The rock shelter at Nauwalbila
Radiocarbon analysis of the early archaeological site of Nauwalabila I, Arnhem Land, Australia: implications for sample suitability and stratigraphicintegrity
Auteur(s) / Author(s)
BIRD M. I. ; TURNEY C. S. M. ; FIFIELD L. K. ; JONES R. ; AYLIFFE L. K. ; PALMER A. ; CRESSWELL R. ; ROBERTSON S. ;
Résumé / Abstract
This study presents the results of an extensive radiocarbon dating program at the Nauwalabila I site in northern Australia. The results show that the radiocarbon chronology at Nauwalabila is reliable to ∼130 cm depth, but below this depth coarse charcoal has been variably altered during a period in the early Holocene when an ephemeral groundwater table reached close to the ground surface of the time. Below ∼150 cm none of the radiocarbon ages can be considered to indicate reliably the age of deposition of the sediments. Luminescence dates near the surface and at 110 cm are concordant with the radiocarbon chronology in the upper part of the sequence, and hence the aberrant radiocarbon results below ∼ 150 cm do not constitute a reason to doubt the accuracy of the luminescence chronology deeper in the stratigraphy. A conservative estimate of the age of the sequence, based on extrapolation of results from that portion of the sequence where the radiocarbon chronology is considered to be reliable, is consistent with the chronology proposed previously from luminescence dating. Both chronologies therefore suggest occupation of the site before 50,000 years. Based on sediment characteristics and the distribution of quartz, chert, quartzite and quartz crystal’ artefacts, there is no evidence that there has been significant vertical displacement of artefacts relative to the surrounding sand matrix. Both chemical alteration and physical translocation of charcoal contributed to the aberrant ages at depth in the deposit. The results point to the need for careful assessment of the suitability of charcoal for radiocarbon dating prior to analysis and to the dangers of relying on a small number of radiocarbon dates in the development robust site chronologies. Strategies for screening samples for suitability include (i) microscopic examination, (ii) not analysing samples unless they survive the full ABOX pretreatment, (iii) not analysing samples unless the material is significantly larger than the sediment matrix, (iv) using CHN analysis on both untreated and pretreated material to check for organic contamination and (v) using stepped combustion to check for concordancy in the ages of carbon released at successively higher temperatures.
A very old, but not impossibly old, date for Aborigines in Northern Australia. 65,000 isn’t an impossibility. I’ve had a look around and the thermoluminesence dates from the site seem to agree with the 50,000 date, as do dates from Malakunanja II
The case of Roberts et al. has been recently been significantly strengthened by their announcement of a similar age for the basal deposits of a second Arnhem Land site, Nauwalabila I, 65-70 km south of Malakunanja II. This site contains ‘securely stratified’ artefacts in a rubble base below the sand deposits dated by the related but different luminescence technique, optically-stimulated luminescence (OSL) (Jones 1993; 114; Roberts et al. 1993; Roberts et al. in press). At Nauwalabila I a sequence of five OSL dates are also in stratigraphic order The three oldest samples are 30,000+2400 years (OxODK166) from 1.70-1.75 m depth below surface; 53,400+5400 years (OxODK168) from 2.28-2.40 m; and 60,300+6,700 years (OxODK169) from 2.85-3.01 m. This latter date is below both the rubble layer and the lowest artefacts, while the date of 53,400+5400 years dates the sands immediately above the rubble layer.
The central issue is whether Malakunanja II and Nauwalabila I are really >15,000 years older than any other known Australian site as these dates imply. Luminescence dates measure calendrical years and for that part of the radiocarbon range for which we can calibrate radiocarbon determinations against other dating techniques, uncalibrated radiocarbon determinations mainly underestimate calendrical years. Stuiver et al. (1991: 10) suggest this underestimation is c. 2000 at 14,000 years ago. Mazaud et al. (1991) propose a maximum underestimation of 3000 years between 18,000 years ago and 40,000 years ago and a negligible difference between 45,000 years ago and 50,000 years ago. Bard et al. (1993) indicate that a determination of 18,000 radiocarbon years represents almost 22,000 calendar years. Stuiver & Reimer (1993) use this last date as the oldest in their most recent calibration program. In western NSW, Bell (1991: 48) compared four paired radiocarbon determinations and thermoluminescence dates for separate hearths each c. 30,000 years old, where the TL dates were between 3500 and 5100 years older than radiocarbon determinations. However, substantial comparative sequences of radiocarbon determinations and dates produced by alternative radiometric techniques for the crucial period between 20,000 and 40,000 radiocarbon years are not yet available from anywhere in the world.
From this discussion of the dating of Australian sites. It’s entirely possible that humans were in Oz at this time, as there’s some evidence of contemporary occupation in Asia (Luijiang, at 68,000 years old). So a 60,000 year old entry date to Australia is completely possible.
When did humans first arrive in greater Australia and why is it important to know?
James F. O’Connell, Jim Allen
James O’Connell is Professor of Anthropology at the University of Utah. He has conducted ethnographic and archaeological fieldwork in central Australia, western North America and East Africa, and has published extensively on modern hunter-gatherer ecology, ethnoarchaeology, and Greater Australian and North American prehistory. Jim Allen is currently an Australian Research Council Senior Research Fellow in the Archaeology Department, La Trobe University, Melbourne. Prior to this he was the Foundation Professor of the same department. He has conducted extensive archaeological fieldwork in various parts of Australia and Tasmania and in Papua New Guinea since the mid-1960s. Since 1984 he has concentrated on the Pleistocene archaeology of those countries and published widely on that research.
Until recently, archeologists have generally agreed that modern humans arrived on Australia and its continental islands, New Guinea and Tasmania (collectively, Greater Australia), about 35,000 to 40,000 years ago,1 a time range that is consistent with evidence of their first appearance elsewhere in the Old World well outside Africa.2,3 Over the past decade, however, this consensus has been eroded, first by dates of 50,000 to 60,000 years from two sites in Arnhem Land and then, dramatically, by dates of 116,000 to 176,000 years from a third site on the eastern margin of the nearby Kimberley region. If accurate, these dates require significant changes in current ideas, not just about the initial colonization of Australia, but about the entire chronology of human evolution in the late Middle and early Upper Pleistocene. Either fully modern humans were present well outside Africa at a surprisingly early date or the behavioral capabilities long thought to be uniquely theirs were also associated, at least to some degree, with other hominids. Deciding whether these dates are accurate and associated with definite evidence of human activity thus becomes critically important. We think there are good reasons to be skeptical, not only on the basis of the dates and their alleged associations, but because of their mismatch with established sequences, both in Greater Australia and elsewhere. Until these issues are resolved, adjusting the broader global picture to accommodate these early dates is premature. © 1998 Wiley-Liss, Inc.
However, I’m suspicious of a date over 100,000 years. I think it’s rather more likely there was another hominid group (Erectus) in that locale, and Mungo man agrees with me.