Australia: The Land Where Time Began

A biography of the Australian continent 

Ngarrabullgan Cave, a Pleistocene Archaeological Site, Australia - New Optical and Radiocarbon Dates, Implications for Comparability of date and Human Colonisation of Australia 

Site and Stratigraphy

Ngarrabullgan Cave is a rock shelter in Queensland. This is a large flat-topped mountain 18 x 6 km, 100 km west of Cairns, 200-400 m above the surrounding hills and plains, and bounded by high cliffs. This cave, the largest archaeological site on the mountain, has cultural deposits that are more than 37,000 radiocarbon years old (David, 1993). At this site the deposits are very dry and there are few terrestrial mammals on the mountain-top that could have disturbed the sediments, and the strata integrity and preservation of organic materials, especially charcoal and microscopic residues on stone tools, are exceptional (Fullagar & David, 1997).

Dating

A good suite of radiocarbon determinations has been obtained at this site for which there are matches with 2 optically stimulated luminescence (OSL) dates, which the author1 views as an encouraging sign that OSL dates are reliable. According to the author1 humans first arrived in Australia in the period when dating is made difficult by the reduced reliability of radiocarbon dates near the limit of their reliability, dating relying more on a variety of luminescence methods, such as thermoluminescence at Jinmium (Antiquity, December 1996).

Debate has continued concerning the time of arrival of the first humans in Australia since the determinations of OSL and TL dates of 50,000-60,000 BP from northern Australia (Roberts et al., 1990, 1994b). Some have argued that the habitation of Australia took place in this time range (Chappell et al., 1996; Roberts et al, 1994a; Roberts & Jones, 1994), while others argued that these dates are not correct, or are not directly comparable to radiocarbon chronology from other sites in Australia, arguing that there is no conclusive evidence that Australia was occupied before 40,000 BP (Allen, 1994; Allen & Holdaway, 1995). More recently TL dates of 116 ± 12 ka and beyond have been reported for sands containing artefacts (Fullagar et al, 1996), that have lengthened the disputed time frame, though there are some chronostratigraphic uncertainties making these results open to a number of interpretations. For the period beyond 30 ka there are no paired 14C/optical dates available for Australia, partly as the a result of the rarity of sites containing the appropriate sediments and charcoal. In Ngarrabullgan Cave* in north Queensland there are deposits rich in charcoal within a sandy matrix rich in silica that has allowed the obtaining of 2 Pleistocene optical age determinations.

* The spelling of Ngarrabullgan has changed over time (Nurrabullgin, Ngarrabullgin) as a result of instructions from elders of the local Aboriginals.

The deposit in the rock shelter is shallow but finely stratified, with 27 distinct strata over a depth of 43.5 cm, the cultural material being in the top 35.7 cm. The dates obtained for the site are 21 14C dates (accelerator mass spectrometry (AMS) and 4 radiocarbon dates by beta-counting.

See Source 1 for radiocarbon determinations and optical age determinants

Paired age comparison

The best estimate of the optical age of sample ANU.sub.OD122a is 34,700 ± 2,000 BP, which is about 2,200 years older than the mean 14C age of stratum 3, which is about 32,500 BP, a similar degree of underestimation for the 40,000-30,000 BP period has been previously reported. On the basis of coral ages, 234U/230Th (Bard et al., 1993), TL on fireplaces (Bell, 1991) and burnt flints (Boeda et al., 1996), and geomagnetic intensity variations (Guyodo & Valet, 1996; Laj et al., 1996). The AMS 14C ages for stratum 3 are consistent with the optical date, this agreement indicating that the 2 time clocks are broadly comparable over at least this time period (Smith et al., in press). It is implied that the 60,000-50,000 BP dates from TL and optical methods from sediments that were unheated from Malakunanja II (Roberts et al., 1990) and Nauwalabila (Roberts et al., 1994b) do not equate with the 14C dates from less than 40,000 BP, as was suggested (Allen, 1994). The implication of the luminescence dates from Malakunanja II and Nauwalabila I, coupled with the results presented in this article, is that the prehistory of Australia is much older than 39,700 ± 1,000 BP, currently the oldest reliable radiocarbon evidence of the human presence in Australia (O'Connor, 1995).

Sources & Further reading

  1. David, Bruno et al., March 1997, 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, Antiquity, Vol.71, No.271, pp. 183-188

 

Author: M. H. Monroe
Email:  admin@austhrutime.com
Last updated 18/09/2013 
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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading