New Optical and Radiocarbon Dates from Ngarrabullgan Cave, a Pleistocene Archaeological Site in Australia: Implications for the Comparability of Time Clocks and for the Human Colonization of Australia

Article excerpt

The human settlement of Australia falls into that period where dating is hard because it is near or beyond the reliable limit of radiocarbon study; instead a range of luminescence methods are being turned to (such as thermoluminescence at Jinmium: December 1996 ANTIQUITY). Ngarrabullgan Cave, a rock-shelter in Queensland, now offers a good suite of radiocarbon determinations which match well a pair of optically stimulated luminescence (OSL) dates - encouraging sign that OSL determinations can be relied on.

Archaeologists have been divided over the antiquity of human occupation in Australia and Papua New Guinea since the publication of 50,000-60,000-year-old thermoluminescence (TL) and optically stimulated luminescence (OSL) dates from northern Australia (Roberts et al. 1990; 1994b). One group has argued that this time-range approximates the antiquity of habitation on this continent (Chappell et al. 1996; Roberts et al. 1994a; Roberts & Jones 1994). Another has argued that these dates are erroneous or not directly comparable to the 14C chronology from most other Australian sites (Allen 1994; Allen & Holdaway 1995); they argue that there is as yet no conclusive evidence for human occupation in Australia pre-dating 40,000 radiocarbon years b.p. Recently reported TL dates of 116,000[+ or -]12,000 years BP and beyond for artefact-bearing sands (Fullagar et al. 1996) have further lengthened the time-frame at issue, although a number of chrono-stratigraphic uncertainties make these results open to various possible interpretations. No paired 14C/optical dates beyond 30,000 b.p. have hitherto been available in Australia, partly because stratified sites possessing appropriate sediments and charcoal are rare. Charcoal-rich deposits within a silica-rich sandy matrix now enable two Pleistocene optical age determinations to be set alongside a suite of radiocarbon determinations from Ngarrabullgan(1) Cave in northern Australia.

Ngarrabullgan: site and stratigraphy

Ngarrabullgan is a large (18x6 km) table-top mountain 100 km west of Cairns in north Queensland. Rising 200-400 m above the surrounding plains and hills, it is bounded by high cliffs. Ngarrabullgan Cave is the largest archaeological site on top of the mountain, from which cultural deposits more than 37,000 radiocarbon years old have been obtained (David 1993). The deposits at this site are very dry, and there have been few terrestrial mammals on the mountain-top to disturb sediments; the integrity of strata and the preservation of organic materials [TABULAR DATA FOR TABLE 1 OMITTED] (especially charcoal and microscopic residues on stone tools) are exceptional (Fullagar & David in press).

Dating

The rock-shelter deposit is finely stratified but shallow; it possesses 27 distinct strata over a depth of only 43.5 cm, the top 35.7 cm of which is cultural [ILLUSTRATION FOR FIGURE 1 OMITTED]. Twenty-one 14C dates obtained by accelerator mass spectrometry (AMS) and four radiocarbon dates obtained by beta-counting are listed in TABLE 1.

Radiocarbon determinations

All of the Australian Nuclear Science and Technology Organisation (OZB) AMS 14C dates are on single pieces of charcoal; the Beta Analytic beta-counting radiocarbon results are on multiple pieces of charcoal (TABLE 1). The results show that the period of occupation before 37,000 b.p. may be restricted to a single, thin stratum (3E; [ILLUSTRATION FOR FIGURE 1 OMITTED]) consisting of sparse cultural deposits; this initial period of occupation was followed by one or more episodes of low intensity occupation at [approximately]32,500 radiocarbon years b.p. The site was then abandoned and appears not to have been re-occupied until [approximately]5400 b.p. The 14C dates from substrata 3A-3D exhibit a high degree of self-consistency, with none of the individual pieces of dated charcoal being intruded from the overlying Holocene units. This implies that stratum 3, or at least the extant portion, has remained largely undisturbed since deposition. …