Australian sites by luminescence methods replies to Allen's (1994) view of the continent's human chronology, published in the June ANTIQUITY (68: 339-43). They argue the strength of the long chronology with their new optical dates.

The issue

Systematic application of radiocarbon dating to archaeological sites in Australia and Papua New Guinea during the 1960s revolutionized knowledge concerning the antiquity of human presence in the region, with established values being extended from mid-Holocene times (Clark 1961: 243) to c. 33,000 radiocarbon years ago (Jones 1973) in little over a decade. It became apparent during the next 15 years that an apparent 'ceiling' had been reached, whereby radiocarbon dates of between 35,000 years and just short of 40,000 years were obtained from a number of disparate locations across the continent.

Two 1989 papers interpreted these data differently (Allen 1989; Jones 1989). Allen (1989), taking this limit literally, argued that some of the oldest dates came from stratigraphically less secure contexts, such as river terraces and other open deposits. However, one of us (RJ) had been concerned for several years that dates of this order of magnitude were close to the theoretical limits of the method and that contamination by even a tiny amount of modern carbon could change a sample of 'infinite' age into one with an apparent age of 40,000 years or less (Jones 1982: 30).

Geomorphological examples

The same issue has been discussed in geomorphological research, for example by Thom (1973) concerning evidence for old sea levels.

More generally, as Chappell (1991) points out, if a deposit which extends in age across the range 20,000 to 100,000 years were to be uniformly sampled, then some 70% of the results ought to be infinite in radiocarbon terms. Were only 1% modern carbon to be added, no sample would give an infinite result and 80% would appear to have an age of about 35,000-40,000 years; this Chappell (1991: 378) calls an 'event horizon'.

Deep lacustrine sequences in Australia, at Pulbeena Swamp in northwestern Tasmania (Colhoun et al. 1982; Colhoun 1985: 48-9), Lake Terang in western Victoria (D'Costa & Kershaw in press) and Lake Eyre in South Australia (Magee et al. in press), all show this phenomenon: 14C age increases steadily with depth back to about 35,000 years BP, and this apparent finite 14C age then continues into deposits at least 80,000-90,000 years old. At a level in the Lake Eyre sequence at Williams Point, for example, Genyornis eggshell was dated by accelerator mass spectrometry (AMS) radiocarbon, by thermal ionization mass spectrometry (TIMS) uranium series, and by amino-acid racemization; the surrounding sediment was dated by thermoluminescence (TL). While the three latter methods yielded ages of c. 50,000 years, the 14C age was only c. 40,000 years BP. AMS radiocarbon values of c. 45,000 years BP were also obtained from fine-grained charcoal and pollen collected from deposits that are at least last interglacial in age (Magee et al. in press).

The famous pollen sequence from Lake George, on the Southern Tablelands of New South Wales, further illustrates this point. 14C dating of organics yielded ages in correct stratigraphic order back to a value of c. 30,000 years BP at a depth of 2 m (Singh & Geissler 1985: 396). 14C ages continued to fluctuate around this value to a depth of 7 m in deposits, that on other grounds, are believed to date to the last interglacial.

In the geological sequences discussed above, few of the 14C ages are stated to be equivalent to 'background'.

Archaeological examples

Among Australian archaeological sequences, Allen (1994: 341) states he has not seen evidence that, when plotted against depth, 14C ages start to 'flatten out' at slightly less than 30,000 radiocarbon years. However, the Devil's Lair site in southwestern Australia (Dortch 1979; 1984: 40-41) shows just such a pattern, with frequent stratigraphic inversion of radio-carbon ages of 30,000-38,000 years BP in deposits of possibly significantly greater age. …